Organic Syntheses, Vol. 77, 254
Submitted by Paul Binger
1, Petra Wedemann
1, and Udo H. Brinker
2.
Checked by Wenyong Wang and Amos B. Smith, III.
1. Procedure
A. Cyclopropene (1). A
250-mL, three-necked flask is equipped with a
25-mL dropping funnel, a
Dimroth-type reflux condenser (Note
1), an immersed
thermometer, a
magnetic stirring bar, and a
silicon oil gas bubbler with a short connection to the inlet tube of a cold
trap (ampule).
Argon flow (Note
2) is introduced from the top of the condenser. The flask is charged with
sodium bis(trimethylsilyl)amide (Note 3), (Note 4) (35.05 g, 0.192 mol), which is dissolved in
toluene (150 mL) (Note
5). The resulting solution is brought to a vigorous reflux (Note
6) at which time
allyl chloride (Note 7), (Note 8) (13.8 mL, 0.169 mol) is added from the dropping funnel over a period of 45-60 min (Note
9).
Cyclopropene (1) emerges from the flask and is condensed through the inlet tube into an ampule at −80°C. After an additional 30 min at reflux,
2.6 g (d = ca. 0.7 g/mL) (yield
39 %) of
cyclopropene (1) is collected as a colorless liquid (purity > 95%) containing only traces of
allyl chloride as determined by
1H NMR spectroscopy at −80°C (Note
10).
B. Reaction of cyclopropene (1) with cyclopentadiene. A
50-mL, one-necked flask equipped with a
rubber septum and a stirring bar is charged with
pentane (10 mL) (Note
11) and cooled to −80°C, whereupon
cyclopentadiene (3.81 g, 4.76 mL, 0.058 mol) (Note
12) is added. A
25-mL flask is equipped with a rubber septum and charged with
pentane (10 mL). The
pentane is cooled to −80°C, then transferred via
cannula into the cold trap (ampule) containing
cyclopropene (1) (2.1 g, 0.052 mol). The resulting
cyclopropene solution is quickly transferred through a short capillary (steel, 1 mm i.d.) to the
cyclopentadiene solution (Note
13) using a positive pressure of
argon. The reaction mixture is allowed to warm to room temperature within 2 hr. The resulting colorless solution is distilled via a short-path still. At 125°C,
2.80 g of
endo-tricyclo[3.2.1.02,4]oct-6-ene (2) is collected. A second fraction furnished
1.16 g, providing a combined yield of
3.96 g (
72 %) of
2; the purity is >99% as determined from its
1H and
13C NMR spectra (Note
14).
2. Notes
1. An internal water-cooled coil allows for efficient condensation of low-boiling vapors and prevents condensation on the outside, which might seep into joints. The checkers used an
Allihn condenser.
2. A stream of
argon is added after the condenser is in place to drive the
cyclopropene into the cold trap. If the
argon stream is too strong,
cyclopropene will be blown out of the cold trap, thereby affecting the yield.
3.
Sodium bis(trimethylsilyl)amide is commercially available from Aldrich Chemical Company, Inc., or Fluka Chemical Corp., but can be prepared according to reference
3 4.
5.
Toluene (Overlack) was dried over Na/K alloy and freshly distilled before use.
6. The oil bath should be kept between 140°C and 150°C.
7.
Allyl chloride (98%, Fluka Chemical Corp.) was freshly distilled before use.
8. Optimal yields were obtained when using 0.85-0.90 equiv of
allyl chloride.
5
10. The following spectra were obtained:
1H NMR (500 MHz, toluene-d
8, −80°C, round-bottom tube, 5-mm wide) δ: 1.18-1.19 (m, 2 H), 6.69-6.70 (m, 2 H);
13C NMR (125 MHz, toluene-d
8, −80°C) δ: 3.0, 108.5 (2C).
11.
Pentane (> 98%, Aldrich Chemical Company, Inc.) was distilled before use.
12.
Cyclopentadiene (purity > 99%) was obtained by cracking the dimer (Aldrich Chemical Company, Inc.) at 180°C.
13. The NMR spectrum recorded at −80°C showed that no reaction takes place (even after 24 hr).
14. The spectra were as follows:
1H NMR (500 MHz, CDCl
3) δ: 0.37-0.39 (m, 1 H), 0.57-0.62 (m, 1 H), 1.33-1.36 (m, 2 H), 1.69-1.71 (m, 1 H), 1.80-1.82 (m, 1 H), 2.77 (s, 2 H), 5.70-5.71 (m, 2 H);
13C NMR (125 MHz, CDCl
3) δ: 12.3 (2C), 17.0, 42.3 (2C), 63.6, 130.4 (2C).
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
From a preparative point of view, previous attempts at preparing
cyclopropene (1) are either very laborious or low yielding. Over the last 25 years, the submitters have developed simple syntheses for substituted cyclopropenes on a multigram scale.
6,5 17 Here they present their efforts towards an improved synthesis of the parent compound
1.
When
allyl chloride (Note
7), (Note
8), (Note
9) was dropped into a solution of
sodium bis(trimethylsilyl)amide (Note
3), (Note
4) in boiling
toluene (Note
5), (Note
6),
cyclopropene (1) could be isolated in a trap/ampule at −80°C.
18 19 Compared with the published procedure,
16 these conditions proved superior, affording
1 in about 40% yield. Furthermore, as could be established by NMR spectroscopy at −80°C, the
cyclopropene (1) was nearly pure (>95%) (Note
10), containing only traces of
allyl chloride. Compound
1, prepared in this manner, was found to be stable in
toluene solution at −78°C for at least 1 week. Upon warming to −30°C,
1 begins to oligomerize (NMR control).
Acknowledgments U.H.B. thanks the State University of New York at Binghamton for a sabbatical leave and those at the Max-Planck-Institut für Kohlenforschung for their generous hospitality.
Appendix
Chemical Abstracts Nomenclature (Collective Index Number);
(Registry Number)
Cyclopropene (8,9); (2781-85-3)
Cyclopentadiene: 1,3-Cyclopentadiene (8,9); (542-92-7)
Sodium bis(trimethylsilyl)amide: Disilazane, 1,1,1,3,3,3-hexamethyl-, sodium salt (8); Silanamine, 1,1,1-trimethyl-N-(trimethylsilyl)-, sodium salt (9); (1070-89-9)
Allyl chloride: Propene, 3-chloro- (8); 1-Propene, 3-chloro- (9); (107-05-1)
endo-Tricyclo[3.2.1.02,4]oct-6-ene: Tricyclo[3.2.1.02,4]oct-6-ene, endo- (8);
Tricyclo[3.2.1.02,4]oct-6-ene, (1α,2α,4α,5α)- (9); (3635-94-7)
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