THE TANDEM DIELS-ALDER REACTION BETWEEN ACETYLENEDICARBOXYALDEHYDE AND N,N'-DIPYRROLYLMETHANE - AN AB-INITIO STUDY OF THE MOLECULAR MECHANISMS

An extensive exploration at RHF/3-21G and RHF/6-31G levels of the pot ential energy surface for the tandem cycloaddition of acetylenedicarbo xyaldehyde to N,N'-dipyrrolylmethane allows us to characterize the rea ction pathways and the associated stationary points. The formation of the pincer and/or domino adducts can be described as a stepwise mechan ism. The first step, associated with an intermolecular [4 + 2] cycload dition, is the rate determining step and an azanorbornadiene intermedi ate is obtained. The second step is an intramolecular [4 + 2] cycloadd ition. The formation of the pincer adduct is the step which kineticall y controls the global process, due to the low barrier height for the s econd intramolecular Diels-Alder step for the pincer channel. But the presence of a transition structure connecting the azanorbornadiene int ermediates of the domino and pincer reaction pathways with an intrinsi c low energy barrier associated to a configurational inversion on the nitrogen atom, allows the connection between both reactive channels. T he Curtin-Hammett principle is operative in this reaction and by therm olysis the global process gives mainly the thermodynamically more stab le domino product. (C) 1998 Elsevier Science B.V.