[54884-50-3] · C6H10BF4NO2S · Dimethylsuccinimidosulfonium Tetrafluoroborate · (MW 247.02)
(stable sulfonium salt for the mild oxidation of alcohols and catechols in the presence of a tertiary amine; also used for the introduction of a methylthiomethylene substituent in the ortho position of simple phenols, or the transfer of a dimethylsulfonium group to nucleophiles)
Alternate Name: succinimidodimethylsulfonium tetrafluoroborate.
Physical Data: mp 169-171 °C
Solubility: very sol methylene chloride, acetonitrile; insol ether.
Form Supplied in: colorless solid; not commercially available.
Analysis of Reagent Purity: mp, 1H NMR, or elemental analysis.
Preparative Method: an acetonitrile solution of N-Chlorosuccinimide and Dimethyl Sulfide produces the unstable dimethylsuccinimidosulfonium chloride at -25 °C, which is then treated with Triethyloxonium Tetrafluoroborate (eq 1).
Handling, Storage, and Precautions: hygroscopic, but thermally and air stable; can be stored indefinitely under N2 in sealed flasks in the absence of moisture.
The general class of dimethylsuccinimidosulfonium salts are reagents useful for the transfer of the dimethylsulfonium moiety to nucleophilic species.1 The main variation in this group of reagents is the nature of the counteranion (Cl, Br, BF4). The title reagent incorporating the tetrafluoroborate anion is a thermally stable and isolable salt of this series. Both the chloride and bromide are generally generated in situ for the purpose of oxidations of alcohols2 or the sulfonium aminations of anilines.3
NCS (22.7 g, 0.17 mol) was dissolved in freshly distilled acetonitrile under a nitrogen atmosphere. The solution was cooled to -25 °C with stirring while 10 mL (0.2 mol) of DMS was added dropwise. After stirring for 15 min, the reaction mixture was cooled to -50 °C until crystals formed. A solution of 25 g (0.17 mol) of triethyloxonium tetrafluoroborate in 100 mL of acetonitrile was added dropwise to the reaction mixture. After addition was complete (about 1 h), the flask was cooled to -78 °C until the solution solidified. The solid mass was slowly brought to rt over a period of 4 h; then 500 mL of anhyd ether was added slowly to precipitate the dimethylsuccinimidosulfonium tetrafluoroborate as a white solid. Recrystallization from acetonitrile/ether gave 33.6 g (80%) of the product as white needles (mp 169-171 °C).
The advantage of the stable tetrafluoroborate reagent lies in its stoichiometric use in a variety of solvents under controlled reaction conditions in the absence of a nucleophilic anion. A particularly mild and selective oxidation of catechols to o-quinones can be effected with the title reagent and 1 equiv of Triethylamine in acetonitrile below 0 °C (eq 2).
These oxidations can be carried out between -20 and -50 °C in order to extend the lifetime of the sensitive and very reactive o-quinones.4 The mechanism of this oxidation process involves the intermediacy of an aryloxysulfonium cation which undergoes base-promoted elimination of dimethyl sulfide (eq 3). The merits of this procedure include the absence of deleterious byproducts which could react with the quinone product and the lack of opportunity for overoxidation.
The title reagent is also effective for the oxidation of p-hydroquinones to p-quinones in quantitative yields (eq 4). The employment of an amine base is essential for this oxidation process.
The title reagent can also be considered as a source of the dimethylsulfonium cation through reaction with nucleophiles. Thus the nucleophilic addition of a phenolic oxygen atom to the reagent leads to an intermediate aryloxysulfonium species, which rapidly gives up a proton to generate a sulfonium ylide.5 This latter intermediate undergoes a facile [2,3]-sigmatropic rearrangement to produce cyclohexadienones with 2,6-disubstituted phenols (eq 5).6 In the case of unsubstituted phenols, the sigmatropic rearrangement takes place and the dienone rearomatizes to give the ortho-methylthiomethylene phenol.7
Other heteroatoms such as nitrogen and sulfur behave as good nucleophiles in reactions with the title reagent and related salts.8 Stabilized carbanions such as substituted a-cyanoacetates react readily with the succinimidosulfonium reagent to give several rearangement products.8 The initial combination of the reagent with the cyanoacetate anion is thought to generate a s-sulfurane intermediate which could ionize to form the succinimido anion and a sulfonium cation (eq 6). In the presence of a weak base, the sulfonium cation is deprotonated to an ylide which can undergo a [1,2]-rearrangement to a methylthiomethylene derivative of the starting carbanion or, alternatively, the ylide may undergo a [2,3]-sigmatropic rearrangement to yield a substituted ketenimine (eq 7). In most cases, the ketenimine is the major product.
Joseph P. Marino & David P. Holub
The University of Michigan, Ann Arbor, MI, USA