Carbon-hydrogen bonds organolithiums

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. 

All the above reactions involve hydrogen—lithium interconversion (metalation). There are a limited amount of data available indicating that N-chelated organolithium intermediates also undergo nucleophilic addition to carbon-carbon double and triple bonds much more readily than does the organolithium reagent alone. Indeed, this enhanced reactivity toward addition reactions is a key factor in the telomerization of ethylene onto aromatic hydrocarbons (6, 7,8,9). 

Metallation is believed to involve nucleophilic attack by the organolithium reagent on the acidic hydrogen atom. The nucleophilic character of carbon bonded to lithium is enhanced by coordination of the lithium to a base. Organolithium compounds are therefore more reactive when dissolved in ethers than in hydrocarbons. Unfortunately ethers can themselves be metallated. This is especially a problem with the rather basic ether tetrahydrofuran, which is rapidly cleaved by butyllithium at room temperature. 

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