The rates of catalytic dimerization of alkyl acrylates by PdCl2(NCPh)2and by [(C2H4)RhCl]2are greatly increased by the combination of a Lewis acid promoter and a proton source. For the rhodium systems, catalyst life was also enhanced and yields of up to 280 mol/mol Rh with > 98% selectivity to linear dialkyl hexenedioates have been observed. Optimal conditions involved the use of [(C2H4)2RhCl]2, FeCl3and methanol in the proportions 1:5:10 at 80 °C. The isomeric composition of the product is markedly influenced by the combination of catalyst and promoter employed; either cis-Δ2, trans-Δ2, or trans-Δ3hexenedioate may predominate. Thus on changing the promoter from FeCl3to SnCl4in the Rh-catalyzed system, the product distribution changed from 85% trans-Δ2to 62% cis-Δ2hexene-dionate; certain Pd-catalyzed systems produced up to 60% of the isomeric Δ3hexenedioates. © 1985.
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