Implications of CO2 Activation by Frustrated Lewis Pairs in the Catalytic Hydroboration of CO2: A View Using N/Si+ Frustrated Lewis Pairs

A series of base-stabilized silylium species were synthesized and their reactivity toward CO, explored, yielding the characterization of a novel N/Si+ FLP-CO2 adduct. These silicon species are active catalysts in the hydroboration of CO2 to the methoxide level with 9-BBN, catecholborane (catBH), and pinacolborane (pinBH). Both experiments and DFT calculations highlight the role of the FLP-CO2 adduct in the catalysis. Depending on the nature of the hydroborane reductant, two distinct mechanisms have been unveiled. While 9-BBN and catBH are able to reduce an intermediate FLP-CO2 adduct, the hydroboration of CO2 with pinBH follows a different and novel path where the B-H bond is activated by the silicon-based Lewis acid catalyst. In these mechanisms, the formation of a highly stabilized FLP-CO2 adduct is found detrimental to the kinetics of the reaction.