Mechanisms of Co2L8 (L = CO, CNR)-Catalyzed Hydrosilylation of Alkenes: A Theoretical Study

For the hydrosilylation of alkenes catalyzed by Co2L8 (L = CO, CNR), the Type II Chalk-Harrod cycle (the CH-II cycle) starting from HCoL4 ([2H](L)) and the Type II modified Chalk-Harrod cycle (the mCH-II cycle) from Me3SiCoL4 ([2Si](L)) have been proposed as the reaction mechanisms, but neither mechanism is fully consistent with the experimental results. Density functional theory (DFT) calculations verified the reaction pathways of the two catalytic cycles, leading to the conclusion that the behavior of CNR as a ligand is similar to that of CO in catalysis, and a comparison of the activation energies of the rate-determining step of the two cycles suggests that the mCH-II cycle is more favorable. The mCH-II mechanism contradicts the following three experimental results: [2Si](L) is not catalytically active without photolysis; vinylsilane is not formed; and facile alkene isomerization occurs. These discrepancies were resolved by assuming that both HCoL3 ([3H](L)) and Me3SiCoL3 ([3Si](L)) generated from [2H](L) were present in the system and acted as catalysts; [3H](L) was responsible for the alkene isomerization and thermal generation of [3Si](L), whereas [3Si](L) initiated the hydrosilylation of alkenes through the mCH-II cycle. Vinylsilanes could be formed from an intermediate in the mCH-II cycle; however, this pathway is thermodynamically unfavorable.

Automated summary

Density functional theory calculations verified the reaction pathways of the two catalytic cycles and determined that the modified Chalk-Harrod cycle is more favorable. However, this mechanism contradicts some experimental results, which were resolved by assuming the presence of additional catalysts.