Oxidation at the Thiophene Sulfur Atom

The thiophene sulfur atom shows very few of the reactions expected of a sulfide. The oxidation to a sulfone is difficult to achieve, but is of special interest, as knowledge of its aromatic character or lack of it would give information about the ability of sulfur to expand its valence shell beyond eight electrons. 

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 whole reaction pattern of the thiophene-1,1-dioxides is thus characteristic of unsaturated, and not of aromatic, compounds. 

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Low-temperature chlorination of dibenzothiophene with elemental chlorine produces a chlorine adduct at the sulfur atom which is readily decomposed to dibenzothiophene 5-oxide. However, nuclear chlorination of dibenzothiophene has still not been studied. 2-Chlorodibenzo-thiophene has been prepared by the Sandmeyer reaction on 2-amino-dibenzothiophene and this route has now been extended to give... 

The structure of the benzoate derivative 166 was established by X-ray analysis, confirming the selective oxidation of the sulfur atom at the 6-position of thieno[2,3- ]thiophene-2-sulfonamides. The activated double bond of the cyclic vinyl sulfone moiety readily underwent nucleophilic addition, which was exemplified by reaction of 165 with 4-methoxybenzylthiol in the presence of base to provide a mixture of 167 and 168 <1999JHC249>. 

Both the thiophene 1-imide 123 and the 1,1-diimide 124 are thermally stable. X-Ray crystallography showed that the geometry of the former is similar to that of thiophene 1-oxide with a pyramidal configuration at the sulfur atom indicating loss of aromaticity. The C-C bond lengths also confirm this. 

Electrochemical oxidation of thiophene derivatives in methanol at a Pt anode leads to different types of reactions depending upon the electrolytes used. With sodium meth-oxide, 2,5-dimethyl- and tetramethylthiophene [207] afforded 2,5-dimethoxy adducts as a mixture of cisitrcms isomers together with side-chain oxidation products. However, in the presence of sodium acetate, the latter become the main products. In methanol/sulfuric acid solutions, at a carbon anode, thiophene and derivatives [208] oxidations resulted in a ringopening reaction with loss of the sulfur atom, yielding y-dicarbonyl compounds and their... 

The Dewar thiophene 1 was oxidized to its S-Oxide. 19F NMR of the latter compound showed an automerization through the 1,3-shift of the S-oxide group.21 Later, the Dewar thiophene itself was found to automerize at much higher temperature.22 The lone pair on the sulfur atom was proposed to participate in the 1,3-shift. Lemal named it a pseudopericyclic reaction (Scheme 5). The bond order between the lone pair and the C—C double... 

To summarize, electrophilic substitutions and metalations of thiophenes take place preferably at the a-positions due to the electronegativity of the sulfur atom. This is the consequence of the more effective incorporation of lone pair electrons on the sulfur into the aromatic system. In contrast, electrophilic substitution of benzo[fe]thiophenes occurs at the P-position however, the preference is not strong and substitution can result in other positions. Like thiophene, metalation and metal-halogen exchange are favored at the a-position. Also, oxidative couplings of thiophenes and benzothiophenes are not very... 

The most general observation is to be made on the basis of the tracer studies that only a part of surface sulfur is mobile or exchangeable on catalysts with sulfides and oxides. Supported Co and Ni contain only mobile sulfur, if sulfided by thiophene and contain both mobile and immobile sulfur, if they are sulfided by H2S/H2.[ 1 This is interpreted with the differences in the metal-sulfur bond strengths, as the edge S-atoms are held more weakly than the top S-atoms of the slab.I This follows from the results of some radiosulfur tracer studies of metal single crystals that different surface S-species are formed at different surface sulfur concentrations, and on different crystal faces of Pt, and... 

Numerous organic and inorganic sulfur compounds are adsorbed at the oxide-free surfaces of Au and Pt electrodes and can be detected by Mode II [62]. These compounds include thioalcohols, thioethers, thiophenes, thiocarbamates, organic thiophosphates, and numerous inorganic compounds. Adsorption is a prerequisite to detection and therefore at least one nonbonded electron pair must reside on the S-atom. The kinetics for detection of adsorbed S-compounds are quite favorable at pHs from 0 to 14. Since alcohol and amine groups are detected only under highly acidic and/or alkaline conditions, the detection of sulfur compounds under mildly acidic conditions is highly selective. 

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