PREFERENTIAL PROTONATION AND METHYLATION AT THE NITROGEN-ATOMS OF N,N-DIMETHYLAMINO DERIVATIVES OF PYRIDINE

The relative reactivity toward protonation and methylation of the two nitrogen atoms in N,N‐dimethylaminopyridines has been examined by 1H NMR. The ring position of the dimethylamino group has no influence on protonation, which occurs in all the derivatives at the heterocyclic nitrogen. The N‐methylation reaction does not follow a homogeneous behaviour, occurring at the exocyclic nitrogen in the 2‐substituted dimethylamino derivative. The electronic characteristics of the molecules, determined by MO calculations at a semi‐empirical level, indicate that both protonation and methylation should occur at the heterocyclic nitrogen; the calculated relative stabilites, however, of the N‐protonated and N‐methylated forms are in full agreement with the experimental results, and it appears that the anomalous behaviour of 2‐dimethylaminopyridine in the N‐methylation reaction is caused by steric factors. Copyright © 1979 Heyden & Son Ltd.