Disulfonium dications

Furukawa and co-workers conducted an ab initio molecular orbital study of dications of 1,5-dichalcogenacycloocatanes <2000HAC31>. Nenaidenko and co-workers performed ab initio calculations with the HF/6-31G approximation on 1,4-dithiane 114 and a series of disulfonium dications, 115-118 <2003RCB1667>. The bond lengths and interatomic distances in the optimized structures are given in Table 20. 

Table 20 Bond lengths and interatomic distances of optimized structures of various disulfonium dications...

The generalized Woodward-Hoffmann rule suggests that a synchronous addition of disulfonium dications at the double C=C bond of alkenes would be a thermally forbidden process and so would be hardly probable. Simulation of the frontal attack by ethylene on l,4-dithioniabicyclo[2.2.0]hexane 115 gave no optimal structure of an intermediate complex. On the other hand in the lateral approach of the reactants, orbital factors favor attack of the double bond by one of the sulfonium sulfur atoms of the dication. This pattern corresponds to SN2-like substitution at sulfur atom as depicted in Figure 5. Using such a reactant orientation, the structure of intermediate jc-complex was successfully optimized. The distances between the reaction centers in the complex, that is, between the carbon atoms of the ethylene fragment and the nearest sulfur atom of the dication, are 2.74 and 2.96 A, respectively. 

The formation of stable disulfonium dications from certain bis-sulfides provided an impetus for re-evaluating the mechanisms of some old reactions. The isomerization of sulfoxide 124, using trifluoroacetic acid, involved an acid-catalyzed migration of an oxygen atom from a sulfoxide to sulfide (see Equation 36). This was one of the first examples in which an intermediate formation of an S-S dication was proposed. Recently, such a dication 125 was detected in a non-nucleophilic medium such as sulfuric acid <1995HAC145>. 

The differing nucleophilicity of acetate and trifluoroacetate anion determined the manner in which naphtho[l,8-/yt]-l,5-dithiocinc sulfoxide 127 rearranged on treatment with acetic and trifluoroacetic anhydrides. In both cases, the reaction proceeded through formation of a disulfonium dication 128, but the final products were different. When acetic anhydride was used, the reaction afforded the corresponding a-acetylsulfide 130, a normal product of the Pummerer rearrangement, while trifluoroacetic anhydride caused isomerization with formation of dithioacetal 132 (see Scheme 16) <1995HAC559>. 

The most important of organic chalcogen dications are disulfonium dications (also called dithioether dications) - compounds with two positively charged sulfonium atoms connected by a single bond. 

At the beginning of the last century, Bruning and Autenrieth24 suggested for the first time that the reaction of cyclic sulfide 1 with bromine results in formation of a disulfonium dication 2. It was shown later that the actual reaction product is dibromodisulfide 3 (Scheme 2).25... 

The turning point in the understanding of the chemical nature of S-S dications came in 1976 when Musker reported synthesis, isolation and characterization of disulfonium dication 12 formed by the two-electron oxidation of 1,5-dithiacyclooctane 10 (Scheme 4).29,30... 

The unusually facile formation of a disulfonium dication from sulfide 10 is the result of stereochemical features of the eight-membered ring, which favor the formation of a transannular bond.31 According to X-ray data (see in Chapter 7.1 Table 1), the distance between the two sulfur atoms in 1,5-dithiacyclooctane 10 is smaller than the sum of their van der Waals radii (3.75 A), which results in a strong non-bonded interaction between the atoms confirmed by photoelectron spectroscopy and mass spectrometry.32 33 This interaction and the sulfur-sulfur distance can be decreased as a result of bond formation with an electronegative substituent as in sulfoxide 13 or sulfoximine 14.34,35... 

Retro synthetic analysis provides two main approaches to the disulfonium-dication functionality. The first approach involves double alkylation of disulfides so far has been unsuccessful (Scheme 5).36 All currently known methods... 

The second main approach to disulfonium dications is based on nucleophilic substitution at sulfonium sulfur atom by a sulfide. This approach is much more efficient and versatile than the direct oxidation of sulfides.59 61... 

Oae and Numata26 were the first to postulate formation of a disulfonium dication in reaction of concentrated sulfuric acid with monosulfoxide of a bis-sulfide. Later Furukawa et al.62 found that the crystalline hydrosulfate 36 can be prepared by reaction of the corresponding monosulfoxide 13 (or A-tosyli-mide 14) with concentrated sulfuric acid. Formation of a symmetric dication 38 was confirmed by isolation of 1 1 mixture of deuterated sulfoxides 37 and 39... 

Many of these complications can be avoided when monosulfoxides are converted to the dications using triflic anhydride instead of concentrated sulfuric acid. This method was suggested by Furukawa et al.71 in 1987. Reaction of triflic anhydride with a monosulfoxide72 transforms it into trifluoromethanesulfonyloxysulfonium salt 47, which undergoes clean intramolecular nucleophilic substitution of trifluoromethylsulfonate anion by the other sulfur atom with formation of the disulfonium dication 34 in a high yield (Scheme 16).73... 

Monosulfoxide 48 where the transannular interaction of the sulfur atoms is minimal also reacts with triflic anhydride to give a white precipitate of highly strained disulfonium dication 49, which is extremely difficult to isolate.71 Dication 49 exhibits only one 1H NMR signal as a singlet at 3.80 ppm in agreement with a symmetric monomeric structure.71... 

More recently, this method has been extended to preparation of a variety of disulfonium dications from both acyclic and cyclic bis-sulfides, including very labile dications not observed when other methods were used.78 Thus, simple acyclic S-S dications were prepared by an intermolecular reaction of a monosulfide, a monosulfoxide and triflic anhydride.79 In the first step, reaction of triflic anhydride with dimethylsulfoxide generates a highly electrophilic80 complex 50 (dimethyl sulfide ditriflate).81 The latter reacts with dimethyl sulfide to give labile tetramethyldisulfonium dication 51 identified by NMR spectroscopy.79 In a similar manner, bis-(tetramethylene)disulfonium dication 52 is obtained from tetrahydrothiophene and its S-oxide (Scheme 17). 

A Reactions Involving Intermediate Formation of Disulfonium Dications... 

Musker and Doi84 reported convincing kinetic evidence for the intervention of a disulfonium dication during reduction of sulfoxide 13 with iodide in... 

Reaction of -dimethylthiobenzene sulfoxide 56 with trifluoroacetic anhydride results in a mixture of sulfide 65 and the corresponding mono- and disubstituted products of the Pummerer rearrangement 63, 64 via intermediate disulfonium dication 62 (Scheme 23).88... 

Another example that illustrates formation of a disulfonium dication as an intermediate was found in the reaction of sulfoxide 13 and thionyl chloride. Instead of the normal a-chlorosulfide Pummerer product, the reaction leads to a stable chlorosulfonium salt 66.89 Hydrolysis of the salt obtained from 2,2,8,8-tetradeuteriated sulfoxide 37 results in a 1 1 mixture of the two possible isomers, thus indicating that the chlorosulfonium salt does exist in rapid equilibrium with a symmetric S-S dication (Scheme 24). 

Reactions of disulfonium dications with nucleophiles can follow two pathways in which, depending on the nature of the dication and nucleophile, nucleophilic substitution can occur at either the onium sulfur atom or at the a-carbon atom (Scheme 27). 

Decomposition of highly reactive disulfonium dications in sufficiently nucleophilic medium is accompanied by formation of disulfides. Even hydrolysis of disulfonium dications generated from sterically rigid bis-sulfides often affords some disulfide as a by-product in addition to the monosulfoxide main product. For example, hydrolysis of dication 72 yields sulfoxide 74 and trace amounts of heterocycle 73 (Scheme 28).56... 

Dealkylation of labile disulfonium dications 75 derived from 2,2 -bis(alkylthio)-biphenyls gives rise to thiosulfonium salt 76. This reaction can also be classified as nucleophilic substitution at the a-carbon atom. The intermediate dication 75 is highly reactive, but can be detected spectroscopically (Scheme 29).93... 

Nucleophilic substitution at the a-carbon atom does not occur in the case of the most studied and stable bicyclic disulfonium dications.96 Although the reaction of dication 34 with bromide ions formally leads to the S C bond cleavage, the reaction mechanism involves initial nucleophilic attack at the sulfonium atom by the bromide anion. The bromosulfonium salt intermediate... 

Regardless of their structure, all disulfonium dications are easily hydrolyzed to form the corresponding monosulfoxides in quantitative yield. The mechanism involves direct nucleophilic substitution at sulfonium sulfur.29 For example, hydrolysis of monocyclic 84 and bicyclic 36 dications leads to sulfoxides 85 and 10 in high yield (Scheme 31).62... 

Reaction of disulfonium dication 34 with electron donor aromatic compounds also affords products of substitution at the sulfonium atom. For example, reaction of dication 34 with aniline, phenol and triphenylamine leads to the corresponding para-substituted sulfonium salts 86 (Scheme 32).97... 

Disulfonium dications also interact with non-aromatic 71-bonds. This reaction is the only known example in which a 1,2-dication adds to alkenes and... 

According to the stepwise electrophilic reaction mechanism, the differences in the stereochemistries of the products from the reactions of alkenes with cyclic 49 and acyclic 51 disulfonium dications can be explained by the larger rates of the intramolecular reactions. In the case of a cyclic dication, the carbocationic center in intermediate 94, which is formed as the result of initial attack by a S-S dication on a double C=C bond reacts with nucleophile intramolecularly, thus conserving the configuration of the substituents at the double bond. On the other hand, an acyclic dication undergoes transformation to two separate particles (95 and dimethylsulfide) with a consequent loss of stereoselectivity. Additional experiments with deuteretad alkenes confirm that reaction is not stereoselective, lending further support to the stepwise mechanism (Scheme 36).106... 

Reduction of Disulfonium Dications. Reactions with Bases... 

Synthetic approaches to selenium, tellurium and mixed dications are similar to the methods used for disulfonium dications. Oxidation of bis-selenides to diselenonium dications 104 with two equivalents of nitrosonium salts -NOBF4, NOPfV,113 occurs more readily than the oxidation of sulfides and affords the corresponding dications in better yields (Scheme 41).96,114... 

Because the redox potential of selenides is sufficiently low, oxidative generation of Se-Se and S-Se dications on treatment with sulfuric acid is more general than in the case of disulfonium dications. For example, formation of dications 106 and 108 occurs readily when the corresponding bis-selenide 105 and... 

The most versatile approach to disulfonium dications - reaction of triflic anhydride with monosulfoxides of bis-sulfides - has certain limitations in the case of selenium. Most importantly, selenoxides that contain (3-hydrogen atoms are labile.120 122 Trimethylsilyl triflate was used instead of triflic anhydride for synthesis of dication 112 from a selenoxide 111 (Scheme 43).123... 

The lesser electrophilicity of the selenium and tellurium derivatives is also displayed in their hydrolytic stability. In general, all diselenonium dications are more stable toward hydrolysis than the corresponding disulfonium dications. 1,5-Diselenoniabicyclo[3.3.0]octane 113, which is expected to be hydrolyzed to an unstable selenoxide, is stable in water.124 On the other hand, dication 112 is... 

Just as disulfonium dication 34, diselenonium 113 and ditelluronium dication 114 do not undergo deprotonation. Instead, reaction of dication 113 with fluorenyllithium affords bis-selenide and fluorene dimer 103.96 Softer Lewis base such as ra-tolyl magnesium bromide reacts with diselenonium-dication 113 to give 127, a product of nucleophilic substitution at the onium atom (Scheme 48).129... 

Generation of disulfonium dications involves stabilization of the cationic centers on the sulfur atoms through formation of a new S-S bond between the two sulfonium centers. When such an interaction involves more than two chalogen atoms, it leads to formation of new interesting types of dications, which contain hypervalent central atoms, such as sulfurane, selenurane or tellururane.94... 

One can use the same methods for generation of trithiodications as for synthesis of disulfonium dications, i.e. oxidation of suitable trisulfides with concentrated sulfuric acid or with nitrosonium salts as well as reaction of a... 

Common synthesis of disulfonium dications involves formation of a S-S bond by oxidative coupling of two sulfide moieties. Involving for oxidative generation of S-S bond, thiocarbonyl compounds can lead to new-type dications containing disulfide moieties between two positively charged fragments. There are several disulfide dications of this type derived from thioureas, thiocarbon-ates and thioketones as well as selenium derivatives.137 148... 

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