Disulfides, diphenyl eliminative

Similarly, thioselenation of olefins can be achieved with the diphenyl disulfide/ diphenyl diselenide binary system [26] (Eq. 9). The radical chain addition of disulfides to olefins is quite inefficient because of the competition of yff-elimination with the attack of the /S-thioalkyl radical on the disulfide diphenyl diselenide is a far better radical trap. As shown in Eq. (10), the reaction has been extended to thio-telluration. With the exception of norbornene and vinylcyclopropanes, for which the addition step is followed by the fast opening of the three-membered ring, thio-telluration does not proceed with olefins. In contrast, the thiotelluration of acetylenes is highly efficient. 

Salts of dithiocarbamic acid can be prepared by the addition of primary or secondary amines to carbon disulfide. This reaction is similar to 16-9. Hydrogen sulfide can be eliminated from the product, directly or indirectly, to give isothiocyanates (RNCS). Isothiocyanates can be obtained directly by the reaction of primary amines and CS2 in pyridine in the presence of DCC. ° In the presence of diphenyl phosphite and pyridine, primary amines add to CO2 and to CS2 to give, respectively, symmetrically substituted ureas and thioureas ... 

The reaction of enolates with dimethyl disulfides (MeSSMe) or diphenyl disulfides (PhSSPh) leads to the corresponding sulfides. Their oxidation to sulfoxides followed by heating provides a route to functionally substituted alkenes, such as a,[3-unsaturated carbonyl compounds. In the example below, the exocyclic alkene is formed since that is the only possible yn-elimination product. 

Only activated olefins, e.g. a,yS-unsaturated carbonyl compounds, enolethers, and enolacetates, are good radical acceptors in these reactions. The olefins have to be used in five- to tenfold excess. The overall transformation is completed by elimination of the thiophenyl radical. Thus, diphenyl disulfide, the source of thiophenyl radicals, can in principle be employed in substoichiometric amounts. The diaster-eoselectivity of vinylcyclopentane formation is difficult to control, and mixtures of diastereomers are often obtained. A highly stereoselective example is shown in Scheme 3. 

The 14,15-double bond present in tabersonine was introduced into the /3-ethyl isomer of lactam 491 by treatment with LDA-diphenyl disulfide followed by oxidation and elimination to give the a,fl-unsaturated lactam 492. Murphy s law operated at this point, for upon lithium aluminum hydride reduction, 492 gave only a 5% yield of the amino alcohol 493. An alternative procedure was therefore developed. Hydrolysis gave a carboxylic acid, and treatment with ethyl chloroformate and triethylamine followed by sodium borohydride in aqueous THF gave a lactam alcohol 494, which after silylation was reduced with lithium aluminum hydride to amino alcohol 493 in 58% overall yield from 492. Mesylation and elimination of HC1 in refluxing chloroform gave the unsaturated mesylate salt 482. The same salt was prepared previously by Ziegler and Bennett... 

Pyrolysis of sulfoxides provides a convenient method for introducing unsaturation at the position a- to carbonyl compounds. Formation of the enolate and reaction with dimethyl (or diphenyl) disulfide gives the a-methylthio (or phenylthio) derivative. Oxidation with a suitable oxidant, such as mCPBA or NaI04, gives the sulfoxide, which eliminates sulfenic acid on heating to give the a,(3-unsaturated carbonyl compound. For example, the methyl ester of a pheromone of queen honey bees was synthesized from methyl 9-oxodecanoate after initial protection of the ketone as the acetal (2.24). The -isomer usually predominates in reactions... 

Poly(phenylene sulfide) (PPS) has been synthesized by oxidative polymerization of diphenyl disulfide, which was obtained via oxidation of thiophenol (eq. 8) (198-208). PPS is an engineering plastic with a high melting point, which is manufactured by condensation polymerization eliminating a salt. 

Diaryl disulfides have also proven to be a clean source of thiyl radicals by photolysis, and this system was used to study addition reactions using time-resolved spectroscopies [22]. However, the radical chain addition of disulfides to olefins is quite inefficient, due to the effective competition of -elimination with the attack of the )9-thioalkyl radical on the disulfide. An alternative approach is represented by the thioselenation of olefins [23] achieved with a diphenyl disul-fide/diphenyl diselenide mixed system (equation (6)). In this case, the j8-thioalkyl radical adduct is trapped more efficiently by diselenide than disulfide. 5-Benzoyl phenylselenosulfide adds across the double bond of olefins like thioacids to give selenothiocarbonylated products regiospecifically (equation (7)) [24]. The reaction can be initiated either by thermolysis of azo compounds (AIBN at 80°C) or by UV irradiation at room temperature. 

---