Abstract
Catenanes are molecules comprising at least two mechanically
interlocked rings: they cannot be separated, yet do not possess
covalent links between ring constituents. Over the last two decades
several efficient templating mechanisms for the assembly of catenanes,
and other topologically complex molecules, have been developed and
exploited in the synthesis of numerous systems, often with impressive
efficiency. Kinetically controlled assembly routes, employing transition
metal complexation or amide hydrogen bonding interactions, have
proved tremendously successful. A third arena of investigation,
and perhaps the most thoroughly explored and exploited, is the utilisation
of π-complementary components, principally bipyridinium
dications and aromatic ethers. In our work electron deficient bipyridinium
dications were replaced with uncharged, yet electron accepting,
aromatic diimides. This replacement permitted the use of a variety
of ring closing reactions for catenane formation by allowing us
to step away from the structurally powerful but ultimately limited
chemistry demanded by bipyridinium systems. A total of four ring-closing
reactions were employed: acetylenic coupling, Mitsunobu alkylation,
Grubbs’ alkene metathesis, and zinc(II)-bipyridyl ligation.
The first three methods yielded fully covalent interlocked systems,
the fourth a catenane containing a metallomacrocycle. The first
two methods employed irreversible bond forming reactions in catenane
formation, the latter two thermodynamically controlled processes.
This flexible system of interacting components, the synthetic
chemistry used in their preparation, and the structural flexibility
offered by the combination of these factors, is discussed in terms
of a series of model systems leading to the proposition of a method
for the synthesis of a polycatenane. Such systems, polymeric chains
of interlocked rings, are unrealised yet coveted goals of chemists
working in the area of supramolecular topology and are predicted
to exhibit valuable and unusual material properties.
Key words
catenanes - supramolecular chemistry - templating - self-assembly - thermodynamic control