Hydroboration of carbon dioxide with pinacolborane catalyzed by various aluminum hydrides: a comparative mechanistic study

Abstract

In this work, density functional theory (DFT) calculations were performed to probe the catalytic viability of various neutral, cationic and anionic aluminum hydrides (AlH) in the hydroboration of CO₂ with pinacolborane (HBpin). The calculations show that all AlH active species employed are capable of reducing CO₂ to the formate level. However, transmetalation of the resulting Al formate intermediates with HBpin is merely suitable for the neutral and cationic ones, whose Al centre has a tetrahedral configuration with a lower coordination number. Thanks to the Al formate intermediates with these features in charge and structure, the hydride transfer from the four-coordinate boron intermediate to the Al centre in the transmetalation process can proceed smoothly via a cyclic transition state, which not only favors electron transfer but also allows the AlH moiety to suffer from less geometric deformation. Moreover, on the basis of mechanistic studies and turnover frequency (TOF) analysis on the possible catalytic cycles involved in the AlH-catalyzed hydroboration of CO₂, two three-coordinated cationic AlH species are screened as promising catalyst candidates, which exhibit high efficiency and chemoselectivity in the reduction of CO₂ with HBpin to the six-electron reduction product methoxyborane.