Basicity of Phenyl- and Methyl-Substituted 1,2,4-Oxadiazoles

✍ Scribed by Rostislav E. Trifonov; Anna P. Volovodenko; Sergei N. Vergizov; Nabi I. Shirinbekov; Vladimir A. Gindin; Andrei O. Koren; Vladimir A. Ostrovskii

• Publisher: John Wiley and Sons
• Year: 2005
• Tongue: German
• Weight: 175 KB
• Volume: 88
• Category: Article
• ISSN: 0018-019X
• DOI: 10.1002/hlca.200590140

✦ Synopsis

Abstract

The basicity of a series of 3,5‐disubstituted 1,2,4‐oxadiazoles in aqueous H~2~SO~4~ was examined by means of UV and ^1^H‐NMR spectroscopy. The experimental data were analyzed by the modified Yates–McClelland method to yield the following p__K__ values: 3,5‐dimethyl‐1,2,4‐oxadiazole, −1.66±0.06; 3‐methyl‐5‐phenyl‐1,2,4‐oxadiazole, −2.61±0.02; 3‐phenyl‐5‐methyl‐1,2,4‐oxadiazole, −2.95±0.01; 3,5‐diphenyl‐1,2,4‐oxadiazole, −3.55±0.06. A p__K__ value of ca. −3.7 was estimated for the parent unsubstituted 1,2,4‐oxadiazole based on substituents' additivity increments. Possible protonation sites of the compounds were discussed in terms of both experimental data and theoretical calculations (HF/6‐31G**). Generally, protonation is most likely to occur at N(4) of the 1,2,4‐oxadiazole ring. However, concurrent formation of both N(4)‐ and N(2)‐protonated species in comparable amounts is possible in the case of 3‐phenyl‐1,2,4‐oxadiazoles.