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

We investigated the chemical reactions of isodihalomethane (CH2X-X) and CH2 X radical species (where X = Cl, Br, or I) with ethylene and the isomerizat ion reactions of CH2X-X using density functional theory calculations. The C H2X-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 similar to2.9 kcal/ mol for CH2I-I, 6.8 kcal/mol for CH2Br-Br, and 8.9 kcal/mol fo r CH2Cl-Cl. The CH2X reactions with ethylene proceed via a two-step reactio n mechanism to give a cyclopropane product and X atom product with much lar ger barriers to reaction. This suggests that photocyclopropanation reaction s using ultraviolet excitation of dihalomethanes most likely occurs via the isodihalomethane species and not the CH2X species. The isomerization react ions of CH2X-X had barrier heights of similar to 14.4 kcal/mol for CH2I-I, 11.8 kcal/mol for CH2Br-Br, and 9.1 kcal/mol for CH2CI-Cl. We compare our r esults for the CH2X-X carbenoids to results from previous calculations of t he Simmons-Smith-type carbenoids (XCH2ZnX) and Li-type carbenoids (LiCH2X) and discuss their differences and similarities as methylene transfer agents .