Synthesis of chiral organotins suitable for the preparation of asymmetric heterogeneous catalysts

摘要

A key step in the preparation of asymmetric heterogeneous catalysts based on silica-supported rhodium and platinum chemically modified with chiral organotins,is the synthesis of optically pure tin derivatives.We report on the synthetic pathways leading to the synthesis of (-)-menthyltin derivatives without the formation of epimerization by-products.The physical properties (including full ~1H,~(13)C and ~(119)Sn NMR spectra) of the new compounds,containing between one and three (-)-menthyl ligands attached to the tin atom,are reported.The preparation of two catalysts based on silica-supported rhodium and platinum chemically modified with (-)-menthyldiphenylmethyltin,as well as some studies on the catalytic hydrogenation of ethyl pyruvate,are also described.These studies show that the reductions lead to (R)- and (S)-ethyl lactates as the only products,that the (S) enantiomer was the isomer formed preferentially,and that the degree of conversion observed using both catalytic systems was almost quantitative (97-100%).The enantiomeric excesses observed were in the range 7-8%.One important advantage of these catalytic systems is their stability.Recycling of the used catalysts is possible,without any loss in the degree of conversion or enantiomeric excess.