Thioether, oxidation effect

Methionine sulfoxide formation may occur without noticeable changes in physical or immunochemical properties of the protein. Thus reduction of sulfoxide to thioether often completely restores the lost protein function. Many cells, including human polymoprhonuclear neutrophilic leukocytes, contain enzyme methionine sulfoxide reductase, which is able to convert methionine sulfoxide to the reduced methione form in a variety of proteins (B25, F8). Methionine reacting with a strong oxidant effects methionine sulfone production, which in vivo is not reduced back to methionine. 

Thioether oxidation is catalyzed by Ce(IV), whose function is to effect the first step in the above scheme and to oxidize a hydrated form of the cation radical to the sulfoxide. Reoxidation of the... 

Anchimerically assisted redox reactions. Vn. A conparison of the effects of neighboring primary, secondary, and tertiary amine groups on the kinetics and mechanism of thioether oxidation by aqueous iodine, J. Org. Chem. 48 (14), 2371-2374 (1983). 

One of the reasons that thioethers are effective stabilizers is that their oxidation products act as longterm heat stabilizers [103-105]. They can be even more effective hydroperoxide decomposers than the original compounds [ 106]. A proposed mechanism of action of thioethers is shown in Scheme 18.15 [107]. 

The importance of steric effects in determining the oxidation state of the product can be illustrated by a thioether linkage, eg (57). If a methyl group is forced to be adjacent to the sulfur bond, the planarity required for efficient electron donation by unshared electrons is prevented and oxidation is not observed (48). Similar chemistry is observed in the addition of organic nitrogen and oxygen nucleophiles as well as inorganic anions. 

Further evidence for the above-mentioned mechanism of HOMO elevation by group 14 elements is provided by studies of thioethers. The decrease in oxidation potential of silyl ethers as compared to ethers is not realized in the case of a-silylthioethers whereas a-stannyl substituents in thioethers cause a considerable cathodic shift in oxidation potential. Moreover, the effect is geometry-dependent. Values for substituted cyclic dithianes 15 are summarized in Table 21. The difference between Si and Sn in this case is illustrative. The lone nonbonding pair in the 3p orbital of sulfur is much too low in energy compared to... 

Following the synthetic route for the preparation of thioether appended por-phyrazines developed by Schramm and Hoffman (2), a series M[pz(S-Et)g] M = Mn, Co, Fe, were prepared by Ricciardi and co-workers (113-116). Their goal was to study the effect of the central metal and the immediate electronic environment, that is, oxidation state and axial ligand, on the physicochemical properties of the pz (Scheme 10). 

Although sodium sulphide reacts readily with haloalkanes [2] and activated aryl halides (see Chapter 2) [e.g. 3-5] in the presence of a quaternary ammonium catalyst to form symmetrical thioethers (Table 4.1), a major side reaction results in the formation of disulphides owing to the oxidation of the intermediate thiols under the basic conditions. Consequently, little use has been made of this procedure for the synthesis of thioethers [3, 6], but the corresponding reaction of the a,(0-dihaloalkanes to yield cyclic thioethers has proved to be a valuable procedure for the synthesis of thietanes [7] (Table 4.2). The ring closure with the secondary dihaloalkanes is considerably more difficult to effect than is the reaction of the primary dihaloalkanes. 1,3-Dihydrobenzo[c]thiophene (89%) is produced in the reaction of 1,2-bis(bromomethyl)benzene with sodium sulphide (Scheme 4.1) [8]. The direct... 

An effective and mild electrocatalytic procedure for the deprotection of the 1,3-dithiane group of (68), giving the ketone (67), has been developed by using a small amount of tris(/ -tolyl)amine as a homogeneous electron-transfer catalyst (Scheme 26) [86]. The scope and limitations are discussed in detail [87]. The method can be applied also for oxidative removal of the 4-methoxybenzyl thioether protecting group from poly-cystinyl peptides [88]. 

The development of sulfone linkers, the exploration of sulfone based chemical transformations and cleavage strategies are an important objective in soHd-phase organic synthesis. This kind of Hnker (Tab. 3.7) has been used with thioethers [108], sulfoxides [109], sulfones [110], sulfonic acids and their corresponding derivatives [111]. Because carbon-sulfur bonds can be cleaved under very mild conditions, some Hnkers have been based on this effect. They can be cleaved under reductive conditions ]112, 113], photolytic conditions [114, 115] or with strong bases [116]. Various safety catch Hnkers have been developed based on the fact that thiols can be oxidized to sulfoxides and sulfones [112, 113]. 

Unwanted degradation and oxidation processes can be avoided or at least suppressed for some time either by structural modiflcation of the polymer or by special additives. In practice, the addition of so-called antioxidants is particularly effective. Chemical substances that slow down oxidations and the following aging phenomena serve for this purpose. Antioxidants are sufficiently effective even in concentrations below 1 wt% and are added as early as possible to the polymer to be protected, e.g., already during the drying of powdery polymeric materials or during the preparation of granulates. Some of the most important so-called primary antioxidants are sterically hindered phenols and secondary aromatic amines secondary antioxidants are thioethers as well as phosphites and phosphonites. 

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). ... 

The asymmetric oxidation of thioethers as well as kinetic resolution of sulfoxides with 30% H2O2 catalyzed by a stable, recyclable and commercially avialable solid WO3 catalyst provides a simple and effective procedure for the preparation of chiral sulfoxides in good enantimeric purity. The procedure is very easy to perform. 

Again, the precise roles of coordination-compound chemical sensitizers, in most cases, are not understood. In fact, their effects may have little to do with their own coordination chemistry. Many simple salts of gold and other noble metals are effective sensitizers. They also may be added to solutions during silver halide precipitation to produce doped emulsions that have special properties. A variety of compounds that can act as ligands to metal ions are also effective alone as chemical sensitizers, the result of complicated oxidation-reduction, ion replacement and adsorption reactions on the silver halide grain surface. These include polyamines, phosphines and thioether- or thiol-containing compounds. The chemistry of these materials with the silver halide surface is discussed in the reference literature. 

There are still two other factors influencing the reactivity of ketones in the Norrish-Yang reaction. If functional groups with a relatively low oxidation potential (amino, alkenyl or aryl groups, thioethers) are present in the reactants, the excited state may be quenched by partial or complete charge transfer from these groups to the extited carbonyl group. The effects of such processes may vary from a complete loss of reactivity to an entirely new reaction mode - cyclization reactions ini-... 

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