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 and 22.0 kcal/mol, respectively. Although the acetylene + butadiene transition state is more favorable in the interaction energy than the ethylene + butadiene transition state by about 3 kcal/mol due to secondary pi orbital interactions, the stiffer acetylene + butadiene system requires 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 experiment. For the reaction of acetylenedicarboxylic acid and cyclopentadiene, 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.