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.
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