Donor-acceptor-assisted Diels-Alder reaction of anthracene and tetracyanoethylene

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 b etween anthracene and tetracyanoethylene (TCNE) and to characterize the sta tionary points along the path. With only one exception, calculated bond dis tances in anthracene, TCNE, and TCNE.- (the Limiting case of complete elect ron donation to TCNE) are within 3 standard deviations of experiment. We al so predict the geometry of anthracene(.+). Calculations to determine the re action path establish unambiguously that the observed electron donor-accept or complex is an intermediate and that donor-acceptor interactions assist a ttainment of the reaction's transition state by lowering the energy barrier to pyramidalizing about C-9/C-10 of anthracene and the ethylenic carbons o f 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 react ion product.