Conformational polymorphism in thiophenyl substituted quinones and the disappearing NMR spectrum |
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Authors: | Ram P Kashyap Duoli Sun William H Watson |
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Institution: | (1) Department of Chemistry, Texas Christian University, 76129 Fort Worth, Texas |
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Abstract: | The stepwise addition of thiophenol to benzoquinone gives thiophenylbenzoquinone (1b), 2,5-and 2,6-di-(thiophenyl)benzoquinone (2,3),2,3,6-tri(thiophenyl)benzoquinone (4) and 2,3,5,6-tetra(thiophenyl)benzoquinone. (5). Compounds1–4 can be crystallized, and the13C NMR spectra are readily interpreted. Compound5 could not be crystallized easily, and the NMR showed more than twice as many lines as the number of carbon atoms. Slow evaporation in an NMR tube produced three distinct crystals. X-ray analysis of two crystals (5a and5c) showed the compounds to be conformational polymorphs. Direct synthesis of5 from tetrachlorobenzoquinone and thiophenol yielded only conformer5a, and the13C NMR spectrum showed only the 6 lines expected for four equivalent phenyl substituents on benzoquinone. Subjecting5a to a variety of solvents, reagents and temperatures did not regenerate the original13C NMR spectrum. The crystal structures, conformations, and polymorphs are discussed. |
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Keywords: | Thiophenylquinones polymorphism conformers molecular mechanics |
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