Collect. Czech. Chem. Commun. 1999, 64, 633-648

Are 1,4-Dihydropyrazines Antiaromatic? Ab initio Study of 1,4-Dihydropyrazines and Their Tetrahydro Derivatives

Pavel Vlčeka, Zdeněk Havlasb,* and Zdeněk Pavlíčeka

a Department of Physical and Macromolecular Chemistry, Faculty of Science, Charles University, Hlavova 2030, 120 00 Prague 2, Czech Republic
b Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, Flemingovo nám. 2, 166 10 Prague 6, Czech Republic


A set of twenty molecules containing 1,4-dihydro- or tetrahydropyrazine ring was calculated using ab initio methods. This set also includes previously prepared diacetyl- or disilyldihydropyrazines. Structures of 1,4-dihydropyrazine derivatives are strongly dependent on ring substituents and change from planar to heavily distorted boat conformations. In the planar and near-planar structures of some 1,4-diacyl- or 1,4-diformyl-1,4-dihydropyrazines, conjugation of nitrogen lone pairs and ring bond π electrons is small. Structures, bond lengths and bond orders of 1,4-dihydropyrazines and their tetrahydro derivatives are similar. The isodesmic energy shows tendency rather to aromatic then to antiaromatic conjugation. On the basis of structural, electronic and energy arguments it is proposed to classify 1,4-dihydropyrazines as nonaromatic compounds.

Keywords: Antiaromaticity; Aromaticity; Pyrazines; DFT calculations; Ab initio calculations; Isodesmic energy.