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Dichlorocarbene

Dichlorocarbene is a carbene commonly encountered in organic chemistry. This reactive intermediate with chemical formula CCl₂ is easily available by reaction of chloroform and a base such as potassium t-butoxide ^[1] or sodium hydroxide dissolved in water. A phase transfer catalyst for instance benzyltriethylammonium bromide is added to facilitate the migration of the hydroxide in the organic phase. Dichlorocarbene reacts with alkenes in a formal [1+2]cycloaddition to form geminal dichlorocyclopropanes which can subsequently be reduced to proper cyclopropanes or hydrolyzed to a cyclopropanone in a gem halide hydrolysis. The preparation of dichlorocarbene from chloroform and its utility in synthesis was discovered by William von Eggers Doering in 1954 ^[2] based on functionalizion of cyclohexene. A Geuther in 1862 was the first to propose dichlorocarbene.

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Related reactions

The Doering-LaFlamme carbon chain extension ^[3]describes the conversion of alkenes to allenes (a chain extension) with magnesium or sodium metal through initial reaction of the alkene with dichlorocarbene. The same sequence is incorporated in the Skattebøl rearrangement to cyclopentadienes. Dichlorocarbene also features in the Reimer-Tiemann reaction. Closely related is the more reactive dibromocarbene CBr₂.

Synthetic methods

Other dichlorocarbene precursors are ethyl trichloracetate when reacted with sodium in methanol ^[4] and phenyl(trichloromethyl)mercury by thermal decomposition ^[5] . Dichlorodiazirine is an experimental dichlorocarbene precursor ^[6]. It is stable in the dark at room temperature and decomposes into the carbene and nitrogen gas by photolysis.

^[7]

The reaction mechanism for the formation of dichloroaziridine is depicted below:

Dichlorocarbene can also be obtained by reaction of carbon tetrachloride with elemental magnesium with ultrasound chemistry ^[8]. This method is tolerant to esters and carbonyl compounds because it does not involve strong base.

References

^ Organic Syntheses, Coll. Vol. 5, p.874 (1973); Vol. 41, p.76 (1961).Online Article
^ The Addition of Dichlorocarbene to Olefins W. von E. Doering and A. Kentaro Hoffmann J. Am. Chem. Soc.; 1954; 76(23) pp 6162 - 6165; doi:10.1021/ja01652a087
^ A two-step of synthesis of allenes from olefins Tetrahedron, Volume 2, Issues 1-2, 1958, Pages 75-79 W. von E. Doering and P. M. LaFlamme doi:10.1016/0040-4020(58)88025-4
^ Organic Syntheses, , Coll. Vol. 6, p.731 (1988); Vol. 54, p.11 (1974).Online Article
^ Organic Syntheses, , Coll. Vol. 5, p.969 (1973); Vol. 46, p.98 (1966).Online Article
^ Dichlorodiazirine: A Nitrogenous Precursor for Dichlorocarbene Gaosheng Chu, Robert A. Moss, and Ronald R. Sauers J. Am. Chem. Soc., 127 (41), 14206 -14207, 2005 DOI Abstract
^ a) Starting from phenol reaction with cyanogen bromide to phenyl cyanate b) hydroxylamine reaction to the N-hydroxy-O-phenylisourea c) elevate hydroxyl group to leaving group by reaction with mesyl chloride to the mesylate d) intramolecular ring closure with sodium hypochlorite to the aziridine f) nitration with nitronium tetrafluoroborate g) nucleophilic substitution see proposed mechanism with caesium chloride, tetrabutylammonium chloride in ionic liquid
^ A Facile Procedure for the Generation of Dichlorocarbene from the Reaction of Carbon Tetrachloride and Magnesium using Ultrasonic Irradiation Haixia Lin, Mingfa Yang, Peigang Huang and Weiguo Cao Molecules 2003, 8, 608-613 Online Article

Category: Reagents for organic chemistry

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