Sulfur electrophilic, addition with

The electrophilic character of sulfur dioxide does not only enable addition to reactive nucleophiles, but also to electrons forming sulfur dioxide radical anions which possess the requirements of a captodative" stabilization (equation 83). This electron transfer occurs electrochemically or chemically under Leuckart-Wallach conditions (formic acid/tertiary amine - , by reduction of sulfur dioxide with l-benzyl-1,4-dihydronicotinamide or with Rongalite The radical anion behaves as an efficient nucleophile and affords the generation of sulfones with alkyl halides " and Michael-acceptor olefins (equations 84 and 85). 

Addition Reactions with Electrophilic Sulfur and Selenium Reagents... 

Reactants with internal nucleophiles are also subject to cyclization by electrophilic sulfur reagents, a reaction known as sulfenylcyclization.92 As for iodolactonization, unsaturated carboxylic acids give products that result from anti addition.93... 

Additions ofArSX, ArSeX, and Related Reagents with Electrophilic Sulfur... 

Further reactions of P-hydridocyclotriphosphazenes (See Section IV,E) have been described. They undergo insertion reactions with aldehydes, ketones, isothiocyanates and electrophilic olefins. Addition of sulfur or oxidation with KMn04 gives thioxo-or oxo-cyclotriphos-phazene derivatives which are methylated at the chalcogen to afford methylthio- and methoxy- cyclotriphosphazenes (36). Hydridocyclo-phosphazenes can also be oxidized to give symmetric and unsymmetric bis(cyclotriphosphazenyl) oxides (35). 

Undoubtedly, the most widely used method for the synthesis of o.p. sulfoxides is the nucleophilic addition of metal organic reagent to an electrophilic sulfur with preestablished chirality, and the subsequent displacement of the sulfoxide. The reason is that either a good kinetic resolution of the sulfinyl chloride, generated first, or high separation factor of the intermediate diastereoisomers formed, permits the sulfinylating agent to be obtained in 100% de. 

The reaction of phenyl vinyl sulfoxide 234 with isobutene, in the presence of trifluoroacetic anhydride, yielded the to-alkylated product 238 (Scheme 59).128 It was suggested that this reaction proceeded by a different mechanism than the usual additive Pummerer mechanism. The alkene reacts with the electrophilic sulfur atom of intermediate 235, giving, after loss of a trifluoroacetate ion and a proton, the sulfonium ion 236. Thio-Claisen rearrangement of the ion then gives the thonium ion 237 which reacts with a further molecule of isobutene to give the product 238. 

The /ra t-thiiranium ion, in separate experiments, reacted with water to afford /ra t-di-/-butylethene, in amounts depending on the concentration of water and the thietanium ion. The authors estimated rate constants for the reactions of both the as- and the /ra t-thiiranium ions with water at 25 °C and pointed out that the thiiranium salts are models for transition states for addition of electrophilic sulfur compounds to alkenes. A summary of the findings and their interpretation was published in 1997 <1997G177>. 

Chiral epoxides frequently play a key role as intermediates in organic synthesis and the development of methods for the catalytic asymmetric synthesis of such compounds therefore remains an area of intensive research. Methods have focused principally on the asymmetric electrophilic oxidation of alkenes and good enantioselectivity has been achieved [1]. An alternative to oxidative processes for the synthesis of epoxides is the reaction of sulfur ylides with aldehydes and ketones [2,3,4,5,6]. Sulfur ylide epoxidation is a carbon-carbon bond forming reaction and is complementary to oxidative methods. The standard conditions for this reaction utilize the original Corey method treatment of a sulfonium salt with a strong base in the presence of or followed by the addition of an aldehyde... 

Thiophene prefers reactions with electrophilic reagents. Additions and ring-opening reactions are less important than with furan, and substitution reactions are dominant. Some additional reactions, such as oxidation and desulfurization, are due to the presence of sulfur and are thus confined to thiophenes. 

An unusual example of electrophilic addition to trimethylvinylsilane involved the iron carbonyl-catalyzed addition of elemental sulfur. The elemental sulfur reacts with excess trimethylvinylsilane under mild conditions (benzene, 55°C) in presence of Fe3(CO)i2 giving l-trimethylsilyl-2,3,4,5,6-pentathiepane (27), among the other products. These products were isolated chromatographically. The elemental sulfur in the absence of Fe3(CO)i2 does not react with the alkene under conditions described (Equation (21)) <83J0U133,83JOUU3, 88T281>. 

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