Ring-chain Equilibrium Research Articles

Directing Organometallic Ring-Chain Equilibrium by Electrostatic Interactions.

Dynamic chemistry, which falls into the realm of both supramolecular and covalent chemistry, enables intriguing properties, such as structural diversity, self-healing, and adaptability. Due to robustness of covalent bonds compared to noncovalent ones, dynamic covalent chemistry has been exploited to synthesize complex molecular nanostructures at solid/liquid interfaces under ambient conditions, generally responsive to internal factors that directly regulate intermolecular covalent bonds. However, directing dynamics of covalent nanostructures, e.g., the typical ring-chain equilibria, on surface by extrinsic interactions remains elusive and challenging. Herein, we have controllably directed the ring-chain equilibrium of covalent organometallic structures by regulating intermolecular electrostatic interactions, thus achieving on-surface dynamic covalent chemistry under ultrahigh vacuum conditions. Our findings unravel the dynamic mechanism of covalent polymers governed by weak intermolecular interactions at the submolecular level, which not only bridges the gap between supramolecular and covalent chemistry but also offers great opportunities for the fabrication of adaptive polymeric nanostructures that respond to different conditions.

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.

Effects of additional ring-fusion site on dual reactivity based dynamic covalent chemistry

Tautomers are widespread in organic chemistry, and their unique reactivity allows diverse transformations. In this work, the incorporation of nearby amide ring-fusion site offered a versatile platform for controlling dual reactivity based dynamic covalent chemistry (DCC) of aldehyde ring-chain tautomers. A suite of 2-formylbenzosulfonamide derived tautomers were prepared, and the intramolecular ring-chain equilibrium and intermolecular dynamic covalent reactions along the underlying mechanistic foundation were investigated. By regulating the substituent electronic effect and solvent effect, ring-chain tautomerization equilibrium was readily regulated in solution. The adjacent amide NH participates in intramolecular hydrogen bonding, as revealed by crystal analysis. Furthermore, the addition of acid and the change of solvent allowed turning on/off the intramolecular cyclization from the amide group to create fused ring scaffolds. Finally, the reversible covalent reactions with amine nucleophile reagents maintained the intermolecular reactivity successfully. The molecular and pathway diversity reported herein sets the scene for future design and applications.

Visible-Light Driven Control Over Triply and Quadruply Hydrogen-Bonded Supramolecular Assemblies.

Supramolecular polymers offer tremendous potential to produce new "smart" materials, however, there remains a need to develop systems that are responsive to external stimuli. In this work, visible-light responsive hydrogen-bonded supramolecular polymers comprising photoresponsive supramolecular synthons (I-III) consisting of two hydrogen bonding motifs (HBMs) connected by a central ortho-tetrafluorinated azobenzene have been characterized by DOSY NMR and viscometry. Comparison of different hydrogen-bonding motifs reveals that assembly in the low and high concentration regimes is strongly influenced by the strength of association between the HBMs. I, Incorporating a triply hydrogen-bonded heterodimer, was found to exhibit concentration dependent switching between a monomeric pseudo-cycle and supramolecular oligomer through intermolecular hydrogen bonding interactions between the HBMs. II, Based on the same photoresponsive scaffold, and incorporating a quadruply hydrogen-bonded homodimer was found to form a supramolecular polymer which was dependent upon the ring-chain equilibrium and thus dependent upon both concentration and photochemical stimulus. Finally, III, incorporating a quadruply hydrogen-bonded heterodimer represents the first photoswitchable AB type hydrogen-bonded supramolecular polymer. Depending on the concentration and photostationary state, four different assemblies dominate for both monomers II and III, demonstrating the ability to control supramolecular assembly and physical properties triggered by light.

Intramolecular Ring-Chain Equilibrium Elimination Strategy for Pinpointing C═C Positional and Geometric Isomers of N-Alkylpyridinium Unsaturated Fatty Acid Derivatives via Ion Mobility-Mass Spectrometry.

Free unsaturated fatty acids (UFA) are key intermediates of lipid metabolism and participate in many metabolic pathways with specific biological functions. Although various fragmentation-based methods for pinpointing C═C locations in UFA were developed, the current mass spectrometry methods are difficult to simultaneously differentiate geometric isomers and positional isomers in trace samples due to low ionization efficiency, low conversion, and low resolution. Herein, an intramolecular ring-chain equilibrium elimination strategy via 4-plex stable isotope labeling dual derivatization-assisted ion mobility-mass spectrometry was developed, thereby one-pot specifically labeling C═C and carboxyl groups among the trace and unstable UFA with high sensitivity, high efficiency, and good substrate generality. It achieved fast separation of both C═C positional and geometric isomers with high resolution, which benefited from eliminating the intramolecular ring-chain equilibrium by suppressing the formation of salt bridges between free carboxyl groups and pyridine cations. 4-plex stable isotope labeling reagents showed similar reactivity, enabling high-throughput quantitative analysis of omics. This method was successfully applied for accurate and rapid identification of the UFA composition in olive oil extract. These results suggest that the developed method provides new insight for rapid characterization of UFA C═C positional and geometric isomers in complex samples to explore disease biomarkers and food quality control indicators.

Multiple control of azoquinoline based molecular photoswitches.

Multi-addressable molecular switches with high sophistication are creating intensive interest, but are challenging to control. Herein, we incorporated ring-chain dynamic covalent sites into azoquinoline scaffolds for the construction of multi-responsive and multi-state switching systems. The manipulation of ring-chain equilibrium by acid/base and dynamic covalent reactions with primary/secondary amines allowed the regulation of E/Z photoisomerization. Moreover, the carboxyl and quinoline motifs provided recognition handles for the chelation of metal ions and turning off photoswitching, with otherwise inaccessible Z-isomer complexes obtained via the change of stimulation sequence. Particularly, the distinct metal binding behaviors of primary amine and secondary amine products offered a facile way for modulating E/Z switching and dynamic covalent reactivity. As a result, multiple control of azoarene photoswitches was accomplished, including light, pH, metal ions, and amine nucleophiles, with interplay between diverse stimuli further enabling addressable multi-state switching within reaction networks. The underlying structural and mechanistic insights were elucidated, paving the way for the creation of complex switching systems, molecular assemblies, and intelligent materials.

Enhancing hydrolytic stability of dynamic imine bonds and polymers in acidic media with internal protecting groups

Imine bonds are among the most explored building motifs in dynamic chemistry, polymers, and materials, and yet, their acid-resistance remains a longstanding issue. Herein we demonstrate a concept of internal protecting groups for improving the kinetic stability of dynamic imine bonds and polymers. Systematic examination of structure-reactivity relationship of a series of aldehydes/imines bearing a neighboring carboxyl allowed uncovering of required structural features for dynamically masking imine bonds with cyclic structures. Mechanistic studies indicated that noncovalent interactions along with sterics control the ring-chain equilibrium and the stability of imine bonds. The incorporation of internal protecting groups into imine polymers further enabled their controlled stability in acidic media. Moreover, a combination of dynamic covalent network and coordination supramolecular network provided a facile means for the modulation of luminescent and mechanical properties of polymers. The strategies and results reported should be beneficial to molecular assemblies, dynamic polymers, biological delivery, and intelligent materials.

An efficient cyclodepolymerization route for the chemical recycling of poly(ethylene adipate)

We present here a green cyclodepolymerization route for the chemical recycling of poly(ethylene adipate) to its cyclic oligomers based on ring–chain equilibria in solution.

Ring-chain equilibria of dynamic macrocycles with a bis(hindered amino)disulfide linker

The effect of chain length on ring-chain equilibria was investigated to both reveal their dynamic equilibrium reactions and expand the availability of bis(2,2,6,6-tetramethylpiperidin-1-yl)disulfide (BiTEMPS)-based macrocyclic monomers.

Polydioxanone Derivative Bearing Methoxy Groups toward Bio‐Functional Degradable Polymers Exhibiting Hydration‐Driven Biocompatibility

Front Cover: In article 2200192 by Kazuki Fukushima, Yuki Ota, and Takashi Kato, an aliphatic poly(ester-ether) with methoxy side chains is prepared by ring-opening polymerization of the corresponding cyclic monomer derived from a glycerol derivative. The polymerization involves ring—chain equilibrium and thus is conducted at below 0 °C to achieve a high monomer conversion of more than 80%. The polymer exhibits high hydration and hydrolytic properties due to ether groups in the main and side chains. This polymer is promising as a bio-based, sustainable, and biomedical material.

Polydioxanone Derivative Bearing Methoxy Groups toward Bio‐Functional Degradable Polymers Exhibiting Hydration‐Driven Biocompatibility

AbstractAn aliphatic poly(ester‐ether) with methoxy side chains from a glycerol derivative that exhibits increased hydration properties for potential applications in blood‐contacting medical devices is developed. The poly(ester‐ether) is prepared by ring‐opening polymerization (ROP) of an analogue of 1,4‐dioxan‐2‐one (DX) with a methoxy‐containing substituent in the presence of acidic organocatalysts. It is found that the ring–chain equilibrium resides in the ROP of the DX analogue at room temperature and at relatively low monomer concentration (≈2 m). The organocatalytic ROP has enabled more than 80% monomer conversion at temperatures below 0 °C in the bulk state. The methoxy‐bearing poly(ester‐ether) is insoluble in water, amorphous, and viscous, demonstrating a high level of interaction with water when evaluating the thermal properties of the hydrated polymer and water contact angles. The polymer also shows susceptibility to hydrolysis and end‐group dependence on thermal degradation. These results suggest a good potential for methoxy‐bearing poly(ester‐ether) as a biodegradable and biocompatible polymer derived from renewable feedstocks.

Dynamic Ring-chain Equilibrium of Nucleophilic Thiol-yne “Click” Polyaddition for Recyclable Poly(dithioacetal)s

We report a dynamic polymerization system based on the reversible nucleophilic Michael polyaddition of activated alkynes and dithiols. Four poly(dithioacetal)s ( P1 − P4 ) were prepared via the base-catalyzed thiol-yne “click” polyaddition of two dithiols (1,4-butanedithiol ( 4S ) and 1,5-pentanedithiol ( 5S )) and two alkynones (3-butyn-2-one ( Y1 ) and 1-phenyl-2-propyn-1-one ( Y2 )) at high concentrations. We systematically investigated the base-catalyzed polymerization of 4S and Y1 (for polymer P1 ) under different conditions, and found that this polymerization was a highly concentration-dependent dynamic system: polymer P1 was formed at high concentration, while seven-membered dithioacetal, 1-(1,3-dithiepan-2-yl) propan-2-one ( C1 ), was obtained at low concentration. The polymerization of 4S and Y2 (for polymer P4 ) displayed similar polymerization behavior, generating 2-(1,3-dithiepan-2-yl)-1-phenylethanone ( C4 ) at low concentration. On the contrary, polymer P2 (from Y1 and 5S ) was exclusively obtained with no formation of eight-membered dithioacetal. The polymerizations of Y1 with 1,2-ethanedithiol ( 2S ) and 1,3-dimercaptopropane ( 3S ) only afforded corresponding five- and six-membered dithioacetals, 1-(1,3-dithiolan-2-yl) propan-2-one ( C2 ) and 1-(1,3-dithian-2-yl) propan-2-one ( C3 ). This dynamic behavior of P1 and P4 was attributed to the concentration-dependent retro-Michael addition reaction between a thiol and a β-sulfido-α,β-unsaturated carbonyl compound catalyzed by bases. Furthermore, polymers P1 and P4 could be depolymerized into C1 and C4 in yields of 58% and 95%, respectively. The ring-opening polymerization of C1 at high concentration could successfully regenerate polymer P1 . Thus, a new type of closed-loop recyclable poly(dithioacetal)s was developed.

Pattern-free generation and quantummechanical scoring of ring-chain tautomers.

In contrast to the computational generation of conventional tautomers, the analogous operation that would produce ring-chain tautomers is rarely available in cheminformatics codes. This is partly due to the perceived unimportance of ring-chain tautomerism and partly because specialized algorithms are required to realize the non-local proton transfers that occur during ring-chain rearrangement. Nevertheless, for some types of organic compounds, including sugars, warfarin analogs, fluorescein dyes and some drug-like compounds, ring-chain tautomerism cannot be ignored. In this work, a novel ring-chain tautomer generation algorithm is presented. It differs from previously proposed solutions in that it does not rely on hard-coded patterns of proton migrations and bond rearrangements, and should therefore be more general and maintainable. We deploy this algorithm as part of a workflow which provides an automated solution for tautomer generation and scoring. The workflow identifies protonatable and deprotonatable sites in the molecule using a previously described approach based on rapid micro-pKa prediction. These data are used to distribute the active protons among the protonatable sites exhaustively, at which point alternate resonance structures are considered to obtain pairs of atoms with opposite formal charge. These pairs are connected with a single bond and a 3D undistorted geometry is generated. The scoring of the generated tautomers is performed with a subsequent density functional theory calculation employing an implicit solvent model. We demonstrate the performance of our workflow on several types of organic molecules known to exist in ring-chain tautomeric equilibria in solution. In particular, we show that some ring-chain tautomers not found using previously published algorithms are successfully located by ours.

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.

Creation of a PDMS Polymer Brush on SiO2-Based Nanoparticles by Surface-Initiated Ring-Opening Polymerization.

The incorporation of nanoparticles into soft matrices opens a broad spectrum of novel property combinations. However, one of the major challenges for these systems remains the compatibilization of particles with the surrounding matrix by proper surface functionalization. For silicon-based systems or liquid crystalline phases, polydimethylsiloxane (PDMS) brushes at the surface of particles increase the stability against particle agglomeration in such systems. Here, we report a novel approach for the functionalization of particles with a polysiloxane brush by surface-initiated ring-opening polymerization of a cyclosiloxane. For this purpose, surface hydroxy groups of silica and silica-coated hematite particles are used as initiators in combination with phosphazene bases as catalysts. The ring–chain equilibrium of a model-based solution polymerization is investigated in detail to find the appropriate reaction parameters. The corresponding molar masses are determined and compared by 1H-NMR and SEC measurements to confirm the underlying mechanism. In the resulting hybrid nanostructures, a covalently bound PDMS fraction is achieved up to 47 mass %.

Synthesis and Crystal Structure of the Nickel(II) Complex with Aroyl Hydrazone of Ethyl Ether of 5,5-Dimethyl-2,4-Dioxohexane Acid

Coordination complexes of nickel(II) with aroyl hydrazones of ethyl ether of 5,5-dimethyl-2,4-dioxohexane acid (H2L1–H2L6) are prepared and studied. The composition and structure of complexes obtained on the basis of these ligands are established by elemental analysis, IR, 1H NMR spectroscopy, and XRD methods. IR and 1H NMR parameters correlate with each other under the variation of the substituents in the p-position of the benzene ring located in the hydrazide part of ligand molecules. The ring-chain equilibrium in the complexes is established.

Architecture-Controlled Ring-Opening Polymerization for Dynamic Covalent Poly(disulfide)s.

A strategy is reported for controlling the architecture of poly(disulfide)s by ring-opening polymerization. Aryl thiol initiators shift the ring-chain equilibrium to yield cyclic polymers, while alkyl thiols favor linear ones. Control over polymerization enables synthesis of large polymers (630 kDa) and catalytic depolymerization to recycle monomers. This work provides a new avenue to create dynamic covalent polymers with controlled geometry and length, allowing better characterization of structure-property relationships to expand their materials potentials.

Concentration Insensitive Supramolecular Polymerization Enabled by Kinetically Interlocking Multiple-Units Strategy

Concentration Insensitive Supramolecular Polymerization Enabled by Kinetically Interlocking Multiple-Units Strategy

Controllable supramolecular “ring opening” polymerization based on DNA duplex

Supramolecular polymerization is influenced by many factors, especially the stiffness of the polymers, which is directly related to cyclization of the polymers. We have proposed a supramolecular “ring opening” polymerization system with DNA molecules to study the ring-chain equilibrium of supramolecular polymerization. The molecular weight of the supramolecular polymers was characterized by Asymmetric Flow Field Flow Fractionation, and the diffusion coefficient of the supramolecular polymers was characterized by Fluorescent Correlation Spectra. Tuning the ratio of the “ring opening” strand, the length of the DNA monomers and temperature could change the ring-chain equilibrium, thus influence the molecular weight and diffusion coefficient. This strategy has offered us a useful platform to dig into the kinetic mechanism of supramolecular polymerization.

1,2,4-TRIAZINES – PRODUCTS OF REACTION OF THIOCARBOHYDRAZIDE AND ITS ALKYL(ARYL)-SUBSTITUTED DERIVATIVES WITH 1,2-DICARBONYL COMPOUNDS

The reaction of a series of 1,2-dicarbonyl compounds with thiocarbohydrazide and its 1- and 1,4-alkyl-(aryl)-substituted analogs has been studied. Treatment of diacetyl with the polynucleophiles indicated gives monohydrazones and/or 5-methylene-4,5-dihydro-2 H -[1,2,4]triazine-3-thiones. Reaction of phenylglyoxal and benzil with thiocarbohydrazide yields 5-hydroxy- or 5-alkoxy-4,5-dihydro-2 H -[1,2,4]triazine-3-thiones depending on the nature of the solvent. The products of condensation with 1,1-dimethylthiocarbohydrazide showed a thiocarbonohydrazone – 5-hydroxy-4,5-dihydro-2 H -[1,2,4]triazine-3-thione ring-chain type equilibrium. How to Cite Alekseyev, V. V.; Lagoda, I. V.; Yakimovich, S. I.; Yegorova, M. B. Chem. Heterocycl. Compd. 2010 , 46 , 971. [ Khim. Geterotsikl. Soedin. 2010 , 1202.] For this article in the English edition see DOI 10.1007/s10593-010-0610-2