Leventis Nicholas, Rawaswdeh Abdel-Monem M, Zhang Guohui, Elder Ian A, Sotiriou-Leventis Chariklia
Department of Chemistry, University of Missouri-Rolla, Rolla, Missouri 65409, USA.
J Org Chem. 2002 Oct 18;67(21):7501-10. doi: 10.1021/jo020489+.
In anhydrous CH3CN a series of nine 4-(4-substituted-benzoyl)-N-methylpyridinium cations (substituent: -OCH3, -CH3, -H, -SCH3, -Br, -Ctbd1;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 E(1/2) values of both waves and the substituent constants sigma(p)(-)(X). The reaction constants for the two 1-e reductions are rho(1) = 2.60 and rho(2) = 3.31. The lower rho(1) value is associated with neutralization of the pyridinium ring, and the higher rho(2) 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 sigma(p)(-NO)2*- = -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.
在无水乙腈中，一系列九个4-(4-取代苯甲酰基)-N-甲基吡啶阳离子（取代基：-OCH₃、-CH₃、-H、-SCH₃、-Br、-Ctbd1;CH、-CHO、-NO₂和-(+)S(CH₃)₂）在与醌类和紫精等其他主流氧化还原催化剂相同的电位范围内表现出两个化学可逆、分离良好的单电子（1-e）还原。哈米特线性自由能图显示，两个波的E(1/2)值与取代基常数sigma(p)(-)(X)之间具有良好的相关性。两次单电子还原的反应常数分别为rho(1)=2.60和rho(2)=3.31。较低的rho(1)值与吡啶环的中和有关，较高的rho(2)值与第二次电子还原过程中产生的负电荷有关。结构-功能相关性表明，在两次单电子还原中，取代作用都具有纯粹的诱导作用。4-(4-硝基苯甲酰基)-N-甲基吡啶阳离子的情况特别值得注意，因为4-硝基苯甲酰基部分在4-苯甲酰基-N-甲基吡啶体系的第二次还原之前就发生了还原。该化合物的第三个波与其他化合物的第二次电子还原的相关性得出sigma(p)(-NO)2*-=-0.97±0.02，因此将-NO₂-*基团置于最强的电子供体之中。溶剂氘同位素效应以及静电势图（通过PM3计算）作为取代作用的函数表明，双电子还原形式通过O原子与质子供体（如CH₃OH）形成氢键。进入氢键缔合的CH₃OH分子的平均数量随着给电子取代基的增加而增加。氢键作用使第二次还原波更靠近第一次。这具有重要的实际意义，因为它增加了由单电子还原形式歧化产生的双电子还原形式的平衡浓度。
