Azoles definition

Azidouracils. photolysis, 55, 197, 200 Azimines, pyrazole-derived, formation from azomethine imines and nitroso compounds, 60, 34 Azinium azolates, definitions, 60, 200 Aziridine... 

Azolium azolates, definitions, 60, 200 Azolo(I,5-a]pyrimidines, 5,7-diphenyl-, 60, 249... 

Azole Definition of Kt Conclusion Tautomer with highest IX Less basic... 

NMR data for 4-methyloxazole have been compared with those of 4-methylthiazole the data clearly show that the ring protons in each are shielded. In a comprehensive study of a range of oxazoles. Brown and Ghosh also reported NMR data but based a discussion of resonance stabilization on pK and UV spectral data (69JCS(B)270). The weak basicity of oxazole (pX a 0.8) relative to 1-methylimidazole (pK 7.44) and thiazole (pK 2.44) demonstrates that delocalization of the oxygen lone pair, which would have a base-strengthening effect on the nitrogen atom, is not extensive. It must be concluded that not only the experimental measurement but also the very definition of aromaticity in the azole series is as yet poorly quantified. Nevertheless, its importance in the interpretation of reactivity is enormous. 

Palmer devoted several papers to simple azoles [77JST(40)191 78JST(43)33, 78JST(43)203 81ZN(A)1246 87CPH(111)249], succeeding in definitely establishing the tautomeric predominance of 2//-1,2,3-triazole, 1//-1,2,4-triazole, 2//-tetrazole, and l//-indazole in the gas phase. 

Clearly, there is a distinct difference between the kinetic results for the benzologs of azines and azoles. The former definitely show a sizable steric effect in N-methylation reactions when the site of quaternization is a peri position, while any effect for the latter is insignificant. Perhaps this simply reflects the smaller internal bond angles for five- over six-membered rings. 

Comparison of the Bronsted correlation for 3-substituted pyridines [Eq. (12)] and that for the azoles [Eq. (15)1 is most interesting. The slopes are very similar, 0.39 and 0.43, respectively, and represent overlapping values at the uncertainty level of one standard deviation. However, the intercepts, —1.90 and —2.75, respectively, do not overlap. But consideration of the approximations in the plot for the azoles precludes a definite conclusion about whether the differences are truly significant. 

Non-empirical (ab initio) calculations have been used to derive vertical REs for the azole series as (in kJ mol-1) pyrrole (149), indole (308) and isoindole (252) [cf. benzene (212), furan (89), benzofuran (232) and isobenzofuran (147)] <76JCS(P2)81> although the numbers are not directly comparable with the DREs (since the definitions of RE are different) the trends in the differences for the two heterocyclic series are similar. 

The transformation of 1,2-azoles carrying, at C-3, a side-chain of three atoms terminating in a doubly-bonded heteroatom, into isomeric systems with a new five-membered ring is a general process, though there is no definitive view as to the details of its mechanism. 

Synonyms 1-Aza-2,4-cyclopentadiene Azole Divinylenimine Imidole Monopyrrole Definition Constituent of coal tar and bone oil Empirical C4H5N Formula CH=CHNHCH=CH Properties Colorless to yel.-brn. oily liq. chloroform-like odor burning, pungent taste sol. in alcohol, ether, dil. acids, fixed oils, benzene insol. in water, dil. alkalis m.w. 67.09 dens. 0.966 (20/4 C) f.p. -24 C b.p. 129-131 C flash pt. 36 C ref. index 1.5091 (20 C)... 

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