Cation Bridge Mediating Homo- and Cross-Coupling in Copper-Catalyzed Reductive Coupling of Benzaldehyde and Benzophenone.

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.