Isothiazoles photochemistry

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. 

These results reveal that both isothiazoles and pyrazoles phototranspose by the electrocycKc ring closure-heteroatom migration mechanism and that tricycKc zwitterionic intermediates, once thought to be important in thiazole and isothiazole photochemistry, need not be invoked. 

Similarly to the sulphenamides, the photochemistry of isothiazole is initiated by the fission of the weakest S—N bond192 (equation 137 and 138). 

The photochemistry of TV-substituted pyrazoles and of isothiazoles has been of considerable interest <76PHC123 80RGES501 94PP803 94PP1063 970P57> since the first report that 1-methylpyrazole (1) undergoes photoisomerization to 1-methylimidazole (Scheme 1) <67HCA44>. 

EFFECT OF ADDED BASE ON THE PHOTOCHEMISTRY OF ISOTHIAZOLES AND THIAZOLES. 

Although the photochemistry of 4-substituted isothiazoles in the presence of NH3 or TEA is dominated by reactions initiated by cleavage of the S-N bond, 5-phenyl- and 3-phenylthiazoles, (56) and (54), react under these conditions by competing photochemical pathways . 

The Photochemistry of Pyrazoles and Isothiazoles Table 5 Phototransposition of Methylthiazolium Ions... 

Convincing evidence for the involvement of an isocyanide in the P4 isothiazole -> thiazole phototransposition was obtained by studying the photochemistry of 4-benzylisothiazole 55. In this case, irradiation in methanol containing ammonia led to the formation of the nitrile photocleavage product 56, which was trapped by reaction with benzylbromide to yield benzylthioether 57,... 

Topics which have formed the subjects of reviews this year include photoinduced organic synthesis, photoisomerisations involving super-cyclophanes, regioselec-tive and stereoselective [2+2] photocycloadditions, position- and stereoselective photocyclisation, the photochemistry of indoles, five-membered heterocyclic compounds of the indigo group, pyrazoles and isothiazoles, and heterocyclic N-oxides, photochromic reactions of naphthopyran derivatives, photodegradation reactions of photochromic spirooxazines and 2H-chromenes, ° and chiral photo-chromic compounds. ... 

Topics reviewed during the year include the photochemistry of indoles, sulfoxides, pyrazoles and isothiazoles, (S-hetero)cyclic unsaturated carbonyl compounds, photoinduced single electron transfer (SET) reactions of amines and of azo compounds, SET reactions of organosilanes and organostannanes with Qo and ketones, photochromic polypeptides and di(hetero)arylethenes, processes in chromophore sequences on a-helical polypeptides,aryl-aryl coupling in furans, thiophenes and pyrroles," [3+2]cycloaddition of aromatic nitriles (and esters) with alkenes, and reactions of benzylsilane derivatives. ... 

The photoisomerization of isothiazole 5 to thiazole 6 (Scheme 3) was the first reported phototransposition in the isothiazole-thiazole heterocyclic system. Later work published during the period 1971 to 1978 by Lablache-Combier and co-workers, by Vernin and colleagues,"" andby Maeda and Kojima " also showed that methylisothiazoles phototranspose to methylthiazoles and that phenyl-substituted isothiazoles and thiazoles also undergo phototransposition reactions. A surprising finding, however, was that phenyhsothiazoles were not observed to react via the N2—C3 interchange pathway, an important pathway in pyrazole photochemistry. Furthermore, a common conclusion of these studies was that these photoisomerizations proceed by way of tricyclic zwitterionic intermediates, which had no counterparts in pyrazole photochemistry. 

These results indicate that the photochemistry of pyrazoles and isothiazoles can be understood in terms of one general mechanistic scheme involving competition between the electrocyclic ring clo-sure-heteroatom migration and the N2-C3 interchange pathwa). Although in the case of isothiazoles the addition of a base substantially alters the competition between these pathways, similar effects are not observed in pyrazole photochemistry. Presumably this is because pyrazoles and imidazoles are already sufficiently basic to deprotonate the azirine as shown in Scheme 17. 

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