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

The main-group Lewis superacid complexes (pinF)2Si·MeCN (1·MeCN) and (pinF)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 (pinF = 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.