Reaction of CCl2 with CH2NH and the formation of dipolar and biradical ylide structures

Abstract

The potential energy surface for the reaction between CH₂NH and CCl₂ has been investigated using ab initio methods. We have performed geometry optimizations at the MP2/6-31G* level of theory and single point calculations at the MP4(SDQ)/6-311++G** level. The reaction step for ylide formation has a free energy of activation predicted to be 5.0 kcal mol^–1. The parallel 1,2-cycloaddition reaction has a calculated free energy barrier of 16.5 kcal mol^–1, indicating that this second pathway is not competitive with ylide formation. The structure of the azomethine ylide formed in the first reaction step is similar to that found for the ylide resulting from the reaction of methylene with ammonia and corresponds to a dipolar species. This is highly unstable and rearranges to its more stable isomer, the biradical azomethine ylide, which has a structure similar to the corresponding carbonyl ylide. This species has a free energy barrier to ring closure calculated to be 21.2 kcal mol^–1, so it has reasonable kinetic stability. The resulting aziridine has a free energy of 24.1 kcal mol^–1 lower than the biradical azomethine ylide, and the activation free energy of ring opening is calculated to be 45.3 kcal mol^–1.