SYNTHESIS OF DITHIA[3.3]BIPHENYLENO(2,2')(1,2)CYCLOPHANE, DITHIA[3.3]BIPHENYLENO(2,2')(1,3)CYCLOPHANE, DITHIA[3.3]BIPHENYLENO(2,2')(1,4)CYCLOPHANE AND THEIR ATROPISOMERISM AND DYNAMIC STEREOCHEMISTRY

The dithiacyclophanes (1), (2), (3) and (4) were prepared by intermolecular cyclization reactions under high dilution conditions. Detailed H-1 NMR spectroscopic analyses and dynamic NMR studies have indicated that pseudorotation of the biphenyl unit is restricted up to a temperature of 443K in (1), (2) and (3) involving energy barriers estimated to be >75 kJ mol-1. At room temperature, a flipping process of the 1,3-bridged ring in (1) and (2) is however clearly evident based on analyses of the aromatic protons in the 1D and 2D H-1 NMR spectra. The dithiacyclophane (4) proves to be conformationally the most mobile among the three members and exhibits two-step conformational processes involving first an inversion process of the 1,2-bridged ring followed by a pseudo-rotation (racemization) of the biphenyl moiety, with an estimated free energy of activation of 61.9 kJ mol-1, at high temperatures.