Epoxide ring opening with alcohols using heterogeneous Lewis acid catalysts: Regioselectivity and mechanism

Abstract

Lewis acidic catalytic materials are investigated for the regioselective ring opening of epoxides with alcohols. For ring opening epichlorohydrin with methanol, the catalytic activity shows a strong dependence on the type of support and Lewis acidic species used. While Sn-SBA-15 is catalytically active, significantly higher catalytic activity can be achieved with hydrothermally synthesized zeolites of which Sn-Beta is 6 and 7 times more active than Zr-Beta or Hf-Beta, respectively. Sn-Beta is determined to be more active and more regioselective for epoxide ring opening of epichlorohydrin with methanol than Al-Beta. For Sn-Beta, the activation energy for the reaction between epichlorohydrin and methanol is determined to be 53 ± 7 kJ mol−1. For epichlorohydrin, the activation energy barrier and experimentally observed regioselectivity are found using DFT to be consistent with a concerted reaction mechanism involving activation of the epoxide on an alcohol adsorbed on the catalytic site and nucleophilic attack by a second alcohol. The epoxide is shown to impact the regioselectivity and the mechanism since isobutylene oxide is selectively ring opened by methanol to form the terminal alcohol. DFT calculations indicate the mechanism for isobutylene ring opening involves epoxide activation and ring opening on an alcohol adsorbed onto the catalytic site. Finally, catalyst reuse testing indicates that Sn-Beta can be used for multiple reactions with no decrease in activity and limited to no leaching of the tin site, demonstrating Sn-Beta is a promising catalytic material for epoxide ring opening reactions with alcohols.