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Producing Dendritic Nanostructures using Dynamic Chemistry

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 [4]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 [4]rotaxane.

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 [4]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.

 

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