Cycloaddition reactions via "on water" protocol reactions: A density functional theory study

In this work, the reactions of quadricyclane with dimethyl azodicarboxylate (DMAD) and of quadricyclane with diethyl azodicarboxylate (DEAD) in gas phase and in water environments were studied by a first-principles investigation within the framework of auxiliary density functional theory (ADFT). For these type of organic reactions is known that water is required to accelerate them. Since the reason of why this occur is still unknown, this work aims to gain insight into this reaction mechanism. For this investigation, the generalized gradient approximation as well as a hybrid functional were employed. The obtained optimized structures for the reactants, of the products and of the transition states are reported, together with the corresponding frequency analysis results and the reaction profiles. Along the proposed concerted reaction mechanism, a critical points search of the electron density and a charge analysis were performed. The calculated potential energy barriers of these reactions in gas phase and in water environments are compared. In agreement with experiment, the obtained results indicate that both reactions occur faster in water than in gas phase. This study shows that there is a change in the polarity of the two most important carbon atoms of the formed compounds along the reactions and that the decrease of the activation energy barrier which occurs in liquid phase in these reactions is because the structures of the main transition states are stabilized by the water environment. Therefore, the here obtained results demonstrate the important role played by the water-molecule framework into the activation energy barrier and structures of the molecules that participate in the DMAD and DEAD cycloaddition reactions.