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

Masafumi Hirano,Hideyuki Kobayashi,Annie L Colebatch,Yuki Hiroi,Takao Ueda,Martin A Bennett,Ryota Abe,Nobuyuki Komine

doi:10.1021/acs.organomet.7b00882

Masafumi Hirano, Hideyuki Kobayashi + Show 6 more

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https://doi.org/10.1021/acs.organomet.7b00882

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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 ...

Highlights

Catalytic cross-dimerization between conjugated dienes and substituted alkenes is a reliable and promising method for production of linear organic molecules with high atom and step economy
We focused on the air- and moisture-stable compound [Ru(acac)2(η4-1,5-COD)] (2a) as a catalyst precursor for the present cross-dimerization
We employed the cross-dimerization of 2,3-dimethylbutadiene with styrene as a model reaction, because this reaction is catalyzed by the Ru(0) complex [Ru(η6naphthalene)(η4-1,5-COD)] (1a) and the reaction mechanism is well-established.20a Table 2 illustrates the results of screening of several reductants for 2a

Summary

Introduction

Catalytic cross-dimerization between conjugated dienes and substituted alkenes is a reliable and promising method for production of linear organic molecules with high atom and step economy. Following the first brief report by Wittenberg of Cocatalyzed cross-dimerizations of dienes,[1] various catalytic systems based on transition metal compounds from groups 10 to 8 have been reported (Table 1).[2−15] We have documented regio- and enantioselective cross-dimerizations between conjugated dienes and substituted alkenes catalyzed by [Ni(η4-1,5-COD)2]/AlEt3/PPh(OMe). [Ru(η6-naphthalene)(η4-cyclic diene)] (1),[16] where the cyclic diene ligand acts as an important ancillary ligand for the activity and selectivity. Typical examples are the cross-dimerization of 1,3-dienes with acrylates,[17] the tail-to-tail dimerization of acrylates,[18] and the chemo- and stereoselective cross-dimerization of methyl methacrylate or methacrylamide with unsaturated five-membered-ring compounds.[19]

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