(111f) A Scalable Synthesis of Dimethyl 2,2'-Bifuran-5,5'-Dicarboxylate Via the Oxidative Coupling of 2-Methyl Furoate

To mitigate the environmental impact of plastic waste and reduce our dependence on fossil fuels, we must find renewable alternatives with desirable combinations of physical and chemical properties. Recent investigations have shown that bifuran-based polyesters could afford such materials: poly(ethylene bifuroate), for example, has shown attractive UV blocking and gas barrier properties. The bifuroate can be prepared by the selective C-C homocoupling of methyl furoate via Heck-type reaction, but this requires halide substitution and inert reaction atmosphere. To prepare bifuroate from less expensive ingredients, alternatives to the selective C-C homocoupling need to be found. A recent report has shown the feasibility of oxidative coupling of furoate using homogeneous Pd(II) catalysts. This reaction mixture, however, has many components and the yields are low for practical purposes.

After optimization of reaction conditions, we found the reaction of methyl furoate, palladium acetate and sodium pivalate—as catalyst promoter—in dimethylacetamide (DMAc) at moderate temperatures (140 °C) generates acceptable yields of the product (11.4%) in only 3.5 min. The selectivity to the 5,5’ isomer is very high, with turnover frequency (TOF) in excess of 500 h^-1. The reaction kinetics were investigated, and a possible mechanism and rate determine step (RDS) are proposed. The amount of catalyst, concentration of reactant, and temperature have all significant impact on the rate and yield, while oxygen pressure shows little influence. We conclude that the reductive elimination step is not the RDS. Trans-metalation of two Pd-activated furoates appears to be the most likely RDS because of the observation of the reaction order of 2.5 on Pd. It is also found that the solubility of product is strongly temperature-dependent, a property that can be used as the basis for a scalable continuous separation method. Recycle tests show that no major deactivation occurs during at least 4 reaction cycles.