Thiazol-2-ylidenes: Versatile N-Heterocyclic Carbene Ligands with Enhanced Electrophilicity for Transition-Metal-Catalysis

Abstract Over the last 20 years, N-heterocyclic carbenes (NHCs) have emerged as a dominant direction in ligand development in transition-metal-catalysis. In particular, strong σ-donation in combination with tunable steric environment make NHCs to be among the most common ligands used for C–C and C–heteroatom bond formation. Thus far, NHC ligand development has been almost exclusively limited to N-aryl-imidazolylidenes, such as IPr, prepared by deprotonation of symmetrical imidazolium salts. However, the molecular structure of imidazolylidene systems is constrained by the substitution with two nitrogen atoms, a limitation that prohibits the development of more active carbene 2 analogues. Herein, we report the first study on steric and electronic properties of thiazol-2-ylidenes. We demonstrate that the ring dissymmetry and enhanced 𝝅-electrophilicity result in a class of highly active carbene ligands for electrophilic cyclization reactions to form valuable oxazoline heterocycles. The evaluation of steric, electron-donating and 𝝅-accepting properties as well as structural characterization and coordination chemistry is presented. This mode of catalysis can be applied to late-stage drug functionalization to furnish attractive building blocks for medicinal chemistry. Considering the key role of N-heterocyclic ligands, we anticipate that N-aryl thiazol-2-ylidenes will be of broad interest as ligands in modern chemical synthesis.