Proton transfer from heterocyclic compounds. Part II. Purine, 9-alkylpurines, and imidazo[4,5-b]pyridine

Abstract

The rates of detritiation of [8-3H]purine, 9-isopropyl[8-3H]purine, 9-t-butyl[8-3H]purine, and [2-3H]imidazo-[4,5-b]pyridine have been measured over a pH range at 85°. The bell-shaped rate–pH profiles for purine and imidazo[4,5-b]pyridine are interpreted in terms of rate-determining attack by hydroxide ion on the conjugate acid, giving rise to an ylide intermediate. The rate–pH profiles for the 9-alkylpurines are also consistent with this pathway, with an additional hydroxide-catalysed exchange of the neutral species becoming increasingly important at high pH. The first-mentioned mechanism requires protonation (or partial protonation) of purine and 9-substituted purines at N-7 rather than at N-1, the generally accepted position.