Towards heterogeneous organocatalysis: chiral iminium cations supported on porous materials for enantioselective alkene epoxidation

Enantiomerically pure iminium cations have been supported on a microporous support (zeolite Y) and on a mesoporous support (Al-MCM-41). These materials are effective asymmetric catalysts for the epoxidation of a range of aryl alkenes, giving high conversions quickly and with enantioselectivities similar to or in some cases even higher than are achievable using the corresponding iminium tetraphenylborates under homogeneous conditions. The catalysts can be simply recycled by filtration and washing. The methodology is illustrated in the synthesis of two natural products, (−)-(3′S)-lomatin and (+)-(3′S,4′R)-trans-khellactone, showing the general efficacy of our approach.