Org. Synth. 1947, 27, 33
DOI: 10.15227/orgsyn.027.0033
[Pyrrole, 2,4-diphenyl-]
Submitted by C. F. H. Allen and C. V. Wilson.
Checked by Arthur C. Cope, Frank S. Fawcett, and Harold R. Nace.
1. Procedure
A suspension of 61 g. (0.26 mole) of α-phenyl-β-benzoylpropionitrile1 (Note 1) in 150 ml. of methanol and 1 level teaspoonful of Raney nickel catalyst2,3 (Note 2) are placed in a 400–500 ml. pressure bottle (Note 3) connected to a low-pressure reduction apparatus. The bottle is alternatively evacuated and filled with hydrogen twice, and the reduction is conducted at 80–90° by shaking with hydrogen at an initial pressure of 50 lb. After 2 moles of hydrogen is absorbed the reduction is discontinued, the solution is filtered to separate the catalyst, and the solvent is removed from the green filtrate by distillation under reduced pressure. The crude 2,4-diphenyl-2-pyrroline (Note 4) is rinsed into a 300-ml. Kjeldahl flask with a little methanol, which is removed by distillation at reduced pressure (water pump) while the flask is immersed in a bath at 100°. Selenium (10 g.) is added, and the mixture is heated in a metal bath at 245–265° for 5 hours while a slow stream of nitrogen is passed over the surface of the liquid. This operation is conducted in a well-ventilated hood. The molten mass is poured from the reaction flask into a beaker (Note 5), and the product is extracted with 300 ml. of boiling toluene. The green toluene solution is filtered through a cotton plug to remove a small amount of suspended selenium, and the filtrate is cooled. The 2,4-diphenylpyrrole separates as light green crystals which melt at 174–176° and weigh 24–26 g. (42–46%) (Note 6). A second crop of 2–3 g. (4–5%), m.p. 160–170°, can be obtained by concentrating the filtrate and cooling.
2. Notes
1. The α-phenyl-β-benzoylpropionitrile1 was used without recrystallization (m.p. 123–125°).
2. The time required for the hydrogenation was 2.5 hours with a very active nickel catalyst, and longer with a less active catalyst. Raney nickel prepared according to 2 is a particularly active catalyst and gives excellent results in this preparation.
3. A 500-ml. Pyrex centrifuge bottle is satisfactory.
4. The submitters state that distillation yields pure 2,4-diphenyl-2-pyrroline, b.p. 170–172°/3 mm., 203–205°/8 mm., which solidifies on cooling (yield 80–90%).
5. If the contents of the flask are not poured into a beaker while molten, it is difficult to remove the cake of selenium. If more than one preparation is to be made, the contents of the flask are allowed to cool and the product is extracted with hot toluene. The selenium can be reused.
6. The product is sufficiently pure for most purposes. Recrystallization from toluene gives a 90–93% recovery of a very light-green product, m.p. 179–180° (cor.). The submitters state that the product obtained in the first crystallization has this melting point if the 2,4-diphenyl-2-pyrroline is purified by distillation before dehydrogenation.
3. Discussion
2,4-Diphenylpyrrole has been prepared only by dehydrogenation of 2,4-diphenyl-2-pyrroline with selenium, Raney nickel, and nickel supported on pumice.4

References and Notes
  1. Org. Syntheses Coll. Vol. 2, 498 (1943)
  2. Pavlic and Adkins, J. Am. Chem. Soc., 68, 1471 (1946).
  3. Org. Syntheses Coll. Vol. 3, 176, 181 (1954).
  4. Rogers, J. Chem. Soc., 1943, 594.

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

methanol (67-56-1)

hydrogen (1333-74-0)

nitrogen (7727-37-9)

Raney nickel (7440-02-0)

toluene (108-88-3)


α-Phenyl-β-benzoylpropionitrile (6268-00-4)

Pyrrole, 2,4-diphenyl- (3274-56-4)