Application of the chiral poisoning strategy: Enantioselective Diels-Alder catalysis with a racemic Ru/BINAP-monoxide Lewis acid

A chiral poisoning strategy has been applied to the catalyst system for the asymmetric Diels-Alder reaction between methacrolein and cyclopentadiene to produce enantioenriched exo-2-methylbicyclo[2.2.1]hept-5-ene-2-carboxaldehyde. When 1 was mixed with AgSbF6, the dicationic Lewis acid [(+/-)-p-cymeneRu-(H2O)(BINPO)](SbF6)(2) (+/-2) was generated. The additions of a number of chiral poisons (P* enantiopure ligand) were found to deactivate one of the enantiomers of the catalyst with varied levels of selectivity. The most effective chiral poison was either L-proline or L-prolinamide. In catalytic trials with L-proline, an ee of up to 54% was observed. In the case where a stoichiometric amount of (+/-)-Ru/methacrolein reacted in the presence of L-prolinamide, the DA product was obtained with ee = 60% (S), de = 96% (exo), and 92% conversion.