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Carbenoid Alkene Insertion Reactions of Oxiranyllithiums

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journal contribution
posted on 05.10.2012, 00:00 authored by Lawrence M. Pratt, Binh Khanh Mai, B. Ramu Ramachandran
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–CH2–O–CH3 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.