Thiolsulfinate decomposition

Figure 3. Effect of base on decomposition of cumene hydroperoxide in the presence of tert-butanethiosulfoxylic add from thiolsulfinate decomposition. Concentrations, mmol/1 in benzene CHP 2.0 0.1 A, CaCOs 0.08 B, tert-BuS(0)S-tert-Bu 0.2 C, tert-BwS(O)S-tert-Bu 0.2, CaCOs 0.08 D, O tert-BuS(0)S-tert-Bu 0.2, CaCOs 2.5 (15).

We have thus established that sulfides, disulfides, and their initial oxidation products are not the actual preventive antioxidants. The active peroxide decomposers are the sulfenic acid from sulfoxide decomposition, the thiosulfoxylic acid from thiolsulfinate decomposition, and the acidic products formed when they react with hydroperoxides. The catalytic... 

Aryl thiolsulfinates, ArS(0)SAr, which cannot give cycloeliminations of the type shown in (57) or (58) do, however, undergo an alternative thermal decomposition (Koch et al., 1970). Although this decomposition has sometimes been shown as having the stoichiometry of a simple disproportionation (73), somewhat more disulfide than thiolsulfonate is actually... 

Second, partial decomposition of an aryl thiolsulfinate specifically labeled (35S) at the sulfinyl sulfur indicates (a) some incorporation of label in ArSSAr, which would not be expected if (75) were the only path for formation of disulfide (b) a significantly unequal distribution of label beween the two sulfurs of ArS02SAr [in its simple form (76) predicts both sulfurs should be equally labeled] (c) some incorporation of label into the sulfenyl sulfur of the recovered unreacted thiolsulfinate. The specific reactions responsible for these variations of the 35S-distribution from the pattern predicted by (74)-(76) cannot be pinpointed with certainty, although Koch et al. (1970) considered that a homolytic decomposition (80) of sulfenyl sulfinate [20], competitive with its... 

Under some conditions. the kinetics of (73) show evidence that induced decomposition of the thiolsulfinate becomes important (Koch et al., 1970). A reaction sequence for the induced decomposition that satisfies the kinetics is (81)—(82). Each of these reactions can be envisaged as occurring in two... 

All possible combinations of methyl, propyl, allyl, and 1-propenyl disulfides (primarily), monosulfides, and trisulfides have been found among the volatile flavor components of onion (28,29, 30,31), garlic (32), caucas Allium victorialis) (33), and other Allium species 28) although proportions vary with species. These compounds are presumably derived from the corresponding thiolsulfinates. This is accomplished either by direct decomposition by an unknown mechanism with evolution of SO2 (32) or by interaction with cysteine to produce a mixed disulfide (15),... 

Recently we reported a study of the peroxide decomposing activity of sulfoxides, sulfenic acids, thiolsulfinates, and their oxidation or decomposition products (2,15). A benzene solution of the sulfenic acid reacted rapidly with both tert-butyl hydroperoxide and cumene hydroperoxide, consuming two moles of hydroperoxide per mole of sulfur compound. A slower catalytic process destroyed many additional moles of hydroperoxide per mole of sulfur compound. A similar solution of cumene hydroperoxide in benzene with thiolsulfinate present in a ratio of 10 moles of hydroperoxide per mole of sulfur compound showed no change for 22 hr at 25°C. At that time a catalytic decomposition started which destroyed many moles of hydroperoxide per mole of sulfur compound. 

Thiolsulfinates and their reaction products play an important role in the preventive antioxidant activity observed with organic sulfides and disulfides. An investigation of the decomposition of cumene hydroperoxide in benzene at 25°C in the presence of tert -butyl tert-butanethiolsulfinate has shown that the actual peroxide decomposer is an acidic species whose activity is affected by the basic character of the S-O group in the parent thiolsulfinate and in the sulfoxides. Alternative mechanisms for generating the acidic species are discussed. Although hydroperoxide decomposition occurs primarily by a polar process, the results also indicate the involvement of radical generating processes. 

The polar nature of the catalytic decomposition of cumene hydroperoxide with added terf-butyl terf-butanethiolsulfinate has been clearly established (I). The thiolsulfinate is converted into an active peroxide decomposer capable of destroying many moles of hydroperoxide per mole of sulfur compound. The acidic character of the active species was demonstrated by its effective neutralization with the added base calcium carbonate. Formation of the active peroxide decomposer may be envisaged as involving one or more of the following three reaction types concerted process, ionic processes, and free-radical processes. 

Reactions 3, 4, 5, and 6 would then ensue with sulfonic acid being the catalytic peroxide decomposer, and the major source of H+ for the induced decomposition of the thiolsulfinate. The disproportionation of thiolsul-finates is accelerated markedly by the addition of substances such as alkyl sulfides that contain a more nucleophilic sulfur atom than found in thiolsulfinates. 

Homolytic processes Evidence also has been presented for the radical-induced decomposition of thiolsulfinates (6,7). Homolytic cleavage is facilitated by the weak S-S bond ( 40kcal). The availability of sulfidic sulfur for radical attack is indicated by the observation that thiolsulfinates strongly retard the free radical polymerization of vinyl monomers (8). 

Figure 3. Effect of various sulfoxides on the decomposition of cumene hydroperoxide in the presence of tert-butyl tert-butane-thiolsulfinate at 25° C (from Ref. 2) O 0.21M CHP and 0.02M tert-BuSS(0)tert-Bu 0.20M CHP, 0.02M C6H5S(0)C6H5, and 0.02M tert-BuSS(0)tert-Bu 0.20M CHP, 0.02M CHsS-(0)CHs, and 0.02M tert-BuSS(0)tert-Bu 0.20M CHP, 0.02M tert-BuS(0)tert-Bu, and 0.02M tert-BuSS(0)tert-Bu...

The effect of water on the rate of cumene hydroperoxide decomposition in the presence of thiolsulfinate is shown in Figure 1. The displacement of thiolsulfinate by water is not the primary means of generating the active peroxide decomposer. To the contrary, water inhibits peroxide decomposition. Water may be expected to hydrate the thiolsulfinate (9) and the acidic decomposing species thereby decreasing the observed hydroperoxide decomposition. 

Since thiolsulfinates are very weakly nucleophilic, cleavage of the S-S bond should be subject to nucleophilic assistance if Reaction 4 operates. Figure 2 shows the effect of added n-butyl sulfide on the thiolsulfinate hydroperoxide reaction. From Figure 2, the addition of sulfide —whether thiolsulfinate is present or absent—results in the consumption of one mole of hydroperoxide per mole of sulfide presumably forming the sulfoxide and thereby preventing further hydroperoxide decomposition. The inhibitory effect of sulfoxides on the thiolsulfinate hydroperoxide reaction has been noted previously (see Figure 3) (2). 

In Figure 3 phenyl sulfoxide has a small but distinct retarding effect on the decomposition of hydroperoxides by thiolsulfinate solutions. Since the thiolsulfinate is comparable in basicity to the phenyl sulfoxide, the thiolsulfinate should exert a similar effect, and no decomposition should occur until sufficient acid is generated. The addition of the more basic methyl sulfoxide or tert-butyl sulfoxide prevents hydroperoxide decomposition by effectively complexing the active acidic species. 

It has been established that the decomposition of cumene hydroperoxide in the presence of thiolsulfinate occurs primarily via a polar process (1). However, a homolytic process may be involved in the conversion of the thiolsulfinate to the active peroxide decomposer. This was probed by adding the radical inhibitors /2-naphthol and 2,6-di-tert-butyl-4-methylphenol (see Figure 5). The inhibitors totally suppressed the decomposition of hydroperoxide by thiolsulfinate. In contrast to /3-naphthol and 2,6-di-terf-butyl-4-methylphenol, the addition of cyclo-hexanol had no significant effect. Similarly, the addition of methanol only reduced the decomposition of hydroperoxide by the thiolsulfinate by 1% after 186 hr. 

The following reaction sequence is suggested. Initially the thiolsul-finate undergoes an intramolecular cycloelimination reaction as in Reaction 1. Extensive supporting evidence for this postulate has been presented by Block (4) from studies on the decomposition of thiolsulfinate under mild conditions (< 100°C). Since Reaction 1 is favored over homolysis and a reactive species, thiosulfoxylic acid, is formed, Reaction 1 is probably the initial step. 

Accordingly, if a more basic species (i.e., an alkyl sulfoxide) is added, the acidic species will be complexed effectively preventing the decomposition of cumene hydroperoxide by thiolsulfinate solutions at 25°C. 

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