Thiophene Sulfones and Sulfoxides

Thiophene sulfones show no aromatic character, they behave as dienes and also show reactions of compounds containing a C = C bond conjugated with an electron-withdrawing group. Thiophene sulfone itself is highly unstable, but alkyl and aryl groups and fused benzene rings increase the stability. 

An azulene synthesis involves the addition of 6-(iV,iV-dimethylamino)fulvene (208) to a thiophene sulfone (77TL639, 77JA4199). 

Benzo [/j] thi ophene sulfone (210) reacts as a vinyl sulfone and forms adducts (209) and (211) when treated with mercury(II) acetate in methanol and with cyclopentadiene, respectively. 

Reactivity of Five-membered Rings with One Heteroatom 

The O-trimethylsilyl derivative (212) of dibenzothiophene 5-oxide on treatment at — 78°C with 2,2 -dilithiobiphenyl gave the first stable tetracoordinated sulfur compound with four C-S bonds (213) in 96% yield (92CC1141). 

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Oxidation of thiophene with peracid under carefully controlled conditions gives a mixture of thiophene sulfoxide and 2-hydroxythiophene sulfoxide. These compounds are trapped by addition to benzoquinone to give ultimately naphthoquinone (225) and its 5-hydroxy derivative (226) (76ACS(B)353). The further oxidation of the sulfoxide yields the sulfone, which may function as a diene or dienophile in the Diels-Alder reaction (Scheme 88). An azulene synthesis involves the addition of 6-(A,A-dimethylamino)fulvene (227) to a thiophene sulfone (77TL639, 77JA4199). 

Melles and Backer " found, from a study of the oxidation of substituted thiophenes with perbenzoic or peracetic acid, that sulfones could be obtained from polysubstituted methyl- and phenyl-thiophenes and that the presence of electron-attracting groups, such as nitro, hindered the oxidation. Oxidation of thiophene - led to a product which was formed through a Diels-Alder reaction between the intermediate thiophene sulfoxide (211) and thiophene sulfone (212) and for which two alternative structures, (213) or (214), were suggested. Similar sesquioxides were also obtained from 2- and 3-methylthiophene and 3-phenylthiophene. The structures were not proved. Bailey and Cummins synthesized thiophene-1,1-dioxide... 

The by far more common and preparatively valuable dioxirane oxidation of divalent sulfur substrates is that of sulfides, to produce either sulfoxides or sulfones . Since sulfoxides are considerably less reactive than sulfides, the reaction outcome may be conveniently controlled by the stoichiometry of the oxidant For example, in the low-temperature oxidation of thiophene by an excess of DMD, the corresponding 1,1-dioxide (sulfone) has been obtained, albeit in low yield (equation 20). This is the first preparatively useful method for isolating this elusive sulfone, which also accentuates the importance of the neutral and anhydrous conditions under which the oxidations with the isolated DMD may be conducted. 

Thiophene sulfoxides and thiophene sulfones can be prepared by direct oxidation of the corresponding sulfides (Section 3.15.9.5.1) with hydrogen peroxide. 2-Thiophene sulfones are also prepared by reaction of arylsulfonyl chlorides with thiophene under mild Friedel-Crafts conditions, or by condensation of 2-thiophenesulfonyl chlorides with aromatic compounds (Section 3.14.2.4). 

The relative order of increasing adsorptivity of the various sulfur compounds and associated hydrocarbon to be adsorbed on sihca gel, given by Haresnape, Fidler, and Lowry (151), is aromatic hydrocarbons, thiophenes, alkanethiols, alkane disulfides, aromatic and cycloalkanethiols, alkane sulfides, and cyclic sulfides. (The more polar sulfur compounds such as sulfoxides, sulfones, and sulfonic acids would be expected to be more readily adsorbed and therefore would fall at the end of the list.)... 

Additional sulfur-containing substances that penetrate the environment include intermediates, byproducts and waste products of the chemical, pulp, coal, and petrol industries, as well as compounds used in dye production such as thiols, thiophenes, sulfides, thiazoles, sulfoxides, sulfones, and sulfonic acids. Agriculture contributes to soil and water pollution through the widespread use of pesticides containing sulfur. Unfortunately, warfare agents are still produced. 

The classical structures of pyrrole, furan and thiophene (31) suggest that these compounds might show chemical reactions similar to those of amines, ethers and thioethers (32) respectively. On this basis, the initial attack of the electrophile would be expected to take place at the heteroatom and lead to products such as quaternary ammonium and oxonium salts, sulfoxides and sulfones. Products of this type from the heteroaromatic compounds under consideration are relatively rare. 

The principal electrophiles to attack ring sulfur are either oxidants or alkylating reagents. Thiophene sulfoxide and sulfone formation is discussed in Section 3.02.2.6. Alkylating agents capable of forming thiophenium salts include trimethyloxonium tetrafluoroborate (MeaO BF ) and alkyl fluorosulfonates (ROSO2F). The salts e.g. 87) are conveniently isolated as hexafluorophosphates (88). 

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