Iridium Complex-Catalyzed Enantioselective Intramolecular [4+2] Cycloaddition of Dieneynes

Abstract

A catalytic and highly enantioselective intramolecular [4+2] cycloaddition of dieneynes was achieved by use of chiral iridium complex prepared in situ from [IrCl(cod)]₂ and BDPP [2,4-bis(diphenylphosphino)pentane]. The reaction proceeded in refluxed tert-butyl acetate to give chiral cyclohexa-1,4-dienes in very high enantiomeric excess.

Key words

catalysis - chiral auxiliaries - cycloadditions - Diels-Alder reactions - Iridium

References

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11

A typical experimental procedure is as follows (Table [2] , entry 9): Stirring of (S,S)-BDPP (17.6 mg, 0.04 mmol) and [IrCl(cod)]₂ (13.4 mg, 0.02 mmol) in t-BuOAc (1 mL) at
40 °C under argon gave a light yellow solution. After addition of a t-BuOAc solution (1 mL) of dieneyne 1a (40.7 mg, 0.205 mmol), the solution was refluxed for 2 h. Solvent was removed under a reduced pressure, then the resulting crude products were purified by thin layer chromatography. Pure 2a was obtained (26.0 mg, 0.131 mmol, 64% yield) and the ee was determined to be 95% by HPLC analysis using a chiral column.

12

Aromatized products 3a-d were also generated (entries 9-12).