Syntheses with Electrogenerated Halogens: Part II . Oxidation of Malonate Esters

The indirect electrochemical oxidation of malonate esters (I) to ethane‐1,1,2,2‐tetracarboxylate esters (II) has been investigated in greater detail than previously reported. With acetonitrile or an alcohol as the solvent, ethenetetracarboxylate (III), propane‐1,1,2,2,3,3‐hexacarboxylate (IV), and ethane‐1,1,2‐tricarboxylate (V) esters were also formed. If the conversion of (I) was limited to 70%, by‐product formation was low and 98% yields of (II) were obtained. In acetonitrile, current efficiency was comparable with the chemical yield, but with alcoholic solvents it was lower (70–80%) due to competing indirect oxidation of the alcohol to the corresponding alkyl formate (VI). The similar coupling of monosubstituted malonate esters (VII ; giving VIII) shows a marked steric effect similar to that observed in some cyanoethylation reactions, suggesting that the coupling occurs via a nucleophilic displacement reaction. The steric effect rationalizes the absence of tetrameric and higher products. The similar cyclization of some bismalonate esters (IX ; giving X) is also reported.