Catalytic transfer hydrogenation of cinnamic acid in alkaline aqueous medium using PdCl2/HCOOH/NaOH

Catalytic transfer hydrogenation can be an effective method for reducing a variety of organic substrates, which avoids some of the technical and safety concerns associated with using compressed hydrogen gas. The heterogeneous mixture of Pd-C and ammonium formate in alcohol solvent has proven to be a particularly effective reagent in organic synthesis. Replacing organic solvents with water offers economic advantages, improves safety, reduces the environmental impact of the waste stream, and in combination with the development of catalytic processes offers great opportunities for `green' chemistry.' We have studied transfer hydrogenation in alkaline aqueous conditions as part of a broad investigation of noble metal-catalyzed reactions of organic substrates in water. Herein, we report a convenient, effective method for reducing unsaturated carboxylic acids using the nonpyrophoric catalyst palladium(II)chloride, formic acid, and sodium hydroxide base in water.

The reduction of cinnamic acid was carried out using 10 mol% PdCl2 and 4 equivalents of hydrogen donor, in 2.5 M aqueous NaOH solution, heated to 65°C for 16 h. A series of potential hydrogen donors was investigated, from which formic acid was the most effective (yield of hydrocinnamic acid was 98%), followed in decreasing order of effectiveness by: glyoxylic acid (80%), gluconic acid (35%), N-(2-hydroxyethyl)ethylenediaminetriacetic acid (20%), glycolic acid (20%), and formaldehyde (5%). Formaldehyde is known to produce hydrogen under alkaline aqueous conditions, but was ineffective for the reduction under these conditions. The reaction was also successful using varying hydroxide concentrations ranging from 0.5 to 5 M; however, no reduction was evident when 2.5 M Na2CO3 was used as base.

In summary, PdCl2 is an effective catalyst for transfer hydrogenations using formic acid in alkaline aqueous media. This procedure is advantageous because no organic solvent is required, the reagents are inexpensive, and the catalyst is nonpyrophoric. These reaction conditions should be suitable for the reduction of a wide variety of organic substrates, and offers an economical, safe, and environmentally benign alternative to available procedures.

Typical procedure: To a stirred solution of cinnamic acid (50 mg, 0.33 mmol) in 2.5 M aqueous NaOH (4 mL) was added PdCl2 (6.0 mg, 0.025 mmol). Formic acid (50 uL, 1.3 mmol) was added dropwise, then heated to 65°C for 16 h. The reaction mixture was neutralized with 2 M HCI, extracted with diethyl ether (2x5 mL), washed with water (10 mL), and dried (Na2SO4). The product was then purified by a short column of silica gel eluted with 25% ethyl acetate in hexanes. Removal of solvent in vacuo gave hydrocinnamic acid (50 mg, 98% yield).