Ambident Reactivities of Formaldehyde N,N-Dialkylhydrazones

摘要

N,N-Dialkylhydrazones 1, which can be considered as 2-aza-enamines, have been employed as synthetically useful nucle-ophiles in a variety of C-C bond-forming reactions (Scheme 1). Like enamines, hydrazones 1 are ambident nucleophiles that react with electrophiles either at the terminal nitrogen or at the azomethine carbon, which adopts nucleophilic character from conjugation of the lone pair on the terminal nitrogen with the C=N double bond (Scheme 1). Electrophilic alkylation at nitrogen yields N,N,N-trialkyihydrazonium salts, the synthetic utility of which has already been explored. When electrophilic addition occurs at the azomethine carbon, hydrazones 1 behave as acyl anion equivalents. This form of umpolung of the carbonyl reactivity (Scheme 1) has been extensively applied in chiral auxiliary based strategy and in asymmetric organo-catalyzed reactions.