Department of Physical, Environmental, and Computer Sciences Medgar Evers College, The City University of New York, 1650 Bedford Avenue, Brooklyn, New York 11225, United States.
J Org Chem. 2012 Oct 5;77(19):8605-14. doi: 10.1021/jo301550y. Epub 2012 Sep 26.
The first computational investigations of the carbenoid reactions of α-lithiated dimethyl ether (methoxymethyllithium) and the intramolecular and intermolecular reactions of lithiated epoxides with the alkene double bond to yield cyclopropane rings are presented. These reactions represent the full spectrum of known carbenoid pathways to cyclopropanation. The reaction of Li-CH(2)-O-CH(3) with ethylene proceeds exclusively through a two-step carbolithiation pathway, the intramolecular reaction of 1,2-epoxy-5-hexene follows either the carbometalation or a concerted methylene transfer pathway (the former is energetically more favorable), and the reaction of lithiated ethylene oxide (oxiranyllithium) with ethylene, the main focus of this paper, appears to proceed exclusively by the methylene transfer mechanism. In the case of these latter reactions, the free energy of activation for cyclopropanation tends to decrease with the higher aggregation states. Formation of tetramers or higher aggregates is favorable in nonpolar solvents, but in strongly coordinating solvents such as tetrahydrofuran (THF), steric factors appear to limit aggregate sizes to the dimer. In the case of 1,2-epoxy-5-hexene, consideration of competing reaction pathways provide an explanation for the observed product distribution.
首次对α-锂化二甲醚（甲氧基甲基锂）的碳烯反应以及环氧化合物与烯烃双键的分子内和分子间反应生成环丙烷环进行了计算研究。这些反应代表了已知的所有碳烯环丙烷化途径。Li-CH(2)-O-CH(3)与乙烯的反应仅通过两步卡罗利蒂安化途径进行，1,2-环氧-5-己烯的分子内反应遵循碳金属化或协同亚甲基转移途径（前者在能量上更有利），而本文的主要重点是氧化亚乙基锂（环氧乙烷）与乙烯的反应似乎仅通过亚甲基转移机制进行。在后一类反应中，环丙烷化的活化自由能往往随聚合态的升高而降低。在非极性溶剂中，形成四聚体或更高的聚集体是有利的，但在强配位溶剂如四氢呋喃（THF）中，空间因素似乎将聚合体大小限制在二聚体。对于 1,2-环氧-5-己烯，考虑竞争反应途径可以解释观察到的产物分布。
