Enhancement of Chain Amplification in Photoreactions of N-Methoxypyridinium Salts with Alcohols†

Abstract

Recently we reported a chain-amplified photochemical reaction, initiated by electron transfer from an excited sensitizer to N-methoxypyridinium salts, which leads to N-O bond cleavage (26). Hydrogen atom abstraction by the methoxy radical from an alcohol yields an alpha-hydroxy radical, which reduces another N-methoxypyridinium molecule and propagates the chain. We now report that the chain amplification can be significantly enhanced in the presence of water. Detailed kinetic studies of the reaction of 4-cyano-N-methoxypyridinium salt (CMP) with benzhydrol (BH) showed that the rate constant for reduction of CMP by the diphenyl ketyl radical (1.1 x 10(6) M(-1) s(-1)) increases by more than an order of magnitude in the presence of water. This increase in the rate constant is the result of coupling of the electron transfer to a proton transfer from the ketyl radical to water, which decreases the endothermicity of the reaction. Unfortunately, this increase in the rate constant for one of the two propagation steps is accompanied by a larger increase in the rate constant(s) of the competing termination reaction(s) of the ketyl radical. The observed enhancement in chain amplification is the result of a significant increase in the ratio of propagation to termination rate constants of the reactions of the methoxy radical. The main chain-terminating reactions of the methoxy radical are deuterium abstraction from the solvent, CD(3)CN, and reaction with the sensitizer, thioxanthone. The effect of increase in the ratios of the propagation rate constant of the methoxy radical (hydrogen abstraction from BH) to those of both termination reactions is larger than the unfavorable effect of water on the reactions of the ketyl radical. The increase in chain amplification depends on the concentration of the reactants; at 0.037 M of both reactants, the quantum yield increases form approximately 16 to approximately 45 in the presence of <1% water. The reaction of 4-phenyl-N-methoxypyridinium (PMP) with 4-methoxybenzyl alcohol does not proceed via chain amplification because of large endothermicity for electron transfer from the alpha-hydroxy radical to the pyridinium salt. However, chain amplification could be induced, simply by addition of water, where at approximately 10% water content, a quantum yield of approximately 5 was obtained. Water-induced, proton-coupled electron transfer increases the rate constant for reduction of PMP from a negligible level to becoming the dominant path.