β‐Alkylation through dehydrogenative coupling of primary alcohols and secondary alcohols catalysed by thioether‐functionalized N‐heterocyclic carbene ruthenium complexes

A new 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 (imidazolebased 1a-l and benzimidazole-based 2a-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 1c, 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.