Halides reaction with organolithium reagents

Seyferth showed in 1959 that allyllithium, which is not available from allyl halides because of competing substitution reactions, can be prepared from the reaction of allylstannanes with organolithium reagents (XM = R Li Section 22.1). These reactions involve nucleophilic attack of R upon tin to form a pentacoordinate intermediate. Lewis acids, MX, on the other hand, react by electrophilic attack at the y-position. 

These mechanisms ascribe importance to the Lewis acid-Lewis base interaction between the allyl halide and the organolithium reagent. When substitution is complete, the halide ion is incorporated into the lithium cluster in place of one of the carbon ligands. Substituted allylic halides give product mixtures resulting from bond formation both at C—1 and C—3 of the allylic system, with the ratio of products favoring reaction at the less substituted site. 

Wurtz coupling is not as much of a problem in ether solvents as it is when hydrocarbon (alkane) solvents are used in the reaction of lithium metal and alkyl halides. Unfortunately, many organolithium reagents slowly react with ethers to give decomposition products. This means that if an organolithium... 

Organocuprates can react with all alkyl halides except tertiary alkyl halides. Because they can react with vinylic halides and aryl halides (an aryl hahde has a hydrogen attached to a benzene ring), they can be used to prepare compounds that cannot be prepared by Sn2 reactions with Grignard reagents or organolithium compounds. (Remember that vinylic and aryl halides cannot undergo nucleophilic attack Section 9.5). 

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