Atropisomerism results from restricted rotation about a single bond, as in biphenyl compounds where large substituents on either side of the central bond cause steric hindrance. This restricted rotation in biphenyls substituted with different groups leads to the formation of chiral atropisomers as enantiomeric pairs. The biphenyl rings are perpendicular to minimize clashes between the ortho substituents, and rotation about the biphenyl bond through the pivotal bond is restricted, demonstrating axial chirality along the biphenyl axis. The stability of atropisomers is determined by factors including the steric demand of substituents near the axis, any bridges present, and mechanisms allowing isomerization beyond physical rotation.

1. C
STEREO ISOMERISM IN BIPHENYL
COMBOUNDS
(ATROPISMERISM)
PREPARED BY
B.ARUN RAJ
U21BP005

2. Atropisomerism are stereoisomers as a result of restricted rotation about a single
bond.
from Greek, a = not and tropos = turn
Biphenyls are compounds whereby a phenyl ring is connected to another through a
central σ bond

3. However, biphenyls with large substituents at the positions on either side of the central σ
bond experience restricted rotation along this bond due to steric hindrance. If the substituents
are different, a chiral molecule existing as a pair of enantiomers called atropisomers is
obtained.
Biphenyl substituted on position, which contains a chiral axis along the biphenyl linkage.
The biphenyl rings are perpendicular to each other in order to minimize steric clashes between
the four ortho substituents meaning that rotation about the biphenyl bond through pivotal bond
is restricted.

4. Atropisomerism is also called axial chirality and the chirality is not simply a centre or a
plane but an axis.

6. The diagram shows a 360° rotation around the pivotal bond. The inter-ring
resonance or the energy of the compound is lowest at 90° angle therefore, when the
torsion angle is 90° & 270° as shown in the figure.
The strict hindrance or repulsion will be greater than those of between atoms of
structure 3 & 4. Thus, the structure (1, 2, 5) will have greater energy than that the
structure (3 & 4) & therefore, the structure (1, 2, 5) will be stable than that of
structure (3 & 4)

7. In these structure (1, 2 & 5) the inter atomic distance between atoms
a-a & b-b and a-b is minimum inter atomic distance.
It the energy barrier exceeds 80-100 KJ/mol, the stereoisomers can be
repeated at normal room temperature because of their greater energy
difference

8. ✓ A rotationally stable axis
✓ Presence of different substituents on both sides of the axis
✓ The configurational stability of axially chiral biaryl compounds is mainly determined
by three following factors:
i. The combined steric demand of the substituent in the proximity of the axis.
ii. The existence, length and rigidity of bridges.
iii. Atropisomerisation mechanism different from a merely physical rotation about the axis, e.g.
photo chemically or chemically induced processes.

9. ✓ Starting with the substituent of highest priority in the closest ring and moving along
the shortest path to the substituent of highest priority in the other ring, the
absolute configuration is assigned P for clockwise and M for counterclockwise

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