Weak interactions in the crystal structures of polyhalogenated enantiopure sulfoxides

In the last decade, we have built up a conspicuous chemical library of enantiopure sulfoxides by applying our innovative methods of asymmetric synthesis [1]. The crystal structures of many of the items of this library were analyzed by X-Ray diffraction. In our early paper on the patterns of enantiopure p-haloaryl sulfoxides [2], both hydrogen and halogen bondings were recognized as the interactions assembling the crystal structures. Recently, as a progress of our investigation on the mechanism of the enantioselective oxidation of sulfides, we oxidized successfully fluorinated substrates [3], and we suggested an oxidation mechanism, supported also by DFT calculations [3], based upon weak interactions due to fluorine atoms. Now we wish to report our observations on the crystal structures of fluorinated enantiopure sulfoxides. The main motif that we found in these structures is the stacking interactions between pentafluorophenyl and aryl groups, since this stacking is a strong and well-known synthon in crystal engineering [4]. Beyond this main interaction, we observed also other short contacts below the sum of the van der Waals radii, between the sulfinyl oxygen and the fluorine atom, and between two different fluorine atoms [5]. The geometric characteristics of the first short contacts belong to the "Type I" classification, as other similar cases previously reported [5]. On the other hand, the second short contact that we observed has been often subjected to controversies [5]. In particular, in the crystals of (R)-benzyl pentafluorophenyl sulfoxide, we observed a short contact between the fluorine atom in the meta-position with another one in the para-position of two different pentafluorophenyl groups. All these interactions will be briefly reviewed and discussed.