Novel Lewis Acid-Assisted Chiral Lewis Acid (LLA) System: Development of Boroxin-Ti-BINOL-Catalyzed Asymmetric Allylation of Aldehydes

Abstract

The formation of hetero multimetallic complex having B-O-Ti bond could increase the Lewis acidity of titanium. A novel LLA system made up of Ti-BINOL and 4-(trifluoromethyl)phenylboroxin dramatically accelerated the allylation reaction of aldehyde with high enantioselectivity.

aldehydes - allylations - asymmetric catalysis - titanium - Lewis acid assisted chiral Lewis acid catalysts

References

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<A NAME="RY03104ST-8">8</A>

Keck has also reported that the use of Ti(Oi-Pr)₄ and BINOL (1:1) catalyze the allylation of benzaldehyde very slowly at -20 °C even in the presence of activated molecular sieves [10 mol% (Ti), 70 h, 88% yield, 95% ee]. See ref. [7c]

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A referee has argued that an ionic complex made up of borane-isopropoxide ate complex and cationic titanium(IV) triisopropoxide could be formed and act as catalyst.

<A NAME="RY03104ST-10">10</A>

Maruoka has reported bis[binaphthoxy(isopropoxy)tita-nium] oxide having Ti-O-Ti bond. In this paper, they assumed this type activation system (LLA) as one of the possible intermediates for the reaction. See ref. [7s] . Another possibility in our system is that the monomer of aryl boroxin would coordinate to an oxygen atom on Ti-BINOL complex and enhance the Lewis acidity of titanium in the same fashion, which seem to be also an example of LLA activation.

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