Organometallic Compounds

Department of Chemistry

SGBAU,Amravati

Mr.Zamir S. ShekhAssistant Professsor

Defination & Importance Of OMC :- Organometallic chemistry is the study of chemical compounds containing at least one bond between a carbon atom of an organic compound and a metal

Organometallic compounds provide a source of nucleophilic carbon atoms which can react with electrophilic carbon to form a new carbon-carbon bond.

Nomenclature:- Organometallic compounds are normally named as substituted metals, e.g. alkyl metal or alkyl metal halide. Organomagnesium compounds are generally referred to as Grignard reagents. Examples:  CH3Li = methyllithium, CH3MgBr = methylmagnesium bromide.

Physical Properties:-Organometallic are usually kept in solution in organic solvents due to their very high reactivity (especially with H2O, O2 etc.)

Structure:-Organosodium and organopotassium compounds are essentially ionic compounds.Organolithiums and organomagnesiums have a σ bond between a C atom and the metal:  C-MThese are very polar, covalent bonds due to the electropositive character of the metals.

Look at the electronegativities of the metals Li, Na, K and Mg compared to C and the other atoms we have seen so far,

The images show the electrostatic potentials for methyl chloride, methyl lithium and methyl magnesium bromide.  The more red an area is, the higher the electron density and the more blue an area is, the lower the electron density.In the alkyl halide, the methyl group has lower electron density (blue), and is an electrophile. In methyl lithium, the methyl group has higher electron density (red) and is a nucleophile.In methyl magnesium bromide, the methyl group is less electron rich that methyl lithium.

Organo Lithium Organolithiurn reagents react with a wide variety of organic substrates to form carbon-carbon bonds and serve as precursors for the preparation of other organometallic reagents.

Important points to consider when preparing and using organolithiums are,

Atmosphere :- inert atmosphere (Ar and He are best)Nature of the halide :- Bromides generally are best; iodides have a tendency to undergo the Wurtz reactionPurity and physical state of the metal:- The rnetal surface should be clean and have a large surface area.Solvent:- Most R-Li reagents are prepared in hydrocarbon solvents. However,phenyllithium, methyllithiumReactivity:- The basicity of organolithium reagents decreases with increasing stability of the carbanion moiety (e.g., t-BuLi > s-BuLi > n-BuLi).

Organolithium reagents exhibit reactivities similar to those of Grignard reagents

Organolithiums Via Iithium-Halogen Exchange

Aryllithium Reagents

Metal-halogen exchange provides an efficient route to aryllithiums and heteroaromatic lithium reagents that are inaccessible by metal-hydrogen exchange

Chemoselectivity

Ortho-Metalation of Substituted Benzenes and Heteroaromatic compounds

Metalation Alkynes

This imparts sufficiently greater acidity to acetylene and 1-alkynes (pKa 24-26) so that bases such as alkyllithiums, lithium dialkylamides, sodium amide in liquid ammonia, and ethylmagnesium bromide may be used to generate the alkynyl anions.

Applications of Organo lithium:-

a) Addition to Carbonyl Compoundsb) Adddition to Acidsc) Addition to Nitriles & CO2d) Displacement of Halo Groupse) Opens Epoxide

Organo Magnesium The Grignard reaction, reported in 1900 by Victor Grignard (Nobel Prize, 1912) provides the synthetic chemist with one of the most powerful tools for connecting carbon moieties.

Although 100 years have passed since Grignard published the preparation of ethereal solutions of organornagnesiurn halides, the actual mechanism for the formation of the reagents and their structures are still not completely understood.

The overall reaction for the formation of Grignard reagents involves an insertion of magnesium into the carbon-halogen bond via an oxidative addition, thereby changing its oxidation state from Mg(0) to Mg(II).

Schlenk equilibrium

Alkynyl Grignard reagents are obtained by deprotonation of 1-alkynes with ethylinagnesium bromide in THF. For the preparation of ethynylrnagnesium bromide (HC _CMgBr), a solution of ethylmagnesium bromide in THF is slowly added to a cooled solution of THF containing the acetylene.

Limitations Of Grignard Reagents :-a) For thte preparation of Allyl & Benzyl Grignard reagennt always used Anthracene,due to High stable speciesb) For the praparation of RMgF &RMgI Always use EtBrc) For the preparation of Alkenyl Grignard use Already synthesized RMgX.d) If any acidic groups present along with halo compounds that also decomposed RMgX.

Organo Magnesiu

m

Flow Chart l

Applications of RMgX

Flow Chart llOrgano Magnesium

Applications of RMgX

Cram’s rule

In 1952, Donald I. Cram (1919-2001, Nobel Prize in Chemistry, 1987) and his cowork-er Fathy A. Abd Elhafez presented an experimental rule for the diastereoface-differentiating reaction of ketones having a chiral center at the vicinal carbon with metal hydride complexes or with organometallic reagents

Cram's rule states that ketones (A; L, M, and S are large, medium, and small groups, respectively) with a chiral center The nucleophile of the reagent Y--M' then attacks the carbonyl carbon at the less hindered diastereoface.

In contrast to the polar nature of C-Li and C-MgX bonds, the C-Zn bond is highly covalent and hence less reactive,allowing the preparation of functionalized derivatives.

Utilization of organozinc reagents in organic synthesis has mainly in preparation and utilization of compounds in organic syntheses (Reformatsky reaction), cyclopropanation (Simmons-Smith reaction),and transmetalations with transition metals

Can not opens epoxide

Organo Zinc

Reformatsky Reaction

The Reformatsky reaction involves condensation of a-haloester with aldehydes or ketones in presence of zinc enolates to give the corsesponding beta hydroxy esters.

The zinc enolates are generated by addition of an a-haloester in THF,DME, Et-O in benzene or toluene to an activated zinc, such as a Zn-Cu couple or zinc obtained by reduction of zinc halides with potassium .

An example of a Reformatsky condensation using Rieke zinc is shown below.

Principal

Examples

Mechanism

l. Generation Of Zinc Enolate Using Alpha Halo Ester With Zn ll. Attack of Enolate on the Carbonyl Carbon(Aldehyde/Ketone))

Enantioselective addition of organozinc reagents to carbonyl cornpounds furnishes chiral alcohols

In 1958, Simmons and Smith reported that treatment of a zinc-copper couple with diiodomethane (CH2I2) in ether produces a reagent that adds to alkenes to form cyclopropanes.

Simmons-Smith cyclopropanation Or Simmons & Smith Reaction

The cyclopropanation reaction of simple alkenes appears to proceed via stereospecific syn-addition of a Zn-carbenoid (carbene-like species) to the double bond without the involvement of a free carbene

The synthetic utility of the reaction stems from the following characteristics

1) Stereospecificity 2) Tolerance of a variety of f~lnctional groups, such as Cl, Br, OH, OR, CO,R, C=O, and CN3) The syn-directing effect of hydroxyl and ether functions4)Chemoselectivity-zinc carbenoids are electrophilic and react

chemoselectively with the more nucleophilic double bond in dienes and polyenes

If any Chelating group is present i.e.OH,OR,NH2 then Cycloproponation prefer on the same side of that group

Organo Copper Use of organocopper reagents offers a very efficient method for coupling of two different carbon moieties. Since copper is less electropositive than lithium and magnesium,the C-Cu bond is less polarized than the C-Li and C-Mg bonds. Organocopper complexes (RCu)Lower-order cuprates (R2CuLi, also known as GilmanreagentsLower-order cyanocuprates (RCu(CN)Li)Higher-order cyanocuprates (R2Cu(CN)Li2) 

The organocopper reagents are more selective and can be acylated with acidchlorides without concomitant attack on ketones, alkyl halides, and estersRelative reactivity: RCOCl > RCHO > tosylates, iodides > epoxides > bromides >> ketones > esters > nitriles

Substitution Reactions

Conjugate Addition Reactions

Enantioselective Variants

In reactions with a,ᵝ-unsaturated carbonyl compounds, the organocopperreagents prefer 1,4-over 1,2 addition.

Tandem 1,4-Addition-Enolate Trapping

Epoxide opening organocuprates is highly selective for the less hindered position

Reaction with Vinyl Halides :-

Acylation :-