Wang Yuwen, Kostenko Arseni, Hadlington Terrance J, Luecke Marcel-Philip, Yao Shenglai, Driess Matthias
Metalorganics and Inorganic Materials, Department of Chemistry , Technische Universität Berlin , Straße des 17. Juni 135, Sekr. C2 , 10623 Berlin , Germany.
J Am Chem Soc. 2019 Jan 9;141(1):626-634. doi: 10.1021/jacs.8b11899. Epub 2018 Dec 19.
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:)Xant 1a [Xant = 9,9-dimethyl-xanthene-4,5-diyl; PhC(N Bu)2] and (LSi:)Fc 1b (Fc = 1,1'-ferrocenyl) as four-electron reduction reagents under mild reaction conditions (RT, 1 atm), affording the corresponding disilylketenes, Xant(LSi)(μ-O)(μ-CCO) 2a and Fc(LSi)(μ-O)(μ-CCO) 2b, respectively. However, the dibenzofuran analogue of 1b, compound 1c, was unreactive toward CO due to the longer distance between the two Si atoms, which demonstrated the crucial role of the Si···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 1a and 1b 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 1a with isocyanide Xyl-NC (Xyl = 2,6-MeCH), isoelectronic with CO. Treatment of 1a 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)(μ-XylNC) intermediate 4. Moreover, compound 3a reacts with 1 molar equivalent of CO to give the disilylketenimine Xant(LSi)(μ-O)(μ-CCNR) 6, representing, for the first time, a selective heterocoupling product of CO with isoelectronic isocyanide.
虽然通过切断极强的CO键将一氧化碳转化为多碳化合物（燃料和有机大宗化学品）主要由过渡金属主导，但在地壳中含量第二丰富的非金属硅在非基质条件下对CO进行分裂和脱氧还原偶联的情况极为罕见，且其反应机理尚不清楚。在此，我们报道了在温和反应条件（室温，1个大气压）下，利用（LSi:）Xant 1a [Xant = 9,9 - 二甲基 - 呫吨 - 4,5 - 二基；PhC(N Bu)2] 和（LSi:）Fc 1b（Fc = 1,1'-二茂铁基）作为四电子还原试剂，二价硅对一氧化碳进行选择性脱氧均偶联反应，分别得到相应的二硅基烯酮，即Xant(LSi)(μ - O)(μ - CCO) 2a和Fc(LSi)(μ - O)(μ - CCO) 2b。然而，1b的二苯并呋喃类似物化合物1c对CO无反应，这是由于两个硅原子之间的距离较长，这表明Si···Si距离对协同CO结合和活化起着关键作用。密度泛函理论（DFT）计算证实了这一点，对1a和1b与CO均偶联反应的进一步理论研究表明，CO结合和断裂的初始步骤涉及CO作为路易斯酸（四电子受体），这与过渡金属介导的CO活化形成鲜明对比，在过渡金属介导的过程中CO作为路易斯碱（双电子供体）。1a与与CO等电子的异腈Xyl - NC（Xyl = 2,6 - MeCH）的反应有力地支持了这一机理。用一分子或两分子Xyl - NC处理1a分别通过可分离的双桥联Xant(LSi)(μ - XylNC)中间体4得到独特的（硅基）（亚氨基）硅烯3a和C═C偶联的双（Xyl - NC）产物5。此外，化合物3a与1摩尔当量的CO反应生成二硅基烯酮亚胺Xant(LSi)(μ - O)(μ - CCNR) 6，这首次代表了CO与等电子异腈的选择性杂偶联产物。
