Photochemistry of pyrazoles

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

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. 

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. 

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>. 

As in previous years, much time and effort have been devoted to the study of the photochemistry of cyclic azo-compounds. Photoreactions of this type offer a unique opportunity to examine the mechanism of this decomposition, and also provide a useful synthetic route to cyclopropane derivatives. Irradiation of several steroidal 4 S,5-dihydro[17a, 16-c]pyrazoles leads exclusively by elimination of nitrogen to cyclopropa[16a,17a] compounds.6 The penicillin derivatives... 

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. ... 

Photochemistry of N-heterocyclics Benzotriazole 1-oxide from pyrazole ring... 

This chapter deals with the photoisomerization reactions of pyrazoles, 1,2,4-oxadiazoles, pyridine, and its simple derivatives and pyridinium cations. The photochemistry of nitrogen-containing heteroaromatic compounds that also contain sulfur are considered in another chapter. 

The photochemistry of 1-methylpyrazoles 12 through 14, in which the various ring carbons are systematically labeled with a second methyl group, has also been studied. The primary products shown in Scheme 8, and the results of deuterium labeling studies shown in Scheme 9 confirm that these dimethyl-pyrazoles undergo only pyrazole-to-imidazole phototransposition by as many as three distinct pathways. 

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. 

Caboni P, Sammelson RE, and Casida JE (2003) Phenyl-pyrazole insecticide photochemistry, metabolism, and GABAergic action Ethiprole compared with fipronil. Journal of Agricultural and Food Chemistry 51 7055-7061. 

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