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.
Read more about this topic: Electrophilic Aromatic Substitution
Famous quotes containing the words aromatic and/or substitution:
“Die of a rose in aromatic pain?”
—Alexander Pope (1688–1744)
“To play is nothing but the imitative substitution of a pleasurable, superfluous and voluntary action for a serious, necessary, imperative and difficult one. At the cradle of play as well as of artistic activity there stood leisure, tedium entailed by increased spiritual mobility, a horror vacui, the need of letting forms no longer imprisoned move freely, of filling empty time with sequences of notes, empty space with sequences of form.”
—Max J. Friedländer (1867–1958)