Basicity of 2-phenyl-5-R-1,3,4-oxadiazoles

Abstract

CF 3 ) in aqueous sulfuric acid solutions. These compounds are weak organic bases . The values of pK BH + determined on the H 0 and X acidity function scales agree well with each other. The substituent at the 5 position has a substantial effect on the basicity of the 1,3,4-oxadiazole ring.Compounds of the 1,3,4-oxadiazole series are widely used in various areas of modern technology [1]. Considering the unique luminescent spectral properties of aryl-substituted 1,3,4-oxadiazoles and their ability to form stable molecular and coordination complexes with various substrates, these compounds may be considered as promising components of new materials for advanced technologies, including as components for active media of lasers [2]. The basicity of 1,3,4-oxadiazoles, reflecting their ability to act as electron density donors, has been insufficiently studied. We previously determined the basicity constants for a series of 2-aryl-5-phenyl-1,3,4-oxadiazoles, and also quantitatively established the effect of the electronic properties of the benzene ring substituents on the pK BH + of these heterocycles [3]. For 2,5-diphenyl-1,3,4-oxadiazole, we determined the basicity constants in the S 0 and S 1 states, and we also studied the effect of the acidity of the medium on its luminescent spectral properties [4].In this paper, we use the spectrophotometric method to determine the basicity constants of a series of 2-phenyl-5-R-1,3,4-oxadiazoles containing different types of substituents at the 5 position of the heterocycle (1-7), plus 2,2'-diphenyl-5,5'-di-1,3,4-oxadiazole (8). N O N R Ph 1-8 1 R = H; 2 R = Ме; 3 R = CH 2 Ph; 4 R = t-Bu; 5 R = CH 2 Cl; 6 R = CCl 3 ; 7 R = CF 3 ; 8 R = 2-phenyl-1,3,4-oxadiazol-5-ylThe pK BH + values for these compounds were calculated by the Yates-McClelland and Cox-Yates methods, using the acidity functions H 0 and X respectively. We consider the dependences of the basicity constants on the substituent constants. __________________________________________________________________________________________

Furazans in Medicinal Chemistry

“…1,2,5-Oxadiazoles have unique physical and electronic properties relative to the 1,2,4- and 1,3,4-isomers . Differences in lipophilicity, basicity, − and magnitude of the dipole moment ,,− are observed (Figure ). Furazans are highly inductive, comparable to a trifluoromethyl or tetrazolyl group ,, (Figure ).…”

1,2,4- and 1,3,4-Oxadiazoles as Scaffolds in the Development of Antiparasitic Agents

“…It was observed that the presence of two phenyl rings considerably increases the observed λ max value. For example, 2-phenyl-1,3,4-oxadiazole (14) has a λ max = 245 nm, 34 whereas the 2,5-diphenyl-1,3,4-oxadiazole (15) analogue has a λ max = 276 nm ( Figure 7). 35 The analysis of UV spectra information allows us to infer that 1,3,4-oxadiazoles have greater aromaticity, presumably because of their symmetry.…”

“…43 It is also possible to find protocols employing ultrasound irradiation to synthesize 1,2,4-oxadiazoles (29), as reported by Bretanha et al 44 Short reaction times (15 min) and high yields (84-98%) were reported for the reaction between trichloroacetoamidoxime (27) and the properly substituted benzoyl chlorides (28) in ethyl acetate as solvent (Scheme 4). 44 In a different approach, Yoshimura et al 45 reacted nitrile oxides (34) and nitriles (31) to obtain 1,2,4-oxadiazoles (32). Hydroxy(aryl)iodonium (IBA-OTf, 33) was employed allowing the formation in situ of nitrile oxides (34) from the respective nitriles (31).…”

“…44 In a different approach, Yoshimura et al 45 reacted nitrile oxides (34) and nitriles (31) to obtain 1,2,4-oxadiazoles (32). Hydroxy(aryl)iodonium (IBA-OTf, 33) was employed allowing the formation in situ of nitrile oxides (34) from the respective nitriles (31). IBA-OTf (33) can be generated in situ by addition of 2-iodo-benzoic acid (35), trifluoromethanesulfonic acid (TfOH) and meta-chloroperbenzoic acid (m-CPBA) as oxidant.…”