Metal-Free Reduction of CO2 with Hydroboranes: Two Efficient Pathways at Play for the Reduction of CO2 to Methanol
Metal-Free Reduction of CO2 with Hydroboranes: Two Efficient Pathways at Play for the Reduction of CO2 to Methanol
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DOI：
10.1002/chem.201400349
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发表时间：
2014-06-02
影响因子：
4.3
通讯作者：
Cantat, Thibault
中科院分区：
文献类型：
--
作者：
Gomes, Christophe Das Neves;Blondiaux, Enguerrand;Cantat, Thibault
胍和脒被证明是高效的无金属催化剂，可使用 9-硼双环[3.3.1]壬烷 (9-BBN) 和儿茶酚硼烷 (catBH) 等氢硼烷将 CO2 还原为甲醇。氮碱，如 1,5,7-三氮杂双环[4.4.0]dec-5-ene (TBD)、7-甲基-1,5,7-三氮杂双环[4.4.0]dec-5-ene (Me-TBD) 和 1,8-二氮杂双环十一碳-7-烯 (DBU) 是这种转化的活性催化剂，Me-TBD 可以催化 CO2 还原为室温下甲氧基硼烷的 TON 和 TOF 分别高达 648 和 33h-1 (25 ℃)。甲酸盐 HCOOBR2 和缩醛 H2C(OBR2)2 衍生物已被确定为 R2BH 还原 CO2 的反应中间体，并且第一个 CH 键的形成是速率决定因素。实验和计算研究表明 TBD 和 Me-TBD 遵循不同的机制。 TBD 的 NH 键可与 9-BBN 发生脱氢偶联反应，并提供新型受阻路易斯对 (FLP)，该对可以激活 CO2 分子并形成稳定的加合物 2，这是催化活性物质，可以促进氢化物从硼原子转移到碳原子。相比之下，Me-TBD 通过氢硼烷试剂的活化促进 CO2 的还原。详细的 DFT 计算表明，计算出的两种机制的能垒与实验结果一致，并解释了不同硼还原剂的反应性。
Guanidines and amidines prove to be highly efficient metal-free catalysts for the reduction of CO2 to methanol with hydroboranes such as 9-borabicyclo[3.3.1]nonane (9-BBN) and catecholborane (catBH). Nitrogen bases, such as 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (Me-TBD), and 1,8-diazabicycloundec-7-ene (DBU), are active catalysts for this transformation and Me-TBD can catalyze the reduction of CO2 to methoxyborane at room temperature with TONs and TOFs of up to 648 and 33h-1 (25 degrees C), respectively. Formate HCOOBR2 and acetal H2C(OBR2)2 derivatives have been identified as reaction intermediates in the reduction of CO2 with R2BH, and the first CH-bond formation is rate determining. Experimental and computational investigations show that TBD and Me-TBD follow distinct mechanisms. The NH bond of TBD is reactive toward dehydrocoupling with 9-BBN and affords a novel frustrated Lewis pair (FLP) that can activate a CO2 molecule and form the stable adduct 2, which is the catalytically active species and can facilitate the hydride transfer from the boron to the carbon atoms. In contrast, Me-TBD promotes the reduction of CO2 through the activation of the hydroborane reagent. Detailed DFT calculations have shown that the computed energy barriers for the two mechanisms are consistent with the experimental findings and account for the reactivity of the different boron reductants.