EfficientN‐Alkyl Enamination of 3‐(Thiobenzoyl)pyrrolo[2,3‐b]quinoxalin‐2‐one,E/ZIsomerisation of the Reaction Products and Potential Fluorescent Sensors for ­Zinc(II) Ion

Abstract

An efficient and rapid procedure for the N‐alkyl enamination from thioketones to push–pull (E/Z)‐enamine derivatives is reported. The starting thioketone 2 was obtained by regioselective thionation of 3‐benzoylpyrrolo[2,3‐b]quinoxalin‐2‐one (1) with Lawesson's reagent (LR). 1H NMR and X‐ray crystal structure analyses of 1‐aryl‐3‐(alkylaminophenylmethylidene)pyrrolo[2,3‐b]quinoxalin‐2‐ones 3–6 confirmed that the E diastereoisomers predominated. Interconversion of the isolated E form of 3b and 4a into the Z configuration and their E/Z equilibria in [D6]DMSO were studied by temperature‐dependent 1H NMR spectroscopy. The photophysical properties of compounds 1–6 are also reported. We found that the non‐fluorescent compounds 3a–6a are chemosensors for the zinc ion. Fluorescent titrations of these sensors with zinc(II) acetate showed that compound 6a has the best working range of zinc concentrations with Kd = 3.77 00 ± 0.51 μM.

Chelate Ring Size Effect as a Factor of Selective Fluorescent Recognition of Zn2+Ions by Pyrrolo[2,3-b]quinoxaline with a Substituted 2-Pyridyl Group Receptor

“…22 Replacement of zinc acetate by zinc acetylacetonate changed the mechanism of complex formation. 1 H NMR titration indicates the formation of two complexes, [(E)-3] 2 Zn and (E)-3-Znacac, upon the addition of 0.2 equiv of Zn(acac) 2 to 3 in CD 3 CN.…”

“…22 Complex formation with deprotonation of the NH enamine group should inhibit E/Z isomerization. The planarization is possible by coordination of the metal ion through enamine and N5 atoms, which was indicated by X-ray analysis of [(E)-2] 2 Zn and (E)-3-ZnOAc.…”

“…The ligand 3 and (E)-3-ZnOAc were synthesized using previously published procedures. 22 Synthesis of (…”

Ratiometric fluorescent Zn2+and In3+receptors of fused pyrazine with an aminopropanol chain in acetonitrile

“…1). 35 Among these [N, N, N] or [N, N, O] tridentate ligands 1a-4a, compound 3a showed the ratiometric turn-on fluorescence response toward zinc cations within the working zinc concentration range of K d = 3.77 AE 0.51 mM and a binding stoichiometry of 1 : 1. This push-pull (E/Z)-enamine undergoes deprotonation while binding zinc cations, yielding a red-shifted excitation wavelength with induced charge transfer signal transduction (ICT).…”

“…This push-pull (E/Z)-enamine undergoes deprotonation while binding zinc cations, yielding a red-shifted excitation wavelength with induced charge transfer signal transduction (ICT). 35 The electronegativity of the oxygen atom in the carbonyl group of imide causes the delocalization of N-1 or enamine nitrogen's lone pairs into the p system, leading to formation of zwitterions A or B, respectively (Scheme 1). Form B is stabilised by an internal hydrogen bond.…”