A search for lone-pair interactions in forward and reverse Menschutkin reactions of some diaza heterocycles

Abstract

The rates of methylation (forward reaction) of phthalazine (1), 1,8-naphthyridine (2), 1,10-phenanthroline (3), and of some ring methyl derivatives are compared with the rates of demethylation of the corresponding N-methyl quaternary iodides. It was observed for (2), and especially for (3), that a nitrogen lone pair in place of a CH group (quinoline and 8-methylquinoline, respectively) aids the forward reaction but does not have a commensurate retarding effect on the reverse process. In contrast to (3), 2,2´-bipyridine showed behaviour typical of a-substituted pyridines. The introduction of a 2-methyl group into (3) had an appreciable rate-enhancing effect on methylation (at N 10) and this is interpreted as evidence for lone-pair cooperativity in the forward reaction.