Metal-containing ditopic receptors for molecular recognition of diammonium cations

Two new ditopic receptors for α,ω-alkanediyldiammonium cations based on a tetraazamacrocyclic (cyclidene) nickel(II) complex bearing two crown-ether residues were synthesized. The studies of the host-guest interaction between the receptors and a series of α,ω-diammonium salts by NMR titration in acetonitrile-d₃ showed that 1:1 complexes are formed with K_assoc∼10³-10⁵ M⁻¹. Receptor 1 with benzo-15-crown-5 arms showed substantial selectivity in binding of trimethylene- and tetramethylenediammonium dications, and 1-2 orders of magnitude weaker binding of shorter (C₂) or longer (C₅ and C₆) diammonium cations. Receptor 2 with benzo-18-crown-6 arms showed higher affinity to all studied diammonium cations, but the recognition of the length of α,ω-diammonium cations was less pronounced.     

Nitronate anion recognition and modulation of ambident reactivity by hydrogen-bonding receptors

Nitronate anions were shown to form complexes in DMSO with hydrogen-bonding receptors such as 1,3-dimethylthiourea 1 (K(a)= 120M(-1)) and bicyclic guanidinium 2 (K(a) = 3200M(-1)). A ditopic bis-thiourea exhibited increased association with substrates, that contained either two nitronates (K(a)= 7000M(-1)) or a combination of nitronate and carboxylate (K(a)=7200M(-1)). Complexation of nitronate resulted in a change in the ambident reactivity during alkylation with p-nitrobenzyl bromide. The predominant reaction pathway was shifted from oxygen alkylation to carbon alkylation as receptor binding strength increased. Kinetic analysis indicated an overall inhibition of nitronate reactivity, and this suggests that greater suppression of the oxygen pathway allows carbon alkylation to predominate.     

Uranyl-salophen based ditopic receptors for the recognition of quaternary ammonium halides

Uranyl-salophen complexes endowed with aromatic side arms behave as very efficient ditopic receptors towards tetralkylammonium halides as a result of a combination of Lewis acid-base and cation-pi interactions.     

The cooperative effect in ion-pair recognition by a ditopic hemicryptophane host

The heteroditopic hemicryptophane 1 , which bears a tripodal anion binding site and a cation recognition site in the molecular cavity, proved to be an efficient ion‐pair receptor. The hemicryptophane host binds anions selectively depending on shape and hydrogen‐bond‐accepting ability. It forms an inclusion complex with the Me₄N⁺ ion, which can simultaneously bind anionic species to provide anion@[ 1? Me₄N⁺] complexes. The increased affinity of [ 1? Me₄N⁺] for anionic species is attributed to a strong cooperative effect that arises from the properly positioned binding sites in the hemicryptophane cavity, thus allowing the formation of the contact ion pair. Density functional theory calculations were performed to analyze the Coulomb interactions of the ion pairs, which compete with the ion‐dipole ones, that originate in the ion–hemicryptophane contacts.     

Thermodynamic aspects of dicarboxylate recognition by simple artificial receptors.

Recognition of dicarboxylates by bis-functional hydrogen-bonding receptors displays divergent thermodynamics in different solvent systems. NMR titration and isothermal titration calorimetry indicated that neutral bis-urea and bis-thiourea receptors form exothermic complexes with dicarboxylates in DMSO, with a near zero entropic contribution to binding. The increased binding strength of bis-guanidinium receptors precluded quantitative measurement of binding constants in DMSO, but titration calorimetry offered a qualitative picture of the association. Formation of these 1:1 complexes was also exothermic, but additional endothermic events occurred at both lower and higher host-guest ratios. These events indicated multiple binding equilibria but did not always occur at a discrete 2:1 or 1:2 host-guest molar ratio, suggesting higher aggregates. With increasing amounts of methanol as solvent, bis-guanidinium receptors form more endothermic complexes with dicarboxylates, with a favorable entropy of association. This switch from association driven by enthalpy to one driven by entropy may reflect a change from complexation involving the formation of hydrogen bonds to that promoted by solvent liberation from binding sites.     

Molecular recognition of carbohydrates by artificial receptors: systematic studies towards recognition motifs for carbohydrates

The synthesis and binding properties for carbohydrates of several artificial, acyclic receptors containing two or three heterocyclic recognition units covalently attached to a phenyl spacer is described. These host molecules having uncharged hydrogen-bonding sites were used in a systematic study towards the evaluation of recognition motifs for carbohydrates. A novel effective, acyclic hydrogen-bonding receptor possessing naphthyridine-amide moieties as heterocyclic recognition units has been developed.     

Molecular recognition of alkylammonium contact ion-pairs using a ditopic receptor

A ditopic, macrobicyclic receptor with adjacent anion and cation binding sites is able to distinguish between various monoalkylammonium salts by binding them as contact ion-pairs. The affinity for linear n-propylammonium chloride is at least 2 orders of magnitude greater than that for n-propylammonium acetate, n-propylammonium p-toluenesulfonate, and branched isopropylammonium chloride. An X-ray structure of the receptor complexed with methylammonium chloride illuminates the basis of the molecular recognition.     

Highly efficient hydrogen-bonding catalysis of the Diels-Alder reaction of 3-vinylindoles and methyleneindolinones provides carbazolespirooxindole skeletons

Carbazolespirooxindole derivatives were synthesized in a high-yielding, atypically rapid, stereocontrolled Diels-Alder reaction catalyzed by a C(2)-symmetric bisthiourea organocatalyst. Simple precursors and mild conditions were used to construct carbazolespirooxindole derivatives with high enantiopurity and structural diversity under H-bonding catalysis. The practical approach recycles the organocatalyst and solvent. This simple and efficient operational procedure will allow diversity-oriented syntheses of this intriguing class of compounds.     

Highly efficient and enantioselective Michael addition of acetylacetone to nitroolefins catalyzed by chiral bifunctional organocatalyst bearing multiple hydrogen-bonding donors

A new efficient catalyst system for the asymmetric addition of acetylacetone to nitroolefins using a chiral bifunctional organocatalyst bearing multiple hydrogen-bonding donors was developed. When using the organocatalyst 2c derived from natural cinchona alkaloid in optimal conditions, up to 98% chemical yield and 98% ee were observed with a variety of aromatic nitroolefins.     

High alpha/beta-anomer selectivity in molecular recognition of carbohydrates by artificial receptors

[structure: see text] New effective, acyclic, pyridine-based receptors 1-3 show remarkable alpha/beta binding selectivity in the recognition of monosaccharides. They are able to participate in cooperative and bidentate hydrogen bonds with sugar hydroxyls as well as in CH-pi interactions with CH's of sugar molecules.     

Cooperative recognition of C60-ammonium substrates by a ditopic oligophenylenevinylene/crown ether host

A new fullerene derivative with an ammonium subunit has been prepared. Its ability to form supramolecular complexes with oligophenylenevinylene derivatives bearing one or two crown ether moieties has been evidenced by electrospray mass spectrometry, and UV-visible and luminescence spectroscopy experiments. Interestingly, the assembly of the C60-ammonium cation with the oligophenylenevinylene derivative bearing two crown ether moieties leads to the cooperative formation of the 2:1 complex owing to intramolecular fullerene-fullerene interactions.     

Highly regioselective and diastereodivergent aminomethylative annulation of dienyl alcohols enabled by a hydrogen-bonding assisting effect

A ligand-controlled palladium-catalyzed highly regioselective and diastereodivergent aminomethylative annulation of dienyl alcohols with aminals has been established, which allows for producing either cis- or trans-disubstituted isochromans in good yields with complete regioselectivity and good to excellent diastereoselectivity. Moreover, the chiral cis-products were also obtained in good yields with up to 94% ee by using a chiral phosphinamide as the ligand. Mechanistic studies revealed that the hydroxyl group plays a key role in facilitating the Pd-catalyzed Heck insertion regioselectively taking place across the internal C C bond of conjugated dienes.

An efficient hydrogen-bonding assisted directing strategy has been identified, which enables the Pd-catalyzed highly regioselective and diastereodivergent 3,4-difunctionalized aminomethylative annulation of dienyl alcohols with aminals.     

Chemical probes for the recognition of cannabinoid receptors in native systems

Receptors made visible: The described biotin-tagged small-molecule probes with excellent affinities for the CB(1) and CB(2) cannabinoid receptors (CB(1)R and CB(2)R) enable direct visualization of these receptors in native cellular systems, including neurons, microglia, and immune cells. This method could overcome some of the limitations of current methodologies and may help to dissect the complexity of the endogenous cannabinoid system.     

Calix[4]arene-based ditopic receptors for simultaneous recognition of fluoride and cobalt(II) ions

A series of calix[4]arene based ditopic receptors possessing bipyridyl and hydrazone units have been synthesized and evaluated for ionic recognition. It has been observed that the synthesized derivatives function as allosteric receptors for simultaneous recognition of Co²⁺and F^? ions through non-covalent interactions. Significant bathochromic shifts in the UV–visible spectrum with a profound colour change promise their use to engineer novel applications.     

Selective recognition of thymidylylthymidine (TpT) and antitumor effects of a macrocyclic dizinc(II) complex

A polyamino dizinc(II) complex, [(N-bisdien)Zn2(II)Cl2](ClO4)2, (LZn), has been synthesized as a new nucleobase receptor molecule in aqueous solution at physiological pH, and shows to be highly selective in recognizing deoxythymidine (dT) and thymidylylthymidine (TpT). The strong acidic Zn(II) ions in LZn at the fifth coordination sites interact with a variety of nucleosides. The binding and recognition processes have been studied by potentiometric titration. The X-ray crystal analysis of LZn shows that the two zinc ions are out of the basal plane of the macrocycle, favoring the effective recognition of TpT on the single strand of DNA. In vitro antitumor investigation shows that LZn is a patent inhibitor of tumor cell growth with IC50 values below 10 micromolar.     

Highly efficient asymmetric Lewis acid catalysis with platinum group complexes of conformationally flexible 1,3-butadiene-bridged diphosphines, NUPHOS

[structure: see text] Palladium and platinum complexes of conformationally flexible 1,3-butadiene-bridged diphosphines NUPHOS can be resolved with (S)-BINOL at elevated temperatures to afford diastereopure delta-[(NUPHOS)M[(S)-BINOL]] (M = Pd, Pt). The homochiral Lewis acid complexes delta-[(NUPHOS)M][OTf](2), generated by protonation of delta-[(NUPHOS)M[(S)-BINOL]] with trifluoromethanesulfonic acid, catalyze the Diels-Alder reaction between acryloyl-N-oxazolidinones and cyclopentadiene to give ee values up to 96%. The corresponding enantiopure dichlorides delta-[(NUPHOS)PtCl(2)] react with AgClO(4) to form highly efficient catalysts that give good endo/exo selectivities and high endo enantioselectivity.     

Hydroxyl group recognition by hydrogen-bonding donor and acceptor sites embedded in a layered metal-organic framework

One of the dominant types of interactions between host and guest molecules is hydrogen-bonding, and these interactions can work selectively for a guest molecule. Here, we demonstrate a metal-organic framework (MOF) having both hydrogen-bonding donor and acceptor sites that are quite effective for selective sorption. The MOF selectively interacts with hydroxylic guests in contrast to aprotic hydrogen-bonding guests and shows a sorption selectivity for protic H(2)O, MeOH, and EtOH guests. Notably, this is the first compound that shows complete selectivity in adsorption not for MeCN and MeCHO but for EtOH, which has similar fundamental properties except for its proticity.     

Highly regio- and stereoselective hydrothiolation of acetylenes with thiols catalyzed by a well-defined supported Rh complex

Highly regio- and stereoselective hydrothiolation of a wide range of alkynes with various thiols was demonstrated in the presence of a well-defined Rh complex supported on mesoporous SBA-15 silica. The catalyst was easily recovered and reused several times without significant loss of activity or selectivity.