Ruthenium(II)-Chitosan, an Enantioselective Catalyst for the Transfer Hydrogenation of N-Heterocyclic Ketones

摘要

The present study aimed at extending the applicability of a recently developed stereoselective catalytic system to the preparation of optically enriched N-heterocyclic alcohols. Chiral ruthenium catalyst formed in situ using the chitosan biopolymer as ligand, which provided good results in the transfer hydrogenation of heterobicyclic compounds, such as 4-chromanone and 4-thiochromanone, was used in reactions of various N-containing prochiral ketones. High enantioselectivities were reached in transfer hydrogenations of bicyclic compounds bearing nitrogen either in aromatic or cycloaliphatic moieties, provided that the amino group was protected or shielded by a nearby substituent. Results were rationalized by interactions of the nitrogen with the metal and/or ligand. N-containing bicyclic compounds having heteroatoms in both rings were also prepared and tested. The detrimental effect of the pyridyl moiety was compensated by the beneficial influence of the heteroatom in the cycloaliphatic ring, as indicated by high rates and good enantioselectivities obtained in reactions of these compounds. Preparation of several N-heterocyclic alcohols, in good yields and high optical purities was achieved using Ru(II)-chitosan complex.