Tuning the Redox Chemistry of 4-benzoyl-N-methylpyridinium Cations through Para Substitution. Hammett Linear Free Energy Relationships and the Relative Aptitude of the Two-electron Reduced Forms for H-bonding
In anhydrous CH3CN a series of nine 4-(4-substituted-benzoyl)-N-methylpyridinium cations (substituent: −OCH3, −CH3, −H, −SCH3, −Br, −C CH, −CHO, −NO2, and −+S(CH3)2) demonstrate two chemically reversible, well-separated one-electron (1-e) reductions in the same potential range as other main stream redox catalysts such as quinones and viologens. Hammett linear free energy plots yield excellent correlation between the E1/2 values of both waves and the substituent constants σp-X. The reaction constants for the two 1-e reductions are ρ1 = 2.60 and ρ2 = 3.31. The lower ρ1 value is associated with neutralization of the pyridinium ring, and the higher ρ2 value with the negative charge developing during the 2nd-e reduction. Structure−function correlations point to a purely inductive role for substitution in both 1-e reductions. The case of the 4-(4-nitrobenzoyl)-N-methylpyridinium cation is particularly noteworthy, because the 4-nitrobenzoyl moiety undergoes reduction before the 2nd reduction of the 4-benzoyl-N-methylpyridinium system. Correlation of the third wave of this compound with the 2nd-e reduction of the others yields σp-NO2•− = −0.97 ± 0.02, thus placing the −NO2•- group among the strongest electron donors. Solvent deuterium isotope effects and maps of the electrostatic potential (via PM3 calculations) as a function of substitution support that 2-e reduced forms develop H-bonding with proton donors (e.g., CH3OH) via the O-atom. The average number of CH3OH molecules entering the H-bonding association increases with e-donating substituents. H-bonding shifts the 2nd reduction wave closer to the first one. This has important practical implications, because it increases the equilibrium concentration of the 2-e reduced form from disproportionation of the 1-e reduced form.
N. Leventis et al., "Tuning the Redox Chemistry of 4-benzoyl-N-methylpyridinium Cations through Para Substitution. Hammett Linear Free Energy Relationships and the Relative Aptitude of the Two-electron Reduced Forms for H-bonding," Journal of Organic Chemistry, American Chemical Society (ACS), Sep 2002.
The definitive version is available at http://dx.doi.org/10.1021/jo020489+
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