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Basicity of Phenyl‐ and Methyl‐Substituted 1,2,4‐Oxadiazoles
Author(s) -
Trifonov Rostislav E.,
Volovodenko Anna P.,
Vergizov Sergei N.,
Shirinbekov Nabi I.,
Gindin Vladimir A.,
Koren Andrei O.,
Ostrovskii Vladimir A.
Publication year - 2005
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.200590140
Subject(s) - chemistry , oxadiazole , protonation , medicinal chemistry , ring (chemistry) , yield (engineering) , aqueous solution , additive function , nuclear magnetic resonance spectroscopy , spectroscopy , computational chemistry , stereochemistry , organic chemistry , ion , mathematical analysis , materials science , physics , mathematics , quantum mechanics , metallurgy
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 BH +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 BH +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.