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Org. Synth. 1966, 46, 44
DOI: 10.15227/orgsyn.046.0044
HEXAPHENYLBENZENE
[Benzene, hexaphenyl-]
Submitted by Louis F. Fieser1
Checked by Chester E. Ramey and V. Boekelheide.
1. Procedure
A 100-ml., round-bottomed, ground-glass flask containing 40 g. of benzophenone is heated over a free flame to melt the bulk of the solid, and then 8.0 g. of tetraphenylcyclopentadienone (0.021 mole) (Note 1) and 8.0 g. of diphenylacetylene (0.043 mole) (Note 2) are introduced through a paper cone so that no material lodges on the neck or walls. An air condenser is attached, and the mixture is heated over a microburner so that it refluxes briskly but without flooding the condenser (the temperature of the liquid phase is 301–303°). Carbon monoxide is evolved, the purple color begins to fade in 15–20 minutes, and the color changes to a reddish brown in 25–30 minutes. When no further lightening in color is observed (after about 45 minutes), the burner is removed and 8 ml. of diphenyl ether is added to prevent subsequent solidification of the benzophenone. The crystals that separate are brought into solution by reheating, and the solution is let stand for crystallization at room temperature. The product is collected and washed free of brown solvent with benzene to give 9.4 g. (84%) of colorless plates, m.p. 454–456° (sealed capillary) (Note 3). A satisfactory solvent for recrystallization is diphenyl ether, using 7 ml. per gram of product.
2. Notes
1. A synthesis of tetraphenylcyclopentadienone is described in Org. Syntheses, Coll. Vol. 3, 806 (1955). However, the following procedure is more convenient. A 250-ml. Erlenmeyer flask is charged with 21 g. of benzil, 21 g. of dibenzyl ketone (Eastman Organic Chemicals, practical grade), and 100 ml. of triethylene glycol, a thermometer is introduced, and the mixture is heated over a free flame until the solid has dissolved. A 10-ml. portion of a 40% solution of benzyltrimethylammonium hydroxide in methanol is made ready, the temperature of the reaction mixture is adjusted to 100°, the basic catalyst is added, and the mixture is swirled once for mixing and let stand. Within 15–20 seconds the liquid sets to a stiff paste of purple crystals and the temperature rises to 115°. After the temperature has dropped to 80°, the mixture is cooled, thinned by stirring in 50 ml. of methanol, and the product is collected and washed with methanol until the filtrate is purple, not brown. The yield of product, m.p. 219–220°, is 35.5 g. (93%). Recrystallization can be accomplished with 92% recovery by dissolving the ketone in triethylene glycol (10 ml./gram) at 220°.
2. Improvements in the preparation and dehydrohalogenation of meso-stilbene dibromide [Org. Syntheses, Coll. Vol. 3, 350 (1955)] are as follows. trans-Stilbene (20 g.) is heated with 400 ml. of acetic acid on the steam bath until dissolved, 40 g. of pyridinium bromide perbromide is added, and the mixture is heated on the steam bath and swirled for 5 minutes. meso-Stilbene dibromide separates at once in pearly white plates. The mixture is cooled to room temperature, and the product is collected and washed with methanol. The yield of dibromide, m.p. 236–237°, is 32.4 g. (86%). A 250-ml., round-bottomed, ground-glass flask is charged with 32.4 g. of the dibromide, 65 g. of potassium hydroxide pellets, and 130 ml. of triethylene glycol. A 15 × 125 mm. test tube containing enough of the same solvent to cover the bulb of a thermometer is inserted in the flask. The flask is supported in a clamp, which is used as a handle for swirling the flask over a free flame to mix the contents and bring the temperature to 160°, when potassium bromide begins to separate. By intermittent heating and swirling the mixture is kept at 160–170° for 5 minutes more to complete the reaction. The test tube is then removed, dipped into 500 ml. of water in a beaker, and the adhering organic material is rinsed into the beaker with 95% ethanol. The hot reaction mixture is poured into the beaker, and the flask is rinsed alternately with water and with ethanol. After cooling, the crude product is collected, washed with water, and air-dried (16.5 g.). The brown solution of this material in 50 ml. of 95% ethanol is filtered from a little dark residue, reheated, and let stand for crystallization. A first crop of diphenylacetylene (11.8 g.) separates in large colorless spars, m.p. 61.5–62.5°. Concentration of the mother liquor yield an additional 2.4 g. of crystals, m.p. 58–59°.
The checkers used diphenylacetylene provided by Aldrich Chemicals.
3. The melting point is determined conveniently with a Mel-Temp apparatus and a 90–510° thermometer designed for use with it (Laboratory Devices, Post Office Box 68, Cambridge 39, Massachusetts). An evacuated capillary containing a sample is sealed close to the sample to prevent sublimation, and repeated determinations are made with the same sample. The figure 456° is the average of two determinations of the temperature of melting; 454° is the average of two observations of the point of solidification. When the amount of diphenylacetylene was reduced to 1.2 times the theory, the yield was the same but the melting point was 450–452°.
3. Discussion
Hexaphenylbenzene has been prepared by heating tetraphenylcyclopentadienone and diphenylacetylene without solvent2 and by trimerization of diphenylacetylene with bis-(benzonitrile)-palladium chloride and other catalysts.3
4. Merits of the Preparation
Hexaphenylbenzene can be prepared satisfactorily by strong heating of a mixture of 0.5 g. each of tetraphenylcyclopentadienone and diphenylacetylene in a test tube, but the method is unsatisfactory on a larger scale because of the high melting point of the product and the poor heat transfer in a flask. The present procedure demonstrates use of benzophenone as solvent for a Diels-Alder reaction requiring a temperature of about 300°. When the reaction is completed, addition of a small amount of diphenyl ether lowers the melting point of benzophenone sufficiently to prevent this solvent from solidifying.
Other solvents tried and the liquid temperatures of the refluxing mixtures are: stearic acid (340–365°), di-n-butyl phthalate (320–325°), phenyl salicylate (290°). The first two solvents are unsatisfactory because of side reactions consuming some of the tetraphenylcyclopentadienone, the third because the addition reaction is too slow.
(Note 1) describes an improvement in the preparation of the starting dienone involving use of a medium of higher solvent power and higher boiling point than ethanol and of a basic catalyst more convenient than potassium hydroxide because it is miscible with the solvent employed. (Note 2) reports two improvements in the preparation of diphenylacetylene. The yield in the conversion of trans-stilbene to the meso dibromide is increased by use of the highly stereoselective reagent pyridinium bromide perbromide. In the dehydrohalogenation step the reaction time is reduced substantially and the yield increased by use of a high-boiling alcohol in place of ethanol.

References and Notes
  1. Department of Chemistry, Harvard University, Cambridge, Massachusetts, 02138.
  2. W. Dilthey and G. Hurtig, Ber., 67, 2007 (1934).
  3. A. T. Blomquist and P. M. Maitlis, J. Am. Chem. Soc., 84, 2329 (1962).

Appendix
Chemical Abstracts Nomenclature (Collective Index Number);
(Registry Number)

pyridinium bromide perbromide

meso dibromide

ethanol (64-17-5)

acetic acid (64-19-7)

Benzene (71-43-2)

methanol (67-56-1)

carbon monoxide (630-08-0)

Benzil (134-81-6)

potassium hydroxide,
potassium hydroxide pellets (1310-58-3)

Benzophenone (119-61-9)

potassium bromide (7758-02-3)

phenyl salicylate (118-55-8)

stearic acid (57-11-4)

dibenzyl ketone (102-04-5)

diphenyl ether (101-84-8)

Diphenylacetylene (501-65-5)

benzyltrimethylammonium hydroxide (100-85-6)

Tetraphenylcyclopentadienone (479-33-4)

triethylene glycol (112-27-6)

Hexaphenylbenzene,
Benzene, hexaphenyl- (992-04-1)

bis-(benzonitrile)-palladium chloride

trans-Stilbene (103-30-0)

meso-stilbene dibromide

di-n-butyl phthalate (84-74-2)