Cyclopropanation Reactions of Halomethyllithium Carbenoids: A Computational Study of the Effects of Aggregation and Solvation

Computational results are presented to support experimental evidence that cyclopropanation reactions of halomethyllithium carbenoids with alkenes occur through the concerted (methylene transfer) pathway rather than the alternate stepwise (carbometalation) pathway in non-polar and polar (THF, ε = 7.85) media. These results complement and confirm the recent findings of Ke, Zhao, and Phillips (KZP) [J. Org. Chem.2007, 72, 848] for ethylene in nonpolar and moderately polar (DME, ε = 4.34) solvents. We also studied carbenoid reactions with 2,3-dimethyl-2-butene. The explanation for the preference for the concerted pathway is provided by detailed examination of the reaction pathways of aggregated carbenoid species, which are the dominant reactive species in both polar and nonpolar solvents. The details of the syn and anti eliminations of lithium halide from the intermediate formed in the two-step mechanism with ethylene are also studied for the first time. Two stable dimeric structures were identified for the carbenoids and both have been studied. By explicit coordination of the monomeric and dimeric organolithium species to THF ligands, we also find that a clear preference for the concerted pathway is shown by both monomers and dimers in polar solvents, and solvent steric factors are dominant in determining the relative stabilities of the aggregates of the carbenoid species in THF solution.