The application of chiral phosphorus ligands in the catalytic asymmetric hydrogenation of heteroaromatic compounds

Abstract

In contrast to the tremendous success in the catalytic asymmetric hydrogenation of prochiral olefins, ketones, and imines, the catalytic asymmetric hydrogenation of heteroaromatic compounds, which is a convenient method for the preparation of enantiomerically pure heterocycles, still remains a challenging task. Successful examples in this kind of asymmetric catalytic reactions are rare. It is highly desirable to develop more efficient catalysts or catalytic systems for high enantioselectivity and high yield. We have synthesized a series of chiral diphosphinite ligands, and examined their performance in asymmetric hydrogenation of quinolines. The results indicated that the introduction of substituents on the 3-and 3'-position of H8-binaphthyl ring would result in the decrease of enantioselectivity. The chiral diphosphinite ligand based on l,l'-spirobinindane framework was found to be highly effective for the asymmetric hydrogenation of quinolines with high substrate/catalyst ratio (up to 5000) and high enantioselectivity (up to 94 %). We have also studied the application of the chiral diphosphinite ligands in the Ir-catalyzed asymmetric hydrogenation of quinoxalines. In this study, it was shown that Ir-H8-BINAPO is an efficient catalyst for the asymmetric hydrogenation of quinoxalines and their derivatives, providing good enantioselectivies up to 95.6%. High turnover number was also achieved under mild conditions. Finally, the asymmetric hydrogenation of pyridines using chiral Ir-P-Phos/I2 catalytic system has been explored. It was found that iodine acted as an important role on the controlling of the enantioselectivity and yield. The catalytic system demonstrated excellent catalytic ability for the preparation of chiral piperidine derivatives, which are not only important starting materials for many biologically active compounds, but also important structural building blocks for many alkaloids. It is our pursuit to develop efficient chiral catalyst for high value chiral products, especially in the field of asymmetric hydrogenation of heteroaromatic compounds. These studies offer a solid foundation in the designing and synthesis of highly active chiral catalysts.