New Molybdenum Catalysts for the Enantioselective Hydrogenation of Arenes and Heteroarenes

Abstract

The synthesis and catalytic activity of a set of new enantioenriched oxazoline imino(pyridine) (OIP) molybdenum catalysts are described. The design of these catalysts has been motivated by the need for more active, enantiospecific hydrogenation catalysts, based on earth-abundant metals to provide an alternative to expensive, unsustainable precious metal catalytic systems. Previous work by the Chirik group has established 4tBu-(OIP)Mo(COD) pre-catalyst as effective for the hydrogenation of 2,6 disubstituted quinolines and naphthalenes, yielding high diastereomeric ratios and high enantioselectivities. In this thesis, the effect on catalytic activity of steric encumbrance on the aryl imine arm of the ligand was investigated. Herein, three novel 4tBu-(OIP)Mo(COD) variants were synthesized, isolated, fully characterized and tested for catalytic performance. Initial screens revealed substrate dependent trends concerning conversion, while the enantioselectivities remained high and unaffected by variations of substituents on the aryl imine arm of the ligand. Notably, increasing the steric bulk furnished a significantly higher selectivity for the fully hydrogenated decalin product over the partially hydrogenated tetralin product for the hydrogenation of 2,6-dimethylnaphthalene. The hydrogenation results presented in this thesis also point towards differing mechanisms for bicyclic (hetero)cycles, as compared to other classes of substrates, further informing future catalyst design principles. Molybdenum-based hydrogenation catalyst design principles are discussed.