Iridium Complex-Catalyzed Intramolecular Ene-Type Reaction of 1,6-Enynes Accelerated in Ionic Liquid

Abstract

Iridium complex catalyzes an intramolecular ene-type reaction of 1,6-enynes to give cyclic 1,4-dienes. The reaction proceeds more efficiently in an imidazolium salt than in toluene and the ionic liquid can be reused.

iridium - cycloisomerization - enynes - dienes - ionic liquid

References

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<A NAME="RU25104ST-11">11</A>

Using 1a- E as an enyne in toluene at 90 °C, no reaction proceeded by [RhCl(cod)]₂ and many unidentified products were obtained by PtCl₂.

<A NAME="RU25104ST-12">12</A>

The higher reactivity of the E-isomer than the Z one is probably because the β-hydrogen is closer to the metal center in the metallacycle derived from the E-isomer. [4] However, the different selectivity of Ir- an Rh-catalyst could not yet be explained.

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<A NAME="RU25104ST-14">14</A>

Typical Experimental Procedure (Table 2, entry 3): Under an atmosphere of argon, [IrCl(cod)]₂ (10.1 mg, 1.5 × 10^-2 mmol, 5 mol%) and allyl propargyl ether 1a- E (54.7 mg, 0.300 mmol) were placed in a flask. After the addition of [BMIM]BF₄ (0.5 mL), the mixture was stirred at 60 °C (bath temperature) for 1 h. The resulting mixture was extracted with Et₂O, then the combined ether phase was evaporated to dryness, and the obtained crude products were purified by thin layer chromatography to give pure 2a (52.0 mg, 0.285 mmol, 95%).

<A NAME="RU25104ST-15">15</A>

In [BMIM]BF₄ at 60 °C, it took 3 h to consume 1a- Z and 2a was obtained in 43% yield.

<A NAME="RU25104ST-16">16</A> No complexation from [IrCl(cod)]₂ and [BMIM]BF₄ could be observed by NMR measurement. However, the polarity of the solvent is not an answer because polar solvents like 1,2-dimethoxyethane and N,N-dimethlyformamide gave poorer results than toluene, therefore the role of imidazolium salts in the present catalytic reaction is unclear. See: Park S. Kazlauskas RJ. J. Org. Chem.  2001,  66:  8395 

<A NAME="RU25104ST-17">17</A> [BMIM]BF₄ was washed with EtOAc and passed through an activated alumina column, then it could be reused

<A NAME="RU25104ST-18">18</A>

Under the same reaction conditions, crotylpropargylmalonate did not give an ene-type product.

Reviews:

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For example, copper-free Sonogashira coupling could be realized in ionic liquid:

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