Origins of Chemoselectivity in the Ni-Catalyzed Biaryl and Pd-Catalyzed Acyl Suzuki-Miyaura Cross-Coupling of N-Acetyl-Amides.

Abstract

The mechanisms and origins of selectivity in the Pd-catalyzed nondecarbonylative and Ni-catalyzed decarbonylative Suzuki-Miyaura cross-coupling of N-acetyl-amides have been explored with density functional theory calculations. The reaction of the two catalysts shares a similar process that contains oxidative addition to break the N-C(O) bond and transmetalation with the Ar'B(OH)2 reagent. Then, the reaction bifurcates at the generated PCy3M(acyl)Ar' (M = Ni/Pd) intermediate. Our results show that the electronegativity of the central metal plays a decisive role in guiding the selectivity of subsequent reactions (acyl reductive coupling versus decarbonylation). Palladium with a higher electronegativity tends to accept electrons for directly C-C reductive elimination, but it is not conducive to the d → π* back donation from the metal to CO to stabilize the decarbonylative process, which restricts the decarbonylation. In contrast, for the nickel-catalyzed system, it prefers to undergo decarbonylation benefiting by the stronger stabilizing effect of d → π* back donation from nickel to CO rather than conduct reductive elimination due to its lower electronegativity. Consequently, the Pd-catalyzed Suzuki-Miyaura cross-coupling of N-acetyl-amides gives biaryl ketones (ArCOAr') but the Ni-catalyzed same reaction generates decarbonylated biaryls (ArAr'). The mechanistic understanding gives useful insight into the further advancement of selective cross-coupling reactions enabled by transition metals.