Kinetic study on photoisomerization of some tetra- and hexasubstituted 4H-thiopyrans

The effects of phenyl and methyl groups at 3,5-positions of tetrasubstitute d 4H-thiopyrans 1a and Ib on photoisomerization are investigated from a kin etic point of view using H-1-NMR spectroscopy. On irradiation of 1a-1d in b enzene-d(6) solution the hexasubstituted 4H-thiopyrans Ic and Id, unlike th ose of the tetrasubstituted analogues 1a and 1b, give the isomeric 2H-thiop yrans 3c and 3d with no detectable signals for intermediates in H-1-NMR spe ctra. The photoisomerization of hexasubstituted 4H-thiopyrans 1c and 1d occ ur with relative rate constants lower than the corresponding tetrasubstitut ed model compounds 1a and 1b. Moreover, the kinetic comparison of 1a with 1 b reveal that the presence of two phenyl groups at 4-position of tetrasubst ituted 4H-thiopyran increases the relative rate constant of photoisomerizat ion.