Effects of the Thiazole Ring on Substituents

Although these effects are considered in connection with each type of substituent in following chapters, it is of interest to discuss some typical and more quantitative aspects of the influence that the ring exerts on some particular substituents. 

The interaction between a substituent and the ring carbon to which it is bonded could be related to some electronic characteristics of the unsubstituted ring and especially to the net charge of its various sites. In that respect the rr-net charges diagram discussed in Section 1.5 indicates that the electron-withdrawing power of the ring-carbon atoms will decrease in the order, 2 4 5. 

This typical behavior of the very unsymmetrical thiazole ring led to a series of studies from the group of H. Erlenmeyer in Basle be studied the H/D exchange of 2,4-dimethyl-5-carboxythia2ole as well as that of similar methylated nitrogen heterocycles (507). The results are shown in Fig. 1-27. 

In conclusion, it appears that in neutral or weakly acidic conditions only the methyl in the 2-position shows pseudoacidic behavior. The same conclusion can be drawn from the base-induced hydrogen-metal exchange reactions discussed in Section III.5.B. 

Nuxnber of hydrogen atoms exchanged against deuterium by dissolution of the sodium salt of the add in D,0 (507). 

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The electron donor-acceptor molecular complexes between iodine and thiazole, benzothiazole, and some derivatives have been studied in several organic solvents by UV spectroscopy <87CJC468>. In all cases, the presence of the thiazole ring produces a displacement of the Amax iodine band at 512 nm towards shorter wavelengths and a decrease of its absorbance. Moreover, a sharp isosbestic point near 470 nm was observed for all iodine-thiazole complexes. 2-Aryl and 2-hetarylbenzothiazoles showed fluorescence, the maxima of emission being between 350 nm and 395 nm. Both substituent and solvent effects on the spectra were observed <93MI 306-02>. The photophysical properties of bis(benzothiazolylidene)squaraine dyes have also been studied <93JPC13625>. 

The amino group activates the thiazole ring toward electrophilic centers. This point is illustrated by the rate constants of the reaction between 2-dialkylaminothiazoles (32) and methyl iodide in nitromethane at 25 C (Scheme 23) (158). The steric effects of substituents on nitrogen are... 

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