Identity Double-Proton Transfer in (3Z)-3-Hydroxy-1,4-di(quinolin-2-yl)but-3-en-2-one

• Published in: Chemistry : a European journal Vol. 9; no. 12; pp. 2710 - 2716
• Main Authors: Ośmiałowski, Borys, Kolehmainen, Erkki, Gawinecki, Ryszard
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 16.06.2003; WILEY‐VCH Verlag; Wiley
• Subjects: ab initio calculations; annulation; Chemistry; Chemistry, Multidisciplinary; identity reaction; NMR spectroscopy; Physical Sciences; Science & Technology; tautomerism; TRIPLE; CHEMICAL-SHIFTS; HYDROGEN-BOND; DELOCALIZED CONJUGATE BASES; SOLID-STATE; tautomerism; ab initio calculations; annulation; X-RAY; identity reaction; NMR spectroscopy; DERIVATIVES; N-15 CPMAS NMR; ALPHA-HETEROCYCLIC KETONES
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200204273

Although there is a very fast (on the NMR timescale) double‐proton transfer in (1Z,3Z)‐3‐hydroxy‐4‐quinolin‐2‐yl‐1‐quinolin‐2(1H)‐ylidenbut‐3‐en‐2‐one (the product of the condensation of ethyl oxalate with 2lithiomethylquinoline), it is the only species present in chloroform solution. Comparison of the product of condensation of ethyl oxalate with 2lithiomethyl derivatives of pyridine (recent studies) and quinoline (present studies) shows that benzoannulation considerably affects the tautomeric equilibrium. The observed changes are not only quantitative but also qualitative. Moreover, contrary to the proton transfer in the pyridine tautomers, this process is fast in the quinoline tautomers. Comparison of the experimental and ab initio/DFT GIAO‐calculated 13C and 15N chemical shifts for the transition states in the proton‐transfer reactions between (1Z,3Z)‐ 3‐hydroxy‐4‐quinolin‐2‐yl‐1‐quinolin‐2(1H)‐ylidenbut‐3‐en‐2‐one and its tautomers support the theory that a concerted identity reaction takes place between the enolimine‐enaminone and enaminone‐enolimine tautomeric forms. As a consequence, the most stable tautomeric form, (1Z,3Z)‐1,4‐di(quinolin‐2‐yl)buta‐1,3‐diene‐2,3‐diol, is not present in the tautomeric mixture. A fast equilibrium takes place between the two identical enolimine‐enaminone tautomeric forms shown as evidenced by experimental and calculated NMR chemical shifts.