Oxidation of Aryl Methyl Sulfoxides by Oxo(salen)manganese(V) Complexes and the Reactivity−Selectivity Principle

Abstract

The oxidation of various para-substituted phenyl methyl sulfoxides with several substituted oxo(salen)manganese(V) complexes follows an overall second-order kinetics that is first-order in sulfoxide and in oxo(salen)manganese(V) complex. Electron-releasing substituents in sulfoxides and electron-withdrawing substituents in oxo(salen)manganese(V) complexes enhance the rate of oxidation. The less nucleophilic sulfoxides are more sensitive to substituent effect (ρ = −2.44) compared to the corresponding sulfides (ρ = −1.86). These results are interpreted with a common mechanism involving the electrophilic attack of the oxidant on the sulfur center of the substrate. Correlation analyses show the presence of an inverse relationship between reactivity and selectivity in the reactions of various sulfoxides with a given oxo(salen)manganese(V) complex and vice versa. Mathematical treatment of the results leads to the conclusion that this redox system falls under strong reactivity−selectivity principle.

Mechanism of oxidation of aryl methyl sulfoxides with sodium hypochlorite catalyzed by (salen)MnIII complexes

“…However, in the oxidation of phenyl methyl sulfoxides by dioxiranes [34] (ρ = −0.76), gemdialkylperoxonium ion [35] (ρ = −0.83), Cr(VI)-oxalic acid [36] (ρ = −0.93), methyltrioxorhenium(VII) catalyzed H 2 O 2 [43] (ρ = −0.65), and oxo(phosphine)ruthenium(IV) complexes [44] (ρ = −0.42), although low ρ-values have been observed, an S N 2 mechanism has been postulated. In the present study, since the observed ρ-value is high, similar to the ρ-value (−2.44) observed for PhIO oxidation of sulfoxides catalyzed by (salen)Mn III complexes [10] and log k 2 values are better correlated with σ p than σ + /σ + , one may anticipate an S N 2 mechanism involving the ratedetermining electrophilic attack of the oxidant on the sulfur of the sulfoxide. The positive ρ-value of 0.50 ± 0.01 obtained from the Hammett correlation of log k 2 versus 2σ p (Fig.…”

“…Acetonitrile (GR, E. Merck) was first refluxed over P 2 O 5 for 5 h and then distilled. The [(salen)Mn III ] + PF 6 − complexes la-f were synthesized according to reported procedure [10,18]. The results of IR and UV-visible spectral studies of all the complexes were found to be identical with literature data [18].…”

“…As the isolation of pure oxo(salen)manganese(V) complexes is difficult [7][8][9][10]15,18], they were generated in situ for the studies reported here.…”

“…: (i) a rate-determining electrophilic attack of the oxidant on the sulfur of the sulfoxide [10,[32][33][34][35][36]; (ii) a single electron transfer from the sulfoxide to the metal ion in the rate-limiting step [37][38][39]; and (iii) a nucleophilic attack of the oxidant on the sulfoxide in the ratedetermining step [3,40,41], have been reported so far for the oxygenation of organic sulfoxides. In the present investigation, let us consider how the oxygen atom is actually transferred from the oxo species to the sulfoxide in the rate-limiting step.…”

“…We have initiated a systematic study on the oxygenation reactions of the organic sulfides and sulfoxides with oxometal complexes by taking Cr, Mn and Ru as metal ions. We have reported the mechanism of oxidation of organic sulfides [7][8][9] and sulfoxides [10] with PhIO catalyzed by (salen)Mn III complexes and also the successful application of the reactivity-selectivity principle (RSP) to these oxidation reactions. The mechanistic studies of (1)…”