Electrosyntheses from aromatic aldehydes in a flow cell. Part II. The cross-coupling of benzaldehydes to...

It is demonstrated that the reduction of mixtures of two benzaldehydes in acidic water-methanol in a membrane flow cell with a lead cathode can lead to a mixture of diols including the unsymmetrical diol, e.g. 4-F-C6H4-CH(OH)-CH(OH)-C6H5. In the conditions employed, the ratio of the three diol products follows the statistical distribution expected for the coupling of two different radical intermediates produced in the same ratio as the concentrations of their precursors. The yield and current efficiency for the diols are excellent when the pH and potential are selected so that a le- reduction of both benzaldehydes occurs. The yield of unsymmetrical dimer based on one reactant can be increased substantially by using the other carbonyl compound as a sacrificial reagent.