5486
Oxidation of Thioethers by Iodine to Sulfoxides. Catalytic Role of Certain Inorganic Nucleophiles’ T. Higuchi and K.-H. Gensch2 Contributionfrom the School of Pharmacy, University of Wisconsin, Madison, Wisconsin. Received June 30,1966 Abstract: The rate of oxidation of several thioethers by iodine in aqueous solution has been found to be a complex function of the concentrations of sulfide, iodine, nucleophilic species, and iodide. In the absence of buffer species the In the presence of species such as rate law agrees with the formulation - d[RSR‘]/dt = k[RSR’][(Iz)t~~l]/~+][I-]2. HP02- the rate has been found, however, to be substantially increased (IO4 times in 0.1 MHP042’) and follows the relationship -d[RSR’]/dt = k[RSR’][02)t,~3[HP042-]/[Hf][I--]2 over a limited range. A mechanism is proposed on the basis that the catalyzed reaction probably involves in this instance an oxidative phosphorylative step.
T
he rate of formation of sulfoxides from the corresponding thioethers by oxidation with free iodine in aqueous solution as represented in eq 1 has R-S-R’
+ Iz + HzO +R-S-R’ + 2H+ + 21.1
(1)
0
been shown to be relatively In the present paper the oxidative rate is shown to be markedly accelerated by certain nucleophiles. The doubly charged anion, HP042-, for example, exhibits a catalytic activity such that the effect of concentrations as low as 1 X l W M is readily detectable. The behavior of these systems suggests that the observed effect such as in the case of phosphate may occur through an oxidative acylative mechanism. Results and Discussion Oxidation in the Absence of Catalytic Buffer Species. Although the rate of oxidation of thioethers by iodine in the absence of catalytic species is relatively slow, these reactions can be conveniently followed for nearly all aliphatic and alicyclic sulfides by determining the amount of acid produced in accordance with eq 1. In the presence of a constant concentration of iodide and hydrogen ions and with a large excess of thioether, the loss of iodine, as reflected in the amount of base required to maintain constant pH, occurred in a simple pseudo-first-order fashion. Since these observed pseudo-first-order rate constants were found to be, as may be expected, directly proportional in all cases to the concentration of the thioethers, it was possible to calculate observed second-order rate constants from these data. Over-all second-order rate constants obtained in this manner for the oxidation of tetramethylene sulfide are shown in Figure 1 at several pH values.6 The observed second-order rate constant at 25 O as defined by the equation d(mmo1es of base consumed per ml) kobsd = 2[RzSl[(I2)tota1ld~ (1) This research was supported in part by grants from the National Institutes of Health under AM-03437 and GM-05830. (2) NATO Scholar during the year 1961. (3) E. Fromm, Ann., 396, 75 (1913). (4) W. Steinkopf and S. Muller, Ber., 56, 1926 (1923). (5) I
