chirality

Examples of chirality in the following topics:

• Enantiomorphism

  • Stereogenic elements may be chiral or achiral.
  • First, try to identify all chiral stereogenic centers.
  • Formulas having one chiral center are always chiral; and if two or more chiral centers are present in a given structure it is likely to be chiral, but in special cases, to be discussed later, may be achiral.
  • The chiral centers will be identified by red dots.
  • Compounds C, D & H have more than one chiral center, and are also chiral.

• Chirality and Symmetry

  • All objects may be classified with respect to a property we call chirality (from the Greek cheir meaning hand).
  • A chiral object is not identical in all respects (i.e. superimposable) with its mirror image.
  • Chiral objects have a "handedness", for example, golf clubs, scissors, shoes and a corkscrew.
  • All dissymmetric objects are chiral.
  • All asymmetric objects are chiral.

• Diastereoselection in Reactions with Chiral Enolates

  • The enolate donor in an aldol reaction may also have a center of chirality, leading to the formation of additional diastereomeric products.
  • The evidence indicates mediocre selectivity, probably resulting from steric differences between large and small substituents (RL and RS) at the chiral center.
  • These Z-enolates are expected to favor 1,2-syn diastereoselectivity of the newly created α & β chiral centers, as noted earlier.
  • Excellent facial selectivity is found in reactions of these nucleophiles Here the 1,2-diastereoselectivity of the newly created α & β chiral centers is strongly anti, as expected.
  • From past observations, the 1,2-diastereoselectivity of the newly created α & β chiral centers is expected to exhibit moderate syn-diastereoselectivity.

• Formulas Using Other Configurational Notations

  • Fischer projection formulas are particularly useful for comparing configurational isomers within a family of related chiral compounds, such as the carbohydrates.
  • When describing acyclic compounds incorporating two or more chiral centers, many chemists prefer to write zig-zag line formulas for the primary carbon chain.
  • These compounds are all chiral and only one enantiomer is drawn (the D-family member).
  • In cases having two adjacent chiral centers, such as this, the prefixes erythro and threo may be used to designate the relative configuration of the centers.
  • The syn-anti nomenclature may be applied to acyclic compounds having more than two chiral centers, as illustrated by the example in the colored box.

• Occurrence of Aldehydes and Ketones

  • With the exception of the first three compounds (top row) these molecular structures are all chiral.
  • When chiral compounds are found in nature they are usually enantiomerically pure, although different sources may yield different enantiomers.

• Compounds Having Two or More Chiral Centers

  • As a general rule, a structure having n chiral centers will have 2n possible combinations of these centers.

• Enantioselective Aldol Reactions

  • Aldol reactions of prochiral donor and acceptor reactants produce racemic mixtures of chiral adducts.
  • The chiral substituent may then be removed, yielding the final enantiomerically pure aldol adduct.
  • A substituent serving this purpose is commonly called a chiral auxiliary.
  • Chiral auxiliaries have been prepared from different kinds of natural products, including amino acids, alkaloids and terpenes.
  • As a rule, Evans' chiral auxiliaries exert a controlling influence in reactions with chiral α-substituted aldehydes, overriding even Felkin-Ahn preferences.

• Stereogenic Nitrogen

  • Since the nitrogen in these compounds is bonded to three different groups, its configuration is chiral.
  • If the nitrogen atom were the only chiral center in the molecule, a 50:50 (racemic) mixture of R and S configurations would exist at equilibrium.
  • If other chiral centers are present, as in the ephedrin isomers, a mixture of diastereomers will result.

• Conformational Enantiomorphism

  • Simply put, any chiral species that are present are racemic.
  • It is interesting to note that chiral conformations are present in most conformationally mobile compounds, even in the absence of any chiral centers.
  • The gauche conformers of butane, for example, are chiral and are present in equal concentration in any sample of this hydrocarbon.
  • The following illustration shows the enantiomeric relationship of these conformers, which are an example of a chiral axis rather than a chiral center.
  • All chiral twisted conformers are present as racemates, so this compound cannot be resolved.

• General Summary of Isomerism and Molecular DescriptorsInclude Relationship of Constitutional and Stereoisomers and Relationships of Stereoisomers

  • The 1,2- and 1,3-dichlorocyclohexanes each have two centers of chirality, bearing the same set of substituents.
  • The cis & trans-1,4-dichlorocyclohexanes do not have any chiral centers, since the two ring groups on the substituted carbons are identical.
  • The chair conformer of the cis 1,2-dichloro isomer is chiral.
  • Since the cis isomer has two centers of chirality (asymmetric carbons) and is optically inactive, it is a meso-compound.
  • The corresponding trans isomers also exist as rapidly interconverting chiral conformations.