Host Amplification in a Dithioacetal-Based Dynamic Covalent Library

Acetal-thiol Click-like Reaction: Facile and Efficient Synthesis of Dynamic Dithioacetals and Recyclable Polydithioacetals

Dithioacetals are heavily used in organic, material and medical chemistries, and exhibit huge potential to synthesize degradable or recyclable polymers. However, the current synthetic approaches of dithioacetals and polydithioacetals are overwhelmingly dependent on external catalysts and organic solvents. Herein, we disclose a catalyst- and solvent-free acetal-thiol click-like reaction for synthesizing dithioacetals and polydithioacetals. High conversion, higher than acid catalytic acetal-thiol reaction, can be achieved. High universality was confirmed by monitoring the reactions of linear and cyclic acetals (including renewable bio-sourced furan-acetal) with aliphatic and aromatic thiols, and the reaction mechanism of monomolecular nucleophilic substitution (SN1) and auto-protonation (activation) by thiol was clarified by combining experiments and density functional theory computation. Subsequently, we utilize this reaction to synthesize readily recyclable polydithioacetals. By simple heating and stirring, linear polydithioacetals withM ‾ ${\bar M}$ w of ~110 kDa were synthesized from acetal and dithiol, and depolymerization into macrocyclic dithioacetal and repolymerization into polydithioacetal can be achieved; through reactive extrusion, a semi-interpenetrating polymer dynamic network with excellent mechanical properties and continuous reprocessability was prepared from poly(vinyl butyral) and pentaerythritol tetrakis(3-mercaptopropionate). This green and high-efficient synthesis method for dithioacetals and polydithioacetals is beneficial to the sustainable development of chemistry.

Photoswitchable Cascades for Allosteric and Bidirectional Control over Covalent Bonds and Assemblies

Studies of complex systems and emerging properties to mimic biosystems are at the forefront of chemical research. Dynamic multistep cascades, especially those exhibiting allosteric regulation, are challenging. Herein, we demonstrate a versatile platform of photoswitchable covalent cascades toward remote and bidirectional control of reversible covalent bonds and ensuing assemblies. The relay of a photochromic switch, keto-enol equilibrium, and ring-chain equilibrium allows light-mediated reversible allosteric structural changes. The accompanying distinct reactivity further enables photoswitchable dynamic covalent bonding and release of substrates bidirectionally through alternating two wavelengths of light, essentially realizing light-mediated signaling cycles. The downfall of energy by covalent bond formation/scission upon photochemical reactions offers the driving force for the controlled direction of the cascade. To show the molecular diversity, photoswitchable on-demand assembly/disassembly of covalent polymers, including structurally reconfigurable polymers, was realized. This work achieves photoswitchable allosteric regulation of covalent architectures within dynamic multistep cascades, which has rarely been reported before. The results resemble allosteric control within biological signaling networks and should set the stage for many endeavors, such as dynamic assemblies, molecular motors, responsive polymers, and intelligent materials.

Thiol-Aldehyde Polycondensation for Bio-based Adaptable and Degradable Phenolic Polymers

Designing sustainable materials with tunable mechanical properties, intrinsic degradability, and recyclability from renewable biomass through a mild process has become vital in polymer science. Traditional phenolic resins are generally considered to be not degradable or recyclable. Here we report the design and synthesis of linear and network structured phenolic polymers using facile polycondensation between natural aldehyde-bearing phenolic compounds and polymercaptans. Linear phenolic products are amorphous with Tg between -9 °C and 12 °C. Cross-linked networks from vanillin and its di-aldehyde derivative exhibited excellent mechanical strength between 6-64 MPa. The connecting dithioacetals are associatively adaptable strong bonds and susceptible to degradation in oxidative conditions to regenerate vanillin. These results highlight the potential of biobased sustainable phenolic polymers with recyclability and selective degradation, as a complement to the traditional phenol-formaldehyde resins.

Chemically Recyclable Dithioacetal Polymers via Reversible Entropy-Driven Ring-Opening Polymerization

In a sustainable circular economy, polymers capable of chemical recycling to monomers are highly desirable. We report an efficient monomer-polymer recycling of polydithioacetal (PDTA). Pristine PDTAs were readily synthesized from 3,4,5-trimethoxybenzaldehyde and alkyl dithiols. They then exhibited depolymerizability via ring-closing depolymerization into macrocycles, followed by entropy-driven ring-opening polymerization (ED-ROP) to reform the virgin polymers. High conversions were obtained for both the forward and reverse reactions. Once crosslinked, the network exhibited thermal reprocessability enabled by acid-catalyzed dithioacetal exchange. The network retained the recyclability into macrocyclic monomers in solvent which can repolymerize to regenerate the crosslinked network. These results demonstrated PDTA as a new molecular platform for the design of recyclable polymers and the advantages of ED-ROP for which polymerization is favored at higher temperatures.

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.

Direct Synthesis of Unsymmetrical Dithioacetals

Dithioacetals are a frequently used motif in synthetic organic chemistry and have recently seen increasing attention as structural motif in promising antiviral agents against plant pathogens. Most existing reports, however, only discuss symmetrical dithioacetals. Examples of mixed dithioacetals are scarce and no general method for the selective synthesis of these compounds exists. Herein, a synthetically simple general one-step protocol was developed for the synthesis of a broad range of unsymmetrical dithioacetals consisting of one aromatic and one aliphatic thiol moiety from the corresponding aldehyde/thiol mixture. The mixed S,S-acetals were obtained in high yields, and a great variety of functional groups was tolerated. Kinetic control enabled an excellent selectivity in regard to the unsymmetrical dithioacetal.

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.

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.

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.

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.