Stereoisomeric 2-butylphenylsulfoxides and their binding modes in the adduct formation with an enantiopure dirhodium tetracarboxylate complex

All stereoisomers of the 2-butylphenylsulfoxides 1a and 2a and their p-substituted derivatives 1b-1e and 2a-2e (X = F, Br, NO2, and OCH3) were synthesized. Absolute configurations were derived from commercial enantiopure 2-butanols used as starting materials, by X-ray diffraction and by polarimetry. Preferred conformations were determined by density functional and second-order Møller-Plesset calculations. Oxygen atoms dominate in the adduct formation equilibria of 2-butylsubstituted sulfoxides and the chiral dirhodium complex Rh∗ although the sulfur atom is, in principle, the stronger donor. This is due to steric shielding of the sulfur atom produced by the aromatic ring and the secondary 2-butyl substituent. Enantiodifferentiation of sulfoxides is easily accomplished by the dirhodium experiment, i.e. recording NMR spectra in the presence of an equimolar amount of Rh∗. Complex formation shifts (Δδ) and diastereomeric dispersion effects (Δν) differ in the dirhodium experiment for nonracemic mixtures of sulfoxides as compared to pure enantiomers. This, however, does not affect the efficiency of the dirhodium experiment at all.