Sterically hindered olefins are efficiently reduced to the
corresponding alkanes by the borane−methyl 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.
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