RambergâBĂ€cklund reaction | |
---|---|
Named after |
Ludwig Ramberg Birger BĂ€cklund |
Reaction type | Rearrangement reaction |
Identifiers | |
Organic Chemistry Portal | ramberg-baecklund-reaction |
RSC ontology ID | RXNO:0000094 |
The RambergâBĂ€cklund reaction is an organic reaction converting an α-halo sulfone into an alkene in presence of a base with extrusion of sulfur dioxide. [1] The reaction is named after the two Swedish chemists Ludwig Ramberg and Birger BĂ€cklund. The carbanion formed by deprotonation gives an unstable episulfone that decomposes with elimination of sulfur dioxide. This elimination step is considered to be a concerted cheletropic extrusion.[ citation needed]
The overall transformation is the conversion of the carbonâsulfur bonds to a carbonâcarbon double bond. The original procedure involved halogenation of a sulfide, followed by oxidation to the sulfone. Recently, the preferred method has reversed the order of the steps. After the oxidation, which is normally done with a peroxy acid, halogenation is done under basic conditions by use of dibromodifluoromethane for the halogen transfer step. [2] This method was used to synthesize 1,8-diphenyl-1,3,5,7-octatetraene.
The RambergâBĂ€cklund reaction has several applications. Due to the nature of elimination, it can be applied to both small rings [3],
and large rings containing a double bond [4].
This reaction type gives access to 1,2-dimethylenecyclohexane [5]
and the epoxide variation [6] access to allyl alcohols.
A recently developed application of the RambergâBĂ€cklund reaction is the synthesis of C-glycosides. The required thioethers can be prepared easily by exchange with a thiol. The application of the RambergâBĂ€cklund conditions then leads to an exocyclic vinyl ether that can be reduced to the C-nucleoside [7].
In a variation, oxidation of a sulfamide generates a azo compound. [1]
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. [8]
The sulfone group contains an acidic proton 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. [9]
The Favorskii rearrangement and the Eschenmoser sulfide contraction are conceptually related reactions.