Linear dimerization of acrylates by palladium and rhodium catalysts: effect of lewis and protic acid additives

Abstract

The rates of catalytic dimerization of alkyl acrylates by PdCl 2(NCPh) 2 and by [(C 2H 4)RhCl] 2 are 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 [(C 2H 4) 2RhCl] 2, FeCl 3 and 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-Δ 2, trans-Δ 2, or trans-Δ 3 hexenedioate may predominate. Thus on changing the promoter from FeCl 3 to SnCl 4 in the Rh-catalyzed system, the product distribution changed from 85% trans-Δ 2 to 62% cis-Δ 2 hexene-dionate; certain Pd-catalyzed systems produced up to 60% of the isomeric Δ 3 hexenedioates.