I ' Substitution Effects and Linear Free Energy Relationships during Reduction of 4 - Benzoyl - N - ( 4 - substituted benzy 1 ) pyridinium Cations

In analogy to 4-b-substituted benzoyl)-N-methylpyridinium cations (1 -X's), the title species (2-X's, -X = -0CH3, -CH3, -H, -Br, -COCH3, -N02) undergo two reversible, well-separated (E1122650 mV) one-electron reductions. The effect of substitution on the reduction potentials of 2-X's is much weaker than the effect of the same substituents on 1-X's: the Hammett p-values are 0.80 and 0.93 for the 1"and 2nd-e reduction of 2-X's vs. 2.3 and 3.3 for the same reductions of 1-X's, respectively. Importantly, the nitro group of 2-NO2 undergoes reduction before the 2nd-e reduction of the 4benzoylpyridinium system. These results suggest that the redox potentials of the 4benzoylpyridinium system can be course-tuned via p-benzoyl substitution and fine-tuned via p-benzyl substitution. Introducing the recently derived substituent constant of the -N02'group ( cp-No; = -0.97) yields an excellent correlation for the 3'd-e reduction of 2-