Deprotonation of methyl-substituted, five-membered aromatic molecules: a surprising case of mixed conjugation, rehybridization, and induction contributions.

Methyl-substituted, six-membered aromatic molecules are deprotonated to benzylic carbanions, which are stabilized by π conjugation. In contrast, deprotonation of 3(5)-methylpyrazole (NH protected) occurs at an endocylic CH group. Computational analyses showed that the reduction of π conjugation in substituted five-membered rings plays a major role, while the reduced bond angles, in addition to the strengthened induction of Csp(2) versus Csp(3), further favor the deprotonation of endocyclic carbon sites rather than that of the methyl group.