Efficient Hydrogenation of Sterically Hindered Olefins with Borane-Methyl Sulfide Complex

Sterically hindered olefins are efficiently reduced to the corresponding alkanes by the boranemethyl sulfide (BMS) complex at room temperature (or below) in dichloromethane containing a mild one-electron oxidant (such as an aromatic cation radical) or by the passage of an anodic current. In an alternative procedure, the hydrogenation of the same (electron-rich) olefins with the BMS complex (in the absence of a one-electron oxidant) is also carried out in the presence of a strong Brønsted acid (such as HBF4). In the oxidative activation, olefin cation radicals are the first observable intermediates, and separate experiments (including transient electrochemistry) confirm the facile reduction of the olefin cation radical by borane to produce the corresponding alkane. The direct protonation of olefins produces carbocationic intermediates which are also efficiently reduced by borane to the corresponding alkanes. The intermediacy of both olefin cation radicals and carbocations in the hydrogenation procedure with borane is discussed.