A Simple and Safe Catalytic Hydrogenation of 4-Vinylbenzoic Acid Shantanu De, Geetu Gambhir, and H. G. Krishnamurthy University Of Delhi, Delhi-110007 Catalytic hydrogenation (both heterogeneous and homogeneous) is very familiar to all organic chemists. It is widely applicable for the reduction of alkenes and alkynes to alkanes. The conversions are essentially quantitative, and the products are isolated in high purity. The method is suitable for any level of operation from microscale to industrial. Although this topic is introduced very early in organic chemistry courses, it is rarely included and illustrated in student's laboratory exercises mainly due to nonavailability of the hydrogenation equipment and the high cost of noble metal catalysts. Recently, Ben Plummer described in this Journal ( I )the catalytic heterogeneous hydrogenation of methyl oleate to methyl stearate by in situ generation of hydrogen gas in an apparatus based on the Brown hydrogenator. CH3(CH2)f2FCH(CH>)f200CH3
PdlC-Hz
room temp.
CH3(CH2)jCH2CH2(CH2)jC0OCH3
(1)
Catalytic Transfer Hydrogenation An alternative procedure to catalytic hydrogenation (catalyst-Hz) is catalytic transfer hydrogenation (CTH).In this technique, the reduction of an organic compound (acceptor A) is achieved with the aid of donor substance (DH) in the presence of a catalyst. CTH can be generalized by an expression such as
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Journal of Chemical Education
DH+A-
AH+D
solvent CTH came into prominence following the publication of a very stimulating review by Breiger and Nestrick (2)in 1974. Its gain in popularity is evident from the appearance of an exhaustive account (3) in 1985. CTH is a good alternative to classical catalytic hydrogenation and chemical hydride transferring agents. The method has greater convenience because no elaborate equipment is required. Reactions are run with common glassware right a t the work bench. The solutions are simply stirred a t room temperature or gently refluxed. Transfer hydrogenations provide selectivity in reductions with proper choice of donor and catalyst. We previously described the hydrogenation of cinnamic acid to dihydrocinnamic acid and benzalacetophenone (chalcone)to dihydrochalcone using sodium or ammonium formate as the hydrogen donor and the commercially available palladium-charcoal (PdICj catalyst (4).We propose here a simple and pedagogically useful heterogeneous hydrogenation experiment that is safe. A tandem Wittig reaction1CTH sequence transforms 4-carboxybenzylbromide (2) to 4-ethylbenzoic acid (5). This is in essence a one-carbon homologation of 4-methylbenzoic acid (1)to 5. The first step is the preparation of 4-vinylbenzoic acid (4)startingfrom 1(eq 3).This sequence is a highly popular preparative organic chemistry experiment (51,and 4 is readily obtained in good yield. The second step is the reaction of 4 with ammonium formate in the presence of 10% PdC. The reaction converts the vinyl side chain to an ethyl group (eq 4).
mixture is refluxed on a water bath with frequent and gentle agitation to ensure thorough mixing of reactants. The refluxing is continued for 3 0 4 0 min. The reaction mixture is then filtered under aspirator pressure to remove the catalyst,' and solvent methanol is removed from the filtrate by simple distillation. Water is added to the residue, and the organic compound is extracted with ether (2 x 10 mL) using a separatory funnel. The ether extract is dried over anhydrous sodium sulfate (15-20 min). Evaporation of the solvent yields a white solid residue. (mp 111112 "C (95 mg, 94%)) I t is recrystallized from hot water if necessary. (mp = 112113 OC; lit. mp = 113.5% (8))
COOH 1
'H NMR: 1.2 (t, 3H, J = 6 Hz, CH3),2.6 (q, 2H, J = 6 Hz, CH2),7.1 (d, 2H, J = 8 Hz, H-3, H-5). 7.8 (d, 2H, J = 8 Hz, H-2, H-6), and 10.4 (lH, COOH). COOH 3
Y
COOH 4
(3)
Reduction Using Cyclohexene a s Hydrogen Donor Caution: Cyclohexene is inflammable and irritant. It should be kept away from spark sources and should be disposed of properly in a waste-solventcan.
A mixture of 4 (100 mg, 0.675 mmol), cyclohexene (2 mL, 18 mmol excess) and PdIC (10%)(100 mg, 9.3 x los g atoms) is taken in a 50-mL round bottom flask.2The flask is equipped with a water-cooled reflux condenser, and the mixtureis refluxed on a water bath for 90 min with occasional shaking of the apparatus. The reaction mixture is filtered under a s ~ i r a t o Dressure r to remove the catalvst.' Cyclohexene is iemovedfrom the filtrate by distillition under reduced uressure. The residue is boiled with ethanol-water (12&)and filtered hot to remove the insoluble polymer. The aqueous filtrate on cooling gave a white solid (mp = 111-112 OC) identical with 5 (40 mg, 39.4%). The polymeric material above melted between 185 and 200°C.
I
COOH
COOH
Remarks 5
(4)
The product 5 is obtained in near quantitative yield and high purity. Hvdrogen transfer with cvclohexene a s the donor also -" gave 5. The reaction requires refluxing of the mixture for more than 1h. The yield of 5 is about 40%. This low yield is due to competing polymerization of 4 (6).
-
Experimental Procedure IH NMR spectra were obtained using a 60-MHz Hitaichi R-600 FT-NMR spectrometer. Samples were run a s dilute solutions in CDC13. Tetramethyl silane was used a s a n internal standard. The chemical shifts are reported in ppm on S scale. Cyclohexene is prepared from cyclohexanol (7). Reagent-grade methanol is used a s a solvent.
4-VinylbenzoicAcid Compound 4 is prepared according to Broos et al. (5) and recrystallized from ethanol-water to a give white solid. (mp = 142-144 "C) 'H NMR: 5.3 (dd, lH, J = 11and 1.0 Hz), 5.7 (dd, lH, J = 18 and 1.0 Hz), 6.6 (four lines, 1H),,7.3(d, 2H, J = 9 Hz), 7.8 (d, 2H, J = 9 Hz)
I n CTH experiments the donor is generally used in excess (five to ten fold). Anv commerciallv available PdlC catalyst works well. The reaction is veryslow a t ambient temoerature hut is com~letein less than 1h a t the boiling of the solvent. he product is obtained in high and is readily identified by its melting point. Baeyer's test (neutral KMnOa test) clearly distinguishes 4 from 5. IR and NMR data can be used to explain the change before and after reduction. The reduction of 4 to 5 is eminently suitable a s a microscale hydrogenation experiment. Acknowledgment The authors thank CSlR (India, for the a w x d of J.R.F. to Shantanu De and Geetu Cambhir. Literature Cited 1. Plummer,B. J Chrm.Edue. 1988,66,516. 2. B"egor. G.; Nesfriek.T J. Chem. Re". 1974.74.567. 3. Johnatonto. A. W ; Wilby. A H.:Entulatle, I. 0. Chem Re". 1985.85, 129. d.. Knahnamn* H G: Sathvanarauan..S. Chemistrv Educ. i l n d d 1987.4.60 . . 5. Broos, R.;Tavemier, D:Anteuni~,M.J . Chrm Educ 1978,55,813. 6. Marvel. C. S.; Overbe~ger.C. G. J. A m r Chem. Soc.1845.67.2250. 7. Fumiss, B. S. ; Hannaford. A J.: Rogers, Y ; Smith, P. W. G.; Tatchell, A. R. V08db M b w k ofPmctico1 Orgonic Chemistry. 4th ed.: Langnan Group Ltd: London, ~~
.~~~~-~~~-
, a,=. " 2.39 A".", r""".
8. Emerson, W. 8.;Heyd, J. W: Lucas, V E.; Lyness,W . I.: Owens. G.R.: Shortridge, R. W J A m e r Cham. Soc.1347,69.1905.
4-EthylbenzoicAcid Reduction Using Ammonium Formate a s Hydrogen Donor (loom g 2 0.675 mmol) is dissolved in methanol (10 mL) in a 50-mL round-bottom flask. To this solution is added ammonium formate (500 mg, 8 mmol) folg atoms). The flask lowed by PdK (10%) (50 mg, 4.8 X is equipped with a water-cooled reflux condenser, and the
'Care is taken to keep the catalyst moist with solvent throughout the filtration to avoid ignition of the dry catalyst, though this is rarely observed in these experiments.The spent catalyst was stored under water in a wide.mouthboRle, 'The excess of cyclohexene used here serves as a hydrogen donor as well as a solvent. Benzene and toluene can be used as cosolvents (3).In our experiments benzene or toluene gave simllar results.
Volume 71 Number 11 November 1994
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