Derivation, hydrogen sulfide-promoted

A derivative of the Claus process is the Recycle Selectox process, developed by Parsons and Unocal and Hcensed through UOP. Once-Thm Selectox is suitable for very lean acid gas streams (1—5 mol % hydrogen sulfide), which cannot be effectively processed in a Claus unit. As shown in Figure 9, the process is similar to a standard Claus plant, except that the thermal combustor and waste heat boiler have been replaced with a catalytic reactor. The Selectox catalyst promotes the selective oxidation of hydrogen sulfide to sulfur dioxide, ie, hydrocarbons in the feed are not oxidized. These plants typically employ two Claus catalytic stages downstream of the Selectox reactor, to achieve an overall sulfur recovery of 90—95%. 

Lew and Kim [66] recently synthesized a sulfide isostere of carbocyclic influenza neuraminidase inhibitor 268, moving from trityl protected aziridine 266, in turn prepared from (-)-quinic acid 159 (Scheme 43). Ring opening of 266 with 1-propanethiol promoted by BF3 etherate, followed by acetylation, gave sulfide 267 which was finally hydrogenated to diamine 268, in a good chemical yield. As the authors expected, mercapto-derivative 268 resulted to be a strong inhibitor of... 

It is tempting to speculate whether the role of promoter HDS metals (Co, Ni) is the C-S activation of the thiophenes whereas the catalytic metal (group 6, Mo, W) would be responsible for the transfer of hydrogen to the activated substrate. This is a very likely pathway to be envisaged on metal sulfide surfaces where sulfides or thiolates bonded to two or more metal atoms are plentiful. However, this type of interaction is obviously unavailable in single-site complexes, and this could explain why ring opening is frequent but complete desulfurization is rare on mononuclear derivatives. 

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