Water-soluble Cavitand Research Articles

An updated synthesis of octa-acid

ABSTRACT Octa-acid 1 is a water-soluble cavitand that has been used to investigate hydrophobic solvation and Hofmeister effects, control photophysical and physicochemical properties, modulate the reactivity of encapsulated guests, and as a tool to engender novel separation protocols. The synthesis of 1 has largely centered around its formation from 2, a host that is itself most readily synthesized on the multi-gram-scale in crude form (>75% purity). In this Methods Article we reveal improvements in the synthesis of 2, as well as a new synthetic strategy that efficiently converts crude 2 into pure 1. This provides access to 1 in only a five-step linear sequence, shortening the total reaction time from previous methods, improving the purity, and increasing the final yield of 1. We therefore anticipate that the described protocols will be of interest to researchers seeking to utilize octa-acid in their studies.

C-H⋅⋅⋅X-C bonds in alkyl halides drive reverse selectivities in confined spaces

Binding of non-activated alkyl halides (2–20) in water-soluble cavitand (1) through supramolecular forces is here reported, with emphasis on the role of size and polarizability of the halogen atom in the formation of intramolecular C-H hydrogen bonds in confined spaces. Rare reverse affinity in water (RI < RBr < RCl) is surprisingly observed for the more water-soluble short alkyl halides in dynamic open-ended containers. Competitive bindings and theoretical calculations confirm the unusual selectivity and the presence of C-H hydrogen bonds in non-activated systems for the first time, pointing out the importance and effect of subtle forces on molecular recognition in confined spaces.

Binding and Assembly of a Benzotriazole Cavitand in Water.

A water-soluble cavitand bearing a benzotriazole upper rim was prepared and characterized. It exists as a dimeric velcraplex in D2 O, but forms host-guest complexes with hydrophobic and amphiphilic guests. Alkanes (C5 to C10), cyclic ketones (C6-C10), cyclic alcohols (C6-C8) and various amphiphilic guests form 1 : 1 cavitand complexes. A cyclic array of hydrogen bonds, bridged by solvent/water (D2 O) molecules, stabilizes the vase conformation of the complexes. With longer alkanes (C12-C15), symmetrical dialkyl amine, urea and phosphate, 2 : 1 host:guest capsules are formed. Computations indicate that additional waters on the upper rim create a self-complementary hydrogen-bonding pattern for capsule formation.

Organic pollutants in water-soluble cavitands and capsules: contortions of molecules in nanospace

Unique binding modes of environmentally relevant long n-alkyl chain alcohols and n-alkylbenzenes within deep cavitands were discovered in water.

Hydrophobic and Metal-Coordinated Confinement Effects Trigger Recognition and Selectivity.

We report the synthesis and characterization of a new water-soluble cavitand 1. The container features 2-aminobenzimidazole panels at the "rim" and pyridiniums at the "feet". In the solid state, a single-crystal X-ray structure of the organic-soluble precursor 2 showed a stable vase form. The structure is stabilized by hydrogen-bonded bridges between adjacent panels through solvents and ions. In aqueous solution, binding of hydrophobic and amphiphilic guest molecules to 1 was investigated using 1H NMR. Alkanes, alcohols, acids, diols, and diacids formed 1:1 host-guest complexes, and the guest conformations were deduced from characteristic chemical shift changes. In the presence of [Pd(ethylenediamine)(H2O)2·2NO3], cavitand 1 formed a complex incorporating two metals. The metal-coordinated cavitand also bound hydrophobic linear alkanes and difluorobenzene isomers in aqueous medium. The metallo-cavitand showed shape and size selectivity and was used to separate o-difluorobenzene from its isomers as observed by 19F NMR spectroscopy. The primary amino function of the cavitands offers possibilities for further elaboration to covalent clusters of these container compounds.

Dual Binding Modes of a Small Cavitand

ABSTRACT The small size and high cohesiveness of water means that water-mediated interactions are strongly context dependent.As a result, there is still much to learn about how non-polar solutes and ions interact with themselves or each other.To help address this issue, we report here on a cavitand host, TMAX-Cl (2). Possessing two different binding sites, a shallow non-polar dish that binds hydrophobes, and a crown of ammoniums that bind anions, TMAX-Cl (2) provides insight into the hydrophobic and Hofmeister effects.We find that binding to the non-polar site is weak, suggesting that a larger surface area is needed for substantial binding.In contrast, binding to the crown of ammoniums is relatively strong, despite the high dielectric of water.These findings provide a better understanding of water-mediated interactions, and define the supramolecular properties of TMAX-Cl 2 as we continue our studies of this host and related water-soluble cavitands.

Recognition of Hydrophilic Cyclic Compounds by a Water-Soluble Cavitand.

A water-soluble deep cavitand bearing amides on the upper rim and trimethyl ammonium groups on the feet was synthesized. The open-ended cavity is stabilized by the intramolecular hydrogen bonds formed between the adjacent amides, and the introduction of trimethylammonium imparts to the cavitand good solubility in water. The cavitand exhibits high binding affinity and selectivity to hydrophilic molecules in water. With certain guests, such as cyclohexyl alcohols, amines and acids, the recognition involves the synergistic action of hydrogen bonding with hydrophobic effects. The binding phenomena are interpreted in terms of a fixed solvent cage presented by the host to the guest.

Theoretical investigation on the binding of alkyl halides and cyclohexyl halides in water-soluble cavitands

Encapsulation complexes of hexyl- and cyclohexyl-halides in water soluble cavitands with urea rims have been investigated by DFT calculations. In agreement with experiment, the most stable conformations of the different encapsulated halides have the halide at the rim of the cavity, or “up”. The possibility of halogen bonding of Br to the resorcinarenes of the cavitand is suggested by the near degeneracy of “up” and “down” conformations found in encapsulated bromides. In the calculations, in addition to the implicit inclusion of the solvent, explicit addition of water molecules at the rim of the tetra-urea cavitand is found to be necessary.

A new water-soluble cavitand with deeper guest binding properties

A new water-soluble cavitand captured hydrophobic or amphiphilic molecules in an aqueous solution owing to its deep aromatic pocket.

Binding orientation and reactivity of alkyl α,ω-dibromides in water-soluble cavitands.

Host-guest complexation of long chain α,ω-dibromides was evaluated in deep water-soluble cavitands 1 and 2. The bound dibromides (C7-C12) tumble rapidly on the NMR timescale and averaged signals were observed. The complexation allows mono hydrolysis of dibromides in aqueous solution. The arrangement of the products in the host-guest complex was fixed in an unsymmetrical manner that protects the guest from further reaction. Up to 93% yields of the mono-alcohols were obtained. The α,ω-dibromides formed a capsule with cavitand 2 and remained unreactive to hydrolysis.

Asymmetric binding of symmetric guests in a water-soluble cavitand

Binding of long aliphatic chains in water-soluble container compounds is driven by the hydrophobic effect: the CH2 groups of the guests are buried from water and solvate the inner aromatic surface of the host. The hydrophobic forces and the CH-π attractions are often sufficient to contort the guests into otherwise unfavorable shapes. Here we show that alkanes, α,ω-diols, α,ω-diacids and their methyl esters assume folded, J-shaped conformations in a water-soluble cavitand. The environment of the guest’s ends interconvert rapidly on the NMR timescale through yo-yo like motions. The applications of water-soluble cavitands to reaction processes are discussed in the context of moving vs. fixed guest ends.

Binding of alkyl halides in water-soluble cavitands with urea rims

Alkyl halide guests in cavitands move rapidly and maintain halide to contact with the aryl surfaces of the host.

Supramolecular self-assembly of water-soluble cavitands: investigated by molecular dynamics simulation

In this study, we have examined supramolecular self-assembly process of a hydrophobic guest with a water-soluble host known by the trivial name octa acid (OA). Two octa acids form a capsular assembly only in presence of a nonpolar guest(s). Size and shape of the guest control the stoichiometry of the capsular complex. Here, all atom molecular dynamics simulation has been utilized to investigate complex formation mechanisms of a nonpolar guest (nonylbenzene) with two OA cavitands. Nonylbenzene was encapsulated into the nonpolar cavity of OA capsule owing to solvophobic interactions. Upon encapsulation it was twisted and bent due to lack of free space within the capsule. These unusual forms obtained from the simulation study were in accord with experimental findings. The post-complexation attributes of the guest were regulated by the available free space within the OA and favorable non-covalent interactions between the guest and the walls of the OA capsule. In the identical simulation condition two OA cavitands did not form a capsule without a guest, thus indicating requirement of a guest during the self-assembly of OA cavitands.

Macrocyclization of Folded Diamines in Cavitands.

Synthetic access to water-soluble cavitands and capsules has moved recognition events from organic solvents into aqueous media. Here we report the binding and reactivity of long-chain α,ω-diamines (C11 to C18) in cavitand hosts. The containers bind the diamines in folded conformations that bury the hydrocarbon chains and expose the amino groups to the aqueous medium. Their acylation with succinic anhydride results in improved yields of monofunctionalized products. The cavitand-bound amino acid products were cyclized to the corresponding macrocyclic dilactams in D2O using water-soluble carbodiimide. Direct reaction of the folded diamines in the cavitand with activated diesters of succinic acid and glutaric acids resulted in 54-96% yields of the 17- to 25-membered dilactams. These cavitand-chaperoned reactions provided 3- to 10-fold improvements over the yields obtained in bulk solution and offer an alternative to high dilution methods. The cavitand induces unlikely conformations in flexible guests and channels their reactivity along otherwise improbable paths.

Water-soluble cavitands promote hydrolyses of long-chain diesters

Water-soluble, deep cavitands serve as chaperones of long-chain diesters for their selective hydrolysis in aqueous solution. The cavitands bind the diesters in rapidly exchanging, folded J-shape conformations that bury the hydrocarbon chain and expose each ester group in turn to the aqueous medium. The acid hydrolyses in the presence of the cavitand result in enhanced yields of monoacid monoester products. Product distributions indicate a two- to fourfold relative decrease in the hydrolysis rate constant of the second ester caused by the confined space in the cavitand. The rate constant for the first acid hydrolysis step is enhanced approximately 10-fold in the presence of the cavitand, compared with control reactions of the molecules in bulk solution. Hydrolysis under basic conditions (saponification) with the cavitand gave >90% yields of the corresponding monoesters. Under basic conditions the cavitand complex of the monoanion precipitates from solution and prevents further reaction.

Preparative scale and convenient synthesis of a water-soluble, deep cavitand.

Cavitands are established tools of supramolecular chemistry and molecular recognition, and they are finding increasing application in sensing and sequestration of physiologically relevant molecules in aqueous solution. The synthesis of a water-soluble, deep cavitand is described. The route comprises six (linear) steps from commercially available precursors, and it relies on the fourfold oligomeric cyclization reaction of resorcinol with 2,3-dihydrofuran that leads to the formation of a shallow resorcinarene framework; condensation with aromatic panels, which deepens the hydrophobic binding cavity; construction of rigid urea functionalities on the upper rim; and the introduction of the water-solubilizing methylimidazolium groups on the lower rim. Late intermediates of the synthesis can be used in the preparation of congener cavitands with different properties and applications, and a sample of such a synthetic procedure is included in this protocol. Emphasis is placed on scaled-up reactions and on purification procedures that afford materials in high yield and avoid chromatographic purification. This protocol provides improvements over previously described procedures, and it enables the preparation of sizable amounts of deep cavitands: 7 g of a water-soluble cavitand can be prepared from resorcinol in 13 working days.

Cavitands as Chaperones for Monofunctional and Ring-Forming Reactions in Water.

Cyclic processes involving medium-sized rings show low rates because internal strains-torsions and transannular interactions-are created during the reactions. High dilution is often used to slow the competing bi- and higher-molecular processes but cannot accelerate the desired cyclization reaction. Here we apply cavitands to the formation of medium- to large-sized rings through conversion of long-chain diisocyanates to cyclic ureas. The reactions take place in aqueous (D2O) solution, where hydrophobic forces drive the starting materials into the cavitands in folded conformations. The guest assumes the shape to fill the space properly, which brings the reacting ends closer together than they are in bulk solvent. Complexation overcomes some of the internal strains involved in precyclization shapes of the guest molecules and accelerates the cyclization. The results augur well for applications of water-soluble cavitands to related processes such as remote functionalization reactions.

Cavitands as Reaction Vessels and Blocking Groups for Selective Reactions in Water.

The majority of reactions currently performed in the chemical industry take place in organic solvents, compounds that are generally derived from petrochemicals. To promote chemical processes in water, we examined the use of synthetic, deep water-soluble cavitands in the Staudinger reduction of long-chain aliphatic diazides (C8 , C10 , and C12 ). The diazide substrates are taken up by the cavitand in D2 O in folded, dynamic conformations. The reduction of one azide group to an amine gives a complex in which the substrate is fixed in an unsymmetrical conformation, with the amine terminal exposed and the azide terminal deep and inaccessible within the cavitand. Accordingly, the reduction of the second azide group is inhibited, even with excess phosphine, and good yields of the monofunctionalized products are obtained. In contrast, the reduction of the free diazides in bulk solution yields diamine products.

Synthesis of tri-arylated cyclotriveratrilenes with ortho- and meta-extended functionality

Aromatic nucleophilic substitution reaction between cyclotriguaiacylene and ortho- or meta-functionalized fluoroarenes affords a series of ortho- or meta-extended cyclotriveratrilene (CTV) cavitands. Further transformation of the functional groups into NH and/or OH moieties has been demonstrated. This enabled us to prepare an amphoteric water-soluble cavitand bearing anilino-NH2 and phenolic-OH substituents. In addition, one molecular structure was successfully determined by crystallographic analysis, which suggests an extended/flattened structure. We propose that vase-shaped conformations with inwardly directed functional groups will soon be possible with the CTV scaffold.

Water-soluble octa acid capsule as a reaction container: Templated photodimerization of indene in water

Abstract In the context of green chemistry, identifying strategies to carry out organic reactions in aqueous medium in the absence of organic and inorganic reagents is important. We show below that octa acid (OA), a water-soluble cavitand, is capable of solubilizing water insoluble reactant in water and yielding selective product by confining the reactants and intermediates during a light initiated reaction. By employing indene as the substrate we have observed that the OA capsule upon irradiation facilitates the formation of anti-head–tail dimer, the isomer that is generally not obtained upon direct excitation and triplet and electron transfer sensitizations in isotropic organic solvents.