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Experimental details are given for the preparation of large self-complementary molecules capable of assembly into pseudospherical capsules. These structures exist as hydrogen bonded dimers in organic solvents, and they form and dissipate on a time scale that permits direct NMR observation of the reversible encapsulation of smaller molecules. The cavity is roomy enough to accommodate more than one molecule, and two solvent molecules such as benzene appear to occupy the resting state of the capsules. Liberation of these solvent molecules is responsible for the unexpected thermodynamic parameters of the encapsulation process. A bimolecular reactionthe Diels−Alder reactionis shown to be accelerated by the capsule as both reactants can occupy the capsule concurrently. Size selectivity, saturation kinetics, and product inhibition studies point to a reaction that takes place within the capsule.

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Encapsulation complexes are reversibly formed assemblies in which small molecule guests are completely surrounded by large molecule hosts. The assemblies are held together by weak intermolecular forces and are dynamic: they form and dissipate on time scales ranging from milliseconds to days-long enough for many interactions, even reactions, to take place within them. Little information is available on the exchange process, how guests get in and out of these complexes. Here we report that these events can be slow enough for conventional kinetic studies, and reactive intermediates can be detected. Guest exchange has much in common with familiar chemical substitution reactions, but differs in some respects: no covalent bonds are made or broken, the substrate is an assembly rather than a single molecule, and at least four molecules are involved in multiple rate-determining steps.Molecule-within-molecule complexes can be regarded as a phase of matter wherein guest molecules are kept inside host structures held together by strong covalent bonds. The earliest systems were prepared a decade ago by Cram and co-workers (1) and Collet and co-workers (2) and were kinetically stable. In these systems-carceplexes and cryptophanes-the hosts can stabilize reactive intermediates (3), produce new forms of stereoisomerism (4), and distinguish between guests and their mirror images (5). Encapsulation complexes are related recently synthesized structures in which hosts are held together by weaker forces, such as hydrogen bonds (6) and metal-ligand interactions (7,8). The systems self-assemble reversibly from subunits and are dynamic (9). They can feature large cavities in which more than one guest can be accommodated (10), behave as nanometric chambers for bimolecular reactions, and offer promise in catalysis (11). We have now examined one such system, the conceptual ''softball'' (12) and address here a fundamental step of assembly and encapsulation-how guests enter and depart. MATERIALS AND METHODSThe solvent p-xylene-d 10 was obtained from Cambridge Isotope Laboratories and was used as received for all guest exchange experiments. All the guests employed for the study were obtained from Aldrich. Adamantane (A) and ferrocene (F) were further purified by sublimation under reduced pressure. Standard solutions of the capsule 1⅐1 (2 mM) and the guests (10-50 mM) were prepared for the kinetic experiments. The guest exchange was monitored by 1 H NMR (600 MHz) with the probe maintained at constant temperature. For the study represented in Fig. 4 each data point was obtained under pseudo-first-order kinetic conditions. The components for all of the samples were mixed at 293 K, and data acquisition was automatically performed at 289 K every 3.5 min. For the experiment represented in Fig. 3 the data points were taken every 5 min. RESULTS AND DISCUSSIONThe softball consists of two self-complementary subunits held together by a seam of 16 hydrogen bonds in a roughly spherical assembly (Fig. 1). In solvents such as p-xylene-d 10 the NMR ...

4-Amino-I -azadienes 1 react with a,&unsaturated Fischer carbene complexes at -40 "C to give stereoselectively a variety of substituted 3H-4,S-dihydroazepines 3; similarly, 1 -hydroxy-1azadienes (cc,#?-unsaturated oximes) 6 afforded the corresponding azepine derivatives 7. Chiral, nonracemic carbene ~~