Synthetic Approaches to Chiral Non-C2-symmetric N-Heterocyclic Carbene Precursors

Abstract N -Heterocyclic carbenes and their metal complexes have found applications in many organic transformations. Apart from the privileged C 2 -symmetry present in modern enantioselective catalysis, ligands bearing C 1 -symmetry have witnessed growing attention due to the better control of process stereoselectivity in many cases. The present review summarizes, for the first time, the seminal synthetic efforts for the preparation of N -heterocyclic carbene precursors exhibiting C 1 -symmetry. The well-established methods will be discussed in the light of recent achievements, giving a direct opportunity for comparison of the existing methods, and simultaneously a chance to find the best synthetic pathway for the ideal chiral ligand. 1 Introduction 2 Five-Membered Rings 2.1 Five-Membered Saturated (Imidazolinium) Ring 2.1.1 Amino Alcohol Derivatives 2.1.2 Amino Acid Derivatives 2.1.3 Amine and Diamine Derivatives 2.2 Five-Membered Unsaturated Ring 2.2.1 Cyclization Strategy 2.2.2 Functionalization of the Existing N -Imidazole Ring 3 Triazolium Salts 3.1 Substitution in Oxadiazolium Salts 3.2 Lactam-Derived Triazolium NHC (The Leeper and Knight Methodology) 3.3 Monoterpenoid-Derived NHC Triazolium Salts 3.4 Modifications of the Existing Triazole Ring 4 Thiazole-Derived Salts 4.1 Cyclization of a Thiazolium Ring 4.2 Modifications of the Existing Thiazole Ring 5 Summary and Outlook