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(\pm)-Indan-1-yl 4-nitrobenzoate upon dissolution in $CH_2Cl_2$, surprisingly displays optical activity-usually levorotation which swings to dextrorotation over several hours, and gradually vanishes. Its crystal structure (redetermined for this study) reveals an unusually close contact (1.471\AA) between the $C_1$ chiral centre in one molecule, and the oxygen atom at $C_1$ in a neighbouring molecule of the same 'R' configuration. Corresponding changes in bond lengths and torsion angles indicate a set of frozen $S_N2$ transition states. These interactions apparently persist in the aggregated solution, enabling a ’serial $S_N2$‘ process which can invert the $C_1$ configurations in the (presumed) chiral aggregates. The selective distortion of the 'R' enantiomer in the crystal, and the spontaneous generation of optical activity upon dissolution, indicate processes that are akin to a second order asymmetric transformation. |
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