2- benzothiazole tautomers

Time-resolved electron paramagnetic resonance served to detected the short-lived triplet state of the keto tautomer (238) of 2-(2-hydroxyphenyl)benzothiazole (239) generated by excited state intramolecular proton transfer (92CC641). 

FeClJ X = S) used to model the active sites of the nonporphyrine iron proteins with the N,S-ligand environment [96MI8 97JCS(CC)1711]. Additional metal cyclic structures are formed upon introduction of an azole framework to the hydrazone systems (291) (92MI8), in which the imino tautomer of the benzothiazole is fixed. 

Benzothiazines can also be synthesized by the ring expansion of dihydroben-zothiazoles bearing a side chain at position 2 which contains an active methylene unit. In some instances the product is contaminated with a benzothiazole (Scheme 110) (81JHC279). A similar reaction occurs with the 5-oxides (248) which afford both the tautomers (249) and (250) when treated with acetic anhydride (81TL1701). 

RISC has been observed by two-color irradiation for a series of photo-tautomers generated by photoinduced proton transfer [40-46]. [The parent molecules, 3-hydroxyflavone (63), 2, 3, 4, 5, 6 -pentamethylphenyl-3-hydroxyflavone (64), 7-hydroxy indanone (65), 2-(2 -hydroxyphenyl)benzothiazole (66), and 2,2 -bipyridine-3,3 -diol (67), are shown in Chart 3.] Following proton transfer in the... 

Azido-thiazole and -benzothiazole exist in equilibrium with a corresponding thiazolo[2,3-e]tetrazole form (Scheme 9). In the case of 2-azidobenzothiazole, at 20 °C in dioxane about the same proportion of both tautomers exist in equilibrium (77CJC1728). 

The tautomeric equilibrium in l-Btz-9-acridone (Btz = substituted benzothiazol-2-ylamino) is completely shifted toward the oxo amino tautomer stabilized by... 

There is only one way in which a benzene ring can be fnsed to each of the three 1,3-azoles, generating IH-benzimidazole, benzoxazole and benzothiazole. Indazole is the only possibility for the analogons fusion to a pyrazole it exists as a H tautomer - the 2/f-tautomer cannot be detected, though 2-substituted 2//-indazoles are known. Two distinct isomers each are possible for the other two benzo-fused 1,2-azoles ... 

Light-induced intramolecular proton transfer has long been suspected to play an important role in the mechanism for photochromism in both crystals and solutions. One class of compounds that has been extensively studied is the A -salicylidene anilines. The main evidence for invoking an excited-state proton-transfer step in the photochromic mechanism comes from the observation that the colored species exists as the keto amine tautomer (in the ground state) while the bleached form exists as an enol imine (in the ground state). Replacement of the labile hydrogen with a methyl group prevents the photochemical reaction that leads to the colored species. Picosecond transient spectroscopy was used to study iV-salycylidene-o-toluidine (NST) and the related compound 2-(o-hydroxyphenyl)benzothiazole (HBT). ... 

The separation by crystaUization of tautomers has been reported for some 2-amidobenzothiazoles (1969CJC4483) and 2-acetamidothiazoles (1959 RJC303, 1982J(P2)939) based on IR spectra of soHds. However, later on, the IR spectral data assignments of 2-acetaminobenzothiazoles and 2-acet-aminothiazoles have been revised. It turned out that only the 20a tautomeric form of 2-(2-chloropropionamido)benzothiazole was isolated in the solid form (1994J(P2)615). 

According to their spectral properties,2-amino- and 2-methylamino-benzothiazoles exist chiefly in the enamino-forms (38a and 39a). Their dissociation constants indicate that the amino-forms (38a and 39a) predominate over the imino-forms (38b and 39b) by factors of the order of 2—3 X 10 in 80% methyl cellosolve and 50% methanol. The free-energy difference between the tautomers of both (38) and (39) is found, from the relation AG = - iJrin Artaut to be approximately 4.4 kcal mol", which is indicative of the loss of aromatic energy in passing from the amino- into the imino-form. As expected, this energy loss is smaller than... 

---