Views:9 Author:Site Editor Publish Time: 2019-04-01 Origin:Site
During our investigations of phenolic oxidation reactions using BDD electrodes, cinnamic acid suppliers observed the generation of solvent-derived methoxy radicals that conducted an oxidation process of the phenol substrate to the corresponding coupling product . In our second investigation on the use of the BDD electrode in organic synthesis, the electrochemical reduction of methyl cinnamate (1a) was investigated to assess the applicability of BDD electrodes under cathodic reduction conditions, and to obtain new neolignan-type bioactive substances.
As shown in Figure 1, the radical intermediate derived from phenylacrylate through a one-electron reduction (right) differs from that obtained by anodic oxidation of 4-hydroxyphenyl-1-propene (left). Therefore, the reductive dimerization of cinnamic acid derivatives was expected to provide access to unprecedented neolignan-type dimeric compounds.
The ester methyl cinnamate (1a) was electrolyzed under constant current electrolysis (CCE) conditions in a divided cell. Solvents used for the reactions played a significant role in providing the desired coupling (Table 1, entries 1–5). Thus, only acetonitrile (Table 1, entry 5) gave the desired coupling product (±)-2  in 4% yield, recovered educt 1a and hydrolyzed product 1b. The undesired hydrolysis could be depressed using a phosphate-buffered solution in the cathodic cell (pH 7, Table 1, entries 7–11), and finally the optimized conditions for the synthesis of 2 (85% yield, racemate/meso = 74:26) were acquired in the case of 2.5 F/mol current (Table 1, entry 11).