Selectivity Control in 3d Transition Metal-Catalyzed C–H Activation

Abstract

Methods for the selective functionalization of otherwise inert C–H bonds have been recognized as a transformative tool in synthetic organic chemistry, with applications ranging from the synthesis of complex bioactive compounds to material sciences. In particular, 3d metal catalysts have emerged in recent years as inexpensive, earth-abundant and less toxic alternatives to their heavier counterparts. However, full selectivity control in base metal-catalyzed C–H activation continues to be challenging. In this context, the development of novel 3d transition metal catalysts enabling chemo- and stereo-selective C–H functionalizations should be investigated. Within this thesis, we became interested in the development of a user-friendly and broadly applicable protocol for synthetically useful cobalt-catalyzed C–H amidations with ample substrate scope. Furthermore, while the enantioselective functionalization of C–H bonds remains largely dominated by noble transition metal catalysts such as palladium, rhodium and iridium, we developed an unprecedented enantioselective iron-catalyzed C–H alkylation by alkene hydroarylation. The design of novel chiral N-heterocyclic carbene (NHC) ligands proved to be crucial to achieve high enantioselectivities. Furthermore, detailed mechanistic studies were conducted to unravel the nature and mode of action of the in situ generated catalyst. Recently, nickel-catalyzed hydroarylation-type C–H activation has emerged as a cost-efficient alternative to expensive rhodium catalysis. However, the intramolecular hydroarylations of unactivated alkenes remain strongly limited by the requirement of pyrophoric organoaluminium additives, significantly compromising their functional group tolerance and synthetic utility. This observation prompted us to investigate the asymmetric cyclization of N-homoallylimidazoles under aluminium-free conditions. Interestingly, the endo product was selectively obtained, which in sharp contrast to previously reported methods. Mechanistic studies were then conducted in order to delineate the unique reactivity of the developed catalytic system.