Asymmetric Electrophilic Aromatic Substitution
Electrophilic aromatic substitutions with prochiral carbon electrophiles have been adapted for asymmetric synthesis by switching to chiral Lewis acid catalysts especially in Friedel-Crafts type reactions. An early example concerns the addition of chloral to phenols catalyzed by aluminium chloride modified with (-)-menthol. A glyoxylate compound has been added to N,N-dimethylaniline with a chiral bisoxazoline ligand - copper(II) triflate catalyst system also in a Friedel-Crafts hydroxyalkylation :
In another alkylation N-methylpryrrole reacts with crotonaldehyde catalyzed by trifluoroacetic acid modified with a chiral imidazolidinone :
Indole reacts with an enamide catalyzed by a chiral BINOL derived phosphoric acid :
In all these reactions the chiral catalyst load is between 10 to 20% and a new chiral carbon center is formed with 80-90% ee.
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