Four Simultaneously Dynamic Covalent Reactions. Experimental Proof of Orthogonality

Dynamic covalent synthesis

Dynamic covalent synthesis aims to precisely control the assembly of simple building blocks linked by reversible covalent bonds to generate a single, structurally complex, product. In recent years, considerable progress in the programmability of dynamic covalent systems has enabled easy access to a broad range of assemblies, including macrocycles, shape-persistent cages, unconventional foldamers and mechanically-interlocked species (catenanes, knots, etc.). The reversibility of the covalent linkages can be either switched off to yield stable, isolable products or activated by specific physico-chemical stimuli, allowing the assemblies to adapt and respond to environmental changes in a controlled manner. This activatable dynamic property makes dynamic covalent assemblies particularly attractive for the design of complex matter, smart chemical systems, out-of-equilibrium systems, and molecular devices.

Anisotropic functionalized platelets: percolation, porosity and network properties

Anisotropic functionalized platelets are able to model the assembly behaviour of molecular systems in two dimensions thanks to the unique combination of steric and bonding constraints. The assembly scenarios can vary from open to close-packed crystals, finite clusters and chains, according to the features of the imposed constraints. In this work, we focus on the assembly of equilibrium networks. These networks can be seen as disordered, porous monolayers and can be of interest for instance in nano-filtration and optical applications. We investigate the formation and properties of two dimensional networks from shape anisotropic colloids functionalized with four patches. We characterize the connectivity properties, the typical local bonding motives, as well as the geometric features of the emerging networks for a large variety of different systems. Our results show that networks of shape anisotropic colloids assemble into highly versatile network topologies, that may be utilized for applications at the nanoscale.

Dual reactivity based dynamic covalent chemistry: mechanisms and applications

Dynamic covalent chemistry (DCC) focuses on the reversible formation, breakage, and exchange of covalent bonds and assemblies, setting a bridge between irreversible organic synthesis and supramolecular chemistry and finding wide utility. In order to enhance structural and functional diversity and complexity, different types of dynamic covalent reactions (DCRs) are placed in one vessel, encompassing orthogonal DCC without crosstalk and communicating DCC with a shared reactive functional group. As a means of adding tautomers, widespread in chemistry, to interconnected DCRs and combining the features of orthogonal and communicating DCRs, a concept of dual reactivity based DCC and underlying structural and mechanistic insights are summarized. The manipulation of the distinct reactivity of structurally diverse ring-chain tautomers allows selective activation and switching of reaction pathways and corresponding DCRs (C-N, C-O, and C-S) and assemblies. The coupling with photoswitches further enables light-mediated formation and scission of multiple types of reversible covalent bonds. To showcase the capability of dual reactivity based DCC, the versatile applications in dynamic polymers and luminescent materials are presented, paving the way for future functionalization studies.

Peptide Macrocyclization Guided by Reversible Covalent Templating

The creation of complementary products via templating is a hallmark feature of nucleic acid replication. Outside of nucleic acid-like molecules, the templated synthesis of a hetero-complementary copy is still rare. Herein we describe one cycle of templated synthesis that creates homomeric macrocyclic peptides guided by linear instructing strands. This strategy utilizes hydrazone formation to pre-organize peptide oligomeric monomers along the template on a solid support resin, and microwave-assisted peptide synthesis to couple monomers and cyclize the strands. With a flexible templating strand, we can alter the size of the complementary macrocycle products by increasing the length and number of the binding peptide oligomers, showing the potential to precisely tune the size of macrocyclic products. For the smaller macrocyclic peptides, the products can be released via hydrolysis and characterized by ESI-MS.

Teaching an Old Compound New Tricks: Reversible Transamidation in Maleamic Acids

Dynamic combinatorial chemistry is a method widely used for generating responsive libraries of compounds, with applications ranging from chemical biology to materials science. It relies on dynamic covalent bonds that are able to form in a reversible manner in mild conditions, and therefore requires the discovery of new types of these bonds in order to progress. Amides, due to their high stability, have been scarcely used in this field and typically require an external catalyst or harsh conditions for exchange. Compounds able to undergo uncatalysed transamidation at room temperature are still rare exceptions. In this work, we describe reversible amide formation and transamidation in a class of compounds known as maleamic acids. Due to the presence of a carboxylic acid in β-position, these compounds are in equilibrium with their anhydride and amine precursors in organic solvents at room temperature. First, we show that this equilibrium is responsive to external stimuli: by alternating the additions of a Brønsted acid and a base, we can switch between amide and anhydride several times without side-reactions. Next, we prove that this equilibrium provides a pathway for reversible transamidation without any added catalyst, leading to thermodynamic distributions of amides at room temperature. Lastly, we use different preparation conditions and concentrations of Brønsted acid to access different library distributions, easily controlling the transition between kinetic and thermodynamic regimes. Our results show that maleamic acids can undergo transamidation in mild conditions in a reversible and tunable way, establishing them as a new addition to the toolbox of dynamic combinatorial chemistry.

Sulfur in Dynamic Covalent Chemistry

Sulfur has been important in dynamic covalent chemistry (DCC) since the beginning of the field. Mainly as part of disulfides and thioesters, dynamic sulfur-based bonds (DSBs) have a leading role in several remarkable reactions. Part of this success is due to the almost ideal properties of DSBs for the preparation of dynamic covalent systems, including high reactivity and good reversibility under mild aqueous conditions, the possibility of exploiting supramolecular interactions, access to isolable structures, and easy experimental control to turn the reaction on/off. DCC is currently witnessing an increase in the importance of DSBs. The chemical flexibility offered by DSBs opens the door to multiple applications. This Review presents an overview of all the DSBs used in DCC, their applications, and remarks on the interesting properties that they confer on dynamic chemical systems, especially those containing several DSBs.

An Orthogonal Dynamic Covalent Chemistry Tool for Ring-Opening Polymerization of Cyclic Oligochalcogenides on Detachable Helical Peptide Templates

A model system is introduced as a general tool to elaborate on orthogonal templation of dynamic covalent ring-opening polymerization (ODC-TROP). The tool consists of 310 helical peptides as unprecedented templates and semicarbazones as orthogonal dynamic covalent linkers. With difficult-to-control 1,2-dithiolanes, ODC-TROP on the level of short model oligomers occurs with high templation efficiency, increasing and diminishing upon helix stabilization and denaturation, respectively. Further, an anti-templated conjugate with mispositioned monomers gave reduced templation upon helix twisting. Even with the "unpolymerizable" 1,2-diselenolanes, initial studies already afford mild templation efficiency. These proof-of-principle results promise that the here introduced tool, recyclable and enabling late-stage side chain modification, will be useful to realize ODC-TROP of intractable or unknown cyclic dynamic covalent monomers for dynamer materials as well as cellular uptake and signaling applications.

Assembling Inorganic Nanocrystal Gels

Inorganic nanocrystal gels retain distinct properties of individual nanocrystals while offering tunable, network-structure-dependent characteristics. We review different mechanisms for assembling gels from colloidal nanocrystals including (1) controlled destabilization, (2) direct bridging, (3) depletion, as well as linking mediated by (4) coordination bonding or (5) dynamic covalent bonding, and we highlight how each impacts gel properties. These approaches use nanocrystal surface chemistry or the addition of small molecules to mediate inter-nanocrystal attractions. Each method offers advantages in terms of gel stability, reversibility, or tunability and presents new opportunities for the design of reconfigurable materials and fueled assemblies.

Simultaneous Formation of a Fully Organic Triply Dynamic Combinatorial Library

Here we report the simultaneous formation of doubly and triply dynamic libraries as a result of exchange reactions between functionalized organic building blocks. A combination of three different reversible covalent linkages involving a boronate ester transesterification along with an imine and disulfide exchange was employed to generate a new type of fully organic triply dynamic molecular assembly.

Colloidal Nanocrystal Gels from Thermodynamic Principles

Gels assembled from solvent-dispersed nanocrystals are of interest for functional materials because they promise the opportunity to retain distinctive properties of individual nanocrystals combined with tunable, structure-dependent collective behavior. By incorporating stimuli-responsive components, these materials could also be dynamically reconfigured between structurally distinct states. However, nanocrystal gels have so far been formed mostly through irreversible aggregation, which has limited the realization of these possibilities. Meanwhile, gelation strategies for larger colloidal microparticles have been developed using reversible physical or chemical interactions. These approaches have enabled the experimental navigation of theoretically predicted phase diagrams, helping to establish an understanding of how thermodynamic behavior can guide gel formation in these materials. However, the translation of these principles to the nanoscale poses both practical and fundamental challenges. The molecules guiding assembly can no longer be safely assumed to be vanishingly small compared to the particles nor large compared to the solvent.In this Account, we discuss recent progress toward the assembly of tunable nanocrystal gels using two strategies guided by equilibrium considerations: (1) reversible chemical bonding between functionalized nanocrystals and difunctional linker molecules and (2) nonspecific, polymer-induced depletion attractions. The effective nanocrystal attractions, mediated in both approaches by a secondary molecule, compete against stabilizing repulsions to promote reversible assembly. The structure and properties of the nanocrystal gels are controlled microscopically by the design of the secondary molecule and macroscopically by its concentration. This mode of control is compelling because it largely decouples nanocrystal synthesis and functionalization from the design of interactions that drive assembly. Statistical thermodynamic theory and computer simulation have been applied to simple models that describe the bonding motifs in these assembling systems, furnish predictions for conditions under which gelation is likely to occur, and suggest strategies for tuning and disassembling the gel networks. Insights from these models have guided experimental realizations of reversible gels with optical properties in the infrared range that are sensitive to the gel structure. This process avoids time-consuming and costly trial-and-error experimental investigations to accelerate the development of nanocrystal gel assemblies.These advances highlight the need to better understand interactions between nanocrystals, how interactions give rise to gel structure, and properties that emerge. Such an understanding could suggest new approaches for creating stimuli-responsive and dissipative assembled materials whose properties are tunable on demand through directed reconfiguration of the underlying gel microstructure. It may also make nanocrystal gels amenable to computationally guided design using inverse methods to rapidly optimize experimental parameters for targeted functionalities.

Gating the photoactivity of azobenzene-type ligands trapped within a dynamic system of an M4L6 tetrahedral cage, an M2L2 metallocycle and mononuclear MLn complexes

Complexation of metal ions by a ligand bearing a azobenzene linker yielded two types of complexes: an [M2L2]4+ metallocycle and an [M4L6]8+ tetrahedral cage, which can be further converted into libraries of mononuclear [M(L′/L′′)n]2+ species.

Capture and Release of Protein-Nanoparticle Conjugates by Reversible Covalent Molecular Linkers

A hybrid approach to covalently detachable molecules for nanoparticle capture and release from several custom-functionalized surfaces is described. This new surface chemistry capability provides a means for reversible binding of functionalized nanoparticles without relying on costly nucleic acid-based complexation. A new surface linker motif was devised wherein custom molecules were synthesized with components for surface anchoring, cleavage, and target capture through biotin-streptavidin binding. All capture-and-release chemistry is performed using physiological conditions (aqueous, pH 7). Covalent cleavage of linker molecules was achieved through incorporation of a tunable orthogonal reversible covalent (TORC) hydrazone functional group which underwent exchange with a competitive hydrazide aided by an aniline catalyst. The influence of the linker architecture on hydrazone exchange and nanoparticle release was probed by altering the distance between hydrazone and biotin groups using different length PEG spacers. Cleavable linkers were used to functionalize microwells, magnetic separation beads, and gold-coated glass surfaces. Upon functionalization, all surface types bound streptavidin and conjugated nanoparticles regardless of the linker structure. Conversely, the extent of hydrazone exchange as well as release of nanoparticles were influenced both by the hydrazone surface density and the linker molecular structure.

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.

Sequence-directed dynamic covalent assembly of base-4-encoded oligomers

As an information-bearing biomacromolecule, DNA is encoded in base-4, where each residue site can be occupied by any one of four nucleobases. Mimicking the information dense, sequence-selective hybridization of DNA, we demonstrate two orthogonal dynamic covalent interactions to effect the selective assembly of molecular ladders and grids from base-4-encoded oligo(peptoid)s.

Adaptive Covalent Networks Enabled by Dual Reactivity: The Evolution of Reversible Covalent Bonds, Their Molecular Assemblies, and Guest Recognition

Adaptive chemistry allows transformation and selection within molecular networks, and adaptive systems composed of different types of dynamic covalent reactions (DCRs) are challenging. Herein, we demonstrate dual reactivity-based covalent networks encompassing the regulation of and switching between C-N- and C-S-based reversible covalent assemblies. The creation and exchange of C-N- or C-S-derived assemblies exhibiting diverse architectures, including linear structures, macrocycles, and cages, were achieved. The shift of reactivity then permitted the interconversion between C-N- and C-S-containing assemblies. Moreover, the adaption of intramolecular and intermolecular scaffolds was feasible via linker design. The latent hemiaminal chirality center offered a pathway for the induction of chirality within assemblies. Finally, switchable structural change and controlled extraction of ions were realized with Hg²⁺ as a guest for macrocycles. The remarkable complexity of networks described herein could open the door for the utility in sophisticated functional systems.

Simultaneous CO2 capture and metal purification from waste streams using triple-level dynamic combinatorial chemistry

A reduction in CO₂ emissions is required to mitigate global warming. Post-combustion carbon capture is one of the most developed technologies that has the potential to meet this goal, but its cost prevents its widespread use. A different approach would be to use CO₂ directly as it is captured, before it is stored. Here we explore spontaneous CO₂ fixation by industrial polyamines as a strategy to generate dynamic libraries of ligands for metal separation and recovery. We identify the CO₂ loadings and solvents promoting the optimal precipitation of each metal from the dynamic libraries of complexes. We demonstrate the separation of lanthanum and nickel using the exhaust gas of an internal combustion engine vehicle, and show that the three metal constituents of the La₂Ni₉Co alloys used to manufacture the batteries of electric vehicles can be separated and recovered by successive CO₂-induced selective precipitations. Beyond the concept of CO₂-sourced multi-level dynamic coordination chemistry, this study provides a potential framework for integrated CO₂ capture and use through sustainable processes.

Exploiting complexity to implement function in chemical systems

This feature article reflects a personal overview of the importance of complexity as an additional parameter to be considered in chemical research, being illustrated with selected examples in molecular recognition and catalysis.

Dynamic Covalent Switches and Communicating Networks for Tunable Multicolor Luminescent Systems and Vapor-Responsive Materials

Molecular switches are an intensive area of research, and in particular, the control of multistate switching is challenging. Herein we introduce a general and versatile strategy of dynamic covalent switches and communicating networks, wherein distinct states of reversible covalent systems can induce addressable fluorescence switching. The regulation of intramolecular ring/chain equilibrium, intermolecular dynamic covalent reactions (DCRs) with amines, and both permitted the activation of optical switches. The variation in electron-withdrawing competition between the fluorophore and 2-formylbenzenesulfonyl unit afforded diverse signaling patterns. The combination of switches in situ further enabled the creation of communicating networks for multistate color switching, including white emission, through the delicate control of DCRs in complex mixtures. Finally, reversible and recyclable multiresponsive luminescent materials were achieved with molecular networks on the solid support, allowing visualization of different types of vapors and quantification of primary amine vapors with high sensitivity and wide detection range. The results reported herein should be appealing for future studies of dynamic assemblies, molecular sensing, intelligent materials, and biological labeling.

Engineering multilayer chemical networks

Dynamic multilevel systems emerged in the last few years as new platforms to study thermodynamic systems. In this work, unprecedented fully communicated three-level systems are studied. First, different conditions were screened to selectively activate thiol/dithioacetal, thiol/thioester, and thiol/disulfide exchanges, individually or in pairs. Some of those conditions were applied, sequentially, to build multilayer dynamic systems wherein information, in the form of relative amounts of building blocks, can be directionally transmitted between different exchange pools. As far as we know, this is the first report of one synthetic dynamic chemical system where relationships between layers can be changed through network operations.

A tetrahedral molecular cage with a responsive vertex

Dynamic covalent chemistry (DCC) is a widely used method for the self-assembly of three-dimensional molecular architectures. The orthogonality of dynamic reactions is emerging as a versatile strategy for controlling product distributions in DCC, yet the application of this approach to the synthesis of 3D organic molecular cages is limited. We report the first system which employs the orthogonality of alkyne metathesis and dynamic imine exchange to prepare a molecular cage with a reversibly removable vertex. This study demonstrates the rational and controlled application of chemical orthogonality in DCC to prepare organic cages of expanded functionality which respond to chemical stimuli.

Chemical Approaches to Dynamically Modulate the Properties of Synthetic Matrices

As knowledge about the dynamic nature of tissues within the human body has increased, the need for cell culture models that mimic the properties of these dynamic microenvironments has grown. Hydrogels are useful platforms for investigating cellular responses to microenvironment cues in disease and regeneration processes and recently have been designed to contain dynamic bonds to regulate the mechanical and biochemical properties of the matrix in three-dimensional cell culture applications. In this Viewpoint, we highlight recent advances in developing hydrogels with dynamic properties for modeling aspects of human tissues, providing control over the properties of the synthetic matrix on multiple length and time scales, and their application for understanding or directing cell response. We conclude by discussing how orthogonal chemistries can be utilized to design dynamic hydrogel platforms for controlling both the mechanical and biochemical environment, affording opportunities to investigate more complex questions associated with disease progression and tissue regeneration.

Tunable Orthogonal Reversible Covalent (TORC) Bonds: Dynamic Chemical Control over Molecular Assembly

Dynamic assembly of macromolecules in biological systems is one of the fundamental processes that facilitates life. Although such assembly most commonly uses noncovalent interactions, a set of dynamic reactions involving reversible covalent bonding is actively being exploited for the design of functional materials, bottom-up assembly, and molecular machines. This Minireview highlights recent implementations and advancements in the area of tunable orthogonal reversible covalent (TORC) bonds for these purposes, and provides an outlook for their expansion, including the development of synthetically encoded polynucleotide mimics.

Supramolecular and dynamic covalent hydrogel scaffolds: from gelation chemistry to enhanced cell retention and cartilage regeneration

This review highlights the most recent progress in gelation strategies of biomedical supramolecular and dynamic covalent crosslinking hydrogels and their applications for enhancing cell retention and cartilage regeneration.

Designing Molecular Building Blocks for the Self-assembly of Complex Porous Networks

The creation of molecular or colloidal building blocks which can self-assemble into complex, ordered porous structures has been long sought-after, and so are the guiding principles behind this creation. The pursuit of this goal has led to the creation of novel classes of materials like metal organic frameworks (MOFs) and covalent organic frameworks (COFs). In theory, a tremendous number of structures can be formed by these materials due to the variety of geometries available to their building blocks. However, most realized crystal structures tend to be simple or homoporous and typically assemble from building blocks with high degrees of symmetry. Building blocks with low degrees of symmetry suitable for assembly into the more complex structures tend to assemble into polymorphous or disordered structures instead. In this work, we use Monte Carlo simulations of patchy vertex-like building blocks to show how the addition of chemical specificity via orthogonally reacting functional sites can allow vertex-like building blocks with even asymmetric geometries to self-assemble into ordered crystallites of various complex structures. In addition to demonstrating the utility of such a strategy in creating ordered, heteroporous structures, we also demonstrate that it can be used as a means for tuning specific features of the crystal structure, accomplishing such aims as the control of relative pore sizes. We also discuss heuristics for properly designing molecules so that they can assemble into target structures.

Adaptable Fast Relaxing Boronate-Based Hydrogels for Probing Cell-Matrix Interactions

Hydrogels with tunable viscoelasticity hold promise as materials that can recapitulate many dynamic mechanical properties found in native tissues. Here, covalent adaptable boronate bonds are exploited to prepare hydrogels that exhibit fast relaxation, with relaxation time constants on the order of seconds or less, but are stable for long-term cell culture and are cytocompatible for 3D cell encapsulation. Using human mesenchymal stem cells (hMSC) as a model, the fast relaxation matrix mechanics are found to promote cell-matrix interactions, leading to spreading and an increase in nuclear volume, and induce yes-associated protein/PDZ binding domain nuclear localization at longer times. All of these effects are exclusively based on the hMSCs' ability to physically remodel their surrounding microenvironment. Given the increasingly recognized importance of viscoelasticity in controlling cell function and fate, it is expected that the synthetic strategies and material platform presented should provide a useful system to study mechanotransduction on and within viscoelastic environments and explore many questions related to matrix biology.

Three Switchable Orthogonal Dynamic Covalent Reactions and Complex Networks Based on the Control of Dual Reactivity

Achieving complexity is central to the creation of chemical systems, inspired by natural systems. Herein we introduce a strategy of switchable orthogonal dynamic covalent chemistry (DCC) toward the regulation of complex dynamic networks. The control of dual reactivity of tautomers and resulting pathways allowed reversible covalent bonding of a large scope of primary amines, secondary amines, alcohols, and thiols with high efficiency. The selection of reaction pathways next enabled the realization of orthogonal but switchable dynamic covalent reactions (DCRs) with nucleophile pairs of amine/alcohol, alcohol/thiol, and amine/thiol by varying protonation and oxidation states. Control experiments confirmed the crucial role of dual reactivity on the stability and switchability of DCRs. The specificity toward amines, alcohols, and thiols, as well as interconversion between their corresponding assemblies, was further accomplished in one vessel, thus creating tunable communicating networks with three types of DCRs. Moreover, the switchable orthogonality combined with differential reactivity of multiple sulfonamides and nucleophiles enhanced the complexity within dynamic libraries. The generality and versatility of our approaches should facilitate their incorporation into many aspects of chemistry endeavors.

N,1,4-Tri(4-alkoxy-2-hydroxybenzyl)-DAZA: efficient one-pot synthesis and labelling with 68Ga for PET liver imaging in ovo

We report the isolation of a new type of 1,4-diazepan-6-amine (DAZA)-based ligand. Condensation of aldehydes with DAZA gives a novel class of 1,5-diazabicyclo[3.2.1]octanes in nearly quantitative yields. Subsequent reductive cleavage of these bicyclic aminal species with sodium borohydride selectively leads to N,1,4-tri(4-alkoxy-2-hydroxybenzyl)-1,4-diazepan-6-amines (alkoxy = Me: TMeOHB-DAZA; alkoxy = Et: TEtOHB-DAZA) via a unique reductive alkylation reaction in which a substituent is added to the DAZA moiety without the presence of an alkylating agent. Mass spectrometry studies of the intermediates suggest that the mechanism involves insertion of in situ released carbonyl species into an aminal bond to form hemiaminal intermediates, and subsequent reduction. TMeOHB-DAZA and TEtOHB-DAZA are hexadentate ligands suitable for effectively coordinating Ga(iii) ions. Chelation of the radionuclide 68Ga was achieved within 5 min at 100 °C. In vitro stability studies in PBS and human serum confirmed the kinetic inertness of the tracers as no 68Ga demetallation was observed over a period of 4 h. Positron emission tomography (PET)/computed tomography (CT) imaging after in ovo administration to incubated ostrich eggs showed a high uptake in the liver, namely 27% (60 min post injection), and subsequent biliary excretion. These results suggest that [68Ga]Ga-TMeOHB-DAZA and [68Ga]Ga-TEtOHB-DAZA have excellent potential as hepatobiliary PET/CT imaging agents.

2,2'-Bipyridine and hydrazide containing peptides for cyclization and complex quaternary structural control

A synthetic peptide containing two N_ε-methyl lysines (Ac-K(N_ε-Me)GYTGYTGK(N_ε-Me)D-OH) was alkylated with bipyridine (bipy) ligands substituted at the fifth (MP-5) and sixth (MP-6) positions, thereby creating Ac-K(N_ε-Me, N_ε-Bipy)GYTGYTGK(N_ε-Me, N_ε-Bipy)D-OH. Peptides with 6-position bipyridine did not bind to Fe²⁺ and Zn²⁺. Peptides with 5-position bipyridine bound these metals, and in the presence of one equivalent of a free bipy derivative folded into a macrocycle. Further, the free bipy derivative could also contain a cyclized peptide derived from hydrazone formation, resulting in complex but controlled quaternary peptide structure.

Redox Control over Acyl Hydrazone Photoswitches

Photoisomerization provides a clean and efficient way of reversibly altering physical properties of chemical systems and injecting energy into them. These effects have been applied in development of systems such as photoresponsive materials, molecular motors, and photoactivated drugs. Typically, switching from more to less stable isomer(s) is performed by irradiation with UV or visible light, while the reverse process proceeds thermally or by irradiation using another wavelength. In this work we developed a method of rapid and tunable Z→E isomerization of C═N bond in acyl hydrazones, using aromatic thiols as nucleophilic catalysts. As thiols can be oxidized into catalytically inactive disulfides, the isomerization rates can be controlled via the oxidation state of the catalyst, which, together with the UV irradiation, provides orthogonal means to control the E/Z state of the system. As a proof of this concept, we have applied this method to control the diversity of acyl hydrazone based dynamic combinatorial libraries.

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.

Versatile Self-Adapting Boronic Acids for H-Bond Recognition: From Discrete to Polymeric Supramolecules

Because of the peculiar dynamic covalent reactivity of boronic acids to form tetraboronate derivatives, interest in using their aryl derivatives in materials science and supramolecular chemistry has risen. Nevertheless, their ability to form H-bonded complexes has been only marginally touched. Herein we report the first solution and solid-state binding studies of the first double-H-bonded DD·AA-type complexes of a series of aromatic boronic acids that adopt a syn-syn conformation with suitable complementary H-bonding acceptor partners. The first determination of the association constant (K_a) of ortho-substituted boronic acids in solution showed that K_a for 1:1 association is in the range between 300 and 6900 M^-1. Crystallization of dimeric 1:1 and trimeric 1:2 and 2:1 complexes enabled an in-depth examination of these complexes in the solid state, proving the selection of the -B(OH)₂ syn-syn conformer through a pair of frontal H-bonds with the relevant AA partner. Non-ortho-substituted boronic acids result in "flat" complexes. On the other hand, sterically demanding analogues bearing ortho substituents strive to retain their recognition properties by rotation of the ArB(OH)₂ moiety, forming "T-shaped" complexes. Solid-state studies of a diboronic acid and a tetraazanaphthacene provided for the first time the formation of a supramolecular H-bonded polymeric ribbon. On the basis of the conformational dynamicity of the -B(OH)₂ functional group, it is expected that these findings will also open new possibilities in metal-free catalysis or organic crystal engineering, where double-H-bonding donor boronic acids could act as suitable organocatalysts or templates for the development of functional materials with tailored organizational properties.