Enantioselective synthesis: chiral auxiliaries, asymmetric catalysis, and ee measurement
Anchor (Master): Ojima — Catalytic Asymmetric Synthesis, 3e (2010)
Intuition Beginner
A chiral molecule and its mirror image are called enantiomers. They have identical physical properties in most ways — same melting point, same solubility in ordinary solvents. But inside a living organism, which is itself built from chiral molecules (L-amino acids, D-sugars), the two enantiomers can behave completely differently. One might cure a disease while the other does nothing — or causes harm.
The thalidomide tragedy made this point devastatingly clear. In the late 1950s the drug was prescribed as a racemix mixture (equal parts of both enantiomers) to treat morning sickness. One enantiomer had the desired sedative effect; the other turned out to be teratogenic, causing severe birth defects. This case drove the pharmaceutical industry to demand methods that produce a single enantiomer.
Enantiomeric excess (ee) is the standard way to quantify how pure a sample is with respect to chirality. A racemic mixture has 0% ee. A sample containing only one enantiomer has 100% ee. The formula is ee = (major − minor) / (major + minor) × 100%, where "major" and "minor" are the amounts of the two enantiomers.
Two main strategies exist for controlling which enantiomer forms during a reaction. A chiral auxiliary is a temporary group attached to the starting material that directs the reaction toward one face of a planar intermediate. After the key bond-forming step, the auxiliary is removed and discarded. Asymmetric catalysis uses a chiral catalyst that remains unchanged and turns over many substrate molecules, directing each one toward the same enantiomer. This is more atom-economical because a small amount of catalyst can produce a large amount of product.
Visual Beginner
Schematic comparison of chiral auxiliary approach (left) versus asymmetric catalytic approach (right). In the auxiliary strategy the directing group is stoichiometric and consumed. In the catalytic strategy the chiral environment is provided by a sub-stoichiometric metal–ligand complex that turns over many times.
Worked example Beginner
A reaction produces (R)-2-butanol and (S)-2-butanol in a 9:1 ratio. What is the ee?
Step 1. Identify the major and minor enantiomer amounts.
- major = 9, minor = 1.
Step 2. Apply the ee formula.
- ee = (9 − 1) / (9 + 1) × 100% = 8 / 10 × 100% = 80%.
Step 3. Interpret the result. An ee of 80% means the sample is enriched in the (R)-enantiomer but still contains 10% of the (S)-enantiomer (since 90% is R and 10% is S, the excess of R over S is 80%).
Check your understanding Beginner
Formal definition Intermediate+
Enantiomeric excess. For a sample containing amounts and of the two enantiomers of a chiral compound, the enantiomeric excess is defined as
and is conventionally reported as a percentage. The sign is assigned according to which enantiomer is in excess.
Enantioselectivity (selectivity factor). For a reaction producing enantiomers R and S, the ratio of rate constants is related to the energy difference between the two diastereomeric transition states:
A of just 1.4 kcal mol at 25 °C corresponds to an ee of approximately 90%. This small energy gap — roughly the strength of a single hydrogen bond — is what chiral auxiliaries and asymmetric catalysts must create between competing transition states.
Key mechanism Intermediate+
Sharpless asymmetric epoxidation
The Sharpless epoxidation converts allylic alcohols to epoxy alcohols with high enantioselectivity using a combination of titanium(IV) isopropoxide, diethyl tartrate (DET), and tert-butyl hydroperoxide (TBHP). The chirality of the product is determined by which enantiomer of tartrate is used: (R,R)-DET delivers oxygen to one face of the alkene, while (S,S)-DET delivers it to the opposite face.
The active species is a Ti–tartrate complex that coordinates both the allylic alcohol substrate and the hydroperoxide oxidant in a well-defined chiral pocket. The oxygen transfer occurs through a cyclic transition state that strongly favors approach from one face. The mnemonic for predicting the outcome: draw the allylic alcohol with the OH in the lower-right corner; if (D)-(−)-DET is used, the epoxide forms from the top face.
Selectivities of 90–99% ee are typical for E-allylic alcohols. The method is robust, tolerates many functional groups, and the reagents are inexpensive.
Asymmetric hydrogenation (Knowles and Noyori)
William Knowles demonstrated that a chiral phosphine–rhodium catalyst could hydrogenate a prochiral enamide to give L-DOPA with high ee. The key insight was that a chiral ligand on the metal center creates a dissymmetric environment, causing one prochiral face of the substrate to bind preferentially. The catalytic cycle involves oxidative addition of H₂ to Rh(I), substrate coordination, migratory insertion, and reductive elimination — the chiral ligand biases the substrate orientation throughout.
Ryoji Noyori extended this concept with BINAP (2,2′-bis(diphenylphosphino)-1,1′-binaphthyl), a C₂-symmetric axially chiral ligand. BINAP–Ru catalysts achieve ee values above 99% for a wide range of substrates, including functionalized and unfunctionalized olefins and ketones. The rigidity of the binaphthyl backbone and the fixed dihedral angle between the two naphthyl rings are critical to stereocontrol.
Exercises Intermediate+
Connections Master
Enantioselective synthesis intersects several areas of modern chemistry and chemical engineering. In process chemistry, the economics of asymmetric catalysis versus resolution determine which route is chosen for commercial-scale production. The sitagliptin synthesis (Merck) exemplifies this: an asymmetric hydrogenation using a Rh–ferrocenyl phosphine catalyst replaced a resolution step, improving yield and reducing waste.
Dynamic kinetic resolution (DKR) combines a racemization catalyst with an enantioselective transformation, allowing theoretical yields of 100% from a racemic starting material. This is relevant to enzymatic processes where lipases or transaminases provide the enantioselectivity while a metal complex or base catalyzes racemization of the slow-reacting enantiomer.
The chiral pool approach — using naturally occurring chiral building blocks (amino acids, sugars, terpenes) as starting materials — remains important in total synthesis. It avoids the need for external stereocontrol when the starting material already contains the required stereocenters.
In pharmaceutical regulation, the FDA and EMA require justification for producing either a single enantiomer or a racemate, including pharmacokinetic and pharmacodynamic data for both enantiomers. This regulatory environment has driven demand for robust, scalable asymmetric methods.
Computational methods (DFT transition-state calculations) now guide catalyst design by predicting values before synthesis, reducing the need for empirical ligand screening.
Historical notes Master
The field of asymmetric synthesis emerged from the convergence of stereochemistry, coordination chemistry, and catalysis. Pasteur's manual separation of sodium ammonium tartrate crystals (1848) was the first chiral resolution, but it was not until the 1960s that chemists began designing reactions that inherently favored one enantiomer.
William Knowles at Monsanto developed the first practical asymmetric hydrogenation catalyst (1972), using a chiral monodentate phosphine ligand (CAMP) to hydrogenate a prochiral enamide en route to L-DOPA with 95% ee. This was the first industrial application of asymmetric catalysis. Knowles shared the 2001 Nobel Prize in Chemistry with Ryoji Noyori and Barry Sharpless.
Noyori's BINAP ligands (1980s) dramatically expanded substrate scope and enantioselectivity. The rigidity and C₂ symmetry of BINAP provided a design principle that influenced thousands of subsequent ligand families.
Sharpless developed both the asymmetric epoxidation (1980) and asymmetric dihydroxylation (1988) reactions, providing general, predictable methods for enantioselective oxidation of olefins. The empirical mnemonic for predicting stereochemical outcome made these methods accessible to non-specialists.
Henri Kagan's DIOP ligand (1971), a chiral diphosphine derived from tartaric acid, was an early demonstration that bidentate chiral ligands could achieve practical levels of enantioselectivity, and it influenced the development of BINAP and related systems.
The Merck sitagliptin process (2009) demonstrated that asymmetric hydrogenation could be applied to demanding substrates (unprotected enamines) at manufacturing scale, achieving >99% ee with low catalyst loading — a benchmark for modern process chemistry.
Bibliography Master
Knowles, W. S. "Asymmetric Hydrogenations." Angew. Chem. Int. Ed. 2002, 41, 1998–2007. (Nobel Lecture)
Noyori, R. Asymmetric Catalysis in Organic Synthesis; Wiley: New York, 1994.
Sharpless, K. B.; Woodard, S. S.; Finn, M. G. "On the Mechanism of the Sharpless Asymmetric Epoxidation." Pure Appl. Chem. 1983, 55, 1823–1836.
Ojima, I., Ed. Catalytic Asymmetric Synthesis, 3rd ed.; Wiley: Hoboken, 2010.
Evans, D. A.; Helmchen, G.; Rüping, M. "Chiral Auxiliaries in Organic Synthesis." In Asymmetric Synthesis — The Essentials; Christmann, M., Bräse, S., Eds.; Wiley-VCH: Weinheim, 2007.
Robinson, D. I. "Control of Impurities: Regulatory and Scientific Considerations." Org. Process Res. Dev. 2010, 14, 946–959.
Savile, C. K. et al. "Biocatalytic Asymmetric Synthesis of Chiral Amines from Ketones Applied to Sitagliptin Manufacture." Science 2010, 329, 305–309.