Aldehydes half-wave potentials

The polarographic reduction of benzo[h]quinolizinium salts varied with pH. At pH 1.9 to 7 two waves were observed, the first (half-wave —0.807 V) a one-electron reversible reduction, the second (half-wave —1.022 V) irreversible and diffusion controlled. The proposed pathway is shown in Eq. (46). The reduction in basic medium also had two waves (half-wave potentials about — 1.5 and — 2.2 V) due to the reduction of aldehyde 103, a known product of nucleophilic attack at position 6 in benzo[h]quinolizinium salts. The changes in reduction potential produced by substituents on the benzo-[hjquinolizinium ion were examined (19 examples).123 Various explanations based on inductive and mesomeric effects were offered for observed variations. 

Mass spectrometry (22, 96,181, 530, 531) and NMR spectroscopy (65, 69,181,209,431) have been recorded. The UV spectra of the dienone alkaloids differ in the extinction maxima (proaporphine alkaloids at 220 nm (e 24800) and 285 nm (c 3000) promorphinane alkaloids 230 nm (e 24,000) and 285 nm (e 7500)). IR Spectroscopy reveals characteristic frequencies of the cross-conjugated dienone at 1655, 1630, and 1613 cm"i. On polarography the proaporphine and the promorphinane dienone compoimds are reduced (335) at a half-wave potential similar to that of aromatic aldehydes. 

The carbonyl group, including aldehydes, ketones, and quinones, produce voltammetric waves. In general, aldehydes are reduced at lower potentials than ketones conjugation of the carbonyl double bond also results in lower half-wave potentials. 

A simple one-electron oxidation process appears to be excluded by the lack of a correlation between the observed reactivities and the half-wave oxidation potentials of the olefins from the literature. What is the role of the sacrificial aldehyde The answer to this question is a good entry point into the mechanism of the transformation. We... 

The electronics and structure of the acceptors, especially a, 3-unsaturated ketones, is also a determinant in 1,2- vs. 1,4-addition processes. In general, substitution of aryl or large groups at the carbonyl unit increases the preference for 1,4-addition,16ab while a, 3-unsaturated aldehydes afford exclusive 1,2-addition and [3,(3-disubstitution suppresses 1,4-addition,l6c presumably due to steric hinderance. House and Seyden-Penne have established good correlations between chemoselectivity and either the half-wave electrolytic reduction potentials,17 or the energy levels of the LUMO of various a, 3-unsaturated ketones.17b... 

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