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Transfer hydrogenation of benzpthalide with limonene/palladium on carbon; 2-(2-phenylethyl) benzoic acid

SyntheticPage 535
DOI: 10.1039/SP535
Submitted Feb 08, 2012, published Feb 08, 2012
Ramesha Ramakrishna (

			Reaction Scheme: Transfer hydrogenation of benzpthalide with limonene/palladium on carbon

Chemicals Used

Benzpthalide, purity Min 98% bright yellow solid
Limonene (racemic, known as dipentene) commercial grade 
10% Pd-C purchased from ArroraMathey, Culcutta, India
Acetone, commercial Grade 


To a 1-L four necked flask carrying a mechanical stirrer, Dean-stark apparatus (to remove traces of moisture in limonene), nitrogen inlet, thermometer pocket and an addition flask, 10% Pd-C (20 g) was charged under nitrogen atmosphere followed by benzphthalide (200 g, 0.9 mol). Under a slow stream of nitrogen, the temperature was raised to 170–180 oC. At about 100 oC, benzpthalide melts and stirring is possible. Once the temperature reaches 175 oC limonene (300 g, 2.2 mol) was added slowly over a period of 1 h (during addition, there is gentle reflux due to exothermic reaction). After the complete addition of limonene, reflux was continued for 2 h. The reaction mass was cooled to 50–60 oC during which the reaction mass solidifies. The solid was dissolved in acetone (1000 mL) and the reaction mixture was filtered under nitrogen. The filter cake containing the Pd-C catalyst was washed with acetone (50 mL). The combined filtrates were mixed and concentrated under reduced pressure. The residue was cooled to 10 oC, filtered and washed with a cold mixture of toluene: petroleum ether (3:7, 300mL) to gave 2 as a white crystalline solid. 175 g(86%). Which is bright white solid and practically pure by NMR.

Author's Comments

10% Pd-C is pyrophoric - It should be handled wet/ under nitrogen
The reaction should be carried out under nitrogen atmosphere.
The side product in the process is para methyl iso propyl benzene.  2-(2-phenylethyl)benzoic acid is sparingly soluble in para methyl iso propyl benzene and therefore product crystalizes out.
Mechanical stirring should be started once inside temperature reaches about 100 degree centigrade.


White solid. Mp 130 – 132 oC , .

IR (KBr): 1685, 3155 cm-1 ,1H NMR (200 MHz, CDCl3): δ 2.94 (t, J = 8.0 Hz, 2H), 3.35 (t, J = 8.0 Hz, 2H), 7.15 – 7.51(m, 8H), 8.09(d, J = 9.0 Hz, 1H); 13C NMR (100 MHz, CDCl3): δ 37.6, 38.6, 126.4, 126.7, 128.6, 128.8, 129.0,132.0, 132.3, 133.5, 142.4, 145.3, 173.9.


Lead Reference

A. R. Ramesha, Anjan K. Roy, Synthetic Communications, 2001, 31(16), 2419–2422

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Keywords: alkenes, aromatics/arenes, carboxylic acids, hydrogenation, Palladium on carbon, reduction, Transfer hydrogenation

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