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

The reactions of a series of structurally related large-ring propellanes wi th iodine monochloride were studied experimentally and computationally. In the case of 1,3-dehydroadamantane (1) and [3.3.1]propellane (2) free-radica l addition was observed. [3.3.2]Propellane (3) and 3,6-dehydrohomoadamantan e (4), which are less prone to radical attack, selectively form products of formal double nucleophilic (oxidative) addition, e.g., dichloro (in ICl/CH 2Cl2), dimettloxy (in ICl/CH3OH), and diacetamino (in ICl/CH3CN) derivative s under otherwise identical conditions. Single-electron transfer pathways i nvolving the alkane radical cations are proposed for the activation step fo r aliphatic hydrocarbons with relatively low oxidation potentials such as c age alkanes. Similar mechanisms are postulated for the activation of the te rtiary C-H bonds of adamantane based on H/D-kinetic isotope effect data. Th e latter compare well to the k(H)/k(D) value for hydrogen atom loss from th e adamantane radical cation (measured 2.78 +/- 0.21 and computed 2.0) and d iffer considerably from the kinetic isotope effects for electrophilic C-H b ond 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. U pon C-C or C-H fragmentation, the incipient alkane radical cations are trap ped by nucleophiles.