Dichlorocarbene

Dichlorocarbene
Names
Preferred IUPAC name
Dichlorocarbene
Systematic IUPAC name
Dichloromethylidene
Other names
Carbon(II) chloride

Carbon dichloride
Carbonous chloride
Dichloro-λ2-methane

Dichloromethylene
Identifiers
3D model (JSmol)
1616279
ChEBI
ChemSpider
200357
MeSH Dichlorocarbene
Properties
CCl2
Molar mass 82.91 g·mol−1
Hazards
Main hazards Highly reactive
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Dichlorocarbene is the reactive intermediate with chemical formula CCl2. Although this material has not been isolated, it is a common intermediate in organic chemistry, being generated from chloroform. This bent diamagnetic molecule rapidly inserts into other bonds.

Preparation

Dichlorocarbene is most commonly generated by reaction of chloroform and a base such as potassium t-butoxide or aqueous sodium hydroxide.[1] A phase transfer catalyst, for instance benzyltriethylammonium bromide, facilitates the migration of the hydroxide in the organic phase.

HCCl3 + NaOH → CCl2 + NaCl + H2O

Other reagents and routes

Another precursor to dichlorocarbene is ethyl trichloracetate. Upon treated with sodium methoxide, it releases CCl2.[2] Phenyl(trichloromethyl)mercury decomposes thermally to also release CCl2:[3]

PhHgCCl3 → CCl2 + PhHgCl

Dichlorodiazirine, which is stable in the dark, decomposes into dichlorocarbene and nitrogen via photolysis.[4]

Dichlorocarbene from dichlorodiazirine [5]

Dichlorocarbene can also be obtained by dechlorination of carbon tetrachloride with magnesium with ultrasound chemistry.[6] This method is tolerant to esters and carbonyl compounds because it does not involve strong base.

Reactions

With alkenes

Dichlorocarbene reacts with alkenes in a formal [1+2]cycloaddition to form geminal dichlorocyclopropanes. These can be reduced to cyclopropanes or hydrolysed to give cyclopropanones by a geminal halide hydrolysis. Dichlorocyclopropanes may also be converted to allenes in the Skattebøl rearrangement.

With phenols

In the Reimer–Tiemann reaction dichlorocarbene reacts with phenols to give the ortho-formylated product.[7] e.g. phenol to salicylaldehyde.

With amines

In the carbylamine reaction dichlorocarbenes can react with primary amines to give isocyanides. For example, the reaction with ethylamine:

History

Dichlorocarbene as a reactive intermediate was first proposed by Anton Geuther in 1862 who viewed chloroform as CCl2.HCl[8] Its generation was reinvestigated by Hine in 1950.[9] The preparation of dichlorocarbene from chloroform and its utility in synthesis was reported by William von Eggers Doering in 1954.[10]

The Doering–LaFlamme allene synthesis entails 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.

Closely related is the more reactive dibromocarbene CBr2.

Chlorocarbene

The related chlorocarbene (ClHC) can be generated from methyllithium and dichloromethane. It has been used in the synthesis of spiropentadiene.

References

  1. ↑ Organic Syntheses, Coll. Vol. 5, p.874 (1973); Vol. 41, p.76 (1961).
  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).
  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:10.1021/ja055656c
  5. ↑ 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 diazirine e) nitration with nitronium tetrafluoroborate f) nucleophilic substitution with caesium chloride, tetrabutylammonium chloride in ionic liquid
  6. ↑ 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
  7. ↑ Wynberg, Hans (1960). "The Reimer-Tiemann Reaction". Chemical Reviews. 60 (2): 169–184. doi:10.1021/cr60204a003. Retrieved 3 January 2014.
  8. ↑ Ueber die Zersetzung des Chloroforms durch alkoholische Kalilösung Annalen der Chemie und Pharmacie Volume 123, Issue 1, Date: 1862, Pages: 121-122 A. Geuther doi:10.1002/jlac.18621230109
  9. ↑ Carbon Dichloride as an Intermediate in the Basic Hydrolysis of Chloroform. A Mechanism for Substitution Reactions at a Saturated Carbon Atom Jack Hine J. Am. Chem. Soc., 1950, 72 (6), pp 2438–2445 doi:10.1021/ja01162a024
  10. ↑ 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
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