Stereospecificity in the Reductive Coupling of 4-Pyridinecarboxaldehyde

Article abstract of DOI:10.1021/jo00322a032

One-electron reduction of 4-pyridinecarboxyaldehyde in aqueous solutions yields a mixture of dl- and meso-pinacols.Controlled-potential electrolysis (using a DME) at a potential corresponding to the limiting current of the first one-electron wave yielded dl/meso ratios of 0.57, 0.35, and 1.92 at pH values of 6.1, 10.6, and 13.3, respectively.The variation of the dl/meso ratios can be attributed to a difference in the surface orientation of radical cations (pH 6.1), uncharged radicals (pH 10.6), and radical anions (pH 13.3) that are involved in the dimerization at these different values of pH.For all dimerizations studied, voltammetric and polarographic results indicate a radical-radical coupling mechanism.