Thiazolyl radical

The thiazolyl radical has never been directly observed. Torsell (284), however, could indirectly prove its existence by the method of spin trapping in the photolysis of 2-iodothiazole ... 

Coupling constants for this 2-thiazolyl radical are following a>j =9.90 gauss, 2.35 gauss, = 0.48 gauss, Uh, = 3.15 gauss. 

These results show that in the phenylation of thiazole with benzoyl peroxide two secondary reactions enter in competition the attack of thiazole by benzoyloxy radicals, leading to a mixture of thiazolyl benzoates, and the formation of dithiazolyle through attack of thiazole by the thiazolyl radicals resulting from hydrogen abstraction on the substrate and from the dimerization of these radicals. This last reaction is less important than in the case of thiophene but more important than in the case of pyridine (398). 

Vernin et al. independently observed that during the irradiation in benzene of 2-iodothiazole to generate 2-thiazolyl radicals, the resulting 2-phenylthiazole (216) isomerized into a mixture of 4-phenylthiazole (214) and 3-phenylisothiazole (213) (Scheme 103) (489). Marking the 4- and... 

The thiazolyl radicals are, in comparison to the phenyl radical, electrophilic as shown by isomer ratios obtained in reaction with different aromatic and heteroaromatic compounds. Sources of thiazolyl radicals are few the corresponding peroxide and 2-thiazolylhydrazine (202, 209, 210) (see Table III-34) are convenient reagents, and it is the reaction of an alky] nitrite (jsoamyl) on the corresponding (2-, 4-, or 5-) amine that is most commonly used to produce thiazolyl radicals (203-206). The yields of substituted thiazole are around 40%. These results are summarized in Tables III-35 and IIT36. 

TABLE III-37. COMPARISON OF THE RELATIVE REACTIVITIES (WITH RESPECT TO BENZENE) OF VARIOUS PYRIDINE SUBSTRATES towards PHENYL AND 2-THIAZOLYL RADICALS AT 70 TO... 

TABLE m-38. REACTIVITY OF ALKYLPYRIDINES TOWARDS 2-THIAZOLYL RADICALS (208)... 

Thiazolylium salts, 2-methyl-3-phenacyl-reactions, 6, 293 Thiazolylium tosylate, 3-methyl-UV spectroscopy, 6, 240 Thiazolyl radicals... 

Of more synthetic interest, at least as regards phenylation, is the reaction of heteroaromatic radicals with benzene. In this case, if the radical source is easily available, it is possible to work with a very large excess of benzene and to obtain complete conversion of the heterocyclic compound into only one phenylated isomer. The method has successfully been used with thienyP and thiazolyl radicals. 

The 2-thiazolyl radical could be indirectly observed by EPR using the method of spin trapping in the photolysis of 2-iodothiazole (Scheme 3) (70T2759). Coupling constants for this 2-thiazolyl-t-butyl nitroxide radical (in gauss) are as follows a (NO) =9.90, a(N-3) =2.35, a(H-4) =0.48, a(H-5) =3.15. 

TT-Electron densities, correlation with proton chemical shift, of alkylthiazoles, 344 of phenylthiazoles, 345 Electronic charges, of thiazole, 31 of aminothiazoles, 44 of chlorothiazoles, 44 of methylthiazoles, 40 Electronic effects, of 5-substituents on quaternization, 390 Electronic paramagnetic resonance, 84 coupling constants for nitrothiazoles, 84 coupling constants for 2-thiazolyl radical, 84... 

Free-radical Reactions.—2-Thiazolylhydrazine and the three isomeric thiazolecarbonyl peroxides may serve as thiazolyl radical precursors, the free radicals being generated by oxidation with silver oxide or by thermal deomposition ... 

The generation of 2-thiazolyl radicals (49) in benzene, bromobenzene, or cumene affords thiazole, together with fair yields of 2-arylthiazoles, but negligible amounts of esters. The use of thiazole-4-carbonyl peroxide in benzene produces, in addition to thiazole, both 4-arylthiazoles and the phenyl ester as main products, indicative of the participation of both 4-thiazolyl radicals (50) and thiazole-4-carboxyl radicals. The 5-peroxide... 

Thiazolyl Radicals and their Reactions.—The reactions have been reported of a large number of 4- and 5-mono- and 4,5-di-substituted thiazol-2-yl radicals (generated by aprotic diazotization of 2-amino-thiazoles) with substrates such as benzene, > pyridine, and thiophen (see Vol. 3, p. 573). The radical... 

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