Chain-amplified photochemical fragmentation of N-alkoxypyridinium salts: Proposed reaction of alkoxyl radicals with pyridine bases to give pyridinyl radicals

摘要

Photoinduced electron transfer to N-alkoxypyridiniums, which leads to N-O bond cleavage and alkoxyl radical formation, is highly chain amplified in the presence of a pyridine base such as lutidine. Density functional theory calculations support a mechanism in which the alkoxyl radicals react with lutidine via proton-coupled electron transfer (PCET) to produce lutidinyl radicals (BH?). A strong electron donor, BH? is proposed to reduce another alkoxypyridinium cation, leading to chain amplification, with quantum yields approaching 200. Kinetic data and calculations support the formation of a second, stronger reducing agent: a hydrogen-bonded complex between BH? and another base molecule (BH??B). Global fitting of the quantum yield data for the reactions of four pyridinium salts (4-phenyl and 4-cyano with N-methoxy and N-ethoxy substituents) led to a consistent set of kinetic parameters. The chain nature of the reaction allowed rate constants to be determined from steady-state kinetics and independently determined chain-termination rate constants. The rate constant of the reaction of CH3O? with lutidine to form BH?, k_1, is ～6 × 10~6 M~(-1) s~(-1); that of CH_3CH_2O ? is ～9 times larger. Reaction of CD_3O ? showed a deuterium isotope effect of ～6.5. Replacing lutidine by 3-chloropyridine, a weaker base, decreases k_1 by a factor of ～400.