The Ramberg–Bäcklund reaction is an organic reaction converting an α-halo sulfone into an alkene in presence of a base with extrusion of

sulfur dioxide Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a colorless gas with a pungent smell that is responsible for the odor of burnt matches. It is r ...

. The reaction is named after the two Swedish chemists Ludwig Ramberg and Birger Bäcklund. The carbanion formed by deprotonation gives an unstable episulfone that decomposes with elimination of

. This elimination step is considered to be a concerted cheletropic extrusion. The overall transformation is the conversion of the carbon–sulfur bonds to a carbon–carbon double bond. The original procedure involved halogenation of a

sulfide Sulfide (also sulphide in British English) is an inorganic anion of sulfur with the chemical formula S2− or a compound containing one or more S2− ions. Solutions of sulfide salts are corrosive. ''Sulfide'' also refers to large families o ...

, followed by

oxidation Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is ...

to the sulfone. Recently, the preferred method has reversed the order of the steps. After the

, which is normally done with a peroxy acid, halogenation is done under basic conditions by use of dibromodifluoromethane for the halogen transfer step. This method was used to synthesize 1,8-diphenyl-1,3,5,7-octatetraene.

Applications

The Ramberg–Bäcklund reaction has several applications. Due to the nature of elimination, it can be applied to both small rings , and large rings containing a double bond . This reaction type gives access to 1,2-dimethylenecyclohexane and the epoxide variation access to allyl alcohols. A recently developed application of the Ramberg–Bäcklund reaction is the synthesis of ''C''-glycosides. The required thioethers can be prepared easily by exchange with a thiol. The application of the Ramberg–Bäcklund conditions then leads to an exocyclic vinyl ether that can be reduced to the C-nucleoside .In a variation,

of a sulfamide generates a azo compound.

Substrates

The necessary α-halo sulfones are accessible through oxidation of the corresponding α-halo sulfides with peracids such as ''meta''-chloroperbenzoic acid; oxidation of sulfides takes place selectively in the presence of alkenes and alcohols. α-Halo sulfides may in turn be synthesized through the treatment of sulfides with halogen electrophiles such as ''N''-chlorosuccinimide or ''N''-bromosuccinimide.

Mechanism

The sulfone group contains an acidic

proton A proton is a stable subatomic particle, symbol , Hydron (chemistry), H+, or 1H+ with a positive electric charge of +1 ''e'' (elementary charge). Its mass is slightly less than the mass of a neutron and approximately times the mass of an e ...

in one of the α-positions which is abstracted by a strong base (''scheme 1''). The negative charge placed on this position (formally a carbanion) is transferred to the halogen residing on the other α-position in a nucleophilic displacement temporarily forming a three-membered cyclic sulfone. This intermediate is unstable and releases sulfur dioxide to form the alkene. Mixtures of cis isomer and trans isomer are usually obtained. The Favorskii rearrangement and the Eschenmoser sulfide contraction are conceptually related reactions.

References

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# # # # # # # {{Alkenes Elimination reactions Olefination reactions Carbon-carbon bond forming reactions Name reactions