Organic Syntheses, CV 5, 211
Submitted by W. Voskuil and J. F. Arens
1.
Checked by Hugh D. Olmstead, James E. Oliver, and Herbert O. House.
1. Procedure
Caution! Because of the sensitivity of the reagents and product to moisture and
oxygen, all manipulations must be performed in an anhydrous, inert atmosphere (Note
1).
A
500-ml., four-necked, round-bottomed flask is equipped with an
efficient stirrer, a
reflux condenser, a
250-ml. dropping funnel, and a
low-temperature thermometer (Note
2). In the flask are placed
34.4 g. (21.8 ml., 0.25 mole) of phosphorus trichloride (Note
3) and
150 ml. of anhydrous ether. A solution of
0.50 mole of isopropylmagnesium chloride in about 150 ml. of ether (Note
4) and (Note
5) is placed in the dropping funnel.
The flask is cooled in a dry ice-acetone bath, and the Grignard reagent solution is added dropwise with rapid stirring at such a rate that the temperature of the reaction mixture remains between −25° and −30° with a bath temperature of −45°; this addition requires about 1.5 hours. After the addition has been completed, the cooling bath is removed, and the mixture is allowed to warm to room temperature. Finally, the reaction mixture is heated to reflux with continuous stirring for 30 minutes.
After the reaction mixture has cooled to room temperature, it is filtered with suction (Note
6) and (Note
7), and the residual salts are washed thoroughly with three
100-ml. portions of anhydrous ether. The combined ethereal filtrates are concentrated under reduced pressure at room temperature, and the residual liquid is fractionally distilled through a
15-cm. Vigreux column. After a small forerun has been collected, the product is obtained as a clear, colorless liquid, b.p.
46–47° (10 mm.),
n20D 1.4752 (Note
8). The yield is
21–23 g. (
55–60%); practically no residue remains in the distillation pot.
2. Notes
1. The submitters used
nitrogen purified by passage through B.T.S. catalyst (B.A.S.F., Ludwigshafen, Germany). The checkers used commercial prepurified
nitrogen without further treatment.
2. The checkers used a
three-necked flask, one neck of which was fitted with an
adapter to accommodate the thermometer. They also used a
pressure-equalizing dropping funnel so that a static
nitrogen atmosphere several millimeters above atmospheric pressure could be maintained in the flask.
3. The submitters used
Merck reagent grade phosphorus trichloride. The checkers used material from Baker and Adamson.
5. The concentration of the Grignard reagent should be estimated by titration. If an excess or less than the stoichiometric amount of the organometallic reagent is added, the yield is lower and the product is less pure. The checkers found the titration procedure of Watson and Eastham
2 to be most convenient. In a typical titration, performed under a
nitrogen atmosphere, a 5.00-ml. aliquot of the Grignard reagent was added to a solution of about
2 mg. of o-phenanthroline in 10 ml. of anhydrous benzene. The resulting purple solution was titrated with a standard solution
(0.999M) of sec-butyl alcohol in xylene until the purple color of the
o-phenanthroline-Grignard reagent charge transfer complex was just discharged. In this procedure the number of millimoles of
sec-butyl alcohol added is equal to the number of millimoles of alkylmagnesium chloride present in the aliquot of Grignard reagent.
6. The checkers performed this filtration and subsequent washing of the precipitate by replacing the dropping funnel in the reaction flask by a sintered-glass filter stick. A slight positive
nitrogen pressure was applied in the reaction flask, and the pressure was reduced in the flask that served as a
receiver for the filtrate passing through the
sintered-glass filter.
7. The checkers found it necessary to dislodge and break up the cake of magnesium salts that formed on the walls of the reaction flask. If this precaution was not observed, a substantial amount of product occluded in the salt cake was not recovered during the washing process.
8. The checkers verified the absence of dichloroalkylphosphine and trialkylphosphine contaminants in this product by obtaining acceptable elemental analytical results and by measuring the mass spectrum of the product, which exhibits a molecular ion peak at m/e 152 (
35Cl) with abundant fragment peaks at m/e 110, 43, and 41.
3. Discussion
4. Merits of the Preparation
Chlorodialkylphosphines are important synthetic intermediates in organophosphorus chemistry. In the chemical literature there is a widespread view that the simple one-step Grignard method is not suitable for the preparation of these compounds because of dominant trisubstitution and the formation of difficultly separable mixtures.
12 Although this is true for the
n-alkyl compounds, the present preparation demonstrates that in the case of branched primary alkyl compounds and secondary and tertiary alkyl compounds the method can be very convenient and can give pure products. The submitters have prepared
13 chlorodiisobutylphosphine (
45–50%),
chlorodi-sec-butylphosphine (
75–80%),
chlorodi-t-butylphosphine (
65–70%), and
chlorodicyclohexylphosphine (
60–65%) in analogous manner.
This preparation is referenced from:
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