Methods for Organic Electrosynthesis - Fundamentals and Applications of Organic Electrochemistry - page 77

4.5.1 Electroauxiliaries Based on
Molecular Orbital Interactions
The orbital interaction is highly effective in
increasing the HOMO level. In principle, the
interaction of the HOMO with a high-energy
filled orbital increases its energy level. For
instance, the energy level of a C-Si σ orbital is
generally much higher than that of C-H and
C-C σ orbitals therefore the C-Si σ orbital
interacts effectively with a non-bonding p
orbital of a heteroatom like N, O or S (σ-n
interaction) if two orbitals align in the same
plane ( Figure 4.11 ) [18,19]. Accordingly, a silyl
group at the α position activates a heteroatom
compound towards anodic oxidation.
One-electron oxidation gives the radical cation,
in which the C-Si σ orbital interacts with a
half-vacant p orbital of the heteroatom to
stabilize the system. Such interaction also
weakens the C-Si bond, and hence the C-Si
bond is cleaved selectively. The resulting carbon
radical undergoes further oxidation to give the
carbocation, which is trapped by a nucleophile
to give the desired product ( Eq. 4.11 ). Thus, the
silyl group not only activates substrates towards
oxidation but also controls the reaction
pathway [18,19].