Neighboring Group Participation in Radicals: Pulse Radiolysis Studies on Radicals with Sulfur-Oxygen Interaction

Neighboring group participation by alcohol and carboxylate groups resulting in kinetic and thermodynamic stabilization of an oxidized sulfur atom in various organic sulfides is reported. The resulting radical intermediates of the general type (-S↔O)' are characterized by an optical absorption in the 400-nm range and exhibit lifetimes of up to several hundred microseconds in aqueous solution under pulse radiolysis conditions. Significant sulfur-oxygen interaction seems to occur, however, only if both heteroatoms are separated by three or four carbon atoms in the unoxidized molecule which enables favorable five- or six-membered ring structures in the radical intermediates. This geometric effect can additionally be favored by minimizing the free rotation of the functional groups through rigid molecular structures, e.g., in norbornane derivatives, and introduction of particular substituents. A most suitable function for stabilization of an oxidized sulfur atom seems to be a carboxylate group where an overall neutral radical of the general structure -S^•+↔^-OOC is formed. In these species stabilization can be envisaged to involve the carboxylate group as a whole rather than only an individual oxygen atom. The bond strength of the sulfur-carboxylate interaction is estimated to be of the order of 50 kJ mol^-1as deduced from the temperature dependence of its dissociation. Further evidence for net sulfur-carboxylate bonding is provided by rate constants of 10⁵–10⁷M^-1s^-1for its proton-assisted decay. These rates are considerably lower than for the diffusion controlled protonation of free carboxylate functions. Oxidation of endo-2-(2-hydroxyisopropyl)-endo-6-(methylthio)bicyclo[2.2.1]heptane yields a transient where sulfur-oxygen interaction is associated with a strong acidification of the alcoholic hydroxyl group. A pK = 5.9 has been measured for the (-S↔OH)^•+⇄(-S↔O)^•+ H⁺equilibrium. All the results on these transient radical intermediates can be viewed in terms of neighboring group participation. Such participation is also clearly evidenced in the formation and properties of intermolecular radical cations, (R₂S.^..SR₂)⁺, derived from these sulfides, and in the kinetics of the primary oxidation process. Absolute rate constants for the one-electron oxidation of various sulfides by CC1₃OO^•radicals, for example, have been found to range from 3 X 10⁸to <10^-6M^-1s^-1depending on the nature of the oxygen containing functional groups. The present results are finally discussed in terms of the general possibility of stabilization of an oxidized sulfur function by other heteroatoms. Several electronic structures including the two-center-three (2σ/lσ*) electron bond can be advanced to describe the physicochemical properties of the radicals with sulfur-oxygen interaction.