Asymmetric synthesis is the branch of organic chemistry concerned with preparing one enantiomer or diastereomer of a compound in preference to others, starting from achiral or prochiral materials. In the usual definition, a reaction counts as asymmetric when it creates one or more new stereogenic units, such as a chiral centre, a chiral axis or a chiral plane, and when the possible stereoisomeric products form in unequal amounts. Because many biologically active molecules, including most drugs, are chiral and their mirror-image forms can behave very differently in living systems, the field underpins the pharmaceutical, agrochemical and fragrance industries.
Definition and terminology
The term was in use by the late nineteenth century. Chemists associate its popularisation with Emil Fischer, who in 1894 described the formation of unequal amounts of stereoisomers when a new asymmetric carbon is generated in a molecule that already contains chiral elements; he defined the idea largely through examples rather than a formal rule. Fischer's concept is now usually called asymmetric induction. A clear general definition took much longer to settle. Early definitions worked well for reactions that yield enantiomers, but they became ambiguous for diastereomeric products, partly because the meaning of "diastereomer" itself was not yet firmly fixed. Later revisions, including one codified by the International Union of Pure and Applied Chemistry (IUPAC), restored clarity by emphasising the formation of a new chirality element and unequal yields of stereoisomers.
Several related terms are used with overlapping meanings. Enantioselective synthesis usually refers to reactions that favour one enantiomer. Diastereoselective synthesis refers to reactions that favour one diastereomer, which may already contain stereocentres. Stereoselective synthesis is the broadest category and includes both. Asymmetric synthesis is therefore best understood as a subclass of stereoselective reactions in which a new chiral unit is generated during the transformation.
Principles
Asymmetric synthesis depends on the fact that the two possible mirror-image pathways from an achiral or prochiral substrate have identical energies unless some chiral influence is introduced. The chiral influence creates diastereomeric transition states, which differ in energy. The lower-energy pathway is followed preferentially, so one stereoisomer accumulates. The size of the energy difference between competing transition states, measured against the reaction temperature, determines how selective the reaction is. Small energy differences give low selectivity, and larger ones give high selectivity.
The degree of enantiopurity in a product is commonly expressed as the enantiomeric excess (ee). For a mixture containing amounts and of two enantiomers, it is defined as
A racemic mixture has an ee of zero, and a single enantiomer has an ee of 100 per cent. Reported ee values are sensitive to analytical methods, so they should be interpreted alongside the measurement technique used.
Strategies
Chemists conventionally group asymmetric methods into three main approaches, although the boundaries between them are not always sharp.
Chiral pool synthesis. A readily available natural chiral material, such as an amino acid, a carbohydrate, tartaric acid or camphor, is incorporated into the target. Its existing stereocentres are carried through the sequence. This approach is efficient when the natural starting material closely resembles the target, but it is restricted to the structures that nature supplies in convenient forms.
Chiral auxiliaries. A chiral, enantiomerically pure group is attached temporarily to the substrate. It shields one face of the reactive centre or organises the transition state so that subsequent bond formation favours one diastereomer. The auxiliary is then cleaved and, ideally, recovered. Early examples include the use of 8-phenylmenthol in the 1970s, and oxazolidinone auxiliaries introduced by David A. Evans in the early 1980s, which became widely used in aldol and alkylation reactions. Because the auxiliary is used in stoichiometric amounts, the method generates extra steps and waste, but it is reliable and predictable.
Asymmetric catalysis. A small amount of a chiral catalyst, often a metal complex with chiral ligands or a small organic molecule, directs the formation of the new stereocentre. The catalyst forms transient diastereomeric complexes with the substrate and then regenerates, so one molecule of catalyst can convert many molecules of substrate. Catalysis is applicable to a wider range of transformations than any other single method of enantioselective synthesis. Catalytic methods are the most attractive for industrial use, although they depend on the availability and cost of the catalyst metal or ligand.
Reagent-controlled methods, in which a stoichiometric chiral reagent transfers chirality to the substrate, and substrate-controlled methods, in which existing stereocentres in the substrate direct the outcome, are also recognised. Many practical syntheses combine several of these strategies in a single route.
History
Fischer's work in the 1890s established the principle of asymmetric induction. Through the twentieth century, chemists developed a range of stereoselective reactions based on chiral reagents and auxiliaries. Some of the early catalytic examples used chiral metal complexes to promote asymmetric cyclopropanation, and the first catalytic enantioselective processes were reported in the 1960s, including an electrochemical reduction using a chiral alkaloid in 1967.
A decisive step was the development of catalytic asymmetric hydrogenation in the 1970s. William S. Knowles, working at Monsanto, showed that a chiral phosphine attached to a rhodium complex could reduce a prochiral alkene to give a product with a modest excess of one enantiomer. The first such catalyst gave an enantiomeric excess of only about 15 per cent, but it demonstrated that catalytic asymmetric hydrogenation was possible. Knowles's team later applied the method to the industrial production of L-DOPA, a precursor of dopamine used in the treatment of Parkinson's disease, which became one of the first commercial asymmetric catalytic processes. Ryoji Noyori extended the hydrogenation work to ruthenium catalysts that were effective for ketones and other substrates. K. Barry Sharpless developed chiral catalysts for asymmetric epoxidation and dihydroxylation of alkenes in the 1980s.
In 2001, the Nobel Prize in Chemistry was awarded to Knowles, Noyori and Sharpless for the development of catalytic asymmetric synthesis, with one half to Knowles and Noyori for chirally catalysed hydrogenation and the other half to Sharpless for chirally catalysed oxidation. In 2021, the prize was awarded to Benjamin List and David W. C. MacMillan for asymmetric organocatalysis, in which small chiral organic molecules, rather than metal complexes, act as catalysts. Organocatalysis, which often exploits the enzyme-like activation of iminium and enamine intermediates, has become an important complementary strand of the field.
Methods and selected applications
Asymmetric catalysis now covers reactions such as hydrogenation, epoxidation, dihydroxylation, aldol and Michael additions, Diels–Alder cycloadditions, allylic alkylations, hydrosilylation, and carbonyl reduction. Oxazaborolidine catalysts of the type developed by Corey, Bakshi and Shibata are widely used for enantioselective reduction of ketones. Sharpless epoxidation, which uses a titanium complex with a chiral tartrate ester to epoxidise allylic alcohols, is a standard textbook example, and its products are used as building blocks for many natural products and drugs.
Asymmetric synthesis is central to pharmaceutical manufacturing. Because the two enantiomers of a drug may differ in pharmacological activity, toxicity or metabolism, regulators have encouraged the development of single-enantiomer drugs. A notable historical case is thalidomide, whose enantiomers were associated with different effects, a fact that contributed to the emphasis on stereochemically defined drugs. Asymmetric methods are also used in agrochemicals, flavours and fragrances, and in materials such as liquid crystals and chiral polymers.
Biological systems provide both models and benchmarks. Enzymes achieve very high selectivity through precisely shaped chiral active sites, and many chemists design catalysts with the aim of approaching that level of control.
Limitations and current research
Asymmetric synthesis has several recognised limitations. Stoichiometric auxiliaries add steps and generate waste. Many catalysts rely on precious or toxic metals such as rhodium, ruthenium and iridium, and their ligands can be costly to prepare. Catalysts that work well for one class of substrates may give poor selectivity for others, so a general catalyst for all substrates remains an aim rather than a reality. Achieving very high enantiomeric excess at low catalyst loading, under mild conditions and at industrial scale, is still challenging.
Current research addresses these problems in several directions. Chemists are developing catalysts based on abundant metals such as iron and copper, recyclable or immobilised catalysts that can be reused, and organocatalysts that avoid metals altogether. Photochemical and electrochemical methods are being combined with chiral catalysts to access new reaction types under renewable energy inputs. Computational modelling of transition states, together with data-driven approaches, is increasingly used to predict selectivity and guide ligand design. Enzymatic and chemoenzymatic routes offer an alternative route to chiral compounds and are often considered alongside chemical catalysis.
Another ongoing theme is the relationship between asymmetric synthesis and the wider goals of sustainable chemistry, since the field's efficiency and waste profile determine its environmental cost in pharmaceutical production.
References
- Definition of the term asymmetric synthesis—History and revision - Helmchen - 2023 - Chirality - Wiley Online Libraryonlinelibrary.wiley.com
- Asymmetric Catalysis: Recent Advances toward a More Sustainable Synthesispubs.acs.org
- Asymmetric Synthesis - an overviewsciencedirect.com
- Enantioselective synthesisen.wikipedia.org
- Chiral auxiliary — Grokipediagrokipedia.com
- Profile of William S. Knowlespnas.org
- Press release: The 2001 Nobel Prize in Chemistry - NobelPrize.orgnobelprize.org
- The Nobel Prize in Chemistry 2001 - Popular information - NobelPrize.orgnobelprize.org
- ASYMMETRIC CATALYSIS WINS: Chemistry Nobel honors Knowles, Noyori, Sharpless for chiral synthesespubs.acs.org