Are 1,4-dihydropyrazines antiaromatic? Ab initio study of 1,4-dihydropyrazines and their tetrahydro derivatives

A set of twenty molecules containing 1,4-dihydro- or tetrahydropyrazine rin g was calculated using ab initio methods. This set also includes previously prepared diacetyl- or disilyldihydropyrazines. Structures of 1,4-dihydropy razine derivatives are strongly dependent on ring substituents and change f rom 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 pi electrons is small. S tructures, bond lengths and bond orders of 1,4-dihydropyrazines and their t etrahydro derivatives are similar. The isodesmic energy shows tendency rath er to aromatic then to antiaromatic conjugation. On the basis of structural , electronic and energy arguments it is proposed to classify 1,4-dihydropyr azines as nonaromatic compounds.