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

Abstract

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.

An umpolung mechanism of B(pin)-mediated Cu/B rearrangement and origin of regioselectivity for NHC-Cu-catalyzed allylation of imines

“…In our previous study on the copper-catalyzed reductive coupling of benzaldehyde and benzophenone, we revealed the mechanism of the Brook rearrangement, which is mediated by organobases (Li/Na/KO t Bu). 8 We also examined in detail the mechanisms of both the aza-Brook rearrangement 9 and the bora-Brook rearrangement. 10 The common mechanism characteristic of these Brook rearrangements is the deprotonation of nitrogen (or removal of a metal ion from oxygen) to form the corresponding anion (Scheme 1b).…”