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
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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
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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.
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