文摘
Photoinduced electron transfer to N-alkoxypyridiniums, which leads to N鈥揙 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 (BHp>鈥?/sup>). A strong electron donor, BHp>鈥?/sup> 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 BHp>鈥?/sup> and another base molecule (BHp>鈥?/sup>路路路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 CH3Op>鈥?/sup> with lutidine to form BHp>鈥?/sup>, k1, is 6 脳 10p>6p> Mp>鈥?p> sp>鈥?p>; that of CH3CH2Op>鈥?/sup> is 9 times larger. Reaction of CD3Op>鈥?/sup> showed a deuterium isotope effect of 6.5. Replacing lutidine by 3-chloropyridine, a weaker base, decreases k1 by a factor of 400.