Thiols using phthalocyanine complexes

A poly(propylenamine) dendrimer (11, Fig. 6.37) functionalised with poly-(N-isopropylacrylamide) (PIPAAm) (see Section 4.1.2) was used as dendritic host for anionic cobalt(II)-phthalocyanine complexes (a, b) as guests, which are held together by supramolecular (electrostatic and hydrophobic) interactions [57]. These dendritic complexes were investigated as catalysts in the above-mentioned oxidation of thiols, where they show a remarkable temperature dependence the reaction rate suddenly increases above 34°C. One attempted explanation assumes that the dendritic arms undergo phase separation and contraction above the Lower Critical Solubility Temperature (LCST). At this temperature the phthalocyanine complex site is more readily accessible for substrates and the reaction rate is therefore higher. 

The continuous oxidation of thiols involved in the sweetening of light naphtha, with air to the disulphides using cobalt phthalocyanine complexes as catalysts, involves the formation of a stable complex between the thiolate ions and the metallocyanine catalyst . 

Photooxidafions are also iudustriaHy significant. A widely used treatment for removal of thiols from petroleum distillates is air iu the presence of sulfonated phthalocyanines (cobalt or vanadium complexes). Studies of this photoreaction (53) with the analogous ziuc phthalocyanine show a facile photooxidation of thiols, and the rate is enhanced further by cationic surfactants. For the perfume iudustry, rose oxide is produced iu low toimage quantifies by singlet oxygen oxidation of citroneUol (54). Rose bengal is the photosensitizer. 

Attention has also been focused on the oxidation of thiols in the presence of solid catalysts. One of the more comprehensive investigations into systems of this type has been made by Wallace et al. [133,145, 146] with a view to the possible use of phthalocyanine type complexes as commercial sweetening catalysts. Comparisons were drawn with metal pyrophosphates, phosphomolybdates, phosphotungstates, and phosphates. Pyrophosphates were found to be effective catalysts, possible due to the existence of six-membered rings involving the cobalt cation [147], which enhances the ability of the cation to donate an electron to oxygen and stabilises each oxidation state of the cation. For a series of pyrophosphates, the order of activity was Co > Cu > Ni > Fe, an activity pattern which was explained in terms of the stability of the 3d electron shells. 

Cobalt complexes of polymeric phthalocyanines have been employed in aqueous alkaline solution as heterogenous catalysts in the oxidation of thiols to disulfides (MEROX, mercaptan oxidation process in the petroleum industry, Eq. 6-12, see Sections 5.2 and 5.4, Experiments 5-11 and 5-12). The catalytic activities of the polymer 31 (M = Co(II)) are higher than those of dissolved low molecular weight phthalocyanines, and both complexes exhibit better activities on charcoal than on Si02 as carrier. This is the result of better electrical contact between different reaction centers, which facilitates a multi-electron process in the oxidation of R-S to R-S-S-R and reduction of O2 to H2O [95]. Another advantage of the heterogeneous catalysts in comparison to the dissolved low molecular weight phthalocyanines is their easy re-use. 

Radiochromatographic techniques have been used to determine the rates of oxidation of cysteine by pertechnate ion, Tc04. The technetium(vu) is reduced by the thiol (and cysteine ethyl ester) to form a Tc complex which involves both S- and 7V-co-ordination of the amino-acid. The rate law is first order with respect to both [Tc ] and [RSH]. A hydrogen-ion dependence observed is attributed to the formation of pertechnic acid, the rate-determining step being the nucleophilic attack by the thiol at the metal centre of HTCO4. The oxidation of RSH (R=Et, Pr, or Bu) has been studied over the range 20—40 °C in aqueous alkaline solutions in the presence of metal phthalo-cyanines. The reaction is zero order with respect to [thiol], first order in phthalocyanin and decreases in the order M = Co>Mn> V>Feii. No effects are observed from the nature of the alkali cation. 

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