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Project Goals:

1) Self-assembly of supramolecular pseudorotaxane polymers,

2) conversion to polymers with permanent mechanical (rotaxane) linkages, and

3) characterization of their physical properties.

Summary of Progress: In this first year we have addressed goal #1; this included development of synthetic routes to a variety of building blocks for this purpose as well as considerations for goal #2. Efforts thus far have focused on use of 4,4'-bipyridinium salts (paraquats) as guest species and crown ethers and crown ether/pyridyl cryptands as hosts, since these systems display the highest binding constants typically.

Use of a dibenzo-30-crown-10-based pyridyl cryptand allows use of relatively small “stoppers” or blocking groups to be used in preparing the rotaxane systems (goal #2); for example, 3,5-di(t-butyl)phenyl units are large enough for this purpose as we demonstrated by synthesis of a model rotaxane from a paraquat diacid chloride and 3,5-di(t-butyl)phenol. We will also employ bis(m-phenylene)-32-crown-10-based pyridyl cryptands, which bind more strongly.

Desymmetrization reactions have been explored as part of these efforts. ω-Bromo primary alcohols are useful for preparation of paraquat-based building blocks. We have demonstrated an efficient method for preparation of these compounds from the corresponding diols in a two phase reaction with HBr. Another useful process for production of linkers for the guest species with the stoppers is the Cannizzaro reaction of linear α,ω-bis(benzaldehyde)s with Ba(OH)2 to form the corresponding α-carboxy-ω-alcohols. We are also developing routes to other cryptands based on unsymmetrically substituted dibenzo crown ethers. Desymmetrization of dialdehyde derivatives via Cannizzaro reactions has been shown to be nearly quantitative for several different crown ether substrates. The resultant acid-alcohol functionalized crown ethers should afford selective and hopefully high yielding routes to new types of cryptands otherwise not currently accessible.

Using the precursors mentioned above we have prepared paraquats with relatively long spacers linked to stoppers, as building blocks for the supramolecular polymers via semirotaxanes, which will then be converted to “slip-link polymers” via reactions of the functional groups on the host and end of the guest with suitable difunctional linkers. Difficulties have arisen because of micelle formation in some cases and low solubility in others, resulting in low yields and purification issues. Nonetheless, several building blocks are in hand. The binding of a dibenzo-30-crown-10-based pyridyl cryptand with one of these paraquats bearing a tris(p-t-butyl)phenylmethyl stopper tethered by a 12-atom linker on one nitrogen and a 6-carbon carboxylic acid on the other nitrogen has been measured at 2 x 10exp4 M-1, high enough to ensure efficient formation of the target supramolecular polymer.

Oligomeric building blocks in the form of butadiene and poly(tetramethylene oxide) have been end functionalized by attachment of paraquat (and other) units. These will be complexed with appropriate ditopic host species.

We are still developing synthetic methods for functionalization of our crown ether/pyridyl cryptands; this has proved to be very challenging. Although we have successfully prepared some useful compounds, low yields have been a problem. Recently, however, we have found a facile route that should allow elaboration via copper catalyzed acetylene-azide cycloaddition reactions. Once these host species are in hand we will advance onward toward goals #2 and 3.