Baciocchi Enrico, Calcagni Alessandra, Lanzalunga Osvaldo
Dipartimento di Chimica and Istituto CNR di Metodologie Chimiche (IMC-CNR), Sezione Meccanismi di Reazione, Sapienza Università di Roma, piazzale A. Moro 5, Rome, Italy.
J Org Chem. 2008 Jun 6;73(11):4110-5. doi: 10.1021/jo8001672. Epub 2008 May 7.
The reactivity of the 2,2-diphenyl-1-picrylhydrazyl radical (dpph*) toward the N-methyl C-H bond of a number of 4-X-substituted- N, N-dimethylanilines (X = OMe, OPh, CH 3, H) has been investigated in MeCN, in the absence and in the presence of Mg(ClO 4) 2, by product, and kinetic analysis. The reaction was found to lead to the N-demethylation of the N, N-dimethylaniline with a rate quite sensitive to the electron donating power of the substituent (rho (+) = -2.03). With appropriately deuterated N, N-dimethylanilines, the intermolecular and intramolecular deuterium kinetic isotope effects (DKIEs) were measured with the following results. Intramolecular DKIE [( k H/ k D) intra] was found to always be similar to intermolecular DKIE [( k H/ k D) inter]. These results suggest a single-step hydrogen transfer mechanism from the N-C-H bond to dpph* which might take the form of a concerted proton-electron transfer (CPET). An electron transfer (ET) step from the aniline to dpph* leading to an anilinium radical cation, followed by a proton transfer step that produces an alpha-amino carbon radical, appears very unlikely. Accordingly, a rate-determining ET step would require no DKIE or at least different inter and intramolecular isotope effects. On the other hand, an equilibrium-controlled ET is not compatible with the small slope value (-0.22 kcal (-1) K (-1)) of the log k H/Delta G degrees plot. Furthermore, the reactivity increases by changing the solvent to the less polar toluene whereas the reverse would be expected for an ET mechanism. In the presence of Mg (2+), a strong rate acceleration was observed, but the pattern of the results remained substantially unchanged: inter and intramolecular DKIEs were again very similar as well as the substituent effects. This suggests that the same mechanism (CPET) is operating in the presence and in the absence of Mg (2+). The significant rate accelerating effect by Mg (2+) is likely due to a favorable interaction of the Mg (2+) ion with the partial negatively charged alpha-methyl carbon in the polar transition state for the hydrogen transfer process.
通过产物分析和动力学分析，研究了2,2 - 二苯基 - 1 - 苦基肼自由基（dpph*）与多种4 - X - 取代 - N, N - 二甲基苯胺（X = OMe、OPh、CH₃、H）的N - 甲基C - H键在乙腈中以及有无Mg(ClO₄)₂存在时的反应活性。发现该反应会导致N, N - 二甲基苯胺发生N - 去甲基化，反应速率对取代基的给电子能力相当敏感（ρ(+) = -2.03）。使用适当氘代的N, N - 二甲基苯胺，测量了分子间和分子内的氘动力学同位素效应（DKIEs），结果如下。发现分子内DKIE [ (kH/kD)intra ] 总是与分子间DKIE [ (kH/kD)inter ] 相似。这些结果表明存在从N - C - H键到dpph的单步氢转移机制，其可能采取协同质子 - 电子转移（CPET）的形式。从苯胺到dpph的电子转移（ET）步骤导致苯胺自由基阳离子的形成，随后是产生α - 氨基碳自由基的质子转移步骤，这种情况似乎极不可能。因此，速率决定的ET步骤不需要DKIE，或者至少分子间和分子内同位素效应不同。另一方面，平衡控制的ET与log kH/ΔG°图的小斜率值（-0.22 kcal⁻¹K⁻¹）不相符。此外，将溶剂换成极性较小的甲苯时反应活性增加，而对于ET机制则预期相反。在Mg²⁺存在下，观察到强烈的速率加速，但结果模式基本保持不变：分子间和分子内DKIEs以及取代基效应再次非常相似。这表明在有和没有Mg²⁺存在时都运行着相同的机制（CPET）。Mg²⁺产生的显著速率加速效应可能是由于Mg²⁺离子与氢转移过程极性过渡态中部分带负电荷的α - 甲基碳发生了有利的相互作用。
