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Int. J. Mol. Sci. 2013, 14(3), 4444-4460; doi:10.3390/ijms14034444

Synthesis and Structural Characterization of Substituted 2-Phenacylbenzoxazoles

1 Department of Chemistry, University of Technology and Life Sciences, Seminaryjna 3, PL-85-326 Bydgoszcz, Poland 2 Department of Chemistry, P.O. Box 35, FI-40014 University of Jyväskylä, Finland 3 Department of Physical Chemistry, Collegium Medicum, N. Copernicus University, Kurpińskiego 5, 85-950 Bydgoszcz, Poland
* Author to whom correspondence should be addressed.
Received: 10 January 2013 / Revised: 4 February 2013 / Accepted: 16 February 2013 / Published: 25 February 2013
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1H and 13C NMR spectra of eleven 2-phenacylbenzoxazoles (ketimine form) show that their CDCl3-solutions contains also (Z)-2-(benzo[d]oxazol-2-yl)-1-phenylethenols (enolimine form). Intramolecular hydrogen bonding in the latter tautomer was found to be significantly weaker than that one in respective (Z)-2-(2-hydroxy-2-phenylvinyl)pyridines. Integrals of the 1H NMR signals were used to evaluate the molar ratio of the tautomers. Strong electron-donating substituents were found to stabilize the ketimine tautomer. pKT (negative logarithm of the equilibrium constant, KT = [ketimine]/[enolimine]) was found to be linearly dependent on the Hammett substituent constant σ. The results of the MP2 ab initio calculations reveal enolimine including an intramolecular OHN hydrogen bond to be the most stable form both with electron-donor and electron-acceptor substituents. The stability of ketimines is an intermediate of those found for enolimines and enaminones i.e., (E)-2-(benzo[d]oxazol-2(3H)-ylidene)-1-phenylethanones. 13C CPMAS NMR spectral data reveal that in the crystalline state the ketimine tautomer is predominant in p-NMe2 substituted congener. On the other hand, enolimine forms were detected there when the substituent has less electron-donating character or when it is an electron-acceptor by character.
Keywords: 2-phenacylbenzoxazole; tautomerism; substituent effect; hydrogen bond; resonance interaction; NMR; quantum-chemical calculations 2-phenacylbenzoxazole; tautomerism; substituent effect; hydrogen bond; resonance interaction; NMR; quantum-chemical calculations
This is an open access article distributed under the Creative Commons Attribution License (CC BY 3.0).

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Skotnicka, A.; Kolehmainen, E.; Czeleń, P.; Valkonen, A.; Gawinecki, R. Synthesis and Structural Characterization of Substituted 2-Phenacylbenzoxazoles. Int. J. Mol. Sci. 2013, 14, 4444-4460.

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