Catalytic cyclization of o-alkynylbenzaldehyde acetals and thioacetals. Unprecedented activation of the platinum catalyst by olefins. Scope and mechanism of the reaction

A general protocol for the synthesis of functionalized indenes from o-alkynylbenzaldehyde acetals and thioacetals has been elaborated. Acetals uniformly give cyclization products having the alkyl group from the starting acetylene migrated to the α-position, whereas the cyclization of the corresponding thioacetals proceeds without alkyl migration. Optimization of the catalytic system for the cyclization of o-alkynylbenzaldehyde acetals revealed an unknown activation effect: PtCl₂ was found to be a better catalyst for the cyclization of acetals in the presence of olefins than without. A similar catalytic system (PtCl₂/ benzoquinone) has been found to be appropriate for the cyclization of cyclic acetals, whereas the optimal catalyst for the reaction of thioacetals is PdI₂. NMR monitoring of two reactions, acetal 3a + Pd(CH₃CN)-Cl₂ in CD₃CN and thioacetal 5j + PdI₂ in CD₂Cl₂, revealed that in both reactions similar cationic species are formed at the early stage of the transformation. Computational data (B3LYP/SDD level of theory) suggest that the difference in the reaction pathways for acetals and thioacetals can be rationalized by taking into account the relative stabilities of the corresponding vinylpalladium intermediates (22 vs 20 and 19 vs 21), which suggests a reversible thermodynamically controlled alkyl migration in the intermediate vinylcationic species.