Organic Syntheses, CV 9, 653
Submitted by Paul J. Fagan and William A. Nugent
1.
Checked by Mark S. Jensen and James D. White.
1. Procedure
A
500-mL, three-necked, round-bottomed flask equipped with a
magnetic stirring bar,
rubber septum-capped pressure-equalizing addition funnel on the center neck,
rubber septum on one side neck, and a
nitrogen inlet with stopcock on the other side neck is charged in a nitrogen-filled glove box (Note
1) with
27.0 g (92.5 mmol) of zirconocene dichloride [(η-C5H5)2ZrCl2] (Note
2),
150 mL of tetrahydrofuran (Note
3), and
16.0 mL (204 mmol) of 2-butyne (Note
4) added via syringe. The apparatus is removed from the glove box and attached via the
nitrogen stopcock to a
nitrogen bubbler. The flask is cooled to −78°C (
dry ice-acetone bath) and
108 mL of 1.72 M butyllithium (186 mmol) in hexane (Note
5) is added via syringe to the addition funnel through the septum. The
butyllithium solution is added dropwise to the stirred mixture in the flask. After the addition is complete, the reaction mixture is stirred at −78°C for 10 min (Note
6). The flask is allowed to warm by removing the dry ice-acetone bath, and the reaction mixture is stirred at room temperature for 2.5 hr at which point the mixture is dark orange-red (Note
7). The flask is again cooled to −78°C, and
17.5 mL (129 mmol) of dichlorophenylphosphine (Note
8) is added in a slow stream via syringe through the septum on the flask. The dry ice-acetone bath is removed and the flask is allowed to warm to room temperature. After 1 hr the orange-red color has dissipated and the septa are replaced with glass stoppers. The nitrogen inlet is connected to a vacuum line (0.1 mm) and the solvent is removed under reduced pressure. The stopcock is closed and the apparatus is placed in a nitrogen-filled glove box (Note
1). The reaction residue is extracted three times with
30-mL portions of hexane, each of which is filtered from the precipitate of (
η-C5H5)2ZrCl2 and combined. The
hexane is removed under reduced pressure (0.5 mm), and the oily liquid that remains is transferred into a
100-mL round-bottomed flask. The flask is attached to a vacuum distillation apparatus with a
10-cm Vigreux column. The distillation apparatus is removed from the glove box and attached to a vacuum line. Distillation at 0.35 mm yields a fraction which is collected between 40–92°C and discarded. A second fraction boiling between
92–104°C (Note
9) is collected; the clear oily liquid is
1-phenyl-2,3,4,5-tetramethylphosphole [
14.9–15.5 g,
75–78% based on (
η-C5H5)2ZrCl2)] (Note
10). The compound is air-sensitive and should be stored under
nitrogen.
2. Notes
1. A nitrogen-filled glove bag may also be used.
2.
Zirconocene dichloride was obtained from Aldrich Chemical Company, Inc. and used without further purification.
3.
Tetrahydrofuran was distilled from
sodium benzophenone ketyl before use.
4.
2-Butyne was obtained from Farchan Chemical Company and dried over 4 Å molecular sieves before use.
6. The reaction mixture may become thick with a white solid at this point. The solid can be loosened by manually shaking the flask, or with the aid of an external permanent magnet using the magnetic stirring bar to break up the solid. The solid loosens some more upon warming, and stirring is not a problem.
7. If desired, the
zirconium metallacycle can be isolated at this point: The flask is attached to a vacuum line via the nitrogen inlet, and solvents are removed from the flask under reduced pressure. With the aid of a 40°C water bath, the reaction residue is thoroughly dried. The flask is sealed and placed in a nitrogen-filled glove box (Note
1). The residue is extracted with small portions of
toluene (total of 50 mL), each portion being filtered and combined.
Toluene is removed from the filtrate under reduced pressure.
Hexane (20 mL ) is added to the solid residue, and after trituration, the solid is collected by filtration, and washed once with
10 mL of hexane. It is dried under reduced pressure to yield
26.0 g (
85%) of crystalline, orange-red
(η-C5H5)2ZrC4(CH3)4 which is >95% pure by spectroscopic analysis. This compound is very stable thermally both in solution and in the solid state; however, it is air-sensitive and should be handled under
nitrogen. It can be used as obtained as a reagent in the synthesis of other heterocycles. The NMR spectrum is as follows:
1H NMR (300 MHz, THF-d
8) δ: 1.54 (s, 6 H, CH
3), 1.57 (s, 6 H, CH
3), 6.15 (s, 10 H, hη-C
5H
5).
9. The boiling point has been reported previously as
105–110°C at 0.5 mm.2
10. The product has the following spectral properties:
1H NMR (300 MHz, THF-d
8) δ: 1.89 (d, 6 H, J = 10.6, CH
3), 1.93 (s, 6 H, CH
3), 7.24 (m, 5 H, phenyl);
13C NMR (75.5 MHz, THF-d
8) δ: 12.9 (dq, J
CH = 126, J
PC = 22, CH
3), 13.9 (q, J
CH = 126, CH
3), 129.2 (dt, J
CH = 160 and 6, J
PC = 8, phenyl), 129.6 (dt, J
CH = 162 and 6, phenyl), 134.3 (ddt, J
CH = 161 and 5, J
PC = 20, phenyl), 134.7 (d, J
PC = 15), 136.4 (s, C=C), 143.4 (m, J
PC = 11);
31P {
1H} NMR (121.7 MHz, THF-d
8) δ: 14 (s). Exact mass: Calcd. for C
14H
17P: m/e = 216.1068. Found: 216.1127.
Waste Disposal Information
All toxic materials were disposed of in accordance with "Prudent Practices in the Laboratory"; National Academy Press; Washington, DC, 1995.
3. Discussion
Phospholes and other related heterocycles are an important class of main group compounds. The chemistry of phospholes and their preparation has been reviewed extensively by Mathey.
3 We provide details here for a simple, one-pot procedure for the preparation of
1-phenyl-2,3,4,5-tetramethylphosphole applying zirconocene chemistry.
4 The procedure involves reduction of (η-C
5H
5)
2ZrCl
2 with
butyllithium in the presence of
2-butyne which (as reported initially by Negishi, et al.
5) forms a
zirconium metallacycle. Addition of
dichlorophenylphosphine to this reaction mixture produces the phosphole. One other procedure for the preparation of
1-phenyl-2,3,4,5-tetramethylphosphole has been reported by Nief, et al.
2 That procedure involved aluminum chloride-coupling of
2-butyne, followed by reaction with
dichlorophenylphosphine to form a
chlorophospholium tetrachloroaluminate which was then reduced with
tributylphosphine to produce the phosphole in 68% yield.
Using a procedure similar to that described here, or using isolated
zirconium metallacycles as reagents, we have been able to prepare not only phospholes, but also arsoles, stiboles, bismoles, siloles, germoles, stannoles, galloles, thiophenes, selenophenes, and borole Diels-Alder dimers.
4 Since a number of other
titanium and
zirconium metallacycles are readily available,
4 these reagents should be useful in the preparation of a variety of heterocycles.
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