Ruthenium-catalyzed β-alkylation of secondary alcohols with primary alcohols: Protic N-heterocyclic carbene's promotional influence

Abstract

This study investigated the catalytic activities of two pincer Ru complexes with bifunctional metal-ligand cooperativity, namely (HO-C5H3N-CH2-C5H3N-NHC)Ru(PPh3)2Cl (1a) and (C5H4N-CH2-C5H3N-NHC)Ru(PPh3)2Cl (2a), for CC bond formation through the borrowing hydrogen process. Surprisingly, complex 2a, which lacks the 2-hydroxypyridyl fragment, exhibited comparable activity to complex 1a with the same catalyst loading in the coupling of a broad range of primary and secondary alcohols. The efficiency of this straightforward system was further demonstrated in the challenging reaction of β-alkylation of 1-phenylethanol with benzyl alcohol, with a minimal catalyst loading of 0.0001 mol.%. The highest TOF observed for the cross-coupling of alcohols was 13,333 h−1. Additionally, complex 2a was found to be effective in catalysing β-alkylation of aliphatic secondary alcohols with primary alcohols and double C-alkylation of cyclopentanol. This finding holds promise for the development of more effective bifunctional catalysts for β-alkylation reactions of primary and secondary alcohols.