Alkylammonium Guests Research Articles

Hexahexyloxycalix[6]arene, a Conformationally Adaptive Host for the Complexation of Linear and Branched Alkylammonium Guests.

Hexahexyloxycalix[6]arene 2b leads to the endo-cavity complexation of linear and branched alkylammonium guests showing a conformational adaptive behavior in CDCl3 solution. Linear n-pentylammonium guest 6a+ induces the cone conformation of 2b at the expense of the 1,2,3-alternate, which is the most abundant conformer of 2b in the absence of a guest. In a different way, branched alkylammonium guests, such as tert-butylammonium 6b+ and isopropylammonium 6c+, select the 1,2,3-alternate as the favored 2b conformation (6b+/6c+⊂2b1,2,3-alt), but other complexes in which 2b adopts different conformations, namely, 6b+/6c+⊂2bcone, 6b+/6c+⊂2bpaco, and 6b+/6c+⊂2b1,2-alt, have also been revealed. Binding constant values determined via NMR experiments indicated that the 1,2,3-alternate was the best-fitting 2b conformation for the complexation of branched alkylammonium guests, followed by cone > paco > 1,2-alt. Our NCI and NBO calculations suggest that the H-bonding interactions (+N-H···O) between the ammonium group of the guest and the oxygen atoms of calixarene 2b are the main determinants of the stability order of the four complexes. These interactions are weakened by increasing the guest steric encumbrance, thus leading to a lower binding affinity. Two stabilizing H-bonds are possible with the 1,2,3-alt- and cone-2b conformations, whereas only one H-bond is possible with the other paco- and 1,2-alt-2b stereoisomers.

The Hexameric Resorcinarene Capsule at Work: Supramolecular Catalysis in Confined Spaces.

The hexameric resorcinarene capsule reported by Atwood in 1997 is able to act as a supramolecular catalyst. Its inner cavity provides a unique environment, in which organic reactions can be efficiently catalyzed, thanks to the confinement effect of the substrates. In addition, different stereo- and regiochemical outcomes can be observed with respect to reactions in the bulk solvent. The hexameric capsule shows some catalytic features reminiscent of natural enzymes. In particular, highlights of the capsule discussed herein include 1) its ability to recognize the substrates (substrate selectivity), 2) the possibility of stabilizing the transition states and intermediates through secondary interactions, 3) an inherent Brønsted acidity, and 4) its ability to act as a hydrogen-bond catalyst. In addition, it is also shown how the catalytic activity of the hexameric capsule can be modulated in the presence of competitive alkylammonium guests, which show high affinities for its internal cavity. These aspects are discussed through a critical examination of data reported in the literature in recent years.

Cation-Induced Molecular Switching Based on Reversible Modulation of Peptoid Conformational States.

Peptoids are oligomers of N-substituted glycines with predictable folding and strong potentials as guest-binding receptor molecules. In this contribution, we investigate the structural features of a series of designed symmetric cyclic octamer peptoids (with methoxyethyl/propargyl side chains) as free hosts and reveal their morphologic changes in the presence of sodium and alkylammonium guests as tetrakis[3,5-bis(trifluoromethyl)phenyl]borate salts, reporting the first case of reversible adaptive switching between defined conformational states induced by cationic guests (Na+ and benzylammonium ion) in the peptoid field. The reported results are based on 1H NMR data, theoretical models, and single-crystal X-ray diffraction analysis. They represent initial steps toward deciphering the unique conformational states of cyclic octamer peptoids as supramolecular hosts with the aim to fully disclose their functional and dynamic properties.

Controlling the Recognition and Reactivity of Alkyl Ammonium Guests Using an Anion Coordination-Based Tetrahedral Cage.

Caged structures have found wide application in a variety of areas, including guest encapsulation and catalysis. Although metal-based cages have dominated the field, anion-coordination-based cages are emerging as a new type of supramolecular ensemble with interesting host-guest properties. In the current work, we report a C3-symmetric tris-bis(urea) ligand based on the 2,4,6-triphenyl-1,3,5-triazine spacer, which assembles with phosphate anions to form an A4L4-type (A = anion, L = ligand) tetrahedral cage, 3, with unusually high packing coefficients (up to 99.5% for the best substrate). Cage 3 is able to adjust its size and shape (from 136 to 216 Å3) by bending of the triphenyltriazine plane. This allows it to accommodate relatively large guests. In the case of DABCO, inclusion within the cage allows the degree of methylation to be controlled and the monomethylated product to be isolated cleanly under conditions where mixtures of the mono- and dimethylated adduct are obtained in the absence of cage 3.

Alkylammonium Guest Induced‐Fit Recognition by a Flexible Dihomo­oxacalix[4]arene Derivative

AbstractThe conformationally mobile tetramethoxy‐dihomooxacalix[4]arene 1c is able to interact with linear and branched alkylammonium guests through an induced‐fit process mainly driven by H‐bonds and cation–π interactions. Although free 1c adopts a 1,4‐alternate conformation in solution, in the presence of linear nBuNH3+ and branched sBuNH3+ and iPrNH3+ guests, it preferentially forms endo‐complexes in a cone conformation through adaptive structural change in response to the presence of the ammonium guests. In accordance with previous results, the DFT‐optimized structures of the complexes indicate that they are mainly stabilized by H‐bonding interactions between the guest ammonium group and the host CH2OCH2 ethereal bridge.

Endo-Complexation of Alkylammonium Ions by Calix[4]arene Cavity: Facilitating Cation−π Interactions through the Weakly Coordinating Anion Approach

Notwithstanding its small dimensions, the narrow cavity of p-tert-butylcalix[4]arene macrocycle is still able to host alkylammonium guests in the presence of the tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (TFPB(-)) weakly coordinating (or "superweak") anion. As an artificial analogue of biological receptors, cone-shaped calix[4]arene 1d is able to recognize a n-BuNH3(+) guest mainly through a 4-fold cation-π interaction aided by a weak H-bonding interaction. This recognition motif can be used to assembly a 2:1 capsular architecture with a linear diammonium axle.

Alkylammonium Cation Complexation into the Narrow Cavity of Dihomooxacalix[4]arene Macrocycle

How big should a calixarene macrocycle be for endo-cavity complexation to occur or to allow through-the-annulus threading? To answer these questions, a complete study on the complexation of primary and secondary (di)alkylammonium cations by 18-membered p-tert-butyldihomooxacalix[4]arene macroring has been performed in the presence of the "superweak" TFPB counterion. Thus, it was found that this macrocycle is currently the smallest calixarene able to host linear and branched alkylammonium guests inside its aromatic cavity. Molecular mechanics calculations revealed that this recognition event is mainly driven by a H-bonding interaction between the guest ammonium group and the host CH(2)OCH(2) bridge. The endo-cavity complexation of chiral s-BuNH(3)(+) guest results in an asymmetric complex in the NMR time scale. The chirality transfer from guest to host is likely due to a restricted guest motion inside the tight cavity. The complexation study with secondary di-n-alkylammonium·TFPB salts revealed that the 18-membered dihomooxacalix[4]arene macroring cannot give the through-the-annulus threading with them because of its small dimension. However, the macrocycle is able to complex such ions, which can only be accommodated in an hook-like conformation characterized by two unfavorable gauche interactions around the CH(2)-NH(2)(+) bonds. The strain generated by this unfavorable folding is very likely compensated by stronger H-bonds and more favorable CH/π interactions between guest and host.

Self-Assembly of Resorcin[4]arene in the Presence of Small Alkylammonium Guests in Solution

We followed the self-assembly of C-alkyl-resorcin[4]arene (1a,1b) in the presence of tetraethylammonium salts (2). The X-ray structure of C-ethyl-resorcin[4]arene (1c) in the presence of TEABr (2b), showed a dimer encapsulating one guest. However, diffusion measurements reveal that two molecules of tetraethylammonium are encapsulated within the hexameric capsule of 1a,b in chloroform, regardless of the anion. Most importantly, the anion affects the role of water in the self-assembly and the nature of the formed capsules.

Ab Initio Study of the Complexation Behavior of Calix[6](aza)cryptand with Alkyl Ammonium Cations

The structures and complexation energies of p-tert-butylcalix[6](aza)cryptand 1 with a series of alkylammonium cations have been calculated by using ab initio HF/6-31G method. After geometry optimizations, B3LYP/6-31G(d) single point calculations of the final structures are carried out including the effect of an electron correlation and the basis set with a polarization function. The calculated complexation efficiencies of 1 for alkylammonium guests are better than those of the previously reported calix[5]arenes, and much better than those of the calix[4]crown-6-ether. The calculation results show that calix[6](aza)cryptand also has much better complexation ability with smaller ammonium cations than with bulky alkylammonium guests. The structural characterizations of the calculated complexes are described as the function of the nature of the alkyl substituents of the ammonium guests.

Ab Initio Study of the Complexation Behavior of p-tert-Butylcalix[5]arene Derivative toward Alkyl Ammonium Cations

The structures and complexation energies of penta-O-tert-butyl ester 1 of p-tert-butylcalix[5]arene toward a series of alkyl ammonium guests have been optimized by ab initio HF/6-31G method. The calculated complexation efficiencies of 1 for alkyl ammonium guests have been found to be similar to the values of previously reported debutylated-calix[5]arene 2. Calculation results show that both of the calix[5]aryl derivatives have much better complexation ability toward ammonium cation without alkyl group over other alkyl ammonium guests. The structural characteristics of the calculated complexes are discussed as a function of the nature of the alkyl substituents of the ammonium guests.

Ab Initio Study of the Complexation Behavior of Calix[5]arene Derivative toward Alkyl Ammonium Cations

The structures and complexation energies of penta-O-alkylated 1b and penta-O-tert-butyl ester 1e of p-tert-butylcalix[5]arene and their simplified structures (2b and 2e) toward a series of alkyl ammonium guests have been calculated by a semi-empirical AM1 method. For AM1 calculations, complexation efficiencies of the simplified host 2e are very similar to the values of host 1e. The complexes of simplified host 2e with alkyl ammonium ions also have been optimized by ab initio HF/6-31G method. The calculated complexation efficiencies for 2e by ab initio method have been found to be bigger in magnitude than the values obtained by AM1 calculations for linear alkyl ammonium guests. Calculation results show that all of the calix[5]aryl derivatives investigated in this study have much better complexation ability toward ammonium cation without alkyl group compared with other alkyl ammonium guests. Ab initio calculations also well duplicate the molecular discriminating behaviors of calix[5]arene derivative 2e between butyl ammonium ions: <TEX>$n-BuNH_3^+\;>\;iso-BuNH_3^+\;>\;sec-BuNH_3^+\;>\;tert-BuNH_3^+$</TEX>.

Ab Initio Study of Conformers of p-tert-Butylcalix[4]crown-6-ether Complexed with Alkyl Ammonium Cations

The structures and energies of p-tert-butylcalix[4]crown-6-ether(1) in various conformers and their alkyl ammonium complexes have been calculated by ab initio HF/6-31G quantum mechanics method. We have tried to obtain the relative affinity of partial-cone and 1,3-alternate conformers of 1 for alkyl ammonium guests by comparison with its cone-shaped analogue. We have also calculated the relative complexation efficiency of these host-guest complexes focusing on the binding sites of <TEX>$crown-\sigma-enther$</TEX> moiety or benzene-rings pocket of the host molecule 1. These calculations revealed that the crown moiey has better complexation efficiency than upper rim part of calyx[4]arene that is in similar trend to the cone-shaped complexes.