Ramberg-Backlund reaction mechanism

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 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... 

The Ramberg-Backlund reaction involves a thiirane-1,1-dioxide intermediate 58 as shown in Scheme 8. J ng etal. determined the mechanisms of reaction of thiirane-1,1-dioxide 59 with hydroxide ion in water <1996PAC825>. Two pathways were identified, the one first order and the other second order in hydroxide. The first step is formation of a trigonal bipyramidal mono-anion 60 formed by attack of OH at S of the sulfone. That anion then reacts with water to afford ethane sulfonate anion 61 or with a second OH to afford ethene and sulfite anion via 62 or 63 (Scheme 9). Some rate constants and equilibrium constants were determined. 

It is now widely accepted that a-halo sulfones are converted into alkenes under the basic conditions of the Ramberg-Backlund reaction via intermediate thiirane 1,1-dioxides (Scheme 7). Experimental data in support of this mechanism were first put forward by Bordwell in 1951, and extensive studies carried out since that time, particularly by Bordwell and Paquette in the late 1960s and early 1970s, have substantiated this overall scheme and have provided further insight into the mechanisms of the individual steps. Very little additional work concerning the mechanism of the reaction has been published since 1975. Consequently, only a summary of the essential features is presented here, and readers are referred to the detailed reviews, written by the leaders of the key research groups themselves, for the full data which support the following broad outline. ... 

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. 

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. 

CHAPTER 8 THE RAMBERG-BACKLUND REACTION Scheme 8.1 Ramherg-Racklund reaction mechanism. 

Ramberg-Backlund reaction and the stereochemistry of the Wittig reaction have been highlighted. Mechanisms of decomposition of a-chloromethyl methyl sulfone, uncatalysed and catalysed by hydroxide ion, have been studied by using the DFT at the B3LYP/6-311+G(d,p) level the uncatalysed reaction proceeds in two steps, and catalysis by hydroxide ion involves three steps. 

Treatment of (a -bromobenzyl)benzyldiphenylphosphonium salts (364) with amine bases has been shown to afford alkenes (366) with Z-selectivity. The reaction is believed to proceed via an e/it-phosphonium species (365) by a mechanism similar to that of the Ramberg-Backlund transformation. 

Reactions of (145) with potassium permanganate, performic acid, aqueous lithium carbonate, and silver acetate in acetic acid are described. Treatment of the cyclopentadiene-methyl a-bromovinyl sulphone adduct with sodium methoxide in dimethyl sulphoxide gives (141) by the Ramberg-Backlund rearrangement, but in refluxing aqueous 2N sodium hydroxide (146) is also formed. Good evidence in the latter case is presented for a homolytic mechanism. With added isopropyl alcohol, in addition to (146), products are obtained by hydrogen-atom abstraction. 

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