Organometallic Compounds Chapter 15 Guest Lecturer: Prof. Jonathan L. Sessler AWOL! Organometallic Compounds • Organometallic compound: a compound that contains a carbon-metal bond • In this chapter, we focus on organometallic compounds of Mg, Li, and Cu – these classes illustrate the usefulness of organometallics in modern synthetic organic chemistry – they illustrate how the use of organometallics can bring about transformations that cannot be accomplished in any other way – several more recent reactions of organometallic compounds are discussed in Chapter 24 Organometallic Reagents The Key Concepts: Make a carbon negatively charged/polarlized so it is nucleophilic. Reaction with electrophilic carbons can make carbon-carbon bonds. This is a Big Deal! The First Organometallic Reagents… Grignard Reagents • Discovered by Victor Grignard in 1900 – Key factors are ethereal solvent and water-free conditions • Awarded Nobel Prize in 1912 Victor Grignard Grignard, Victor , 1871–1935, French chemist. He shared the 1912 Nobel Prize in Chemistry for his work in organic synthesis based on his discovery (1900) of the Grignard Reagent. He taught at the Univ. of Nancy (1909–19) and at the Univ. of Lyons (from 1919 until the end of his career). Grignard Reagents • Grignard reagent: an organomagnesium compound – prepared by addition of an alkyl, aryl, or alkenyl (vinylic) halide to Mg metal in diethyl ether or THF Br + Mg 1-Bromobutane Br + Mg Bromobenzene ether MgBr Butylmagnesium bromide (an alkyl Grignard reagent) ether MgBr Phenylmagnesium bromide (an aryl Grignard reagent) An Alternative to Grignard Reagents are Alkyl Lithiums Both are prepared from alkyl, vinyl, and aryl halides under anhydrous conditions dry ether + Br Mgo n-butyl bromide MgBr or tetrahydrofuran (THF) recall: THF = Grignard Reagent O dry ether Br n-butyl bromide + 2 Lio or tetrahydrofuran (THF) Li alkyl lithium + LiBr Grignard and Organolithium Reagents • Given the difference in electronegativity between carbon and magnesium (lithium), the C-Mg (C-Li) bond is polar covalent, with C- and Mg+ ( Li+ ) – Grignard and organolithium reagents behave like carbanions • Carbanion: an anion in which carbon has an unshared pair of electrons and bears a negative charge - - + Li + MgBr Grignard Reagent alkyl lithium Grignard and Organolithium Reagents • Carbanion: an anion in which carbon has an unshared pair of electrons and bears a negative charge – Carbanions are strong bases--they are easily quenched by even very weak acids (water, alcohols, amines, amides, carboxylic acids, even terminal alkynes). A limitation to utility! pKa = ca. 50 + MgBr Grignard Reagent + - + OH pKa = 16 alkane corresponding to original alkyl halide + O- MgBr+ Grignard and Organolithium Reagents • Carbanion: an anion in which carbon has an unshared pair of electrons and bears a negative charge – Carbanions are strong bases--they are easily quenched by even very weak acids (water, alcohols, amines, carboxylic acids, amides, even terminal alkynes). A limitation to utility! pKa = ca. 50 + Li - + NH2 + alkane corresponding to original alkyl halide + alkyl lithium pKa = 40 NH- Li+ Limitations • Can’t make Grignards with acidic or electrophilic functional groups present in the molecule: • R2NH pKa 38-40 Terminal Alkynes pKa 25 ROH pKa 16-18 Carbonyls & Nitros pKa 11-27 Grignard and Organolithium Reagents • Carbanion: an anion in which carbon has an unshared pair of electrons and bears a negative charge – Carbanions are also great nucleophiles. This is the reason for their great utility! • Key Point: Grignard and Organolithium Reagents • Great nucleophiles that add efficiently to electrophilic carbons, such as epoxides and carbonyl group of aldehydes, ketones and esters. However, their basicity can be a limitation! • Epoxides illustrate how many common organic functional groups contain electrophilic carbons Grignard and Organolithium Reagents • Carbanions (nucleophiles) can react with electrophilic carbon centers in favorable cases. The net result is a carbon-carbon bond--a big deal! • Grignards and organolithium reagents react with many oxygen-containing electrophiles, but not with alkyl halides. • We’ll illustrate this with epoxides. • Recall, acidic protons will “kill” our reagents and/or won’t allow them to be generated in the first place Grignard reagents react productively with: formaldehyde to give primary alcohols aldehydes to give secondary alcohols ketones to give tertiary alcohols esters to give tertiary alcohols CO2 to give acids epoxides to give primary alcohols The one we are choosing for the sake of initial illustration Epoxides: The Example We Want to Stress New C-C bond - MgBr + - O Considered Retrosynthetically: + OH O MgBr H O+ 3 Dilute THF OH MgBr + These is an extremely valuable reaction… Worth celebrating! O Prof. Iverson Would Normally Play His Trumpet Here! But, he is gone… Another Kind of Celebration! • Cost of Attending USC (per year): $50,000 • Gymnastics, Tumbling and Cheerleading Lessons: $20,000 • Keeping Hair Just The Right Shade of Blond: $10,000 • Cheering for the Wrong Team: Priceless! Back to Work: A Related Example Detailed Mechanism Highlighting Retention of Stereochemistry New C-C bond Key Points to Note: Attack at least hindered carbon Mechanism is SN2-like in initial step Single enantiomeric product Chiral center not affected by reaction Relief of ring strain helps drive reaction H3O+ breaks up initial salt Two More Examples of Additions to Epoxides 1. + MgBr O - New C-C bond + 2. HCl, H2O OH Grignard Reagent 1. + Li Organolithium Reagent O New C-C bond - OH + H H H H 2. HCl, H2O Note: Stereochemistry at epoxide retained in product Key point: Orientation Matters! Gilman Reagents • Lithium diorganocopper reagents, known more commonly as Gilman reagents – prepared by treating an alkyl, aryl, or alkenyl lithium compound with Cu(I) iodide diethyl ether 2 CH3 CH2 CH2 CH2 Li + CuI or THF Butyllithium Copper(I) iodide ( CH3 CH2 CH2 CH2 ) 2 Cu - Li Lithium dibutylcopper (a Gilman reagent) + + LiI Gilman Reagents • Coupling with organohalogen compounds – form new carbon-carbon bonds by coupling with alkyl and alkenyl chlorides, bromides, and iodides. (Note that this doesn’t work with Grignard or organolithium reagents. THEY ARE TOO BASIC AND DO E2 ELIMINATIONS.) R'Br + R2 CuLiBr diethyl ether or THF R'-R + RCu + LiBr •Example I 1 . Li, pent ane 2 . CuI 2 1-Iod od ecane d ie th yl ethe r Br or THF 2-Methyl-1-dodecen e CuLi New C-C bond Gilman Reagents – coupling with a vinylic halide is stereospecific; the configuration of the carbon-carbon double bond is retained I + t rans-1-Iodo-1-nonene 2 CuLi Lithiu m dib utylcopper d iethyl eth er or THF New C-C bond trans -5-Tridecen e Gilman Reagents • A variation on the preparation of a Gilman reagent is to use a Grignard reagent with a catalytic amount of a copper(I) salt CH3 (CH2 ) 7 (CH2 ) 7 CH2 Br C C H H (Z)-1-Bromo-9-octadecene + Cu THF + CH3 ( CH 2 ) 4 MgBr CH3 (CH2 ) 7 (CH2 ) 1 2 CH3 C C H H (Z)-9-Tricos ene (Muscalu re) Gilman Reagents • Reaction with epoxides – regioselective ring opening (attack at least New hindered carbon) C-C O OH bond 1 . ( CH2 =CH) 2 CuLi 2 . H2 O, HCl S tyrene oxide (racemic) 1-Ph enyl-3-buten-1-ol (racemic) Interim Summary of Introduction to Organometallic Reagents… Organolithium reagents and Grignard reagents are very basic but also great nucleophiles. They react with epoxides at the less hindered site to give a two-carbon chain extended alcohol. They do not couple with alkyl-, aryl-, or vinyl halides. Gilman reagents react with epoxides as do organolithium reagents and Grignard reagents. However, they also add to alkyl-, aryl-, and vinyl halides to make new C-C bonds. Feeling Lost? Fortunately, Dr. Iverson will be back on Monday! Meanwhile…. Back to Grignard Reagents… • Addition of a Grignard reagent to formaldehyde followed by H3O+ gives a 1° alcohol H H - + CH3CH2 MgBr C O H - THF + CH3CH2 C O MgBr H H3O dil. + H +2 CH3CH2 C OH + Mg H • This sequence (mechanism) is general and important! Grignard Reactions CH3CH2 + MgBr O C O - THF + CH3CH2 C O MgBr O H3O + dil. These are valuable and important reactions… Please add to your card stock! +2 CH3CH2 C OH + Mg O Grignard reagents react with esters – R R' •• + OCH 3 •• C MgX O •• •• R' diethyl ether R C •• OCH3 •• • O • + MgX • •• •– but species formed is unstable and dissociates under the reaction conditions to form a ketone Grignard reagents react with esters – R R' •• + OCH 3 •• C R' diethyl ether R OCH3 •• • O • + MgX • •• •– MgX O •• •• this ketone then goes on to react with a second mole of the Grignard reagent to give a tertiary alcohol C •• –CH3OMgX R R' C O •• •• Example O 2 CH3MgBr + (CH3)2CHCOCH3 1. diethyl ether 2. H3O+ OH (CH3)2CHCCH3 CH3 (73%) Two of the groups attached to the tertiary carbon come from the Grignard reagent Practice OH MgBr O + H H Practice OH MgBr O + O MgBr + H H The Same Chemistry is seen With Organolithium Reagents O H2 C CHLi + CH 1. diethyl ether 2. H3O+ CHCH OH CH2 (76%) Practice O OH MgBr + O C O Organometallics are one of the classic ways of activating CO2 End of Lecture. Any Questions?