Simple and Efficient Microwave-Assisted Hydrogenation Reactions at ­Moderate Temperature and Pressure

Abstract

A generally applicable method for the introduction of gaseous hydrogen into a sealed reaction system to perform hydrogenation reactions under microwave irradiation has been developed. Several different types of substrates are easily reduced in short reaction times with moderate temperatures between 80 °C and 100 °C with 50 psi of hydrogen. The use of simultaneous cooling is also applied to the hydrogenation of more difficult substrates.

Key words

hydrogenations - microwave - palladium - reductions - ­simultaneous cooling

References and Notes

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Experimental Set-up.
All reactions were performed with a CEM Discover single mode microwave reactor equipped with a 300 W power source. A 10 mL fiber optic accessory was equipped with a gas inlet to allow introduction of hydrogen gas to the reaction vessel, and each of the reactions was performed in a CEM 10 mL microwave reaction vial. All temperature measurements were performed with a fiber optic probe, and 2 mL of solvent was used for each reaction to ensure ample submersion of the fiber optic probe.
General Procedure.
To a solution of trans,trans-1,4-diphenyl-1,3-butadiene (1, 103 mg, 0.500 mmol) in 2.0 mL of EtOAc was added Pd/C (10 wt%, 5 mg, 0.005 mmol). The reaction vessel was purged three times with hydrogen, charged to 50 psi, and then closed off to the source of hydrogen. The reaction was heated under microwave irradiation to 80 °C with 100 W of power and held for 5 min. Upon cooling to ambient temperature, the reaction mixture was filtered through Celite^® and condensed to give 105 mg (>99%) of 1,4-diphenylbutane. ¹H NMR (400 MHz, CDCl₃): δ = 7.23-7.27 (m, 4 H), 7.14-7.17 (m, 6 H), 2.62 (t, J = 6.6 Hz, 4 H), 1.65 (dt, J = 7.2, 3.8 Hz, 4 H). ¹³C NMR (100 MHz, CDCl₃): δ = 142.66, 128.53, 128.38, 125.77, 35.93, 31.20. Characterization data is consistent with that of commercially available material.

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Isolated yields were difficult due to the low volatility of the products.