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Evidence for inclusion complexation in solution between cucurbituril (C36H36N24012) and various alkyl-and aryl-substituted ammonium ions is elaborated. Induced NMR chemical shifts and UV spectral perturbations of guests complexed within cucurbituril are noted and may be used to quantitate binding. Dissociation constants (K¿) for over 50 guests are recorded, and the kinetics of complexation have also been investigated. On the basis of selectivity among bound species, a detailed model for the structure of the host-guest complexes is deduced. The cavity within cucurbituril has dimensions equivalent to the size of a para-disubstituted benzene ring. Successful inclusion is attributable to hydrophobic interactions (freeing of solvent molecules upon complexation) and to a charge-dipole attraction between ammonium cations and the electronegative oxygens of the urea moieties in cucurbituril.

Chemical Research (RIKEN) for donation of rutheniumcatalysts. We are also indebted to Professor S. Suzuki of our department for the suggestion for the malonaldehyde assay. We also thank Professor K. Hirao of our University for his helpful guidance in conducting MO calculations.A mechanistic investigation is described for the cycloaddition induced between RNH2+CH2C=CH and RNHz+CH2CHzN3 (R = H, t-Bu) consequent to encapsulation by the polycyclic molecular receptor cucurbituril (C3BH9sNU012). The reaction is shown to be substantially accelerated (ca. lo5-fold), and the kinetic characteristics of catalytic saturation behavior, substrate inhibition, and slow product release are documented. For substrates with R = t-Bu a rotaxane product results, and inhibition kinetics with NH3+CH2CH2C(CH3), are also examined. A rate enhancement attributed to bound-substrate destabilization is detected. The significance of this effect and its connection with the phenomenon of nonproductive binding in catalytic systems are discussed.Understanding enzymic catalysis represents perhaps the most severe challenge of mechanistic organic chemistry. Because of the complexity of proteins, analogues which mimic aspects of biochemical catalysis are desirable. Excellent progress has been made in devising synthetic molecular receptors, and much is being learned about spe-(1). Taken in part from the Ph.D. Thesis of M. Adhya, University of Illinois, Chicago, 1986; Diss. Abstr. Int. B 1986, 47, 1056. 0022-3263/89/1954-5302$01.50/0 0 cificity in molecular recognition from imaginatively engineered models of this naturee2 Even more arduous is (2) Recent reviews: Brealow, R. Adv. Enzymol. Relat. Areas Mol. Biol. 1986, 58, 1. Franke, J.; Vogtle, T. Top. Curr. Chem. 1986, 132, 135. Sutherland, I. 0. Chem. SOC. Rev. 1986, 15, 63. Cram, D. J. Angew,

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