Stepwise fabrication and architecture of heterogeneous 9-thiourea epiquinine catalyst with excellent enantioselectivity in the asymmetric Friedel–Crafts reaction of indoles with imines

Heterogeneous 9-thiourea epiquinine catalysts are prepared by covalently anchoring 9-(3,5-bis(trifluoromethyl)phenylthiourea)epiquinine on mesoporous silica surfaces via mercapto linker. It is found that 9-thiourea epiquinine moieties are located comparably on the interior and exterior surfaces of SBA-15 while preferentially on the exterior surface of MCM-41. The heterogeneous 9-thiourea epiquinine are applied as catalysts in the asymmetric Friedel–Crafts reaction of indoles with imines. SBA-15 supported 9-thiourea epiquinine, with more catalytic sites inside the channels, is found more chemselective and enantioselective than the counterpart supported on MCM-41. The effects of solvents and molecule dimensions of both substrates on the reaction effectiveness and selectivity are also discussed.