Abstract
Irradiation of 2,4,4,6-tetraaryl-4H-thiopyrans 4a-4c in methanol solutions led to a mixture of 1,3,5,6-tetraaryl-2-thiabicyclo[3.1.Olhex-3-ene intermediates 5a-5c and the thermodynamically more stable 2,3,4,6-tetraaryl-2H-thiopyran isomers 6a-6c with no signs of byproducts. The intermediates 5a-5c were separated and the photoisomerization of 4a-4c investigated from a kinetic point of view using HPLC. The relative rate constants reveal that the formation of the intermediate is the rate-determining step in the photoisomerization of tetraarylsubstituted 4H-thiopyrans. It is also found that the electron donating paramethoxy substituents on 2,6-aryl groups, unlike those of the migratory 4,4-aryl groups, decrease both the formation and conversion relative rate constants of the corresponding intermediates.
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