Organic Syntheses, Coll. Vol. 6, p.424 (1988); Vol. 58, p.64 (1978).
has been identified as a carcinogen. OSHA has issued emergency standards on its use. All procedures involving benzene
should be carried out in a well-ventilated hood
, and glove protection is required. A hood should be employed for the chlorination.
A 1-l., three-necked flask
is charged with 95 g. (0.36 mole) of triphenylphosphine (Note 1)
and 500 ml. of anhydrous benzene
, and fitted with a gas-inlet (Note 2)
, a mechanical stirrer
, and a condenser with attached drying tube
. The flask is cooled in an ice bath
, stirring is begun, and chlorine
is introduced through the gas-inlet. Dichlorotriphenylphosphorane
separates as a white solid or as a milky oil; the flow of chlorine
is discontinued when the mixture develops a strong lemon-yellow color (Note 3)
. The gas inlet is quickly replaced by an addition funnel
, and a solution of 10 g. of triphenylphosphine in 60 ml. of benzene
is added dropwise fairly rapidly (Note 4)
. A solution of 24.5 g. (0.250 mole) of 1,2-epoxycyclohexane (Note 5)
in 50 ml. of benzene
is then added dropwise over ca.
20 minutes. The ice bath is replaced by a heating mantle
, and the mixture, which consists of two liquid phases, is stirred and refluxed for 4 hours. It is then cooled, and excess dichlorotriphenylphosphorane
is destroyed by the slow addition of 10 ml. of methanol (Note 6)
. The mixture is concentrated on a rotary evaporator
100 mm., and the residue, which may be a white solid or a viscous oil, is triturated with 300 ml. of petroleum ether
(30–60°). The solid triphenylphosphine oxide
that separates is collected by suction filtration. The cake is thoroughly broken up with a spatula
and washed with three 100-ml. portions of petroleum ether
. The combined filtrates, from which a little more triphenylphosphine oxide
precipitates, are refiltered, then washed with 250-ml. portions of aqueous 5% sodium bisulfite (Note 7)
and with water. The organic phase is dried over magnesium sulfate
, filtered, concentrated on a rotary evaporator at ca.
100 mm., and distilled through a 20-cm. Vigreux column
. There is very little forerun before 27–28 g.
) of cis-1,2-dichlorocyclohexane
is collected at 105–110° (33 mm.)
1.4977 (Note 8)
A sharp endpoint is not observed. A simple test for complete chlorination is as follows: the flow of chlorine
and the stirrer are stopped, and the mixture is allowed to settle. Chlorine
is then admitted without stirring. If unreacted triphenylphosphine
is present, a visible clouding (formation of dichlorotriphenylphosphorane
) will occur at the gas–liquid interface.
Although a slight excess of chlorine
does not appear to be deleterious, a substantial excess is avoided by adding the last portion of triphenylphosphine
at this point.
The reaction mixture may be allowed to stand overnight before addition of methanol
The distilled cis-dichlorocyclohexane
tends to become colored if the solution is not washed with a reducing agent.
The checkers, using a 10-cm. Vigreux column
, found that it was necessary to take a wider boiling range fraction (105–115°, 33 mm.) to obtain similar yields. The product is virtually free of trans-1,2-dichlorocyclohexane
(the isomeric 1,2-dichlorocyclohexanes are readily separated by GC on Carbowax 20M or on diethylene glycol succinate columns).
This procedure is general for the conversion of epoxides to dichlorides with inversion of configuration at each of the two carbons and, in effect, provides a method for the cis
-addition of chlorine
to a double bond.3 cis-1,2-Dichlorocyclohexane
has also been prepared from 1,2-epoxycyclohexane
and sulfuryl chloride
but the stereospecificity of the reaction appears to be extremely sensitive to reaction conditions, and the yield is lower than that obtained by the method described here. Other methods give cis-1,2-dichlorocyclohexane
contaminated with considerable amounts of the trans
-isomer. This method has been used to convert cis
- and trans
-4,5-epoxyoctanes to meso
- and d,l
-4,5-dichloroöctanes, respectively, and trans-7,8-epoxyoctadecane
. These conversions were carried out on smaller amounts of material, and the products were purified by column chromatography on silica gel. Yields were 51–63%.
Halogenations with dihalotriphenylphosphoranes have been reviewed briefly by Fieser and Fieser.5 Dibromotriphenylphosphorane
appears to have been studied somewhat more than the dichloro compound, but both reagents effectively convert alcohols to alkyl halides, carboxylic acids and esters to acid halides, etc. The reaction of 1,2-epoxycyclohexane
under conditions similar to those described here gives a mixture of cis
- and trans
-1,2-dibromocyclohexanes. A reagent prepared from triphenylphosphine
and carbon tetrachloride
has been used for similar transformations.6
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