Imidazo[1,5-a]pyridin-3-ylidene/Thioether Mixed C/S Ligands and Complexes Thereof

Abstract

The synthesis of chiral imidazo[1,5-a]pyridinium salts containing thioether-functionalized side chains was accomplished by a direct alkylation of the heterocycle with (R)-(1-bromo-3-methylbutan-2-yl)(cyclohexyl)sulfane or by alkylation of suitable formamides with halomethyl pyridines followed by POCl3-promoted cyclization of the resulting products. These compounds readily react with Ag2O to afford the corresponding silver carbenes in excellent yields, and the latter were used as carbene transfer reagents in reactions with [PdCl2(CH3CN)2], [Pd(η3-C3H5)(COD)]+SbF6-, or [Rh(COD)2]+SbF6- for the synthesis of several Pd and Rh complexes. Simple alkyl-imidazol-2-ylidene analogues were also synthesized for comparison purposes. Studies on the catalytic behavior of the Pd complexes in asymmetric allylic alkylations revealed a strong influence of the nature of the heterocycle in the sense of the asymmetric induction, which reached up to 91% ee in the presence of Bu4NBr as an additive. Structural studies demonstrate that the formation of the C/S complexes proceeds in a stereoselective way: preferential coordination of one of the lone pairs of the thioether sulfur atom to the metal affords a boat-like chelate with S-configuration at sulfur and relative anti i-Pr/Cy configuration, while the conformation of the chelate depends on the backbone substitution pattern.