title: Chain-amplified photochemical fragmentation of N-alkoxypyridinium salts: proposed reaction of alkoxyl radicals with pyridine bases to give pyridinyl radicals.
abstract: 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•, k1, is ~6 × 10(6) 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 k1 by a factor of ~400.
authors.affiliation: Research Laboratories, Eastman Kodak Company, Rochester, New York 14650, United States. deepak.shukla@kodak.com
authors.forename: Deepak
authors.initials: D
authors.lastname: Shukla
authors.forename: Shashishekar P
authors.initials: SP
authors.lastname: Adiga
authors.forename: Wendy G
authors.initials: WG
authors.lastname: Ahearn
authors.forename: Joseph P
authors.initials: JP
authors.lastname: Dinnocenzo
authors.forename: Samir
authors.initials: S
authors.lastname: Farid
mesh_terms: D000438:Alcohols / Q000737:chemistry*; D002412:Cations / Q000737:chemistry*; D006860:Hydrogen Bonding; D015394:Molecular Structure; D055668:Photochemical Processes; D011725:Pyridines / Q000737:chemistry*; D011726:Pyridinium Compounds / Q000737:chemistry*
chemical_list: D000438:Alcohols; D002412:Cations; D011725:Pyridines; D011726:Pyridinium Compounds; C059688:alkoxyl radical