Stoichiometric and catalytic cross dimerization between conjugated dienes and conjugated carbonyls by a ruthenium(0) complex: Straightforward access to unsaturated carbonyl compounds by an oxidative coupling mechanism

Article abstract of DOI:10.1021/om300234d

A series of stoichiometric and catalytic cross dimerizations between conjugated dienes and conjugated carbonyls are studied. The reaction of Ru(η⁴-cisoid-1,3-butadiene)(η⁴-1,5-COD)(NCMe) (2a) with methyl acrylate gives a Ru(0) complex, Ru[methyl η⁴-cisoid- (2E,4E)-hepta-2,4-dienoate](η⁴-1,5-COD)(NCMe) (3aa) in 97% yield. Similar treatments of 2a with a series of tert-butyl acrylate, methyl crotonate, 3-buten-2-one, and N,N-dimethylacrylamide produce similar analogues of 3ac. When (E)-1,3-pentadiene complex 2d is employed in the reaction with methyl acrylate, the branched coupling product Ru[methyl η⁴- cisoid-(2E,4E)-4-methylhepta-2,4-dienoate](η⁴-1,5-COD)(NCMe) (3da-b) is dominantly obtained in 65% yield along with the linear product in 19% yield. In the case of the (E)-2,5-dimethylhexa-1,3-diene complex 2e, the corresponding branch product is exclusively obtained in 86% yield. The catalytic cross dimerizations between conjugated dienes and conjugated carbonyls are established by 2. The origin of the present chemoselectivity is the η⁴-coordination of a conjugated diene and η²- coordination of an electron-deficient alkene to formal 6e coordination sites at Ru(0), and the regioselectivity being prone to giving branched products is interpreted as an oxidative coupling mechanism, involving nucleophilic attack of the coordinated diene to the coordinated electron-deficient alkene.