Thiols oxygen

Thiol-Oxygen-Co-Oxidation (TOCO) and Related Processes 479... 

Beckwith pioneered the use of a Thiol-Oxygen-Co-Oxidation (TOCO) process for the transformation of 1,4-dienes and 1,3,6-trienes to 1,2-dioxolanes [90], As illustrated by the example in Scheme 51 [90a], this process involves phenylthio radical addition to the least substituted double bond, oxygen entrapment, peroxyl radical cyclization, oxygen entrapment and hydrogen atom transfer from the thiol. In accord with the Beckwith-Houk transition state model [91, 92], cyclization provides preferentially the c/s-3,5-disubstituted 1,2-dioxolanes. 

Bachi has extended this strategy to the synthesis of naturally occurring endoper-oxides with antimalarial activity via the thiol-oxygen cooxidation of dienes [47] (Scheme 8) the reaction involves the formation of a peroxy radical 4, which adds intramolecularly to the double bond to give the endoperoxide function required in the final product. 

M) ethane thiol oxygen NaOH (30°C) (kcal mole 1 ) ... 

A major difference between alcohols and thiols concerns their oxidation We have seen earlier m this chapter that oxidation of alcohols gives compounds having carbonyl groups Analogous oxidation of thiols to compounds with C=S functions does not occur Only sulfur is oxidized not carbon and compounds containing sulfur m various oxida tion states are possible These include a series of acids classified as sulfemc sulfimc and sulfonic according to the number of oxygens attached to sulfur... 

Although a variety of oxidizing agents are available for this transformation it occurs so readily that thiols are slowly converted to disulfides by the oxygen m the air Dithiols give cyclic disulfides by intramolecular sulfur-sulfur bond formation An example of a cyclic disulfide is the coenzyme a lipoic acid The last step m the laboratory synthesis of a lipoic acid IS an iron(III) catalyzed oxidation of the dithiol shown... 

Bivalent Sulfur. The prefix thio, placed before an affix that denotes the oxygen-containing group or an oxygen atom, implies the replacement of that oxygen by sulfur. Thus the suffix -thiol denotes — SH, -thione denotes —(C)=S and implies the presence of an =S at a nonterminal carbon atom, -thioic acid denotes [(C)=S]OH [(C)=0]SH (that is, the O-substituted acid and the 5-substi-... 

Meta.1 Complexes. The importance of Ni complexes is based on their effectiveness as quenchers for singlet oxygen. Of disadvantage is their low colorfastness and their lower ir-reflectance compared to cyanine dyes (qv) therefore they are used in combination with suitable dyes. Numerous complexes are described in the Hterature, primarily tetrathiolate complexes of Pt or Ni, eg, dithiolatonickel complexes (3). Well known is the practical use of a combination of ben2othia2ole dyes with nickel thiol complexes in WORM disks (Ricoh, TDK) (17). 

Although aminyl radicals are stable towards oxygen, they can oxidi2e other aromatic amines, phenols and thiols (10), and regenerate the diarylamine. Thus, mixtures of phenols and diarylamines frequendy show better antioxidant activity than either one alone. This is called synergism. 

Oxidation. Disulfides are prepared commercially by two types of reactions. The first is an oxidation reaction uti1i2ing the thiol and a suitable oxidant as in equation 18 for 2,2,5,5-tetramethyl-3,4-dithiahexane. The most common oxidants are chlorine, oxygen (29), elemental sulfur, or hydrogen peroxide. Carbon tetrachloride (30) has also been used. This type of reaction is extremely exothermic. Some thiols, notably tertiary thiols and long-chain thiols, are resistant to oxidation, primarily because of steric hindrance or poor solubiUty of the oxidant in the thiol. This type of process is used in the preparation of symmetric disulfides, RSSR. The second type of reaction is the reaction of a sulfenyl haUde with a thiol (eq. 19). This process is used to prepare unsymmetric disulfides, RSSR such as 4,4-dimethyl-2,3-dithiahexane. Other methods may be found in the Hterature (28). 

When oxygen is used as the oxidant, a basic catalyst is required for the lighter thiols (31) and a transition metal co-catalyst may be required for the heavier thiols (32). Oxidation using sulfur as the oxidant requires a basic 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. 

Thiol esters, which are more reactive to nucleophiles than are the corresponding oxygen esters, have been prepared to activate carboxyl groups for both lactoniza-tion and peptide bond formation. For lactonization S-f-butyl and S-2-pyridyP esters are widely used. Some methods used to prepare thiol esters are shown below. The S-r-butyl ester is included in Reactivity Chart 6. 

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