铂催化2-烯炔苯甲醛水合环化反应的机理及化学选择性的理论研究

采用密度泛函理论(DFT)的计算方法, 研究了铂催化2-烯炔基苯甲醛水合环化反应的微观机理及化学选择性的根源. 计算结果表明, 首先炔基被催化活化而发生亲核环化生成吡喃铂中间体; 接着吡喃铂中间体与烯烃双键发生3+2]环加成生成铂-碳卡宾复合物; 之后, 反应将沿2条路径进行, 得到产物3a或4a, 其中4a的生成需经两步水分子辅助的质子转移过程. 生成产物3a需要克服的活化能垒为146.5 kJ/mol; 对4a的生成, 烯醇式和酮式互变异构是决速步聚, 当一个水分子参与反应时, 对应的能垒为185.8 kJ/mol, 当2个和3个水分子参与反应时, 能垒分别降低到128.1和64.9 kJ/mol. 因此, 水分子参与催化得到产物4a的路径是有利的. 另外, 反应的选择性与在异构化过程中水的共催化作用有关. 以上结果很好地解释了实验现象, 并为铂催化水环化反应提供新的见解.

Theoretical Studies on the Mechanism of Water-dependent Chemoselectivity in the Pt-Catalyzed Hydrative Cyclization of 2-Enynylbenzaldehydes

With the aid of density functional theory(DFT) calculations, we made a detailed mechanism study on the origin of chemoselectivity in Pt-catalyzed hydrative cyclizations of 2-enynylbenzaldehydes. The calculations indicate that the formation of platinum-pyrylium intermediate is initiated by the activation of alkyne. Successive 3+2] cycloaddition with a double bond leads to the platinum-carbene complex. After that, the reaction proceeds along either pathway Ⅰ or pathway Ⅱ to yield products 3a and 4a, depending on the subsequent two-step water-assisted proton-transfer process. The calculated barrier leading to product 3a is 146.5 kJ/mol. For the formation of product 4a, the tautomerization(from enol to keto form) is the rate-determining step with a barrier of 185.8 kJ/mol when one water molecule is involved. However, when two and three water molecules are involved in catalysis, the barrier is reduced to 128.1 and 64.9 kJ/mol respectively. Therefore, the reaction preferentially proceeds along the pathway Ⅱ leading to product 4a. Water molecules that act as a cocatalyst in the tautomerization process are mainly responsible for the good selectivity. This result rationalizes well the experimental observations and provides a new insight into the Pt-catalyzed hydrative cyclizations.