The high reactivity of 2‐methylene‐1,2‐dihydropyridines also known as 2‐methylpyridinium derived N‐heterocyclic olefins (2‐pyNHOs) has been recognized in organic synthesis, yet a quantification of their nucleophilicity is lacking. Herein we used stopped‐flow photometry to determine the nucleophilicity of a series of 2‐pyNHOs from the kinetics of their reactions with quinone methides and benzhydrylium ions as reference electrophiles in four organic solvents at 20 °C. The kinetic data was evaluated by using the Mayr‐Patz equation, lg k(20 °C) = sN(N + E), which gave nucleophilicity parameters N (and sN). With N in the range of 19.4 to 21.2 (in DMSO), 2‐pyNHOs exceed the reactivity of classical enamines, such as pyrrolidino‐cyclopent‐1‐ene. The addition of 2‐pyNHOs to quinone methides resulted in the formation of zwitterionic adducts with pyridinium and phenolate moieties. Subsequent tautomerization yielded entirely neutral pyridine‐2(1H)‐ylidene‐phenol species in several cases. Formation of the zwitterionic adducts from 2‐pyNHOs and neutral electrophiles was almost equally fast in the polar solvents acetonitrile and DMSO, but proceeded one to two orders of magnitude slower in the less polar solvents dichloromethane or THF.

中文翻译：

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2-亚甲基-1,2-二氢吡啶（2-pyNHOs）：高亲核烯胺

2-亚甲基-1,2-二氢吡啶也称为2-甲基吡啶衍生的N-杂环烯烃（2-pyNHOs）的高反应性已在有机合成中得到认可，但缺乏对其亲核性的量化。在此，我们使用停流光度法，根据 2-pyNHO 在四种有机溶剂中于 20 °C 下与醌甲基化物和二苯甲基鎓离子作为参考亲电子试剂的反应动力学，确定了一系列 2-pyNHO 的亲核性。使用 Mayr-Patz 方程 lg k(20 °C) = sN(N + E) 评估动力学数据，得出亲核参数 N（和 sN）。当 N 的范围为 19.4 至 21.2（在 DMSO 中）时，2-pyNHO 超过了经典烯胺的反应活性，例如吡咯烷-环戊-1-烯。将 2-pyNHO 添加到醌甲基化物中，导致与吡啶鎓和酚盐部分形成两性离子加合物。随后的互变异构化在一些情况下产生了完全中性的吡啶-2(1H)-亚基-苯酚物质。在极性溶剂乙腈和 DMSO 中，2-pyNHO 和中性亲电子试剂形成两性离子加合物的速度几乎相同，但在极性较小的溶剂二氯甲烷或 THF 中，形成速度要慢一到两个数量级。