Sulfur-extrusion reaction Sulfuric acid

Nucleophilic substitutions involving sulfur, and sulfur extrusion reactions. Sulfur is much more reactive than oxygen as a consequence of the greater acidity of the SH functionality. Intermediate thioethers may be oxidised to the corresponding S=0 or S02 derivatives, and pyrolysis results in sulfur extrusion to give the corresponding C— C bonded species, such as 6.58 (Scheme 6.7). 

The role of the thiophile is to assist extrusion of the sulfur atom from the episulfide intermediate to produce the unsaturated product. Studies have established that the presence of a thiophile affects the rate as well as the yield of the sulfur-extrusion reaction. This participation was observed by measuring the difference in reaction rates and yields upon inverting the order of addition of the base and thiophile to a given a-thioiminium salt. Better results were obtained when the thiophile was allowed to stand in the presence of the a-thioiminium salt prior to addition of base. Presumably a sulfur-phosphine complex is established that enhances the acidity of the a-proton slated for abstraction and results in shorter reaction times and better yields. 

Treatment of ethyl 2,7-di-/ert-butylthiepin-4-carboxylate (24) with 3-chloroperoxybenzoic acid at — 78 °C results in the benzene derivative 25 only, and no sulfur-oxidized products 80 however, the stable 2,7-di-ter/-butylthiepin (26) can be oxidized with 0-benzyl 00-hydrogen monoper-oxycarbonate at — 78 °C to give the corresponding S-oxide 27, which was monitored by HNMR spectroscopy at — 40°C. At —15 C, sulfoxide 27 was converted, via extrusion of sulfur monoxide, with a half-life of 5.5 hours to the benzene derivative 28.87 The oxidation reaction of 26 with excess of the monoperoxycarbonate did not proceed to the S,S-dioxide, even though the parent thiepin 1,1-dioxide is known to be stable at room temperature.15... 

Both compounds 190 and 193 are reduced to 9,10-diphenylanthracene (205) by zinc and acetic acid. However, more interest attaches to the formation of anthracenes from anthracen-9,10-imines in nonreducing conditions. The iV-ethoxycarbonyl derivative (192) decomposed at 215° in cyclohexane to 33% of 205, although curiously this product was not obtained if the solvent was previously degassed. Whether or not the reaction involves simple extrusion of ethoxycarbonyl nitrene could not be established, since the expected iV-cyclohexylurethane was not detected. The 9,10-epithioanthracene (194) loses sulfur thermally to give 205. ... 

More decisive evidence is provided by the interconvertibility of N-aryl-JV-arylamidinothioureas (28) and Hector s bases by oxidation-reduction.63, B4,67b The former compounds are accessible (as salts) (i) by the condensation of arylthioureas (24) with arylcyanamides (23),63 (ii) by the extrusion of sulfur from the recently described40,41 s-diaryldithioformamidine hydrobromidesB4a (22), (tit) by the oxidation of arylthioureas (24) with 0.5 moles of hydrogen peroxide in the presence of mineral acids,B4a and (iv) by the mild reduction of Hector s bases by hydrogen sulfide in acid media.B4a The first of these four reactions limitB the structure of the products to the three alternatives 25, 28, and 30. Of these, 25 is excluded by the non-identity of the product with authentic67 N-phenyl-i -phenylamidinothiourea (25 R = Ph). The monosulfide structure (30) is not reconciled as readily with the observedB4a hydrolytic fission of the products into diaryl-guanidines (29) and thiocyanic acid as is structure 28. Indeed, as in the case of thioamides and nitriles (see Section II, C, 1), the present condensation may involve the primary formation of an intermediate diimido-monosulfide (30) and its isomerization to 28. 

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