Structure and mechanism of hydrolysis of diaryl(acylamino)(chloro)-λ4-sulfanes and diaryl(acylamino)sulfonium salts

Abstract

Aryl (methylaminocarbonylaryl) sulfides were converted by t-BuOCl to diaryl(acylamino)(chloro)-λ⁴-sulfanes or the corresponding diaryl(acylamino)sulfonium chlorides depending on the substituent of the S-aryl group. ¹H NMR data showed that chloro-λ⁴-sulfanes exist only in CDCl₃ and DMSO-d₆ solvents, whereas in CD₃OD complete ionic dissociation takes place, leading to sulfonium chlorides. Both the chemical shifts of ¹H NMR signals and NOE data suggest that chloro-λ⁴-sulfanes and sulfonium salts having an o-MeO, o-Cl or o-Me substituent on the phenyl ring assume a skew conformation, whereas the aryl ring in compounds without an ortho-substituent can rotate practically free about the S–C(1′) axis. In o-MeO-substituted derivatives there exists an equatorial 1,4 type S ⋯ O close contact. Sulfonium salts with axial 1,5 type S ⋯ O close contacts involving neighbouring COOMe, CONHMe, COMe or NO₂ groups occur in butterfly conformation, like spiro-λ⁴-sulfanes. There is a correlation between the ¹⁵N chemical shift of the amide-nitrogen and the elongation of the S–N covalent bond, by which the interdepending S–N, S–Cl and S ⋯ O bonds can be characterized. Effective intermolecular S ⋯ O interaction was detected between the sulfonium centre and solvent molecules having a negatively polarized oxygen atom. The hydrolysis of sulfonium salts yielding sulfoxides was investigated by a kinetic method in 98∶2 (v/v) dioxane–water mixture and in water. On the basis of medium, substituent (ρ +1.03), steric, salt and kinetic isotope effects detailed mechanisms involving a hydroxy-λ⁴-sulfane intermediate are proposed. The more reactive sulfonium salts with a five-membered hetero ring are hydrolyzed by water, whereas the sulfonium centre of the less reactive analogues with a six-membered ring is attacked only by OH⁻ ions.