Dichlorocarbene

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Dichlorocarbene formation and reaction with cyclohexene
Dichlorocarbene formation and reaction with cyclohexene

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, hydrolyzed to a cyclopropanone in a gem halide hydrolysis or converted to another carbene in the Skattebøl rearrangement.

Dichlorocarbene also features in the Reimer-Tiemann reaction.

Closely related is the more reactive dibromocarbene CBr2.

[edit] Synthetic methods

Other dichlorocarbene precursors are ethyl trichloracetate when reacted with sodium in methanol [2] and phenyl(trichloromethyl)mercury by thermal decomposition [3]. Dichlorodiazirine is an experimental dichlorocarbene precursor [4]. It is stable in the dark at room temperature and decomposes into the carbene and nitrogen gas by photolysis.

Dichlorodiazirine synthesis: 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
Dichlorodiazirine synthesis: 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

Dichlorocarbene can also be obtained by reaction of carbon tetrachloride with elemental magnesium with ultrasound chemistry [5]. This method is tolerant to esters and carbonyl compounds because it does not involve strong base.

Dichlorodiazirine: formation reaction mechanism
Dichlorodiazirine: formation reaction mechanism


[edit] External links

[edit] References

  1. ↑  Organic Syntheses, Coll. Vol. 5, p.874 (1973); Vol. 41, p.76 (1961).Online Article
  2. ↑  Organic Syntheses, , Coll. Vol. 6, p.731 (1988); Vol. 54, p.11 (1974).Online Article
  3. ↑  Organic Syntheses, , Coll. Vol. 5, p.969 (1973); Vol. 46, p.98 (1966).Online Article
  4. ↑  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
  5. ↑  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
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