Atropisomerism in Compounds other than Biphenyls: Phenomenon of atropisomerism in compounds other than biphenyls, Detection, rules and examples

1. DOS&R IN ORGANIC CHEMISTRY
TUMKUR UNIVERSITY
By
PRUTHVIRAJ K
Faculty
DOS&R in Organic Chemistry
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Atropisomerism in Compounds
other than Biphenyls

2. Atropisomerism in Compounds other
than Biphenyls
• Some of the molecules which different than biphenyl also show the atropisomerism. These
molecules are linked together through a pivotal bond and rotation around the pivotal bond
is restricted. The atoms joined through the pivotal bond are usually sp2 hybridized.
• One or both of the phenyl groups are replaced by other heteroaromatic or aromatic rings.
Appropriately substituted molecules (15) are resolvable
• 3,3’-bipyridyl (16) can also resolved and it exits in two enantiomer
due to atropisomerism
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3. • Two phenyl groups (A and B) are introduced on p-substitution at
biphenyls to form para terphenyl derivative and restricted rotation
may arise around two pivotal bonds so two terminal phenyl rings (A
and B) are co-planar as well as co-axial
• Molecule 17 is Cis, where both bromide on phenyl ring A and ring B
are on same side. The molecule is C2 symmetric and also resolvable
and chiral axis is passing through ring A and middle ring of the
biaryl and another chiral axis is passing through ring B and middle
ring of the biphenyl. The cis molecule has two chiral axes so it will
have enantiomers as well as diasetreromers. If molecule B is trans
than it will possess the inversion center and Ci point group and
compound will be meso (same like in two chiral center tartaric acid)
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4. • If one of the planar ring is replaced by an acyclic grouping
(substituted alkene) which is two dimensionally. This type of
molecules 19 may give atropisomerism if sufficient steric hindrance is
created around the pivotal bond.
• The sp3-sp3 single bond is restricted through various extents but
the energy barrier is too low so such type of molecules cannot be
isolated. In the triptycene type molecules, however the barrier to
rotation around a 9-substituted bond may be quite high and these
atropisomers (20 and 20’) can be isolate at room temperature
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5. • One or both of the phenyl groups are replaced by other aromatic or
heteroaromatic rings, thus appropriately substituted N-
Phenylpyrrole, N,N’-bipyrryl, 1,1’-binapthyl are resolvable.
• The two phenyl groups are interposed by a phenylene ring forming
para terphenyl derivtaives and restricted rotation may arise around
two pivotal bonds so that two terminal phenyl groups may coaxial
as well as planar.
• This corresponds to a planar combination of two two-dimensional
chiral units and both diastereomerism and enantiomerism may occur.
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6. • Thus the terphenyl deriavatives in which all the 8 orthopositions are
substituted exists in two achiral diastereomers which can be described
as cis and trans with respected to two Br atoms, both the cis and
trans isomers have C2 axis, the cis has two σv planes and the trans one
σh plane: the trans, in addition has centre of symmetry, belongs to C2v
& C2h pount groups respectively.
• In the case of lower moieties cis belongs to C2 and trans remains with
Centre of symmtery and has meso like property of tartaric acid.
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7. • One of the planar ring is replaced by an acyclic grouping which is
two dimensionally chiral usually due to a dissymmetrical substituted
trigonal atom, may exhibit atropisomerism if sufficient steric
hindrance is created over pivotal bond.
•
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8. • Atropisomerism in nonbridged and bridged biaryl compounds due to
restricted rotation through pivotal bond.
• Biaryl compound with appropriate different ortho susbsituent on
each aryl ring will so the atropisomerism. The bulkier groups on
ortho position of the biaryl ring restrict the rotation through C-C
bond gives two enantiomers and resolvable at room temperature.
• In bridged biaryls effect of bridging on the restricted rotation of
biaryl system is depends on the ring size. Some of the molecules
which different than biphenyl also show the atropisomerism.
• The sp3-sp3 single bond is restricted through various extents and
triptycene type molecules the barrier to rotation around may be
quite high and these atropisomers can be isolate at room
temperature.
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