Sulfur-extrusion reaction—

Thermodynamic parameters have been obtained from kinetic HNMR spectroscopic studies of the thermal decomposition of ethyl 2,7-di-to7-butyl-5-methylthiepin-4-carboxylaten and two 1-benzothiepin compounds.12 The activation parameters for sulfur extrusion are AH = 93.7 kJ mol - 1 and AS = — 112.6 J Kmol-1 (in [2H18]Decalin) for the thiepin derivative,11 and AH = 75.3 and 87.9 kJ mol1 and AS = —100.4 and —104.6J Kmol-1 (in [2Hs]toluene) for the benzothiepin compounds.12 The large negative activation entropy values are consistent with a high degree of order in the anticipated thianorcaradiene transition state of the sulfur extrusion reaction. 

At present, the problems in thiepin chemistry awaiting solution are (i) how to construct the thiepin skeleton under mild reaction conditions, (ii) what are the structural effects on thermal stability of thiepin, (iii) whether the thianorcaradiene is an intermediate of sulfur extrusion reaction or not, (iv) what is the molecular structure of the thiepin (planar or nonplanar), (v) what is the antiaromaticity of the thiepin ring. 

Secondly, in view of the generally accepted mechanism for the sulfur extrusion reaction, which involves valence isomerization of the thiepin ring to its corresponding thianorcaradiene (benzene episulfide) isomer followed by irreversible loss of sulfur,... 

The synthetic procedures outlined in Section 3 provide ready access to a wide variety of benzo[7>]thiepins in which the substituents are methyl, formyl and ethoxy-carbonyl. The thermal stability of the thiepins was conveniently evaluated from the half-lives of their sulfur extrusion reactions by means of H-NMR spectroscopy. The half-lives of a series of substituted benzo[b]thiepins and naphtho[2,3-f>]thiepins thus obtained are summarized in Table 1. 

One of the most important problems that has to be solved in the thiepin chemistry is the mechanism of the sulfur extrusion reaction. Ready loss of sulfur of the simplest thiepins presumably occurs by valence isomerization to the corresponding thianorcaradiene, which requires a [4n -I- 2] disrotatory electrocyclic process leading to a c/s-fused three-membered ring, followed by cheletropic loss of sulfur. A lot of evidence supporting the above mechanism, though inconclusive, is available to date. 

C-C bond of the resulting thiirane. Therefore, when assuming a thianorcaradiene intermediate in the sulfur extrusion reaction of the thiepin, the relative stability of 4 and 73 can reasonably be explained. 

Fourth, kinetic data of the sulfur extrusion reaction of thiepin will provide direct evidence for the transition state of the process. Data on the conversion of the thiepin 34 into its corresponding naphthalene derivative are available 2SK The substantially large negative activation entropy (AS si —24 cal mol-1 deg-1) points to the existence of a highly ordered transition state, namely a thianorcaradiene, in the reaction. 3,4-Bis(methoxycarbonyl)-5-hydroxybenzo[/>]thiepin 33 thermally... 

Aminorhodanines 284 reacted with ethyl 2-bromo-3,3-diethoxypropionate to provide 2,3-dihydro-pyrazolo[5,1 -b -thiazoles 285. The mechanism proposed for the cyclization involved via a sequential condensation-sulfur extrusion reaction (Equation 125) <1994JHC1719>. 

In their full paper 58b), they reported the preparation of a diastereomeric mixture of ethyl [10.6]betweenanenecarboxylates via a sequence of conversions involving Ramberg-Backland s sulfur extrusion reaction. 

Thermodynamic parameters have been obtained from kinetic studies of the thermal decomposition of thiepins (74T2431). The sulfur extrusion reaction of (53) was examined kinetically and showed a large negative AS value (-24 J K-1 mol-1) consistent with a decrease in entropy in the transition state which would be anticipated to be close to a thianorcaradiene structure. Thus while the latter valence tautomer of any thiepin has yet to be detected directly, the kinetic data, allied to the other evidence discussed, support the view that thiepins are analogous to oxepins in undergoing spontaneous tautomerization to their episulflde or epoxide forms. 

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). 

Dihydropyrazolo[5,l-b]thiazoles (48) were simply prepared from 3-aminorhodanines (47) and ethyl 2-bromo-3,3-diethoxypropionate under toluene reflux for 2 hr in fair yield, accompanying with tandem condensation, and sulfur extrusion reaction under mild condition. [94JHC1719]... 

Typical sulfur-extrusion reactions can lead to quinolines after an initial cyclocondensation producing the intermediate seven-membered ring (Scheme 129) <2000SL595>. Additional examples generated 2-trifluoromethyl-4-formyl derivatives <1997T641>. 

A remarkable sulfur extrusion reaction was discovered by Fang-haenel.46 The dithiazine (51), if heated to about 180°, yields the... 

The sulfur extrusion reaction of unprotected A -acetonylthioinosine with triphenylphosphane and potassium tert-butoxidc gives 6-acetonylpurine riboside 19, though in low yield,... 

The literature search for this review turned out to be complicated. Reaction data bases such as REACCS or ChemInformRX worked well but, as expected, could not give a complete survey of the relevant literature. Chemical Abstracts key words such as sulfur extrusion or desulfurization revealed numerous references dealing with industrial desulfurization of stock chemicals in addition to the citations of interest nevertheless, these turned out to be incomplete (for similar problems in searching for sulfur extrusion reactions, see 90MI2). 

In sulfur extrusion reactions mediated by Raney nickel or nickel complexes (90MI1) or in reactions carried out under photolytic conditions (69JOC1233), biradical intermediates such as 44 and 47 are presumed to be present (Scheme 9). However, there is evidence that lithium-assisted desulfurization of phenothiazines 49 proceeds via open-chain intermediates 50 (58JA380) (Scheme 10). 

Benzo[b]thiepin structures are also assumed to be intermediates in sulfur extrusion reactions of cyclopropabenzothiopyrans (61JA4034 69JOC56). Phenanthrenes 74 are formed when dibenzo[b,/]thiepins 73 are desulfurized under drastic conditions (57JCS3814) (Scheme 17). 

Two interesting sulfur extrusion reactions have been reported for derivatives of 239. Rossi and Pagani,150 upon heating l,4-dimethyl-2,l-benzothiazine 2,2-dioxide (264) with copper powder at approximately 300° C, isolated the indole 265 and o-isopropenyl-)V-methylaniline 266 (Eq. 56). 

The coupling reaction is referred to commonly as the sulfide-contraction or sulfur-extrusion reaction. However, this reaction is distinct from other sulfur-eliminating, carbon-carbon bond-forming... 

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. 

The decomposition is best performed in diethyl ether at room temperature (ca. 24 h).1 In refluxing toluene yields decrease sharply, because sulfur extrusion reactions lead to the corresponding isothiazolcs35 (see Section 1.4.5.2.) ... 

Although C-S bond cleavage reactions between thiophenes and osmium clusters have not been observed, selenophene and tellurophene undergo ring opening reactions with [Os3(CO)io(NCMe)2] to give complexes 28a and 28b (Scheme T). " It is likely that the sulfur-extrusion reactions of iron and ruthenium carbonyl clusters with thiophenes proceed via ring-opened intermediates of this type. 

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