Mechanistic competition variations due to the substituents in the lithium carbenoid promoted cyclopropanation reactions

Abstract

The cylcopropanation reactions of the LiCH 2X (X = F, Cl, Br and I) carbenoids with ethylene were investigated at the CCSD(T)/6-311G ∗∗//B3LYP/6-311G ∗∗ level of theory along two reaction pathways: methylene transfer and carbometalation. There exists a competition between these two reaction pathways for the different substituted lithium carbenoids. Interestingly, the substituent has different effect on the methylene transfer and carbometalation pathways. The trend of the activation energies for the methylene transfer pathway is LiCH 2F (9.8 kcal/mol) > LiCH 2Cl (7.6 kcal/mol) ≈ LiCH 2Br (7.4 kcal/mol) ≈ LiCH 2I (7.5 kcal/mol), whereas the activation energies for the carbometalation pathway increases in this order: LiCH 2F (6.1 kcal/mol) < LiCH 2Cl (7.1 kcal/mol) < LiCH 2Br (8.2 kcal/mol) < LiCH 2I (8.5 kcal/mol). The different effect mainly arises from that the substituent of the lithium carbenoid influences the hybridization character of the C 1 atom. The mechanistic competition varies due to the different substituents of the lithium carbenoids during the cyclopropanation reactions. This result is revelatory for us to control mechanistic competition to obtain target product by modifying the substituents of the lithium carbenoids.