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 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.

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

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

Reactions
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 based on the functionalization of cyclohexene.

In the Reimer–Tiemann reaction dichlorocarbene reacts with phenol to salicylaldehyde.

History
Dichlorocarbene as a reactive intermediate was first proposed by Anton Geuther in 1862 who viewed chloroform as CCl2.HCl and again by Hine in 1950. Dichlorocarbene was first trapped by Doering in 1954.

Related reactions
The Doering-LaFlamme carbon chain extension 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.

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