For over 40 years, the synthesis of
exotic molecules such as catenanes, rotaxanes, knots and Borromean rings with
aesthetically appealing and useful applications has been proceeding apace.
During the same period of time, the synthetic protocols employed by chemists
have evolved from being all statistical in the beginning, to progressively
covalent, coordinative, and noncovalent templating strategies under both kinetic
and thermodynamic control.
A series of rotaxanes compounds
including mechanically interlocked dendrimers have been successfully prepared in
high yields through a template-directed thermodynamic synthesis. Dynamic
covalent chemistry has been exploited in the template-directed syntheses of both
catenanes and rotaxanes under thermodynamic control. This method can provide
proof-reading and error checking for the equilibrium reaction to yield almost
quantitatively the product.
Recently, employing dynamic covalent
chemistry, different types of mechanically interlocked structure (Box)
have been prepared by the thermodynamic “clipping” approach. An exotic
mechanically interlocked molecular structure in which eight components cooperate
to form a jumbo-sized cycle, namely a rotaxane was synthesized (Chem. Eur.
J. 2005, 11, 4655-4666) by heating the suspension containing
the components. The yield of formation is over 95% determined by nuclear
magnetic resonance (NMR) spectroscopy. Also, the crude product could be further
purified by recrystallization to achieve a very pure sample of the branched
In the context of constructing
non-classical mechanically interlocked dendrimers by employing a convergent
templation procedure, the same “clipping” thermodynamic approach have been
explored to introduce steric bulky dendrons onto a trivalent ammonium ion core
with seven-component self-assembly (J. Am. Chem. Soc. 2005, 127,
5808-5810; highlighted in Science 2005, 308, 326) Four
generations of mechanically interlocked dendrimers from generation zero to
generation three up to a molecular weight over 8,000 gmol-1 were synthesized in
a one-pot reaction by simply mixing the seven components together. As for the
rotaxanes, the dendrimers form in excess of 95% yield. The mechanically
interlocked core of the dendrimers were able to be modified and transformed into
kinetically stable dendrimers, which were characterized by massspectrometry.