Phototranspositions

The phototransposition reaction in the isoindazole series (187) to give benzimidazoles (188) has been carefully investigated when R = Me, Et, Pr , Bu, Bz and Ph (67HCA2244). The reaction is sensitive to steric effects thus for r = Bu and R = Me, the l-t-butyl-7-methylbenzimidazole is not formed. 

Beckmann rearrangement, 4, 292 phototransposition, 4, 204 synthesis, 4, 223 Wittig reaetion, 4, 294 Wolff-Kishner reduetion, 4, 291 Pyrrole, 1-aeyl-barrier to rotation, 4, 193 IR speetra, 4, 21, 181 rearrangement, 4, 41 synthesis, 4, 82 thermal rearrangement, 4, 202 Pyrrole, 2-aeyl-aeidity, 4, 297 cleavage, 4, 289 conformation, 4, 33... 

Thiophene, 3-methyl-4-phenyl-phototranspositions, 4, 743 Thiophene, 2-(methylthio)-3,4-disubstituted synthesis, 4, 876, 931 Thiophene, nitro-... 

Thiophene, 3-pentadeuterophenyl-chemical shifts, 4, 730 Thiophene, 2-phenyl-oxidation, 4, 800 phototranspositions, 4, 743 rearrangement, 4, 42 reduction, 4, 775 synthesis, 4, 865, 914 UV spectrum, 4, 735 Thiophene, 3-phenyl-photochemical rearrangements, 4, 735 phototranspositions, 4, 743 lsmeier formylation, 4, 759 Thiophene, 2-pivaloyl-Birch reduction, 4, 775 Thiophene, polybromo-reactivity, 4, 829 Thiophene, polylithio-synthesis, 4, 831 Thiophene, (propargylthio)-rearrangement, 4, 746 Thiophene, 2-(3-pyridinyl)-synthesis, 4, 781 Thiophene, 2-(5-pyrimidinyl)-synthesis, 4, 781 Thiophene, 3-pyrrolidinyl-cycloaddition reactions, 4, 68 with dimethyl acetylenedicarboxylate, 4, 788-789... 

A final example that is so remarkable to deserve mention is the photoisomerization of alkenyne 61 to its isomer 63 (equation 21)149. In a recent reinvestigation of the reaction, Johnson and coworkers have provided computational evidence for the intermediacy of the 1,2-cyclobutadiene isomer 62 in the phototransposition reaction150. 

Compound 122 represents the product of a transposition of the substituents on the benzene ring. Such products are quite characteristic for this photodecarboxylation reaction. Their origin could be explained a priori by the well-known photoisomerization of substituted benzenes320 of type 121 and 126, which are formed primarily. Alternatively, it has been suggested60 that the phototransposition reaction occurs during the course of the decarboxylation. This supposition is based on two facts. First, no photo-Fries... 

Although evidence for thermally induced migrations points to a sigmatropic shift of the substituent, as discussed above, the photochemical conversion of 2-cyanopyrrole into 3-cyanopyrrole proceeds by a [1,3] sigmatropic walk of the heteroannular atom by route (a), as shown in Scheme 2. 2-Cyano-3-methylpyrrole and 2-cyano-4-methylpyrrole undergo similar phototranspositions to produce 3-cyano-4-methylpyrrole (by route a) and 4-cyano-2-methylpyrrole (by route a), respectively (75CC786), whilst 2-cyano-5-methylpyrrole produces... 

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]... 

At that time it was suggested that the isomerization occurred by way of an initial photo-ring contraction to an undetected 2-(A-methylimino)-2//-azirine intermediate (2) and subsequent ring expansion to the observed product, 1-methylimidazole (3). Although the intermediacy of acylazirines has been adequately demonstrated in the analogous isoxazole-to-oxazole phototransposition, such iminoazirines have not been detected in a pyrazole to imidazole isomerization, and thus, this mechanistic suggestion has never been experimentally substantiated. 

Direct irradiation of Z-enaminonitrile (8) and Z-enaminoisocyanide (9) revealed (Scheme 5) that both undergo Z— isomerization and photocyclization to 7, the N-2 -C-3 interchange product. In addition, cyclization of Z-9 to 7 was observed when the former was heated to 80 °C. Although the quantum yields shown in Scheme 3 reveal that the photocyclization of enaminonitrile Z-8 to imidazole 7 is a very inefficient reaction, chemical and quantum yields show that the photocleavage-photocyclization pathway via an isocyanide intermediate is a major route for the N-2-C-3 interchange phototransposition reaction <97JOC8325>. 

In order to study the 1-methylpyrazole to 1-methylimidazole phototransposition process with minimum substituent perturbation, the phototransposition chemistry of 3,4-dideuterio-l-methylpyrazole (1-3,4d2) has been studied. This labelling pattern allows distinction between the three pathways since Scheme 17 shows that the conversion of 1 to 2 via the three pathways is accompanied by transposition of C-5 of the reactant to ring position 5 by the N-2-C-3 interchange pathway or to ring positions 2 or 4 if the transposition occurs by electrocyclic ring closure followed by one or two nitrogen migrations respectively. 

The photoisomerization of isothiazole (47a) (Scheme 24) to thiazole (48a) was the first reported phototransposition in the isothiazole-thiazole heterocyclic system <69CC1018>. 

Although methylthiazoles do not undergo phototransposition upon irradiation in a variety of neutral solvents, methylthiazolium cations (49a-c), formed by dissolving the corresponding neutral methylthiazoles in trifluoroacetic acid (TFA), do undergo (Scheme 25) phototransposition to methylisothiazolium ions (50a-c) <93JOC3407>. 

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. 

N-2-C-3 interchange pathway. Although phototransposition was enhanced by the addition of base to the reaction medium, when the irradiation was carried out in ether or in methanol containing a small quantity of aqueous HC1 (Scheme 34), the phototransposition was completely quenched and the only product observed was a large yield of the photocleavage product (57H). 

As a result of these studies it has been suggested that the N-2-C-3 interchange phototransposition of isothiazoles to thiazoles is mechanistically similar to the analogous... 

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