Kharasch and Metalloporphyrin Catalysis in the Functionalization of Alkanes, Alkenes, and Alkylbenzenes by t-BuOOH. Free Radical Mechanisms, Solvent Effect, and Relationship with the Gif Reaction

New results for the oxidation of alkanes, alkenes, and alkylbenzenes by t-BuOOH according to the Kharasch reaction and Mn(III)— and Fe(III)—porphyrin catalysis are reported. Chemical and kinetic evidence show the role of t-BuO* and t-BuOO· radicals in these oxidations and the strict relationship with the Gif reaction, for which recently a free-radical mechanism has been emphasized in contrast with the Barton interpretation. A key step appears to be the hydrogen abstraction from t-BuOOH by t-BuO· very fast in nonbasic solvents, but inhibited by solvents which form hydrogen bonds with t-BuOOH. As a consequence, the generally accepted Kochi mechanism of the Kharasch reaction must be modified, and the similar mechanistic features of the three types of catalysis (Kharasch, metalloporphyrins, and Gif) are discussed.