These class of compounds do not contain any formal chiral centre but exhibit stereoisomerism due to presence of either a chiral axis or a chiral plane and are called stereoaxis and stereoplanes respectively

1. DOS&R IN ORGANIC CHEMISTRY
TUMKUR UNIVERSITY
Optical activity in
Catenanes & Rotaxanes
By
PRUTHVIRAJ K
Faculty
DOS&R in Organic Chemistry
KPR. DOS&R in ORGANIC CHEMISTRY TUT

2. Introduction
• Stereoisomerism in organic compounds has
been established on the basis of chiral
centres acting as stereogenic bases. Two
other elements of chirality, namely as axes
and planes, also behaves as stereogenic units
according to CIP rules. Appropriately
substituted allenes, alkylidenecycloalkanes,
spiranes, adamantoids, biaryls, trans-
cycloalkenes, cyclophanes, catenanes,
rotaxanes and their analogues.
KPR. DOS&R in ORGANIC CHEMISTRY TUT

3. • These class of compounds do not contain any
formal chiral centre but exhibit
stereoisomerism due to presence of either a
chiral axis or a chiral plane and are called
stereoaxis and stereoplanes respectively
• Molecules having helix structure possesses an
inherent chirality being non-superimposable
with its mirror image. This introduces another
element of chirality known as helicity
KPR. DOS&R in ORGANIC CHEMISTRY TUT

4. Principle of axial and planar
1. Elongated tetrahedron approach
KPR. DOS&R in ORGANIC CHEMISTRY TUT

5. • 2. Approach based on two dimensional chiral
complex.
KPR. DOS&R in ORGANIC CHEMISTRY TUT

6. Catenanes & Rotaxanes
• Catenane is a compound consisting of two or more rings that are
interlocked mechanically without there being necessarily any chemical
interaction/bond between the two. Generally, without breaking a
chemical bond, the rings cannot be separated. Catenane is derived from
the Latin catena meaning "chain". In recent times the terminology
"mechanical bond“ has been coined and it is the connection between
the macrocycles of a catenane.
• Rotaxanes are long, fairly linear molecules consisting of a "dumbbell
shaped molecule" threaded through a macrocyclic ring, like cotton
threads through the eye of a needle. The name is derived from the Latin
for wheel (rota) and axle (axis). Same as catenanes, rotaxanes also
cannot decompose into ring and chain without breaking chemical
bonds. Hence, the bulky groups terminate the linear, chain part of the
molecule and it is too large to fit through the cyclic fragment.
Rotaxanes without such physical barriers, in which the thread can
leave the needle, are called pseudorotaxanes. Pseudorotaxanes are
necessary precursors for both rotaxanes and catenanes.
KPR. DOS&R in ORGANIC CHEMISTRY TUT

7. Chemical Topology of Catenanes
and Rotaxanes
A compound can be usually described,
unequivocally, by:
1. The order in which given numbers of atoms are
joined
2. The type of bonds which connect them
3. The configuration at asymmetric atoms or rigid
centers
4. The conformation
5. The topology
KPR. DOS&R in ORGANIC CHEMISTRY TUT

8. • Chemical topology deals with the structure and the property differences of
compounds which are identical with regard to the fore mentioned first three
points and which in spite of that cannot be interconverted by conformational
changes, such as rotation about an axis or modification of bond angles.
Compounds which can be suitably classified in this manner were called as
‘Topological Isomers’ by Frisch and Wasserman. Catenane 1 and the twice
threaded catenane 6 are thus topological isomers and for similar reasons as
stated above, a rotaxanes is not isomeric with its molecular subunits.
KPR. DOS&R in ORGANIC CHEMISTRY TUT

9. Nomenclature of Catenanes and Rotaxanes
• The nomenclature of catenanes is decided by the number of
rings, i.e. how many number of rings interlocked to each other,
e.g. a [2]-catenane consists of two interlocked rings (fig. 2).
Analog ‘ane’ used in the end which is similar to alkanes. A
catenane mainly consists of an organic fragment, it rarely
consists of hydrocarbon moieties. The terms [n]-catenand and
[n]-catenate are also used analogously with cryptand and
cryptate, for a metal center which is suitably interlocked in the
ring system of a catenane acting as a ligand. The catenand is the
free ligand that forms a catenate complex in the presence of
metal center. Rotaxanes can be named in an analogous manner
too. In the case of unbranched species such a numbering system
is not necessary.
• The names of the rings forming a branch to the main chain are
numbered with subscripts and placed together with the ring on
which the branching takes place in one bracket.
KPR. DOS&R in ORGANIC CHEMISTRY TUT

10. • In catenanes and rotaxanes containing multiple windings it is
also necessary to designate the winding number α. The
quantity α represents the number of times one macrocycle
winds about the other. Catenanes which form a ring with
themselves as designated as cyclocatenanes. The examples
below will clarify the concept of nomenclature.
KPR. DOS&R in ORGANIC CHEMISTRY TUT

11. KPR. DOS&R in ORGANIC CHEMISTRY TUT

12. KPR. DOS&R in ORGANIC CHEMISTRY TUT

13. Stereochemistry of Catenanes and Rotaxanes
• Stereochemistry of catenanes and rotaxanes is not a very highly
explored topic except for discussion on simple structures of catenanes
in particular. Rotaxanes have been very minimally dealt with in this
area of stu
The stereochemistry of simpler carbocyclic [2]-catenanes
can be categorized as follows:
 Catenanes bearing no substituents donot occur as antipodes
(antipodes are synonymous with enantiomers or optical isomers)
 Catenanes having substituents in only one of the two rings are
stereochemically related to the corresponding uncatenated
macrocycles.
 Catenanes in which each subunit contains two substituents located on
the same ring atom are stereochemically related to allenes and spiro
compounds. If each ring has two substituents A and B, as shown in
formula in the figure, a necessary and sufficient condition for the
occurrence of enantiomers is that A≠B.
KPR. DOS&R in ORGANIC CHEMISTRY TUT

14.  Catenanes where each subunit contains two substituents located on
different ring atoms, generally exist as an enantiomeric pair. However,
Doornbos pointed out that this may not always be true. Consider
catenane shown in the figure as an example. In this catenane, each ring
has the same two substituents R on different ring atoms. The segments
of the individual rings of the catenane have to be of such a nature that
the molecule possesses a fourfold alternating axis of symmetry. In this
case the catenane is achiral, although the component rings are
dissymmetrical.
KPR. DOS&R in ORGANIC CHEMISTRY TUT

15. KPR. DOS&R in ORGANIC CHEMISTRY TUT
 A type of enantiomerism closely related catenane mentioned in point 3 was
first pointed by Closson, and in more generalized form by Prelog et al and
later by Cruse. Consider the following two catenanes which are
cycloenantiomers. In order that this difference be pertinent, each ring of a
catenane must consist of atleast three different segments, even though every
segment may appear in both rings.

16. KPR. DOS&R in ORGANIC CHEMISTRY TUT
 Special cases may arise if one of the two rings of a catenane is so small, or
has substituents so large, that free rotation of one ring within the other is
hindered. This case is shown in the figure below wherein the enantiomers
arise because the large substituents A and B prevent free rotation of the
rings.
 As shown in figure, the small ring is fixed on right side by two bulky groups
A and B in one enantiomer while in the other enantiomer it is fixed on left
side. Both stereoisomers should be separable into antipodes.
 The [2]-catenane shown below in the figure having a winding number α=2, is
not identical with its mirror image and should therefore be optically active.
This isomerism corresponds to the mirror image relation which exists
between an α- and β- helices.

17. KPR. DOS&R in ORGANIC CHEMISTRY TUT
Stereochemistry of the [3]-catenanes, as well as of higher catenanes, has not been
considered, except for some special cases. A possibility to differentiate the [3]-
catenanes as depicted in the figure below by means of optically active
compounds, was pointed first by Frisch and Wasserman.

18. KPR. DOS&R in ORGANIC CHEMISTRY TUT
Rotaxanes:
The stereochemistry of rotaxanes closely resembles that of catenanes as illustrated
with catenane mentioned above in point 3. In the figure of the rotaxanes shown below,
R≠R’ and A≠B is achiral, it exhibits geometric isomerism.

19. KPR. DOS&R in ORGANIC CHEMISTRY TUT
 Catenanes and rotaxanes differ from all other organic compounds
synthesized to this date in a way that molecular subunits are
linked mechanically.
 Catenane is a compound consisting of two or more rings that are
interlocked mechanically without there being necessarily any
chemical interaction/bond between the two.
 Rotaxanes consist of a long, fairly linear molecule threaded
through a macrocyclic ring, like cotton through the eye of a
needle.
 Chemical topology deals with the structure and the property
differences of compounds which are identical with regard to the
aforementioned first three points and which in spite of that
cannot be interconverted by conformational changes, such as
rotation about an axis or modification of bond angles. In this
module we have classified catenanes and rotaxanes according to
their chemical topology

20. KPR. DOS&R in ORGANIC CHEMISTRY TUT
 Stereochemistry of catenanes and rotaxanes is not a very
highly explored topic except for discussion on simple
structures of catenanes in particular. Rotaxanes have been
very minimally dealt with in this area of study. In this module,
we have discussed all possible cases of stereochemistry of
catenanes and rotaxanes.
 Catenanes have been synthesized by incorporation of many
functional units, including redox-active groups (e.g., viologen,
TTF = tetrathiafulvalene), photoisomerizable groups (e.g.,
azobenzene), fluorescent groups and chiral groups. Many of
these units have been used to create molecular switches as
described above, as well as for the fabrication of molecular
electronic devices and molecular sensors.
 Rotaxane-based molecular machines have been of particular
interest for their potential uses in molecular electronics as
logical molecular switching elements and also as molecular
shuttles.