Thietane 1,1-dioxides thermolysis

The photolysis of various substituted thiete dioxides under similar conditions resulted in the formation of the unsaturated ketones (255)264, most probably via a vinyl sulfene intermediate followed by a loss of sulfur monoxide as shown in equation 96. The same results were obtained in the thermolysis of 6e (R1 = R3 = Ph R2 = R4 = H)231, which further demonstrates that similar mechanisms are operative in thermolyses and pho-tolyses of thietane dioxides and thiete dioxides. 

The first diradical intermediate is the same as that involved in the photolysis and thermolysis of thietane l,l-dioxides ° and interconversion of the y-sultine and thietane dioxide often accompanies cyclopropane formation. In flash-vacuum pyrolysis, formation of the alkene may compete with the desired cyclopropane-forming reaction. Examples of this reaction are summarized in Table 11. 

Thermolysis and Photolysis of Thietan Dioxides. These reactions frequently lead non-stereospecifically to cyclopropanes and olefins, the relative yields of which vary markedly with the constitution of the thietan... 

A few special syntheses of thiirane derivatives from four-membered sulfur heterocycles are known. Methylenethiiranes are derived by thermolysis of the tosylhydrazones of thietan-3-ones (Scheme 143) (81TL4815), and 2,2,3,3-tetrakis(trifluoromethyl)thiirane is obtained by thermolysis of 2,2,4,4-tetrakis(trifluoromethyl)-l,3-dithietane 1,1-dioxide with loss of sulfur dioxide. 2,2,3,3-Tetrakis(trifluoromethyl)thiirane 1,1-dioxide is obtained in low yield by heating a mixture of 3,3-bis(trifluoromethyl)-l,2,4-oxadithietane 2,2,4,4-tetroxide and bis(trifluoromethyl)ketene. The mono-episulfide of norbornadiene is obtained from thietane derivative (65 Scheme 144) (73JOC649). 

Thietanes can also be prepared by ring contraction of higher-membered heterocyclic rings. Thermolysis of the dithiolane dioxide 129 in benzene leads to trans-2-phenyl-3-benzoylthietane (130) and benzalacetophenone (Eq. 20). 

Thermolysis of thiete 1,1-dioxide (116), either in the vapor phase or in benzene solution, gives the oxathiole (117) in over 80% yield. The most likely mechanism involves a sulfene intermediate as shown in Scheme 38 (70CJC3704). Not surprisingly, the flash thermolysis of thietane 1,1-dioxide, the saturated analog of (116), gives only cyclopropane and propene. 

Thermolysis of 4-benzoyl-3-phenyl-l,2-dithiolane 2,2-dioxide at 230° in a sealed tube gives 3-benzoyl-2-phenylthietane in 55% yield. ° Photolysis of 1,2-dithiolanes gives low yields of thietanes. ... 

In addition to the unsubstituted sulfene itself, the various substituted sulfenes used in the preparation of thietane 1,1-dioxides are listed in Table 4. Although the usual method for generating the sulfenes is by treatment of a methanesulfonyl chloride with triethylamine, sulfenes have been obtained by the reaction of phenyl-methanesulfonyl fluoride with phenyllithium, by treatment of a-chloro-ethanesulfinic acid with refluxing triethylamine,by treatment of 4-nitro-phenyl or 2-chloro-4-nitrophenyl, esters of arylmethanesulfonic acids with potassium t-butoxide or 2,6-dimethylpyridine, respectively, by treatment of diazoalkanes with sulfur dioxide" (e.g., to give 146) and by thermolysis of a Diels-Alder adduct of sulfene." ... 

Gas-phase thermolysis of thietane S-oxide at 500-1130°C and lO Torr provides one method for generation of 1,2-oxathiolane (5) <81JCS(P2)1I66>. As described in the first edition, thermal ring expansion of thiete 1,1-dioxide provides the most convenient access to 1,2-oxathiole 2-oxide (7) and this reaction has also been used to obtain the 3-phenyl- <72TL4781>, 3,5-diphenyl- <77JOC3506>, and... 

Reactions.—Thermolysis of thietan 1,1-dioxide yields mainly cyclopropane and sulphur dioxide, with some propene. Flash vacuum thermolysis of (49) gives... 

Oxathietans, Oxathiets, and Dioxathietans.—Oxathietan intermediates have been considered in the reaction of thiocarbonyl ylides with diphenylketen and in the vapour-phase thermolysis of S-methoxymethyl thioacetates. The behaviour of solutions of monothiobenzil may indicate isomerization to an oxathiet. The photo-oxidation of tetramethyl-3-thiocyclobutane-l,3-dione by oxygen may proceed via a l,2-dioxa-3-thietan. The l,3-dioxa-2-thietan 2,2-dioxide (210) was obtained by treatment of 3,3,3-trifluoro-2-tri-fluoromethylpropanoic acid with sulphur trioxide. It is cleaved by various nucleophiles to bis-trifluoromethylketen, which dimerizes, and its reactions with fluoroethylenes have been determined. Thermolysis of (210) yields anhydride (211). ... 

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