Chiral reagents

 A chiral auxiliary is a chiral molecule attached to the starting material of the reaction; diastereoselective reactions of compounds from the chiral pool are likewise controlled by chirality in the starting material, and we call this type of stereocontrol substrate control. But is it also possible for enantioselective reactions to be controlled by chiral reagents. For example, a typical achiral base will just remove a proton from a substrate, but an enantiomerically pure chiral base can select one of two enantiotopic protons and form a product enantioselectively. The product of course has to be chiral, so we can’t use a chiral base to make planar enolates enantioselectively, for example, but we can a chiral base to make chiral organolithiums. Alkyllithiums are sufficiently strong as bases to remove the protons adjacent to the nitrogen atom of N-Boc pyrrolidine, shown in the margin. The product of deprotonation is an organo lithium which is a chiral molecule: the lithium-bearing carbon is chiral. Alkyllithiums can be turned into chiral bases in quite a simple way—by complexation with a chiral ligand. A widely used example is the tetracyclic diamine (–)-sparteine. Sparteine’s structure looks complex, but it is a relatively widely available natural product which folds around the lithium atom of an alkyllithium and places the base in a chiral environment.

This chiral base can now choose to remove from the pyrrolidine substrate just one of the enantiotopic protons adjacent to nitrogen, and form a chiral, enantiomerically enriched organolithium. The stereochemistry of the organolithium is preserved through its reactions with electrophiles such as the ketone shown here.

One of the reasons this reaction is so useful is that the products happen to be derivatives of the less readily available (R)-proline. But, as with chiral auxiliaries, if you use a chiral reagent you need a full equivalent of the source of enantiomeric purity (here, (–)-sparteine) which can get prohibitively expensive on a large scale. It is for this reason that the real pinnacles of achievement in asymmetric synthesis make use of asymmetric catalysis, which we turn to next.

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مواضيع ذات صلة

Catalytic asymmetric reduction of ketones

Enantiomeric excess

Alkylation of enolates

Resolution can be used to separate enantiomers

Nature is asymmetric

Palladium catalysis in the total synthesis of a natural alkaloid

Alcohols and amines as intramolecular nucleophiles

Oxypalladation and the Wacker oxidation

Alkenes coordinated to palladium (II) are attacked by nucleophiles

Palladium-catalysed amination of aromatic rings

Intramolecular alkylations make rings

Vinyl epoxides provide their own alkoxide base