A novel
mechanism of organobase-mediated Brook rearrangement and
C–C coupling in the copper-catalyzed reductive coupling of
benzaldehyde and benzophenone is proposed. The results demonstrate
that this reaction proceeds mainly through five sequential elementary
steps: transmetalation, carbonyl addition, σ-bond metathesis,
Brook rearrangement, and C–C coupling. The organobases played
a significant role not only in forming the active catalyst but also
in mediating the Brook rearrangement and chemoselectivity in homo-
and cross-coupling. Brook rearrangement mediated by organobases is
more favored than that without organobases. In the C–C coupling
step, the cation bridge combines two O atoms with the same electronegativity
to form a pre-reaction complex. Moreover, a significant charge difference
is a major factor in the selectivity of carbonyl addition and C–C
coupling.