Density functional theory investigation of the reactions of isodihalomethanes (CH2X-X where X = Cl, Br, or I) with ethylene: Substituent effects on the carbenoid behavior of the CH2X-X species

Abstract

We investigated the chemical reactions of isodihalomethane (CH 2X-X) and CH 2X radical species (where X = Cl, Br, or I) with ethylene and the isomerization reactions of CH 2X-X using density functional theory calculations. The CH 2X-X species readily reacts with ethylene to give the cyclopropane product and an X 2 product via a one-step reaction with barrier heights of ̃2.9 kcal/mol for CH 2I-I, 6.8 kcal/mol for CH 2Br-Br, and 8.9 kcal/mol for CH 2Cl-Cl. The CH 2X reactions with ethylene proceed via a two-step reaction mechanism to give a cyclopropane product and X atom product with much larger barriers to reaction. This suggests that photocyclopropanation reactions using ultraviolet excitation of dihalomethanes most likely occurs via the isodihalomethane species and not the CH 2X species. The isomerization reactions of CH 2X-X had barrier heights of ̃14.4 kcal/mol for CH 2I-I, 11.8 kcal/mol for CH 2Br-Br, and 9.1 kcal/mol for CH 2Cl-Cl. We compare our results for the CH 2X-X carbenoids to results from previous calculations of the Simmons-Smith-type carbenoids (XCH 2ZnX) and Li-type carbenoids (LiCH 2X) and discuss their differences and similarities as methylene transfer agents.