Thioethers, catalytic oxidation

Figure 27.18 Common configuration for postcolumn reactors with electrochemical analysis. (A) LC-chemical reaction-EC. Postcolumn addition of a chemical reagent (for example, Cu2+ or an enzyme). (B) LC-enzyme-LC. Electrochemical detection following postcolumn reaction with an immobilized enzyme or other catalyst (for example, dehydrogenase or choline esterase). (C) LC-EC-EC. Electrochemical generation of a derivatizing reagent. The response at the second electrode is proportional to analyte concentration (for example, production of Br2 for detection of thioethers). (D) LC-EC-EC. Electrochemical derivatization of an analyte. In this case a compound of a more favorable redox potential is produced and detected at the second electrode (for example, detection of reduced disulfides by the catalytic oxidation of Hg). (E) LC-hv-EC. Photochemical reaction of an analyte to produce a species that is electrochemically active (for example, detection of nitro compounds and phenylalanine). Various combinations of these five arrangements have also been used. [Reprinted with permission from Bioanalytical Systems, Inc.]...

The material 6 showed a remarkable catalytic activity in the oxidation of thioethers 13 to sulfoxides 14 by urea hydroperoxide (UHP) or H O (Scheme 2). Although the conversion and selectivity (for 14 over 15, >90%) was reasonable with UHP for the substrates with smaller substituents, 13a and 13b, the ones with bulkier substrates 13c and 13d failed to produce any measurable conversion. The conversion increases to 100% by changing UHP with H O. The catalytic activity of 6 for selective sulfoxidation remains similar even after 30 cycles. Despite the fact that no asymmetric induction was found in the catalytic sulfoxidations, enantioen-riched sulfoxides were obtained by enantioselective sorption of the resulting racemic mixture by the chiral pores of 6, which occured simultaneously with the catalytic process. Thus, after catalytic oxidation of 13a, (5)-14a was preferentially absorbed by the pore of 6 leaving exactly equal amount of the excess ) -enantiomer in the solution phase (-20% ee). The combination of high catalytic activity and enantioselective sorption property of 6 provides a unique opportunity to device a one-step process to produce enantioenriched products. 

The catalytic oxidation of sulfur-containing compounds has been studied mostly on the examples of thioethers and thiols, which are most often encountered in petrochemical industry. Elimination of the foul odor arising from thiols (mercaptans) contained in some oils ("sweetening") is mostly carried out by oxidation of thiols to disulfides. The oxidation of sulfur salts is also of practical importance in the processing of sulfide ores and handling the... 

NO used for thioether oxidation to sulphoxides is unique in that it can be employed as a reagent in stoichiometric amounts or as a catalyst for autoxidation. Oxygen alone cannot oxidise thioethers at ambient temperatures and atmospheric pressures, so NO must be the active agent in the catalytic oxidation. There is strong correlation between the oxidative conversion of thioethers to sulphoxides and the concentration of [R S, NO ] complexes. The enhanced reactivity of an alkyl thioether such as dibutyl sulphide relative to diphenyl sulphide directly parallels the difference in the steady-state concentrations of the corresponding nitrosonium donor-acceptor complexes [22] [Bu S, NO ], [Ph S, NO ]. The same trend in the structure/reactivity relationship is more clearly shown in the oxidative conversion of the homologous thioethers in... 

Key words Amino acids. Catalytic oxidation under ambient conditions, 2-chloroethyl ethyl sulfide, Co-catalysis by copper. Decontamination, Effect of ligands, Gold complexes. Mustard gas, Perfluorinated solvents. Solvent effect. Sulfoxide, Thioether... 

It is worth mentioning that thioethers are catalytically oxidized to sulfoxides in nitromethane-aqueous nitric acid in the presence of Bu NAuCl with the rate-limiting step being the reoxidation of Au(I) to Au(III). This involves the use of an excess of NOj" relative to An however, in our systems O2 and not NOa is the terminal oxidant. This follows from the fact that only one equivalent of NO3 is used in our reactions, but 200 equivalents of CEESO product per equivalent ofNOj can be obtained. 

The oxidation of thioesters of phospho-rus(III) acids (S -ethyldiphenylphosphi-nite, S, S -diethylphenylphosphonite and triethylthiophosphite) in AN/NaCl04 occurs at potentials characteristic for thioethers with substituents other than for phosphorus(III) groups, which suggests the contribution of the lone pair of sulfur to the HOMO to be predominant. The process is thought to proceed via an intermediate cation radical with the number of electrons n varying from 0.65 to 0.85, which suggests a catalytic mechanism [12-15]. 

In an earlier experiment, Jori et al. (14) reported that methionyl residues are important in maintaining the tertiary structure of lysozyme. The introduction of a polar center into the aliphatic side chain of methionine, as a consequence of the conversion of the thioether function to the sulfoxide, may bring about a structural change of the lysozyme molecule which, in turn, reduces the catalytic efficiency. When ozonized lysozyme was treated with 2-mercaptoethanol in an aqueous solution according to the procedure of Jori e al. (14), the enzyme did not show any increase in its activity. This may be explained in two ways. In one, such reactions are complicated by many side reactions, e.g. sulfhydryl-disulfide interchange, aggregation and precipitation of the modified enzyme (24-26). In the other, the failure to recover the activity of the enzyme may by associated with the extensive oxidation of other residues. 

Chiral sulfoxides have emerged as versatile building blocks and chiral auxiliaries in the asymmetric synthesis of pharmaceutical products. The asymmetric oxidation of prochiral sulfides with chiral metal complexes has become one of the most effective routes to obtain these chiral sulfoxides.We have recently developed a new heterogeneous catalytic system (WO3-30% H2O2) which efficiently catalyzes both the asymmetric oxidation of a variety of thioethers (1) and the kinetic resolution of racemic sulfoxides (3), when used in the presence of cinchona alkaloids such as hydroquinidine 2,5-diphenyl-4,6-pyrimidinediyl diether [(DHQD)2-PYR], Optically active sulfoxides (2) are produced in high yields and with good enantioselectivities (Figure 9.3). ... 

Table XXXVI is a list of some catalytic photochemical redox transformation of organic reactants by (Q or H)3PW 204o. In the presence of UV light, Q3PW12O40 reacts with paraffins, arenes, alcohols, alkyl halides, ketones, nitriles, thioethers, and water. Under either anaerobic or aerobic conditions, decarboxylation, dehydrogenation, dimerization, polymerization, oxidation, and acylation takes place.

Catalytic studies have extended the applications of TS-1 from early aromatic hydroxylation to the oxidation of olefins, alkanes, alcohols, amines and thioethers, and to other minor reactions. A combination of three factors is the basis of its effectiveness the high activity of Ti peroxy species, the organophilic properties of the surface and the size of the pores in the range of molecular dimensions. For... 

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