Proton transfer reactions of a bridged bis-propyl bis-imidazolium salt

Abstract

Tetraazafulvalene 1 has found broad application in reduction and other related transformations and is conveniently generated from bis-propyl bis-imidazolium salt 4 with a strong base in a non-protic solvent. The proposed mechanism for the formation of 1 involves initial deprotonation at C(2) to give a mono-carbene 9 followed by intramolecular reaction at the second azolium centre. Herein, we report the second-order rate constants for deuteroxide-catalysed exchange in aqueous solution of the C(2)-hydrogens of bis-propyl bis-imidazolium di-iodide salt 4 and related monomeric dipropyl imidazolium iodide 10 of kDO = 1.37 × 104 and 1.79 × 102 M−1 s−1, respectively, and used these data to calculate pKa values of 21.2 and 23.1. The greater C(2)-H acidity of the doubly bridged bis-propyl bis-imidazolium salt 4 relative to 10 may be attributed to the inductive or electrostatic destabilization of the conjugate acid dicationic azolium ion 4 relative to the monocationic carbene 9, which is enhanced by bis-tethering. Formation of tetraazafulvalene 1 was not observed under the aqueous conditions employed highlighting that carbene reprotonation significantly outcompetes dimerization under these conditions.