A-Halosulfone, 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). 

The a -halosulfone, required for the Ramberg-Backlund reaction, can for example be prepared from a sulfide by reaction with thionyl chloride (or with N-chlorosuccinimide) to give an a-chlorosulfide, followed by oxidation to the sulfone—e.g. using m-chloroperbenzoic acid. As base for the Ramberg-Backlund reaction have been used alkoxides—e.g. potassium t-butoxide in an etheral solvent, as well as aqueous alkali hydroxide. In the latter case the use of a phase-transfer catalyst may be of advantage. ... 

The Ramberg-Backlund Reaction. a-Halosulfones undergo a related rearrangement known as the Ramberg-Backlund reaction.91 The carbanion formed by deprotonation gives an unstable thiirane dioxide that decomposes with elimination of sulfur dioxide. This elimination step is considered to be a concerted cycloelimination. 

Treatment of an a-halosulfone 1 with base leads to extrnsion of sulfur dioxide and formation of an alkene 2. This reaction is referred to as the Ramberg-Backlund reaction, it usnaUy yields a mixture of E- and Z-isomers of the alkene. 

Sulfones have been shown to be versatile intermediates for the introduction of carbon-carbon double bonds [410]. One of the best known routes to do so is the Ramberg-Backlund reaction of an a-halosulfone with an appropriate base. 

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. 

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. 

Halosulfones are important intermediates in the Ramberg-Backlund reaction (1940) which converts dialkyl sulfones into alkenes (87) with extrusion of sulfur dioxide. 4 The reaction requires a leaving group (X) at the a-position and an a -hydrogen atom it is generally carried out with a-chloro sulfones (91), although other halogens or sulfonates are effective (Scheme 38)... 

The Ramberg-Backlund reaction is the base-mediated conversion of a-halosulfone 1 to olefin 1 with extrusion of S02.1-8 It is also often referred to as the Ramberg-Backlund rearrangement. The thermodynamically stable. E-olefin is often obtained when a weak base (e.g., NaH) is used whereas a strong base often results in the Z-olefin. 

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. 

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... 

Review articles have featured advances in the cycloisomerization of l,n-allenynes and Rn-allenenes, " reactions for synthesis of quaternary centres bearing a nitrogen substituent, " the Ramberg-Backlund reaction transforming a-halosulfones into alkenes, " and ring expansion of l,2-benzisoxazol-3-ones with a Vilsmeier reagent system, POCI3 and dimethylformamide (Scheme 75). " ... 

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. 

One of the most important reactions of sulfones in general is the Ramberg-Backlund rearrangement, which involves the conversion of a-halosulfones to olefins with accompanying loss of hydrogen halide and sulfur dioxide under basic conditions (equation 48) ... 

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