Design, synthesis and application of carbazole macrocycles in anion sensors

Carboxylate sensing solid-contact ion-selective electrodes (ISEs) were created to provide a proof-of-concept ISE development process covering all aspects from in silico ionophore design to functional sensor characterization. The biscarbazolylurea moiety was used to synthesize methylene-bridged macrocycles of different ring size aiming to fine tune selectivity towards different carboxylates. Cyclization was achieved with two separate strategies, using either amide synthesis to access up to –[CH₂]₁₀– macrocycles or acyl halides to access up to –[CH₂]₁₄– macrocycles. Seventy-five receptor–anion complexes were modelled and studied with COSMO-RS, in addition to all free host molecules. In order to predict initial selectivity towards carboxylates, ¹H NMR relative titrations were used to quantify binding in DMSO-d₆/H₂O solvent systems of two proportions – 99.5%:0.5% m/m and 90.0%:10.0% m/m, suggesting initial selectivity towards acetate. Three ionophores were selected for successful sensor prototype development and characterization. The constructed ion-selective electrodes showed higher selectivity towards benzoate than acetate, i.e., the selectivity patterns of the final sensors deviated from that predicted by the classic titration experiments. While the binding constants obtained by NMR titration in DMSO-d₆/H₂O solvent systems provided important guidance for sensor development, the results obtained in this work emphasize the importance of evaluating the binding behavior of receptors in real sensor membranes.

The Influence of Binding Site Geometry on Anion-Binding Selectivity: A Case Study of Macrocyclic Receptors Built on the Azulene Skeleton

An understanding of host-guest noncovalent interactions lies at the very heart of supramolecular chemistry. Often a minute change to the structure of a host molecule's binding site can have a dramatic impact on a prospective host-guest binding event, changing the relative selectivity for potential guest molecules. With the overall goal of aiding the rational design of selective and effective receptors for anions, we have studied the influence of small perturbations in binding site geometry for a series of five closely related 20-membered macrocyclic tetra-amide receptors, constructed from two building blocks from a pool of azulene-5,7-bisamide, azulene-1,3-bisamide, and dipicolinic bisamide units. The solid-state structures revealed that the conformational preferences of the free receptors are driven by the inherent preferences of the building blocks, yet in some cases the macrocyclic topology is able to over-ride these to promote pre-organized conformations favorable for anion binding. The solid-state structures of the chloride complexes of these receptors revealed that although all the receptors can adapt to binding to the challenging small Cl^- guest with all the NH groups, only receptors containing azulene-5,7-bisamide units form short and linear, and therefore strong, hydrogen-bonding interactions. These conclusions are further supported by studies in solution. Although all the receptors showed high affinities toward a series of anions (H₂ PO₄^- , PhCO₂^- , Cl^- , and Br^- ), even in a highly competitive polar medium (DMSO/25 % MeOH), only receptors containing azulene-5,7-bisamide units exhibited non-inherent selectivity for Cl^- over PhCO₂^- , breaking the Hofmeister trend of selectivity. The data presented herein highlight the privileged properties of the azulene-5,7-bisamide building block for binding to chloride anions and provide guidelines for the construction of selective and efficient anion receptors with prospective practical applications.

An intramolecular tryptophan-condensation approach for peptide stapling

Stapled peptides are gaining tremendous interest as next-generation therapeutic agents to target protein–protein interactions. Herein, we report an intramolecular peptide stapling method which links two tryptophan residues at C2 position of the indole moieties via acid-mediated condensation with an aldehyde.

Beyond classical reactivity patterns: hydroformylation of vinyl and allyl arenes to valuable β- and γ-aldehyde intermediates using supramolecular catalysis

In this study, we report on properties of a series of rhodium complexes of bisphosphine and bisphosphite L1-L7 ligands, which are equipped with an integral anion binding site (the DIM pocket), and their application in the regioselective hydroformylation of vinyl and allyl arenes bearing an anionic group. In principle, the binding site of the ligand is used to preorganize a substrate molecule through noncovalent interactions with its anionic group to promote otherwise unfavorable reaction pathways. We demonstrate that this strategy allows for unprecedented reversal of selectivity to form otherwise disfavored β-aldehyde products in the hydroformylation of vinyl 2- and 3-carboxyarenes, with chemo- and regioselectivity up to 100%. The catalyst has a wide substrate scope, including the most challenging substrates with internal double bonds. Coordination studies of the catalysts under catalytically relevant conditions reveal the formation of the hydridobiscarbonyl rhodium complexes [Rh(Ln)(CO)2H]. The titration studies confirm that the rhodium complexes can bind anionic species in the DIM binding site of the ligand. Furthermore, kinetic studies and in situ spectroscopic investigations for the most active catalyst give insight into the operational mode of the system, and reveal that the catalytically active species are involved in complex equilibria with unusual dormant (reversibly inactivated) species. In principle, this involves the competitive inhibition of the recognition center by product binding, as well as the inhibition of the metal center via reversible coordination of either a substrate or a product molecule. Despite the inhibition effects, the substrate preorganization gives rise to very high activities and efficiencies (TON > 18,000 and TOF > 6000 mol mol(-1) h(-1)), which are adequate for commercial applications.

A carbohydrate-anion recognition system in aprotic solvents

A carbohydrate–anion recognition system in nonpolar solvents is reported, in which complexes form at the B-faces of β-d-pyranosides with H1-, H3-, and H5-cis patterns similar to carbohydrate–π interactions. The complexation effect was evaluated for a range of carbohydrate structures; it resulted in either 1:1 carbohydrate–anion complexes, or 1:2 complex formation depending on the protection pattern of the carbohydrate. The interaction was also evaluated with different anions and solvents. In both cases it resulted in significant binding differences. The results indicate that complexation originates from van der Waals interactions or weak CH⋅⋅⋅A⁻ hydrogen bonds between the binding partners and is related to electron-withdrawing groups of the carbohydrates as well as increased hydrogen-bond-accepting capability of the anions.

Precise supramolecular control of selectivity in the Rh-catalyzed hydroformylation of terminal and internal alkenes

In this study, we report a series of DIMPhos ligands L1-L3, bidentate phosphorus ligands equipped with an integral anion binding site (the DIM pocket). Coordination studies show that these ligands bind to a rhodium center in a bidentate fashion. Experiments under hydroformylation conditions confirm the formation of the mononuclear hydridobiscarbonyl rhodium complexes that are generally assumed to be active in hydroformylation. The metal complexes formed still strongly bind the anionic species in the binding site of the ligand, without affecting the metal coordination sphere. These bifunctional properties of DIMPhos are further demonstrated by the crystal structure of the rhodium complex with acetate anion bound in the binding site of the ligand. The catalytic studies demonstrate that substrate preorganization by binding in the DIM pocket of the ligand results in unprecedented selectivities in hydroformylation of terminal and internal alkenes functionalized with an anionic group. Remarkably, the selectivity controlling anionic group can be even 10 bonds away from the reactive double bond, demonstrating the potential of this supramolecular approach. Control experiments confirm the crucial role of the anion binding for the selectivity. DFT studies on the decisive intermediates reveal that the anion binding in the DIM pocket restricts the rotational freedom of the reactive double bound. As a consequence, the pathway to the undesired product is strongly hindered, whereas that for the desired product is lowered in energy. Detailed kinetic studies, together with the in situ spectroscopic measurements and isotope-labeling studies, support this mode of operation and reveal that these supramolecular systems follow enzymatic-type Michaelis-Menten kinetics, with competitive product inhibition.

"Unclosed cryptands": a point of departure for developing potent neutral anion receptors

Six macrocyclic lariat-type compounds, representing a new class of anion receptors, were synthesized in a simple approach. We identified the optimal macroring size and the position of the hydrogen bond donating center in the lariat arm offering the best affinities toward chloride and carboxylate anions. The anion-binding properties of such systems were investigated by applying (1)H NMR titrations in DMSO/water and methanol/DMSO mixtures.