Hydrogen Atom Abstraction Selectivity in the Reactions of Alkylamines with the Benzyloxyl and Cumyloxyl Radicals. The Importance of Structure and of Substrate Radical Hydrogen Bonding

A time-resolved kinetic study on the hydrogen abstraction reactions from a series of primary and secondary amines by the cumyloxyl (CumO(center dot)) and benzyloxyl (BnO center dot) radicals was carried out. The results were compared with those obtained previously for the corresponding reactions with tertiary amines. Very different hydrogen abstraction rate constants (k(H)) and intermolecular selectivities were observed for the reactions of the two radicals. With CumO(center dot), k(H) was observed to decrease on going from the tertiary to the secondary and primary amines. The lowest k(H) values were measured for the reactions with 2,2,6,6-tetramethylpiperidine (TMP) and tert-octylamine (TOA), substrates that can only undergo N-H abstraction. The opposite behavior was observed for the reactions of BnO center dot, where the k(H) values increased in the order tertiary < secondary < primary. The k(H) values for the reactions of BnO were in all cases significantly higher than those measured for the corresponding reactions of CumO center dot, and no significant difference in reactivity was observed between structurally related substrates that could undergo exclusive alpha-C-H and N-H abstraction. This different behavior is evidenced by the k(H)(BnO center dot)/k(H)(CumO(center dot)) ratios that range from 55-85 and 267-673 for secondary and primary alkylamines up to 1182 and 3388 for TMP and TOA. The reactions of CumO(center dot) were described in all cases as direct hydrogen atom abstractions. With BnO center dot the results were interpreted in terms of the rate-determining formation of a hydrogen-bonded prereaction complex between the radical alpha-C-H and the amine lone pair wherein hydrogen abstraction occurs. Steric effects and amine HBA ability play a major role, whereas the strength of the substrate alpha-C-H and N-H bonds involved appears to be relatively unimportant. The implications of these different mechanistic pictures are discussed.