Recyclable mesoporous silica-supported chiral ruthenium-(NHC)NN-pincer catalysts for asymmetric reactions

Abstract

New stable chiral [RuHClCO((NHC)NN)] and [RuCl2(p-cymene)((NHC)NN)] pincer complexes with a pendant silyloxy group ((NHC)NN: (S)-1-((6-((3-aryl-2,3-dihydro-1H-imidazol-1-yl)methyl)pyridin-2-yl)methyl); R-N-(3-(triethoxysilyl)propyl)pyrrolidine-2-carboxamide, aryl = mesityl, 2,6-diisopropylphenyl; R = Me, Ph) were synthesized and grafted onto a mesoporous silica MCM-41 through the siloxane linkage, and the resulting supported ruthenium catalysts were highly active and recyclable catalysts for the asymmetric hydrogenation of alkenes and cyclopropanation of styrenes. The solid catalysts were characterized with a variety of methods including solid state cross-polarization magic-angle spinning (CP MAS) 13C NMR, FT-IR, and elemental analysis. Excellent activity and enantioselectivity was observed for these supported Ru-catalysts owing to readily accessible and uniform catalytic sites within the large channels of MCM-41 and short diffusion lengths for the organic compounds. The high accessibility introduced by the structure of the support allows the preparation of highly efficient immobilized catalysts which can duplicate the activity of the homogeneous analogues. No deactivation of the catalysts was observed after repeated recycling.