The thermal sulfenate-sulfoxide rearrangement: A radical pair mechanism

The thermal reaction of sulfenates (RS-OR), yielding their corresponding sulfoxides (RS(=O)R), was studied experimentally. The first step of the reaction was found to be the formation a radical pair by homolytic cleavage of the carbon-oxygen bond of the sulfenate. The two transient radicals formed then recombine to form the carbon-sulfur bond of the sulfoxide. The thermolysis of cinnamyl-4-nitrobenzenesulfenate has a positive entropy of activation (Delta S = 6.4 +/- 2.0 eu in toluene), characteristic of a dissociative pathway, A normal secondary kinetic isotope effect (k(H)/k(D) = 1.19 +/- 0.04) was also measured with this substrate, Finally, a trapping experiment allowed the isolation and characterization of a product coming from the coupling of the cinnamyl radical and TEMPO, These studies confirm a mechanism that was proposed earlier based on computational studies. The experimentally determined bond dissociation energy of the carbon-oxygen bond of similar to 28 kcal.mol(-1) is in good agreement with the computed value of similar to 26 kcal.mol(-1). These studies confirm a unique structural feature of the sulfenate moiety, where the weakest bond of the molecule in the ground state is not the heteroatom-heteroatom bond intuitively considered to be the weakest based on the analogy to peroxides or disulfides. Radical stabilizing substituents are expected to have a large effect on the thermal reactivity of sulfenates. Evidence for a competing acid-catalyzed mechanism has also been observed.