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

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.