Enantioselectivity promotion by achiral surface functionalization on SiO2-supported Cu-bis(oxazoline) catalysts for asymmetric Diels-Alder reactions

Novel SiO2-supported chiral Cu-bis(oxazoline) (BOX) complexes for asymmetric Diels-Alder reactions were prepared by combining metalcomplex immobilization with surface functionalization using achiral silane-coupling reagents on SiO2. We found that the surface functionalization of a SiO2-supported Cu-BOX catalyst with achiral 3-methacryloxypropyltrimethoxysilane dramatically increased enantioselectivity in the asymmetric Diels-Alder reaction of cyclopentadiene and 3-acryloyl-2-oxazolidinone. The Cu-BOX complexes on bare and functionalized SiO2 surfaces were characterized by XAFS, ESR, FT-IR, UV/vis, and Si-29 solid-state MAS NMR. The large increase in enantioselectivity by achiral surface species without chiral center may be due to a glue effect, creating a new chiral ensemble structure at the surface. (c) 2006 Elsevier Inc. All rights reserved.