H-Coupled Electron Transfer in Alkane C−H Activations with Halogen Electrophiles

The mechanisms for the reactions of isobutane and adamantane with polyhalogen electrophiles (HHal(2)(+), Hal(3)(+), Hal(5)(+), and Hal(7)(+), Hal = Cl, Br, or I) were studied computationally at the MP2 and B3LYP levels of theory with the 6-31G (C, H, Cl, Br) and 3-21G (I) basis sets, as well as experimentally for adamantane halogenations in Br(2), Br(2)/HBr, and I(+)Cl(-)/CCl(4). The transition structures for the activation step display almost linear C...H...Hal interactions and are characterized by significant charge transfer to the electrophile; the hydrocarbon moieties resemble the respective radical cation structures. The regiospecificities for polar halogenations of the 3-degree C-H bonds of adamantane, the high experimental kinetic isotope effects (k(H)/k(D) = 3-4), the rate accelerations in the presence of Lewis and proton (HBr) acids, and the high kinetic orders for halogen (7.5 for Br(2)) can only be understood in terms of an H-coupled electron-transfer mechanism. The three centered-two electron (3c-2e) electrophilic mechanistic concept based on the attack of the electrophile on a C-H bond does not apply; electrophilic 3c-2e interactions dominate the C-H activations only with nonoxidizing electrophiles such as carbocations. This was shown by a comparative computational analysis of the electrophilic and H-coupled electron-transfer activation mechanisms for the isobutane reaction with an ambident electrophile, the allyl cation, at the above levels of theory.