Synthesis of chiral organotins suitable for the preparation of asymmetric heterogeneous catalysts
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, 13C and 119Sn 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. Copyright © 2005 John Wiley & Sons, Ltd.