[2++4] Cycloadditions of Iminium Ions − Concerted or Stepwise Mechanism of Aza Diels−Alder Reactions?
作者:Herbert Mayr、Armin R. Ofial、Jürgen Sauer、Bernhard Schmied
DOI:10.1002/1099-0690(200006)2000:11<2013::aid-ejoc2013>3.0.co;2-a
日期:2000.6
kinetics of these hetero Diels−Alder reactions, which have been followed by dilatometry and 1H-NMR spectroscopy, obey second-order rate laws. Since the observed rate constants are only 30−300 times larger than those calculated for the stepwise process by the linear free-enthalpy relationship lg k = s (E + N), it is concluded that these cycloadditions proceed in a stepwise manner or through pericyclic
N,N-二甲基亚甲基铵离子 1 与 1,3-二烯 2a-2f 发生区域和立体选择性反应,生成 1,2,5,6-四氢吡啶鎓离子 3。这些杂 Diels-Alder 反应的动力学具有其次是膨胀法和 1H-NMR 光谱,服从二阶速率定律。由于观察到的速率常数仅比通过线性自由焓关系 lg k = s (E + N) 为逐步过程计算的速率常数大 30-300 倍,因此可以得出结论,这些环加成以逐步方式或通过周环进行两个新 σ 键的协同形成没有显着稳定的过渡态。相比之下,亚胺离子 1 与环戊二烯 (2g) 的反应产生 2,2-二甲基-2-azoniabicyclo[2.2。