title: Diels-Alder Reactivities of Benzene, Pyridine, and Di-, Tri-, and Tetrazines: The Roles of Geometrical Distortions and Orbital Interactions.
abstract: The cycloadditions of benzene and ten different azabenzenes (pyridine, three diazines, three triazines, and three tetrazines) with the ethylene dienophile have been explored with density functional theory (M06-2X) and analyzed with the distortion/interaction model. Activation barriers correlate closely with both distortion energies and interaction energies over an activation energy range of 45 kcal/mol. The replacement of CH with N increases Diels-Alder reactivity due not only to the more favorable orbital interaction, but also to a decrease in distortion energy. The rates of reactions are greatly influenced by the nature of the bonds being formed: two C-C bonds > one C-C bond, and one C-N bond > two C-N bonds. The activation energy of Diels-Alder reactions correlates very well with reaction energies and with the NICS(0) values of the aromatic dienes. The distortion energy of the Diels-Alder reaction transition states mostly arises from the diene out-of-plane distortion energy.
authors.affiliation: Department of Chemistry and Biochemistry, ‡Department of Chemical and Biomolecular Engineering, University of California , Los Angeles, California 90095, United States.
authors.forename: Yun-Fang
authors.initials: YF
authors.lastname: Yang
authors.affiliation: Department of Chemistry and Biochemistry, ‡Department of Chemical and Biomolecular Engineering, University of California , Los Angeles, California 90095, United States.
authors.forename: Yong
authors.initials: Y
authors.lastname: Liang
authors.affiliation: Department of Chemistry and Biochemistry, ‡Department of Chemical and Biomolecular Engineering, University of California , Los Angeles, California 90095, United States.
authors.forename: Fang
authors.initials: F
authors.lastname: Liu
authors.affiliation: Department of Chemistry and Biochemistry, ‡Department of Chemical and Biomolecular Engineering, University of California , Los Angeles, California 90095, United States.
authors.forename: K N
authors.initials: KN
authors.lastname: Houk