THIOPHENE 1-OXIDES .5. COMPARISON OF THE CRYSTAL-STRUCTURES AND THIOPHENE RING AROMATICITY OF 2,5-DIPHENYLTHIOPHENE, ITS SULFOXIDE AND SULFONE

The detailed preparation of 2,5-diphenylthiophene 1-oxide (2) is repor ted as well as the comparative study of the crystal structures of 2,5- diphenylthiophene, 1, its sulfoxide 2 and sulfone 3 obtained by X-ray diffraction. This work represents the first experimental study of a co mplete heterocyclic series, including a thiophene derivative, and the corresponding sulfoxide and sulfone. On the basis of the geometrical p arameters, the first unequivocal experimental parameters obtained for a thiophene 1-oxide derivative, we have also examined the evolution of the aromatic character of the thiophene ring when oxidizing the sulfu r atom to the sulfoxide and the sulfone. Paolini's bond orders and Jul g and Francois's aromaticity indices have also been calculated for the three compounds and compared to those previously calculated for relat ed thiophene derivatives by semi-empirical or ab initio methods [6], [ 7]. All the data examined showed that, in spite of its non planarity, the thiophene ring of 2,5-diphenylthiophene 1-oxide 2 could still exhi bit some delocalization of its pi electrons indicating a certain degre e of aromaticity lower than in thiophene 1 but higher than in the sulf one 3.