5-Phenylisothiazole, phototransposition

Syntheses and reactions of lithiated isothiazoles and thiazoles were reviewed. [95H533] Phenylisothiazoles and phenylthiazoles are known to undergo a variety of phototranspositions upon irradiation in benzene solvent. These reactions were reinvestigated. [94JA2292]... 

A more significant difference between 1 -methylpyrazole and isothiazole photochemistry, however, appears to be the minor role of the N-2-C-3 interchange pathway in isothiazole chemistry. Thus, although N2-C3 interchange is a major transposition pathway in pyrazole chemistry, it is only a minor pathway upon irradiation of phenylisothiazoles in benzene solution. In fact, 4-phenylisothiazole (55), the compound most expected to react via the N-2-C-3 interchange pathway, was the only isomer that did not yield a transposition product upon irradiation in benzene solution (Scheme 27). This is not due to the photostability of the compound. Indeed, 55 is the most reactive of the six isomers. Nevertheless, even after consumption of 85% of 55, no phototransposition product could be detected. 

Pavlik and coworkers reinvestigated the phototransposition chemistry of these compounds. Their results differ from those reported by Vernin in several ways. Most importantly, careful capillary colunm gas chromatographic analysis, including coinjections with authentic samples of the three isomeric phenylisothiazoles 47-49, confirmed that 5-phenylthiazole 52 transposes to 4-phenylisothiazole 48 but that 5-phenylisothiazole 49 and 3-phenylisothiazole 47 are not formed in this reaction. As a consequence, Pavlik and colleagues concluded that it was not necessary to evoke the Kellogg mechanism to explain the formation of any products in these photoreactions [36]. 

These results show that within the tetrad (Scheme 9) the intermediate that incorporates deuterium is on both the 2-phenylthiazole 50 and 5-phenyl-isothiazole 49 to 3-phenylisothiazole 47 pathway but is not on the 4-phenylthiazole 51 to 3-phenylisothiazole 47 pathway. Accordingly, these photodeuteration results, which Maeda and Kojima concluded required the intermediary of tricyclic zwitterion intermediates, actually preclude their involvement in the phototransposition pathway. 

Vernin and colleagues conducted extensive studies of the phototransposition chemistry of phenylisothiazoles 13,14, and 15 and phenylthiazoles 16,17, and 18 in benzene solution (see also Table 98.2). ... 

A significant difference between the photochemistry of pyrazoles and the photoreactions of phenyKsothi-azoles in benzene solutions is the almost complete absence of transposition of the latter via the N2-C3 interchange reaction pathway. Indeed, although 4-phenyKsothiazole 14 is expected to be the most reactive isomer by this pathway, it is the only isomer that was not observed to yield a phototransposition product upon irradiation in benzene solution. This is not due to the low reactivity of the isomer because the quantum yield for disappearance of this compound (O = 0.41) indicates that it is the most reactive of the phenylisothiazole isomers. 

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