Disulfonium dications reactions

Reduction of Disulfonium Dications. Reactions with Bases... 

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 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>. 

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... 

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... 

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... 

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). 

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... 

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... 

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... 

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... 

In an aprotic solution, the mechanism of oxidation of diaryl disulfides was shown to be more complex than a direct cleavage of the S—S linkage [116,117,123], The occurrence of two consecutive reactions being of second kinetic order if potential-determined and of first order if current-determined, was established for the two-electron transfer steps. Dimerization of the cation radicals occurs on the ArS fragment, whose contribution to the HOMO is more important, and produces an intermediate disulfonium dication. The subsequent cleavage of the latter results in two ArS+ cations and a molecule of a disulfide (the same as a starting disulfide in the case of symmetrical compounds). This mechanism, EC2C1E (E = electrochemical, C = Chemical), has... 

In contrast to the vicinal-oxonium dications, vicinal-sulfonium dications are well known dicationic systems. Many examples have been prepared and studied. As discussed in review articles,91 the disulfonium dications can be readily prepared by electrochemical or chemical oxidation of appropriate disulfides (eq 66), by reactions of mono-S-oxides with acidic reagents (eq 67),... 

In analogy to the properties of ordinary sulfonium salts, reaction of disulfonium dications with bases may lead to either a- or p-deprotonation. Deprotonation of the S-S dication formed during the Pummerer rearrangement of monosulfoxides of certain bis-sulfides 13, 59, 56 is a key step of the overall process. Furukawa and co-workers " also suggested that direct formation of disulfonium dication ylides occurred upon treatment of sulfinyli-mines of 14 and 41 with potassium tert-butoxide. Reaction of triflic anhydride with sulfoxide 100 affords sulfonium salt 102 through deprotonation of the corresponding S-S dication 101 (Scheme 39). ... 

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