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

Abstract

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.