A theoretical study of the cycloaddition between cyclopentadiene and allylideneammonium cation: an AM1 study

The Diels-Alder reaction between cyclopentadiene and allylideneammonium cation has been studied by means of AMI semiempirical method. Allylideneammonium cation can add to cyclopentadiene via its either C=C or C=N bond. For each cycloaddition, four reactive channels have been characterized corresponding to the endo-s-trans, endo-s-cis, exo-s-trans and exo-s-cis approach modes. The results indicate that C=C bond of allylideneammonium cation is more reactive as dienophile than its C=N bond in reaction with cyclopentadiene and the reaction takes place along a stepwise mechanism. The first and rate-limiting step is the nucleophilic attack of cyclopentadiene to the beta-carbon atom of the imminium ion to give an intermediate cation, and the second step is associated to the ring closure of this intermediate cation to afford the final cycloadduct. The transition structures, energetic aspects and other factors controlling the reaction outcome are discussed as well. (C) 2002 Elsevier Science B.V. All rights reserved.