Synthesis of Chiral Ligands and Their Applications in Combinatorial Asymmetric Catalysis

Abstract

Enhanced effectiveness of pharmaceuticals and agrochemicals is the most sought-after attribute in the field of enantioselective synthesis of chiral molecules where single enantiomer (R or S) is required, for which asymmetric catalysis is an attractive way. The synthesis of enantiomerically pure compounds in traditional way is difficult, costly and time-consuming process involving the running of long columns which create a lot of waste that causes water pollution. The selectivity of products are equally depends upon the catalyst and conditions of the reaction which makes the task harder for selecting and finding new catalytic system. Present work deals with the development of six families of chiral ligands libraries for rapid recognition of the best system, by using combinatorial chemistry, during hydrogenation of ketones, a starting material and as intermediate in a number of pharmaceutical processes. The ligands were both generated in situ and in pure form for the comparison of results and were obtained by simple condensation, providing one of the starting materials being chiral. Six families namely; bisimine, diamine, aminal, imine, iminophosphine and oxazoline were synthesized and tested in presence of Zn(II), Rh(I) and Rh(III) by using polymethylhydrosiloxane (PMHS), diphenylsilane and formic acid/sodium formate as hydrogen donor species for transfer hydrogenation during synthesis of selective chiral alcohols from achiral acetophenone. Overall 1566 catalytic systems were developed and subsequently tested and the yield and enantiomeric excesses were found by using 1H NMR and chiral High Performance Liquid Chromatography (HPLC) respectively with more than 90% ee and 99% conversion as the best result for imine and aminal ligands. It has been proven that ligands synthesized in-situ gave comparable and almost same results to purified version with best reproducibility.