Conformationally constrained derivatives of 1,3‐dithianes (5‐tert‐butyl‐ and 4,6‐dimethyl‐1,3‐dithiane and dithiadecalin) have been oxidised with various oxidants to yield axial and equatorial sulfoxides, disulfoxides and sulfones. Axial sulfoxides were prepared by nucleophilic hydroxide addition to the corresponding 2‐alkylidene derivatives with subsequent retro‐aldol‐type elimination. The reaction outcome is related to the relative energies of the derivatives, as demontstrated by DFT calculations. The NMR spectroscopic data of the compounds obtained were analysed to identify 4J couplings. It was found that only 4J W couplings can be observed regardless of whether there is an axial or an equatorial sulfoxide, sulfide or sulfone group present in between the respective C–H moieties. A previously postulated γ‐gauche effect of axial sulfoxides (but not of equatorial sulfoxides or sulfones) leading to the shielding of carbon atoms is not unambiguously supported by the NMR spectroscopic data of conformationally fixed derivatives.
Though vinyl sulfoxides behave in some respects like alpha,beta-unsaturated carbonyl compounds, the mode of stabilization is significantly different. Interaction of the C=C double bond (acting as a donor) with the electron-withdrawing S=O bond is only possible when the p orbitals of the double bond are collinear with the S-O sigma* orbital. The maximum UV absorbance wavelengths in conformationally constrained substrates bearing an S=O bond collinear with the p orbitals of a C=C double bond are 2-14 nm higher than those in analogous compounds with a roughly orthogonal sulfoxide moiety. Similarly, a lone pair evolving during nucleophilic attack on vinyl sulfoxides is stabilized only if it is oriented anti to a S=O bond, which has significant impact on the stereoselectivities and reaction rates for nucleophilic attack on vinyl sulfoxides. The differing reaction rates of differently configured vinyl sulfoxides were measured in a competition experiment.
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