No abstract

PreambleClassically, 1 the term synthesis has implied the construction of molecular systems as a result of the sequential formation of covalent bonds. The total syntheses of elaborate molecular compounds, such as brevetoxin B, 2 palytoxin, 3 and the calichearubicins 4 sto cite but a few examplessare triumphs for contemporary synthetic methodology. Nonetheless, the chemistry of the covalent bond has now almost been stretched to its conceptual limits. Even the best present-day synthetic chemists cannot hope to fabricate complicated nanosystemssanalogous to those witnessed in naturesusing only the currently available repertoire of covalent bond-making tools. It is time for them to look further afield for fresh challenges. 5 In order for the synthetic chemist to be able to build nanosystems, the likes of which are commonplace in the natural world, (s)he must learn to control another type of bonds specifically, the intermolecular, noncovalent bond. The chemist's drive toward the synthesis of nanoscale com-posites, employing the noncovalent bond, has led to the birth of a highly interdisciplinary field of chemical researchsviz., supramolecular chemistry 6 sin recent years. This branch of contemporary science is concerned with advancing structural complexitysbeyond the molecules from inclusion complexes toward ordered oligo-and polymolecular entities which are held together using noncovalent, intermolecular bonds. The ultimate aim of supramolecular chemistry is to become the "science of informed matter", 6a i.e., it seeks to create functioning, organized nanoscale devices 7 which will be able to stockpile and process information, by analogy with the countless marvelous examples of machine-like systems present in nature. There are two facets of modern-day chemical synthesis which are influenced by supramolecular chemistry. These are 6a (1) the creation of multicomponent supramolecular architectures utilizing noncovalent bonding interactions, i.e., supramolecular synthesis, 8 and (2) the synthesis of discrete molecular entitiessheld together using wholly covalent and mechanical 9 bondssaided and abetted by intermolecular, noncovalent interactions, i.e., supramolecular assistance to molecular synthesis. The impetus for the development of both of these aspects of synthetic supramolecular chemistry has been selfassembly, 8b,10 the spontaneous generation of well-defined supramolecular and molecular architectures from specifically "engineered" building blocks.

A series of secondary dialkylammonium ions (RCH2)2NH2 + have been prepared, and their binding properties toward the macrocyclic polyether dibenzo[24]crown-8 (DB24C8) evaluated. By using this information, a route to a kinetically stable rotaxane-like entitystabilized by noncovalent bonding interactions between the DB24C8 macroring and the ammonium centerwas established, in which the crown ether slips over a dialkylammonium ion's stopper groups (R). However, we have found that the kinetic stability of this rotaxane-like entity is extremely dependent on the nature of the solvent in which it is dissolved, suggesting that pseudorotaxanes lie in the fuzzy domain between two sets of extremes, wherein a beadlike macrocycle and a dumbbell-like component may either (1) exist as a rotaxane or (2) be completely disassociated from one another.

No abstract