A Hydride-Shuttle Mechanism for the Catalytic Hydroboration of CO2.

Herein we report our investigation into the mechanism of CO2 reduction by HBpin catalyzed by [Ru(CO)H(L)(PPh3)2] (2; L is the 4,5-diazafluorenyl ligand with a Bpin functional group at the 9-position) through computational studies using the model complex [Ru(CO)H(L)(PMe3)2] (A1). The reaction consists of four stages: (1) CO2 insertion into the C-B bond of A1 to form A4, (2) the reduction of A4 by HBpin to afford HCOOBpin (P2) and regenerate A1, (3) the reduction of P2 by HBpin to HCHO (P5), and (4) the reduction of P5 to CH3OBpin (P6). We found that Lewis adduct formation plays a key role in all stages of the mechanism, in that it forms more relaxed rings in the key transition states and makes the hydride more hydridic. Oftentimes, the hydride and Bpin moieties can transfer within the Lewis adducts in a concerted manner in our proposed hydride-shuttle mechanism. The energy spans for all stages of our proposed mechanism are within the range of 15.7-22.6 kcal/mol in terms of Gibbs free energy. In contrast, the direct hydroboration and σ-bond metathesis mechanisms proposed in the literature have extremely high energy barriers because of the highly strained four-membered rings in the transition states and the unactivated hydride in HBpin.