Silicon-Mediated Selective Homo- and Heterocoupling of Carbon Monoxide

While the transformation of carbon monoxide to multicarbon compounds (fuels and organic bulk chemicals) via reductive scission of the enormously strong CO bond is dominated by transition metals, splitting and deoxygenative reductive coupling of CO under nonmatrix conditions using silicon, the second most abundant nonmetal of the earth's crust, is extremely scarce and mechanistically not well understood. Herein, we report the selective deoxygenative homocoupling of carbon monoxide by divalent silicon utilizing the (LSi:)(2)Xant 1a [Xant = 9,9-dimethyl-xanthene-4,5-diyl; PhC((NBu)-Bu-t)(2)] and (LSi:)(2)Fc 1b (Fc = 1,1'-ferrocenyl) as four-electron reduction reagents under mild reaction conditions (RT, 1 atm), affording the corresponding disilylketenes, Xant(LSi)(2)(mu-O)(mu-CCO) 2a and Fc(LSi)(2)(mu-O)(mu-CCO) 2b, respectively. However, the dibenzofuran analogue of lb, compound lc, was unreactive toward CO due to the longer distance between the two Sin atoms, which demonstrated the crucial role of the Si center dot center dot center dot Si distance on cooperative CO binding and activation. This is confirmed by density functional theory (DFT) calculations, and further theoretical investigations on CO homocoupling with la and lb revealed that the initial step of CO binding and scission involved CO acting as a Lewis acid (four-electron acceptor), in sharp contrast to CO activation mediated by transition metals where CO serves as a Lewis base (two-electron donor). This mechanism was strongly reinforced by the reaction of la with isocyanide Xyl-NC (Xyl = 2,6-Me2C6H3), isoelectronic with CO. Treatment of la with one or two molecules of Xyl-NC furnished the unique (silyl)(imido)silene 3a and the C=C coupled bis(Xyl-NC) product 5, respectively, via the isolable doubly bridged Xant(LSi)(2)(mu-XylNC)(2) intermediate 4. Moreover, compound 3a reacts with 1 molar equivalent of CO to give the disilylketenimine Xant(LSi)(2)(mu-O)(mu-CCNR) 6, representing, for the first time, a selective heterocoupling product of CO with isoelectronic isocyanide.