Enhanced aromaticity of the transition structures for the Diels-Alder reactions of quinodimethanes: Evidence from ab initio and DFT computations

The Diels-Alder reactions of various quinodimethanes with ethylene are stud ied by means of ab initio molecular orbital and density functional theory ( DFT) to show the effect of aromaticity on the reaction path. The calculatio ns reveal that these reactions are both kinetically and thermodynamically m uch more favored than the prototype butadiene-ethylene Diels-Alder reaction due to the aromatization process in the transition state (TS) and product. A progressive aromaticity gain is noticed during the reaction, and hence t he partial pi -delocalized peripheral diene ring function is coupled with t he six-electron sigma,pi -delocalized cyclic unit resulting in an enhanced aromaticity of the TS. The magnetic criteria such as magnetic susceptibilit y exaltation and nucleus independent chemical shift provide definitive evid ence for and fully support the aromatization process and the aromaticity of the TS. The extent of sigma-pi delocalization and the bond make-break at t he TS are consistent with each other, and this is strongly influenced by th e adjacent pi -aromatization process. Moreover, the aromaticity trends in t he resulting TSs and products parallel the activation and reaction energies ; the extent of aromatization increases with increasing reaction rate and e xothermicity. This confirms that aromaticity is the driving factor governin g cycloadditions involving quinodimethanes.