The effect of salt formation on structure and charge distribution in imines. Part 4. Energy barriers to isomerisation about the C–N bond in 2,6-dimethyl-4-aryliminopyrans and their salts: solvent and substituent effects, and evidence for isomerisation mechanisms

Abstract

Free energies of activation, ΔG^‡, have been calculated for isomerisation about the CN bond in a series of 2,6-dimethyl-4-aryliminopyrans (1), and their hydro-[(2) and (3)] and metho-[(4) and (5)] salts. Independent values were measured from coalescence of two different sets of ¹H n.m.r. signals in each compound. The effects of temperature, solvent, and aryl substituent X on ΔG^‡ indicate that the metho-salts isomerise by CN bond rotation, the imines by inversion or a mechanism intermediate between inversion and rotation, and the hydro-salts by the previously unreported deprotonation–imine isomerisation–reprotonation mechanism. Hydro-salts (3) having a –M substituent in the aryl para-position give anomalous results for ΔG^‡ at the lower coalescence temperature.