THEORETICAL-STUDIES OF DIELS-ALDER REACTIONS OF ACETYLENIC-COMPOUNDS

Activation barriers in the Diels-Alder reactions of acetylenic systems have been calculated and discussed using the ab initio G2MS and IMOMO (G2MS:MP2) methods. Activation energies for the reaction of acetylene + butadiene and acetylene + cyclopentadiene are predicted to be 25.8 a nd 22.0 kcal/mol, respectively. Although the acetylene + butadiene tra nsition state is more favorable in the interaction energy than the eth ylene + butadiene transition state by about 3 kcal/mol due to secondar y pi orbital interactions, the stiffer acetylene + butadiene system re quires about 5 kcal/mol more reactant distortion energy. Thus, overall , the acetylene + butadiene barrier is about 2 kcal/mol higher and the predicted rate is slower than ethylene + butadiene, consistent with e xperiment. For the reaction of acetylenedicarboxylic acid and cyclopen tadiene, which possesses a very asymmetric transition state, the best IMOMO(G2MS:MP2) method predicts an activation barrier of 9.2 kcal/mol and reveals significant electronic effects of substituents outside the model system of acetylene + butadiene.