The Real Role of N-Heterocyclic Carbene in Reductive Functionalization of CO2: An Alternative Understanding from Density Functional Theory Study

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• 作者：Qinghai Zhou ; Yuxue Li
• 刊名：Journal of the American Chemical Society
• 出版年：2015
• 出版时间：August 19, 2015
• 年：2015
• 卷：137
• 期：32
• 页码：10182-10189
• 全文大小：704K
• ISSN：1520-5126

文摘

The mechanisms of reductive functionalization of CO₂ to formamide catalyzed by N-heterocyclic carbene (NHC) were comprehensively studied with DFT calculations. New activation mode with much lower energy barrier than those proposed before was discovered. In this reaction, NHC acts as neither a CO₂ nor a silane activator, but as a precursor of the real catalyst, i.e., the in situ formed ionic liquid [NHCH]⁺[Carbamate]^{鈭?/sup>. In this loose contact ion pair, the negatively charged O atom of the carbamate anion becomes the new active site and is free to do nucleophilic attack. When amine is absent, CO₂ will be converted into methanol. In this case, the NHC-CO₂ adduct is the real catalytic species, the active site shifted from the carbene C atom to the negatively charged O atom. These new activation modes follow a pattern of 鈥淪_N2@Si-Acceptor鈥? in which the Si鈥揌 bond is activated via concerted backside S_N2 nucleophilic attack by the negatively charged O atom, and the leaving hydride is directly accepted by a free CO₂ molecule. The advantages of these new activation modes originate from the following points: (1) The ionic liquid [NHCH]+[Carbamate]鈭?/sup> and NHC-CO₂ adduct are thermodynamically more stable than NHC. (2) The active site of the NHC catalyst is extended outside a lot. Consequently, the large steric effect between the NHC arms and the substrates in transition state can be avoided to some extent. (3) The O atom has good silicon affinity. In addition, a free CO₂ molecule, whose carbon atom is more electrophilic than those of the CO₂ moieties in NHC-CO₂ adduct and carbamate, acts as an efficient hydride acceptor.}