REACTIONS OF AEENEDIAZONIUM SALTS IL-WOO YANG, B . S . A THESIS IN CHEMISTRY Submitted to the Graduate Faculty of Texas Tech University in Partial Fulfillment of the Requirements for the Degree of MASTER OF SCIENCE Approved Accepted December, 1979 ^t^-k%^\, mi ACKNOWLEDGEMENTS The author wishes to express his appreciation to Professor Richard A, Bartsch for his friendly advice and guidance through the cource of this work. He also wishes to express his thanks to' Professors John N, Marx and Wayne H. Smith for their helpful suggestions. 11 CONTENTS ACKNOWLEDGEMENTS LIST OF TABLES I. GENERAL INTRODUCTION 1 Introduction 1 Formation of Arenediazonium Salts 2 Stability of Arenediazonium Salts i Reactions of Arenediazonium Ions ? •General Concepts ^ Replacement of Nitrogen by Muclecphiles (Dediazoniation) 3 Reaction of Nucleophiles at the TerTir.al Nitrogen 10 Nucleophilic Aromatic Substi-u-icr. Activated by the Diazonium Group 1^ Metal Catalyzed Reactions 15 Complexation of Arenediazoniu.T. Zens by Multidentate Ligands Solubilization of Arenediazonium Salts by Macrocyclic Polyethers l6 16/ Crown Ethers as Phase Transfer Catalysts in Arenediazonium Salt Reactions Acyclic rolyethers Research Plan Polyethylene Glycol as a Phase Transfer Catalyst for Arenediazonium Ion Reactions Reactions of Arenediazonium Ions ^axal^/'zed by Iron Pentacarbonyl 19 19 19 -•.r\ II. POLYETHYLENE GLYCOL AS A PHASE TRANSFER CATALYST FOR ARENEDIAZONIUM REACTIONS EXPERIMENTAL 22 General Methods 22 Reagents and Chemicals 22 Solvent Purification 23 Gas Chromatographic Analysis 23 Product Identification 23 Yield Determination 2^ Synthesis of Substrates 24 £-Bromobenzenediazonium Tetrafluoroborate 24 £-Nitrobenzenediazonium Tetrafluoro borate 25 Synthesis of Authentic Samples 26 Synthesis of 2-Bromoiodobenzene 26 Synthesis of £-Nitrobiphenyl 2? Phase Transfer Catalytic Synthesis of Aryl Bromides and Aryl Iodides from Arenediazonium Tetrafluoroborates: General Procedure 2? Synthesis of ^-Dibromobenzene 28 Synthesis of £-Bromonitrobenzene 28 Synthesis of ^-Bromoiodobenzene 29 Synthesis of ^-lodonitrobenzene 29 Phase Transfer Catalytic Synthesis of Unsymmetrical Biaryls from Arenediazonium Tetrafluoroborates: General Procedure Synthesis of ^-Bromobiphenyl iv 30 30 Synthesis of ^-Nitrobiphenyl RESULTS AND DISCUSSION III. 32 ATTEMPTED SYNTHESIS OF ARYL CHLORIDES BY PHASE TRANSFER CATALYZED REACTIONS OF ARENEDIAZONIUM IONS EXPERIMENTAL AND RESULTS 37 General Methods 37 Solvents and Chemicals 37 Synthesis of Substrate: £-Bromobenzenediazonium Tetrafluoroborate 38 General Procedure for the Reactions of £-Bromobenzenediazonium Tetrafluoroborate with Potassium Acetate in the Presence of a Phase Transfer Catalyst and a Chlorine Atom Source 38 DISCUSSION IV. 3I REACTIONS OF ARENEDIAZONIUM IONS CATALYZED BY IRON PENTACARBONYL EXPERIMENTAL AND RESULTS General Methods 41 ^ i44 Reagent and Chemicals I4J4, Purification of Iron Pentacarbonyl 45 Gas Chromatographic Analysis 4^ Synthesis of Arenediazonium Salts 4^ Protodediazoniation of Arenediazonium Ions in Methanol Catalyzed by Iron Pentacarbonyl; General Procedure Chlorodediazoniation of ^-Bromobenzenediazonium Tetrafluoro borate: General Procedure i^ U,Q Bromodediazoniation of £-Bromobenzenediazonium T e t r a f l u o r o b o r a t e : General Procedure 51 C a t a l y t i c Decomposition of £-Bromobenzenediazonium T e t r a f l u o r o b o r a t e i n the Presence of Benzene 52 C a t a l y t i c Decomposition of £-Bromobenzenediazonium T e t r a f l u o r o b o r a t e i n the Presence of Air 52 DISCUSSION 53 Possible Mechanism of the Protodediazoniation of Arenediazonium Ions in Methanol Catalyzed by Iron Pentacarbonyl 53 Substituent Effects in Catalytic Decomposition of Arenediazonium Ions in Methanol 59 Other Reactions of Arenediazonium Ions Catalyzed by Iron Pentacarbonyl 60 Concluding Remarks 61 REFERENCES AND NOTES 62 VI LIST OF TABLES 1. Yields of Aryl Halides from Reactions of Arenediazonium Tetrafluoroborates with Potassium Acetate and CBrCl^ or CH I in Chloroform 3^ 2. Yields of Unsymmetrical Biaryls from Reactions of Arenediazonium Tetrafluoroborates with Potassium Acetate in Benzene 35 3. Chlorodediazoniation Yield of £-Bromobenzenediazonium Tetrafluoroborate Initiated by Potassium Acetate in the Presence of Phase Transfer Catalyst 40 4. Preparation of Arenediazonium Tetrafluoroborates 47 5. Yields of Reduction Products in the Decomposition of Arenediazonium Tetrafluoroborates in Methanol using Iron Pentacarbonyl as a Catalyst 49 6. Chlorodediazoniation Yields of £-Bromobenzenediazonium Tetrafluoroborate Catalyzed by Iron Pentacarbonyl 50 7. Bromodediazoniation of £-Bromobenzenediazonium Tetrafluoroborate in CBrCl^-DMF solution Catalyzed by Iron Pentacarbonyl 51 vii CHAPTER GENERAL 1 INTRODUCTION Introduction The importance of arenediazonium salts in the dye industry, as well as their use as synthetic intermediates in numerous reactions, has encouraged a great deal of investigation in this field of chemistry since the initial identification of dlazotized picramic acid by Peter 1 Griess in 1858. In subsequent work, Griess established many of the main lines of diazonium salt chemistry. Much research followed that of Griess and chemists now have a rather thorough knowledge of the chemistzy and structure of arenediazonium ions. Nevertheless, investigations involving arenediazonium salts continue to be an active research area. Arenediazonium salts are usually very reactive and unstable. Often they are used only as intermediates. Their isolation was generally avoided until the reasonably stable arenediazonium tetrafluorobo2 rates were first prepared in 1913 by Bart . Reasonable stability of arenediazonium tetrafluoroborates in the solid state made diazonium chemistry more versatile and useful. Most of the arenediajzonium ion chemistry involves either loss of N with replacement by various other groups (such as OH in the hydroly- tic decomposition, hydrogen in the protodediazoniation, an aryl group in the Gomberg-Bachman reaction, halogen in the Sandmeyer reaction etc.) or a change of the N^ group as in the reduction to phenylhydrazine or coupling to form ArN^X (where X may be carbon, oxygen, nitrogen, sulphur, phosphorus, etc.). The mechanisms of some of these reactions are somewhat complicated and are still controversial. Due to their ionic nature, reactions of arenediazonium salts are usually conducted in aqueous media or in highly polar organic solvents, such as methanol or dimethyl sulfoxide. Unfortunately, such common polar solvents are also good nucleophiles which may attack aromatic diazonium compounds or intermediates formed from them leading to undesiable side reactions. Therefore, the use of crown ethers as phase transfer catalysts in reactions of arenediazonium salts in nonpolar 4 solvents is currently receiving considerable attention. Formation of Arenediazonium Salts Diazotization is one of the oldest and most extensively used reactions in organic chemistry. Nearly all primary aromatic amines can be converted into their diazonium salts. The original method by which Griess diazotized picrajnic acid consisted of passing nitrous acid gas, prepared by the reduction of nitric acid with starch or arsenious anhydride, into a solution of the amine in cold water or alcohol. Although this method has been replaced by a number of simpler procedures, the basic reaction by which arenediazonium salts are formed is expressed in its simplest and most general way as follows: ArNH^ + HX + HNO^ > ArN^X" + 2H2O (l-l) 3 There can be a number of variations in the operating technique according to the differing properties of amines and the purpose for which the products are to be used: The nitrous acid may arise from different sources; the association of amine and acid may be altered; and the reaction may be caiiried out in aqueous solution or in solution or suspension in various solvents. The simplest and the most widely used method for conducting the diazotization reaction consists of treating the amine dissolved in an aqueous inorganic acid with an alkali metal nitrite at low temperature (O-IO C ) . An acid medium is essential during the diazotization. On the one hand, acidic conditions prevent the shift of the ammonium ;F^ amine equilibrium to the right, which makes the amine insoluble. On the other hand, such conditions are necessary in -order to form the most active nitrosating agent (which is discussed below). Finally, a distinctly acidic medium in the diazotization process prevents the formation of certain side products. The molar proportion of acid usually employed is 2.5 - 3» but for many weakly basic amines, such as £-nitroaniline, which bear electron-withdrawing substitituents on the aromatic nucleus, as many as 6 - 7 moles of acid can be used with advantage. Hydrolysis of the diazonium compounds generated from these amines may occur in aqueous solution with lower acidity. The alkali metal nitrites are used in strictly stoichiometric amounts. An excess, as well as a deficiency, has unfavorable effects on the diazonium compound formed. 4 It was proven that a small amount of free amine in equilibrium with the corresponding ammonium ion is the species that is diazotized. However, publications have appeared recently in which the possibility of direct diazotization of certain arylammonium salts is postulated. 7 In either case , the first stage in the diazotization process is the attachment of a nitroso funtion to the amino group. The nitrous acid formed in the first instant of the reaction between the alkali metal nitrite and the inorganic acid may subsequently react with the inorganic acid, giving irise to a number of equilibria which can be represented o as follows: HNO^^=^H^NO^ ^==^=^=^ N^O^ + H^O ^^_^^ NO"^ + H^O and in the presence of hydrogen halides; H"*" HNO^ Hal" "- H^NO^ V - Hal-NO + H^O (1-3) Thus, many possible nitrosating agents are present in the nitrous acidinorganic acid system. The possible nitrosating agents are arranged in the following sequence with respect to their reactivity towards primary amines. 5 NO nitrosonoum ion IJ5-W0 protonated nitrous acid Br^O nitrosyl bromide 0~NO-N0 V HO-NO nitrous anhydride /' nitrous acid Experimental evidence for the ordering of this sequence was provided in studies by Hammett, Ingold, and others.°' The general features of the conversion of a nitrosamine into a diazonium ion are fairly clear. The nitrosamine is in tautomeric equilibrium with the diazohydroxide, which may react with the inorganic acid and be converted into the diazonium ion: Ar-^H-NO .^ +H A2-N=N-0H .+ -^ Ar-^, (1-^) -HjO The completion of diazotization reactions may be determined by numerous methods and the excess of nitrous acid may easily be removed by adding urea or sulphamic acid. 11 Stability of Arenediazonium Salts As a consequence of the easy loss of molecular nitrogen, many simple arenediazonium salts are more or less explosive in the solid state, especially the most common arenediazonium chlorides. The explosive character is enhanced by oxidizing substituents on the aromatic nucleus, as well as by oxidizing anions. Therefore, arenediazonium salts in which the counter ion is nitrate, chromate, perchlorate, etc. do not exhibit a high degree of stability. Conversely sulphates, 6 hexafluorophosphates, and tetrafluoroborates tend to be more stable. 12 Arenediazonium ions which have a potetial leaving group ortho to the diazonium function are often dangerous because of the ease with which they can form benzynes. In case of the £-C0p and £-S0p~ substituted benzenediazonium ions, fragmentation furnishes the neutral molecules N^ and COp or SOp which have high heats of formation. Several diazo compounds have been patented as explosives. For example, 13 4,6-dinitrobenzene-2-diazo-l-oxide , the substance prepared by diazo- tization of picramic acid, is less sensitive to friction than mercury fulminate or lead azide but equal or superior to both as a detonator. It is well-known that certain metallic salts which form double salts with dlajzonium compounds have the effect of retarding decomposition, Griess 14 described the complex salts from benzenediazonium chloride and auric or platinous chloride and Michaelis double salts with tetravalent platinum. 15 prepared the Using Werner's coordination + theory, these salts are described as (ArNp) AuCl^ - 2+ and (ArNp) 2'^'^^^^ In many cases, caJ.cium, cuprous, tin, and zinc chlorides also yield double salts which have only slight water solubility. Double salts with mercu3:y chlorides have significance in the preparation of organometallic compounds and they have been intensively investigated by Nesmeyanov's school. Recently, it has been possible to affect stabilization of arene4 diazonium ions by the use of macrocyclic polyethers. A more detailed description of the stabilization of the arenediazonium ion by complexation with polyethers is given in the other section of this thesis. 7 Reactions of Arenediazonium Ions General Concepts The reactions of arenediazonium ions have been the subject of 17 extensive study and controversy for the past 70 years. The variety and complexity of behaviour shown by the dia^;onium group is very great indeed. Competing equilibria may occur in solution and sevei^ paral- lel reactions (e.g. ionic, free radical), whose relative importance can be changed by small variations in solvent or reaction conditions, are often possible, Because of the probability that some of the reactions lie on a mechanistic borderline, they are usually classified according to the overall process rather than mechanism. 18 The main types of arenediazonium ion reactions are summarized in Equations 1-5 to 1-8. These include: replacement of nitrogen by a nucleophile (via phenyl cation, S^2, or benzyne formation); reactions of a nucleophile at the terminal nitrogen; nucleophilic aromatic displacements activated by the strong electron-withdrawing diazonium group; and free radical reactions (where Y donor). may be a metal or other electron > Z / VY.N, (1-5) > z- (/ V)—N=N-": (1-6) V-Np"^ 2 ++Z (1-7) > > Y—/ Z—(/ V* + No + Y* (1-8) 8 Replacement of Nitrogen by Nucleophiles (Dediazoniation) There are three possible ionic pathways for the replacement of nitrogen from arenediazonium ion, _la, by a nucleophile Y~. Reaction 1-9 is analogous to the S-^l mechanism and is characterized slow -N, • > ••^^^^ • v> ^ ^^^^^ la (1-9) Ic la > -> Of- l£ (1-10) ,N^ r^^"^ Ji^ >1< -> le by a free phenyl cation, _lb, in the reaction pathway. HY ^ ^ (1-11) Reaction 1-10 is a bimolecular nucleophilic aromatic substitution in which jd can be either a transition state (synchronous loss of Np with attack by Y ) or an intermediate. In the latter case, either the formation or breakdown of Id. can be rate-determing. The elimination-addition pathway 1-11 involves the formation of an airyne. If, followed by the addition of HY. Again any of the steps on this reaction sequence could concei- vably be rate-determing. The three pathways represent mechanistic extremes and many reactions may be on the borderline. 9 The validity of the S^l nature of the reaction is based primarily on the insensitivity of the observed rate of decomposition to the concentration and the nature of added nucleophile Y". Kinetic investigation by several groups have shown that the rate of decomposition of benzenediazonium ion in water is first order in la and shows little 19 20 21 dependence on the concentration of added nucleophile. * ' Mechanistic studies on the well-known Schiemann reaction 22 , which is a good preparative method for fluoroarenes by the thermal decomposition of arenediazonium fluoroborates, have shown that the actual nucleophile in methylene chloride is the ion BFr rather than F and that reaction occurs via the rate-determining formation of a singlet 23 aryl cation (Reaction 1-9). Lewis and coworkers 24 25 26 ' have demonstrated in an elegant series of experiments that the hydrolysis of arenediazonium salts is accompanied by a slower exchange of nitrogens within the diazonium salt itself. This interesting rearrangement was also reported by other groups. ' The rearrangement is now believed to involve a phenyl cation which recaptures nitrogen before the species become separated by solvent. In spite of the general acceptance of the S^^l mechanism in nucleophilic dediazoniation reactions of arenediazonium ions, there is a considerable amount of experimental deta which seems to be more consistant with a mechanism having a bimolecular rate-determining step. Lewis found in several cases that the rate of dediazoniation 10 could be increased by increasing the concentration of anions. 31 Zollinger e_t. al. also observed second-order kinetics for the heterolytic arylation of toluene, benzene, trifluoromethlbenzene, and anisole with benzenediazonium tetrafluoroborate in trifluoroethanol. The distinction between mechanisms 1-9 and 1-10 remains an area of controversy and active research interest. Several studies have shown that the benzyne route, Equation 1-11, is not important in reactions of simple arenediazonium ions in aqueous solution at moderate pH. This is most simply demonstrated by the absence of rearranged products. However, decomposition of £-benzene- 32 33 34 diazonium carboxylate, ' ^^ £-benzenesulphinic acid, or £-benzeneboric acid 35 clearly proceeds by benzyne formation since the benzyne intermediate can readily be trapped either as a dimer or by cycloaddition to a four-pi electron donor. Reaction of Nucleophiles at the Terminal Nitrogen Nucleophiles may attack the terminal nitrogen of arenediazonium ion to fomi azo adducts. The fate of these azo adducts is greatly influenced by the characteristics of the nucleophile and the medium. AzNsN + X" ^ ^ Ar-N=N-X ^ stable product (1-12) Ar* + N^ + X' When X~is a good leaving group and the medium is highly polar. 11 the equilibrium is shifted towards the arenediazonium ion side. Good nucleophiles which can form stable radicals by electron transfer give rise to aryl radicals by hemolytic cleavage of the C-N bond. Stabilization of the azo adduct can be achieved by using good nucleophiles which are relatively poor leaving groups or by conversion to a derivative, such as a diazotate, which is resistance to loss of X~. In the reactions of nucleophiles with arenediazonium ions, the kinetically- controlled cis azo compounds are, in many cases, formed preferentially in spite of the fact that the trans isomers are thermo3 dynamically more stable. Zollinger has explained this preferential formation of cis isomers according to the reactivity of the attacking nucleophile. If the nucleophile is a more reactive one (such as CN , 2SO^ , and OH ), the transition state-of the coupling reactions will come earlier; while a less reactive nucleophile (such as aromatic amines or phenols) will have a late trasition state. Thus the stability of the products will be less important in the former case which yields the kinetically-controlled products, cis isomers. 3^ Bunnett at. al, ' 37 have shown that £-nitrobenzenediazonium ion reacts with sodium methoxide in methanol to yield the trans diazomethoxide and nitrobenzene in approximately equal amounts (Equation 1-13). (1-13) KO,-/ 2 \ Ih , 0 - / ^ ^OMe li 12 Since the relative amounts of l]i and jl were independent of the methoxide concentration, this suggests that there is a common intermediate for these two products. The cis isomer, l£, the common intermediate proposed, is formed rapidly from £-nitrobenzenediazonium ion (k = 3x10 1 mol~ sec" at 23*0), The formation of Ih and jl from l£ occurs at approximately the same rate, A radical mechanism was suggested for the reductive decomposition of l£_ to form nitrobenzene since deuterium was not incorporated when methanol-0-D was used in place of methanol. The equilibrium constant for the formation of a covalent azo compound 1J_ from acetate and benzenediazonium ion lies very much to 39 -5 the side of the starting ion. An estimated value of K = 10 40 (Equation 1-14) has been made. 0 q Ar-N^ + '0-G-CH^ ^ — ^ Ar-N=N-0-G-CH (l-l4) li ^0 Ph-N=N-0-C-CH^ 3 Vf MeCO^ Ph-N=N-0" + > Ph-N=N-0" + (CH^C0)„0 " ^''' ^ Ar-N^ ^ = ± ' y (1-15) Ik Ph-N=N-0-N=N-Ph 11 Ph-N=N-0' + N Im + Ph* (l-l6) 13 In spite of the low concentration of the diazoacetate ^ , it has been used as the reactive intermediate in radical halogenation and arylation reactions. Subsequent attack of phenyl diazoacetate by acetate ion yields a diazotate, which is also a good nucleophile for attacking 4l 42 another arenediazonium ion. * The phenyl diazotate radical, jjn, formed by hemolysis of a diazoanhydride jl plays a key role as hydrogen abstractor in a subsequent step (Equation 1-17) to give diazohydroxide Jjn, which continues the chain process (Equation l-l6, 1-17, 1-18). Ph- + / \ > \ i^lL-^Ph-Ph + Ph-N=N-0H ^ ^ =/ Ph-N=N-0H + CH^COO" Y ^ ^1.17) "^ Ph-N=N0" + CH^CO^H (1-18) 43 Cadogan has demonstrated that the chain reaction can be suppr- essed by the addition of suitable radical traps such as 1,1-diphenylethylene. Aryne foirmation involving acetate-induced elimination from benzenediazonium acetate j ^ can be a competing reaction (Equation 1-19). 6Ph-N=N-0Ac : = I Z ± PhNt AcO" ^ .1 ^ -OAc The formation of benzyne by acetate catalysis appears to be a concerted E2 type elimination. Thus, when the diazonium ion is labelled with deuterium in both positions ortho to the diazonium group. 14 ca, 50^ of the deutermm is retained in the anthracene by which the aryne was trapped. Also the observed primary isotope effect is consistent with a concerted E2 mechanism. The bulk of research concerning coupling reactions has been done in aqueous media, under conditions of differing acidity. The use of buffer solutions improved the understanding of these systems since the reaction rate is dependent on pH. A detailed discussion of those reactions, as well as those reactions involving other nucleophiles (e.g. CN , OH , ArS , nitrogen nucleophiles, and carbon nucleophiles) is beyond the scope of this introduction. Nucleophilic Aromatic Substitution Activated by the Diazonium Group The diazonium group is by far the most strongly electron-withdrawing substituent known, and its effect is approximately equivalent to two nitro groups (<J^=1.9, <^=1.7) . The electron-withdrawing character of the diazonium group activates the aromatic ring towards nucleophilic attack and displacement of suitable leaving groups can occur (Equation 1-20). "^-f>;-y>.-i'-f>*. <-' The rate-determining step of this reaction may be the formation of the Meisenheimer complex. When the effect of the diazonium group is augmented by other 15 electron-withdrawing groups, particulary facile displacement can occur and such reactions may take place in aqueous solution at normal diazo47 48 tization temperatures. * Metal Catalyzed Reactions Replacement of the diazo group by the halogen elements usually requires a special catalyst, such as a cuprous salts (Sandmeyer reaction) or metallic copper (Gatteimann reaction). Various copper salts are also used in the arylation of ethylenic compounds (Meerwein reaction) and related reactions to give biaryls, azoarenes, and arenes. The existence of radical intermediates in these reactions (at least under most conditions) is now generally accepted. 46 Waters has suggested that Sandmeyer reaction is a non-ionic decomposition of the diazonium cation, brought about by single electron transfer 3 frxDm the catalyst. Zollinger has pointed out the simil- ality of this sequence to electron transfer by an inner sphere mechanism in which one of the ligands acts as a bridge between reductant and oxidant, as proposed by Taube.^7 The rate-determing step is pro- Cu^Cl + Cl" ^ > Cu^Cl^ (1-21) ArNg + Gu^Gl" > ArN=NClCu^Cl (1-22) ArN=NClCu^Cl > Ar* + N^ + ClCu^Cl (l-23) ArCl + Cu^Cl (1-24) Ar* + Cu'^Cl^ > 16 posed to be the initial coordination step (Equation 1-22) and the observed inverse proportionality of the rate of the chloroarene 48 formation to the concentration of Cl~ ion is explained by the conversion of OuCl^ (the active form of the catalyst) to CuCl^". The catalytic function of copper metal in Gatermann reactions is believed to be an initial electron transfer to form Cu^ as the 49 active catalyst species. Both addition and substitution products result in the metal catalyzed arylation of unsaturated compounds. The reaction is firstorder in CuCl^ and ArN^, but generally independent of the unsaturated 48 substrate. This suggests that formation of Ar' is rate-determining. Arenediazonium salts are known to react with various kinds of transition metal complexes. The formation of arylazo- or arylmetal complexes is often realized by direct reaction of the arenediazonium salt with a transition metal complex(ionic or neutral) with or without the displacement of an existing ligand. Some of the resulting complexes 51 52 53 54 55 such as those of Pd , Ni , Fe etc. have been found to be useful in organic synthesis. Complexation of Arenediazonium Ions by Multidentate Ligands Solubilization of Arenediazonium Salts by Macrocyclic Polyethers 56 In 1973> Gokel and Cram had reported complexation of benzene- diazonium tetrafluoroborates by crown ethers in chloroform. Arenediazonium ions which usually are insoluble in nonpolar solvents due to 17 their ionic nature were found to have increased solubility in non-polar solvents by formation of lipophilic insertion complexes. A suggested geometry for these complexes has the form presented below. X lo Using proton magnetic spectroscopy integration, it was demonstrated that a solution of l8-crown-6 in CDCl^ dissolved 0.8 mole of solid £-toluenediazonium tetrafluoroborate per mole of crown ether. 56 The effectiveness of complexation of this arenediazonium cation with other crown ethers varied according to the ratio of cation diameter to crown ' ether cavity diameter. Usually a ratio of about 0.8 - 0.9 was preferred (the diameter of the diazonium group is estimated to be 2.4 A ). The influence of aryl group substituents upon the ability of crown ethers to solubilize benzenediazonium tetrafluoroborates in chlorocarbon solvents has been examined in two laboratories. ' All £-substituted benzenediazonium tetrafluoroborates are solubilized to a much greater extent than is the unsubstituted salt. This indi- cates that the primary effect of the substituent is to increase the lipophilicity of the diazonium ion portion of the complex. 18 Crown Ethers as Phase Transfer Catalysts in Arenediazonium Salt Reactions Since 1977, l8-crown-6 and dicyclohexano-l8-crown-6 have been utilized as phase transfer catalysts for a variety of arenediazonium salt reactions (e.g. replacement of the diazonium function by hydrogen, halogen, or an aryl group and azo coupling reaction) in non-polar organic solvents. Hartman and Biffar report that benzenediazonium tetrafluoro- borates bearing electron-wlthrawing groups (£-NOp, 2,4-difluoro-, 3-nitro-4fluoro-) are rapidly reduced by powdered copper in dichloromethane in the presence of catalytic quantities (10 mole%) of dicyclohexano-l8-crown-6. No reaction occured in the absence of the crown ether. ^? ^1 In an elegant series of papers, Gokel and coworkers ' ^3 * developed new methods of protodediazoniation, halodediazoniation, and biaryl formation reactions in organic solvents of low polarity. These reactions utilized 18-crown-6 to phase transfer aryldiazonium tetrafluoroborates and the reaction-initiating potassium acetate into chloroform and benzene. The mechanism for generating aryl radicals by a nucleophilic acetate ion was discussed previously (page 12). The l8-crown-6 is believed to play a crucial role in the metathetical gegen ion exchange 63 process resulting in formation of the transient diazoacetate. Ar-N^BF;^ + ( K ^ OAc" ^ = ± KBF^ + Ar-N=N-OAc (1-26) 19 Acyclic Polyethers Acyclic polyethers are the open chain analogs of crown ethers. They are formed by the repetitive unit (-Y-CH -CH -) where in most cases Y=0 (the so-called glymes). It is important to recognize that open chain compounds may behave very much like crown ethers under some conditions. In order to function like crown ethers, these acyclic compounds must overcome a loss of entropy in wrapping about a cation, this might be accomplished if there are strong interactions between the heteroatoms of the chain and the Lewis acid being complexed. Oligoethylene glycols, CH^(CH CH O) CH , have been used for the 64-68 Recently, complexation of alkali and alkaline earth cations. 69 Bartsch, Juri and Mills reported complexation constants for interact- ions of p-tert-butylbenzenediazonium tetrafluoroborate with polyethylene glycols and their mono- and dimethyl ethers in 1,2-dichloroethane. Research Plan Polyethylene Glycol as a Phase Transfer Catalyst for Arenediazonium Ion Reactions Due to their ionic nature, reactions of arenediazonium salts usually have been conducted in highly polar solvents. As has been mentioned previously, the usual solubility properties and reactivity of this important class of organic reagents are modified markedly by the complexation with crown ethers. Thus, in the presence of suitable crown ethers, ionic arenediazonium salts may be solubilized 20 in nonpolar organic solvents such as chloroform."^ The crown ether l8-crown-6 has been utilized as a phase transfer catalyst for reactions of arenediazonium salts in chloroform and benzene. Although the initial solubilization studies of Gokel and Cram"^ indicated that the cyclic polyether structure was requisite for efficient aryldiazonium ion complexation, Bartsch and coworkers ' "^^ have recently observed that substantial complexation may be achieved with acyclic polyethers. The complexation constant measured for a strongly interacting glycol, polyethylene glycol 1,000 was approximately one fifth of that for the crown ether 18-crown-6. This result indicates that polyethylene glycol 1,000 might provide the modified solubility and reactivity of arenediazonium salts previously obtained with crown ethers. This possibility has important economic ramifications since polyethylene glycol 1,000 is a readily-available, industrial product of very low cost compared with crown ethers. For those reasons stated above, it was decided to investigate the effectiveness of polyethylene glycol 1,000 in reactions of arenediazonium ions which had previously utilized l8-crown-6 as a phase transfer catalyst. Reactions of Arenediazonium Ions Catalyzed by Iron Pentacarbonyl As has been mentioned previously, some transition-metal carbonyls 53 react directly with arenediazonium ions. Clark and Cookson examined the reactions of arenediazonium salts with nickel carbonyl and found 21 that gradual addition of the mixture of nickel carbonyl and ethanol to a suspension of an arenediazonium salt in ethanol favored reduction to arenes over carbonylation. Schrauzer reported in I96I that aqu- eous arenediazonium chloildes reacted with iron pentacarbonyl in acetone or methanol to give the carboxylic acids together with diarylketones and aryl chlorides. 71 More recently, Parlman -^ observed that £-bromobenzenedlazonium tetrafluoroborate can be reduced to bromobenzene in good yield by the iron pentacaxbonyl-catalyzed decomposition of the salt in methanol. + _ ArN^BF^ + CH^OH Fe(CO)2 _ ^ ArH (1-27) The protodediazoniation of arenediazonium salts in alcohol is often accompanied the formation of appreciable quantities of the aryl 37 alkyl ethers. Thus, Parlman's observation indicated a useful syn- thetic method for the deamination of aromatic amines via arenediazonium ions. Therefore, it was decided to investigate the reactions of arenediazonium ions in methanol with iron pentacarbonyl more generally using various aromatic amine substrates to see the effects of the substituents in the aromatic ring on the protodediazoniation and to determine the possible mechanisms of this reaction. CHAPTER II POLYETHYLENE GLYCOL AS A PHASE TRANSFER CATALYST FOR ARENEDIAZONIUM ION REACTIONS EXPERIMENTAL General Methods Melting points were determined on a Mel-Temp apparatus (Laboratory Devices) and are uncorrected. Infrared spectra were recorded on a Perkin-Elmer 457 spectrophotometer using sodium chloride plates. All infrared absorptions are reported in wavenumbers (cm ) . Gas- liquid phase chromatography was performed on a Varian Aerograph Series 2400 flame ionization detector gas chromatograph or an Antek 400 flame ionization detector gas chromatograph. Nitrogen was used as a carrier gas for all gas chromatographic analysis and unless otherwise stated, the flow-rate of the carrier gas was 30 ml./min. through 1/8" diameter columns. Reagents and Chemicals Bromotrichloromethane, ir.ethyl iodide, £-bromochlorobenzene, "D-bromofluorobenzene, and £-dibromobenzene were obtained from Aldrich Chemical Co., Inc. Cyclohexylbenzene, iluoroboric acid, and potassium acetate were obtained from J.T.Baker Che.-lcal Co. D-Bromoaniline, £- nitroaniline, £-nitrobipher.yl, and £-iodor.itrobenzene were obtained 22 23 from Eastman Organic Chemicals. Sodium fluoroborate was obtained from Spectrum Chemical Mfg. Co. Scientific Co. Sodium nitaite was obtained from Fisher l8-Crown-6 was obtained from Fluka AG. Buchs BG. Polyethylene glycol 1,000 was obtained from Wilkens Instrument and Research Inc. The dimethyl ether of polyethylene glycol 1,000 was prepared by Dr. P.N.Jurl. Potassium acetate was powdered and dried in an oven at 120''C for several hours before use. Solvent Purification Commercial chloroform was shaken repeatedly with concentrated sulfuric acid until no further color developed in the acid. The resulting chloroform was washed with a solution of sodium bicarbonate and then with water, dried with calcium chloride, and fractionally distilled. ACS certified thiophene-free benzene was fractionally distilled and the fraction of boiling point range 79.5 - 81 C was collected. Gas Chromatographic Analysis Product Identification All reaction products were analyzed by gas-liquid phase chromatography. Columns used were: a 5' x l/8" column of 5% SE 30 on Chromosorb P (Column A); and a 10' x l/8" column of 20^ SE 30 on Chromosorb W (Column B ) . Products were identified by the comparison of the retention times with those of authentic samples. In some cases, use of the two 24 different columns or two different column temperatures for a single column was helpful for increasing the reliability of product identification. Yield Determination The yields of the reaction products were determined by the internal standard method and corrected for the detector response using pre-determlned molar responses of internal standards and authentic samples according to following equation: Molar Response = ^^-^^^ ^^ i n t e r n a l standard moles of authentic sample peak area of authentic sample peak area of i n t e r n a l standard Average values of peak area i n t e g r a t i o n s from more than five i n j e c t i o n s were used for molar response and yield calculations in order to minimize errors. Synthesis of Substrates p-Bromobenzenediazonium Tetrafluoroborate Diazotization of £-bromoaniline was done according to the l i t e r 72 ature procedure by dropwlse addition of a solution of sodium n i t r i t e in water to an aqueous solution of £-bromoaniline and a 2.5 molar excess of hydrochloric acid. The r e s u l t i n g diazonium s a l t solution was f i l t e r e d and added slowly with vigorous s t i r r i n g to a solution 25 containing an excess of sodium fluoroborate in water. The crude £bromobenzenediazonium tetrafluoroborate was purified by dissolving it in a minimum amount of acetone and then precipitating it by addition of diethyl ether. The resulting white arenediazonium salt was daried quickly in the air by spreading thinly on porous paper, which was supported on a wire netting and located near a hood. The yield was 77^ of theoretical value with mp. 134-5*'c (sealed tube, decompose. Lit. 137-138*0, decompose 73) . The infrared spectrum (Nujol mull) showed bands at 3110 cm (aromatic C-H stretching vibration), 2300 cm" (N -1 cm stretching), 1590 -1 (aromatic C=G), 1555 cm" (asymmetric NO stretching), a broad band centered at approximately IO5O cm" 830 cm (BFf), and another band at (aromatic out-of-plane C-H bending). p-Nitrobenzenediazonium Tetrafluoroborate The title compound was prepared according to the literature procedure 74 by dropwise addition of a solution of sodium nitrite in water to the aqueous solution of £-nitro aniline and fluoro boric acid. Due to the continuously thickening precipitate which formed duilng the addition, efficient stirring was required throughout the reaction. After the addition was completed, the crude diazonium salt was collected by suction filtering on a sintered-glass filter. The solid diazonium salt was washed once with cold fluoro boric acid, twice with 95^ ethanol, and several times with diethyl ether. The crude £-nitrobenzenediazonium 26 tetrafluoroborate was purified by dissolving it in a minimum amount of acetone, precipitating it by addition of ether, and air dried. A 95^ yield was obtained, mp. 143-144*C. (sealed tube, decompose. Lit. mp. 157-158'C, decompose"^^). The infrared spectrum (Nujol mull) exhibited bands at 3110 cm" (aromatic C-H stretching vibration), 2295 cm" (Np stretching), 1595 -1 -1 cm (aromatic C=C stretching), 1520 cm" (asymmetric NO stretching), 1300 cm (symmetrical NO stretching), a broad band centered approxi- -1 1 mately at 1020 cm (BF."), and 845 cm (C-N stretching). Synthesis of Authentic Samples The following authentic samples are prepared according to Gokel's 61 62 methods ' and separated by column chromatography. Synthesis of p-Bromoiodobenzene Potassium acetate (O.6O g., 6.0 x 10 mol.) was added in one portion to a stirred mixture of £-bromobenzenedlazonium tetrafluoro-3 -^ borate (O.8O g., 3 x 10 ^ mol.), l8-crown-6 (0.04 g., 1.5 x 10 mol.), and methyl iodide (10 ml., O.I6 mol.) in 30 ml. of chloroform. The mixture was stirred for 3 hrs. and then filtered. The filtrate was washed with 10^ aqueous sodium bisulfite and dried with sodium sulfate. The solvent was removed in vacuo and the resulting orange-red solid was chromatographed on a 30 cm. column of alumina (Fisher Adsorption) using hexane as the eluant. £-Bromoiodobenzene was obtained as white 27 plates: Yield 0.64 g. (75^, mp. 91-92*'c (Lit. mp. 92*'C). Synthesis of 4-Nitrobiphenyl To a suspension of 7.0 g. (0.03 mol.) of £-nitrobenzenediazonium tetrafluoroborate and 0.40 g. (0.015 mol.) of 18-crown-6 in 300 ml. of benzene was added 6.0 g. (0.06 mol.) of potassium acetate in one portion. After 2 hrs. of stirring, the reaction mixture was filtered and washed with 10^ aqueous sodium bisulfite and then dried with sodium sulfate. The resulting darkish-red solution was chromatographed on a 30 cm. X 1.2 cm. alumina (Fisher Adsorption) column using methylene chloride as the eluant. The yellow solution which eluted was evaporated and the resulting yellowish-brown solid was recrystallized in ethanol. Yield 3.88 g. (65^), mp. 112-ll4'C (Lit. mp. 114-114.5''C). Phase Transfer Catalytic Syntheses of Aryl Bromides and Aryl Iodides from Arenediazonium Tetrafluoroborates; General Procedure A 25 ml. flask fitted for nitrogen purge and magnetic stirring was wrapped with aluminum foil. Stirring and the nitrogen purge were started after adding arenediazonium tetrafluoroborate (0.60 mmol.) and polyethylene glycol 1,000 (varing amounts) or l8-crown-6 (0.03 mmol.) to 6 ml. of an appropriate solvent combination. After the mixture was allowed to stir ca. 5 min., powdered potassium acetate (1.20 mmol.) was added in one portion. In most cases, the solution turned 28 yellow immediately. After stirring for appropiriate time intervals (l- 4 hrs.) at room temperature, an internal standard was added to the reaction mixture and the solution was analyzed by gas chromatography. Synthesis of p-Dibromobenzene £-Bromobenzenediazonium tetrafluoroborate (O.I63 g., O.6O mmol.) was stirred in solvent combination (CHCl , 5 ml.; CBrCl , 1 ml.) with potassium acetate (0.12 g., 1.20 mmol.) in the presence of varing amounts of polyethylene glycol 1,000 (O.03 mmol., 0.10 mmol., and 0.20 mmol.) for 2 hrs. as described in the general procedure. The yields of reaction products were determined by gas chromatographic analysis of the reaction mixture using cyclohexylbenzene as an internal standard on Column A. Yields of £-dibromobenzene were 47^ (with 0.03 mmol.), 55% (with 0.10 mmol.), and (i'^fo (with 0.20 mmol.) according to the amount of the polyethylene glycol 1,000 used. When reaction was carried out using l8-crown-6 as a phase transfer catalyst, a 70^ yield of £-dibromobenzene was revealed in gas chromatographic analysis. Synthesis of p-Bromonitrobenzene The title compound was synthesized by stirring £-nitrobenzenediazonium tetrafluoroborate (0.142 g., O.6O mmol.) in the solvent combination of 5 ml. of chloroform and 1 ml. of bromotrichloromethane 29 with potassium acetate (0.12 g., 1.20 mmol.) in the presence of 0.20 mmol. of polyethylene glycol 1,000, The yield of the reaction was determined by gas chromatographic analysis using £-dibromobenzene as an internal standard on Column A. The yield of £-bromonitrobenzene was 84^. Another reaction was carried out as above using l8-crown-6 as a phase transfer catalyst and the formation of £-bromonitrobenzene was realized in 63^ yield. Synthesis of p-Bromoiodobenzene £-Bromobenzenedlazonium tetrafluoroborate (O.I63 g.i O.6O mmol.) was stirred in 6 ml. of solvent combination (CHCl^, 4.5 ml.; CH^I, 1.5 ml.) with potassium acetate (0.12 g., 1.2 mmol.) in the presence of polyethylene glycol 1,000 (0.20 g., 0.20 mmol.) for 2 hrs. as described in the general procedure. The reaction mixture was analyzed by gas chromatography using £-dibromobenzene as the internal standard on Column A. The yield of £-bromoiodobenzene was 79^. When the reaction was carried out employing l8-crown-6 as the phase transfer catalyst, a 68^ yield of £-bromoiodobenzene was reallized. Synthesis of p-Iodonitrobenzene The title compound was synthesized by stirring £-nitrobenzenediazonium tetrafluoroborate (0.142 g., O.6O mmol.) in the solvent 30 combination of 4.5 ml, of chloroform and 1.5 ml. of methyl iodide with potassium acetate (0.12 g,, 1,2 mmol.) in the presence of 0.20 mmol. of polyethylene glycol 1,000. After 2 hrs. of stirring £-dibromobenzene (internal standard) was added to the reaction mixture and gas chromatographic analysis was conducted using Column A. The yield of £-iodonitrobenzene was 1^%, The similar reaction conducted in the presence of 0.03 mmol. of l8-crown-6 showed the formation of the title compound in (:>^% yield. Phase Transfer Catalytic Synthesis of Unsymmetrical Biaryls from Arenediazonium Tetrafluoroborates; General Procedure Potassium acetate (0,12 g., 1.20 mmol.) was added in one portion to a stirred, colorless mixture of axenediazonium tetrafluoroborate (0.60 mmol.) and polyethylene glycol 1,000 (0.03 mmol., 0.10 mmol., or Q.20 mmol.) or l8-crown-6 (0.03 mmol.) in benzene (6 ml.) at room temperature in a 25 ml. flask protected from the light and purged by nitrogen. After stirring for 2 hrs., a known amount of gas chromatographic internal standard was added. The yields of biphenyls were determined by comparison with the internal standard using gas chromatography. Synthesis of p-Bromobiphenyl The title compound was synthesized by stirring £-bromobenzene- 31 diazonium tetrafluoroborate ( O.I63 g., O.6O mmol.) with potassium acetate (0.12 g., 1,20 mmol.) in 6 ml. of benzene in the presence of 0.03 mmol., 0.10 mmol., or 0.20 mmol. of polyethylene glycol 1,000. Gas chromatographic analysis (internal standard, biphenyl; Column A) showed the formation of £-bromobiphenyl in 28% (with 0.03 mmol.), 40% (with 0.10 mmol.), and 65% (with o.20 mmol.) yields according to the amount of polyethylene glycol 1,000 used. When a reaction employing 0.03 mmol. of l8-crown-6 as the phase transfer catalyst was conducted, the formation of the title compound in 80% yield was realized. Synthesis of p-Nitrobiphenyl £-Nitrt)benzenediazonium tetrafluoroborate (0,142 g., O.6O mmol.) was stirred in 6 ml. of benzene with potassium acetate ( 0.12 g., 1.20 mmol.) in the presence of polyethylene glycol 1,000 (0.20 g., 0.20 mmol.) for 2 hrs. as described in the general procedure. The yield determined by gas chromatography using £-bromobiphenyl as an internal standard and Column A was (^5% of £-nitrobiphenyl. The yield of the reaction carried out emplojring O.O3 mmol. of l8-crown-6 as a phase transfer catalyst was RESULTS AND DISCUSSION One of the earliest sources of aryl radicals used was N-nitrosoacetanilide, whose rate-determining rearrangement to the diazoacetate is followed by rapid dissociation of the diazotate to give aryl 75 radicals. Another important source of aryl radicals is the Gomberg reaction, in which sodium hydroxide is added to a vigorously stirred solution of the cold diazonium salt and aromatic substrate. The Gomberg reaction is believed to involve formation of the covalent diazohydroxide which decompose to give aryl and hydroxyl radicals. A somewhat cleaner modification of the Gomberg reaction, developed by Hey, involves the use of sodium acetate instead of sodium hydroxide.76 The Gomberg and Gomberg-Hey procedures suffer from the disadvantage that a heterogeneous system is used. This problem was overcome by using crown ether as a phase transfer catalyst in organic solvents. Thus, the crown ether catalyzed phase transfer of acetate ion and aryl diazonium ions into a non-polar medium followed by reaction of the two species has been found to be an effective and mild method 63 for the generation of aryl radicals. The mechanism whereby diazo- acetate breaks down to give aryl radical and the role of crown ether in this system has been discussed in detail in the previous chapter. The use of glymes and oligoethylene glycol dimethyl ethers, CH^(CH CH 0) CH^, for complexation of alkali and alkaline earth cations and as phase transfer catalysts in reactions involving these 64-67 salts is currently receiving considerable attention. 32 33 From the experimental results shown in Table 1 and Table 2, it becomes clear that polyethylene glycol 1,000 is also an effective agent for phase transfer catalytic generation of aryl radicals from arenediazonium salts as initiated by potassium acetate in chloroform or benzene solution. Aryl radicals thus generated react with various halogen atom sources such as methyl iodide and bromotrichloromethane to form aryl halides. The reaction in benzene solution leads unsymmetrical biaryls. In Table 1, it is noted that the yield of bromodediazoniation product from £-bromobenzenediazonium tetrafluoroborate is increased from 47% to (>1% by increasing the amount of polyethylene glycol 1,000 used. The same trend is shown in Table 2, where the yield of £-bromobiphenyl is'the highest in the reaction which employs 0.20 mmol. of polyethylene glycol 1,000. In view of the diazonium ion complexation 70 by polyethylene glycols, investigation of Bartsch, Juri, and Mills , it seems that the product yields shown here are mainly determined by the effectiveness of complexation of diazonium ions by multidentate ligands. Yields of bromodediazoniation and iododediazoniation products obtained from arenediazonium ions with the higher concentrations of polyethylene glycol 1,000 shown in Table 1 are equal to or greater than those obtained with l8-crown-6 as the catalyst. However, for the biaryl foinning reactions listed in Table 2, polyethylene glycol 1,000 appears to give somewhat lower yields than found with l8-crown-6 3^ C O •H -P TD '^ O 0) H r^ cd EH o CQ c O •H -P O cd <D « g s CH CQ 0) Tj •H rH a C^ vO 0) rH O £ o c; •H (r> o o o o o s O TH O C\2 O O JJ .» C^ o o a\ (N. CO \o ^ c^ • 3 1 -P G 0) tjD H K cn O >» rH O <D tH W 03 i (T^ rH fH "S EH pq 0) CQ 03 o •Tj fi o3 1 1 1 £ P^ O <N2 (T^ O o Cv2 O O O <r\ o o C^2 o O o o o O O O iH •<H .. H o O z * ;>5 CD MD 1 c 3 P U O 1 CO TH fi Q) H >s x: -p 0) vO 1 C 5 * • P ^ O 1 CO >5 rH o PH •H H rH o o !>> o o !>5 H O K ;3 •H CO ca c^ O O O (D C 0 H TJ >> Xi (^•"V 0) c: CD 0) rH >s J3 -P <D \o1 ^ -C -P CD ;>j O 1 00 -tH !>5 o P^ cj p fH rH > j iH o ^ vo1 c ^ p ^ o1 00 TH -P CD § • •3 H O P PH TH • "^.^ C^ (H •2 pq o -p o < CQ 03 C V (H iZ! 0) fciC O o !^ 13 o =fi PQ «. » «. _ _ H t r . . X 4^ •H js 8 o rH 5: e o ^ x^ ^ pq 1 PM ^ . V O 1 PM ^ ^ K\o O vO - "S pq 1 PM o ,i4 S tj CQ G O •H -P O 03 CD O . o o 1 PM - > rH o o3 G •H • •a >^ • ^ ft CD ft G o •H -P O o3 CD ft ft CD G O CD • -P rH 03 CQ CD ;3 -p ?5 CD • O •H x: -p ft x*-^ CQ (D ft <; Tf fcuD (D B e CD e ;3 •H G o fcq 03 •H T3 -P G e o o P ft Ti CD -p 03 o3 iH -P o K o •3 CQ CQ ^ CQ •H CQ >> rH T3 § • e •H G O s • •»H o CD O ft iH c CD CH T3 CD O o H CD iH O (H CD •H t>i xi v^^ 0^ • CQ o3 § •H -p CD CD Tj G Id 5 O •H -P 03 C •H CO ft U C o O CD r-i H !>5 S U U O o3 g • o ft f4 o3 • ^ CD fclD -P •H G m o • G Tj 0) •H J-l /'"S H rH o o H C -P :3 c H CD >H •n •H Ti PH ^ «% -p <; S CD > TH iH O O ^ O O e o s..^^ < !^ -P CD H (D .> TH ^ o G •H -p O O O CQ r—1 CD •H CO vO iH O O o r-i H ^H O <D <D <D O £>O^ OVA v O ^ C^ O P ?H -p < 1 m ,n ,o <+H •H tH 1 s S ^ < 1 fH EH ^ 1 • o ;3 r-{ l+H Oj ^ -P 0) h ^ ?4 ^ <; rQ o I >r) o fP u o tq 03 •H Ti rH 1 •H >H C ?H <; TJ rH <D •H c; •H o3 TJ iH CD CQ 0 -P g H tjD i e T3 CD G *^ ft CO vO CD -P o3 • (D o 5 X* sft -p o c Table 2 35 Yields of Unsymmetrical Biaryls from Reactions of Aryldiazonium Tetrafluoroborates with Potassium Acetate in Benzene.^ Aryl Grou-p £-BrC^H^ 18-Crown-6 (0.03) 80 II Polyethylene Glycol 1,000 (0.03) 28 11 Polyethylene Glycol 1,000 (0.10) 40 II Polyethylene Glycol 1,000 (0.20) ^5 II Polyethylene Glycol 1,000 Dimethyl Ether (0.20) 52 l8-Crown-6 (0.03) 78 Polyethylene Glycol 1,000 (0.20) (^5 S-NOjCgH^ II All reactions were carried out at a diazonium ion concentration of 0.1 molar at room temperature in the dark under a nitrogen atmosphere. b Yield ^. determined by glpc analysis based on diazonium s a l t . 36 even at the higher concentration of polyethylene glycol 1,000. Substitution of the dimethyl ether of polyethylene glycol 1,000 for polyethylene glycol 1,000 produced a lower yield of biaryl than found the glycol itself. In conclusion, the results of the present experiments demonstrate that polyethylene glycol 1,000 is an effective agent for phase transfer catalyzed reactions of arenediazonium salts initiated by potassium acetate in chloroform and benzene. Although substantially higher catalyst concentrations are required to achieve the same yields with polyethylene glycol 1,000 compared with l8-crown-6, the low cost of polyethylene glycol 1,000 relative to l8-crown-6 is an important compensating factor. CHAPTER III ATTEMPTEID SYNTHESIS OF ARYL CHLORIDES BY PHASE. TRANSFER CATALYZED REACTIONS OF ARENEDIAZONIUM IONS EXPERIMENTAL AND RESULTS General Methods The same instruments and methods similar to those described in the previous chapter were used for identification and analysis of the compounds in this chapter. Molar responses for authentic samples and materials used as internal standards were determined and used for calculating product yields from the gas chromatographic analysis data. All reaction mixtures were analyzed utilizing a 5' x l/8" column containing 5% SE 30 on Chromosorb P with the flow rate of the carrier gas (nitrogen) being maintained at 30 ml./min. Solvents and Chemicals Chloroform was purified by the same method as in the preceding chapter. Carbon tetrachloride (NMR grade, Norrel Chemical Co., Inc.) was used directly without fu2rbher purification. Commercial methylene chloride was purified following a literature procedure by washing with water and sodium carbonate solution, drying over calcium chloride, and fractionally distilling. 77 37 38 N-Chlorosuccinimide was obtained from Parish Chemical Co. tertButyl hypochlorite was obtained from Chemalog Chemical Dynamics CO. Sulfuryl chloride and 1,1,2-trichlorotrifluoromethane was obtained from Aldrich Chemical Co. N-Chlorodiisopropyl amine was prepared by mixing equimolar quantities of diisopropyl amine (Eastman) and aqueous sodium hypochlorite i^5*'^%, Purex Co.) at 0 - 5*C, stirring for 30 min., and washing the separated organic layer with 5% aqueous sulfuric acid. The sources of l8-crown-6, polyethylene glycol 1,000, £-dibromobenzene, £-bromochlorobenzene, and potassium acetate were given in the preceding chapter. Potassium acetate was powdered and dried in an oven at 120"c for several hours before use. Synthesis of Substrate; p-Bromobenzenediazonium Tetrafluoroborate The title compound was synthesized according to the method described in the preceding chapter by diazotization of £-bromoaniline in aqueous HCl solution with sodium nitrite, followed by addition of sodium fluoroborate. Identity of the purified white powder of the diazonium salt was confirmed by its melting point and infrared spectrum. General Procedure f o r the Reaction of p-Bromobenzenediazonium T e t r a f l u o r o b o r a t e with Potassium Acetate in the Presence of a Phase T r a n s f e r G a t a l y s t and a Chlorine Atom Source 39 A 25 ml. flask fitted for nitrogen purge and magnetic stirring was wrapped with aluminum foil. Stirring and nitrogen purge were started after adding £-bromobenzenediazonium tetrafluoroborate (O.I63 g., 0.60 mmol.) and l8-crown-6 (8.0 mg., O.O3 mmol.) or polyethylene glycol 1,000 (0.20 g., 0.20 mmol.) to 6 ml. of solvent which sometimes contained an additional chlorine atom source. The resulting mixture was allowed to stir ca. 5 min. Powdered potassium acetate (0.12 g., 1.20 mmol.) was added in one portion to the stirred mixture. The solution turned yellow immediately in most cases. After 2 hrs. of stirring the internal standard (£-dibromobenzene, 0.140 g., O.6O mmol.) was added. If needed, the reaction mixture was centrifuged before injecting into the gas chromatographic column in order to prevent the syringe being plugged by the solid material suspended in the reaction mixture. The yields of the reaction products were determined according to the method described in the preceding chapter and summarized in Table 3. 40 ir ^ rQ •afl o o ft o :3 -P 0 EH e ^ •H G O N 03 •H T3 0 iH EH o m p ft PQ 1. PM •H -P o3 •H G o N 03 .r-l P-i n o oft <N2 iH TH ^ \o 1 1 1 • vO ^ 00 CO CO ^ T-t O T-l u>> C\? ^ C^i o Cvi cv G 0 CQ 0 fi 0 JS -P l+H G O •H -P o3 ft -P G o CQ 0 O o o 0 O 0 0 G •H ft O H •H ^ O 1 1 s •H G •H O O ;3 CQ p ft O o o •H -P en •H s 'xi o Ti - p o1 <; IS < 0 O G O O TJ 0 rH o o3 ft C :3 o ; o Cv2 H (H O O o C^ :3 pq o 1. en • 3 -PI •d oH O ffi ^ fi G O •H (M fe 1 (H O CJ 1 ;3 •H G O O 05 •H H •ii S feC\2 TJ G o3 •H -P 03 0 -P 13 •H CQ CQ 03 -p o ft >5 -P 3 ft 0 l+H CQ o T3 § -p ft CQ >s 0 H CQ o3 03 -P 03 EH £ vO C 5o ft o1 CO O o o •k T-i o ^ o o o ft TH 1 c; e o o1 CO TH 0 •H ft 1 •k • • r ; c:5 s c ^ p ft oI s ao ; 0 ft 0 is 00 TH CQ rO -p •H G G M > rH o m c\i H O c*^ iH o ffi O - ; - OvJ PC o (T^ (H O K o ^ : S iH O O ^ H O O 0 G •H g ft 0 -P 0 Ti 1=) rH 0 •H O PH o3 G O •H -P •H tj ^e rH O o •n • '^ TH o3 • O CM fe iH 0 ft 0 CQ -P O O x: PH 0 H o3 feCsJ CQ O -e p o3 G 0 fjO O ft -P •H G o3 ft 0 T3 G p '¥^ ft o3 Tj 0 G > •H :3 CT< 0 o • -P rH o3 CQ e 13 •H N o3 •H -r) G •H T3 0 -P G o •H -P 03 G •H rQ o o -p c 0 G O G P« S CQ 03 ts O o 0 CQ 03 ,Q CQ •H C^i tH O O EH x: -p H 0 fH O O 0 G •H ft o rH r-\ CH P ft rH r-i < o3 O -p 03 .Q O O feCvj • • B >H H C^ fe H-> G 0 r-\ i>D rQ o EH •HH o >i ft g ft S § o3 o o -p o3 pq CQ 0 O <tj o •H c^ X"^ 0 <+H 'T::^ G ^ CO w^ rH 0 •H rH >H O \C 03 •P >H £>- ^ vr\ T3 ft o (H 0 •H CO o Lated 0 0 N G 0 eroded: c^ G o3 -P o3 O ^ ature ft "ii^ ^ actio 0 -p o3 cti -P CQ rH ^C O ft *•—' PP 1 > (H o (72 T3 • 0 CQ 3 were K CQ 0 >> o ft iH o3 ^ 3 o CQ DISCUSSION The replacement of a diazonium function by a chlorine atom from an organic solvent via the diazoacetate was reported in 1937. Waters prepared dry benzenediazoacetate and allowed it to decompose in chlorocarbon solvents. In this mechanistic study which demonstrated the existence of aryl radicals in this system, the fo2:mation of chlorobenzene in 15 - 30% yields was realized. Experimental results shown in Table 3 indicate that in most of the cases examined in this work, reduction of the diazonium salt to arene is a major pathway, except for the reactions employing sulfuryl chloride and tert-butyl hypochlorite as an additional chlorine atom source. In these cases, both the yields of reduction and chlorine atom abstraction products are low and the forTnation of undetermined complex product mixture was evident. Since the phenyl radicals proposed as intermediates in the decomposition of the diazoacetate are extremely reactive, they probably react the first molecule encountered. Consequently any reaction occuring in a solvent would be principally a reaction of a phenyl radical with a solvent molecule, no matter what other dissolved substances might be added. Therefore, those reactions conducted in chloroform and methylene chloride solution show the formation of reduction compound as a major product. 41 42 Gokel and Korzeniowski 6? suggested that in bromo- and iodo- dediazoniation of arenediazonium ions i n i t i a t e d by potassium acetate in the presence of l8-crown-6 in chloroform solution, the aryl r a d i c a l s are s e l e c t i v e for bromine and iodine atoms r a t h e r than hydrogen and chlorine atoms. According to the r e s u l t s shown in Table 3, the aryl r a d i c a l s produced in the current reaction system seem to be highly s e l e c t i v e for hydrogen atoms r a t h e r than chlorine atoms. The nature of chlorine atom source seems to be not so important in t h i s system. The y i e l d s of £-bromochlorobenzene in the reactions employing an a d d i t i o n a l chlorine atom source, such as N-chloro su coinimlde, sulfuryl chloride, and t e r t - b u t y l hypochlorite were almost the same within experimental e r r o r . The increased yield of £-bromochloro- benzene in a reaction which employed N-chlorodiisopropylamine as an a d d i t i o n a l chlorine atom source i s d i f f i c u l t to explain. The yield of £-bromochlorobenzene obtained from the reaction in pure carbon t e t r a c h l o r i d e was r e l a t i v e l y high (24%), but s t i l l unsatisfactory. Phase t r a n s f e r of arenediazonium ion and acetate ion i n t o carbon t e t r a c h l o r i d e and CCI^FCCIF^ by l8-crown-6 appeared to be not as effective as with chloroform solvent. The poor ion-solvating power of carbon t e t r a c h l o r i d e and 1,1,2-trichlorotrifluoroethane can be one of the reasons which cause lower y i e l d s . As a conclusion, i t i s c l e a r t h a t aryl r a d i c a l s are s e l e c t i v e for hydrogen atoms r a t h e r than chlorine atoms in the systems studied ^3 here. One of the major factors in the failure to obtain high yields of chloroarene in the non-hydrogen containing solvents, such as CGl^ and CCl FCCIF , may be the poor efficiency of transfer of diazonium ion and acetate ion by the catalyst into those solvents. CHAPTER IV REACTIONS OF ARENEDIAZONIUM IONS CATALYZED BY IRON PENTACARBONYL EXPERIMENTAL AND RESULTS General Methods All reactions involving iron pentacarbonyl must be conducted in a well-ventilated hood. Iron pentacarbonyl was added to a reaction mixture through a rubber septum via a syringe. Reaction mixtirres were analyzed by gas liquid partition chromatography, employing a Varian Aerograph Series 2400 gas chromatograph equipped with flame ionization detector. Reagents and Chemicals £-Bromoaniline,ra-bromoaniline,£-bromoaniline, £-chloroaniline, m-toluidine, o-toluldine, £-nitroaniline, o-nitroaniline, bromobenzene, and iodobenzene were obtained from Eastman Kodak Co. £-Chloroaniline, m-chloroaniline, £-iodoaniline, £-toluidine, £-anisidine, m-anisidine, m-nltroaniline, toluene, anisole, and nitrobenzene were obtained from Aldrich Chemical Co. 3,5-Di chloro aniline and m-dichlorobenzene were obtained from Ishlhaxa Sangyo Co. Ltd. Iron pentacarbonyl was obtained from Alfa Products. Commercial methanol was purified according to a literature 44 ^5 On procedure by means of magnesium activated with iodine. The sources and purification methods for other solvents used in this chapter were described in the previous chapters of this thesis. Purification" of Iron Pentacarbonyl Commercial iron pentacarbonyl (Alfa Products, 99.5%) foirms black platelets in the course of elongated storage. The commercial product was purified before use by distillation into an amber bottle under aspirator vacuum. Gas Chromatographic Analysis Product analysis was conducted using the method described in the preceding chapters utilizing gas chromatography with appropriate internal standards. Molar responses for authentic sample and internal standard were determined beforehand in each rrin and used for the yield calculation. The columns used for gas chromatograph were: a 5' x l/8 " column of 3% SE 30 on Chromosorb P (Column A); a 10' x l/8" column of 8% Carbowax 20 M on Chromosorb P (Column B); and a 5' x l/8" column of 3% SE 30 on Varaport 30 (Column C). Synthesis of Arenediazonium Salts Seventeen arenediazonium tetrafluoroborates were prepared according to the general procedure given below. To a 400 ml. beaker containing the appropriate amount of concent- 46 rated HGl (3O-I5O ml.) and 30 ml. of H^O was dissolved 0.10 mol. of the aniline. The mixture was cooled externally with an ice-salt bath. With continuous stirring, a solution of 0.10 mol. of NaNO in 15 ml. of water was added dropwlse at a rate such that the temperature of the solution did not exceed 5''c. The resulting solution was filtered and added slowly with stirring to 0.155 mol. of NaBF, 4 in 15 ml. of water. The mixture was stiorred for an additional 5 min., then filtered and washed with 50 ml. of cold water and then 50 ml. of diethyl ether. The crude arenediazonium tetrafluoroborate was purified by dissolving it in a minimum amount of acetone and precipitating the salt by addition of diethyl ether. The purified arenediazonium salt was dried quickly in the air by spreading the solid thinly on a porous paper supported on a wire gauze near a hood. The arenediazonium salts prepared by the above method are summarized in Table 4. Protodediazoniation of Arenediazonium Ions in Methanol Catalyzed by Iron Pentacarbonyl; General Procedure In a 10 ml., 3-necked flask fitted for nitrogen purge and magnetic stirring was placed O.6O mmol. of an arenediazonium tetrafluoroborate in 6.0 ml. of methanol. The solution was flushed with nitrogen gas for 5 min. and then 3-5 microliter of iron pentacarbonyl was added dropwise via a 10 microliter syringe to the vigorous stirred mixture. ^7 Table 4 Preparation of Arenediazonium Tetrafluoroborates , NaNOo + _ NaBFk X-•Ar-NH^ + HCl -^^^-^ X-Ar-N^Cl > X-Ar-N^BF;^ Aniline X-C^H^NH, Yield, g^ {%) H 18(88) 0.10 30 108-110 £-Br 22(80) 0.10 30 139 m-Br 19(70) 0.10 30 138-139 £-Br 24(88) 0.10 30 156 £-Cl 39(84) 0.20 60 136-138 m-Cl 0.20 60 146-150 £-Cl 35(75) 28(62) 0.20 60 171 £-1 9(68) 0.04 12 119-120 3,5-cl2 37(71) 0.20 60 170-80 £.CH3 31(75) 0.20 60 108-110 m-CH^ 27(65) 0.20 60 97-101 o-CH^ 28(69) 0.20 60 106 £-0CH3 0.10 30 140 m-OCH 17(65) 16(61) 0.10 30 99-100 £-N02 26(87) 0.125 60 157-158 m-NO^ 26(88) 0.125 60 170 0-NO2 28(9^) 0.125 60 130 ^ i e l d % based on a n i l i n e . Moles of Aniline Volume of HCl, ml. Obsd. mp. C C ) ^ All diazonium s a l t s were white except for the m-methoxy-, £ - n i t r o - , and £-nitrodiazonium s a l t s which were l i g h t •u, yellow. Melting points were determined using sealed capillaries. The diazonium salts melted with decomposition. 48 Effervescence of gas started immediately in most cases and the solution turned yellow. After the evolution of gas finished (10-60 min.), an internal standard (0,60 mmol.) was added to the reaction mixture. Quantitative gas chromatographic analysis was conducted and the yield of reduction product was calculated according to the method described previously. The yields of reduction products obtained by this method are listed in Table 5. Chlorodediazoniation of p-Bromo benzenediazonium Tetrafluoroborate; General Procedure In a 10 ml., 3-necked flask fitted for nitrogen purge was placed 0.163 g. (0.60 mmol.) of £-bromobenzenediazonium tetrafluoroborate in 6 ml. of solvent or solvent combination. The suspension was flushed with nitrogen gas for 5 min. and 3-5 liL of iron pentacarbonyl was added dropwlse from a microliter syringe to the vigorously stirred mixture. After continuous stirring for the designated time, the reaction mixture was subjected to a quantitative gas chromatographic analysis (Column A with £-dlbromobenzene as internal standard). The results of the attempted chlorodediazoniation reactions of the diazonium salt are listed in Table 6. 49 Table 5 Yields of Reduction Products in the Decomposition of Arenediazonium Tetrafluoroborates in Methanol using Iron Pentacarbonyl as a Catalyst^ R- of ^^eV^^^'k Reaction Time Product £.N03 10 min, m-NO 2 30 min. II o-NO^ 10 min. II Yield, % 99 82 98 £-Cl 11 m-Cl II II 89 £-Cl II II 71 £-Br II BrC^H m-Br II •1 72 £-Br II II 73 £-1 II 3,5-01^ II H 96 89 42 ^°6«5 60 min. 3,5-C1^0^\ 83 68 °6«6 £-CH3 II m-CH^ 30 min. •1 48 o-CH 60 min. II 31 £-0GH^ II m-OCH^ 11 73 CHjOCgH^ II ^3 25 All reactions were carried out at a diazonium ion concentration of 0,10 molar employing a catalytic amount of iron pentacarbonyl (4-6 mole %) at room temperature in the dark. glpc analysis based on diazonium salt. Yield determined by 50 Table 6 Chlorodediazoniation Yields of £-Bromobenzenedlazonium a Tetrafluoroborate Catalyzed by Iron Pentacarbonyl "U. Solvent Composition (v/v) Reaction Time Yield % Br-.C^H^-Cl Br-C^ «5 GCli, 60 min. 0 0 CCl^-MeOH ( 1 : 5 ) 30 min. 39 10 (5:1) 60 min. 10 24 60 min. 0 0 CHCl^ CHCl^-MeOH ( 1 : 5 ) II 53 1 (5-.1) II 20 3 All r e a c t i o n s were carried out a t a diazonium Ion concentration of 0.10 molar employing a c a t a l y t i c amount of Iron pentacarbonyl (iv-6 mole ^ a t room temperature In the dark. glpc analysis based on diazonium s a l t . ^ H e l d d e t e r g e d by 51 Bromodediazoniation of p-Bromobenzenediazonium T e t r a f l u o r o b o r a t e ; General Procedure I n a 10 m l . , 3-necked f l a s k f i t t e d f o r n i t r o g e n purge was placed 0.163 g. ( 0 . 6 0 mmol.) of £-bromobenzenediazonium tetrafluoroborate i n 6 . 0 ml, of CBrCl^-DMF mixed s o l v e n t i n d i f f e r i n g p r o p o r t i o n s . A f t e r s t i r r i n g f o r 30 min., t h e r e a c t i o n mixture was analyzed by gas chromatography (Column A with £-bromochlorobenzene as i n t e r n a l s t a n dard) . The r e s u l t s of t h e attempted bromodediazoniation r e a c t i o n s of t h e diazonium s a l t a r e summarized in Table 7. Table 7 Bromodediazoniation of £-Bro mo benzenediazonium T e t r a f l u o r o b o r a t e i n CBrCl^-DMF S o l u t i o n Catalyzed by Iron Pentacarbonyl a o -. ^ (v/v) / /\ Solvent ^ ' ' D +• Time rp. Reaction CBrCl^-DMF (1:5) Yield ,^ -n-^ (%)\ . of £-Dibromobenzene 30 min. 42 (3:3) " 47 (5:1) " 37 d i a z o n i u m s a l t used was O.6O mmol. i n 6.0 ml. of s o l v e n t . •I- Yield based on diazonium s a l t . 52. Catalytic Decomposition of p-Bromobenzenediazonium Tetrafluoroborate in the Presence of" Benzene To a suspension of £-bromobenzenediazonium tetrafluoroborate (0.163 g., 0.60 mmol.) in 6.0 ml. of 50:50 mixture (v/v) of benzenemethanol was flushed with N gas for 5 min. and 5 AI1 of iron penta- carbinyl was added with vigorous stirring. The reaction mixture was allowed to stir for 1 hr. and then analyzed by gas chromatography (Column C with biphenyl as internal standard). 4-Bromobiphenyl was formed in 38% yield together with 30% of bromobenzene. Catalytic Decomposition of p-Bromobenzenediazonium Tetrafluoroborate in the Presence of Air A mixture of £-bromobenzenediazonium tetrafluoroborate (O.I63 g., 0.60 mmol.) in 6.0 ml. of methanol was decomposed under air by adding 3 n\ of iron pentacarbonyl. After stirring 10 min., the reaction mixture was analyzed by gas chromatography and showed the formation of bromobenzene in 7^% yield. DISCUSSION Possible Mechanism of the Protodediazoniation of Arenediazonium Ions in Methanol Catalyzed by Iron Pentacarbonyl In 1864, Griess reported that arenediazonium ions could be reduced to arenes with alcohol. He observed that the reduction of benzenediazonium nitrate in ethanol was relatively sensitive to experimental conditions and was favored by the presence of water, use of diazotate rather than diazonium ion, and the presence of electron-withdrawing substituents. The reduction of arenediazonium salts in alcohol is often accompanied the formation of appreciable quantities of the aryl alkyl ether. Bunnett £t al, observed that in acidic methanol solution the atmosphere over the reaction mixture and substituents and their positions on the aryl group can be factors in determining the competition between radical (protodediazoniation) and ionic (methoxydediazoniation) pathways. 56 In alkaline alcohol solvent, the major reaction product was that of protodediazoniation.37 As demonstrated in Table 5 the main products from reations of arenediazonium salts in methanol catalyzed by iron pentacarbonyl are benzene derivatives resulting from protodediazoniation (Equation 4-1). (^-1) 53 5^ The formation of reduction products is consistent with radical intermediates. Aryl radicals are known to arylate benzene, forming biphenyl derivatives. Therefore dediazoniation of a diazonium salt by a redical mechanism in a solution containing benzene should afford biphenyl derivatives as well as the usual reduction product. Therefore, the catalytic decomposition reaction of £-bromobenzenediazonium tetrafluoroborate in 50% benzene-50% methanol (v/v) was investigated. The experimental result shows the formation of 38% of 4-bromobiphenyl as well as 30% of bromobenzene. This observation clearly demonstrates that a major fraction of the reaction must occur by a radical mechanism. In high concentrations, molecular oxygen often interferes with radical reactions. It may combine with radicals to interrupt a chain reaction sequence, or to form different ultimate products. It is therefore expected that a radical dediazoniation should be sensitive to oxygen in the system. In fact, the catalytic decomposition reaction of £-bromobenzenediazonium tetrafluoroborate in methanol under air gave only a 7^% yield of bromobenzene. This compares with an 88% yield under nitrogen atmosphere. It is beyond the scope of this thesis to establish the exact radical mechanism that this reaction follows. But it seems reasonable to expect that the mechanism of this reaction may be the propagation 81 sequence postulated by DeTax and Turetzky . The essential chainpropagating steps of this mechanism are illustrated in Equations 55 ^-2, 4-3 and 4-4. Equation 4-3 involves electron transfer from the C^H^- + CH^OH ^ G^H^ + -CH^OH ^6V^2"^ "• ' ^ V ^ > C^H^-N=N- + CH^OH"*" C^H^-N=N- _ ^ (4-2) (4-3) C^H^- + N^ (4-4) 'CH^OH radical to the diazonium ion. The nature of initiation and termination steps is also unclear. The possibility of direct electron transfer from iron pentacarbonyl to the diazonium ion was eliminated by the results of a cyclic op v o l t a m e t r i c d e t e r m i n a t i o n of e l e c t r o d e p o t e n t i a l s f o r the two s p e c i e s ( E q u a t i o n s 4 - 5 and 4 - 6 ) . Based upon t h e s e measurements, the thermo- + e-^B^—(/ f^e(CO) + e' ^ Fe(CO)- V)—N=N* E=-0.3 V (4-5) E=+1.1V (4-6) dynamically unfavorable direct electron transfer from iron pentacarbonyl to the diazonium ion (E=-1.4 V) appears to be highly unlikely. Another possible initiation step involves the formation of a complex of the diazonium ion and iron pentacarbonyl which may involve strong back donation of electrons from the metal. Arenediazonium ions are known to act as ligands which may be viewed formally as three 5^ electron donor terminal ligands or equivalently, as the arenediazonium ion coordinated through the sigma-lone pair on the terminal nitrogen together with strong back donation of electrons from the metal. A conventional, simplified picture of the electronic structure is shown in Scheme 4-1 (a). In the presently-available X-ray structure (a) (b) M*—:N=f-Ar -» M P= S = N ' \ Ar Scheme 4-1 Backbonding in Arenediazonium Ion Complexation by Transition Metals determinations for several complexes o f aryldiazenato ligands bound to transition metals, the N N C angle is nearly 120* , which attests to the importance o f back-bonding in these complexes (Scheme 4-1, b ) . Some metal complexes react directly with arenediazonium ions by replacement o f a ligand, such as CO o r PR^, by ArN^ . An example is shown in Equation 4-7. T h e complex Fe{'^^kr){CQ)^{VVh trigonal bipyramid with apical phosphines. PPh OC. 84 PPh 3 -Fe ArN, OC^ CO acetone-C,H^ GO' PPh. 3 ) ^ "*" is a 00** 3 Fe • I PPh, 3 N^Ar (4-7) 57 To date, attempts to prepare isolable aryldiazenato derivatives of unsubstituted metal carbonyls have been unsuccessful. However, possible existence of such species at low temperatures was suggested 84 '^'^ by Carrol and Lalor. Clark and Cookson^^ also proposed a complex of a diazonium salt with nickel carbonyl as an intermediate (Scheme 4-2). Diazonium salts did not react with nickel carbonyl itself, but addition of hydroxylic solvent such as ethanol, acetone, tertbutyl alcohol, or acetic acid induced vigorous effervescence of nitrogen and carbon monoxide. The suggested mechanism is as follows: ArN^^ + Ni(CO)^ > ROH ArH Ar2C0 < ArN,"^ < ^— Ar-N=N-Si(CO)^ ^ ArNi(CO)., ROH ArCO-Si(CO)-^ > ArC02R Scheme 4-2 Suggested Mechanism for the Reactions of Arenediazonium Ions with Nickel Carbonyl Since it is obvious that iron pentacarbonyl acts as a catalyst (4-6 mole %) in this system, it might be proposed that the aryl radicals are formed by the decomposition of a labile Fe-C bond of a diazonium ion-iron pentacarbonyl complex and the reaction then 58 follows the chain propagation steps suggested in Equation 4-2, 4-3, and 4-4. Two alternative modes by which arenediazonium ions may form a complex with iron pentacarbonyl are presented in Equations 4-8 and ^-9. Equation 4-8 involves initial coordination of arenediazonium Ar-SsN: -.Fe(CO) ^ > Ar-N=i=;Fe(CO) 4a ^ > Ar-N=N-fe(CO) 4b -5 Ar-SrN 0-6(00) ^ > Ar-N=N-^e(CO)4b -5 (4-8) (4-9) ion through the lone pair on the terminal nitrogen followed by strong back donation of electrons from the metal (4a), or even going beyond the back donation stage to a complete transfer of two electrons from iron to the ligand (4b). In Equation 4-9, the arenediazonium ion acts as a Lewis acid and attack an electron rich iron atom. Lewis acid coordination is typical for those transition metals for which oxidative addition o are important (d -in and d zero-valent complexes). The next step may involve the expulsion of a neutral CO molecule from the complex 4b forming the pentavalent complex 4c or 4d (Equation 4-10). Ar-N=N-f e=C=0 *-• Ar-N=N-Fe^55 ^ ^ 4b (^0)4 (GO)^ Ar-N=S='Fe(CO)^ 4c * Ar-N=N-t'e(CO)^ 4d (4-10) 4b, 4d • Ar' (^-11) 59 It is reasonable to expect that electron-withdrawing substituents in the aromatic ring would speed up the electron transfer process (Equation 4-10) or the addition step in Equation 4-9. In fact, it was observed that protodediazoniation of arenediazonium ions which contain electron-withdrawing substituents in the aromatic ring was fast and vigorous in methanol and nitrogen evolution ended in a few minutes; while those which contained electron-donating substituents reacted much slower (nitrogen evolution continued for almost an hour). It was also observed that diazonium ions do not react directly with iron pentacarbonyl in the absence of methanol. Therefore, solvent molecules must play some role. Substituent Effects in Catalytic Decomposition of Arenediazonium Ions in Methanol It was noted that electron-withdrawing substituents such as NOp-, Br-, and Cl- increase the yields and rates of protodediazoniation of diazonium ions, while electron-donating groups decrease them. Elofson and Gadalla studied the substituent effects on the half- wave potentials of benzenediazonium salts in sulfolane and found that diazonium salts with electron-withdrawing substituents are more easily reduced than those with electron-donating substituents. Their observation was explained according to the nature of the diazonium ions. Diazonium ions which have electron-withdrawing substituents are good 60 e l e c t r o n a c c e p t o r s and t h e r e f o r e a r e good o x i d i z i n g a g e n t s . I f the decomposition of diazonium s a l t s i n methanol c a t a l y z e d by i r o n p e n t a carbonyl i n v o l v e s a temporal i n t e r m e d i a t e such as 4b o r 4d, e l e c t r o n withdrawing s u b s t i t u e n t s w i l l f a c i l i t a t e decomposition of those comp l e x e s by hemolytic cleavage as well as t h e i r formation. Other R e a c t i o n s of Arenediazonium Ions Catalyzed by I r o n Pentacarbonyl Since a r y l r a d i c a l s are b e l i e v e d to be involved i n the decomp o s i t i o n of diazonium s a l t s i n methanol catalyzed by i r o n p e n t a c a r b o n y l , replacement of the diazonium function by halogen atoms u s i n g v a r i o u s s o l v e n t combinations was attempted. Table 6 shows t h e r e s u l t s of c a t a l y t i c c h l o r o d e d i a z o n i a t i o n of £-bromobenzenedlazonium t e t r a f l u o r o b o r a t e i n a s o l v e n t and s o l v e n t combinations. occurred. I n pure carbon t e t r a c h l o r i d e o r chloroform, no r e a c t i o n T h i s experiment again shows t h a t p r o t i c s o l v e n t s (methanol i n t h i s experiment) p l a y some r o l e i n t h e r e a c t i o n p r o c e s s . The c a t a - l y t i c decomposition of £-bromobenzenediazonium t e t r a f l u o r o b o r a t e in CCl,-MeOH ( 5 : 1 ) gave a 24% y i e l d of £-bromochlorobenzene. 4 I n Table 7, t h e y i e l d s from c a t a l y t i c bromodediazoniation of £-bromobenzenediazonium t e t r a f l u o r o b o r a t e in CBrCl^-DMF mixed s o l v e n t are l i s t e d . I t was noted t h a t £-bromo benzenediazonium t e t r a f l u o r o - b o r a t e decomposed r a p i d l y with e v o l u t i o n of n i t r o g e n upon a d d i t i o n 61 of catalytic amount of iron pentacarbonyl. The yield of £-dlbromobenzene varied from 37% to 47% according to the solvent composition. In these halodediazoniation reactions, reduction to bromobenzene was a competing reaction when protic solvents were utilized. Abstraction of hydrogen from the solvent by aryl radicals presents a problem in obtaining high yields of £-halobenzenes. Concluding Remarks It was found in this research that arenediazonium salts containing electron-withdrawing substituents in the aromatic ring can be reduced effectively by the catalytic decomposition in methanol using iron pentacarbonyl. Although the mechanism of the reaction remalnes uncertain at the present time, a possible mechanism is proposed. REFERENCES AND NOTES 1. J. P. Griess, Ann, Chem.. 106, 123 (1858), 2. H, Bart, Ger, Pat, 281, 055 ; Chem. Abst.. 9, 1830 (1915). 3. H, Zollinger, Ace, Chem. Res.. 6, 335 (1973). 4. R, A, Bar-tsch, "Progress in Macrocyclic Chemistry," vol. 2, R. M, Izatt and J, J. Christensen (ed.), Wiley-Interscience, New York, N. Y., 1980, in press. 5. R. Putter, "Methoden der Organischen Chemle," vol. 10, part 3, +1-1 4 ed., Houben-Weyl-Muller (ed.), Georg Thieme Verlag, Stuttgart, 1965. 6. E. D. Hughes, C. K. I n g o l d , and J . H. Ridd, J . Chem. S o c , 58 (1958). 7. J . H. Ridd e t a l . , J . Chem. S o c , 533 (1966); i b i d . , 273 ( 1 9 6 7 ) . 8. B. A, P o r a l - K o s h i t s , Russ, Chem, R e v s , , 3 i , 283 (1970). 9. L. P. Hammett, " P h y s i c a l Organic Chemistry," McGraw-Hill, New York, N. Y,, 19^0, p p . 29^. 10. C. K, I n g o l d , e t a l . , J . Chem. S o c . 929 (1938); i b i d . 2400 (1950); i b i d . 28 ( 1 9 5 2 ) . 1 1 . K, Schank, "Methodicum Chimicum," v o l . 6, F . Zymal Kowski ( e d . ) . Academic P r e s s , New York, N. Y., 1975, PP. l 6 ^ . 12. K, H, Saunders, "The Aromatic Diazo Compounds," 2 Arnold, London, 1948. 1 3 . L. V. Clark, I n d . Eng. Chem., 25, 663 ( 1 9 3 3 ) . 62 e d . , Edward 63 1^. J. P, Griess, Ann, Chem.. 137. 52 (I866), 15. A, Michaelis and J. Ruhl, Ann, Chem., 270, 114 (1892), 16. A. N. Nesmeyanov and E. J. Kahan, Chem. Ber.. 62, 1018 (1929). 17. H. Zollinger, "Azo and Diazo Chemistry," Interscience, New York, N, Y,, 1961, Chapter 7. 18. A. F, Hegarty, "The Chemistry of Diazonium and Diazo Groups," part 2, S. Patal (ed.), Interscience, New York, N. Y., 1978, pp. 511-591. 19. D. F. DeTar and S. K. Wong, J. Am. Chem. Soc. 78, 3921 (1956). 20. E, A, Moelwyn-Hughes and P, Johnson, Trans. Faraday Soc, 36, 9^8 (19^0). 21. C, G, Swain, J. E, Sheats and K, G, Harbison, J. Am. Chem. Soc, 6^, 1400 (19^0), 22. G, Balz and G, Schiemann, Chem, Ber,, 6OI3, 1186 (1927). 23. C, G, Swain and R, J. Rogers, J. Am, Chem, Soc, £7, 799 (1975). 24. E, S. Lewis and J, M, Insole, J. Am. Chem. Soc. 86, 32, 3^ (1964). 25. E, S. Lewis and R. E. Holliday, J, Am, Chem. Soc, 88, 5043 (1966). 26. E. S, Lewis and P. E, Kotcher, Tetrahedron, 25, ^873 (1976). 27. R, G, Bergstrom, C. H, Wahl and H, Zollinger, Tet. Lett., 2975 (197^). 28. C, G, Swain, J. E, Sheats and K, G, Harbison, J. Am, Chem. Soc, 2Z» 796 (1975). 29. E. S. Lewis and W. H, Hinds, J. Am. Chem. Soc, 74» 304 (1952). 30. E. S, Lewis, L, D. Hartung and B, M, Mckay, J. Am, Chem. Soc, £1, 419 (1969). 64 31. P. B u m and H. Zollinger, Helv. Chlm, Acta,. ^6, 2204 (1973). 32. W. T, Ford, J . Org, Chem.. 36, 3979 (1971). 33. M. Stiles, R. G. Miller and U. Burckhardt, J. Am. Chem. Soc, 85, 1792 (1963). 3^. S, Yaroslavsky, Chem, Ind. (London). 765 (I965). 35. G. Wittig and R. W, Hoffmann, Chem, Ber,. 95, 2718 (I962), 36. W. J. Boyle, T, J, Broxton and J. F. Bunnett, Chem. Commun.. 1^9 (1971). 37. J. F. Bunnett and H. Takayama, J. Org. Chem., 32, 1924 (I968). 38. C, D, Ritchie and P. 0, I. Virtanen, J, Am, Chem. Soc. 95, 1882 (1973). 39. H, Suschitzky, Angew, Chem. Int. Ed.. 6^, 596 (I967). 40. E. Muller and H. Haiss, Chem. Ber.. £5, 1255 (1962). 41. E. Bamberger, Chem. Ber.. 29. 446 (I896). 42. E. Bamberger, Chem, Ber,, 31, 3188 (I900), 43. J. I, G. Cadogan, C. D, Murray and J, T, Sharp, J. Chem. Soc. Chem. Commun., 901 (197^). 44. J, I. G, Cadogan, R, M, Paton and C, Thomson, Chem, Commun., 133, (197^). 45. E. S, Lewis and M. D, Johnson, J, Am, Chem, Soc, 81^, 2070 (1959). 46. W, A, Waters, "Chemistry of Free Radicals," 2^ ed,, Oxford University Press, London, 19^8, pp, I63. 47. H, Taube, J. Chem, Ed,, 45, 452 (I968), 48. W, A. Cowdrey and D. S, Davies, Quart, Revs,, 6_, 358 (1952). ^5 9. 50. R, G, R. Bacon and H, A. 0, Hill, Quart, Revs.. 19, 95 (1965). D, Sutton, Chem, Soc Revs.. 4, 443 (1975). 51. K. Kikukawa and T. Matsuda, Chem. Letts. (Japan). 159 (1977). 52> M. F. Semmelhack, P. M. Helquist, and L. D. Jones, J. Am. Chem. Soc., 93, 5098 (1971). 53. J, C, Clark and R. C. Cookson, J. Chem, Soc. 686 (I962). 5^. T. A. Manuel, J. Org, Chem.. 27, 39^1 (I962). 55» R. M. Parlman, "The Use of Organometallies in Organic Synthesis," Ph. D, Dissertation, Department of Chemistry, Washington State University, 1977. 56. G. W, Gokel and D, J, Cram, J. Chem, Soc. Chem, Comm,. 481 (1973). 57. A, R, Butler and P. T. Shepherd, J. Chem. Res.(s). 339 (1978); J, Chem, Res. ( M ) . 4471 (1978), 58. R, A, Baxtsch, H. Chen, N. F. Haddock, and P. N. Juri, J. Am. Chem. Soc. . 98, 6753 (1976). 59. C, Rommlng, Acta Chem, Scand,. JL3, I26O (1959). 60. C D . Hartman and S. E. Biffar, J. Org. Chem.. 42, 1468 (1977). 61. S. H. Korzeniowski and G. W. Gokel, Tet. Lett., 1637 (1977). 62. S. H. Korzeniowski and G. W. Gokel, Tet. Lett., 3519 (1977). 63. S. H. Korzeniowski and L. Blum and G. W. Gokel, Tet. Lett.. 1871 (1977). 64. H. Lehmkuhl, F. Rabet and K. Hauschild, Synthesis. 184 (1977). 65. F. Vogtle, Angew. Chem. Int. Ed. Engl., 12, 197 (1978). 66. H. Sieger and F. Vogtle, Angew. Chem. Int. Ed. Engl., j7,198 (19?8). 66 7. D. G. Lee and V. S. Chang, J. Org. Chem.. 43, 1532 (1978). 68. U. Takai and J. Smid, J. Am, Chem. Soc. 96, 2588 (197^). 69. R. A. Bartsch and P. N, Juri, Tet. Lett.. 407 (1979). 70. R. A. Bartsch and P. N. Juri, and M. A. Mills, Tet. Lett., 2499 (1979). 71. G. N. Schrauzer, Chem. Ber.. 9^, I89I (I96I), 72. A. Roe, Organic Reactions. 5, 203 (1949). 73. D. Schulte-Frohlinde and H. Blume, Z. Phys. Chem. (Frankfurt am Mein). 59, 299 (1968). 7^. E. B. Starkey, "Organic Syntheses," Coll. vol. 2, John Wiley and Sons, New York, N.Y., 1943, pp, 225, 75. E, Bamberger, Chem. Ber., _28, 403 (1895). 76, W. E. Bachmann and R. A. Hoffman, Organic Reactions. _2, 244 (1944). 77. J. H. Mathews, J. Am. Chem, Soc. 48, 562 (1926). 78, W. A. Waters, J. Chem. Soc, 113 (1937). 79. E. C. Evers and A. G. Knox, J. Am Chem, Soc, 73, 1739 (l95l). 80, J. P. Griess, Phil. Trans., 15^, 683 (1864). 81, D. F. DeTar and M. N. Turtzky, J. Am. Chem. Soc, 77, 1745 (1955); ibid.. 78, 3925, 3928 (1956). 82. Cyclic voltametry deta were provided by Dr. W. H. Smith. The data were obtained at a Pt electrode in DMF solvent. Potentials -1 versus the aqueous S.C.E. are quoted for a scan rate of 0.2 VS and a concentration of 2x10 ^ M for each species. 83 . J. A. Ibers and B. L. Haymore, Inorg. Chem., l4, 1369 (1975). •f: 67 8^. W. E, Carroll and F. J. Lalor, J, Chem, Soc. Dal ton. 175^ (1973). 85. R. M. Elofson and F, F. Gadalla, J, Org. Chem.. 3^, 85^ (1969).