Ramberg-Bäcklund reaction

The Ramberg-Bäcklund Reaction is an organic reaction converting a α-halo sulfone into an alkene in presence of a base with extrusion of sulfur dioxide ^[1].

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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.

This reaction type gives access to 1,2-dimethylenecyclohexane (scheme 2) ^[2]:

and the epoxide variation (scheme 3) ^[3] access to allyl alcohols.

The Favorskii rearrangement is a conceptually related reaction.

References

^ L. Ramberg, B. Bäcklund Ark. Chim., Mineral Geol., 1940, 27 Vol 13A, 1- 50
^ Cyclohexane, 1,2-bis(methylene)- Eric Block and Mohammad Aslam Organic Syntheses, Coll. Vol. 8, p.212 (1993); Vol. 65, p.90 (1987) Article
^ The Epoxy-Ramberg-Bäcklund Reaction (ERBR): A Sulfone-Based Method for the Synthesis of Allylic Alcohols European Journal of Organic Chemistry Volume 2006, Issue 7, Date: April 2006, Pages: 1740-1754 Paul Evans, Paul Johnson, Richard J. K. Taylor Abstract