The Preparation of 4-Hydroxy-2,3,4,5-tetraphenyl-2cyclopenten-l-one and Its Base-Catalyzed Conversion into %,3,4,5-~etraphenylcyclopentadienone An Organic Laboratory Experiment Ernest A. Harrison, Jr. Pennsylvania State University, York Campus, York, PA 17403 Despite t h a t fact t h a t the base-catalyzed dehydration of a p-hydroxy carbonyl compound is one of t h e rare examples of a reaction in which t h e leauing m o u p is hydroxide, few (if any) laboratory experiments have h e m published that perm i t the direct observation' of this pedagogically interesting transformation. I n an effort to rectify this situation, we have developed a short synthetic sequence wherein t h e 8-hydroxyketone 4-hydroxy-2,3,4,5-tetraphenyl-2-cyclopentenone (3) is first prepared from benzil(1) a n d 1,3-diphenyl-2-propanone (2) and then dehydrated using (hot) ethanolic potassium hydroxide2to yield t h e well-known 2,3,4,5-tetraphenylcyclopentadienone (4)3.
In addition t o serving t h e pedagogical purpose stated above, this short seauence has t h e following features that make it well suited fa; use i n t h e undergraduate laboratory. (1) Both reactions are easily run and require only simple setups. (2, The progress of hoth reartiunscan be rradily mmitmed by TI.? using mhrn gel plates and rnethylrne chluride as vhent.' (3) The products (3 and 4) may he characterized by IR and NMR spectroscopy. (4) The sequence can certainly be completed in two lab periods and probably in less (e.g., 11/~).8 151 The reaction conditions described herein for the meoaration of , . . 3 lead to the erclusiue formation of the cis 4,d-diphenyl isomer', thus the addition reaction is an example of astereoselectiue reaction.
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Experlmenlal
4-Hydroxy-2,3,4,5-tetraphenyl-2-cyclopenten-l a n e (3) Potassium hydroxide (0.05 g) and anhydrous ethanol (10 mL) were placed in a 50-mL Erlenmeyer flask, and the mixture was stirred mechanically until solution was achieved. To the vigorously stirred alkaline solution was then added b e n d (1.0 g; 5.0 mmol) and, before all of the henzil has completely dissolved, 1,3-diphenyl-2propanone (1.0 g; 5.0 mmol). Stirring was continued for an additional 20 min during which time the product 3 precipitated as a white solid. The mixture was cooled (ice bath), suction-filtered, and the white solid so obtained washed with cold ethanol (4 X 5 mL) and sucked dry. The yield, after air-drying the solid overnight, was 1.6 g (80%),mp 187-192 "C (lit.8 165'C,lit.9208DC, lit.'0204-205'C). 1R (paraffin oil, em-'): 3450 (OH), 1685 (C=O). NMR (CDCL, 6 from Mel%): 6.7-7.4 (m, 20H, CsHs's), 4.5 (s, l H , CsHsCHC=O), 2.60 (s, l H , OH). EI-MS: M/Z (relative intensities) 402 (loo%, Mt),385 (11%,M-OH). The crude product was sufficiently pure to use in the preparation of 4; however, if desired, 3 can be recrystallized from toluene (6mLI 0.6 g).
828
Journal of Chemical Education
2,3,4,5-TetraphenyIcycIopentadienone(4) from 3. A 25-mL round-bottomed flask was equipped with a water-cooled condenser" and charged with 0.90 g (2.2 mM) of 3 and 8 mL of absolute ethanol. The contents of the flask were heated on a steam bath with frequent swirling, and, when it appeared that boiling had just commenced,a solution of ethanolic KOH (0.50 mL, prepared by dissolving 0.05 g KOH in 0.50 mL of absolute ethanol) was slowly addedI2through the top of the condenser over a period of 15-20 s. After completion of the addition, the reaction mixture was then heated at reflux (frequent swirling required) for 15-20 min or until TLC analysis (CHICII, silica gel plates) showed that 3 had been consumed. The mixture was cooled (ice bath), suction-filtered, and the purple solid was washed with ice-cold 95% ethanol (2 X 5 mL) andsucked dry. The yield ofair-dried (overnight) 4 was0.15g (81%), mp 214-216 OC (lit.9 219-220 OC). The IR spectrum of the product was superposable on that of an authentic sample of tetraphenylcycopentadienone. Attempted Dehydration of 3 Using Neat Ethanol a t Reflux The same quantities of starting materials and the same conditions were employed as were used for the preparation of 4 from 3, except that the ethanolic KOH was omitted. TLC analysis ( C H d h CHzC1~:hexane= 1:1, CHnClz:hexane = 4:1, silica gel plates) of the white solid obtained on filtration of the reaction mixture and that obtained on evaporation of the filtrate verified the fact that no conversion to 4 bad occurred; only starting material (3) was in evidence. This result was further confirmed by IR analysis of the solids.
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The aldol condensation experiments fomd in most contemporary laboratory texts provide only indvect ev dence for the oase-catalyzed dehydration. In order to reinforce the fact that base (and not heat alone, for example) is required for the dehydration,the instructor may choose to have half ofthe class carry out the reaction using ethanolic KOH while the other half emolovs ethanol alone. (See Exoerimental.) . Tne preparat on of compouno 4 is, of c o m e , a critical step in sequential experiments presented in a nmber of popular laboratory textsd, one I m s , however, no mention of compound 3 as a reaction intermediate in any of the discussions accompanying these experiments. 'See, for example: (a) Fieser, L. F.; Williamson, K. L. Organic Experiments. 6th ed.; Heath: Lexington, MA, 1987: p 348. (b) Pavia, D.; Lampman, G. M.; Kriz, G. S. Introduction to Organic Laboratory Technioues. 2nd ed.: Saunders: Philadelohia. 1982: o 237. under these co"ditions 3, 1, and 4 iave'~,va~;ksof 0.40, 0.80, and 0.90, respectively. If the instructor wishes to devote only one lab period to the sequence the class can be divided into groups of three with each student assigned one of the reactions to run. O t h e r preparations of 39,'0yield mainly 4 andsmaller quantities of both isomers of 3. 8Dilthey,W.; Thewalt, I.; Trosken, 0.Ber. 1934, 678, 1959. Henderson, G. G.: Corstorphine, R. H. J. Chem. Soc. 1901, 79,
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'' Fookes, C. J. R.; Gallagher. M. J. J. Chem. Soc. Perkin 11975, ~
1R7fi -. " Ground glass joints
should be greased to prevent freezing. Cautlon: too rapid an addition rate may cause extremely vigorous boiling of the reaction mixture. '2
