9-Thiourea Epi-Quinine Supported On Mesoporous Silica

Inspired by the high efficiency and selectivity of enzymes which can activate both nucleophilic and electrophilic functional groups under mild conditions,organocatalysts for chiral C-C bond formation have been attracting much attention in fields such as catalytic chemistry,organic chemistry,and green chemistry.The readily perfect separation of catalysts from the reaction media and subsequently facile recycle in organic catalytic processes is urgently desired concerning the striving for environmentally benign chemical processes or methodologies and economic benefits.Herein,attempts have been made to enhance the enantioselectivity through the immobilization of 9-thiourea cinchona alkaloid on mesoporous silica,in addition to preparing a readily recycled and regenerated heterogeneous catalyst.1.9-thiourea epi-quinine has been supported on SBA-15 and MCM-41,respectively, using a mercapto group as linker.The resulting materials are denoted SBA-15-SQT and MCM-41-SQT.9-thiourea epi-quinine moieties are distributed on both interior and exterior surface of SBA-15,while mainly on the exterior surface of MCM-41.2.In the asymmetric Friedel-Crafts reaction of indole and imine,conjugate addition reaction of nitromethane and chalcone,and asymmetric addition reaction of dimethyl malonate and trans-beta-nitrostyrene,SBA-15-SQT exhibits higher ee value than its homogenous counterpart.MCM-41-SQT shows higher conversion and yield but lower selectivity and ee value than SBA-15-SQT in the asymmetric Friedel-Crafts reaction of indole and imine.3.The enatioselectivity of SBA-15-SQT remains basically unchanged after being recycled for five times.4.The immobilization of 9-thiourea epi-quinine is further controlled to occur inside the nanosized channels of SBA-15 by pre-protecting the exterior surface.The resulting material is denoted CH₃-SBA-15-SQT.In the conjugate addition reaction of nitromethane and chalcone,CH₃-SBA-15-SQT shows higher selectivity and ee value than both of its homogenous counterpart and SBA-15-SQT.But no obvious enhancement of selectivity and ee has been observed in the asymmetric Friedel-Crafts reaction,in comparison with SBA-15-SQT.