DFT studies on the mechanism of the cycloaddition reaction between methyleneketene and 5-methylene-1,3-dioxan-4,6-dione: regioselectivity and solventeffect

Three different reaction schemes for the cycloaddition reaction between met hyleneketene and 5-methylene-1,3-dioxan-4,6-dione were studied by means of the B3LYP/6-31G* methods both in the gas phase and in a solvent of dichloro methane. All the geometries of the stationary points on the reaction paths were optimized by energy gradient technique, and all transition states opti mized by the Berny technique. Transition states were ascertained by frequen cy analysis. The results can be summed up as follows: there are three possi ble sites of cycloaddition in methyleneketene, i.e., 2,3-C=C, 1,2-C=C and C =O. In the gas phase, the cycloadditions of different double bonds of methy leneketene with 5-methylene-1,3-aioxan-4,6-dione are all concerted but asyn chronous, taking place through twisted transition states. The activation ba rriers for reactions (1), (2) and (3) are calculated to be 2.96, 0.25, and 21.81 kcal/mol at the B3LYP/6-31G* level, respectively. The computational r esults show that the energy barrier for the reaction leading to 1,2-adduct is the lowest one, which is in consistence with the regioselectivity of the reactions observed by experiments. For comparison, the solvent effect was also studied in the solvent of dichloromethane using self-consistent reacti on field model. (C) 1999 Elsevier Science B.V. All rights reserved.