NMR study of the (Z)-phenylhydrazones of 5-alkyl- and 5-aryl-3-benzoyl-1,2,4-oxadiazoles: support for the interpretation of kinetic results on the rearrangement of 1,2,4-oxadiazoles to 1,2,3-triazoles

An analysis of the 1H, 13C, and 15N NMR substituent chemical shifts (SCSs) of the title compounds in CDCl3 solution has been carried out by using the SCSs of other classes of compounds (arenes, methyl esters or amides), Hammett substituent constants or kinetic data. The results obtained provide information concerning the ground-state electronic distribution of the compounds examined. The results relevant to the carbon and nitrogen atoms of the 1,2,4-oxadiazole ring can be considered of special interest, as the effects of the substit- Introduction For several years our research group has been involved in the use of NMR spectroscopic data (1H, 13C, 15N, and 17O) to obtain information about the ground-state electronic distribution of different classes of organic compounds and to evaluate the influence of substituents on this distribution in order to correlate NMR and reactivity data.[1,2] Thus, the study of the SCSs (substituent-induced chemical shifts) of aryl (or heteroaryl) carboxylic acids[1a–1c] and their derivatives (esters[1d–1g] and amides[1f]) or ketones,[1d,1i] arylhydrazines[2a–2c] or (Z)-arylhydrazones of 3-benzoyl-5- phenyl-1,2,4-oxadiazole[2d] by using both the one- and the two-parameter [DSP (dual substituent parameter)] treatment, has furnished free-energy relationships (FERs) that are useful for an analysis of substituent effects. At the same time, we have deeply investigated the mechanisms of the “monocyclic rearrangement of heterocycles” (MRH or Boulton–Katritzky reaction, BKR)[3] by collect- [a] Dipartimento di Chimica Organica “A. Mangini”, Università di Bologna, Via San Giacomo 11, 40126 Bologna, Italy Fax: + 39-051-209-5688 E-mail: domenico.spinelli@unibo.it [b] Dipartimento di Chimica e Tecnologie Farmaceutiche, Università di Palermo, Via Archirafi 32, 90123 Palermo, Italy [c] Dipartimento di Chimica Organica “E. Paternò”, Università di Palermo, Viale delle Scienze, Parco d’Orleans II, 90128 Palermo, Italy © 2005 Wiley-VCH Verlag GmbH & Co. 3980 KGaA, Weinheim DOI: 10.1002/ejoc.200500236 Eur. J. Org. Chem. 2005, 3980–3986 uents on the chemical shifts of N-4 and C-5 appear in line with kinetic results collected in the study of the rearrangement (Boulton–Katritzky reaction) of the title compounds into the relevant 1,2,3-triazoles, thereby strongly supporting the importance of the leaving-group ability of the N-4/C-5/O-1 system.