Episulfones

Thiirane 1,1-dioxides extrude sulfur dioxide readily (70S393) at temperatures usually in the range 50-100 °C, although some, such as c/s-2,3-diphenylthiirane 1,1-dioxide or 2-p-nitrophenylthiirane 1,1-dioxide, lose sulfur dioxide at room temperature. The extrusion is usually stereospeciflc (Scheme 10) and a concerted, non-linear chelotropic expulsion of sulfur dioxide or a singlet diradical mechanism in which loss of sulfur dioxide occurs faster than bond rotation may be involved. The latter mechanism is likely for episulfones with substituents which can stabilize the intermediate diradical. The Ramberg-Backlund reaction (B-77MI50600) in which a-halosulfones are converted to alkenes in the presence of base, involves formation of an episulfone from which sulfur dioxide is removed either thermally or by base (Scheme 11). A similar conversion of a,a -dihalosulfones to alkenes is effected by triphenylphosphine. Thermolysis of a-thiolactone (5) results in loss of carbon monoxide rather than sulfur (Scheme 12). 

Recent developments in Ramberg-Backlund rearrangement and episulfone chemistry 99CC217. 

An a-halosulfone 1 reacts with a base by deprotonation at the a -position to give a carbanionic species 3. An intramolecular nucleophilic substitution reaction, with the halogen substituent taking the part of the leaving group, then leads to formation of an intermediate episulfone 4 and the halide anion. This mechanism is supported by the fact that the episulfone 4 could be isolated. Subsequent extrusion of sulfur dioxide from 4 yields the alkene 2 ... 

Other interesting three-component cycloadditions are the following Sulfur dioxide and diazo compounds lead to episulfones (equation 75)436—in a special case to 4,5-dihydrothiepine S,S-dioxides437 sulfur dioxide, ketene, and arylimine lead to thiazole derivatives438 (equation 76) sulfur dioxide, quinone, and alkenes lead to benzoxathiane derivatives439 (equation 77). 

Remarkable solvent effects on the selective bond cleavage are observed in the reductive elimination of cis-stilbene episulfone by complex metal hydrides. When diethyl ether or [bis(2-methoxyethyl)]ether is used as the solvent, dibenzyl sulfone is formed along with cis-stilbene. However, no dibenzyl sulfone is produced when cis-stilbene episulfone is treated with lithium aluminum hydride in tetrahydrofuran at room temperature (equation 42). Elimination of phenylsulfonyl group by tri-n-butyltin hydride proceeds by a radical chain mechanism (equations 43 and 44). 

The Michael induced Ramberg-Backlund reactions gives a mixture of ( )- and (Zy dienes via extrusion of S02 from the episulfone intermediate 57 (equation 55)47. 

Alkanesulfonyl chlorides, when treated with diazomethane in the presence of a base (usually a tertiary amine), give episulfones (67). The base removes HCl from the sulfonyl halide to produce the highly reactive sulfene (66) (17-13), which then adds CH2. The episulfone can then be heated to give off SO2 (17-20), making the... 

The reaction of an a-halo sulfone with a base to give an alkene is called the Ramberg-Bdcklund reaction. The reaction is quite general for a-halo sulfones with an (x hydrogen, despite the unreactive nature of a-halo sulfones in normal 8 2 reactions (p. 437). Halogen reactivity is in the order I>Br>Cl. Phase-transfer catalysis has been used. In general, mixtures of cis and trans isomers are obtained, but usually the less stable cis isomer predominates. The mechanism involves formation of an episulfone, and then elimination of SO9. There is much evidence for... 

Dihydrothiophene-1,1-dioxides (42) and 2,17-dihydrothiepin-1,1-dioxides (43) undergo analogous 1,4 and 1,6 eliminations, respectively (see also 17-38). These are concerted reactions and, as predicted by the orbital-symmetry rules (p. 1067), the former is a suprafacial process and the latter an antarafacial process. The rules also predict that elimination of SO2 from episulfones cannot take place by a concerted mechanism (except antarafacially, which is unlikely for such a small ring), and the evidence shows that this reaction occurs by a non-concerted pathway.The eliminations of SO2 from 42 and 43 are examples of cheletropic reactions, which are defined as reactions in which two a bonds that terminate at a single atom (in this case the sulfur atom) are made or broken in concert. 

Treatment of a-halosulfones 357 bearing a -hydrogens with base gives olefins by extruding SO2 from the episulfone intermediates 359. The reaction was found by Ramberg and Backlund in 1940 and named after them, and it has been very useful for the preparation of tailor-made olefins. The reaction has been investigated in detail " and utilized widely for olefin syntheses. An excellent review on this reaction by Paquett covers the literature up to 1975, so only recent studies are dealt with in this section. The generally accepted mechanistic path is shown below. 

Finally, the most significant mechanistic feature of the Ramberg-Backlund rearrangement is the stereoselective formation of ds-olefin products, as a result of the preferential cis-positioning of the pair of R groups in the episulfone-forming transition state, variously attributed to London forces , to diastereoselectivity in carbanion formation and to steric attraction . However, with the use of stronger bases such as potassium t-butoxide °, the trans-olefin predominates (equation 52), apparently due to prior epimerization of the kinetically favoured cts-episulfone, and subsequent loss of the sulfur dioxide. Similarly, when the episulfone intermediates possess unusually acidic... 

Alkylidene sulfenes (75), generally prepared by the dehydrohalogenation of alkylsulfonyl chlorides, add readily to electron-rich multiple bonds. For example, with enamines, the thietane dioxide (e.g., 76) is formed diazoalkanes yield thiirane dioxides (episulfones) and imines (Schiff bases) afford 1,2-thiazetidine 1,1-dioxides. There are available numerous reviews of sulfenes, including cycloaddition reactions.102... 

The checkers observed that the episulfone decomposed slowly at room temperature and, on one occasion, during evaporation at 15°. 

The crude episulfone (3.0 g.) is placed in a 10-ml. round-bottomed flask fitted with a reflux condenser and is heated at 95° for 30 minutes, when it decomposes with loss of sulfur dioxide. The reflux condenser is replaced with a distillation head (Note 6), and the yellow residue is distilled under reduced pressure (water aspirator). d -2- Oxo-7,7-dimethyl -1 -vinylbicyclo [2.2.1] heptane... 

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