Molybdenum Catalysts for the Asymmetric Hydrogenation of Naphthalene

Abstract

There is an increasing need for sp3 carbons in industrial and pharmaceutical sectors. A higher sp3 content can be achieved via hydrogenation reactions that require catalysts with high chemo- and enantioselectivities, typically based on precious metals. Previous work in the Chirik group has developed pre-catalysts of oxazoline imino(pyridine) (OIP) and pyridine (diimine) (PDI) ligand frameworks based on the earth-abundant metal molybdenum. This work aims to integrate features of both PDI and OIP catalysts into the PDI ligand framework for the asymmetric hydrogenation of naphthalene. A pathway for the ligand synthesis of C1-symmetric PDI ligand was developed. C1- and C2-symmetric PDI(Mo)COD pre-catalysts were synthesized, and their catalytic activity was evaluated for 2,6- substituted naphthalene. Results revealed that the C1- and C2-symmetric 4-tBu-((S)-Cy,MePDI)Mo(COD) were active hydrogenation catalysts for 2,6-dimethylnaphthalene, and selective for the partially reduced product. While the C2-symmetric catalyst induced no enantioselectivity, the C1-symmetric 4-tBu-((S)-Cy,MePDI)Mo(COD) achieved an enantiomeric excess of 13%. Preliminary work has been done on developing C1-symmetric PDI ligands substituted with chiral anilines to increase enantioselectivity.