The Effect of HSAB on Stereoselectivity: Copper- and Gold-Catalyzed 1,3-Phosphatyloxy and 1,3-Halogen Migration Relay to 1,3-Dienes

The origin of stereodivergence between copper- and gold-catalyzed cascade 1,3-phosphatyloxy and 1,3-halogen migration from α-halo-propargylic phosphates to 1,3-dienes is rationalized with density functional theory (DFT) studies. Our studies reveal the significant role of the relative hardness/softness of the metal centers in determining the reaction mechanism and the stereoselectivity. The relative harder Cu­(I/III) center prefers an associative pathway with the aid of a phosphate group, leading to the (<i>Z</i>)-1,3-dienes. In contrast, the relative softer Au­(I/III) center tends to undergo a dissociative pathway without coordination to a phosphate group, resulting in the (<i>E</i>)-1,3-dienes, where the <i>E</i> type of transition state is favored due to the steric effect. Our findings indicate the intriguing role of hard–soft/acid–base (HSAB) theory in tuning the stereoselectivity of metal-catalyzed transformations with functionalized substrates

Hydrogenation of Carbon Dioxide Using Half-Sandwich Cobalt, Rhodium, and Iridium Complexes: DFT Study on the Mechanism and Metal Effect

The hydrogenation of carbon dioxide catalyzed by half-sandwich transition metal complexes (M = Co, Rh, and Ir) was studied systematically through density functional theory calculations. All metal complexes are found to process a similar mechanism, which involves two main steps, the heterolytic cleavage of H₂ and the hydride transfer. The heterolytic cleavage of H₂ is the rate-determining step. The comparison of three catalytic systems suggests that the Ir catalyst has the lowest activation free energy (13.4 kcal/mol). In contrast, Rh (14.2 kcal/mol) and Co (18.3 kcal/mol) catalysts have to overcome relatively higher free energy barriers. The different catalytic efficiency of Co, Rh, and Ir is attributed to the back-donation ability of different metal centers, which significantly affects the H₂ heterolytic cleavage. The highest activity of an iridium catalyst is attributed to its strong back-donation ability, which is described quantitatively by the second order perturbation theory analysis. Our study indicates that the functional group of the catalyst plays versatile roles on the catalytic cycle to facilitate the reaction. It acts as a base (deprotonated) to assist the heterolytic cleavage of H₂. On the other hand, during the hydride transfer, it can also serve as Brønsted acid (protonated) to lower the LUMO of CO₂. This ligand assisted pathway is more favorable than the direct attack of hydride to CO₂. These finds highlight that the unique features of the metal center and the functional ligands are crucial for the catalyst design in the hydrogenation of carbon dioxide