铜催化CO2和1-苯基丙炔的氢羧基化反应机理及区域选择性

采用密度泛函理论(DFT)方法研究了在还原剂(EtO)₃SiH存在下, 铜(I) (Cl₂IPrCuF)催化CO₂插入1-苯基丙炔生成α,β不饱和羧酸的反应机理. 计算结果表明, Cl₂IPrCuF 首先与(EtO)₃SiH 生成活性催化剂Cl₂IPrCuH,然后经历三个步骤完成催化反应: (1) Cl₂IPrCuH 与1-苯基丙炔加成生成烯基铜中间体. 由于炔烃的不对称性,烯基铜中间体有两种同分异构体, 最后可导致生成两种对应的α,β不饱和羧酸衍生物; (2) CO₂插入烯基铜中间体得到羧基铜中间体; (3) (EtO)₃SiH 与羧基铜中间体发生σ转位反应形成最终产物, 同时重新生成催化剂Cl₂IPrCuH. 理论研究还表明, 生成两种α,β不饱和羧酸衍生物的反应路径所对应的决速步骤不同, 在Path a 中炔烃插入反应和CO₂插入反应都可能是整个催化反应的决速步骤, 自由能垒分别为68.6 和67.8 kJ·mol^-1, 而在Path b中, 仅炔烃插入反应是整个催化反应的决速步骤, 自由能垒为78.7 kJ·mol^-1. 此结果很好地给出了实验上两种α,β不饱和羧酸衍生物收率不同的原因. 炔烃与Cl₂IPrCuH的加成决定了反应的区域选择性, 其中电子效应是影响反应区域选择性的主要原因.

Reaction Mechanism and Regioselectivity of Cu(I)-Catalyzed Hydrocarboxylation of 1-Phenyl-propyne with Carbon Dioxide

Density functional theory (DFT) calculations have been used to conduct a detailed study of the mechanism involved the copper(I)-catalyzed hydrocarboxylation of 1-phenyl-propyne using CO₂ and hydrosilane. Theoretical calculations suggested that the activated catalyst Cl₂IPrCuH is initially generated in situ by the reaction of Cl₂IPrCuF with (EtO)₃SiH, and that the entire catalytic reaction involves three steps, including (1) the addition of Cl₂IPrCuH to 1-phenyl-propyne to afford two isomeric copper alkenyl intermediates, which lead to the formation of the corresponding final α,β-unsaturated carboxylic acid derivatives; (2) CO₂ insertion to give the corresponding copper carboxylate intermediate; and (3) σ-bond metathesis of the copper carboxylate intermediate with a hydrosilane to provide the corresponding silyl ester with the regeneration of the active catalyst. The results of our calculations show that the rate-limiting steps for the two paths leading to the two α,β-unsaturated carboxylic acid derivatives are different. In Path a, the alkyne and CO₂ insertion steps were both identified as possible rate-limiting steps, with free energy barriers of 68.6 and 67.8 kJ·mol^-1, respectively. However, in Path b, the alkyne insertion step was identified as the only possible rate-limiting step with an energy barrier of 78.7 kJ·mol^-1. These results were in agreement with the experimental observations. It was also found that the alkyne insertion step controlled the regioselectivity of the products, and that electronic effects were responsible for the experimentally observed regioselectivity.