Molecular clips and cages derived from glycoluril

Cycloglycolurils: Hybrid Glycoluril-Cyclobenzil Macrocycles

Two novel glycoluril macrocycles have been synthesized from cyclotetrabenzil and cyclotribenzoin precursors using solvent-free condensations with urea. The crystal structure of the cyclotetra(p-phenylene)glycoluril macrocycle shows a twisted ring conformation, while that of the cyclotri(m-phenylene)glycoluril hybrid exhibits a distinct tubular supramolecular packing. These structures establish a potentially broad new class of macrocycles with intriguing guest binding properties owing to their available N-H motifs.

X-ray structure of an unusual glycoluril derivative conformation: Structural analysis and influencing factors

The synthesis and X-ray structure of a novel glycoluril derivative are described. The crystal of the glycoluril derivative is obtained by slow evaporation in a refrigerator at about 5 °C. X-ray crystallographic analysis revealed that the compound crystallized in an unusual as conformation instead of the usual clip-shaped aa conformation. Structural analysis shows that the solvation effect, hydrogen bonding, and space-occupying effect influenced the crystallization conformation of the derivative. The X-ray crystallographic analysis in this work provides additional crystallographic evidence of the as conformation of the 3U glycoluril derivative and shows the effect of solvent water on the conformational behavior.

An original self-assembly using a tetrathiafulvalene-based molecular clip for the recognition of fullerene C60

A glycoluril-based molecular clip incorporating tetrathiafulvalene sidewalls self-assembles with fullerene C₆₀ in a 2 : 1 stoichiometry in solution.

Synthesis and separation of cucurbit[n]urils and their derivatives

Cucurbit[n]uril chemistry has become an important part of contemporary supramolecular chemistry since cucurbit[n]urils (Q[n]s) are not only able to encapsulate various guests, but are also capable of coordinating to a wide range of metal ions, leading to the establishment of Q[n]-based host-guest chemistry and coordination chemistry. Each of these impressive developments can be attributed to the growth of protocols for obtaining Q[n]s. In this review, we survey synthetic procedures for obtaining cucurbit[n]urils and their substituted derivatives together with the separation and purification of these remarkable compounds. The coverage is aimed at both existing workers in the field as well as at those requiring an "entry" into Q[n]-based research.

Glycoluril-tetrathiafulvalene molecular clips: on the influence of electronic and spatial properties for binding neutral accepting guests

Glycoluril-based molecular clips incorporating tetrathiafulvalene (TTF) sidewalls have been synthesized through different strategies with the aim of investigating the effect of electrochemical and spatial properties for binding neutral accepting guests. We have in particular focused our study on the spacer extension in order to tune the intramolecular TTF···TTF distance within the clip and, consequently, the redox behavior of the receptor. Carried out at different concentrations in solution, electrochemical and spectroelectrochemical experiments provide evidence of mixed-valence and/or π-dimer intermolecular interactions between TTF units from two closed clips. The stepwise oxidation of each molecular clip involves an electrochemical mechanism with three one-electron processes and two charge-coupled chemical reactions, a scheme which is supported by electrochemical simulations. The fine-tunable π-donating ability of the TTF units and the cavity size allow to control binding interaction towards a strong electron acceptor such as tetrafluorotetracyanoquinodimethane (F4-TCNQ) or a weaker electron acceptor such as 1,3-dinitrobenzene (m-DNB).

Fused glycoluril-tetrathiafulvalene molecular clips as receptors for neutral electron acceptor guests

Glycoluril-based molecular clips incorporating tetrathiafulvalene (TTF) sidewalls have been synthesized, and the efficient binding ability in solution of this host architecture toward m-dinitrobenzene through donor-acceptor interaction has been demonstrated.

Molecular clips and tweezers hosting neutral guests

Intense current interest in supramolecular chemistry is devoted to the construction of molecular assemblies displaying controlled molecular motion associated to recognition. On this ground, molecular clips and tweezers have focused an increasing attention. This tutorial review points out the recent advances in the construction of always more sophisticated molecular clips and tweezers, illustrating their remarkably broad structural variety and focusing on their binding ability towards neutral guests. A particular attention is brought to recent findings in dynamic molecular tweezers whose recognition ability can be regulated by external stimuli. Porphyrin-based systems will not be covered here as this very active field has been recently reviewed.

Molecular recognition of organic ammonium ions in solution using synthetic receptors

Ammonium ions are ubiquitous in chemistry and molecular biology. Considerable efforts have been undertaken to develop synthetic receptors for their selective molecular recognition. The type of host compounds for organic ammonium ion binding span a wide range from crown ethers to calixarenes to metal complexes. Typical intermolecular interactions are hydrogen bonds, electrostatic and cation-π interactions, hydrophobic interactions or reversible covalent bond formation. In this review we discuss the different classes of synthetic receptors for organic ammonium ion recognition and illustrate the scope and limitations of each class with selected examples from the recent literature. The molecular recognition of ammonium ions in amino acids is included and the enantioselective binding of chiral ammonium ions by synthetic receptors is also covered. In our conclusion we compare the strengths and weaknesses of the different types of ammonium ion receptors which may help to select the best approach for specific applications.

Glycoluril trimers: selective synthesis and supramolecular properties

The first selective synthesis of glycoluril trimers is described. Trimers framed by o-xylylene walls represent new supramolecular hosts which are able to encapsulate bispyridinium ethylene and methylviologen guests in the solid state and aqueous solution.

Glycoluril dimers bearing hydrogen atoms on their convex face and their self-assembly in the solid state

A selective method for the synthesis of 1,6-substituted glycolurils has been developed. The glycolurils have been used for the synthesis of methylene-bridged glycoluril dimers bearing hydrogen atoms on their convex face. Depending on the side walls of the dimers, different modes of self-assembly in the solid state have been described using X-ray crystallography.

Aromatic glyoxalimines in criss-cross cycloaddition reactions

Aromatic 1,4-diazabuta-1,3-dienes in glacial acetic acid with thiocyanates produce via criss-cross cycloaddition reactions the corresponding perhydroimidazo[4,5-d]imidazole-2,5-dithiones. When a mixture of thiocyanate and cyanate in a proper ratio was reacted together, nonsymmetrical 5-thioxo-perhydroimidazo[4,5-d]imidazole-2-ones were isolated. With cyanates substituted aromatic 1,4-diazabuta-1,3-dienes afforded product of acetic acid addition to primary formed 1,3-dipole intermediate 5-(4-substituted phenylamino)-3-(4-substituted phenyl)-2-oxoimidazolidin-4-yl acetate.

Adjusting the binding thermodynamics, kinetics, and orientation of guests within large synthetic hydrophobic pockets

Kinetic analysis of the host guest complexation of a large, open molecular basket and a highly complementary adamantoid guest reveals that for these types of systems a dissociative mechanism is in operation. Hence, the resident adamantyl guest must completely vacate the cavity before another guest molecule can move in to replace it. As a result of the rigid nature of the host, the energy barrier to this process is relatively high, about 16 kcal mol(-1) at room temperature. Modifying the cavity of the host by dangling either a methyl group or a hydroxyl group from the portal rim alters the thermodynamic binding profile of these hosts. (1)H NMR shift data analysis also reveals that these functional groups can adjust the orientation that monosubstituted guests adopt within the cavity. Additionally, (1)H NMR studies of the binding of (E)1,4-dibromoadamantane allow the observation of two energetically similar diastereomeric complexes. An examination of this guest binding to the three hosts reveals that the interchange between the isomers is much faster than the entry and egression rates, and that the functional groups at the rim of each cavity influence both the rates of reorientation and the equilibrium relating the isomers.

Molecular clips based on propanediurea: exceptionally high binding affinities for resorcinol guests

A series of new receptor molecules derived from 2,4,6,8-tetraazabicyclo[3.3.1]nonane-3,7-dione (propanediurea) is described. These molecules possess a cavity which is defined by two nearly parallel aromatic side walls positioned on top of a bis-urea framework. The resulting "U-shaped" clip molecules are ideal hosts for the complexation of flat aromatic guest molecules. The affinity of these new propanediurea based molecular clips for dihydroxybenzene derivatives is exceptionally high, with association constants up to K(a) = 2 400 000 L mol(-)(1). Comparison of the binding mechanism of a variety of clip and half clip hosts, in conjunction with NMR, IR, and X-ray studies, has enabled the reason for this high binding to be elucidated. It is shown that subtle sub-angstrom changes in the geometry of the clip molecules have a great impact on their binding properties.

Diastereoselective Formation of Methylene-Bridged Glycoluril Dimers

The acid-catalyzed formation of methylene-bridged glycoluril dimers yields the C(2)(v)()-diastereomer selectively. Product resubmission experiments establish that the selectivity is the result of thermodynamic control. A modified synthetic route is presented that allows for the preparation of unsymmetrically substituted dimers. We present the X-ray crystal structures of both diastereomers. This class of compounds is useful for studies of self-assembly in aqueous solution.

Article

The crystal structure of 3,4,7,8-tetramethylglycoluril (5) was determined by X-ray diffraction. The structure reveals a hydrogen-bonding motif in the crystal lattice that differs from that present in related glycolurils. The two sides of each molecule form part of two independent, but parallel, infinite helical chains. These chains are formed by the NH donor and C==O acceptor on one side of a glycoluril molecule, forming H-bonds to two different molecules at adjacent positions within the helix. On the other side of the same molecule, a similar motif generates another helix of opposite helicity to the first. The molecule has a crystallographic plane of symmetry through the two bridgehead carbon atoms and the two bridgehead methyl groups, which are syn-periplanar. Thus, 5 is similar to 3,4-dimethylglycoluril (3), but differs from some glycolurils, where there is a significant dihedral angle between the two bridgehead-to-bridgehead substituent bonds. Bond lengths and angles in 5 resemble those reported for 3, but bond lengths around the bridgehead positions are slightly lengthened relative to 3.Key words: glycoluril, 1,2,5,8-tetramethyl-2,6,7,8-tetraazabicyclo[3.3.0]octane-3,7-dione, X-ray diffraction, crystal structure, hydrogen-bond array.