A-Halosulfones

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). 

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

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 ... 

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. ... 

Bordwell and Cooper211 drew attention to the inertness of a-halosulfones and related compounds towards nucleophilic displacements of the halogen. Thus chloromethyl p-tolyl sulfone reacts with potassium iodide in acetone at less than one-fiftieth of the rate for n-butyl chloride. On the other hand, l-(p-toluenesulfonyl)-3-chloro-l-propene reacts about 14 times faster than allyl chloride. This contrast (and other comparisons) led the authors to attribute the inertness of a-halosulfones to steric hindrance, which was eliminated when the sulfonyl group was more remote from the reaction center. 

One of the most important reactions of sulfones in general is the R amberg 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)151. 

Bordwell and coworkers63 87 have studied the reaction of 9-fluorenyl carbanions (9-RFP) with a series of electron acceptors and in particular a-halosulfones and sulfoxides, in dimethyl sulfoxide solution. The overall reaction is characterized by the formation of the 9,9 -bis-fluorenyl derivative and the reduction of the halogenated acceptor. A family of 9-substituted fluorenyl carbanions covering a basicity range of 9.1 pKa units was used and... 

TABLE 8. Second-order rate constants for reactions of 9-R-fluorenyl carbanions with a-halosulfones and sulfoxides in Me2SO solution at 25 °c63a-87... 

Finally, we must mention the extensive work of Jarvis and colleagues on the kinetics of nucleophilic displacements on the halogen atoms of a-halosulfones . The reactions studied have often been of the type of equation 22. 

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. 

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. 

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