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.