Org. Synth. 1970, 50, 81
Submitted by W. J. Middleton1
and H. D. Carlson.
Checked by L. Scerbo and W. D. Emmons.
Caution! This procedure should be conducted in a good hood to avoid exposure to ammonia and hexafluoroacetone.
A 3-l., four-necked, round-bottomed flask is equipped with a thermometer (−50° to 150°), a dry ice-cooled reflux condenser (protected from the atmosphere through a T-tube that is also connected to a nitrogen source and a Nujol bubbler), and a gas-inlet tube above the liquid level. The flask and condenser are heated in an air oven at 125° for several hours and flamed under nitrogen with a Bunsen burner. Pyridine (1.2 l.), previously dried over potassium hydroxide pellets, is added to the flask. A nitrogen atmosphere is maintained in the system, the pyridine is cooled to −40°, and 462 g. (2.78 moles) of hexafluoroacetone (b.p. −28°) is added from a cylinder through the gas-inlet tube over 30 minutes (Note 1), (Note 2), and (Note 3). Liquid ammonia (58.3 ml., 47.6 g., 2.80 moles), previously distilled into a cold trap and measured at −78°, is distilled into the pyridine solution over a period of 1 hour ((Note 4)). During this addition the bath is held at −45° to −40°, keeping the solution at −25° to −30°.
As soon as the ammonia has been added, the gas-inlet tube is replaced with a 250-ml. pressure-equalized dropping funnel and the reaction mixture is heated with a heating mantle to 40° over 30 minutes or as quickly as possible (Note 5). The dry ice–cooled condenser is then replaced with a 24-in. water-cooled bulb condenser with Tygon tubing joining the top of the condenser to a 300-ml. cold trap protected from the atmosphere with a calcium chloride drying tube and cooled in a bath maintained at −30°. The condenser is cooled with 18–20° water. The heating mantle is turned off, and the dropping funnel is charged with 394 g. (235 ml., 2.57 moles) of phosphorus oxychloride, which is added dropwise at a rate to maintain a gentle reflux. The imine (b.p. 16°) collects in the cold trap. When addition is complete, the reaction mixture is heated to 100° over 20 minutes and maintained at this temperature for 30 minutes. The cold trap collects 320–360 g. of crude liquid product (Note 6), which is distilled through a Podbielniak still with a reflux head temperature of about 0° (Note 7), yielding 254–291 g. (55–65%) of the purified imine, b.p. 15.5–17° (Note 8). It can be stored indefinitely in a stainless-steel cylinder.
Hexafluoroacetone may be obtained from E. I. du Pont de Nemours and Company, Inc., or Allied Chemical Corporation
If it is inconvenient to add the hexafluoroacetone
directly from a cylinder, it may first be condensed in a calibrated trap containing a boiling chip and cooled in an acetone–dry ice bath
. When cooled to −78°, 462 g. of liquid hexafluoroacetone has a volume of ca. 280 ml.
can be added to the reaction mixture by allowing it to boil slowly from the trap.
The bath is maintained at −40°, and the pyridine
solution is maintained below −20°. Acetone
, to which pieces of dry ice are added periodically to adjust the temperature, is a convenient cooling bath.
can be prevented from bumping with a magnetic stirrer
in the trap. Heat for the distillation can be obtained from the air of the laboratory and by occasionally flushing the outside wall of the trap with room-temperature acetone
Prolonged heating decreases the yield.
For storage until Podbielniak distillation can be carried out, the product can be drained into an evacuated, 500-ml. stainless-steel cylinder
The checkers used a 60-cm., vacuum-jacketed column packed with glass helices
, with satisfactory results.
This preparation has been run in the submitters' laboratory at four times this scale with yields as high as 67%
has been prepared by the reaction of hexafluoroacetone
with triphenylphosphine imine
by the pyrolysis of N-phenyl-2,2-diaminohexafluoropropane
by the reaction of hexafluorothioacetone
with hydrazoic acid
and by the reaction of ammonia
and phosphorus oxychloride
The latter method, described here, is the most convenient, as it does not require preparation of several intermediates or use of pressure equipment. This method has also been used to prepare the imines of other fluoroketones, including those of chloropentafluoroacetone
, and perfluorodiethyl ketone
Substitution of methylamine
in this procedure gives the N-methyl imine
This preparation is referenced from:
Chemical Abstracts Nomenclature (Collective Index Number);
Phosphorus Oxychloride (21295-50-1)
potassium hydroxide (1310-58-3)
2-Propanimine, 1,1,1,3,3,3-hexafluoro- (1645-75-6)
N-methyl imine (2053-29-4)
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