Lewis superacids for catalytic reductions of stronger element–oxygen double bonds with hydrosilanes

Abstract

The main-group Lewis superacid complexes (pin^F)₂Si·MeCN (1·MeCN) and (pin^F)₂Ge·MeCN (2·MeCN) were successfully applied as promoters in the catalytic reduction of phosphine oxides (e.g., Me₃PO, Bu₃PO, and Ph₃PO), a sulfoxide (i.e., Me₂SO), and an amide (i.e., Me₂NCHO) to furnish the respective phosphines, dimethyl sulfide, and trimethylamine using silanes (e.g., PhSiH₃ and (EtO)₃SiH) as hydrogen sources (pin^F = perfluoropinacolato). These substrates target difficult to reduce representatives of oxo compounds in comparison with, for example, the ketones or aldehydes often targeted in such types of catalytic reductions. As benchmark promoters, we also studied B(C₆F₅)₃ and HNTf₂ as reference (soft) Lewis superacid and Brønsted superacid, respectively (Tf = SO₂CF₃). Among the combinations of (pre)catalyst, substrate, and reducing agent investigated, the silicon complex 1·MeCN turned out to be the most versatile system, being the by far most potent (DMSO) or just slightly underperforming (R₃PO and DMF) promoter. Moreover, the hitherto undescribed Lewis acid base adducts 1·Me₂NCHO and 2·Me₂SO were synthesized, isolated, and structurally investigated using NMR spectroscopy and single-crystal XRD analysis.