Chain Amplification in Photoreactions ofN-Alkoxypyridinium Salts with Alcohols:  Mechanism and Kinetics

Abstract

Photosensitized electron transfer from a variety of singlet- and triplet-excited donors to N-methoxypyridinium salts leads to N-O bond cleavage. Hydrogen atom abstraction by the resulting methoxy radical from an added alcohol generates an alpha-hydroxy radical that reduces another pyridinium molecule, thus leading to chain propagation. For example, thioxanthone-sensitized reactions of 4-cyano-N-methoxypyridinium, P1, with several aliphatic and benzyl alcohols gave quantum yields for products formation (an aldehyde or a ketone and protonated 4-cyanopyridinium) of approximately 15-20, at reactant concentrations of approximately 0.02-0.04 M. The reaction can also be sensitized with triplet benzopheone, which in this case acts as an electron donor. Energetic limitations on chain propagation are imposed by the relationship between the oxidation potential of the alpha-hydroxy radical and the reduction potential of the pyridinium salt. The chain reactions proceed despite approximately 0.25 eV endothermicity for the electron-transfer step. Chain reactions with the harder-to-reduce 4-phenyl-N-methoxypyridinium, however, are limited in scope because of increased endothermicity for electron transfer. The thioxanthone-sensitized reaction of P1 with benzhydrol was studied in detail by a combination of steady state and transient kinetics. The bimolecular rate constants for the chain propagation reactions:hydrogen atom abstraction by the methoxy radical and electron transfer from the diphenylketyl radical to P1 are approximately 6 x 10(6) and 1.1 x 10(6) M(-1) s(-1), respectively. The kinetic data indicate that deuterium atom abstraction by the methoxy radical from the solvent, acetonitrile-d(3), is a dominant chain-terminating process. Because of a large deuterium isotope effect, approximately 7, the quantum amplification is strongly suppressed when the reaction is carried out in acetonitrile.