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.
Key words
aldehydes - allylations - asymmetric catalysis - titanium - Lewis acid assisted chiral Lewis acid catalysts
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8 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.
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9 A referee has argued that an ionic complex made up of borane-iso propoxide ate complex and cationic titanium(IV) triiso propoxide could be formed and act as catalyst.
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. 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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