Donor−Acceptor-Assisted Diels−Alder Reaction of Anthracene and Tetracyanoethylene

Abstract

We report hybrid Hartree−Fock/density functional B3LYP/6-31G(d) and B3LYP/6-31+G(d,p) calculations to determine the path of the Diels−Alder reaction between anthracene and tetracyanoethylene (TCNE) and to characterize the stationary points along the path. With only one exception, calculated bond distances in anthracene, TCNE, and TCNE•- (the limiting case of complete electron donation to TCNE) are within 3 standard deviations of experiment. We also predict the geometry of anthracene•+. Calculations to determine the reaction path establish unambiguously that the observed electron donor−acceptor complex is an intermediate and that donor−acceptor interactions assist attainment of the reaction's transition state by lowering the energy barrier to pyramidalizing about C9/C10 of anthracene and the ethylenic carbons of TCNE. Combined with thermodynamic integration calculations in chloroform solvent, B3LYP calculations of the activation energy (20.1 kcal/mol) agree quantitatively with the experimentally derived activation energy (20.0 kcal/mol). For the retro-Diels−Alder reaction, the calculated activation energy underestimates the experimental value by 4.6−5.0 kcal/mol, suggesting that B3LYP/6-31G(d) and B3LYP/6-31+G(d,p) calculations understabilize the reaction product.