GAIN OR LOSS OF AROMATICITY IN DIELS-ALDER TRANSITION-STATES AND ADDUCTS - A THEORETICAL INVESTIGATION

Semiempirical, ab initio and DFT investigations on the mechanism of th e Diels-Alder reactions of a set of masked dienes (ring-fused dienes), viz. o-quinodimethane, anthracene and alpha-vinylnaphthalene, with et hylene, and reactions of alpha-vinylnaphthalene with maleic anhydride and p-benzoquinone were performed with a view to understanding the rol e of masking factors on the activation and reaction energies. The reac tions were found to occur in a concerted fashion through synchronous t ransition states (TSs) in the first two reactions and through asynchro nous TS in the other case. Of the compounds studied, o-quinodimethane is the most reactive diene both in a kinetic and a thermodynamic sense . alpha-Vinylnaphthalene reacts faster with maleic anhydride than p-be nzoquinone, as expected, and two possible stereoselective TSs, one end o and the other exo, have been located for the reactions of alpha-viny lnaphthalene with the above dienophiles. Calculations show that the re lative gain or loss of aromatic stabilization of the benzonoid ring in the transition state and in the product seems to play a major role in the kinetic and thermodynamic control of these reactions. (C) 1998 Jo hn Wiley & Sons, Ltd.