What is enantiomeric excess?

Enantiomeric excess (ee) is the percentage by which one enantiomer exceeds the other in a mixture; an ee of 90 percent means 95 percent of one enantiomer and 5 percent of the other, and it is the standard measure of how selective an asymmetric reaction is.

Why is catalytic asymmetric synthesis so prized?

A single chiral catalyst can convert large amounts of achiral material into one enantiomer, so chirality is multiplied catalytically rather than supplied in full stoichiometric amount, making the process more economical and atom-efficient.

Asymmetric Synthesis

Asymmetric synthesis produces one enantiomer of a chiral product preferentially, using chiral catalysts, auxiliaries, or reagents to control the new stereocenter.

Definition

Asymmetric synthesis is the preparation of a chiral compound as a single enantiomer (or in enantiomeric excess) by means of a chiral influence that differentiates the two possible stereochemical outcomes.

Scope

This topic covers the strategies of asymmetric synthesis — chiral auxiliaries, chiral reagents, and catalytic asymmetric reactions — including asymmetric hydrogenation, epoxidation, and organocatalysis, and the measurement of enantioselectivity by enantiomeric excess.

Core questions

How can a chiral catalyst or auxiliary bias a reaction toward one enantiomer?
Why is catalytic asymmetric synthesis especially valuable compared with using stoichiometric chiral reagents?
How is enantioselectivity quantified and improved?

Key theories

Catalytic asymmetric synthesis: A substoichiometric chiral catalyst differentiates the enantiomeric transition states, delivering high enantiomeric excess from achiral starting materials; landmark examples include asymmetric hydrogenation and epoxidation.
Chiral auxiliaries and organocatalysis: Chiral auxiliaries are temporarily attached to a substrate to direct stereochemistry and then removed, while small-molecule organocatalysts achieve enantiocontrol through well-defined transition states without metals.

Mechanisms

Enantioselectivity arises when a chiral environment makes the two diastereomeric transition states leading to the enantiomeric products unequal in energy. Catalysts such as chiral phosphine–metal complexes or amine organocatalysts present a defined chiral pocket that favors one approach of the substrate, while chiral auxiliaries convert enantioselection into a more controllable diastereoselection.

Clinical relevance

Because drug enantiomers can differ greatly in activity and safety, asymmetric synthesis is central to producing single-enantiomer pharmaceuticals efficiently; the field's importance was recognized by the 2001 Nobel Prize to Knowles, Noyori, and Sharpless and the 2021 Prize for asymmetric organocatalysis.

History

Knowles's asymmetric hydrogenation enabled the industrial synthesis of L-DOPA; Noyori and Sharpless extended catalytic asymmetric methods broadly, and the later rise of organocatalysis (List and MacMillan) added a metal-free dimension, marking decades of Nobel-recognized progress.

Key figures

William S. Knowles
Ryoji Noyori
K. Barry Sharpless
Benjamin List
David MacMillan

Seminal works

noyori2002
careysundberg2007b

Frequently asked questions

What is enantiomeric excess?: Enantiomeric excess (ee) is the percentage by which one enantiomer exceeds the other in a mixture; an ee of 90 percent means 95 percent of one enantiomer and 5 percent of the other, and it is the standard measure of how selective an asymmetric reaction is.
Why is catalytic asymmetric synthesis so prized?: A single chiral catalyst can convert large amounts of achiral material into one enantiomer, so chirality is multiplied catalytically rather than supplied in full stoichiometric amount, making the process more economical and atom-efficient.