Thermolysis and photolysis of 1-anilino-2-methyl-4,6-diphenylpyridinium tetrafluoroborate in arene–trifluoroacetic acid mixtures: a novel route to singlet and triplet phenylnitrenium ions

Abstract

Thermolysis of 1-anilino-2-methyl-4,6-diphenylpyridinium tetrafluoroborate (1) in trifluoroacetic acidbenzene or–toluene yielded 2-methyl-4,6-diphenylpyridine (2) and the N-substitution products 2-(2-anilinophenyl)-6-methyl-4-phenyl pyridine (3) and diphenylamine (4a)[or 2- and 4-methyldiphenylamine (4b) and (5b)], via a singlet phenylnitrenium ion formed in the unimolecular decomposition of the salt (1). The mechanism was inferred from the kinetics of decomposition of the salt (1), the o-/p-direction in the formation of (4b) and (5b), the large partial rate factors for ortho- and para-substitution of toluene, and analysis of the products of the reaction of (1) with diethyl sulphide. The preferred production of the 2-anilinophenyl derivative (3) rather than the corresponding 4-anilinophenyl compounds, and the relative formation rates of (2) and (3) suggest that the product (3) is formed by an intimate contact reaction between pyridine and phenylnitrenium ion formed initially; the results give relative reactivities of the nitrenium ion with benzene, toluene, naphthalene, and dimethyl sulphide. Photolysis of the salt (1), however, gave the pyridine (2), aniline (9), and hydrogen abstraction products [biphenyl (10) or the bitolyls (11)]via a triplet phenylnitrenium ion, without contamination by compounds (3)–(5). The triplet mechanism was proved by observation of inter- and intra-molecular heavy atom effects.