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Axial Chirality

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Chirality in Systems Lacking Chiral Centres
Axial Chirality
Axial chirality refers to stereoisomerism resulting from the non-

planar arrangement of four groups in pairs about a chiral axis
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What are Allenes?
Definition
Allenes are compounds with two or more double bonds sideby-side
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These are distinct from the alternate double-single-double or

conjugated bonds
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The central carbon is sp-hybridized and the two terminal
carbons are sp2-hybridized
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The geometry of the π-bonds causes the groups attached to the
end carbon atoms to lie in perpendicular planes
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Structure of Allene
The Geometry and Symmetry
Since the bond angle formed by the three carbons is 180°, the
cummulated carbons of the allene are in a linear geometry
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There is a
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mirror plane passing through both CH2 planes
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Note that allenes with odd numbers of cumulated bonds are
not chiral; only those with even numbers are
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How do you assign configurations to the optically active allenes?
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Chirality of Allenes
Configurations of Axially Chiral Systems
The configurations in axially chiral systems are specified using
Ra and Sa or P (plus) and M (minus)
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The two "near" and two
"far" substituents on the axial unit are ranked
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Chirality of Allenes
Configurations of Axially Chiral Systems
The model below shows the side views and representations of
the allene along the chiral axis
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Configurations of Chiral Allenes
Determining the Sense of Rotation (Ra vs Sa)
The sense of rotation is determined starting with the end of the
allene that is closest, then finishing with the end that is
farthest
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Configurations of Chiral Allenes
Determining the Sense of Rotation (P vs M)
The sense of rotation is determined by looking only at the high
priority substituents starting with the end of the allene that is
closest, then finishing with the end that is farthest
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Axially Chiral Systems
Biphenyl Systems
Biphenyls are compounds whereby a phenyl ring is connected
to another through a central σ bond
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Axially Chiral Systems
Biphenyl Systems (Atropisomerism)
However, biphenyls with large substituents at the ortho
positions on either side of the central σ bond experience
restricted rotation along this bond due to steric hindrance
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The enantiomers can be made racemic by heating
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Axially Chiral Systems
Atropisomerism
Atropisomers are stereoisomers that can be interconverted by
rotation about single bonds but for which the barrier to rotation
is large enough that the stereoisomers do not interconvert

readily at room temperature and can be separated
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Chirality of Biphenyl Systems
Assigning Configurations of Atropisomers
Configurations of atropisomers are assigned in a similar fashion
as those of chiral allenes using Ra and Sa or P (plus) and M
(minus)
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Chirality of Biphenyl Systems
Assigning Configurations of Atropisomers (Ra vs Sa)
By generating appropriate models of each atropisomer, the
sense of configuration can be determined
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Assignment
Configurations of Axially Chiral Systems
Determine the configurations of the following axially chiral
systems using both the Ra/Sa and P/M notations
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Allenes are compounds with two or more cumulated double
bonds
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The configurations in axially chiral systems are specified
using Ra and Sa or P (plus) and M (minus)
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The stereoisomerism caused by restricted rotation of a
carbon-carbon single bond is called atropisomerism

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CAT 1
STEREOCHEMISTRY

THURSDAY, 24TH OCTOBER 2013

8-9 AM
CHEM LAB 1

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