CONFORMATIONAL POLYMORPHISM IN THIOPHENYL SUBSTITUTED QUINONES AND THE DISAPPEARING NMR-SPECTRUM

The stepwise addition of thiophenol to benzoquinone gives thiophenylbe nzoquinone (1b), 2,5-and 2,6-di-(thiophenyl)benzoquinone (2,3),2,3,6-t ri-(thiophenyl)benzoquinone (4) and 2,3,5,6-tetra-(thiophenyl)benzoqui none (5). Compounds 1-4 can be crystallized, and the C-13 NMR spectra are readily interpreted. Compound 5 could not be crystallized easily, and the NMR showed more than twice as many lines as the number of carb on atoms. Slow evaporation in an NMR tube produced three distinct crys tals. X-ray analysis of two crystals (5a and 5c) showed the compounds to be conformational polymorphs. Direct synthesis of 5 from tetrachlor obenzoquinone and thiophenol yielded only conformer 5a, and the C-13 N MR spectrum showed only the 6 lines expected for four equivalent pheny l substituents on benzoquinone. Subjecting 5a to a variety of solvents , reagents and temperatures did not regenerate the original C-13 NMR s pectrum. The crystal structures, conformations, and polymorphs are dis cussed.