Self-Assembled Capsules of Unprecedented Shapes**

Reversible encapsulation allows the temporary isolation of molecules in very small spaces. There, molecular behavior is quite different than that in bulk solvent; in capsules recognition can be amplified, reactive intermediates can be stabilized, reactions can be accelerated or even catalyzed and new reaction pathways can appear. Accordingly, a multitude of capsules have been devised over the last two decades. The main cohesive forces—covalent bonds, hydrogen bonds metals and ligands, or simple hydrophobic effects—are used to hold the capsules together and a variety of structures are available. Yet the self-assembly processes by their very nature of incorporating multiple subunits tend to create cavity shapes of high symmetry such as spheres, polyhedra and cylinders. We report here examples of new capsules featuring “S”and “banana”-shapes that arise from insertion of propanediureas 3 into cylindrical capsule 1.1 (Figure 1a). An extensive and mutual induced fit behavior is displayed by these systems. The cylindrical capsule host 1.1 (Figure 1a) spontaneously assembles around appropriate guests in apolar organic solvents such as mesitylene. The complex is held together through a seam of bifurcated hydrogen bonds and attractive forces between guest and host. When glycoluril 2 (Figure 1b) is present, new assemblies emerge: the glycolurils act as spacer elements that increase the length and capacity of the inner space. The glycolurils offer superior hydrogen bond acceptors to the imides NH donors and four glycolurils integrate into the middle of the capsule in a twisted “belt” arrangement that results in a chiral assembly 1.24.1. The elongation of 1.1 with 2 is not limited to a single belt: longer guests can drive the assembly toward further extension with 2, 3 or 4 belts of glycoluril spacers incorporated. The twisted belt arrangement is apparently due to a geometric mismatch between the adjacent walls of cavitand 1, that are at right angles to each other (Figure 1e), and the ureido functions of 2 (Figure 1b) that are presented at the considerably larger angle of approximately 1138. The corresponding angle of the propanediurea 3 (Figure 1c) is smaller (ca. 998) and more appropriate as a complement to the right angles of the cavitand. Accordingly, we expected the interaction of 1.1 and 3 but were nonetheless surprised by the results. The insertion of propanediurea (PD) 3 (see Supporting Information for synthesis and characterization of 3) into capsule 1.1 was revealed by the use of commercially available n-alkanes as guest probes: Figure 2 shows the H NMR spectra of all capsular assemblies obtained. The spectra were recorded at 280 K, since all the assemblies exhibited sharp NMR peaks at that temperature. The shortest alkane leading to an extension of the original capsule 1.1 was ntetradecane (n-C14H30), which unexpectedly gave two new assemblies (labeled I and II ; Figure 2 line 1). Both were formed by insertion of four molecules of 3 between the two halves of capsule 1.1 as indicated by integration of the H NMR spectra (see Supporting Information, Figure SI4). The spectrum features the anticipated upfield shifts of the guest signals, and the identical signals for C1/C14, C2/C13, C3/C12 and C4/C11 indicate some symmetry: the two ends of the capsules have the same magnetic environment. The spacing of the alkane guest s methylene signals indicates an extended conformation with little or no compression (coiling). Both assemblies appear achiral on the NMR timescale at Figure 1. a) Models of the known cylindrical capsules 1.1 and 1.24.1. b) Structure and model of glycoluril 2. c) Structure and model of propanediurea 3 used for the studies described herein. d) Structure of cavitand 1. e) Model of cavitand 1. (Peripheral alkyl and aryl groups have been deleted for easier viewing.)