Dichlorocarbene

Not to be confused with Methylene dichloride.
"Carbon dichloride" redirects here. For the chemical compound with empirical formula CCl2, see Tetrachloroethylene.
Dichlorocarbene
Wireframe model of dichlorocarbene
Wireframe model of dichlorocarbene
Ball and stick model of dichlorocarbene
Ball and stick model of dichlorocarbene
Names
Preferred 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
UNII
  • InChI=1/CCl2/c2-1-3
    Key: PFBUKDPBVNJDEW-UHFFFAOYAT
  • [C](Cl)Cl
Properties
CCl2
Molar mass 82.91 g·mol−1
Hazards
Occupational safety and health (OHS/OSH):
Main hazards
 Highly reactive
Related compounds
Related compounds
 C2Cl4
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

Dichlorocarbene is the reactive intermediate with chemical formula CCl2. Although this chemical species 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 tert-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 trichloroacetate. Upon treatment with sodium methoxide it releases CCl2.[2]

Phenyl(trichloromethyl)mercury decomposes thermally to release CCl2.[3]

PhHgCCl3 → CCl2 + PhHgCl

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

Dichlorocarbene from dichlorodiazirine: phenol and (a) to phenyl cyanate; phenyl cyanate and (b) to N-hydroxy-O-phenylisourea; N-hydroxy-O-phenylisourea and (c) to N-mesylate-O-phenylisourea; N-mesylate-O-phenylisourea and (d) intramolecular ring-closure to the diazirine; aryl ring nitration; nucleophilic substitution to dichlorodiazirine; and then decomposition
(a) Cyanogen bromide (b) hydroxylamine (c) mesyl chloride (d) sodium hypochlorite (e) nitronium tetrafluoroborate (f) caesium and tetrabutylammonium chlorides in an ionic liquid

Dichlorocarbene can also be obtained by dechlorination of carbon tetrachloride with magnesium with ultrasound chemistry.[5] 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.

Dichlorocarbene formation and reaction with cyclohexene

With phenols

In the Reimer–Tiemann reaction dichlorocarbene reacts with phenols to give the ortho-formylated product.[6] e.g. phenol to salicylaldehyde.

The Reimer-Tiemann reaction

With amines

Dichlorocarbene is an intermediate in the carbylamine reaction. In this conversion, a dichloromethane solution of a primary amine is treated with chloroform and aqueous sodium hydroxide in the presence of catalytic amount of the phase-transfer catalyst. Illustrative is the synthesis of tert-butyl isocyanide:[7]

Me3CNH2 + CHCl3 + 3 NaOH → Me3CNC + 3 NaCl + 3 H2O

History

In 1835, the French chemist Auguste Laurent recognised chloroform as CCl2•HCl (then written as C8Cl8•H4Cl4)[a] in his paper on analysing some organohalides. Laurent also predicted a compound seemingly consisting of 2 parts dichlorocarbene which he named Chlorétherose (possibly Tetrachloroethylene, which was not known to exist at the time.)[8]

Dichlorocarbene as a reactive intermediate was first proposed by Anton Geuther in 1862 who viewed chloroform as CCl2.HCl[9] Its generation was reinvestigated by Hine in 1950.[10] The preparation of dichlorocarbene from chloroform and its utility in synthesis was reported by William von Eggers Doering in 1954.[11]

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.

See also

Explanatory notes

  1. ^ It was common for French chemists of 19th century to write molecular weights twice, seemingly Laurent also counted 2 molecules of chloroform. Combined with the inaccurate molecular weight of carbon in the early 19th century (considered half of what it really is), these resulted in a count of 8 carbons for 2 molecules of chloroform.

References

  1. ^ "2-Oxa-7,7-dichloronorcarane". Organic Syntheses. 41: 76. 1961. doi:10.15227/orgsyn.041.0076.
  2. ^ "1,6-Methano[10]annulene". Organic Syntheses. 54: 11. 1974. doi:10.15227/orgsyn.054.0011.
  3. ^ "Phenyl(trichloromethyl)mercury". Organic Syntheses. 46: 98. 1966. doi:10.15227/orgsyn.046.0098.
  4. ^ Gaosheng Chu; Robert A. Moss; Ronald R. Sauers (2005). "Dichlorodiazirine: A Nitrogenous Precursor for Dichlorocarbene". J. Am. Chem. Soc. 127 (41): 14206–14207. doi:10.1021/ja055656c. PMID 16218614.
  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
  6. ^ Wynberg, Hans (1960). "The Reimer-Tiemann Reaction". Chemical Reviews. 60 (2): 169–184. doi:10.1021/cr60204a003.
  7. ^ Gokel, G.W.; Widera, R.P.; Weber, W.P. (1988). "Phase-transfer Hofmann carbylamine reaction: tert-butyl isocyanide". Organic Syntheses. 55: 232. doi:10.15227/orgsyn.055.0096.
  8. ^ Auguste Laurent, Note sur les Chlorure, Bromure et Iodure d'Aldehydène (1835), Annales de Chimie et de Physique, p. 327
  9. ^ 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
  10. ^ 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
  11. ^ 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