Cobalt, Iron, and Molybdenum Catalysts for the Asymmetric Hydrogenation of Alkenes and Arenes

Abstract

The transition-metal catalyzed asymmetric hydrogenation of unsaturated organic molecules is a simple yet powerful transformation for the synthesis of optically active molecules, which has found extensive application in the pharmaceutical, agrochemical, and fine chemical industries. State-of-the-art catalysts for this transformation are typically derived from the precious metals such as Rhodium, Iridium, or Ruthenium, which have demonstrated incredible activity, selectivity and utility in the synthesis of small molecules. In recent years, the development of catalysts based on more sustainable, earth-abundant transition metals has been of great interest. Besides the environmental and economic advantages to the use of such metals, the introduction of earth-abundant metals into the catalyst space has led to the discovery of unique, orthogonal reactivity to precious metals. Thus, understanding the structure, behavior and application of earth-abundant transition-metal catalysts for asymmetric hydrogenation will give rise to the access of new chemical reactivity. In this dissertation, the design, structure and applications of earth-abundant transition-metal catalysts for asymmetric hydrogenation will be described. Cobalt catalysts were applied to the synthesis of enantioenriched 1,1-diborylalkanes - powerful synthons enabling the iterative synthesis of complex chiral structures. These studies revealed unique activating effects of the boron substituents which allowed for the hydrogenation of sterically hindered alkenes. Enantioenriched iron complexes were prepared and structurally characterized, and the downfalls of the catalytic activity were identified as a catalyst decomposition pathway. During this investigation, a powerful C-H activation reaction was discovered and transformed into a new iron-catalyzed method for the hydrogen isotope exchange of C-H bonds using benzene-d6 as a deuterium source. The final studies in this dissertation consist of the conception and design of asymmetric arene hydrogenation catalysts based on Molybdenum. The catalysts exhibit remarkable enantioselectivity for the hydrogenation of aromatic hetero- or carbocycles without coordinating functionality and are capable of fully hydrogenating naphthalenes and quinolines to their respective decalin and decahydroquinoline products.