Design of a Chiral Mesoporous Silica and Its Application as a Host for Stereoselective Di-pi-methane Rearrangements

摘要

A chiral periodic mesoporous organosilica (ChiMO) was prepared,in which a l,2-bis-(ureido)-cyclohexyl linker (38 wt%) is introduced into the walls of the hybrid organic-inorganic material.This silica was used as a host for 11-formyl-12-methyldibenzobarrelene (2),and the stereoselectivity of the di-pi-methane rearrangement of 2 within this host-guest complex was studied.At low conversions,the only product was the corresponding dibenzosemibullvalene.An enantiomeric excess of 24% at 11% conversion was obtained using the ChiMO as host.These values compare well with those achieved using a system based on conventional faujasitcs (LiY and NaY)as hosts incorporating a chiral auxiliary.We tested S-phenylglycine,S'-proline,S-camphanic acid,and S-mandelic acid as chiral auxiliaries.In contrast to the behavior of the ChiMO material,adsorption of dibenzobar-relene in purely siliceous mesoporous MCM-41 silica (3.2 nm pore size) containing ephedrine failed,a failure that can be explained as arising from the large internal silanol population and high hydrophilicity of the siliceous MCM-41 sample.