Preparation and Electrooxidative SO-Extrusion of Halogenated 7-Thiabicyclo[2.2.1]heptene 7-Oxides†

Abstract

Halogenated thiophenes have been cycloadded oxidatively to maleimides to give halogenated thiabicyclo[2.2.1]heptene S-oxides which have been subjected to an electrochemical extrusion of SO to give various halogeno-substituted phthalimides. Torssell1 and Fallis and co-workers 2 have shown that oxidative cycloaddition occurs on treating alkyl-substituted thiophenes with m-chloroperbenzoic acid (m-CPBA) in the presence of an electron-poor dienophile. We have studied the reaction in greater detail and found it valuable to synthesize novel alkyl-substituted 7-thiabicyclo[2.2.1]heptene 7-oxides in connection with crown ether systems. 3 Recently, it was shown that the yield for the oxidative cycloaddition of alkyl-substituted thiophenes, proceeding via intermediate thiophene S-monoxides, could be greatly enhanced by adding BF 3 Á Et 2 O to m-CPBA.4 While the uncatalysed reaction can be run at 0 8 8C to ambient temperature, the catalysed reaction is run typically at À78 8 8C. The greater stability of thiophene S-oxides towards further oxidation under catalysed conditions was demonstrated. 4,5 The further oxidation of thiophene S-monoxides to the S,S-dioxides is a main side reaction in the oxidative cycloaddition.Oxidation of thiophenes at sulfur proceeds readily when the thiophenes are substituted with electron donors such as alkyl groups. Electron-withdrawing or more ambidentate substituents hinder the oxidation. For this reason the oxidative cycloaddition of halogenated thiophenes does not proceed well at room temperature. Nevertheless, at more elevated temperatures, 7-thiabicyclo[2.2.1]heptene S-oxides from readily in the reaction of 2,5-chlorinated or brominated thiophenes with such idenophiles as maleimides or maleic anhydride (Scheme 1). The products are not very soluble in dichloromethane and partly precipitate during the reaction. Further precipitation can be induced by addition of diethyl ether. Apart from diligent washing with diethyl ether, the products need no additional puri®cation for a further transformation, although for analytical purposes column chromatography and recrystallization have been performed.It has been found that the addition of BF 3 Á Et 2 O has no bene®cial eect on the reaction, if the thiophene is halogenated at position 2 and/or 5. In the case of 3,4-dibromo-2,5-dimethylthiophene 1d, however, an acceptable yield of the cycloadduct was obtained when the reaction was run at À20 8 8C in presence of BF 3 Á Et 2 O. In the cycloaddition ®ve stereocentres are created. Nevertheless, only one isomer is isolated. Although no crystal structural analysis has been performed, it is thought to be the endo-product with the lone-electron pair of the sulfur directed to the newly formed ole®nic bond. This is evidenced by comparison to compounds formed in cycloadditions with donor-substituted compounds. On the other hand, 3 may be viewed as precursors for corresponding arenes. Photochemical 6 and oxidative extrusion 3 of SO in 3 has been found to proceed at much lower temperatures than the purely thermal e...

Thiophene S-Oxides

“…A viable mechanism involves a cyclic oxathiin, where the first step within the photochemical reaction is initiated by the homolytic ring cleavage α to the sulfoxy group [92][93][94]. A rearrangement of thiophene Soxides to produce furans can also proceed thermally as found by Thiemann et al [18] in the transformation of thiophenophane S-oxide 110 to furanophane 111 (Scheme 26) and by Thiophene S-Oxides http://dx.doi.org/10.5772/intechopen.79080…”

“…This gave cycloadducts 13 and 14 (Scheme 2) [13]. Further groups [11,12,[14][15][16][17][18][19] used this strategy to react thiophene S-oxides such as 11, prepared in-situ with alkenes and alkynes in [4 + 2]-cycloadditions (Schemes 3 and 4). In the reaction with alkenes, 7-thiabicyclo[2.2.1]heptene S-oxides such as 13 were obtained, while the reaction of thiophene S-oxides with alkynes led to cyclohexadienes and/or to aromatic products, where the initially formed, instable 7-thiabicyclo[2.2.1]hepta-2,5-diene S-oxide system 21 extrudes its SO bridge spontaneously (Scheme 4).…”

“…Even before thiophene S-oxides could be isolated in pure form, it was evident that thiophene Soxides are good dienes in cycloaddition reactions, as "trapping" by cycloaddition reaction was one of the standard techniques to gauge the presence of thiophene S-oxide intermediates and provided a versatile preparative entry to 7-thiabi-cyclo[2.2.1]heptene S-oxides 68. These in turn could be converted to substituted arenes 71 by either pyrolysis [15], photolysis [51], or PTCcatalyzed oxidative treatment with KMnO 4 [15] or electrochemical oxidation [18] Thiophene S-Oxides http://dx.doi.org/10.5772/intechopen.79080…”

The sonoelectrooxidation of thiophene S-oxides

“…3 show loss of oxidation current and loss of peak definition upon cycling over a limited potential range 0.5-2.5 V, suggesting electrode fouling or some other interference phenomenon at the electrode, we examined the influence of simultaneous ultrasonic irradiation upon voltammetric traces. Previously, in a related reaction, namely in the electro-oxidative extrusion of SO in sulphoxy bridged bicycles at a Pt anode [27], where fouling of the electrode had been observed, insonation had been found to efficiently maintain electrolysis current. Fig.…”

The chemistry of thiophene S-oxides1 and related compounds

“…Ultrasonication of the substrates at 40 and 850 kHz during the electro-oxidation enhanced the current for both tetraphenylthiophene S-oxide and tetracyclone, 43 but did not mitigate 23 the fouling processes that are often associated with the oxidative processes. Indications that 2-substituted benzothiophene-S-oxides ring-open oxidatively have been obtained.…”