Imidazo[1,5-a]pyridinium salts from phenylchlorocarbene and 2-pyridyl schiff bases: synthesis, reaction mechanism and effect of rotamerism
Résumé
3-Phenylimidazo[1,5-a]pyridinium salts are easily prepared in excellent yield by thermolysis of phenylchlorodiazirine in the presence of 2-pyridyl Schiff bases (2-Pyr-CH=NR, where R is an aromatic or alkyl group). When the reaction is conducted in an alkane solvent, the salts precipitate and are easily obtained with a high level of purity after filtration and repeated washing of the precipitate with a nonpolar solvent. A laser-flash photolysis study of the reaction mechanism reveals that the phenylchlorocarbene produced by decomposition of the diazirine reacts with the Schiff base to give two different ylides. The intramolecular cyclization of these two ylides yields the same intermediate product, which rapidly eliminates a chlorine anion. Neither semi-empirical calculations of the energy barriers for the formation and cyclization of the various possible ylides, nor comparison of their calculated and experimental absorption spectra, allow us to determine whether the two ylides are rotamers of a pyridinium ylide or a pyridinium ylide and an iminium ylide. Only the comparison of systems where R is a cyclohexyl or a tert-butyl group indicates that the two ylides are rotamers of a pyridinium ylide.