Chemical shift thiazole derivatives

The chemical shifts of in natural abundance have been measured for thiazole and many derivatives (257,258). They are given in Tables 1-37 and T38. These chemical shifts are strongly dependent on the nature of the substituent CNDO/2 calculations have shown (184) that they correlate well with the ((t+tt) net charge of the atom considered. As a consequence, the order of the resonance signals is the same for protons and for carbon atoms. 

The molar diamagnetic susceptibility of thiazole and some derivatives was initially determined by the classical Curie-Cheneveau method (5,315,316) and later confirmed by a method (317) based on the difference of NMR proton chemical shift of a sample of tetramethylsilane immersed in the liquid to be investigated, according to the shape (cylindrical or spherical) of the sample tube (Table 1-47) (318),... 

No nitration of thiazole occurs with the classical nitration reagents, even in forcing conditions (341-343). In a study concerning the correlation between the ability of thiazole derivatives to be nitrated and the HNMR chemical shifts of their hydrogen atoms, Dou (239) suggested that only those thiazoles that present chemical shifts lower than 476 Hz can be nitrated. From the lowest field signal of thiazole appearing at 497 Hz one can infer that its nitration is quite unlikely. Thiazole sulfonation occurs... 

The work of Taddei et al.230 on imidazol2,1 -6]thiazole 337 and derivatives has interesting implications on the structure of azapen-talenes, and an important aspect of this study is that the molecular geometry used for calculations on 6-phenylimidazo[2,l-6]thiazole 417 was obtained from X-ray structure determinations130b (Section V,A). The reactivity of this system (Scheme 18, Section IV,C,4,b) is better correlated with Tr-electron densities than with total charges, and 7r-bond orders (by the PPP method) show that the thiazole part of the molecule is more localized than the imidazole part (Section VII). Proton chemical shifts, except that of the H2 proton a to sulfur (Section V,G,2), vary linearly with the total charge carried by the ring carbon atoms. 

NMR study of nitrothiazoles has been the subject of few number of works [525, 530-540], The proton chemical shifts of about 30 thiazole derivatives including 5-nitrothiazole and 2-nitrothiazole are presented in [536], The authors have found the coupling constants between H-2 and II-4 in most 5-substituted thiazoles to be negligible with the exception of 5-nitrothiazole and 5-thiazolecarboxylic acids [536], The change in the H-4 chemical shifts of the 2-substituted 5-nitrothiazole fragment enables the ratio of isomeric products of the reaction of 5-nitrothiazole 2-(T,3 -dicarbonyl) derivatives with hydrazine and hydroxylamine to be established (Scheme 3.8) [537],... 

TABLE 1-38. CARBON CHEMICAL SHIFTS OF SUBSTITUENTS IN THIAZOLE AND SOME OF ITS DERIVATIVES (257)... 

Thiazole shows a N chemical shift of 57.00 ppm relative to nitromethane when the spectrum is registered in pure liquid. The shielding of nitrogen remains practically unchanged in the fused ring of benzothiazole ((5 N of benzothiazole 61.00 ppm, acetone). Nitrogen-15 spectroscopy has been used in the characterization of neutral methanate complexes (ML2) between Zn and 2-thiazoline or benzothiazole derivatives <91G365>. 

The natural abundance C chemical shifts for thiazole (and its 2-, 4-, 2,4-, and 4,5-methyl derivatives) have been measured at 25.15 MHz against 50% enriched 82. The results confirm the accuracy of the C-H coupling constants, and show that the methyl groups exert an appreciable substituent effect on the carbon atoms to which they are attached. 2-Thiazoline (and its 2-methyl, 5-methyl, and 5-methyl-2-ethyl homologues) have also been studied from this point of view. ... 

A detailed study of n.m.r. spectra has provided extensive data on chemical shifts and coupling constants in respect of a series of monophenyl-thiazoles, and their derivatives bearing alkyl and functional substituents in their hetero-rings. The chemical shifts were correlated with the net from electron density and energy diagrams, and the overall results were compared with those previously obtained in respect of thiazole and its methyl derivatives. 

---