H. Rahmani, H. Pirelahi
Dec 1, 1997
Citations
0
Influential Citations
17
Citations
Journal
Journal of Photochemistry and Photobiology A-chemistry
Abstract
Abstract The selective photoisomerization of tetrasubstituted 4-aryl-4-methyl-2,6-diphenyl-4 H -thiopyrans ( 4a-4f ) are investigated from a kinetic point of view using 1 H-NMR spectroscopy. On exposure to a monochromatic UV light at λ = 254 nm, these compounds rearrange to 6-aryl-5-methyl-1,3-diphenyl-2-thiabicyclo[3.1.0] hex-3-en intermediates ( 5a-5f ), then convert to thermodynamically more stable isomers, 2-aryl-4-methyl-3,6-diphenyl-2 H -thiopyrans ( 6a-6f ) in quantitative yields. In the course of photoisomerization, only the intermediates 5a-5f and unreacted 4 H -thiophyrans 4a-4f are present in the reaction meida. It is found that the electron donating or electron withdrawing groups on the para position of the migratory 4-aryl group increase the relative rates of photoisomerizations in methanol-d 4 solutions. The efficiencies of photoisomerizations in methanol-d 4 are greater than in benzene-d 6 solutions, depending upon the substituents used. On the basis of the observed intermediates, the effects of substituents and the polarity of solvents, a mechanism is proposed for the photoisomerization of 4 H -thiopyrans 4a-4f to bicyclic intermediates 5a-4f .