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  • Methylene Transfer or Carbometalation? A Theoretical Study toDetermine the Mechanism of Lithium Carbenoid-PromotedCyclopropanation Reactions in Aggregation and Solvation States
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  • Density functional theory calculations for the lithium carbenoid-promoted cyclopropanations in aggregationand solvation states are presented in order to investigate the controversy of the mechanistic dichotomy,that is, the methylene-transfer mechanism and the carbometalation mechanism. The methylene-transfermechanism represents the reaction reality, whereas the carbometalation pathway does not appear to competesignificantly with the methylene-transfer pathway and should be ruled out as a major factor. A simplemodel calculation for monomeric lithium carbenoid-promoted cyclopropanations with ethylene in thegas phase is not sufficient to reflect the reaction conditions accurately or to determine the reactionmechanism since its result is inconsistent with the experimental facts. The aggregated lithium carbenoidsare the most probable reactive species in the reaction system. The calculated reaction barriers of themethylene-transfer pathways are 10.1 and 8.0 kcal/mol for the dimeric (LiCH2F)2 and tetrameric (LiCH2F)4species, respectively, compared with the reaction barrier of 16.0 kcal/mol for the monomeric LiCH2Fspecies. In contrast, the reaction barriers of the carbometalation pathways are 26.8 kcal/mol for the dimeric(LiCH2F)2 and 33.9 kcal/mol for the tetrameric (LiCH2F)4 species, compared with the reaction barrier of12.5 kcal/mol for the monomeric LiCH2F species. The effects of solvation were investigated by explicitcoordination of the solvent molecules to the lithium centers. This solvation effect is found to enhancethe methylene-transfer pathway, while it is found to impede the carbometalation pathway instead. Thecombined effects of the aggregation and solvation lead to barriers to reaction in the range of 7.2−9.0kcal/mol for lithium carbenoid-promoted cyclopropanation reactions along the methylene-transfer pathway.Our computational results are in good agreement with the experimental observations.
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