The effect of the active site and substrate structure in preparation of substituted tetrahydropyrans via intramolecular cyclization
Abstract
Substituted tetrahydropyrans (THPs) are important biologically active compounds and synthetic intermediates. Their prevalence and diverse applications make them desirable targets in synthetic organic chemistry.
Current synthetic routes for THP synthesis involve homogeneous catalysts and usually multiple steps and the use of expensive heavy metal catalysts, which pose environmental concerns. Therefore, there is a need for alternative approaches that can enhance efficiency, eco-friendliness, and cost-effectiveness This study focuses on the acid-catalyzed synthesis of THPs through the intramolecular cyclization of substituted (E)-5-phenylpent-4-en-1- olsover zeolites as heterogeneous catalysts.
The study particularly focused on investigating the influence of functional groups, as well as substrate size, on the reaction outcome. The results demonstrated that electron-donating groups enhance the conversion of the reactants and increase selectivity towards THPs, while electron-withdrawing groups have the opposite effect.
Furthermore, the cyclization reaction also occurred on substrates containing different aromatic cycles. However, the conversion of a bulky substrate containing naph-thyl group decreased due to mass transfer limitations.
The catalytic performance of different types of zeolites with diverse acid sites was also examined. Bronsted acidic zeolites such as Al-BEA and Ga-BEA exhibited promising catalytic performance and 94 % and 83 % selectivity towards THPs, respectively.
In contrast, Lewis acidic zeolites such as Sn-BEA, Ti-BEA, and Zr-BEA showed conversions below 6.5 % and poor selectivity, showing that Bronsted acidic zeolites are more suitable for the cyclization reaction than Lewis acidic zeolites.