β‐Alkylation through Dehydrogenative Coupling of Primary Alcohols and Secondary Alcohols Catalyzed by Thioether‐Functionalized N‐Heterocyclic Carbene Ruthenium Complexes

Abstract

AbstractA catalytic system for the direct β‐alkylation of secondary alcohol with primary alcohol has been investigated. In this work, a series of cationic Ru(II)(η6‐p‐cymene) complexes with thioether‐functionalized N‐heterocyclic carbene ligands (imidazole‐based 1 a–l and benzimidazole‐based 2 a–e) have been successfully synthesized and evaluated as catalysts. This investigation shows that modifications in the ligand moiety (thioether group and/or NHC core) have a strong effect on both selectivity and reactivity. Imidazole‐based complex 1 c, with only 1 mol % of catalyst loading, displayed the best catalytic activity as well as the highest selectivity for the β‐alcohol up to 98 : 2 for this tandem borrowing hydrogen/aldol methodology. Applied to a wide range of substrates, β‐alkylated secondary alcohols have been obtained in moderate yields, but generally with complete conversion and very high selectivity.