Oscillation of spontaneously generated optical activity in (±)-1-(p-nitrobenzoyloxy)indane solutions: 'serial SN2 reactions' in aggregates supported by the crystal structure. Frozen SN2 transition states and enantioselective distortions

(±)-Indan-1-yl 4-nitrobenzoate upon dissolution in CH2Cl2, 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 Å) between the C1 chiral centre in one molecule, and the oxygen atom at C1 in a neighbouring molecule of the same 'R' configuration. Corresponding changes in bond lengths and torsion angles indicate a set of frozen SN2 transition states. These interactions apparently persist in the aggregated solution, enabling a 'serial SN2' process which can invert the C1 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.