López Carlos Silva, Nieto Faza Olalla, Souto José A, Alvarez Rosana, De Lera Angel R
Departamento de Química Orgánica, Facultade de Química, Universidade de Vigo, Lagoas Marcosende, E-36310, Vigo, Galicia, Spain.
J Comput Chem. 2007 Jun;28(8):1411-6. doi: 10.1002/jcc.20620.
Antara-antara [1,5]-methylene sigmatropic shifts of propenylidene cyclopropane and its heteroanalogues are shown to proceed through concerted or stepwise mechanisms depending on a delicate balance that is directly related to the presence or absence of heteroatoms at the termini atoms of the rearrangement. Lowering of activation energies upon heteroatom replacement are more significant for the oxacyclopropane analogues 1D, 1E, and 1F but double heterosubstitution is still needed to achieve a pseudopericyclic reaction. In fact only after reaching a pseudopericyclic process the energy balance favors the concerted pathway. Negligible NICS values and a clear disconnection in the ACID plot as well as some structural and energetical parameters reveal the pseudopericyclic nature of this concerted antara-antara 1,5 sigmatropic shift. Although the product of the pseudopericyclic process is unprecedented, similar cycloisomerizations after allene activation should be considered within this hitherto undescribed reactions.
亚丙基环丙烷及其杂类似物的[1,5]-亚甲基迁移反应显示，根据与重排末端原子上杂原子的存在与否直接相关的微妙平衡，反应可通过协同或分步机制进行。对于氧杂环丙烷类似物1D、1E和1F，杂原子取代后活化能的降低更为显著，但仍需要双重杂取代才能实现假周环反应。实际上，只有在达到假周环过程后，能量平衡才有利于协同途径。可忽略不计的NICS值、ACID图中的明显断开以及一些结构和能量参数揭示了这种协同的亚丙基-亚丙基1,5迁移反应的假周环性质。尽管假周环过程的产物是前所未有的，但在这个迄今为止未描述的反应中，应考虑丙二烯活化后的类似环异构化反应。
