In chemistry, an enantiomer, also known as an optical isomer, antipode, or optical antipode, is one of a pair of molecular entities which are mirror images of each other and non-superposable.
In chemistry, a molecule or ion is called chiral if it cannot be superposed on its mirror image by any combination of rotations, translations, and some conformational changes. This geometric property is called chirality. The terms are derived from Ancient Greek χείρ (cheir) 'hand'; which is the canonical example of an object with this property.
Enantioselective synthesis, also called asymmetric synthesis, is a form of chemical synthesis. It is defined by IUPAC as "a chemical reaction in which one or more new elements of chirality are formed in a substrate molecule and which produces the stereoisomeric products in unequal amounts."
Tröger's base is a white solid tetracyclic organic compound. Its chemical formula is (CH
3C
6H
3NCH
2)
2CH
2. Tröger's base and its analogs are soluble in various organic solvents and strong acidic aqueous solutions due to their protonation. It is named after Julius Tröger, who first synthesized it in 1887.
Homochirality is a uniformity of chirality, or handedness. Objects are chiral when they cannot be superposed on their mirror images. For example, the left and right hands of a human are approximately mirror images of each other but are not their own mirror images, so they are chiral. In biology, 19 of the 20 natural amino acids are homochiral, being L-chiral (left-handed), while sugars are D-chiral (right-handed). Homochirality can also refer to enantiopure substances in which all the constituents are the same enantiomer, but some sources discourage this use of the term.
In organic chemistry, helicenes are ortho-condensed polycyclic aromatic compounds in which benzene rings or other aromatics are angularly annulated to give helically-shaped chiral molecules. The chemistry of helicenes has attracted continuing attention because of their unique structural, spectral, and optical features.
Sumanene is a polycyclic aromatic hydrocarbon and of scientific interest because the molecule can be considered a fragment of buckminsterfullerene. Suman means "sunflower" in both Hindi and Sanskrit. The core of the arene is a benzene ring and the periphery consists of alternating benzene rings (3) and cyclopentadiene rings (3). Unlike fullerene, sumanene has benzyl positions which are available for organic reactions.
Atropisomers are stereoisomers arising because of hindered rotation about a single bond, where energy differences due to steric strain or other contributors create a barrier to rotation that is high enough to allow for isolation of individual rotamers. They occur naturally and are of occasional importance in pharmaceutical design. When the substituents are achiral, these conformers are enantiomers (atropoenantiomers), showing axial chirality; otherwise they are diastereomers (atropodiastereomers).
In stereochemistry, a chiral auxiliary is a stereogenic group or unit that is temporarily incorporated into an organic compound in order to control the stereochemical outcome of the synthesis. The chirality present in the auxiliary can bias the stereoselectivity of one or more subsequent reactions. The auxiliary can then be typically recovered for future use.
In organic chemistry, kinetic resolution is a means of differentiating two enantiomers in a racemic mixture. In kinetic resolution, two enantiomers react with different reaction rates in a chemical reaction with a chiral catalyst or reagent, resulting in an enantioenriched sample of the less reactive enantiomer. As opposed to chiral resolution, kinetic resolution does not rely on different physical properties of diastereomeric products, but rather on the different chemical properties of the racemic starting materials. The enantiomeric excess (ee) of the unreacted starting material continually rises as more product is formed, reaching 100% just before full completion of the reaction. Kinetic resolution relies upon differences in reactivity between enantiomers or enantiomeric complexes.
In analytical chemistry, a chiral derivatizing agent (CDA), also known as a chiral resolving reagent, is a derivatization reagent that is a chiral auxiliary used to convert a mixture of enantiomers into diastereomers in order to analyze the quantities of each enantiomer present and determine the optical purity of a sample. Analysis can be conducted by spectroscopy or by chromatography. Some analytical techniques such as HPLC and NMR, in their most commons forms, cannot distinguish enantiomers within a sample, but can distinguish diastereomers. Therefore, converting a mixture of enantiomers to a corresponding mixture of diastereomers can allow analysis. The use of chiral derivatizing agents has declined with the popularization of chiral HPLC. Besides analysis, chiral derivatization is also used for chiral resolution, the actual physical separation of the enantiomers.
In chemistry, absolute configuration refers to the spatial arrangement of atoms within a molecular entity that is chiral, and its resultant stereochemical description. Absolute configuration is typically relevant in organic molecules where carbon is bonded to four different substituents. This type of construction creates two possible enantiomers. Absolute configuration uses a set of rules to describe the relative positions of each bond around the chiral center atom. The most common labeling method uses the descriptors R or S and is based on the Cahn–Ingold–Prelog priority rules. R and S refer to rectus and sinister, Latin for right and left, respectively.
Diastereomeric recrystallisation is a method of chiral resolution of enantiomers from a racemic mixture. It differs from asymmetric synthesis, which aims to produce a single enantiomer from the beginning, in that diastereomeric recrystallisation separates two enantiomers that have already mixed into a single solution. The strategy of diastereomeric recrystallisation involves two steps. The first step is to convert the enantiomers into diastereomers by way of a chemical reaction. A mixture of enantiomers may contain two isomers of a molecule with one chiral center. After adding a second chiral center in a determined location, the two isomers are still different, but they are no longer mirror images of each other; rather, they become diastereomers.
In chemistry, inherent chirality is a property of asymmetry in molecules arising, not from a stereogenic or chiral center, but from a twisting of the molecule in 3-D space. The term was first coined by Volker Boehmer in a 1994 review, to describe the chirality of calixarenes arising from their non-planar structure in 3-D space.
Jacobsen's catalyst is the common name for N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediaminomanganese(III) chloride, a coordination compound of manganese and a salen-type ligand. It is used as an asymmetric catalyst in the Jacobsen epoxidation, which is renowned for its ability to enantioselectively transform prochiral alkenes into epoxides. Before its development, catalysts for the asymmetric epoxidation of alkenes required the substrate to have a directing functional group, such as an alcohol as seen in the Sharpless epoxidation. This compound has two enantiomers, which give the appropriate epoxide product from the alkene starting material.
Asymmetric addition of alkenylmetals to aldehydes is a chemical reaction in enantioselective synthesis that reacts an alkenylmetal with an aldehyde to give an allyl alcohol. The stereoselectivity in the reaction is typically controlled by the asymmetric ligands used providing a strategy to introduce controlled asymmetry into the molecule. Controlled molecular asymmetry is crucial for controlling the bioactivity of the synthesized molecules and demanded by drug authorities in drug synthesis. In this case the ligands chelate to the transition metal to create a chiral environment which enables the selective formation of a particular enantiomer. Various transition metals such as Zinc, Nickel, Chromium, and Rhodium have been used in this reaction.
In chemistry, pyramidal inversion (also umbrella inversion) is a fluxional process in compounds with a pyramidal molecule, such as ammonia (NH3) "turns inside out". It is a rapid oscillation of the atom and substituents, the molecule or ion passing through a planar transition state. For a compound that would otherwise be chiral due to a stereocenter, pyramidal inversion allows its enantiomers to racemize. The general phenomenon of pyramidal inversion applies to many types of molecules, including carbanions, amines, phosphines, arsines, stibines, and sulfoxides.
Martin Quack is a German physical chemist and spectroscopist; he is a professor at ETH Zürich.
Hydrogen ditelluride or ditellane is an unstable hydrogen dichalcogenide containing two tellurium atoms per molecule, with structure H−Te−Te−H or (TeH)2. Hydrogen ditelluride is interesting to theorists because its molecule is simple yet asymmetric and is predicted to be one of the easiest to detect parity violation, in which the left handed molecule has differing properties to the right handed one due to the effects of the weak force.
Jeanne Crassous is a French chemist who is a Professor and Director of Research at the French National Centre for Scientific Research (CNRS). She leads the Marie Skłodowska-Curie Actions International Training Network HEL4CHIROLED.
