Photochemistry of 3- and 5-Phenylisothiazoles. Competing Phototransposition Pathways

Abstract

5-Phenylisothiazole undergoes phototransposition via the electrocyclic ring closure-heteroatom migration pathway and by the N(2)-C(3) interchange reaction pathway. The latter route is enhanced by the addition of triethylamine (TEA) to the reaction medium and by increasing the polarity of the solvent. In addition to phototransposition, 5-phenylisothiazole also undergoes photocleavage to 2-cyano-1-phenylethenethiol which was trapped by reaction with benzyl bromide to yield 2-cyano-1-phenylethen-1-ylbenzyl thioether. 3-Phenylisothiazole also phototransposes by both reaction pathways, but the product distribution is not affected by the addition of TEA or by changing the solvent polarity.

Theoretical study of photoinduced ring-isomerization in the 1,2,4-oxadiazole series

“…2 For a given heterocycle, competing pathways involving both ring contraction and internal cyclization have been documented, and they have been shown to depend on the structure of the starting ring and on the photoreaction medium. 4,5 In some cases, e.g. in the isothiazole series the occurrence of the above competing pathways was also affected by the presence of a base such as triethylamine (TEA) in the irradiation medium.…”

“…in the isothiazole series the occurrence of the above competing pathways was also affected by the presence of a base such as triethylamine (TEA) in the irradiation medium. 5 A theoretical approach has been used to rationalize many of these photochemical isomerizations. In this respect, we recently proposed a unified description by using semiempirical calculations, 6 -9 in some case also confirmed by ab initio studies.…”

Photochemistry of 5-nitro-1,2-benzisothiazole derivatives: effects of substituents, solvents and excitation wavelength

“…Five-membered aromatic compounds consisting of two heteroatoms, such as isothiazoles-thiazoles [4][5][6][7] or isooxazoles-oxazoles [4][5][6][7] have been studied in depth and are known to undergo the reversible photoisomerization. However, the photochemical behavior of their condensed ring analogues has not been widely studied.…”

Photochemistry of 4‐ and 5‐Phenyl Substituted Isoxazoles.

“…Thus, although the phototransposition chemistry 1-methyl-5-phenylpyrazole (29) [4] and 5-phenylisothiazole (30) [10] is known to involve competition between heterocyclic ring closure-heteroatom migration ( Path A, Scheme 1) and N2-C3 interchange (Path B, Scheme 1), this study reveals that 5-phenylisoxazole (4) phototranposes only via the P 4 pathway (Path B, Scheme 1).…”