Restricted Rotation Involving the Tetrahedral Carbon. XLIV. Atropisomers of 2,3-Dichloro-9-(1-hydroxy-1-methylethyl)triptycene and Related Compounds

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Abstract
Atropisomers of 2,3-dichloro-9-(1-hydroxy-1-methylethyl)triptycene were obtained as stable compounds at room temperature. The barrier to rotation (±sc→ap) was 34.0 kcal/mol at 153 °C. Other 9-(1-hydroxy-1-methylethyl)triptycenes carrying a substituent in the 1-position of the skeleton gave ±sc isomers only. Attempted isomerization of the ±sc forms failed because the free energy difference between the ±sc and ap forms is great due to the steric effect. ±sc-9-(1-Methoxy-1-methylethyl)-1,4-dimethyltriptycene was prepared in a hope that increasing the steric size of the hydroxyl group would help to produce an ap-isomer on heating. The ap isomer was detected but the population ratio ap⁄±sc was too small to isolate the ap.

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