Thiirene-l-oxides

Dipolar additions to thiirene-l-oxide or thiirene-1,1-dioxide, to produce monocyclic thiirane derivatives, were not found. A cycloaddition to a thiirene, to produce a bicyclic thiirane, was reported in 2002 <2002JA8316>. 

It was reported that the diphenylthiirene-1-oxide, when heated in toluene, gave benzil, which is formed by air oxidation of an unstable oxothioketone <1996CHEC-II197>. 2,2 -Disubstituted thiirene-l-oxides, when heated in refluxing toluene, isomerized in high yields to corresponding a-oxothioketones (Scheme 24), which were air stable <2002HAC424>. 

Thiirene-l-oxides 132, substituted at the 2- and 3-positions, reacted with trifluoroacetic anhydride (TFAA) followed by /)-toluenesulfonamide to give sulfilimines 133 via episulfurenes 134. The latter undergo spontaneous ring expansion to form 1,2-thiazetes 135 that isomerize into a-iminoketones 136 and a-oxothioketones 137 (Scheme 25) <2002CL314>. 

Reaction of disubstituted thiirene-l-oxides 251 with Lawesson s reagent, in dichloromethane at room temperature for 15 or 30 min, gave the corresponding a-dithiones 252 in 20% yield. Only when R = 1-adamantyl was the a-dithione isolated in pure form (Scheme 71) <2003JA12114>. 

Methylene thiirane is obtained by thermolysis of several spirothiirane derivatives which are formally Diels-Alder adducts of methylenethiirane and cyclopentadiene or anthracene <78JA7436). They were prepared via lithio-2-(methylthio)-l,3-oxazolines (c/. Scheme 121). A novel synthesis of the allene episulfide derivatives, 2-isopropylidene-3,3- dimethylthiirane (good yield) or its 5-oxide (poor yield), involves irradiation of 2,2,3,3-tetramethyl-cyclopropanethione or its 5-oxide (81AG293). Substituents on the thiirane ring may be modified to give new thiiranes (Section 5.06.3.9). The synthesis of thiirane 1-oxides and thiirane 1,1-dioxides by oxidation is discussed in Section 5.06.3.3.8 and the synthesis of 5-alkylthiiranium salts by alkylation of thiiranes is discussed in Section 5.06.3.3.4. Thiirene 1-oxides and 1,1-dioxides may be obtained by dehydrohalogenation of 2-halothiirane 1-oxides and 1,1-dioxides (Section 5.06.4.1.2). 

All unexpected reaction is the formation of dithiete 175, thiirene 1-oxide 185, and thioketone 186 by the treatment of di-l-adamantylacetylene with S2CI2 (99UPl).Tlie yield of each compound is 30% at most. 

The reaction of 1,2-diadamantylacetylene with disulfur dichloride led to the formation of the dithiete 232 (mentioned above), l,2-diadamantyl-2-thiooxoethanone 233, and the thiirene 1-oxide derivative 234 in 21%, 33%, and 27% yield, respectively (Equation 50) <2000TL8349, 2003JA12114, 1998BCJ1181 >. 

When di(l-adamantyl)acetylene 285 was used in this reaction, a mixture of l,2-di(l-adamantyl)-2-thioxoethanone 286, 3,4-di(l-adamantyl)-l,2-dithietene 287, and 2,3-di(l-adamantyl)thiirene 1-oxide 288 was obtained in 4-33%, 8-27%, and 3-35% yields, respectively. Small amounts of starting 285 were recovered (3-35%) (Scheme 82). 

Replacement of disulfur dichloride by sulfur dichloride in reaction with acetylenes in dichloromethane led to formation of 2,3-dichlorothiiranes in quantitative yield <2001TL4017>. Acid hydrolysis of 2,3-dichlorothiirane (R = R = f-Bu) gave a mixture of 2,2,5,5-tetramethyl-4-oxo-3-hexanethione 289 and 2,3-di-f-butylthiirene-l-oxide 290 (Scheme 83). Exclusive formation of thiirene-A-oxides was observed during alkaline hydrolysis of 2,3-dichloro thiiranes. 

A detailed examination of the IR and Raman spectra of thiiranes and thiirenes has appeared in previous reviews <83HC(42/l)333,84CHEC-I(7)131>. The absorption frequencies observed for thiiranes and thiirenes tend to be characteristic for the three-membered ring and are not significantly shifted upon its fusion to carbocycles. However, the S—O bond stretch occurs at 1115 cm-1 for fused ring thiirene oxides but is observed at 1060 cm-1 in 2,3-diphenylthiirene-l-oxide. 

Interestingly, benzonitrile oxide does not react with thiirene dioxide 19b even in boiling benzene, whereas the electron-rich diene l-piperidino-2-methyl-l, 3-pentadiene (177) does react under the same reaction conditions to give the expected six-membered [4 + 2] cycloadduct 178, accompanied by sulfur dioxide extrusion and 1,3-hydrogen shift to form the conjugated system 179175 (equation 70). 

Extrusion (or elimination) of sulfur from thiiranes and thiirenes is a facile process. Virtually all thiiranes and thiirenes, as well as their V-oxides and V,V-dioxides, undergo thermal extrusion of the sulfur moiety with increasing facility according to the trend S << SO < S02. Sulfur monoxide produced in this way can be utilized in cycloaddition processes, for example, from 2 in refluxing toluene with 2,3-dimethyl-l,3-butadiene in a chelotropic addition <1997JOC8366>. 

Formation.—Experimental details are now available for the synthesis of diphenylthiiren 1,1-dioxide (32) from a,a -dibromodibenzyl sulphone by treatment with triethylamine. It is also formed by base-catalysed dehydrohalogenation of a,a-dichlorobenzyl benzyl sulphone, whilst similar treatment of a,a -dibromobenzyl sulphoxide (18) afforded diphenylthiiren 1-oxide. Alkyl-substituted thiiren 1,1-dioxides (33), (34), and (35), which could not be synthesized by this method, were obtained by dehydro-bromination of the appropriate bromothiiran 1,1-dioxides (36), (37), and (38), catalysed by triethylamine or l,5-diazabicyclo[4,3,0]non-5-ene. The bromothiiran 1,1-dioxides were synthesized from the appropriate... 

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