4.1
Introduction
The dream of the “Gluttonous Snake” not only inspired Friedrich Kekule with
the structure of benzene ring but also led chemists to begin to understand
the macrocyclic molecules. Interesting and biologically active circular natural
molecules, like terpenoids, not only enlightened chemists to develop a variety of
methods to synthesize them [1, 2] but also prompted the designation and synthesis
of organic compounds with novel structures that are not naturally existing
[3]. In the 1960s, Wasserman first synthesized and discovered a mechanically
interlocked molecule ring and named it catenane using the latin etymological catenof the chains and put forward the concept of “chemical topology” with Frisch later [4,
5]. However, due to the low synthesis efficiency of this kind of mechanically
interlocked molecules, the research of which had always been ignored until the
year of 1983. That was the year, Savage et al. [6] proposed a new synthesis strategy
using metal ion as template, which greatly simplified the synthesis route and improved
the synthesis efficiency of the mechanically interlocked molecules. Subsequently, the
research on mechanically interlocked architecture exploiting supramolecular template
strategy developed rapidly. And the construction of various organic molecules with
novel and complex topologies greatly promoted the research on molecular machine.
Pseudo[1]rotaxane and pseudo[1]catenane could be simply prepared from a
pseudo[2]rotaxane by connecting the axle with the macrocyclic part. The most
important feature of this kind of structure is that it includes mechanical bond in a
single covalent component. Pseudo[1]rotaxane is the simplest mechanically selflocked molecular architecture. When a molecule contains a macrocycle covalently
linked to a molecular chain, it will have two interesting molecular conformations.
One is the chain segment lies behind the macrocycle like a tail, and the other one is
that the chain part bends back threading through the cavity of the macrocycle, which
is given the name – self-inclusion complex. When the other end of the chain
molecule in its self-included conformation is connected with a large molecular
fragment (also known as the end-capping group), we call it [1]rotaxane. When the
chain part penetrates through the macrocyclic molecule and both ends are connected
to the same segment of the macrocyclic molecule, we call it [1]catenane. If the
two ends of the chain part link to two different segments of the macrocyclic
molecule, we call it molecular figure-of-eight. Therefore, based on the topology
structure of the mechanically self-locked molecular architecture, we classify this
kind of architectures into pseudo[1]rotaxanes, pseudo[1]catenanes, molecular figures-of-eight, pretzelanes, and double-lasso molecules to discuss.
4.2
Pseudo[1]rotaxanes
Compared to pseudo[1]rotaxanes, [1]rotaxane has an bulky end-capping group and
it seems that it can keep the self-included conformation. However, due to the reverse
of the macrocyclic fragment, the axial part that passes through the macrocycle
could escape from the macrocycle and change its conformation as shown below
84
S.-H. Li et al.
Introduction
The dream of the “Gluttonous Snake” not only inspired Friedrich Kekule with
the structure of benzene ring but also led chemists to begin to understand
the macrocyclic molecules. Interesting and biologically active circular natural
molecules, like terpenoids, not only enlightened chemists to develop a variety of
methods to synthesize them [1, 2] but also prompted the designation and synthesis
of organic compounds with novel structures that are not naturally existing
[3]. In the 1960s, Wasserman first synthesized and discovered a mechanically
interlocked molecule ring and named it catenane using the latin etymological catenof the chains and put forward the concept of “chemical topology” with Frisch later [4,
5]. However, due to the low synthesis efficiency of this kind of mechanically
interlocked molecules, the research of which had always been ignored until the
year of 1983. That was the year, Savage et al. [6] proposed a new synthesis strategy
using metal ion as template, which greatly simplified the synthesis route and improved
the synthesis efficiency of the mechanically interlocked molecules. Subsequently, the
research on mechanically interlocked architecture exploiting supramolecular template
strategy developed rapidly. And the construction of various organic molecules with
novel and complex topologies greatly promoted the research on molecular machine.
Pseudo[1]rotaxane and pseudo[1]catenane could be simply prepared from a
pseudo[2]rotaxane by connecting the axle with the macrocyclic part. The most
important feature of this kind of structure is that it includes mechanical bond in a
single covalent component. Pseudo[1]rotaxane is the simplest mechanically selflocked molecular architecture. When a molecule contains a macrocycle covalently
linked to a molecular chain, it will have two interesting molecular conformations.
One is the chain segment lies behind the macrocycle like a tail, and the other one is
that the chain part bends back threading through the cavity of the macrocycle, which
is given the name – self-inclusion complex. When the other end of the chain
molecule in its self-included conformation is connected with a large molecular
fragment (also known as the end-capping group), we call it [1]rotaxane. When the
chain part penetrates through the macrocyclic molecule and both ends are connected
to the same segment of the macrocyclic molecule, we call it [1]catenane. If the
two ends of the chain part link to two different segments of the macrocyclic
molecule, we call it molecular figure-of-eight. Therefore, based on the topology
structure of the mechanically self-locked molecular architecture, we classify this
kind of architectures into pseudo[1]rotaxanes, pseudo[1]catenanes, molecular figures-of-eight, pretzelanes, and double-lasso molecules to discuss.
4.2
Pseudo[1]rotaxanes
Compared to pseudo[1]rotaxanes, [1]rotaxane has an bulky end-capping group and
it seems that it can keep the self-included conformation. However, due to the reverse
of the macrocyclic fragment, the axial part that passes through the macrocycle
could escape from the macrocycle and change its conformation as shown below
84
S.-H. Li et al.