Butadiene, hydrogenation, sulfur effect

The reaction of crotonaldehyde and methyl vinyl ketone with thiophenol in the presence of anhydrous hydrogen chloride effects conjugate addition of thiophenol as well as acetal formation. The resulting j3-phenylthio thioacetals are converted to 1-phenylthio-and 2-phenylthio-1,3-butadiene, respectively, upon reaction with 2 equivalents of copper(I) trifluoromethanesulfonate (Table I). The copper(I)-induced heterolysis of carbon-sulfur bonds has also been used to effect pinacol-type rearrangements of bis(phenyl-thio)methyl carbinols. Thus the addition of bis(phenyl-thio)methyllithium to ketones and aldehydes followed by copper(I)-induced rearrangement results in a one-carbon ring expansion or chain-insertion transformation which gives a-phenylthio ketones. Monothioketals of 1,4-diketones are cyclized to 2,5-disubstituted furans by the action of copper(I) trifluoromethanesulfonate. ... 

For 1,3-butadiene hydrogenation, the toxicity of sulfur is 3 (Fig. 13). which is lower than the toxicity for olefin hydrogenation. The hydrogenation of 1-butyne has also been studied for various ratios of sulfur over palladium. As was already published (86), the 1-butyne hydrogenation rate increases with time. The same effect has been observed on sulfided palladium. The turnover number is consequently presented for 1-butyne hydrogenation versus the sulfur content for various 1-butyne conversions (see Fig. 14). During the first minutes of reaction (0-25% conversion), the toxicity of sulfur appears close to 1 the rates are proportional to the free surface. However, at higher conversion, the rate becomes independent from the sulfur ratio. The toxicity is zero. 

Divergent views have been expressed on the way in which the tin acts. Its role in limiting the size of platinum ensembles is not in question what is at issue is whether there is any electronic modification of the active centre. It was claimed that, in the reaction of MCP with hydrogen, tin produced positive effects on dehydrogenation and aromatisation that were not shown by either carbon or sulfur they were attributed to an electronic action, for which Mossbauer spectroscopy provided some evidence. In view of the proposal interpretation of the effect of sulfur on butadiene hydrogenation (Section 8.3) it would not be surprising if tin also influenced the platinum ensembles to some degree. 

The way sulfur treatment is applied gives rise to pronounced differences in selectivities21. A decrease in the overall alkene yield was observed when 1,3-butadiene was hydrogenated on presulfided Pd. In contrast, much improvement in selective alkene formation from isoprene was achieved when sulfur was present in the feed. In the latter case adsorption competition was suggested to account for the favorable effect of sulfur. 

Fig. 13. Effect of degree of sulfuration of palladium on the hydrogenation rate of 1,3-butadiene (7 = 30°C, P = 10 bars).

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