Axial chirality
In chemistry,axial chiralityis a special case ofchiralityin which amoleculecontains two pairs of chemical groups in a non-planar arrangement about anaxis of chiralityso that the molecule is not superposable on its mirror image.[1][2]The axis of chirality (orchiral axis) is usually determined by a chemical bond that is constrained against free rotation either bysteric hindranceof the groups, as in substitutedbiarylcompounds such asBINAP,or bytorsional stiffnessof the bonds, as in the C=C double bonds inallenessuch asglutinic acid.Axial chirality is most commonly observed in substituted biaryl compounds wherein the rotation about the aryl–aryl bond is restricted so it results in chiralatropisomers,as in various ortho-substitutedbiphenyls,and in binaphthyls such asBINAP.
Axial chirality differs fromcentral chirality(point chirality) in that axial chirality does not require achiral centersuch as anasymmetric carbonatom, the most common form of chirality inorganic compounds.Bonding to asymmetric carbon has the form Cabcd where a, b, c, and d must be distinct groups. Allenes have the formabC=C=Ccdand the groups need not all be distinct as long as groups in each pair are distinct: abC=C=Cab is sufficient for the compound to be chiral, as inpenta-2,3-dienedioic acid.Similarly, chiral atropisomers of the formabC−Ccdmay have some identical groups (abC−Cab), as in BINAP.
Nomenclature
[edit]Theenantiomersof axially chiral compounds are usually given the stereochemical labels (Ra) and (Sa),[1]sometimes abbreviated (R) and (S).[3]The designations are based on the sameCahn–Ingold–Prelog priority rulesused for tetrahedral stereocenters.[3]The chiral axis is viewed end-on and the two "near" and two "far" substituents on the axial unit are ranked, but with the additional rule that the two near substituents have higher priority than the far ones.[4]
Helical chirality
[edit]The chirality of a molecule that has a helical, propeller, or screw-shaped geometry is calledhelicity[5]orhelical chirality.[6][7]Thescrew axisor theDn,orCnprinciplesymmetry axisis considered to be the axis of chirality. Some sources consider helical chirality to be a type of axial chirality,[7]and some do not.[6][8]IUPACdoes not refer to helicity as a type of axial chirality.[5][1]
Enantiomers having helicity may labeled by using the prefix notation (P) ( "plus" ) or Δ (from Latindexter,"right" ) for a right-handed helix, and (M) ( "minus" ) or Λ (Latinlevo,"left" ) for a left-handed helix.[5][3][9]TheP/Mor Δ/Λ terminology is used particularly for molecules that actually resemble a helix, such as thehelicenes.This notation can also be applied to non-helical structures having axial chirality by considering the helical orientation of the Cahn–Ingold–Prelog group rankings of the "front" groups compared to the "back", when viewed from either direction along the axis.
External links
[edit]References
[edit]- ^abcIUPAC,Compendium of Chemical Terminology,2nd ed. (the "Gold Book" ) (1997). Online corrected version: (2006–) "axial chirality".doi:10.1351/goldbook.A00547
- ^Eliel, Ernest L.; Wilen, Samuel H.; Mander, Lewis N. (1994).Stereochemistry of organic compounds.New York: Wiley.ISBN0-471-01670-5.OCLC27642721.
- ^abcCross, L. C.; Klyne, W. (1976)."Rules for the Nomenclature of Organic Chemistry. Section E: Stereochemistry (Recommendations 1974)".Pure Appl. Chem.45(1): 11–30.doi:10.1351/pac197645010011.ISSN1365-3075.
- ^Cross, L. C.; Klyne, W. (1976)."Rules for the Nomenclature of Organic Chemistry. Section E: Stereochemistry (Recommendations 1974)"(PDF).Pure Appl. Chem.45:11–30.doi:10.1351/pac197645010011.
Chiral axis. The structure is regarded as an elongated tetrahedron and viewed along the axis—it is immaterial from which end it is viewed; the nearer pair of ligands receives the first two positions in the order of preference
- ^abcIUPAC,Compendium of Chemical Terminology,2nd ed. (the "Gold Book" ) (1997). Online corrected version: (2006–) "helicity".doi:10.1351/goldbook.H02763
- ^abTan, Bin (2021).Axially Chiral Compounds: Asymmetric Synthesis and Applications.ISBN978-3-527-82517-2.OCLC1264474520.
- ^abClark, Andrew; Kitson, Russell R. A.; Mistry, Nimesh; Taylor, Paul; Taylor, Matthew; Lloyd, Michael; Akamune, Caroline (2021).Introduction to Stereochemistry.ISBN978-1-78801-315-4.OCLC1180250839.
- ^Zhang, Dawei; Mulatier, Jean-Christophe; Cochrane; et al. (2016-05-02). "Helical, Axial, and Central Chirality Combined in a Single Cage: Synthesis, Absolute Configuration, and Recognition Properties".Chemistry - A European Journal.22(24): 8038–8042.doi:10.1002/chem.201600664.ISSN0947-6539.PMID27037555.
- ^"VLU: Additional Chirality Elements - Chemgapedia".Archived fromthe originalon 2011-07-18.Retrieved2007-08-06.