Synthesis, Characterization, and Catalysis of Planar Chiral Ferrocene-based N-Heterocyclic Carbene Ligands and Its Metal Complexes

Abstract

This study presents the development of a novel planar chiral ferrocene-based NHC ligand and its corresponding metal complexes. The key unit of the imidazolium backbone, planar chiral ortho-isopropyl ferrocenyl amine, was synthesized starting from (S)-Ugi's amine. Imidazolium was obtained through the sequential condensation of glyoxal with ferrocenyl amine, followed by the ring-closure of 1,3-diferrocenyldiimine with chloromethylethyl ether. Chiral imidazolylidene NHC (IFcPr) metal complexes were prepared by treating imidazolium with silver oxide or base followed by transmetalation with appropriate metal salts. The steric and electronic properties of the IFcPr ligand were assessed using %Vbur and TEP, respectively. These assessments indicated that the ligand was as bulky as the adamantyl NHC ligand and exhibited strong donor ability similar to that of a triazolylidene NHC ligand. The catalytic performance of the copper and palladium complexes was evaluated in the asymmetric borylation of ethyl cinnamate and Suzuki–Miyaura coupling reactions, respectively. The IFcPr−Cu complex catalyzed the borylation of ethyl cinnamate with bis(pinacolato) diboron, followed by oxidation to yield the hydroxy ester with 58 % yield and 30 % ee. In contrast, the IFcPr−Pd-PEPPSI complex provided the axial chiral binaphthyl compound with 59 % yield and 51 % ee at a low catalyst loading (0.1 mol %, TON=590).