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

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.