Effects of Structure on Rates and Quantum Yields in Photoreduction of Fluorenone by Amines. Catalysis and Inhibition by Thiols

Article abstract of DOI:10.1021/ja00376a031

Rate constants, k_ir, quantum yields, φ_red, and effects of thiol have been studied in the photoreduction of fluorenone by amines in benzene.Hydroxyethylamines show increased φ_red as compared with unsubstituted ethyl analogues, φ_red ca.0.8 with triethylamine, but ca.2, the maximum theoretical value, with triethanolamine and 2-(diethylamino)ethanol.Abstraction is predominantly from unsubstituted ethyl groups in the latter, and increased k_ir and φ_red are attributed to solvation by hydroxyl, present only at the reaction site, of polar intermediates or transition states.Secondary amines have high k_ir and low φ_red, 0.01-0.2, attributed to predominant abstraction of H from N and disproportionation of ketyl and aminyl radicals.This reduction is strongly catalyzed by aliphatic thiol, as aminyl radical abstracts H from S and thiyl radical abstracts H from α-C of amine.Primary amines have low k_ir and φ_red.This reduction is not catalyzed by thiol, indicating that H is not abstracted by triplet fluorenone from N of primary aliphatic amines.Aniline inhibits this reduction, but aniline plus thiol leads to strong catalysis.Triplet fluorenone abstracts H from N of aniline, anilino radical abstracts H from S of thiol, and thiol abstracts H from α-C of amine, leading to effective reduction.Thiol catalysis may be diagnostic for the formation of aminyl radicals.Inhibition and catalysis by thiols are discussed.