Characterization of 2‐aryl‐1,3,4‐oxadiazoles by ¹⁵N and ¹³C NMR spectroscopy

Abstract

15 N NMR data are a suitable and sensitive means for characterizing long-range electronic substituent effects. Additionally, 13 C NMR data for these compounds are presented.

17O NMR studies of substituted 1,3,4‐oxadiazoles

“…In previous work, we have shown that the δ 15 N values are much more influenced by the resonance interactions (the ρ R parameters are about twice higher than the ρ I ones). [8] This confirms the low aromaticity of the 1,3,4-oxadiazole ring and a low coupling of oxygen atom lone electron pairs with the delocalized electron system, formed by phenyl ring and C N π -electrons.…”

“…They were about one-fifth of those detected for nitrogen atoms of oxadiazole ring. [8,9] This is a good experimental proof of the low aromaticity or even non-aromatic character of 1,3,4-oxadiazoles, postulated by some wileyonlinelibrary.com/journal/mrc authors on the basis of theoretical calculations and structural data. [12,13] The aromaticity indices published for unsubstituted 1,3,4-oxadiazole were 23.6-25.3 (benzene: 100; furan: 28.3-32.2; pyrole: 70.9-73.0).…”

“…[7,8] For the C ipso -NO 2 (OMe) atom the poor correlation (R 2 0.75 and 0.81, respectively) of the chemical shifts with Hammett parameters was assumed. Similar tendencies were assumed for 1,3,4-oxadiazolines.…”

“…In our previous papers, we have reported the 13 C and 15 N NMR studies of 2-aryl-1,3,4-oxadiazoles [8] and substituted 2-(phenylamino)-5-phenyl-1,3,4-oxadiazoles. [9] This paper presents the results of a study of 17 O chemical shifts of 2-aryl-1,3,4-oxadiazoles, 2,5-diaryl-1,3,4-oxadiazoles and 2-(arylamino)-5-aryl-1,3,4-oxadiazoles.…”

Substituent effects on 15N and 13C NMR chemical shifts of 5‐phenyl‐1,3,4‐oxathiazol‐2‐ones: a theoretical and spectroscopic study

“…2 Hammett correlations established the nature of electronic delocalization, and density functional theory calculations predicted chemical shifts and heterocyclic ring bond orders. We report, herein, the application of these methods to 5-phenyl-1,3,4-oxathiazol-2-ones (1a-1h), for which substituent effects have not previously been assessed.…”