A computational study of aromaticity-controlled Diels–Alder reactions

Abstract

The prime role of aromaticity in Diels–Alder reactions is studied computationally by ab initio and DFT methods using various masked dienes and ethylene. The reactions under consideration yield both aromatic stabilized and destabilized products through a concerted transition state due to the effect of 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 functionalization; therefore they are kinetically as well as thermodynamically much more favorable than the typical butadiene–ethylene reaction. A series of these reactions affirms that the degree of aromatization increases with decreasing barrier and increasing exothermicity of a reaction. In reactions of benzo[c]heterocycles, aromaticity is lost due to the reacting heterocycle, but is gained by the adjacent hexagon during the reaction course. A partly occurring aromatic stabilization process in these reactions seems to facilitate the cycloaddition, but the remaining aromatic destabilization decreases the reaction rate and energy as compared to quinodimethane reactions. In the reactions of polyaromatic hydrocarbons viz. styrene, anthracene and pentacene, only loss of aromaticity occurs by virtue of aromatic defunctionalization. The progress of aromatization as well as dearomatization is evidenced from the nucleus independent chemical shifts (NICS) values whereas the aromaticity of the transition state and product is quantified by magnetic susceptibility exaltation (MSE) calculations. Calculations thus establish with both magnetic and energetic criteria that the aromatic stabilization process as well as the aromatic ring function of the masked diene accelerates the reaction to the maximum extent through an ‘early’ TS, but the aromatic destabilization deactivates the cycloaddition via a ‘late’ TS.