Organolithium reagents, reaction with hydrazones

The reaction of alkyllithium reagents with acyclic and cyclic tosylhydrazones can lead to mixtures of elimination (route A) and addition (route B) products (Scheme 22). The predominant formation of the less-substituted alkene product in the former reaction (Shapiro Reaction) is a result of the strong preference for deprotonation syn to the N-tosyl group. Nucleophilic addition to the carbon-nitrogen tosyl-hydrazone double bond competes effectively wiA a-deprotonation (and alkene formation) if abstraction of the a-hydrogens is slow and excess organolithium reagent is employed. Nucleophilic substitution is consistent with an Su2 addition of alkyllithium followed by electrophilic capture of the resultant carbanion. 

The reactions of organolithium, -magnesium, -zinc, -copper, and -titanium reagents with aldehydes, ketones, acetals, imines and hydrazones will be described in this section. The reactions of enolates and enamines will be subsequently examined ( 6.8 to 6.10). 

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