Effect of <Emphasis Type="Italic">ortho</Emphasis>-methyl groups in the benzene rings of the macrocyclic frame on the chemistry of phosphacavitands

Effect of ortho-methyl groups in the benzene rings of the macrocyclic matrix on the chemistry of cavitands with phosphorous amide bridges in the upper rim is studied. The presence of ortho-methyl groups is shown to prevent formation of phosphacavitands of C _4v symmetry and favor formation of macrocyclic systems of C _2v symmetry, enhance solubility of phosphacavitands in organic solvents, hinder oxidation of phospha(III)cavitands and decrease the yield of phospha(V)cavitands, prevent formation of binuclear molybdenum complexes of phosphorous amide cavitands, and favor formation of their tetranuclear analogs.     

Conformational behavior of pyrazine-bridged and mixed-bridged cavitands: a general model for solvent effects on thermal "vase-kite" switching

The controllable switching of suitably bridged resorcin[4]arene cavitands between a "vase" conformation, with a cavity capable of guest inclusion, and a "kite" conformation, featuring an extended flattened surface, provides the basis for ongoing developments of dynamic molecular receptors, sensors, and molecular machines. This paper describes the synthesis, X-ray crystallographic characterization, and NMR analysis of the "vase-kite" switching behavior of a fully pyrazine-bridged cavitand and five other mixed-bridged quinoxaline-bridged cavitands with one methylene, phosphonate, or phosphate bridge. The pyrazine-bridged resorcin[4]arene cavitand displayed an unexpectedly high preference for the kite conformation in nonpolar solvents, relative to the quinoxaline-bridged analogue. This observation led to extensive solvent-dependent switching studies that provide a detailed picture of how solvent affects the thermal vase-kite equilibration. As for any thermodynamic process in the liquid phase, the conformational equilibrium is affected by how the solvent stabilizes the two individual states. Suitably sized solvents (benzene and derivatives) solvate the cavity of the vase form and reduce the propensity for the vase-to-kite transition. Correspondingly, the kite geometry becomes preferred in bulky solvents such as mesitylene, incapable of penetrating the vase cavity. As proposed earlier by Cram, the kite form is preferred at low temperatures due to the more favorable enthalpy of solvation of the enlarged surface. Furthermore, the kite conformation is more preferred in solvents with substantial hydrogen-bonding acidity: weak hydrogen-bonding interactions between the mildly basic quinoxaline and pyrazine nitrogen atoms and solvent molecules are more efficient in the open kite than in the closed vase form. Vase-to-kite conversion is entirely absent in dipolar aprotic solvents lacking any H-bonding acidity. Thermal vase-kite switching requires fully quinoxaline- or pyrazine-bridged cavitands, whereas pH-controlled switching is also applicable to systems incorporating only two or three such bridges.     

Resorcin[4]arene-derived mono-, bis- and tetra-imidazolium salts as ligand precursors for Suzuki-Miyaura cross-coupling

Eleven resorcinarene cavitands bearing either one, two or four (3-R-1-imidazolylium)-methyl substituents (R = (n)Bu, Ph, Mes, (i)Pr(2)C(6)H(3)) anchored at resorcinolic "ortho" positions have been synthesised from the appropriate bromomethylated precursor. Combining the imidazolium salts with palladium acetate and Cs(2)CO(3) gave active Suzuki-Miyaura cross coupling catalysts. The highest activities were observed with the doubly functionalised cavitands, which all have the imidazolylium groups attached to proximal resorcinol units.     

State-of-the-art and recent progress in resorcinarene-based cavitand

Compartmentalization in the biological world brings excellent efficiency and specificity to the formation of complex compounds, inspiring supramolecular chemists to continuously search for defined spaces that can mimic such natural binding sites. Bowl-shaped cavitands built up from resorcinarenes (RA) present rigid and preorganized concave surfaces, which are capable of mimicking the molecular recognition properties of enzymes. The versatile scaffold of RA endows the cavitand with terrific variety and excellent binding behavior. This review provides a comprehensive overview over the structural modification to date in the high attention field of RA-based cavitands development. Different strategies for synthesizing diverse cavitands, such as small cavity cavitands, wider cavity cavitands, deep cavity cavitands, biscavitands, and asymmetric cavitands, are discussed in details. Furthermore, insights into their applications including catalysis, separations and sensing are provided.     

Synthesis of calixarene-based cavitands and nanotubes by click chemistry

The synthesis of a variety of calixarene-based cavitands (capped and functional calixarenes) and calix nanotubes is easily performed in good to high yields using "click chemistry" methodology through the Cu(I)-catalyzed ligation of adequate bis-alkyne and bis-azide derivatives.     

Synthesis of nanoscale carceplexes from deep-cavity cavitands

Deep-cavity cavitands were shown to undergo carceplex reactions in which two cavitand tetrols were covalently linked using bromochloromethane; the efficiency of the "dimerization" was closely tied to the templating guest molecule incarcerated within the host.     

A new [4]carceplex, and a crystal structure and dynamic combinatorial chemistry of a [5]carceplex

A new [4]carceplex (2·guest) is reported. It is composed of two cavitands linked by four disulfide bonds. It forms twistomers, which interconvert on a millisecond timescale. The energy barrier for interconversion of twistomers is guest-dependent. Formation of [4]carceplex 2·guest is template dependent. The selectivity in templates is flat relative to most previous related template work. Larger kin [5]carceplex 1·guests were reinvestigated. A crystal structure confirms the twist between the hemispherical cavitands. Use of a redox buffer allowed dynamic combinatorial chemistry to be performed between pairs of templates.     

Synthesis and some properties of tetrakis-3,5-di-<Emphasis Type="Italic">tert</Emphasis>-butyl-4-hydroxybenzylated calix[4]resorcinols

A method for the synthesis of new calix[4]resorcinols tetra-3,5-di-tert-butyl-4-hydroxybenzyl derivatives is developed. Their interaction with methyldichlorophosphonate, dimethyldichlorosilane in the presence of a base leads to formation of organophosphorus-organosilicon cavitands. Acetylation of hydroxybenzylated calix[4]resorcinols with acetic anhydride leads to products of either incomplete or full acetylation depending on experimental conditions.     

Cyclic Phosphorochloridites and Phosphorochloridates Derived from Bromocalix[4]resorcinolarenes

Reactions of bromocalix[4]resorcinolarenes with PCl₃ and POCl₃ in the presence of triethylamine give the corresponding phosphorochloridito and phosphorochloridato cavitands in good yields. Phosphorochloridito cavitands were converted into phosphoramidito cavitands by treatment with piperidine.     

Supramolecular sensing with phosphonate cavitands

Phosphonate cavitands are an emerging class of synthetic receptors for supramolecular sensing. The molecular recognition properties of the third-generation tetraphosphonate cavitands toward alcohols and water at the gas-solid interface have been evaluated by means of three complementary techniques and compared to those of the parent mono- and diphosphonate cavitands. The combined use of ESI-MS and X-ray crystallography defined precisely the host-guest association at the interface in terms of type, number, strength, and geometry of interactions. Quartz crystal microbalance (QCM) measurements then validated the predictive value of such information for sensing applications. The importance of energetically equivalent multiple interactions on sensor selectivity and sensitivity has been demonstrated by comparing the molecular recognition properties of tetraphosphonate cavitands with those of their mono- and diphosphonate counterparts.     

Self-complexed deep cavitands: alkyl chains coil into a nearby cavity

[structure: see text]. A series of self-complexing tetrabenzimidazole cavitands have been synthesized. Alkyl chains appended to the rim helically coil into the cavity in competitive organic solvents. These "flexible arms" exchange with each other, a process shown by 2D NMR to proceed via an intermediate solvent-containing species.     

Lower rim functionalized resorcinarenes: useful modules for supramolecular chemistry

[formula: see text] A synthetic scheme for the selective functionalization of all-cis (rccc) resorcinarene platform at the "lower rim" was developed. Self-folding and self-complementary cavitands were prepared for molecular recognition and self-assembly, bearing functionality at remote sites. These molecules promise applications on solid support and as polymeric capsules.     

Deep cavitand vesicles - multicompartmental hosts

The synthesis and characterization of vesicles assembled from deep cavitands in water is reported. These vesicles act as hosts for three different types of guests: the cavitands bind small guest molecules, the bilayer attracts larger hydrophobic guests and the inner aqueous compartment contains hydrophilic molecules.     

Macrocycles and cavitands containing phosphorus

Macrocycles and cavitands containing phosphorus represent two interesting and promising classes with special properties and applications as catalysts or in biology. In this paper we tried to review the progress in synthesis of macrocycles and cavitands containing phosphorus.     

Expanding cavitand chemistry: the preparation and characterization of [n]cavitands with n>=4

The preparation of cavitands composed of 4, 5, 6, and 7 aromatic subunits ([n]cavitands, n=4-7) is described. The simple, two-step synthetic procedure utilized readily available starting materials (2-methylresorcinol and diethoxymethane). The two cavitand products having 4 and 5 aromatic subunits exhibited highly symmetric cone conformations, while the larger cavitands (n = 6 and 7) adopt conformations of lower symmetry. 1H NMR spectroscopic studies of [6]cavitand and [7]cavitand revealed that these hosts undergo exchange between equivalent conformations at room temperature. The departure of these two cavitands from cone conformations is related to steric crowding on their Ar-O-CH2-OAr bridges and is predicted by simple molecular mechanics calculations (MM2 force field). X-ray diffraction studies on single crystals of the [4]cavitand, [5]cavitand, and [6]cavitand hosts afforded additional experimental support for these conclusions.     

Gold nanoparticles functionalized with deep-cavity cavitands: synthesis, characterization, and photophysical studies

In this report, we present methods of functionalization of AuNP's with deep-cavity cavitands that can include organic molecules. Two types of deep-cavity cavitand-functionalized AuNP's have been synthesized and characterized, one soluble in organic solvents and the other in water. Functionalized AuNP soluble in organic solvents forms a 1:1 host-guest complex where the guest is exposed to the exterior solvents. The one soluble in water forms a 2:1 host-guest complex where the guest is protected from solvent water. Phosphorescence from thiones and benzil included within heterocapsules attached to AuNP was quenched by gold atoms present closer to the guests included within deep-cavity cavitands. During this investigation, we have synthesized four new deep-cavity cavitands. Of these, two thiol-functionalized hosts allowed us to make stable AuNP's. However, AuNP's protected with two amine-functionalized cavitands tended to aggregate within a day.     

ESI-FTICR mass spectrometric study of alcohol complexation properties of mono- and diphosphonate-bridged cavitands

Alcohol complexation properties of eight mono- and diphenyl phosphonate-bridged cavitands were studied by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FTICR) and theoretical calculations. The cavitands varied in number and position of phenyl phosphonate bridges and their orientation with respect to the cavity, length of the lower rim alkyl chains, and substituents at apical positions of the resorcarene skeleton. The specificities of the different cavitands toward primary, secondary, and tertiary alcohols varying long of the alkyl chain were investigated, together with the stabilities of the formed complexes. The number, position, and orientation of the P = O moieties affected the complex formation of the cavitands and stability of the complexes dramatically. Methyl groups at apical positions of the resorcarene skeleton also affected the complexation properties. Although length and branching of the alkyl chain of the alcohol influenced the complex formation, the effect on stability of the complexes was negligible.     

‘Cloverleaf’ crown ether resorcin[4]arenes

Crown ether resorcin[4]arenes 1a, 1b, and 2b with a novel connectivity have been prepared from diquinoxaline cavitands in yields of 68%, 73%, and 25%, respectively. The reaction involves bridging the contiguous oxygen atom pairs with tri-, tetra- or pentaethylene glycol ditosylates.     

Recent Advances in the Applications of Water-soluble Resorcinarene-based Deep Cavitands

We review here the use of container molecules known as cavitands for performing organic reactions in water. Central to these endeavors are binding forces found in water, and among the strongest of these is the hydrophobic effect. We describe how the hydrophobic effect can be used to drive organic molecule guests into the confined space of cavitand hosts. Other forces participating in guest binding include cation−π interactions, chalcogen bonding and even hydrogen bonding to water involved in the host structure. The reactions of guests take advantage of their contortions in the limited space of the cavitands which enhance macrocyclic and site-selective processes. The cavitands are applied to the removal of organic pollutants from water and to the separation of isomeric guests. Progress is described on maneuvering the containers from stoichiometric participation to roles as catalysts.     

Synthesis, structure, fullerene-binding and resolution of C3-symmetric cavitands with rigid and deep cavities

An efficient palladium-catalyzed Suzuki-Miyaura coupling method involving the reaction between CTV-Br(3) and a variety of aryl and heteroaryl boronic acids in the presence of indolyl phosphane ligands has been developed. This reaction procedure provided a series of C(3)-symmetric aryl-extended rigid cavitands for the first time. X-ray crystal structure analysis revealed that the phenyl substituted cavitand 5a has much larger rim edges and cavity height. This macrocyclic host adopts a linear head-to-tail "hand-shake" self-inclusion arrangement in the crystalline state. The fluorescence of 5a was considerably quenched upon the addition of C(60), with a binding constant of 78,700 ± 2300 dm(3) mol(-1) and a 1:1 stoichiometry according to the Job's plot. The interaction of C(60) with 5a in the excited state is stronger than that with CTV, which could be attributed to more binding sites in the extended arms of 5a. Moreover, optically active C(3)-symmetric cavitands (+)- and (-)-6 were easily obtained with high efficiency through chemical resolution.