Phototransposition chemistry of phenylisothiazoles and phenylthiazoles. 1. Interconversions in benzene solution

Article abstract of DOI:10.1021/ja00085a008

Phenylthiazoles 1-3 and phenylisothiazoles 4-6 undergo phototransposition in benzene solvent mainly by P₅, P₆, and P₇ permutation pathways. Phenylisothiazoles 5 and 6 also transpose via a P4 permutation process to yield phenylthiazoles 2 and 3 in less than 1% yield. In benzene saturated with D₂O, 2-phenylthiazole (1) and 5-phenylisothiazole (6) each phototranspose to yield 4-deuterio-3-phenylisothiazole (4-D-4) and 4-phenylthiazole (2) without deuterium incorporation. Irradiation of 4-phenylthiazole (2) under these conditions results in rapid photodeuteration to yield 2-deuterio-4-phenylthiazole (2-D-2), which subsequently phototransposes to 5-deuterio-3-phenylisothiazole (5-D-3). These experimental results can be rationalized by a mechanism involving initial electrocyclic ring closure and sigmatropic shift of sulfur around the four sides of the azetine ring. Rearomatization of each bicyclic intermediate thus allows sulfur to insert into each position in the carbon-nitrogen sequence. As a consequence, these compounds divide into a tetrad in which isomers 1, 2,4, and 6 interconvert mainly via P₅, P₆, and P₇ pathways and a dyad of two compounds in which 3 phototransposes to 5 via P₅ and P₇ pathways. Within the tetrad, BC-6, the bicyclic intermediate derived from 5-phenylisothiazoles (6), is postulated to undergo deuteration with simultaneous sigmatropic shift of sulfur when the reaction is carried out in benzene-D₂O. This mechanistic view provides one coherent interpretation for the observed phototransposition and photodeuteration reactions.