In Situ Routes to Catalytically Active Ru(0) Species by Reduction of Readily Available, Air-Stable Precursors

Abstract

Cross-dimerization of a conjugated diene with a substituted alkene catalyzed by in situ reduction of an air-stable Ru(II) catalyst precursor has been achieved. Reaction of 2,3-dimethylbutadiene with styrene is catalyzed by [Ru(acac)2(η4-1,5-COD)] (2a) (5 mol %) with BuLi (10 mol %) at 50 °C for 6 h in hexane, giving the cross-dimers in 99% yield ((E)-4,5-dimethyl-1-phenylhexa-1,4-diene (3a)/(E)-4,5-dimethyl-1-phenylhexa-2,4-diene (3b)/isomers = 84/9/7). Because neither 2a nor BuLi separately catalyzes the cross-dimerization and reduction of 2a with BuLi in the presence of naphthalene produces [Ru(η6-naphthalene)(η4-1,5-COD)] (1a), the active species in this catalysis is considered to be a Ru(0) compound. Interestingly, this in situ reduction method of Ru(II) using BuLi can be applied to the cross-dimerization using an ester such as methyl acrylate. Alternatively, an air-stable Ru(II) complex having a labile arene ligand such as [RuCl2(η6-anisole)]2 (5c) (5 mol %) with Na2CO3 (40 mol %) in the presence of 1,5-COD (20 mol %) at 100 °C for 6 h in 2-butanol also catalyzes the same cross-dimerization in 62% yield. These protocols provide facile methods for production of unsaturated linear compounds by the cross-dimerization using air-stable Ru(II) catalyst precursors. Show more