A Simple Procedure for the Base-Catalyzed Cleavage of Benzopinacolone to Triphenylmethane An Undergraduate Experiment Michael Siranberg and J.-P. Anselme University of Massachusetts at Boston. Boston, MA 02125
The reductive photodimerization of henzophenone 1 to benzopinacol2 and subsequent rearrangement of the latter to benzopinacolone 3 (1) have been "traditional" first experiments in the Spring semester of our undergraduate organic laboratories. Our desire to illustrate the laboratory interconversions of structural types led us to attempt to d e v e l o ~a nrocedure for the cleavaee of henzo~inacolone3 to triphenylkethane 4.' The base-c&lyzed cleavage of nonenolizable ketones, known as the Haller-Bauer reaction (2) is generally not mentioned in undergraduate courses and hitherto has been little used in organic synthesis. However, coincidental with our efforts was the appearance of a series of interesting papers by Paquette and his students (3),who have demonstrated the utility of this reaction.
Unfortunately, the Haller-Bauer reaction involves heating the ketone with sodium amide (or potassium t-butoxide) 'The Beckmann fraamentation of benzoDlnawlone oxime lobthe reaction oiohenvlmaanesium talned ~- ~ bv -,~~ ~ , ~ b;omide ~ on tri~henvlicb .. . tonitrile oxide) to give triphenylcarbinol (Wisland. H.; Rosenfeld. B. Liebig's Ann. Chem. 1930. 484.236) wasconsidered for this purpose. Thw reaction would have a1 owed students to compare the product to the triphenylcarbinol obtained in another traditional experiment, namely the addition of phenylmagnesium bromide to methyl benzoate (or benzophenone). However, oximatlon of benzopinacolone falls. most likely because of steric hindrance. We have carried out the Haller-Bauer of benzoDlnawlone under me conditions ne.'~ P.. Rec. Trav. ..-. - - described ~ ~bv Montaane l ~ o n t a a~. Chim. Pays-Bas 1905, 24: 1051: while a slmnlar yield (90%) was obtained in ethanol and soalum hydroxiaeafter two hours of reflux.the crude product had to be recrystallized toafford purematerial, mp. 88~~~
-
~
~~~
".
~0
~
.
~
~-
".
01 O P
Due caution should be exercized when using ethylene glycol: the experiment should be performed in a wellventilated area and avoid inhaling vapors of this solvent.
616
Journal of Chemical Education
and, as such, is hardly suitable as an undergraduate organic experiment. Our initial attempts involving heating benzo~inacolonewith calcium oxide in testtubes over a n onen flame led only to intractable tars. In the hope of minimiiing the harshnessof the conditions, ethyleneglycol was used as a solvent that could he heated to high temperatures. After three days of heating henzopinacolone in ethylene glycol with potassium hydroxide, followed by cooling to room temperature, white cwstals mecidtated out of solution. The crystals &ere showb to he-nea& pure triphenylmethane by infrared and nuclear mametic resonance s ~ e c t r o ~ h o t o metry: by acidification of the aqueous filtrate, a- small could be isolated. A amount of benzoic acid, mp. 1It?-120 T, procedure suitable for an undergraduate laborarory was developed whereby a better than 905 yield of triphenylmethane could be isolated in less than two hours.' This reaction was also successful with ru,u,n-tris-(p-chloropheny1)-pchloroacetophenone and a,a,a-tris-@-toly1)-p-methylacetophenone to the corresponding triphenylmethanes ( 4 , 5 ) . Experimental Section Triphenylmethane. A mixture of 3.0 g (8.62 mmol) of benzopinacolone, 0.4 g (7.13 mmol) of potassium hydroxide, and 15 mL ethylene glycol in a 50-mL round-bottom flask, equipped with a magnetic
stirringbar, a condenser,and a thermometer,was heated at 175-180 'C for 1h.3 After cooling to room temperature, the colorless solution deposited colorless crystals of triphenylmethane, which were eollected and washed with cold ethanol. The solid was allowed to air dry and yielded 1.9 g (92%) of triphenylmethane,mp 90-93 OC, lit. (5)mp 91-92 "C;if desired, the product may he recrystallized from ethanol. The filtrate was acidified with conc. hydrochloric acid until no mare precipitate appeared to form. A very small amount of crude henzaie acid, mp 118-120 OC, was collected. Literature Clted 1. Fiaaer, L . F.: Williamson, K. L. OlRonic Experimenlr, 5th ed.: Heath: Lexington. M A , 1983. 2. Hamlin, K. E.: Weafon, A. W. OPE. React. 1957, 9, 1: Kaiser. E. M.: Warner. C. D. Synthesis 1915,395. 3. Gilday, J. P.:Paquette,L.A. Org. PrepProred. Int. 1990,22,167. 4. Tousley, N.E.:Gornberg, M. J, Amer.Chem. Sor. 1904,26,1516. 5. Janes. L. W.;Seymour. M . W. J.Am. Chsm Soc. 1928,JO. 1150.
