Theoretical Study of Samarium (II) Carbenoid (ISmCH2I) Promoted Cyclopropanation Reactions with Ethylene and the Effect of THF Solvent on the Reaction Pathways

Abstract

A computational study of the cyclopropanation reactions of divalent samarium carbenoid ISmCH2I with ethylene is presented. The reaction proceeds through two competing pathways: methylene transfer and carbometalation. The ISmCH2I species was found to have a "samarium carbene complex" character with properties similar to previously investigated lithium carbenoids (LiCH2X where X = CI, Br, I). The ISmCH 2I carbenoid was found to be noticeably different in structure with more electrophilic character and higher chemical reactivity than the closely related classical Simmons-Smith (IZnCH2I) carbenoid. The effect of THF solvent was investigated by explicit coordination of the solvent THF molecules to the Sm (II) center in the carbenoid. The ISmCH 2I/(THF)n (where n = 0, 1, 2) carbenoid methylene transfer pathway barriers to reaction become systematically lower as more THF solvent is added (from 12.9 to 14.5 kcal/mol for no THF molecules to 8.8 to 10. 7 kcal/mol for two THF molecules). In contrast, the reaction barriers for cyclopropanation via the carbometalation pathway remain high (> 15 kcal/mol). The computational results are briefly compared to other carbenoid reactions and related species.