Solid states structures of 1,4-diacetyl-1,4-dihydroquinoxaline and 1,2-diacetyl-1,2-dihydrobenzo[c]cinnoline; two potential antiaromatic heterocyclic compounds

Abstract

A crystallographic investigation shows that the 1,4-diacetyl-1,4-dihydroquinoxaline 2 possesses a plane of symmetry perpendicular to the N(1)—N(4) axis and that the 1,4-dihydropyrazine ring exists under a well developed boat conformation. This allows one to reject an azahomoaromatic structure with six delocalized n electrons and a pair of electrons localized on a nitrogen atom. The C(2)=C(3) double bond is localized. 2 shows a ZZ conformation and the molecule indicates an angular shape analogous to that of 1,5-dihydroalloxazines.In the solid state 1,2-diacetyl-1,2-dihydrobenzo[c]cinnolineis not planar. The dihedral angle between the two benzenic rings is 19.5°. The N-acetyl groups present an E,E-conformation and their stereochemical arrangement prevents the nitrogen pairs from being delocalized toward the neighbouring benzenic groups. This shows that 3 is not an antiaromatic molecule.