Gold(I) N-heterocyclic carbene based initiators for bulk ring-opening polymerization of l-lactide

Synthesis, structures, and catalysis studies of gold(I) complexes of N-heterocyclic carbenes namely, a di-O-functionalized 1-(2-hydroxy-cyclohexyl)-3-(acetophenone)imidazol-2-ylidene], a mono-O-functionalized 1-(2-hydroxy-cyclohexyl)-3-(benzyl)imidazol-2-ylidene] and a non-functionalized 1,3-di-i-propyl-benzimidazol-2-ylidene], are reported. Specifically, the gold complexes, 1-(2-hydroxy-cyclohexyl)-3-(acetophenone)imidazol-2-ylidene]AuCl (1c), 1-(2-hydroxy-cyclohexyl)-3-(benzyl)imidazol-2-ylidene]AuCl (2c), and 1,3-di-i-propyl-benzimidazol-2-ylidene]AuCl (3b), were prepared from the respective silver complexes 1b, 2b, and 3a by treatment with (SMe₂)AuCl in good yields following the commonly used silver carbene transfer route. The silver complexes 1b, 2b, and 3a were synthesized from the respective imidazolium halide salts by the reactions with Ag₂O. The N-heterocyclic carbene precursors, 1-(2-hydroxy-cyclohexyl)-3-(acetophenone)imidazolium chloride (1a) and 1-(2-hydroxy-cyclohexyl)-3-(benzyl)imidazolium chloride (2a), were synthesized by the direct reactions of cyclohexene oxide and imidazole with chloroacetophenone and benzyl chloride respectively. The gold (1c, 2c, and 3b) and the silver (3a) complexes along with a new O-functionalized imidazolium chloride salt (1a) have been structurally characterized by X-ray diffraction. The structural studies revealed that geometries around the metal centers were almost linear in these gold and silver complexes. The gold (1c, 2c, and 3b) complexes efficiently catalyze ring-opening polymerization (ROP) of l-lactide under solvent-free melt conditions producing polylactide polymer of moderate to low molecular weights with narrow molecular weight distributions.