Chain-Amplified Photochemical Fragmentation of <i>N</i>‑Alkoxypyridinium Salts: Proposed Reaction of Alkoxyl Radicals with Pyridine Bases To Give Pyridinyl Radicals

Abstract

Photoinduced electron transfer to <i>N</i>-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 <i>N</i>-methoxy and <i>N</i>-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^•, <i>k</i>₁, is ∼6 × 10⁶ M^–1 s^–1; that of CH₃CH₂O^• is ∼9 times larger. Reaction of CD₃O^• showed a deuterium isotope effect of ∼6.5. Replacing lutidine by 3-chloropyridine, a weaker base, decreases <i>k</i>₁ by a factor of ∼400