Synthesis of dithia[3.3]biphenyleno(2,2′)(1,2)-, (1,3)-, (1,4)cyclophanes and their atropisomerism and dynamic stereochemistry

Abstract

The dithiacyclophanes (1), (2), (3) and (4) were prepared by intermolecular cyclization reactions under high dilution conditions. Detailed 1H NMR spectroscopic analyses and dynamic NMR studies have indicated that pseudo-rotation 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 1H 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. © 1993.