Diastereoselectivity in addition of nitrile-stabilized carbanions to Schiff bases and in subsequent alkylation reactions

The stereochemical course in the addition of lithiated benzylcyanide and propionitrile to aromatic Schiff bases, as well as of the subsequent alkylation reaction has been investigated. The stereochemical ratios in the condensation reaction are proved to result from the intermediacy of a prochiral carbanionic intermediate, produced by a fast proton shift in the initially formed azanion. Subsequent one-pot alkylation reaction with a variety of electrophiles leads in high to moderate yields to diastereoselective formation of a second carbon–carbon bond at the same carbon center. Diastereoselectivity in alkylation, which varies from outstanding to high and poor is rationalized in terms of open-chain, product-like or reactant-like transition state models.