Conjugated and Cross-Conjugated Mesomeric Betaines. Correlation of Electroreduction with Structure and Physiological Activity
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Electroreduction studies were performed on several cross- conjugated mesomeric betaines containing the fused pyrazolium (2) and fused imidazolium (3) ring systems. Studies at acidic pH were of principal interest. Substituent effects for 2 were in line with prior findings, and reduction potentials were comparatively negative (−0.96 to −1.34 V). Reduction potentials fit the modified Hammett equation. Compound 3 was more readily reduced (−0.88 V). The related ψ-oxatriazoles (6) gave values in the range of −0.85 to −1.22 V. The electrochemical characteristics are compared with those of the mesoionic syndnones (4) and synoneimines (5). These mesoionic compounds were generally reduced at more positive potentials than (2) and )3). A relatiosnhip between electroreduction and physiological activity is proposed. The overall results are in keeping with the hypothesis of widespread participation of iminium-type species in biological systems.