α,β-Alkylation Reactions Catalyzed by Ruthenium Triphenylphosphine Complexes Bearing Substituted Pyridine–Quinoline Based Ligands

Herein, a series of ruthenium(II) bis-triphenylphosphine complexes comprising substituted pyridine–quinoline based ligands for the efficient α-alkylation of ketones with primary alcohols, via the borrowing hydrogen methodology, are reported. β-alkylation of secondary alcohols by primary alcohols is also reported. The reactions are catalyzed by the ruthenium complexes [RuCl₂(8-Mepq)(PPh₃)] (1), [RuCl₂(6′-Mepq)(PPh₃)₂] (2), [RuCl₂(4,6′-Me₂pq)(PPh₃)₂)] (3), including [RuCl₂(4-Mepq)(PPh₃)₂] (4), and [RuCl₂(pq)(PPh₃)₂] (5). Mass spectrometry, elemental analysis, and ³¹P NMR data reveal that 1 is a five-coordinate 16-electron ruthenium(II) complex with one triphenylphosphine ligand, as opposed to 2–5, which hold two and are six-coordinate. Theoretical calculations suggest that 1 is stabilized by a nonclassical RuCl(1)···H(2) interaction between a hydrogen atom of the appended methyl group and an adjacent chlorine atom. By NMR spectroscopy, different isomers of 2 and 3 have been detected. For the α-alkylation of benzyl alcohol with acetophenone, all complexes selectively provided 1,3-diphenylpropan-1-one as the sole product, within an hour. The β-alkylation of 1-phenylethanol with benzyl alcohol under the same conditions is less selective. As salient features of this report we could mention that these catalytic systems operate with easily accessible and in high yields air-stable catalysts, within a shorter period compared to known ruthenium complexes.