Applications of dynamic combinatorial chemistry for the determination of effective molarity

Hydrogen-Bond Catalysis of Imine Exchange in Dynamic Covalent Systems

The reversibility of imine bonds has been exploited to great effect in the field of dynamic covalent chemistry, with applications such as preparation of functional systems, dynamic materials, molecular machines, and covalent organic frameworks. However, acid catalysis is commonly needed for efficient equilibration of imine mixtures. Herein, it is demonstrated that hydrogen bond donors such as thioureas and squaramides can catalyze the equilibration of dynamic imine systems under unprecedentedly mild conditions. Catalysis occurs in a range of solvents and in the presence of many sensitive additives, showing moderate to good rate accelerations for both imine metathesis and transimination with amines, hydrazines, and hydroxylamines. Furthermore, the catalyst proved simple to immobilize, introducing both reusability and extended control of the equilibration process.

Controlling the Length of Cooperative Supramolecular Polymers with Chain Cappers

The design and the characterization of supramolecular additives to control the chain length of benzene-1,3,5-tricarboxamide (BTA) cooperative supramolecular polymers under thermodynamic equilibrium is unraveled. These additives act as chain cappers of supramolecular polymers and feature one face as reactive as the BTA discotic to interact strongly with the polymer end, whereas the other face is nonreactive and therefore impedes further polymerization. Such a design requires fine tuning of the conformational preorganization of the amides and the steric hindrance of the motif. The chain cappers studied are monotopic derivatives of BTA, modified by partial N-methylation of the amides or by positioning of a bulky cyclotriveratrylene cage on one face of the BTA unit. This study not only clarifies the interplay between structural variations and supramolecular interactions, but it also highlights the necessity to combine orthogonal characterization methods, spectroscopy and light scattering, to elucidate the structures and compositions of supramolecular systems.

Template effects of vesicles in dynamic covalent chemistry

Vesicle lipid bilayers have been employed as templates to modulate the product distribution in a dynamic covalent library of Michael adducts formed by mixing a Michael acceptor with thiols. In methanol solution, all possible Michael adducts were obtained in similar amounts. Addition of vesicles to the dynamic covalent library led to the formation of a single major product. The equilibrium constants for formation of the Michael adducts are similar for all of the thiols used in this experiment, and the effect of the vesicles on the composition of the library is attributed to the differential partitioning of the library members between the lipid bilayer and the aqueous solution. The results provide a quantitative approach for exploiting dynamic covalent chemistry within lipid bilayers.

Engineering Supramolecular Polymer Conformation for Efficient Carbon Nanotube Sorting

Supramolecular polymer sorting is a promising approach to separating single-walled carbon nanotubes (CNTs) by electronic type. Unlike conjugated polymers, they can be easily removed from the CNTs after sorting by breaking the supramolecular bonds, allowing for isolation of electronically pristine CNTs as well as facile recycling of the sorting polymer. However, little is understood about how supramolecular polymer properties affect CNT sorting. Herein, chain stoppers are used to engineer the conformation of a supramolecular sorting polymer, thereby elucidating the relationship between sorting efficacy and polymer conformation. Through NMR and UV-vis spectroscopy, small-angle X-ray scattering (SAXS), and thermodynamic modeling, it is shown that this supramolecular polymer exhibits ring-chain equilibrium, and that this equilibrium can be skewed toward chains by the addition of chain stoppers. Furthermore, by controlling the stopper-monomer ratio, the sorting yield can be doubled from 7% to 14% without compromising the semiconducting purity (>99%) or properties of sorted CNTs.

The canonical behavior of the entropic component of thermodynamic effective molarity. An attempt at unifying covalent and noncovalent cyclizations

The statistically corrected entropic component of effective molarity (EM_S*) complies with the “canonical” values expressed by the log plot of EM_S* vs. the number n of single bonds in the ring product.

Visible-light-induced metathesis reaction between diselenide and ditelluride

A visible-light-responsive metathesis reaction between diselenide and ditelluride proceeding through a radical mechanism without catalysts.

Upper-Rim Monofunctionalisation in the Synthesis of Triazole- and Disulfide-Linked Multicalix[4]- and -[6]arenes

Covalently linked multiple calixarenes are valued in supramolecular chemistry. This work reports an easy and versatile synthetic route to covalently linked double and triple calix[4]arene and calix[6]arenes by a novel DMF-controlled selective alkylation of a convenient and readily available upper-rim dimethylaminomethyl-substituted tetrahydroxy and hexahydroxy calix[4]arene and -[6]arenes. Synthetic routes to upper-rim functionalised redox active disulfide-linked double-, tetra- and peptidohybrid-calixarenes employing either redox chemistry (CH₂ SH) or thiolates (CH₂ S^- ) are also opened up from the same key starting material.

Inherently chiral cone-calix[4]arenes via a subsequent upper rim ring-closing/opening methodology

Access to chiral calix[4]arenes can unlock novel supramolecular architectures for enantioselective catalysis and molecular recognition. However, accessibility to these structures has been significantly hindered so far. We report herein the synthesis and characterization of di- and trifunctionalized cone-calix[4]arenes featuring a lactone moiety spanning the distal positions at the upper rim. The lactones force the whole skeleton to assume pinched-cone conformations. The ring-closure is favored by the high conformational flexibility of the calixarene scaffold. The new lactones are remarkably stable in the solid state, while a quick hydrolysis to restore the parent carboxylic acids occurs in solution under acidic/basic conditions. Slow aminolyses of lactones 2-3 yield inherently chiral products featuring three different functionalities at the upper rim, at room temperature. The subsequent ring-closing/opening methodology presented here highlights the versatility of these lactones as powerful synthons for the preparation of a variety of threefold upper rim functionalized, inherently chiral calix[4]arenes fixed in the cone structure.

Quantitative Analysis of the Self-Assembly Process of Hexagonal PtII Macrocyclic Complexes: Effect of the Solvent and the Components

The self-assembly process of three Pt^II -linked hexagonal macrocycles consisting of dinuclear Pt^II complexes and organic ditopic ligands was investigated in polar and less polar solvents by a recently developed approach: quantitative analysis of the self-assembly process (QASAP). In polar CD₃ NO₂ , for all the three macrocycles, an ML₂ complex was the dominant intermediate during self-assembly, as a result of high positive allosteric cooperativity for the ligand exchange on the Pt^II centers of the dinuclear Pt^II complexes. On the other hand, in less polar CD₂ Cl₂ , the self-assembly process was affected by the components. For two of the three macrocycles, the chainlike oligomers that contain fewer metals and ligands than the corresponding macrocycles grew with time and the type of the chainlike intermediates formed correlated with the allostericity of the two binding sites in the organic ditopic ligands. In every case, no long oligomers containing more components than the macrocycles themselves were produced during the self-assembly even though free rotation around single bonds in the chainlike oligomers allows them to adopt various conformations that do not facilitate the cyclization. This result suggests that electrostatic and/or steric factors besides rigidity of the components make the cyclization advantageous not only thermodynamically but also kinetically.

Diasteroselective multi-component assemblies from dynamic covalent imine condensation and metal-coordination chemistry: mechanism and narcissistic stereochemistry self-sorting

Self-assembly of a modified tris(2-pyridylmethyl)amine TPMA ligand, zinc(ii) or cobalt(ii) ions, and amino acids have been used effectively as stereo dynamic optical probes for the determination of the enantiomeric excess of free amino acids either using Electronic or Vibrational Circular Dichroism (CD and VCD). Herein, we report the mechanistic and stereochemical study of the self-assembly process which reveals a complex equilibrium in solution where even small variations in the experimental conditions can profoundly affect the final products of the reaction. In particular, variation on the metal stoichiometry switch give rises to an entirely enantio narcissistic self-assembly of the structure.

Enantio narcissistic self-assembly is observed in a multi-metallic structure used as stereodynamic probe for CD measurements.

Antiparallel Dynamic Covalent Chemistries

The ability to design reaction networks with high, but addressable complexity is a necessary prerequisite to make advanced functional chemical systems. Dynamic combinatorial chemistry has proven to be a useful tool in achieving complexity, however with some limitations in controlling it. Herein we introduce the concept of antiparallel chemistries, in which the same functional group can be channeled into one of two reversible chemistries depending on a controllable parameter. Such systems allow both for achieving complexity, by combinatorial chemistry, and addressing it, by switching from one chemistry to another by controlling an external parameter. In our design the two antiparallel chemistries are thiol-disulfide exchange and thio-Michael addition, sharing the thiol as the common building block. By means of oxidation and reduction the system can be reversibly switched from predominantly thio-Michael chemistry to predominantly disulfide chemistry, as well as to any intermediate state. Both chemistries operate in water, at room temperature, and at mildly basic pH, which makes them a suitable platform for further development of systems chemistry.

Formation of Imidazo[1,5-a]pyridine Derivatives Due to the Action of Fe2+ on Dynamic Libraries of Imines

An imidazo[1,5-a]pyridine derivative was unexpectedly obtained through the action of Fe²⁺ on a dynamic library of imines generated in situ via condensation of benzaldehyde and 2-picolylamine. The reaction product was easily isolated as the only nitrogen-containing product eluted from the chromatographic column. A reaction mechanism is proposed, in which combined kinetic and thermodynamic effects exerted by Fe²⁺ on the various steps of the complex reaction sequence are discussed. The Fe²⁺ nature of the added metal cation was found to be pivotal for the achievement of the imidazo[1,5-a]pyridine derivative as well as its amount in the reaction mixture. When the electronic effects were evaluated, gratifying yields were obtained only in the presence of moderately electron-releasing or moderately electron-withdrawing groups on the aldehyde reactant. No traces of imidazo[1,5-a]pyridine derivatives were obtained for p-OCH₃ and p-NO₂ benzaldehyde.

Experimental Binding Energies in Supramolecular Complexes

On the basis of many literature measurements, a critical overview is given on essential noncovalent interactions in synthetic supramolecular complexes, accompanied by analyses with selected proteins. The methods, which can be applied to derive binding increments for single noncovalent interactions, start with the evaluation of consistency and additivity with a sufficiently large number of different host-guest complexes by applying linear free energy relations. Other strategies involve the use of double mutant cycles, of molecular balances, of dynamic combinatorial libraries, and of crystal structures. Promises and limitations of these strategies are discussed. Most of the analyses stem from solution studies, but a few also from gas phase. The empirically derived interactions are then presented on the basis of selected complexes with respect to ion pairing, hydrogen bonding, electrostatic contributions, halogen bonding, π-π-stacking, dispersive forces, cation-π and anion-π interactions, and contributions from the hydrophobic effect. Cooperativity in host-guest complexes as well as in self-assembly, and entropy factors are briefly highlighted. Tables with typical values for single noncovalent free energies and polarity parameters are in the Supporting Information.

Implications of dynamic imine chemistry for the sustainable synthesis of nitrogen heterocycles via transimination followed by intramolecular cyclisation

An application of dynamic imine chemistry to the sustainable synthesis of N-heterocycles from N-aryl benzylamines or imines and ortho-substituted anilines is described.

Orthogonal breaking and forming of dynamic covalent imine and disulfide bonds in aqueous solution

Orthogonal bond-breaking and forming of dynamic covalent disulfide and imine bonds in aqueous solution is demonstrated. Through judicious choice of reaction partners and conditions, it is possible to cleave and reform selectively these bonds in the presence of each other in the absence of unwanted competing processes.

Mechanisms of imine exchange reactions in organic solvents

The state of the art in the mechanisms operating in imine chemistry in organic solvents is critically discussed in the present review. In particular, the reaction pathways involved in imine formation, transimination and imine metathesis in organic media are taken into account, with the aim of organizing the poor, and sometimes scattered, information available in the literature. It is shown that 4-membered cyclic transition states, either polar or apolar, can be considered a leitmotif for the chemistry of imines in organic solvents. However, it is pointed out that further investigations will be necessary to reach an adequate degree of knowledge of the mechanisms involved in such important reversible processes.