Sulfenyl reagents Lewis acides

Electrc hilic reagents capable of Mnging about Pummerer rearrangement include inoiganic and organic acids, carboxylic anhydrides, acyl halides, isocyanates, caibodiimides, irimethylsilyl halides and triflate, sulfonyl and sulfenyl halides, phosphorus pentoxide and many typical Lewis acids such as boron... 

Alkane-and arenesulfenyl chlorides add to alkenes and alkynes thus, the addition of 2,4-dinitrobezenesulfenyl chloride (61) may be used to prepare solid derivatives of alkenes, e.g. the adduct (62) (Scheme 37). The Markownikoff addition of a sulfenyl chloride to an alkene is stereospecifically trans the adduct may be sequentially treated with a Lewis acid and a nucleophilic reagent to achieve the introduction of a new carbon-carbon bond (64) via the episulfenium ion intermediate (63) (Scheme 38). The procedure can also be applied to the synthesis of unsaturated sulfides, e.g. (65) (Scheme 39). 

Chiral Lewis acids derived from complexes between copper(I) chloride with phosphino sulfenyl ferrocenes are efficient catalysts for enantioselective aza-Diels-Alder reaction of A-sulfonyl imines with Danishefsky-type dienes (eq 47), and asymmetric conjugate addition of Grignard reagents to cyclic enones (eq 48).136... 

The less polar nature of disulfides and sulfenamides makes the addition of these reagents to alkenes inefficient. Although thiolates can be generated under reducing conditions,the activation of disulfides and sulfenamides is routinely performed by Lewis acids to enable the introduction of new u bonds other than C—S bonds. As a consequence, the stereospecificity and reactivity observed in the Lewis-acid-promoted addition of disulfides and sulfenamides to alkenes is similar to that of sulfenylation by sulfenyl halides and thi-osulfonium salts. Likewise, the accepted reaction mechanism includes the coordination of Lewis acid, resulting in a thio-sulfonium-like species that can further react with alkene either directly or indirectly by way of dithiosuhonium ions. 

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