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

Mesoporous inorganic/metalorganic hybrid materials combining grafted rare earth metal (yttrium) centers with different chiral ligands including L-(-)-3-(perfluorobutyryl)-camphorl 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 degreesC); 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. (C) 2001 Elsevier Science B.V. All rights reserved.