Reactions with Sulfenylating Reagents

Yields of the sulfenylation products are generally excellent (except in the case of arylthiocyanates [58] which may undergo attack on the nitrile carbon) irrespective the fact if disulfides, thiosulfonates or thiocyanates are employed [59]. In some 

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Titanocene pentasulfide (1) has been used as a transfer reagent for the S5 unit to prepare numerous inorganic homo- and heterocycles. More recently, cyclic organic polysulfanes have also been obtained from (1) by reaction with bis(sulfenyl chlorides) (equation 99). ... 

J-Alkyl- or -arylthio amines were obtained by nucleophilic attack of an amine on episulfonium ions generated by reaction of sulfenyl halides or tetrafluoroborates with alkenes15-18. An effective azasulfenylation was also induced by dimethyl(methylthio)sulfonium tetrafluorobo-rate (DMTSF) in the presence of amines. With this latter reagent ammonia also adds to a double bond19. ant/ -Addition products are exclusively formed. 

Sulfenic acid derivatives (RSX) such as esters, amides and halides are reactive owing to the relative weakness of the S-X bond, and they consequently react with nucleophilic reagents (Nu-) (Scheme 33). In such nucleophilic substitutions at the sulfenyl sulfur atom, the reaction mechanism may be SN1 (Scheme 34). 

The formation of the bridged intermediate has been represented as an SN2-like displacement of the leaving group from the sulfenyl sulphur of 85 , or alternatively, as reported in equation 90 in agreement with the addition of other electrophiles to alkenes, it has been proposed that the reaction involves the initial formation of 7r-complex 86 in a rapid equilibrium with the reagents . 

The asymmetric a-sulfenylation of ketones is a particularly challenging reaction, as demonstrated by the poor success reported in the stereoselective variants via classical enolate/azaenolate reaction with an electrophilic sulfur reagent [71]. An umpolung approach has been devised by Coltart and co-workers [72] to effect the first asymmetric a-sulfenylation of ketones with arene thiols. Nitroso alkene derivatives, in i/tM-generated under basic conditions from a-chloro oximes, reacted with arene thiols in the presence of cinchona thiourea 27, which promoted the conjugate addition of thiophenol (Scheme 14.25). The chiral nonracemic a-sulfenylated oximes were directly hydrolyzed by IBX to ketones in high yield and good enantioselectivity. 

Bis-sulfenylation of y-butyrolactone, as well as of cyclic ketones, has been achieved with this reagent via the corresponding lithium enolates. This reaction has been utilized in an efficient synthesis of 3-substituted 2-buten-4-olides (eq 4). 

Haas and coworkers performed a series of studies on the reactivity of the perituoroalkyl sulfenyl chlorides with electron rich heterocyclic compounds, to give CFjS derivatives. Pyrroles are good substrates for reaction with trifluoromethyl sulfenyl chloride due to higher nucleophilicity compared to benzene [45]. An excess of reagent gives bis-CCFjS) pyrrole derivatives as shown in Scheme 16. 

Unsymmetrically substituted sulfides can be accessed by reactions between sulfenyl derivatives R -S-X [R = alkyl, aryl X = Cl, OCH3, A-imidazolyl] with an organolithium reagent. Organolithiums react with selenium and tellurium compounds e.g., with diphenyl diselenide or with phenyltellurium bromide ) in the same way as with sulfur compounds if sometimes less effectively. 

Sulfenyl halides have been found to be specific, mild reagents for modification of tryptophan containing peptides and proteins 123, 124). By reaction with a sulfenyl halide like 2-nitrophenylsulfenyl chloride... 

Such a reversible blocking of the 2-position by sulfenylation was of particular interest in the synthesis of bromo-derivatives of indolyl-3-propionic acid. Specific bromination or nitration of the benzene ring of an indole is normally difficult to achieve, since the pyrrole ring also will react with electrophilic reagents. As an example, 5-bromoindole-3-propionic acid has been prepared using this reversible protection, that is by bromination of the 2-sulfenylated compound and removal of the arylthio group by the two consecutive reactions with NBS and NaBKi (727). 

From intermediate 12, the path to periplanone B (1) is short but interesting. Enolization of 12 with lithium bis(trimethylsilyl)amide at -78 °C, followed by sulfenylation using Trost s reagent,12 affords a 16 1 mixture of regioisomeric monosulfenylated ketones favoring intermediate 17. The regioselectivity displayed in this reaction is... 

Alkali acetylides react very easily with disulfides, R SSR, thiocyanates, R SC=N, and thiosulfonates, R SS02R The reactions can be carried out in liquid ammonia as well as in organic solvents and generally give excellent yields of the acetylenic sulfides, RChCSR [2,105]. Although sulfenyl halides seem suitable reagents for the introduction of alkylthio- or arylthio groups, they are seldom used for this reaction, because of their sensitivity and the... 

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