PREFERRED ORIENTATIONS OF THE S-O BOND IN METHYLSULFINYL DERIVATIVES OF FURAN AND THIOPHENE - AN EXPERIMENTAL-STUDY BASED ON H-1, C-13, AND O-17 NMR-SPECTROSCOPY

The preferred conformations of the methylsulphinyl group bonded to the heterocycles furan and thiophene have been studied with NMR techniques in solution. The presence of one halogen atom ortho to the methylsulphinyl group determines the orientation of the S-O bond in the opposite direction with respect to the halogen, and the degree of coplanarity of this bond with the ring depends on the substitution-pattern. The higher degree of coplanarity is found when the methylsulphinyl group and the halogen substituent are in position 3 and 4. Two ortho substituents determine structures with large twists of the methylsulphinyl group. The chemical shifts of H-1, C-13 and O-17 nuclei of the methylsulphinyl group and referred to spectra in solution do not evidence trends of general validity for all the compounds examined as a function of the orientation of the methylsulphinyl group with respect to the ring. Such trends were present in the case of the corresponding phenyl derivatives. Nevertheless, by examining the behaviour of long-range (n)J(C,H) coupling constants, involving the carbon nucleus of the methyl group, together with chemical shifts, conclusions regarding the conformational behaviour of these molecules in solution can be derived. Possible perturbations of the equilibrium caused by the polarity of the medium are evidenced as well. The results indicate that in the 2-methylsulphinyl