Hydrogen Abstraction from Cyclic Amines by the Cumyloxyl and Benzyloxyl Radicals. The Role of Stereoelectronic Effects and of Substrate/Radical Hydrogen Bonding

A kinetic study on the hydrogen abstraction reactions from cyclic amines and diamines (pyrrolidines, piperidines, morpholines, and piperazines) by the cumyloxyl (CumO^•) and benzyloxyl (BnO^•) radicals was carried out. The reactions with CumO^• were described in all cases as direct hydrogen abstractions. The differences in the hydrogen abstraction rate constant (k_H) were explained in terms of the different number of abstractable hydrogen atoms, the operation of stereoelectronic effects, and, with the morpholines, on the basis of polar effects. Significantly higher k_H values were measured for the reactions of the amines with BnO^•. This behavior was explained on the basis of a mechanism that proceeds through the rate-determining formation of a hydrogen bonded pre-reaction complex between the radical α-C–H and the nitrogen lone pair followed by hydrogen abstraction within the complex. A decrease in k_H was observed going from secondary to tertiary amines and, with tertiary amines, on increasing steric hindrance at nitrogen, pointing toward the important role of steric and electronic effects on pre-reaction complex formation. These results expand previous findings contributing to a detailed mechanistic description of the reactions of alkoxyl radicals with amines, showing that structural effects in both the substrate and the radical can play a dramatic role and providing new information on the role of substrate/radical interactions on these processes.