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Episulfones, Ramberg-Backlund reaction

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). [Pg.141]

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. [Pg.775]

The reaction of an a-halo sulfone with a base to give an olefin is called the Ramberg-Backlund reaction.309 The reaction is quite general for a-halo sulfones with an a hydrogen, despite the unreactivity of a-halo sulfones in normal Sn2 reactions (p. 344). Halogen reactivity is in the order I > Br > Cl. Phase transfer catalysis has been used.310 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... [Pg.1030]

Recently, intermediary episulfones have been isolated from low-temperature Ramberg-Backlund reactions of a-iodosulfones [414-416], opening the way to new perspectives for synthetic applications via these episulfones, although a-iodosulfones are not the most convenient a-halosulfones to prepare and handle. [Pg.73]

Unlike episulfides, the related episulfones (or thiirane S-dioxides) are usually very unstable. Interest in these compounds centered mainly on their proposed intermediacy in the Ramberg-Backlund reaction of a-halosulfones with strong bases to give alkenes - , viz. [Pg.715]

Sutherland, A. G., Taylor, R. J. K. The first Isolation of an episulfone Intermediate from a Ramberg-Backlund reaction. Tetrahedron Lett. 1989, 30, 3267-3270. [Pg.661]

The generally accepted mechanism of the Ramberg-Backlund reaction (Scheme 39) involves the deprotonation of the a-chlorosulfone by the base to give the sulfonyl carbanion (92). The latter undergoes an intramolecular nucleophilic attack on the a-carbon atom with elimination of the chloride anion and formation of the episulfone (93), which is unstable and extrudes sulfur dioxide to yield the alkene (Scheme 39).6 The Ramberg-Backlund reaction, unlike the Julia reaction, yields mainly the (Z)-alkene from acyclic starting materials. [Pg.201]

Alfylicalcohols. Epoxy sulfones undergo the Ramberg-Backlund reaction in which the epoxide acts as the leaving group. Decomposition of hydroxylated episulfones affords the allylic alcohols. [Pg.215]

If the fragmentation of the two atoms from the group, to which they were bonded, takes place in a concerted process resulting in the formation of a conjugated system, this is a cheletropic extrusion. Thus, the two possibilities can be found in the case of the loss of a SO2 group SO2 extrusion from an episulfone, as in the Ramberg-Backlund reaction, is an extrusion reaction, but SO2 extrusion from a cyclic sulfolene is a cheletropic reaction. ... [Pg.2]

The mechanism of the Ramberg-Backlund reaction is rather straightforward. When a-halosulfone 1 is treated with a strong base, deprotonation rapidly takes place to give a-anion 3, which undergoes a backside displacement (intramolecular nucleophilic substitution, SNi) to provide thiirane dioxide 4 (also known as episulfone) as the key intermediate.10 The Swi reaction with loss of halide is the rate-limiting step. Finally, the unstable 4 releases sulfur dioxide and the ring strain to deliver alkene 2. [Pg.387]

To date, there is no report on the isolation of an episulfone intermediate in the Ramberg-Backlund reaction. Taylor and Sutherland have recently succeeded in the isolation of an episulfone50. Thus, when iodo-sulfone 89 was treated with potassium t-butoxide, episulfone 90 was obtained as a white crystalline solid in 69% yield as illustrated in equation 48. Treatment of 90 with excess potassium t-butoxide at — 20 °C to room... [Pg.420]

When the sulfone has an a-halo substituent, formation of the carbanion is often accompanied by loss of the SO2 moiety via the so-called Ramberg-Backlund reaction.335 An illustration is the conversion of 1-bromo-ethyl ethyl sulfone (327) to 2-butene.336 Initial reaction with potassium hydroxide generated the sulfone carbanion (328), which displaced bromide intramolecularly to give an episulfone (329). Under the reaction conditions, 329 lost sulfur dioxide (SO2) to give the alkene. Meyers introduced a modification of this reaction in which the sulfone was converted directly to the alkene without isolating the a-halosulfone, using KOH in CCl4.337,338 Sulfuryl chloride and bromine can also be used.338... [Pg.632]

The 1,5-thiazocine 566 was oxidized to chlorosulfone 457 by treatment with A -chlorosuccinimide in CCI4 and ot-MCPBA. The Ramberg-Backlund rearrangement involved conversion of the a-chlorosulfone 567 into the azepine 569 under basic conditions through the formation of episulfone 568 followed by extrusion of S02. The reaction was conducted using different bases, such as KO/-Bu (66% yield), aqueous KOH (43% yield), and alumina-supported KOH (63% yield) (Scheme 113) <2000JOC8367>. [Pg.412]

Additions to Electron-Deficient Species. Diazomethane will also add to highly electrophilic species such as sulfenes or im-minium salts to give the corresponding three-membered ring heterocycles. When the reaction is performed on sulfenes, the products are episulfones which are intermediates in the Ramberg-Backlund rearrangement, and are therefore precursors for the synthesis of alkenes via chelotropic extrusion of S02- The sulfenes are typically prepared in situ by treatment of a sulfonyl chloride with a mild base, such as Triethylamine (eq 47). Similarly, the addition of diazomethane to imminium salts has been used to methyle-nate carbonyls. In this case, the intermediate aziridinium salt is treated with a strong base, such as n-Butyllithium, in order to induce elimination (eq 48). [Pg.122]

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. [Pg.649]


See other pages where Episulfones, Ramberg-Backlund reaction is mentioned: [Pg.176]    [Pg.622]    [Pg.650]    [Pg.650]    [Pg.152]    [Pg.176]    [Pg.860]    [Pg.176]    [Pg.622]    [Pg.860]    [Pg.109]    [Pg.89]    [Pg.622]    [Pg.376]    [Pg.622]    [Pg.2298]    [Pg.691]    [Pg.691]    [Pg.117]    [Pg.372]    [Pg.397]    [Pg.649]   
See also in sourсe #XX -- [ Pg.387 ]




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