THE ROLE OF CUMULENIC STRAIN ON THE KINETIC AND THERMODYNAMIC CONTROLOF THE DIELS-ALDER REACTIONS INVOLVING ALLENES AS DIENES

Diels-Alder cycloaddition reactions of vinylallene and diallene with e thylene, acrylonitrile, maleic anhydride, p-benzoquinone and 1,4-napht haquinone have been investigated at the semiempirical level and with l imited ab initio and DFT calculations. Calculations predict that these reactions are concerted processes and these cumulated dienes are less reactive than the non-cumulated diene, With increase in cumulenic str ain the reactivity decreases while the exothermicity of the reaction i ncreases. This happens because the transition states occur 'early' and the cumulenic strain is therefore released only when the reaction pro ceeds from transition state to product, Deformation energy analysis ex plains that these cumulated dienes have to deform to a greater extent during the reaction and thus are responsible for the increase of the a ctivation energy, Activation barriers predict the reactivity trends, r egio- and stereoselectivities reasonably.