DMSO-catalyzed CO2 reduction with 9-BBN: selective formation of either formoxy- or methoxyborane under mild conditions and C-methylenation of indoles†

Abstract

The reduction of CO₂ (1 atm) with 9-BBN in DMSO selectively produces either formoxy- or methoxyborane in excellent yields, depending on the amount of DMSO in the reaction at room temperature. CO₂ reacts with 9-BBN in DMSO as a solvent to selectively produce HCOOBBN in 98% NMR yield. On the contrary, the reduction of CO₂ with 9-BBN in the presence of 4.5 or 7 mol% of DMSO in toluene or THF produces CH₃OBBN as the sole product. In both cases, DMSO first reacts with 9-BBN to form a reactive adduct in situ, through which these reduced products are formed. The adduct formation is supported by the structural characterization of the DMSO adduct of formoxyborane, representing the first X-ray structure of its type, which is analysed using DFT calculations. A mechanistic study suggested that CH₃OBBN is formed from HCOOBBN via its acetal precursor. Similar reduction reactions of CO₂ with 9-BBN in the presence of various sulfoxides, sulfone, sulfide and pyridine showed that not all adducts are able to reduce CO₂, and only sulfoxide adducts reduce CO₂ to CH₃OBBN up to an NMR yield of 60%. Furthermore, the adduct formation is controlled by the steric factor of sulfoxides. As a proof of concept, the methylene group of the acetal product was transferred to N-methyl- and N-unsubstituted indoles to produce bis(indolyl)methane derivatives in good isolated yields with detailed mechanistic studies.