Co₂(CO)₈ Catalyzed Pauson-Khand Reaction under Microwave Irradiation [1]

Abstract

Microwave irradiation is used to accelerate Pauson-Khand reactions. The conditions for the Pauson-Khand reaction, catalytic in Co₂(CO)₈ under microwave irradiation, were optimized. It is possible to obtain various types of [2+2+1] cycloaddition products in 5 minutes without additional carbon monoxide.

microwave - Pauson-Khand reaction - catalysis - cobalt - cycloaddition

References

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9

It should be stated that low boiling solvents usually cannot be heated to 200 °C but in our case the desired temperature could be reached in all cases except dichloromethane, which reached a maximum at 140 °C.

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12

When conventional heating of the sealed vessel was provided under identical conditions the yield did not exceed 40% even after 4 h.

13

Typical Experimental Procedure: To a 10 mL glass vial 942 mg (10 mmol, 5 equiv) norbornene 1 and 137 mg (0.4 mmol, 0.2 equiv) Co₂(CO)₈ were added under an inert gas atmosphere in a glove box and sealed with a Teflon septum and an aluminum crimp top. After the addition of 2 mL toluene (freshly distilled from sodium), 220 µL (2 mmol) phenylacetylene 2 and finally 275 µL (2.4 mmol, 1.2 equiv) cyclohexylamine were added through the Teflon septum. The vessel was then heated to 100 °C under microwave irradiation using the Smith Synthesizer (monomode microwave cavity at 2.45 GHz; temperature control by automated adjustment of irradiation power in a range from
0 to 300 W). After 300 s the vial was cooled to r.t. by gas jet cooling. The reaction mixture was then subjected to a typical aqueous workup. The dried organic phase was then liberated from solvent and purified by flash chromatography on silica eluting with EtOAc/petroleum ether to give 363 mg (1.62 mmol, 81%) of exo-3.

All spectral data were in full accordance with those reported in literature:

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14d

Entry 2: ¹H (CDCl₃, 400 MHz): δ = 7.00 (d, J = 2.4 Hz, 1 H), 2.54 (sept., J = 6.6 Hz, 1 H), 2.48 (m, 1 H), 2.30 (m, 1 H), 2.08 (m, 2 H), 1.58 (m, 1 H), 1.51 (m, 1 H), 1.21 (m, 2 H), 1.01 (d, J = 6.6 Hz, 6 H), 0.86 (m, 2 H). ¹³C (CDCl₃, 100 MHz): δ = 210.7, 156.5, 155.4, 54.1, 47.7, 38.9, 37.9, 30.8, 29.0, 28.3, 24.5, 21.5, 21.2.