Mechanism, Reactivity, and Selectivity in Palladium-Catalyzed Organosilicon-Based Cross Coupling Reaction

A comprehensive density functional theory (DFT) study has been performed to elucidate the mechanistic details of the Pd-catalyzed strain-release organosilicon based cross coupling reaction of four- and five-membered silacycles. The reaction is proposed to proceed through Ar-Br oxidative addition (OA), Br-to-tBuO ligand exchange (LE), transmetalation, and reductive elimination (RE) steps, and among which the transmetalation is the rate-determining step. Mechanistically, after ligand exchange, the reaction would lead to two distinct pathways: C-O RE to form the C-O coupling product or transmetalation to provide the C-C coupling product, in which the former is often found to be favored in many other transformations. The experimentally observed uncommon C-C coupling selectivity over the C-O RE in the title reaction is elucidated by an unusual “thermodynamic control scenario”, that is, although the C-O RE has a slightly lower barrier than the transmetalation, however, the product of the transmetalation is much more stable compared to the C-O RE. The computational results from different density functional methods as well as the experimental details verified our findings. The steric effect in the rate-determining step accounts for the contrasting reactivity with different nucleophiles in the reactions with silacyclopentane. The amine base was found to facilitate the transmetalation process by forming a penta-coordinate silacycle. We predict that the -NMe2 containing base would further promote the reaction by decreasing the activation barrier.