Silyl cation mediated conversion of CO2 into benzoic acid, formic acid, and methanol.
Silyl cation mediated conversion of CO2 into benzoic acid, formic acid, and methanol.
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DOI：
10.1002/anie.201107958
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发表时间：
2012-03
影响因子：
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通讯作者：
A. Schäfer;W. Saak;D. Haase;T. Müller
中科院分区：
文献类型：
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作者：
A. Schäfer;W. Saak;D. Haase;T. Müller
使用CO2作为可再生和环境友好的C1源来合成羧酸和燃料如甲醇和甲烷是当前感兴趣的主题。[1-4]CO2的高热力学稳定性显然要求高效活化，这必须与强大的热力学驱动力相结合，以确保不可逆的固定。虽然在过去过渡金属化学在二氧化碳转化化学中发挥了主导作用，[5]近年来已经出现了用于二氧化碳还原的有机催化方法。[6-14]例如，N-杂环卡宾已被用于CO2的亲核活化，随后通过硅烷还原所得的咪唑鎓羧酸盐产生甲醇。[9]此外，已经报道了CO2的化学计量还原和催化还原，其利用受抑的刘易斯对（FLP）[10]进行活化，并使用二氢、硅烷或氨硼烷作为氢源。[11-14]鉴于甲硅烷基阳离子[15]及其与溶剂和抗衡阴离子的络合物的高亲电活性，我们对在CO2活化中利用这种极端反应性的可能性感兴趣。硅烷然后将是逻辑的氢源的还原，并将提供所需的热力学驱动力，通过形成的siloxies.Here，我们报告的三烷基硅烷，R3 SiH（R= Et，iPr），使用化学计量的硼酸三苯甲基酯[Ph 3C][B（C6 F5）4]的无金属还原CO2。在环境条件下，还原速度很快，但会形成不同的产物（取决于所用的溶剂）。在氯苯（PhCl）中，形成二甲硅烷基化的甲酸1或二甲硅烷基甲基氧鎓离子2，这取决于硅烷上的取代基R。这些化合物的简单水解产生甲酸和甲醇（方案1）。在苯（PhH）中，CO2与预先形成的甲硅烷基苯鎓盐[Et 3Si（C6 H6）][B（C6 F5）4]（3 [B（C6 F5）4]）的反应导致CO2的进一步官能化。[8b]在这种情况下，形成苄基阳离子4，其可以通过水解容易地转化为苯甲酸（PhCO 2 H，6），或者通过小心地去质子化容易地转化为甲硅烷基酯5（方案1）。
The use of CO2 as a renewable and environmentally friendly C1 source for the synthesis of carboxylic acids and fuels such as methanol and methane is a topic of current interest.[1–4] The high thermodynamic stability of CO2 clearly calls for highly efficient activation which must be combined with a strong thermodynamic driving force to ensure irreversible fixation. While in the past transition-metal chemistry played a dominant role in CO2 conversion chemistry,[5] recent years have seen the emergence of organocatalytic methods for CO2 reduction.[6–14] For example N-heterocyclic carbenes have been applied for the nucleophilic activation of CO2, and subsequent reduction of the resulting imidazolium carboxylates by silanes yielded methanol.[9] In addition stoichiometric and catalytic reductions of CO2 have been reported which utilize frustrated Lewis pairs (FLPs)[10] for the activation, and dihydrogen, silanes, or ammonia borane as the hydrogen source.[11–14] In view of the high electrophilic activity of silyl cations [15] and their complexes with solvents and counteranions, we were intrigued by the possibility of exploiting this extreme reactivity in CO2 activation. Silanes then would be the logical hydrogen source for the reduction and would provide the desired thermodynamic driving force through the formation of siloxanes.Here we report on a metal-free reduction of CO2 by trialkylsilanes, R3SiH (R= Et, iPr), using stoichiometric amounts of trityl borate [Ph3C][B (C6F5) 4]. The reduction is fast under ambient conditions, but different products—depending on the applied solvent—are formed. In chlorobenzene (PhCl) either disilylated formic acid 1 or the disilylmethyl oxonium ion 2 is formed, depending on the substituent R at the silane. Simple hydrolysis of these compounds yields formic acid and methanol (Scheme 1). In benzene (PhH), the reaction of CO2 with the preformed silylbenzenium salt [Et3Si (C6H6)][B (C6F5) 4](3 [B (C6F5) 4]) leads to further functionalization of CO2.[8b] In this case the benzylic cation 4 is formed, which can be easily transformed either by hydrolysis into benzoic acid (PhCO2H, 6), or, by careful deprotonation, into the silylester 5 (Scheme 1).