Intermolecular Cobalt-Mediated Pauson-Khand Reactions in Ionic Liquids

Abstract

Intermolecular Pauson-Khand reactions of strained al­kenes 1a-d with alkynes 2, 4, 6 and Co₂(CO)₈ were performed in [bmim]PF₆ either thermally or in the presence of NMO. For comparison thermal reactions were done in toluene and NMO-promoted reactions in CH₂Cl_2. The results show that the ionic liquid is a suitable substitute for CH₂Cl₂. Yields of the NMO-promoted reaction could be improved by using a two-phase system [bmim]PF₆/methylcyclohexane.

Keywords

Pauson-Khand reaction - ionic liquids - amine N-oxides

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1

New address: Institut für Organische Chemie, Universität Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart, Germany

7

Due to HF elimination [bmim]PF₆ should not be heated above 50 °C: Seddon K. R., personal communication. See also ref. [2c]

8

Further extraction of the ionic liquid layer with CHCl₃ (3 × 20 mL) resulted in a slight improvement of the yields, particularly for ester-substituted cyclopentenone 3c. However, when CHCl₃ was directly used for the extraction procedure without previous treatment with Et₂O, separation of the layers was very tedious.