KINETIC STUDY ON PHOTOISOMERIZATION OF SOME TETRA- AND HEXASUBSTITUTED 4H-THIOPYRANS

Abstract

The effects of phenyl and methyl groups at 3,5-positions of tetrasubstituted 4H-thiopyrans 1a and 1b on photoisomerization are investigated from a kinetic point of view using ¹H-NMR spectroscopy. On irradiation of 1a-1d in benzene-d₆ solution the hexasubstituted 4H-thi-opyrans 1c and 1d, unlike those of the tetrasubstituted analogues 1a and 1b, give the isomeric 2H-thiopyrans 3c and 3d with no detectable signals for intermediates in ¹H-NMR spectra. The photoisomerization of hexasubstituted 4H-thiopyrans 1c and 1d occur with relative rate constants lower than the corresponding tetrasubstituted model compounds 1a and 1b. Moreover, the kinetic comparison of 1a with 1b reveal that the presence of two phenyl groups at 4-position of tetrasubstituted 4H-thiopyran increases the relative rate constant of photoisomerization.