Indene and thiophenol

Co-oxidation of indene and thiophenol in benzene solution is a free-radical chain reaction involving a three-step propagation cycle. Autocatalysis is associated with decomposition of the primary hydroperoxide product, but the system exhibits extreme sensitivity to catalysis by impurities, particularly iron. The powerful catalytic activity of N,N -di-sec-butyl-p-phenylenediamine is attributed on ESR evidence to the production of radicals, probably >NO-, and replacement of the three-step propagation by a faster four-step cycle involving R-, RCV, >NO, and RS- radicals. Added iron complexes produce various effects depending on their composition. Some cause a fast initial reaction followed by a strong retardation, then re-acceleration and final decay as reactants are consumed. Kinetic schemes that demonstrate this behavior but are not entirely satisfactory in detail are discussed. 

Co-oxidation of indene and thiophenol takes place readily if the reactants in benzene solution are shaken with oxygen at temperatures in the range 20° to 40°C. (7). The major primary product has been shown to be frans-2-phenylmercapto-1 -indanyl hydroperoxide, I, which rearranges spontaneously to the two racemes of frans-2-phenylsulfinyl-l-indanol, II (8), and a tentative reaction scheme involving a three-step radical chain based on the suggestion of Kharasch, Nudenberg, and Mantell (11) was proposed for the formation of I. These three products accounted for 86% of the oxygen absorbed. 

Figure 2. Effect of iron complexes on co-oxidation of indene and thiophenol in benzene at 20°C.

We conclude that although Scheme 1 represents a basic mechanism for co-oxidation of indene and thiophenol, the system is so sensitive to the effects of catalysts and inhibitors that interpreting its behavior necessitates studying the initiation and termination mechanisms. 

The co-oxidation of indene and thiophenol in benzene proceeds by a three-step cyclic free radical chain reaction. Autocatalysis associated with the hydroperoxide which is the main primary product occurs, but the effect is complicated by other trace components. The reaction is extremely sensitive to catalysts and inhibitors, and its kinetic features are determined by the initiation and termination processes. 

The cooxidation of thiophenol with indene by air in hydrocarbon solvents provides l-hydroperoxy-2-phenylthioindane 84 in 77% yield. Subsequent rearrangement afforded a mixture of trans and cis 2-phenylsulphinyl-l-indanols 85148 149 (equation 47). 

Examination of Co-Oxidation Products. Co-oxidation products were identified and hydroperoxide and sulfoxide yields were measured as previously described (7, 8). The observed stoichiometric ratio of indene, thiophenol, and oxygen consumed in the whole reaction is 1 1.06 1 (8). 

Addition of DSBPD during Co-Oxidation at 20°C. A co-oxidation in the absence of DSBPD (5 ml. solution, 0.15M in thiophenol and indene) was allowed to proceed until 0.08 mole of 02 per mole of PhSH had been absorbed the reaction mixture was then frozen. A solution of 5 ml. of benzene containing 9 X 10"4 gram of DSBPD was added, and the cooxidation was restarted. The subsequent rate obtained was 0.0086 compared with an initial rate of 0.015 mole of 02 per mole of PhSH per minute obtained when a similar solution of DSBPD was added at the start of a co-oxidation. 

Co-Oxidation in absence of Added Catalysts or Inhibitors. Figure 1 (curves A,A, B,B ) shows typical oxygen uptake vs. time curves obtained after using the procedures for purifying indene, thiophenol, and benzene and cleaning the reaction vessel as described above. On the experience of several hundred runs, the reproducibility expected was such that the time for uptake of 0.5 mole of 02 per mole of PhSH would be in the range 150 to 200 minutes. Despite all efforts, it was estimated that iron... 

Co-Oxidation in Presence of Added N,N -di-sec-butyl-/>-phenylene-diamine. Figure 3 shows typical oxygen uptake curves obtained if DSBPD is added at concentrations around 10"2 to 10 4M to the indene-thiophenol reaction mixture. The reproducibility was good. The products after oxygen absorption was complete and the hydroperoxide had been allowed to decay were substantially the same as those obtained in the absence of DSBPD. Uptake of oxygen was extremely slow when either of the main reactants was omitted. 

Consumption of the DSBPD results from further oxidation of the >NO radical (Reaction 12) but may also occur by hydrogen abstraction from the alkyl group leading to imine formation (5) or by disproportionation reactions yielding quinonoid structures (9). The existence of this reaction system may therefore be transitory. However, the main chain is long, and the effect of 7.5 X 10"3M DSBPD lasts almost throughout the oxidation of 0.15M indene-thiophenol (see Figure 3). 

A wide range of iron compounds catalyze the indene-thiophenol co-oxidation reaction—e.g., FeCl3 and a variety of complexes formed by chelation. Differences in their behavior are believed to be correlated approximately with the ease of displacement of ligands by thiols or with their susceptibility to attack by peroxide, but the lack of reproducibility which has made much of this work extremely tedious makes quantitative deductions difficult. 

The reaction involves the transient formation of an a-thio hydroperoxide (cf. Houben-Weyl Vol. E13, p 98), which undergoes rearrangement to the final reaction product. For example, the cooxidation of thiophenol with indene yields initially 2,3-dihydro-l-hydroperoxy-2-phenylthio-1/f-indene (1) in 74% yield subsequent rearrangement affords a mixture of trans- and cis-2,3-dihydro-2-phenylsulfinyl-lf/-indcn-l-ol (2) in ca. 98 2 ratio65. 

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