Solution, solid state structure and fluorescence studies of 2,3-functionalized quinoxalines: evidence for a pi-delocalized keto-enamine form with N-Hcenter dot center dot center dot O intramolecular hydrogen bonds

Three quinoxaline derivatives 3, 4 and 5 were prepared by condensation of t etraones 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. H-1, C-13 and N-15 NMR studies show that these derivatives are best described as their keto-enamin e form with N-H . . .O intramolecular hydrogen bonds. This preferred tautom eric form is confirmed by X-ray structural studies of 3 and 5. Compounds 3 and 4, containing an extended delocalized pi -system, exhibit fluorescence properties. In contrast, fluorescence is inhibited in 5, when the central a romatic part is replaced by a cyclohexyl group.