From 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 formation of alkenes from thiirane dioxides may not be stereospecifically controlled in the presence of a sufficiently strong base and sufficiently acidic protons in the three-membered ring. Under such conditions (essentially those typical for the Ramberg Backlund reaction), epimerization via a carbanion intermediate produces an equilibrium mixture of thiirane dioxides19,99 and consequently a mixture of cis- and trans-alkenes. 

Block and Aslam441 reported a novel variant of the Ramberg-Backlund reaction in which a,/J-unsaturated a -bromoalkyl sulfones 368 afforded 1,3-dienes upon treatment with base. Since a -bromoalkyl sulfones 368 can be obtained readily by the initial treatment of olefins with bromomethanesulfonyl bromide under photoirradiation and subsequent treatment with triethylamine, this reaction was utilized for preparation of dienes which contain one additional carbon over that of the corresponding olefin. Starting from the cyclic olefins 369-372 the 1,3-dienes 373-376 were obtained442. 

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. 

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. 

The epoxy-Ramberg-Backlund reaction (ERBR) has been used for the conversion of a,/3-epoxy sulfones into a range of mono-, di-, and tri-substituted allylic alcohols.34 Modification of this method has permitted the preparation of enantio-enriched allylic alcohols following the diastereoselective epoxidation of enantio-enriched vinyl sulfones that were accessed efficiently from the chiral pool. 

A novel tandem process has been reported " for the preparation of allylic amines, ethers, and sulfides from a-bromo-a, jS-unsaturated sulfones. The process is believed to proceed via an initial conjugate addition followed by proton exchange and Ramberg-Backlund rearrangement (see Scheme 75). A new variant of the Ramberg-Backlund reaction has been described in which a, /f-epoxy sulfones (319), on treatment with base, are converted into a range of mono-, di-, and tri-substituted allylic alcohols (320). 

Carotene has been prepared by treating the a,a -dianion generated from starting sul-phone with bromine or iodine followed by rearrangement (equation 25). Utilizing the Ramberg-Backlund reaction, an interesting cyclic enediyne has been made (equation 26). ... 

Cao, X., Yang, Y., Wang, X. A direct route to conjugated enediynes from dipropargylic sulfones by a modified one-flask Ramberg-Backlund reaction. J. Chem. Soc., Perkin Trans. 1 2002, 2485-2489. 

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. 

The Ramberg-Backlund reaction using hydroxide as base can also be performed under phase-transfer conditions, the rate of reaction varying markedly from one substrate to another (equations 2 and 3). One advantage of using these conditions is that ester groups are not hydrolyzed (equation 4 compare equation 46). 

The precise mechanisms by which thiirane 1,1-dioxides are converted stereospecifically into alkenes and SO2 (or products derived from SO2) are not clearIn typical Ramberg-Backlund reactions both thermal and base-catalyzed mechanisms are likely to be in operation, with the latter generally predominating. The importance of the base-catalyzed pathway accounts for the fact that thiirane 1,1-dioxides... 

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. 

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. ... 

Thiirane dioxide 4 as the reaction intermediate was unambiguously confirmed after it was isolated from the Ramberg-Backlund reaction. In 1989, the Taylor group at the University of York isolated thiirane dioxide 6 as the major product when they treated a-iodosulfone 5 with 1.2 equivalents of potassium r-butoxide at low temperature (-78 to 0 °C).11,12 Not surprisingly, thiirane dioxide 6 was readily converted to alkene 7 (in 88% yield) in the presence of potassium /-butoxide at warmer temperature (-20 °C to room temperature). 

Decarboxylative Ramberg-Backlund reaction is relatively rare. Sulfonyl ester 30 was subjected to the Myers s conditions to produce olefin 32, likely from intermediate 31. It was established that decarboxylation of 30 did not take place in the absence of... 

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