Phenylisothiazoles, isomerization irradiation

Vernin et al. independently observed that during the irradiation in benzene of 2-iodothiazole to generate 2-thiazolyl radicals, the resulting 2-phenylthiazole (216) isomerized into a mixture of 4-phenylthiazole (214) and 3-phenylisothiazole (213) (Scheme 103) (489). Marking the 4- and... 

In the dyad, because of the symmetry of the 3-phenylazetine ring in BC-53 (Scheme 29), insertion of sulfur between ring positions C-l and C-4 or CC-1 and C-2 leads to the same compound, 5-phenylthiazole (53). Similarly, insertion of a sulfur atom between N3 and C4 or N3 and C2 leads to 4-phenylisothiazole (55). Accordingly, because of this symmetry, only a dyad results. This symmetry is removed, however, in the case of 2-deuterio-5-phenylthiazole (53-2d). Thus, irradiation of 53-2d resulted in the formation of three isomeric products (Scheme 30), viz., 4-deuterio-5-phenylthiazole (53-4d), 5-deuterio-4-phenylisothiazole (55-5d), and 3-deuterio-4-phenylisothiazole (55-3d). D euterium 1 abelling h as t hus e xpanded t he d yad i nto a tetrad. These results are entirely consistent with the electrocyclic ring closure - heteroatom migration mechanism shown in Scheme 31 <94JA2292>. 

According to the observed photochemical products and the results of deuterium labeling studies, the six isomeric phenylisothiazoles and phenyl-thiazoles can be organized into a tetrad of four isomers that interconvert mainly via P5, Pg, and P, transposition pathways and a dyad in which 5-phenylthiazole 52 transposes via P5 and P7 pathways to 4-phenylisothiazole 48 (Scheme 8), the only isomer that did not yield a transposition product upon irradiation in benzene solution. With one minor exception, no interconversions between the tetrad and dyad were observed. In that case, in addition to transposing to members of the tetrad, 5-phenylisothiazole 49 also transposed to 5-phenylthiazole 52, the first member of the dyad, in less than 1 % yield. This conversion was assumed to occur via a P4 permutation process. 

DjO [37-40]. These workers reported that irradiation of 2-phenylthiazole 50, 4-phenylthiazole 51, or 5-phenylisothiazole 49 under these conditions resulted in the formation of 3-phenylisothiazole 47-4d with deuterium incorporation into ring position 4. In the case of 2-phenylthiazole 50, 4-phenylthiazole 51 was also formed but without deuterium incorporation. Finally, they reported that none of the reactants underwent photodeuteration prior to isomerization. 

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