Ab initio study of the cycloaddition reaction between hexatriene and ethylene

Abstract

The [2+2] and [2+4] cycloaddition reactions between butadiene and ethylene have been studied using the Gaussian-3 (G3) and G3(MP2) methods. In this study, both of the concerted and stepwise pathways have been investigated. It is found that the most favorable mechanism is, as expected, the concerted [2+4] pathway. The G3 and G3(MP2) barriers are in very good agreement with the experimental estimate. On the other hand, the reaction between cZc-hexatriene and ethylene has three possible processes: the [2+2], [2+4], and [2+6] cycloadditions. Once again, the concerted and stepwise pathways of these three reactions have been studied by the G3(MP2) method. Other than these three possible cycloaddition reactions, the well-known electrocyclic formation of cyclohexadiene, is another competitive reaction. This reaction has also been studied by the same method in the present work. It is found that the most favorable pathway is the concerted [2+4] mechanism, while the barrier of the ring closing process of cZc-hexatriene itself is about 5 kJ mol −1 higher. A detailed discussion on the pathways of the three cycloadditions and the electrocyclic reaction is given.