Effect of ortho substituents on the internal rotation processes and conformational preferences of 1,2-diaryl-1,1,2,2-tetrachloroethanes: a 1H and 13C NMR variable temperature and X-ray structural study

Abstract

The conformational behaviour of 1,2-diaryl-1,1,2,2-tetrachloroethanes, ortho,ortho′-disubstituted with CN (1), F (2) and CO₂CH₃(3), and of the mesityl derivative (4), has been studied in solution by variable temperature ¹H and ¹³C NMR spectroscopy. The internal rotation processes around the exocyclic C–C and central C–C bonds appear to have similar energy barriers and thus behave differently from the compounds without ortho substituents, studied previously, which have a higher barrier for the interconversion of anti/gauche conformers. At low temperature, compounds 1–3 exist as mixtures of gauche and anti conformers with the former prevailing; compound 4 is almost exclusively in the gauche form. The solid state crystal and molecular structure has been determined for compounds 2 and 3. The molecule of compound 2 has point symmetry , which defines perfect staggering around the central C–C bond with the aromatic rings anti. The structure is affected by statistical disorder due to two alternate orientations of the phenyl rings. The crystal structure of compound 3 indicates one asymmetrical molecule in a gauche conformation. Statistical disorder involves both the methoxycarbonyl substituents at the phenyl rings; these latter display almost the same orientation with respect to the C(sp³) carbons, in both compounds.