Electrochemical Reduction of 4-benzoyl-N-(4-substituted Benzyl)pyridinium Cations: Substitution Effects and Linear Free Energy Relationships
In analogy to 4-(p-substituted benzoyl)-N-methylpyridinium cations (1-Xs), the title species (2-Xs, X=---OCH3, ---CH3, ---H, ---Br, ---COCH3, ---NO2) undergo two reversible, well-separated (ΔE1/2≥650 mV) one-electron reductions. The effect of substitution on the E1/2s of 2-Xs is much weaker than the effect of the same substituents on 1-Xs: the Hammett ρ-values are 0.80 and 0.93 for the first- and second-electron reduction of 2-Xs vs. 2.3 and 3.3 for the same reductions of 1-Xs, respectively. The difference has been attributed to the different polarizability of π- vs. σ-electrons, and to the fact that in 1-Xs the electronic properties of the substituents are transmitted through the π-system, while in 2-Xs there is a ---CH2--- spacer. From a practical standpoint, these results suggest that the 4-benzoylpyridinium system can be used as an almost continuously tunable redox couple, whose redox potential can be course-tuned via p-benzoyl substitution and fine-tuned via p-benzyl substitution. Importantly, the nitro group of 2-NO2 undergoes reduction before the second-electron reduction of the 4-benzoylpyridinium system. Introducing the recently derived substituent constant of the ---NO2−√ group (σp-NO2−√−&z.rad;=−0.97) yields an excellent correlation for the third-electron reduction of 2-NO2 (corresponding to the reduction of the carbonyl group) with the second-electron reduction of the other 2-Xs, and confirms the electron-donating properties of ---NO2−√.
N. Leventis et al., "Electrochemical Reduction of 4-benzoyl-N-(4-substituted Benzyl)pyridinium Cations: Substitution Effects and Linear Free Energy Relationships," Electrochimica Acta, Elsevier, Aug 2003.
The definitive version is available at http://dx.doi.org/10.1016/S0013-4686(03)00414-6
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