Dichlorocarbene

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Dichlorocarbene
Wireframe model of dichlorocarbene Ball and stick model of dichlorocarbene
Identifiers
CAS number 1605-72-7 YesY
PubChem 6432145 YesY
ChemSpider 4937404 YesY
MeSH Dichlorocarbene
ChEBI CHEBI:51370
Beilstein Reference 1616279
Gmelin Reference 200357
Jmol-3D images Image 1
Properties
Molecular formula CCl2
Molar mass 82.92 g mol−1
Hazards
Main hazards Highly reactive
Except where noted otherwise, 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[edit]

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[edit]

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
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[edit]

With alkenes[edit]

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 gem halide hydrolysis. Dichlorocyclopropanes may also be converted to allenes in the Skattebøl rearrangement.

Dichlorocarbene formation and reaction with cyclohexene

With phenols[edit]

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

The Reimer-Tiemann reaction

With amines[edit]

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

The carbylamine reaction with ethyl amine

History[edit]

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]

Related reactions[edit]

The Doering–LaFlamme carbon chain extension[11] 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. Dichlorocarbene also features in the Reimer–Tiemann reaction. Closely related is the more reactive dibromocarbene CBr2.

Chlorocarbene[edit]

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

External links[edit]

  • [3] English translation of 1969 Polish patent on preparation of dichloropropane derivatives

References[edit]

  1. ^ Organic Syntheses, Coll. Vol. 5, p.874 (1973); Vol. 41, p.76 (1961).[1]
  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).[2]
  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
  11. ^ 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