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.