Solution, solid state structure and fluorescence studies of 2,3-functionalized quinoxalines: evidence for a π-delocalized keto-enamine form with N–H···O intramolecular hydrogen bonds

Abstract

Three quinoxaline derivatives 3, 4 and 5 were prepared by condensation of tetraones RC(O)–CH₂–C(O)– (O)–CH₂–C(O)R [1, R = Ph; 2, R =  neo-Pen] with o-phenylenediamine or (R,R)-1,2-diaminocyclohexane. ¹H, ¹³C and ¹⁵N NMR studies show that these derivatives are best described as their keto-enamine form with N–H···O intramolecular hydrogen bonds. This preferred tautomeric form is confirmed by X-ray structural studies of 3 and 5. Compounds 3 and 4, containing an extended delocalized π-system, exhibit fluorescence properties. In contrast, fluorescence is inhibited in 5, when the central aromatic part is replaced by a cyclohexyl group.