Organic Chemistry

470 CHAPTER 12 Reactions of Alcohols, Ethers, Epoxides, and Sulfur-Containing Compounds

polar. (The carbon–magnesium bond is about 52% ionic.) Lithium (1.0) is even less

electronegative than magnesium (1.2). Thus, the carbon–lithium bond is more polar

than the carbon–magnesium bond. Therefore, an organolithium reagent is a more

reactive nucleophilic reagent than a Grignard reagent.

The names of organometallic compounds usually begin with the name of the alkyl

group, followed by the name of the metal.

A Grignard reagent will undergo transmetallation(metal exchange) if it is added to a

metal halide whose metal is more electronegative than magnesium. In other words, metal

exchange will occur if it results in a less polar carbon–metal bond. For example, cadmium

(1.5) is more electronegative than magnesium (1.2). Consequently, a carbon–cadmium

bond is less polar than a carbon–magnesium bond, so metal exchange occurs.

PROBLEM 32

What organometallic compound will be formed from the reaction of methylmagnesium

chloride and (Hint:See Table 12.3.)

12.12 Coupling Reactions

New carbon–carbon bonds can be made using an organometallic reagent in which the

metal ion is a transition metal. Transition metals are indicated by purple in the periodic

table on the last page of this book. The reactions are called coupling reactionsbecause

two groups (any two alkyl, aryl, or vinyl groups) are joined (coupled together).

The first organometallic compounds used in coupling reactions were Gilman

reagents, also called organocuprates. They are prepared by the reaction of an organo-

lithium reagent with cuprous iodide in diethyl ether or THF.

Gilman reagents are very useful to synthetic chemists. When a Gilman reagent

reacts with an alkyl halide (with the exception of alkyl fluorides, which do not

undergo this reaction), one of the alkyl groups of the Gilman reagent replaces the

halogen. Notice that this means that an alkane can be formed from two alkyl

halides—one alkyl halide is used to form the Gilman reagent, which then reacts with

the second alkyl halide. The precise mechanism of the reaction is unknown, but is

thought to involve radicals.

Gilman reagents can be used to prepare compounds that cannot be prepared by using

nucleophilic substitution reactions. For example,SN 2 reactions cannot be used to

organolithium

reagent

CH 3 Li CuI (CH 3 ) 2 CuLi

Gilman reagent

THF

2 + + LiI

SiCl 4?

ethylmagnesium

chloride

2 CH 3 CH 2 MgCl CdCl 2

diethylcadmium

+ (CH 3 CH 2 ) 2 Cd + 2 MgCl 2

CH 3 CH 2 MgBr CH 3 CH 2 CH 2 CH 2 Li (CH 3 CH 2 CH 2 ) 2 Cd (CH 3 CH 2 ) 4 Pb

ethylmagnesium dipropylcadmium tetraethyllead

bromide

butyllithium

Henry Gilman (1893–1986)was

born in Boston. He received his B.A.

and Ph.D. degrees from Harvard

University. He joined the faculty at

Iowa State University in 1919, where

he remained for his entire career.

He published over 1000 research

papers—more than half after he lost

almost all of his sight as the result of

a detached retina and glaucoma in

1947. His wife, Ruth, acted as his
      eyes for 40 years.

CH 3 CH 2 CH 2 CH 2 Br (CH 2 CH 2 CH 2 ) 2 CuLi CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH 3 CH 3 CH 2 CH 2 Cu

heptane

THF

++

+ LiBr

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