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（英文）

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

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 k J/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 k J/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 k J/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.

With the aid of density functional theory （DFF） calculations, we made a detailed mechanism study on the origin of ehemoseleetivity in Pt-eatalyzed hydrative cyelizations of 2-enynylbenzaldehydes. The calculations indicate that the formation of platinum-pyrylium intermediate is initiated by the activation of alkyne. Successive [ 3 ＋ 2 ] eycloaddition with a double bond leads to the platinum-earbene complex. After that, the reaction proceeds along either pathway I or pathway 1[ 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 lI leading to product 4a. Water molecules that act as a eocatalyst in the tautomerization process are mainly responsiblefor the good selectivity. This result rationalizes well the experimental observations and provides a new insight into the Pt-catalyzed hydrative cyclizations.