Controlling Selectivity in the Hydroboration of Carbon Dioxide to the Formic Acid, Formaldehyde, and Methanol Oxidation Levels

Abstract

The factors that influence the selectivity of pincer supported group 10 transition metal hydride catalysts for CO2 hydroboration are investigated. We demonstrate that selective CO2 reduction to either the two-electron boryl formate reduction product, the four-electron bis(boryl)acetal reduction product, or the six-electron methoxy borane reduction product can be achieved by varying either the identity and concentration of the organoborane reductant, the nature and loading of the catalyst, or the presence of a Lewis acid cocatalyst. In fact, using one specific catalyst, (tBuPCP)NiH (tBuPCP = 2,6-C6H3(CH2PtBu2)2), we can selectively form either the two-, four-, or six-electron CO2 reduction products by changing either the nature of the reductant or the reaction conditions. Additionally, we show that Lewis acid cocatalysts can be used to alter the selectivity of CO2 hydroboration, which is a new method to control the selectivity of this type of hydroboration reaction. All of our results on selectivity are consistent with CO2 hydroboration being a tandem reaction, in which it is possible to either trap the kinetic two- or four-electron reduction products or form the thermodynamic six-electron reduction product. We also explore the formation of off-cycle κ2-borohydride species through the reaction of the transition metal hydride with the borane reductant and show that this can impact selectivity. Overall, our work provides detailed guidelines for designing even more active and selective catalysts for CO2 hydroboration and may also be relevant for the improvement of catalysts for related reactions such as CO2 hydrosilylation.