Organoaluminum reagents racemic

Chloral. Yamamoto has found that the organoaluminum reagent prepared from MeaAl and (/ )-(+)-3,3 -bis(triphenylsilyl)binaphthol catalyzes the ene reaction of chloral and pentafluorobenzaldehyde with 1,1-disubstituted alkenes at -78 °C, giving the expected ene adducts in 40-90% yield and 60-90% enantiomeric excess. Use of the sterically hindered chiral auxiliary is necessary. Low yields of racemic products are obtained with 3,3 -diphenylbinaphthol. 

Generation of Chiral Organoaluminum Reagent by Discrimination of the Racemates with Chiral Ketone In the previous section, we have demonstrated a new, chiral organoaluminum catalyst which is highly effective for the introduction of chirality into... 

The racemic organoaluminum reagent was prepared in dichloromethane. Sequential treatment of this catalyst with chiral ketone, diene and benzaldehyde at low temperature and stirring of the mixture for 3 h afford hetero-Diels-Alder adduct after acidic workup. The optical purity of the major cis adduct was determined by HPLC analysis after conversion to the MTPA ester. 

Among several terpene derived chiral ketones examined, 3-d-bromocamphor was found to be most satisfactory and 3d-bromocamphor is responsible for the generation of (S)-catalyst. Combination of the racemic catalyst and chiral ketone in a 1 1 ratio gave a better results, suggesting that decomplexation of one enantiomeric organoaluminum reagent and the chiral ketone is more readily facilitated by the addition of aldehyde, thereby allowing the enantioselective activation of the aldehyde for the asymmetric hetero-Diels-Alder reaction. 

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