Implications of X-Ray Crystallographic Results of 1,2,3,4-Tetrachloro-9-(2-oxopropyl)triptycene Rotamers

Abstract
Crystals of 1,2,3,4-tetrachloro-9-(2-oxopropyl)triptycene grown from tetrahydrofuran-hexane were found to contain 1:1 ap and ±sc rotational isomers about the C9–CPr bond by the X-ray diffraction method. The molecular structures had the following features. The acetyl group in the 9-substituent takes an O-inside, relative to the triptycene skeleton, conformation in both forms. The benzene rings are deformed by the steric effect to form boats. The 2-oxopropyl groups in both forms were planar within the experimental error but the C2–C1 bond in the 2-oxopropyl group was appreciably shorter in the ±sc conformation than that in the ap. The results are discussed on the ground of steric repulsion and the existence of the interaction between the C=O and the Cl groups which are located only 3.035 Å apart in the ±sc form.

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