Heterogeneous asymmetric nitro-Mannich reaction using a bis(oxazoline) ligand grafted on mesoporous silica

Abstract

A chiral bis(oxazoline) ligand was immobilized on mesoporous silica (SBA-15) and examined in an asymmetric heterogeneous nitro-Mannich reaction. Depending upon the size of the alkyl chain in the nitroalkane substrates, enantioselectivities comparable to and higher diastereoselectivities (syn/anti ratio) than those obtained from homogeneous reactions were observed. In the case of the long chain substituted nitroalkane substrate (nitrohexane), the best selectivities (diastereoselectivity: syn/anti = 98/2, and enantioselectivity: 93% and 82% ee's for syn- and anti-isomers, respectively), were observed. Recycling of the catalyst in subsequent reactions was carried out and gradually diminishing levels of both diastereo and enantioselectivities were observed after each recycle. (C) 2004 Elsevier Ltd. All rights reserved.