Intramolecular hydrogen transfer as the key step in the dissociation of hydroxyl radical adducts of (alkylthio)ethanol derivatives

Article abstract of DOI:10.1021/ja00068a010

The reaction of the hydroxyl radical with dimethyl sulfide (DMS), 2-(methylthio)ethanol (2-MTE) 2,2′ dihydroxydiethyl sulfide (2,2′-DHE), and 3,3′-dihydroxydipropyl sulfide (3,3′-DHP) has been investigated in H₂O and D₂O As an initial step hydroxyl radicals add to the sulfur moiety. These hydroxyl radical adducts subsequently decay via a thioether concentration-dependent and a thioether concentration-independent pathway. The hydroxyl radical adduct of DMS dissociates into a sulfur radical cation and HO^- in the thioether concentration-independent pathway (k_H/k_D = 2.09), whereas a rate-limiting proton transfer from water operates in the thioether concentration-dependent mechanism (k_H/k_D = 5.40), as deduced from the measured solvent kinetic isotope effects. In contrast the hydroxyl radical adducts of 2-MTE and 2,2′-DHE decompose via elimination of water, formed through a rapid intramolecular hydrogen transfer from the adjacent hydroxyl groups. This mechanism leads to the formation of (alkylthio)ethoxy radicals The latter undergo α,β-fragmentation into formaldehyde and α-thioether radicals as well as hydrogen abstraction from a δ-methylene group, analogous to a hydrogen transfer in the Barton reaction, leading to α-thioether radicals. The overall rate constants for these unimolecular reaction sequences were determined to be k_12,H =(6.32 ± 0.7) × 10⁷ s^-1 for 2-MTE and k_15,H = (1-17 ± 0.2) × 10⁵ s^-1 for 2,2′-DHE. Neither of them show an appreciable kinetic isotope effect, suggesting that the actual hydrogen transfer is not the rate-determining step in the overall process.