Arrieta Ana, Cossío Fernando P., Lecea Begoña
Farmazi Fakultatea, Euskal Herriko Unibertsitatea, P.K. 450, 01080 Vitoria-Gasteiz, Spain.
J Org Chem. 1999 Mar 19;64(6):1831-1842. doi: 10.1021/jo9815002.
The different reaction paths associated with the formal [2 + 2] thermal cycloaddition between several keteniminium cations and imines has been studied computationally. It is found that the reaction takes place via stepwise mechanisms that involve the sequential formation of the N1-C2 and the C3-C4 bonds. In some cases, the second step of the reaction is subjected to torquoelectronic effects and determines its stereochemical outcome. Under these conditions, preferential or exclusive formation of cis cycloadducts is predicted, in good agreement with part of the experimental evidence available. When chloroenamines are used as precursors of the keteniminium ions, the step in which the C3-C4 bond is formed can consist of an intramolecular S(N)2 reaction. Under these conditions, the stereocontrol of the reaction is significantly lower and the trans isomer is preferentially formed. According to our results, generation of keteniminium cations from enamines having good leaving groups is recommended to improve the stereocontrol of the reaction.
通过计算研究了几种烯酮亚胺阳离子与亚胺之间形式上的[2 + 2]热环加成反应的不同反应路径。结果发现，该反应通过逐步机理进行，涉及N1-C2键和C3-C4键的依次形成。在某些情况下，反应的第二步受到扭转电子效应的影响，并决定其立体化学结果。在这些条件下，预测会优先或专一地形成顺式环加成产物，这与部分现有实验证据吻合良好。当氯代烯胺用作烯酮亚胺离子的前体时，形成C3-C4键的步骤可能由分子内S(N)2反应组成。在这些条件下，反应的立体控制明显较低，优先形成反式异构体。根据我们的结果，建议从具有良好离去基团的烯胺生成烯酮亚胺阳离子，以改善反应的立体控制。
