Structure in the solid state and conformational analysis in solution of 1,4-diacetyl-1,4-dihydro-2,3-diphenylpyrazine, a potential antiaromatic heterocyclic compound

Abstract

It is shown by X-ray crystallography that the 1,4-dihydropyrazine skeleton of 1,4-diacetyl-1,4-dihydro-2,3-diphenylpyrazine 2 has a boat shape. The C(2)–C(3) and C(5)–C(6) double bonds are localized; therefore 2 does not exist as an azahomoaromatic entity with 6 electrons delocalized on the ring and two electrons localized on one nitrogen atom. In the solid state 2 is in a Z, Z conformation. The 1H and 13C nmr spectra indicate that 2 exists as an equilibrium mixture of Z,E, E,E, and Z,Z conformers in CD2Cl2 at −80 °C. An unambiguous assignment of the different sets of signals has been obtained by nOe experiments performed at −80 °C. The conformer distribution is the following: 65% (Z,E), 22% (E,E), and 13% (Z,Z). The relatively low barrier to rotation about the carbonyl nitrogen bonds of the two amide groups [Formula: see text] is in line with a rather large C—N bond length (~1.375 Å) in the crystal.