Department of Chemistry, School of Science, Tianjin Key Laboratory of Molecular Optoelectronic Sciences , Tianjin University , Tianjin 300072 , P. R. China.
J Org Chem. 2019 May 3;84(9):5514-5523. doi: 10.1021/acs.joc.9b00471. Epub 2019 Apr 1.
Density functional theory calculations were performed to investigate the copper-catalyzed borocyanation of 2-aryl-substituted 1,3-dienes. The computations show that the regioselectivity of the overall reaction is governed by the combination of the inherent regioselectivity of the borocupration and electrophilic cyanation steps. The π-conjugation effect of the 1,3-diene makes the terminal carbon atoms more electrophilic compared with the internal carbon atoms, which coupled with the steric effect results in the 4,3- and 1,2-borocupration being intrinsically more favorable than the other possibilities. The steric repulsion around the breaking Cu-C bond was found to be the key factor in determining the regioselectivity of the electrophilic cyanation. The origins of the experimentally observed ligand-controlled regioselectivity were ascribed to the electronic and steric effects. For the bulky XantPhos ligand, the 4,3-borocupration was found to be more favorable than the 1,2-borocupration due to the steric repulsion around the forming Cu-C bond, resulting in the formation of the 4,3-borocyantion product. On the other hand, the reversed regioselectivity with a small PCy ligand is mainly caused by the electronic effect that the π-electron-withdrawing aryl group at the C atom makes the C atom more electrophilic than the C atom, enabling the 1,2-borocupration to be more favorable than the 4,3-borocupration.
采用密度泛函理论计算研究了铜催化的 2-芳基取代 1,3-二烯的硼氰化反应。计算表明，总反应的区域选择性由硼铜化和亲电氰化步骤的固有区域选择性组合决定。1,3-二烯的π共轭效应使末端碳原子比内部碳原子更具亲电性，再加上空间位阻效应，导致 4,3-硼铜化和 1,2-硼铜化本质上比其他可能性更有利。发现断裂 Cu-C 键周围的空间排斥是决定亲电氰化区域选择性的关键因素。实验观察到配体控制区域选择性的起源归因于电子和空间效应。对于大体积的 XantPhos 配体，由于形成 Cu-C 键周围的空间排斥，4,3-硼铜化比 1,2-硼铜化更有利，导致形成 4,3-硼氰酸根产物。另一方面，带有小 PCy 配体的相反区域选择性主要是由于电子效应引起的，即 C 原子上的吸电子芳基使 C 原子比 C 原子更具亲电性，从而使 1,2-硼铜化比 4,3-硼铜化更有利。
