Structure and conformational processes of bis(o-cumyl)sulfide, sulfoxide and sulfone

The NMR solution spectra of the title sulfide and sulfone show decoalescence of the geminal methyl signals of the isopropyl groups at low temperature (−178 °C for the ¹³C signal of sulfide at 150.8 MHz and −147 °C for the ¹H signal of sulfone at 600 MHz). The barriers for the related dynamic processes were measured (4.3 and 7.0 kcal mol⁻¹ for the sulfide and sulfone, respectively). The preferred conformer of sulfide has a propeller shape with a C₁ symmetry, as suggested by Molecular Mechanics (MM) calculations. In the case of sulfone the preferred conformer has a propeller shape with a C₂-anti symmetry, as indicated by calculations and supported by X-ray crystallographic determination. The computed contour map of the potential energy shows that in both cases the dynamic processes take place via correlated rotations (cogwheel mechanism) of the two aromatic substituents about the Ar-S bonds. Dynamic processes could not be observed by NMR in the title sulfoxide, which was also found to adopt a propeller shaped conformation, as indicated by MM calculations and X-ray diffraction.