Thio Stabilizers

Thio stabilizers are also used as peroxide decomposers, Fig. 3.5. The resulting numerous reaction possibilities go beyond the scope of this work. Thio stabilizers are generally used as costabilizers for sterically hindered phenols. Their reaction mechanisms depend on the chemical structure of the organo-sulfur compounds, the oxidizing substrate, and oxidation conditions. 

The substances typically used are sulfides, dialkyl dithiocarbonates or thiodipro-pionates, such as dilauryl thiodipropionate (DLTDP) and distearyl thiodipropionate (DSTDP) as well as disulfides. These products develop their effect mainly in the range of long-term service temperatures, i. e., at moderate temperatures, and they exhibit few process stabilizing effects (with the exception of disulfides). 

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The synthetic utility of a-phosphorus- and a-thio-stabilized carbanions is the subject of numerous reviews.21 Notable are additions of phosphonium ylides (237),183 sulfonium ylides (238),l84 ° oxosulfo-nium ylides (239)184 " and sulfoximine ylides (240)184,1 to electron-deficient alkenes which afford nucleophilic cyclopropanation products. In contrast, with a-(phenylthio)-stabilized carbanions, which are not acyl anion equivalents, either nucleophilic cyclopropanation or retention of the hetero substituent occurs, depending on the acceptor and reaction conditions used. For example, carbanion (241) adds to 1,1-... 

Carbanions stabilized by phosphorus and acyl substituents have also been frequently used in sophisticated cyclization reactions under mild reaction conditions. Perhaps the most spectacular case is the formation of an ylide from the >S-lactam given below using polymeric Hflnig base (diisopropylaminomethylated polystyrene) for removal of protons. The phosphorus ylide in hot toluene then underwent an intramolecular Wlttig reaction with an acetyl-thio group to yield the extremely acid-sensitive penicillin analogue (a penem I. Ernest, 1979). 

Another use of 4-/ f2 octylphenol is ia the productioa of uv stabilizers. 4-/ f2 -Octylpheaol reacts with sulfur dichloride to yield the thio-hisphenol derivative, which thea reacts with nickel acetate to form 2,2 -thiobis(4-/ f2 octylphenolate)-A/-butylamiQe nickel [14516-71 -3]. This type of stabilizer is widely used in the production of outdoor carpeting based on polypropylene fibers. Nickel compounds give a green discoloration which limits their apphcatioas. A second class of uv stabilizers based on the benzotriazole stmcture. 2-(2 -hydroxy-5 -/ f2 octylphenyl)benzotriazole [3147-75-9] is produced from 4-/ i -octylphenol (55). 

The values of constants of stability of complexes AuBr Thio, AuBiyr/rio AuBrThio, AuBrThio have been calculate. They equal 8,90-lO " 2,81-10" 2,23-10"" 5,60-10"". 

The pA of 1,3-dithiane is 36.5 (Cs" ion pair in THF). The value for 2-phenyl-1,3-dithiane is 30.5. There are several factors which can contribute to the anion-stabilizing effect of sulfur substituents. Bond dipole effects contribute but carmot be the dominant factor because oxygen substituents do not have a comparable stabilizing effect. Polarizability of sulfur can also stabilize the carbanion. Delocalization can be described as involving 3d orbitals on sulfur or hyperconjugation with the a orbital of the C—S bond. MO calculations favor the latter interpretation. An experimental study of the rates of deprotonation of phenylthionitromethane indicates that sulfur polarizability is a major factor. Whatever the structural basis is, there is no question that thio substituents enhance... 

Freparatively useful induced diastereoselectivities have been reported mainly for 1,1-di-substituted allyllithium derivatives which bear carbanion-stabilizing substituents. l-[Methyl-thio-l-(trimethylsilyl)-2-propenyl]lithium106 and the appropriate 1-phenylthio107 derivative, generated from the allylic sulfide with sec-butyllithium, in the reaction with tetrahydropyranyl-protected pregnolone, furnish a single diastereomer. 

The mechanism of secondary stabilization by antioxidants is demonstrated in Figure 15.5. TnT-nonylphenyl phosphites, derived from PCI3 and various alcohols, and thio-compounds are active as a secondary stabilizer [21], They are used to decompose peroxides into non-free-radical products, presumably by a polar mechanism. The secondary antioxidant is reacting with the hydroperoxide resulting in an oxidized antioxidant and an alcohol. The thio-compounds can react with two hydroperoxide molecules. 

FIGURE 15.5 Secondary stabilization by phosphates and thio-compounds. 

A novel nitrilase was purified from Aspergillus niger K10 cultivated on 2-cyanopyridine. It was found to be homologous to a putative nitrilase from Aspergillus fumigatus Af293. The nitrilase exhibited maximum activity at 45 °C and pH 8.0 with much less activity observed at slightly acid pH. Its substrate preference was for 4-cyanopyridine, benzonitrile, 1,4-dicyanobenzene, thio-phen-2-acetonitrile, 3-chlorobenzonitrile, 3-cyanopyridine, and 4-chlorobenzonitrile. ( )-2-Phenylpropionitrile was only poorly converted by this enzyme and with minimal enantioselectivity. The enzyme was shown to be multimeric (>650 kDa) and be stabilized in the presence of sorbitol and xylitol [57]. 

Tellurium sources, 22-24 Thermodynamics in cyclo-oligomerization, 185-186 butadiene insertion, 187-188 reductive elimination, 193, 194 selectivity control, 212 polysilane isomerisation, 158-160 see also Stability Thermolysis, 135, 136, 158 THF (tetrahydrofuran), 97, 150, 153 Thio-Wittig reaction, 37 Tin, 121... 

Even if the extraction of FeMoco in nmf slightly modifies the outer coordination of the MoFe7S9 cluster, in agreement with the experimental observation that the FeMo-protein can exist in the oxidized, semi-reduced and reduced forms, it exhibits two successive one-electron reductions (E st = -0.32 V and is d = -1.00 V,vj. NHE). In the absence of thio-phenol, the first reduction appears to be complicated by self-oxidation processes, whereas the presence of thiophenol apparently stabilizes the different oxidation states. 

Partial fluorination of 4-arylthio-l,3-dioxolan-2-ones occurs preferentially at the carbon atom adjacent to the thio group [67]. However, a remarkable solvent effect is encountered. In the more polar solvent, dimethoxyethane substitution occurs, while in the less polar dichloromethane a larger portion of the desulfurization with cleavage of the phenylthio group takes place. This is attributed to the fact that the intermediate radical cation is more stable in the polar solvent and undergoes deprotonation, while in the less polar solvent, the less stabilized radical cation dissociates into a dioxolane cation and a phenylthio radical. 

A comparison of the relative strength of functional groups to cause a-cleavage is summarized in Table 6.5. [6] This also corresponds to a rough measure of relative charge-stabilizing capability of the respective substituent, e.g., the ratio H2C=0H /H2C=NH2 from 2-amino ethanol molecular ion is 2.3/100 and the ratio of H2C=0H /H2C=SH from 2-thio ethanol molecular ion is 42/70. [20]... 

Bis [(trifluoromethyl)thio] acetaldehyde (83a) has been prepared from an enam-ine precursor (84), although refluxing in aqueous ethanolic HCl is required to effect this reaction.The aldehyde is less stable than its enol tautomer (83b), and many reactions typical of aldehydes fail. For example, addition of aqueous silver nitrate immediately yields the silver salt of (83b), rather than giving precipitation of (elemental) silver. The (trifluoromethyl)thio substituent has pseudohalogenic character and, together with the hydroxy group, stabilizes the alkene tautomer in the manner of a push-pull alkene. The enol-aldehyde equilibrium mixture in acetonitrile shows an apparent of 2.6 when titrated with aqueous hydroxide. 

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