Sulfur dioxide 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 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. 

Exercise 21-24 a. Sulfur dioxide is an angular molecule that can be represented as having a nonbonding electron pair in an sp2 hybrid orbital and one vacant p orbital on sulfur. Use this formulation to derive a thermally allowed transition state for the reversible 1,4-cycloaddition of S02 to 1,3-butadiene (Section 13-3C). b. The three-membered ring sulfone, shown below, is very unstable and rapidly dissociates to S02 and ethene. This process is used for the synthesis of alkenes by the dissociation of cyclic sulfones (Ramberg-Backlund reaction). Determine whether the transition state for the thermally favorable reaction is conrotatory or disrotatory. 

Treatment of o ,/ -epoxysulfones with bases, such as KO-/-Bu, lithium diisopropylamide (LDA), LiHMDS, or LiO-/-Bu in THE, led to the epoxy-Ramberg-Backlund reaction and gave allyl alcohols via thiirane 1,1-dioxides (HMDS = hexamethyldisilazide). The latter lose sulfur dioxide under the reaction conditions (Scheme 22) <1997TL3055>. 

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 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. 

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... 

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) ... 

Thiirane A-oxides are effective precursors for sulfur monoxide (SO) under thermal conditions <1997JOC8366>. The sulfur monoxide has been trapped by a variety of dienes to form cyclic sulfoxides. Extmsion of SO2 from thiirane A,A-dioxides to form alkenes under thermal conditions is well known. The reaction takes place readily at room temperature in the presence of KO/-Bu in tetrahydrofuran (THF). It constitutes the final step of the Ramberg-Backlund rearrangement <20030R357, 1999CC217>. 

Internal alkylation takes place on reaction of a-haloalkyl alkyl sulfones with bases and produces epi-sulfones as intermediaries. These intermediates are usually unstable and provide alkenes by thermal extrusion of sulfur dioxide. This reaction, known as the Ramberg-Backlund rearrangement, has been widely used in synthesis and has been thoroughly reviewed (Scheme 107, entry e).36-3. 43<.473... 

A very different sulfur dioxide extrusion protocol has been applied to the total synthesis of (+)-estradiol (30) by Rigby and coworkers. In the event, the trimethylsilyl-substituted chromium thiepin complex 27 participated in a [6 r+4 r] cycloaddition reaction to provide a 70% yield of the structurally complex dihy-drothepin 28. Subsequent sulfur dioxide extrusion via a Ramberg-Backlund-type ring contraction gave tetracycle 29. The total synthesis of (+)-estradiol (30) was completed using a four-step sequence of standard reactions (eq 12). ... 

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