Electrosyntheses from Aromatic Aldehydes in a Flow Cell. Part II. The Cross-Coupling of Benzaldehydes to Unsymmetrical Diols.

Electroreductive C–C Coupling of Furfural and Benzaldehyde on Cu and Pb Surfaces

“…Similar cathodic shifts have been observed in the reduction peaks of organic halides upon halide anion (Cl – , Br – , and I – ) introduction . The strong competitive adsorption on Pb contrasts with a previous cross-coupling investigation in which the addition of 4-fluourbenzaldehdye did not significantly affect benzaldehyde reduction . In that case, the strong structural and electrochemical similarities between the two species (< 0.01 V difference in half wave potential) may explain the negligible effect of species interactions on the reduction.…”

“…This stronger benzaldehyde coupling supports the higher reduction onset potential for benzaldehyde in the mixed system as inferred from the CV data (Figure C and D). Additionally, the dimer predominance agrees with the sequential, one-electron reductions previously suggested for benzaldehyde reduction on Pb. ,, The presence of benzyl alcohol at −0.5 V (0.13 μmol h –1 cm –2 ) does suggest similar reduction potentials for the two steps, similar to the reduction on Cu, in agreement with the voltammograms (Figure C and D) and previous reactivity studies. , Stepping the potential further down results in a transition from benzaldehyde to furfural as the predominant self-coupling reaction and in the predominant reduction path from dimerization to alcohol production. The benzaldehyde to furfural transition occurs sharply between −0.6 and – 0.7 V, in agreement with the −0.63 and – 0.68 V reduction peaks observed for benzaldehyde and furfural, respectively, in the Pb voltammograms (Figure C and D).…”

“…The lack of radical peaks could result from the higher rate of radical consumption in the present case, with total dimerization rates 3.5 times larger than those of the previous work under acidic conditions (pH 4.6) . Alternatively, the smaller difference in aldehyde and radical reduction potentials at higher pH , may play a role by limiting the potential window in which radicals predominate. In addition to growth in product peaks, the reactant peaks decrease at lower potentials.…”

“…Cross-coupling remains less understood than self-coupling. To our knowledge, only one study covers the electroreductive cross-coupling of aldehydes, involving benzaldehyde, 4-fluorobenzaldehyde, and benzophenone on Pb . The study suggests stochastic control of dimer product selectivities, with a strong dependence on the reactant concentration.…”

“…Electrochemical parameters, such as pH, , catalyst, ,− and potential, ,, control the selectivity of the two pathways. Cross-coupling of different aldehydes can also occur electrochemically . Electroreductive coupling has several advantages as an upgrading technique.…”