Screening of rare earth metal grafted MCM-41 silica for asymmetric catalysis

Abstract

Mesoporous inorganic/metalorganic hybrid materials combining grafted rare earth metal (yttrium) centers with different chiral ligands including l-(−)-3-(perfluorobutyryl)-camphor, 2,2′-methylenebis-[(4S)-4-tert-butyl-2-oxazoline], (S)-(−)-2-(diphenyl-hydroxymethyl)pyrrolidine, (R)-(+)-1,1′-bis-2,2′-naphthol, (1R,2S)-(−)-N-methylephedrine, and (1S,2R,5S)-(+)-menthol were generated via surface-mediated ligand exchange reactions from silylamide modified MCM-41 materials. The hybrid materials were characterized by elemental analysis, FTIR spectroscopy, nitrogen physisorption, and their catalytic performance in the asymmetric hetero-Diels–Alder cyclization of Danishefsky’s diene (trans-1-methoxy-3-trimethylsilyloxy-1,3-butadiene) with benzaldehyde. The immobilized catalysts displayed markedly increased catalytic activity (65–85% yield after 10 h) compared to their molecular congeners. The l-(−)-3-(perfluorobutyryl)-camphor derivative [MCM-41]Y((−)-hfc)x(THF)y produced the highest diastereomeric and enantiomeric excesses (67% de, 37% ee (−35°C); cf. Er[(−)-hfc]3: 68% de, 55% ee). Particular consideration was given to various methods of silylation (“in situ”, “post”) and their influence on product selectivity.