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C. Cyclopropenone. To a stirred solution of 3.0 g. (0.030 mole) of
3.3-dimethoxypropene in 30 ml. of dichloromethane at 0° is added dropwise 5 ml. of cold water containing 3 drops of concentrated sulfuric acid. The reaction mixture is stirred at 0° for an additional 3 hours. Then 30 g. of anhydrous sodium sulfate is added in portions to the solution with stirring at 0°. The drying agent is removed by filtration, and the solvent is evaporated at 50-80 mm. with a water bath maintained at 0-10°. The brown, viscous residue is then distilled at 1-2 mm. With the
bath temperature at 10°, the distillate collected in a receiver cooled to
— 78° is a mixture of methanol and dichloromethane. A new receiver is attached, and the bath temperature is gradually raised to 35° (Note 6), whereupon cyclopropenone collects in the receiver as a white solid. The yield is 1.42-1.53 g. (88-94%). The compound has b.p. 26° (0.46 mm.) and m.p. — 29° to — 28° (Note 7).
Cyclopropenone prepared in this way is quite pure and suitable for most chemical purposes. It can be repurified by crystallization from 3 volumes of ethyl ether at — 60° using a cooled filtering apparatus. The residual ethyl ether is then removed by evaporation at 1-2 mm. and 0°; very pure cyclopropenone is obtained in 60—70% recovery from the above distilled material.
1. Commercial material was used without further purification. The reflux condenser is used to decrease evaporative losses of this material.
2. Proton magnetic resonance (carbon tetrachloride) 8 (number of, protons, multiplicity): 3.63 (2, singlet), 3.48 (2, singlet), 3.27 (6, singlet). The infrared and mass spectra are also as reported.2
3. Any crystals which may form at the tip of the addition funnel are scraped off and allowed to drop into the reaction flask.
4. The checkers found it inconvenient to complete Part Â in one day, and thus they stored this ethereal solution overnight in the freezer compartment of a refrigerator.
5. The product usually contains small amounts of ether, as judged by proton magnetic resonance. The yields given are based on pure cyclopropenone ketal. Proton magnetic resonance (chloroform-rf) S (number of protons, multiplicity): 7.88 (2, singlet), 3.33 (6, singlet).
6. The bath temperature should be raised slowly to prevent decomposition of cyclopropenone.
7. Infrared (chloroform) cm.-1: 1870, 1840, 1493; proton magnetic resonance (chloroform-d) 8: 9.11 (singlet).
Cyclopropenone was first synthesized3-5 by the hydrolysis of an equilibrating mixture of 3,3-dichlorocyclopropene and 1,3-dichloro-cyclopropene (prepared by reduction of tetrachlorocyclopropene with tributyltin hydride). This procedure has been adapted5,4 to prepare
labeled and deuterated cyclopropenone for physical studies. The current procedure is somewhat more convenient. It is closely based on the work of Baucom and Butler,2 who have described this synthesis of dimeth-oxycyclopropene and shown that this ketal can be hydrolyzed to cyclopropenone. The isolation of pure cyclopropenone by hydrolysis of the ketal is parallel to the Breslow and Oda procedure5 for its isolation by hydrolysis of the dichlorocyclopropenes.
Cyclopropenone is a molecule of considerable theoretical interest, since it combines remarkable stability with extreme strain. Various physical studies® suggest that much of its stability is derived from the special conjugative stabilization in a two-pi-electron system which is related to the cyclopropenyl cation. In addition, cyclopropenone has a number of interesting chemical properties7,8 which suggest that it could be a useful synthetic intermediate. It has been used in the synthesis of cyclopropanone derivatives7 and of tropones,7 the latter by rearrangement of products derived from Diels-Alder reactions. In addition, it undergoes a very interesting cyclization-rearrangement reaction with diazo compounds which leads to the overall insertion of three carbons between the diazo group and its original attachment point.7 Perhaps the most remarkable reaction of cyclopropenone so far reported is its conversion with Grignard reagents into 2-substituted resorcinols.8 This reaction seems to be of some generality, and it represents a simple way to elaborate a resorcinol ring (all six carbons of the resorcinol system are derived from two molecules of cyclopropenone) onto a variety of alkyl groups. The ready availability of this compound should lead to other synthetic applications.
1. Department of Chemistry, Columbia University, New York, N.Y. 10027.
2. Ê. B. Baucom and G. B. Butler, J. Org. Chem,, 37, 1730 (1972).
3. R. Breslow and G. Ryan, J. Amer. Chem. Soc., 89, 3073 (1967).
4. R. Breslow, G. Ryan, and J. T. Groves, J. Amer. Chem. Soc., 92, 988 (1970).
5. R. Breslow and M. Oda, J. Amer. Chem. Soc., 94, 4787 (1972).
6. R. C. Benson, W. H. Flygare, M. Oda, and R. Breslow, J. Amer. Chem. Soc., 95, 2772 (1973), and references therein.