Development of Transition Metal Catalysts bearing N-Heterocyclic Carbene for Bond-Forming Reactions

Abstract

N-Heterocyclic carbenes (NHCs) have been widely utilized in organic synthesis since the first isolation of a representative free carbene by Arduengo in 1991.Their strong σ-donating ability and tunable steric hinderance have established NHCs as essential ligands in transition-metal catalysis. In 2005, Glorius and Lassaletta introduced Imidazo[1,5-a]pyridin-3-ylidene (ImPy) as a bicyclic, backbone-modified variant of NHCs. ImPy uniquely enables substituents to be positioned in close proximity to the metal coordination sphere, while simultaneously allowing the introduction of a substituent on the N-wingtip side. This orthogonality inherent to ImPy has facilitated the development of novel catalysts, which have been applied to a variety of reactions. In Part 1, the thesis aims to review the development and applications of previously reported ImPy ligands. The key feature of ImPy is its orthogonality, which allows for flexible combinations of its C5 and N2 positions with a wide variety of substituents. Examples of substituents introduced at each position will be categorized. The ImPy derivatives have been used to introduce new catalytic concepts and enhance reactivity in transition-metal- catalyzed transformations. Part 2 focuses on the development of bifunctional ImPy palladium complexes, where ImPy is modified with an ether linkage at the N2-wingtip. These complexes are then applied in the activation of the deprotonation step in Pd-catalyzed amination reactions. Various NHCs have been developed as catalysts for Pd-catalyzed amination, typically following the common strategy. This common strategy involves the strong σ-donating ability of NHC ligands accelerating oxidative addition, while steric effects control the reductive elimination step. We propose that specifically targeting the deprotonation step could introduce novel improvements in catalytic performance while maintaining the advantages of existing strategies. By comparing the reactivity of bifunctional ImPy-Pd complexes, we sought to validate this hypothesis. The molecular structure of ImPy-Pd complexes was confirmed through single-crystal X-ray diffraction studies. Based on these crystal structures, we further supported our findings with PhD/CH 20172015 buried volume analysis and computational calculations. In Part 3, an in-situ generated phosphine-chelated ImPy nickel complex is employed in the catalytic conversion of ethylene and CO2 into acrylates. This reaction presents an innovative method of transforming CO2, a major greenhouse gas, into acrylates, which are traditionally produced from petrochemical sources. However, significant challenges remain in achieving this transformation efficiently. We aim to address these challenges by employing a rigid, highly planar catalyst, which has demonstrated the highest reported turnover number (TON) for this reaction while maintaining high yields. This chapter will detail the development of this catalytic system and the optimization process that led to its exceptional reactivity.