A computational study of aromaticity-controlled Diels-Alder reactions

The prime role of aromaticity in Diels-Alder reactions is studied computati onally by ab initio and DFT methods using various masked dienes and ethylen e. The reactions under consideration yield both aromatic stabilized and des tabilized products through a concerted transition state due to the effect o f ring functions embedded in the diene framework. Computations reveal that the cycloadditions involving various quinodimethanes achieve a progressive aromaticity gain during the reaction by the influence of aromatic functiona lization; therefore they are kinetically as well as thermodynamically much more favorable than the typical butadiene-ethylene reaction. A series of th ese reactions affirms that the degree of aromatization increases with decre asing barrier and increasing exothermicity of a reaction. In reactions of b enzo[c]heterocycles, aromaticity is lost due to the reacting heterocycle, b ut is gained by the adjacent hexagon during the reaction course. A partly o ccurring aromatic stabilization process in these reactions seems to facilit ate the cycloaddition, but the remaining aromatic destabilization decreases the reaction rate and energy as compared to quinodimethane reactions. In t he reactions of polyaromatic hydrocarbons viz. styrene, anthracene and pent acene, only loss of aromaticity occurs by virtue of aromatic defunctionaliz ation. The progress of aromatization as well as dearomatization is evidence d from the nucleus independent chemical shifts (NICS) values whereas the ar omaticity of the transition state and product is quantified by magnetic sus ceptibility exaltation (MSE) calculations. Calculations thus establish with both magnetic and energetic criteria that the aromatic stabilization proce ss as well as the aromatic ring function of the masked diene accelerates th e reaction to the maximum extent through an 'early' TS, but the aromatic de stabilization deactivates the cycloaddition via a 'late' TS.