Oxidative single-electron transfer activation of σ-bonds in aliphatic halogenation reactions

The reactions of a series of structurally related large-ring propellanes with iodine monochloride were studied experimentally and computationally. In the case of 1,3-dehydroadamantane (1) and [3.3.1]propellane (2) free-radical addition was observed. [3.3.2]Propellane (3) and 3,6-dehydrohomoadamantane (4), which are less prone to radical attack, selectively form products of formal double nucleophilic (oxidative) addition, e.g., dichloro (in ICl/CH2Cl2), dimettloxy (in ICl/CH3OH), and diacetamino (in ICl/CH3CN) derivatives under otherwise identical conditions. Single-electron transfer pathways involving the alkane radical cations are proposed for the activation step for aliphatic hydrocarbons with relatively low oxidation potentials such as cage alkanes. Similar mechanisms are postulated for the activation of the tertiary C-H bonds of adamantane based on H/D-kinetic isotope effect data. The latter compare well to the k(H)/k(D) value for hydrogen atom loss from the adamantane radical cation (measured 2.78 +/- 0.21 and computed 2.0) and differ considerably from the kinetic isotope effects for electrophilic C-H bond activations (i.e., hydride abstraction) or for loss of a proton from a hydrocarbon radical cation (k(H)/k(D) = 1.0-1.4; computed 1.4). Hence, the reactions of alkanes with elementary halogens and other weak electrophiles (but strong oxidizers) do not necessarily involve three-center two-electron species but rather occur via successive single-electron oxidation steps. Upon C-C or C-H fragmentation, the incipient alkane radical cations are trapped by nucleophiles.