Trapping of 1,2-cyclohexadiene: A DFT mechanistic study on the reaction of 1,2-cyclohexadiene with olefins and nitrones

Abstract

The mechanistic aspects of cycloaddition reactions of 1,2-cyclohexadiene with olefins and nitrones have been investigated with DFT calculations. The results show that the cycloaddition reactions of 1,2-cyclohexadiene with olefins do not go through a concerted pathway (one-step mechanism) but rather a stepwise one involving the formation of a biradical intermediate which then closes to form final cycloadduct. Electron-withdrawing substituents on the 1,2-cyclohexadiene decrease the activation barrier of the biradical-forming step but increase the barrier of the product-forming step and product stability, while electron-donating substituents on the 1,2-cyclohexadiene increase the barriers for both the biradical-forming step and the product-forming step but decrease the product stability. In the reaction of 1,2-cyclohexadiene with nitrones, the four pathways investigated have activation barriers within 1 kcal/mol of one another, the lowest being 10.45 kcal/mol and the highest 11.04 kcal/mol, indicating that these reactions are very unselective. Electron-withdrawing groups on the nitrone increase the stability of the resulting products whereas electron-donating group on the nitrone decrease the stability of the resulting products. The [3 + 2] cycloadduct proceeds to the formation of a more stable formal [5 + 2] cycloadduct if a phenyl substituent is present on the nitrogen of the nitrone.