20.9^20.9. Organometallic Reagents^739^742^,,^31846^31931%
Organometallic Reagents

We will now discuss the reactions of carbonyl compounds with organometallic reagents, another class of nucleophiles.

Lithium, magnesium, and copper are the most commonly used metals in organometallic reagents, but others (such as Sn, Si, Tl, Al, Ti, and Hg) are known. General structures of the three common organometallic reagents are shown. R can be alkyl, aryl, allyl, benzyl, sp2 hybridized, and with M = Li or Mg, sp hybridized. Because metals are more electropositive (less electronegative) than carbon, they donate electron density towards carbon, so that carbon bears a partial negative charge.

Electronegativity values for carbon and the common metals in RM reagents are C (2.5), Li (1.0), Mg (1.3), and Cu (1.8).

Because both Li and Mg are very electropositive metals, organolithium (RLi) and organomagnesium reagents (RMgX) contain very polar carbon–metal bonds and are therefore very reactive reagents. Organomagnesium reagents are called Grignard reagents, after Victor Grignard, who received the Nobel Prize in Chemistry in 1912 for his work with them.

Organocopper reagents (R2CuLi), also called organocuprates, have a less polar carbon–metal bond and are therefore less reactive. Although organocuprates contain two alkyl groups bonded to copper, only one R group is utilized in a reaction.

Regardless of the metal, organometallic reagents are useful synthetically because they react as if they were free carbanions; that is, carbon bears a partial negative charge, so the reagents react as bases and nucleophiles.

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Preparation of Organometallic Reagents

Organolithium and Grignard reagents are typically prepared by reaction of an alkyl halide with the corresponding metal, as shown in the accompanying equations.

With lithium, the halogen and metal exchange to form the organolithium reagent. With magnesium, the metal inserts in the carbon–halogen bond, forming the Grignard reagent. Grignard reagents are usually prepared in diethyl ether (CH3CH2OCH2CH3) as solvent. It is thought that two ether oxygen atoms complex with the magnesium atom, stabilizing the reagent.

Organocuprates are prepared from organolithium reagents by reaction with a Cu+ salt, often CuI.

Problem 20.19

Write the step(s) needed to convert CH3CH2Br to each reagent: (a) CH3CH2Li; (b) CH3CH2MgBr; (c) (CH3CH2)2CuLi.

Acetylide Anions

The acetylide anions discussed in Chapter 11 are another example of organometallic compounds. These reagents are prepared by an acid–base reaction of an alkyne with a base such as NaNH2 or NaH. We can think of these compounds as organosodium reagents. Because sodium is even more electropositive (less electronegative) than lithium, the CNa bond of these organosodium compounds is best described as ionic, rather than polar covalent.

An acid–base reaction can also be used to prepare sp hybridized organolithium compounds. Treatment of a terminal alkyne with CH3Li affords a lithium acetylide. Equilibrium favors the products because the sp hybridized CH bond of the terminal alkyne is more acidic than the sp3 hybridized conjugate acid, CH4, that is formed.

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Problem 20.20

1-Octyne (HC≡≡CCH2CH2CH2CH2CH2CH3) reacts rapidly with NaH, forming a gas that bubbles out of the reaction mixture, as one product. 1-Octyne also reacts rapidly with CH3MgBr, and a different gas is produced. Write balanced equations for both reactions and identify the gases formed.

Reaction as a Base

The electron pair in the carbon–metal bond is used to form a new bond to the proton. Equilibrium favors the products of this acid–base reaction because H2O is a much stronger acid than the alkane product.

Similar reactions occur for the same reason with the OH proton in alcohols and carboxylic acids, and the N—H protons of amines.

Because organolithium and Grignard reagents are themselves prepared from alkyl halides, a two-step method converts an alkyl halide into an alkane (or another hydrocarbon).

Problem 20.21

Draw the product formed when each organometallic reagent is treated with H2O.

  1. (CH3)3CMgBr

  2. CH3CH2C≡≡CLi

Reaction as a Nucleophile

Organometallic reagents are also strong nucleophiles that react with electrophilic carbon atoms to form new carbon–carbon bonds. These reactions are very valuable in forming the carbon skeletons of complex organic molecules. The following reactions of organometallic reagents are examined in Sections 20.10, 20.13, and 20.14: