n → π* interactions as a versatile tool for controlling dynamic imine chemistry in both organic and aqueous media

Selective Protein (Post-)modifications through Dynamic Covalent Chemistry: Self-activated SNAr Reactions

SNAr reactions were remarkably accelerated using a pretargeting and activating unit based on dynamic covalent chemistry (DCvC). A Cys attack at the C-F bond on the aromatic ring of salicylaldehyde derivatives was only observed upon iminium formation with a neighboring Lys residue of model small peptides. Such self-activation was ascribed to the stronger electron-withdrawing capability of the iminium bond with respect to that of the parent aldehyde that stabilized the transition state of the reaction, together with the higher preorganization of the reactive groups in the cationic aldiminium species. This approach was further applied for the functionalization of two antibodies. In both cases, the presence of the aldehyde group in close proximity to the reactive C-F bond resulted in a noteworthy increase in bioconjugation yields, with excellent chemo-selectivity. Whereas the modification of an IgG1 antibody led to stochastic product distributions, microenvironment selectivity was noted when employing IgG4, in line with the lower number of Lys residues in the hinge region of the latter. Additionally, the postfunctionalization of the modified antibodies was attained through the dynamic covalent exchange of the tethered iminium derivative with hydrazides, representing an unprecedented "tag and modify" selective bioconjugation strategy based on DCvC.

Structure-Reactivity Relationships in a Small Library of Imine-Type Dynamic Covalent Materials: Determination of Rate and Equilibrium Constants Enables Model Prediction and Validation of a Unique Mechanical Softening in Dynamic Hydrogels

The development of next generation soft and recyclable materials prominently features dynamic (reversible) chemistries such as host-guest, supramolecular, and dynamic covalent. Dynamic systems enable injectability, reprocessability, and time-dependent mechanical properties. These properties arise from the inherent relationship between the rate and equilibrium constants (RECs) of molecular junctions (cross-links) and the resulting macroscopic behavior of dynamic networks. However, few examples explicitly measure RECs while exploring this connection between molecular and material properties, particularly for polymeric hydrogel systems. Here we use dynamic covalent imine formation to study how single-point compositional changes in NH2-terminated nucleophiles affect binding constants and resulting hydrogel mechanical properties. We explored both model small molecule studies and model polymeric macromers, and found >3-decade change in RECs. Leveraging established relationships in the literature, we then developed a simple model to describe the cross-linking equilibrium and predict changes in hydrogel mechanical properties. Interestingly, we observed that a narrow ≈2-decade range of Keq's determine the bound fraction of imines. Our model allowed us to uncover a regime where adding cross-linker before saturation can decrease the cross-link density of a hydrogel. We then demonstrated the veracity of this predicted behavior experimentally. Notably this emergent behavior is not accounted for in covalent hydrogel theory. This study expands upon structure-reactivity relationships for imine formation, highlighting how quantitative determination of RECs facilitates predicting macroscopic behavior. Furthermore, while the present study focuses on dynamic covalent imine formation, the underlying principles of this work are applicable to the general bottom-up design of soft and recyclable dynamic materials.

Anion-Carbonyl Interactions

Presented herein is the exploration of a novel non-covalent anion-carbonyl (X-···C═O) interaction using aromatic imides as receptors and halides as lone pair donors. Combined theoretical calculations and experimental methods including 13C NMR, IR, and crystallographic analyses were performed to provide the physical origin and experimental evidence of anion-carbonyl interactions. The EDA analysis (energy decomposition analysis) based on DFT calculation indicates that electrostatic terms are the dominant contributions for the binding energy while electron delocalization also significantly contributes alongside the electrostatic attraction. Orbital interaction (n → π*) involving the delocalization of halide lone pairs on the carbonyl antibonding orbitals was visualized with NBO (Natural Bond Orbital) analysis. 13C NMR and IR spectra demonstrated upfield chemical shifts and red-shift frequency of hosts upon the addition of halides, reflecting the effect of orbital overlap between the halide lone pairs and π* of carbonyl (n → π* contribution). The anion-carbonyl interactions were directly revealed by X-ray structural analysis of anion and benzene triimide complexes.

Dynamic Covalent Bond-Based Polymer Chains Operating Reversibly with Temperature Changes

Dynamic bonds can facilitate reversible formation and dissociation of connections in response to external stimuli, endowing materials with shape memory and self-healing capabilities. Temperature is an external stimulus that can be easily controlled through heat. Dynamic covalent bonds in response to temperature can reversibly connect, exchange, and convert chains in the polymer. In this review, we introduce dynamic covalent bonds that operate without catalysts in various temperature ranges. The basic bonding mechanism and the kinetics are examined to understand dynamic covalent chemistry reversibly performed by equilibrium control. Furthermore, a recent synthesis method that implements dynamic covalent coupling based on various polymers is introduced. Dynamic covalent bonds that operate depending on temperature can be applied and expand the use of polymers, providing predictions for the development of future smart materials.

Key structural features to favour imines over hydrates in water: pyridoxal phosphate as a muse

Imination reactions in water represent a challenge not only because of the high propensity of imines to be hydrolysed but also as a result of the competing hydrate formation through H2O addition to the aldehyde. In the present work we report a successful approach that allows for favouring imitation reactions while silencing hydrate formation. Such remarkable reactivity and selectivity can be attained by fine-tuning the electronic and steric structural features of the ortho-substituents of the carbonyl groups. It resulted from studying the structure-reactivity relationships in a series of condensation reactions between different amines and aldehydes, comparing the results to the ones obtained in the presence of the biologically-relevant pyridoxal phosphate (PLP). The key role of negatively-charged and sterically-crowding units (i.e., sulfonate groups) in disfavouring hydrate formation was corroborated by DFT and steric-hindrance calculations. Furthermore, the best-performing aldehyde leads to higher imine yields, selectivity and stability than those of PLP itself, allowing for the inhibition of a PLP-dependent enzyme (transaminase) through dynamic aldimine exchange. These results will increase the applicability of imine-based dynamic covalent chemistry (DCvC) under physiological conditions and will pave the way for the design of new carbonyl derivatives that might be used in the dynamic modification of biomolecules.

Aromatic-Carbonyl Interactions as an Emerging Type of Non-Covalent Interactions

Aromatic-carbonyl (Ar···C═O) interactions, attractive interactions between the arene plane and the carbon atom of carbonyl, are in the infancy as one type of new supramolecular bonding forces. Here the study and functionalization of aromatic-carbonyl interactions in solution is reported. A combination of aromatic-carbonyl interactions and dynamic covalent chemistry provided a versatile avenue. The stabilizing role and mechanism of arene-aldehyde/imine interactions are elucidated through crystal structures, NMR studies, and computational evidence. The movement of imine exchange equilibria further allowed the quantification of the interplay between arene-aldehyde/imine interactions and dynamic imine chemistry, with solvent effects offering another handle and matching the electrostatic feature of the interactions. Moreover, arene-aldehyde/imine interactions enabled the reversal of kinetic and thermodynamic selectivity and sorting of dynamic covalent libraries. To show the functional utility diverse modulation of fluorescence signals is realized with arene-aldehyde/imine interactions. The results should find applications in many aspects, including molecular recognition, assemblies, catalysis, and intelligent materials.

Neighboring Effects of Sulfur Oxidation State on Dynamic Covalent Bonds and Assemblies

The impact of a varied sulfur oxidation state (sulfide, sulfoxide, and sulfone) on imine dynamic covalent chemistry is presented. The role of noncovalent interactions, including chalcogen bonds and CH hydrogen bonds, on aldehyde/imine structures and imine exchange reactions was elucidated through experimental and computational evidence. The change in the sulfur oxidation state and diamine linkage further allowed the regulation of imine macrocycles, providing a platform for controlling molecular assemblies.

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.

Intermolecular n→π* Interactions in Supramolecular Chemistry and Catalysis

The n→π* interactions describing attractive force between lone pairs (lps) of nucleophile and carbonyl or polarized unsaturated bonds have recently attracted growing attentions in various disciplines. So far, such non-covalent driving force are mainly concentrated to intramolecular systems. Intermolecular n→π* interactions in principle could produce fascinated supramolecular systems or facilitate organic reactions, however, they remain largely underexplored due to the very weak energy of individual interaction. This review attempts to give an overview of the challenging intermolecular n→π* interactions, much efforts emphasize the supramolecular systems, catalytic processes and spectroscopic measurements.

Amphiphile desymmetrisation-induced steric relief governs self-assembly pathways in aqueous media

Herein, we show that a straightforward desymmetrisation of a bolaamphiphilic chromophore can tune aromatic interactions and exciton coupling upon self-assembly. As a result, multiple assembled states become accesible offering a facile approach to induce pathway complexity in aqueous media.

Double n→π* Interactions with One Electron Donor: Structural and Mechanistic Insights

Double n→π* interactions between one common electron donor of the carbonyl oxygen and two individual acceptor aldehyde/imine units are presented. The structural and mechanistic insights were revealed through a collection of experimental and computational evidence. The orientation and further energetic dependence of orbital interactions were facilely regulated by the size of cyclic urea scaffolds, the bulkiness of aldehydes/imines, and the flexibility of imine macrocycles.

Intermolecular n→π* Interactions Based on a Tailored Multicarbonyl-Containing Macrocycle

Towards unexplored intermolecular n→π* interactions, presented herein are the synthesis, structure, self-assembly and function of a multicarbonyl-containing macrocycle calix[2]arene[2]barbiturate 1. X-ray single crystal diffraction reveals the presence of Cl⋅⋅⋅C=O interactions in CH₂ Cl₂ ⊂1 host-guest complex and multiple intermolecular C=O⋅⋅⋅C=O interactions between molecules 1 in crystalline state. The intermolecular C=O⋅⋅⋅C=O interactions as attractive driving force led to unprecedented self-assembly of nanotube with diameter around 1.4 nm and inner surface engineered by aromatic rings. SEM and TEM images of the self-assembly of 1 demonstrated temperature-dependent morphologies which allows the observation of spheres at 25 °C and rods at 0 °C, respectively. XRD analysis indicated consistent hexagonal patterns in the self-assembly and single crystal lattice, indicating the nanotubes driven by C=O⋅⋅⋅C=O interactions constitute the basic structural architectures of both aggregates. The nanoscopic tubes (pores) formed in the rodlike single crystal engendering the separation of moving dyes were preliminarily investigated by a single-crystal chromatography and crystal-packed column chromatography.

Internal and External Catalysis in Boronic Ester Networks

In dynamic materials, the reversible condensation between boronic acids and diols provides adaptability, self-healing ability, and responsiveness to small molecules and pH. The thermodynamics and kinetics of bond exchange determine the mechanical properties of dynamic polymer networks. Here, we investigate the effects of diol structure and salt additives on the rate of boronic acid-diol bond exchange, binding affinity, and the mechanical properties of the corresponding polymer networks. We find that proximal amides used to conjugate diols to polymers and buffering anions induce significant rate acceleration, consistent with an internal and external catalysis, respectively. This rate acceleration is reflected in the stress relaxation of the gels. These findings contribute to the fundamental understanding of the boronic ester dynamic bond and offer molecular strategies to tune the macromolecular properties of dynamic materials.

Interplay between chalcogen bonds and dynamic covalent bonds

A combination of chalcogen bonds, one type of emerging non-covalent bonding force, and imine bonds, allow the control of the dynamic covalent chemistry with orbital interactions and the reversal of kinetic and thermodynamic selectivity.

nN → π*Ar interactions stabilize the E-ac isomers of arylhydrazides and facilitate their SNAr autocyclizations

We describe a novel mechanism of stabilization of the E-ac isomer of an arylhydrazide via n_N → π*_Ar interactions. We further show that when a leaving group (F) is present at the ortho-position of the carbonyl group of such an arylhydrazide, the n_N → π*_Ar interaction facilitates an S_NAr autocyclization reaction to produce indazolone, an important heterocycle with biological activity. Faster autocyclization of arylhydrazide is observed when an electron withdrawing group is present in the aryl ring, which is a characteristic of S_NAr reactions.

Enantioselective Dynamic Exchange Reactions of Imines

Although dynamic reactions of imines have been extensively studied, the dynamic behaviors manipulated by chirality remain nearly unexplored. In this work, enantioselective amine exchange reactions were demonstrated as a first example via the reaction of enantiomeric chiral amines such as natural amino acids with a series of innovative axially chiral 1,1'-binaphthyl-2,2'-diamine (BNDA)-based imines that were prepared from the condensation reactions between BNDA and salicylaldehyde (SA) or its derivatives. This enantioselective dynamic behavior can be directly indicated by the degree of the fluorescence response of the R-configuration of imines to the d-enantiomer of chiral amine, because the released BNDA can serve as the fluorescence signal output when the amine exchange reaction occurs, which is far higher than the response to its l-enantiomer under identical experimental conditions. For the S-configuration of chiral imines, the fluorescence response is the opposite. The enantioselective exchange reaction can be tuned by altering the electron-withdrawing or electron-donating capability of the substituent at position 4 or 5 of the SA part of chiral imines. Not only o-OH groups in SA-based imines but also protic solvents used as reaction media were found to be important to the dynamic behavior at high rates.

Main Group Metal-Mediated Transformations of Imines

Main-group-metal-mediated transformations of imines have earned a valued place in the synthetic chemist's toolbox. Their versatility allows the simple preparation of various nitrogen containing compounds. This review will outline the early discoveries including metallation, addition/cyclisation and metathesis pathways, followed by the modern-day use of imines in synthetic methodology. Recent advances in imine C-F activation protocols are discussed, alongside revisiting "classic" imine reactivity from a sustainable perspective. Developments in catalytic methods for hydroelementation of imines have been reviewed, highlighting the importance of s-block metals in the catalytic arena. Whilst stoichiometric transformations in alternative reaction media such as deep eutectic solvents or water have been summarised. The incorporation of imines into flow chemistry has received recent attention and is summarised within.

Spectroscopic evidence of n → π* interactions involving carbonyl groups

n → π* has emerged as an important noncovalent interaction that can affect the conformations of both small- and macromolecules including peptides and proteins. Carbonyl-carbonyl (COCO) n → π* interactions involving CO groups are well studied. Recent studies have shown that the COCO n → π* interactions are the most abundant secondary interactions in proteins with a frequency of 33 interactions per 100 residues and, among the various secondary interactions, n → π* interactions are expected to provide the highest enthalpic contributions to the conformational stability of globular proteins. However, n → π* interactions are relatively weak and provide an average stabilization of about 0.25 kcal mol-1 per interaction in proteins. The strongest n → π* interaction could be as strong as a moderate hydrogen bond. Therefore, it is challenging to detect and quantify these weak interactions, especially in solution in the presence of perturbation from other intermolecular interactions. Accordingly, spectroscopic investigations that can provide direct evidence of n → π* interaction are limited, and the majority of papers published in this area have relied on X-ray crystallography and/or theoretical calculations to establish the presence of this interaction. The aim of this perspective is to discuss the studies where a spectroscopic signature in support of n → π* interaction was observed. As the "n → π* interaction" is a relatively new terminology, there remains the possibility of there being earlier studies where spectroscopic evidence for n → π* interactions was obtained but it was not discussed in light of the n → π* terminology. We noticed several such studies and, as can be expected, these studies were often overlooked in the discussion of n → π* interactions in the recent literature. In this perspective, we have also discussed these studies and provided computational support for the presence of n → π* interaction.

Effects of n → π* Orbital Interactions on Molecular Rotors: The Control and Switching of Rotational Pathway and Speed

The role of n → π* orbital interactions in the rotational pathway and barrier of biaryl-based molecular rotors was elucidated through a combined experimental and computational study. The n → π* interaction in the transition state can lead to the acceleration of rotors. The competition between the n → π* interaction and hydrogen bonding further enabled the reversal of the pathway and greasing/braking the rotor in response to acid/base stimuli, thereby creating a switchable molecular rotor.

Physical organic studies and dynamic covalent chemistry of picolyl heterocyclic amino aminals

A dynamic covalent system of the picolyl heterocyclic amino aminals has been studied. The aminals are characterized as a metastable species and easily switch to other forms via external stimuli. The solvent, temperature, acid-base and substituent effects have been examined to evaluate the dynamic covalent system. The results reveal that a more polar solvent, a lower temperature, basic conditions and an electron-withdrawing moiety contribute to the stabilities of aminals. The existence of the n → π* interaction between acetonitrile and the C[double bond, length as m-dash]N moiety makes the N-pyrimidyl imine (4c and 4d) yield higher in CD3CN. In a similar fashion, all aminals tend to convert to the corresponding hemiaminal ethers in a methanol environment. According to these findings, we successfully synthesized the following species: (a) N-2-picolylpyrimidin-2-amine 6c obtained by reduction using acetonitrile as the specific solvent; (b) a picolyl aromatic amino aminal 3e prepared from 2-pyridinecarboxaldehyde and the electron withdrawing 2-methoxy-5-nitroaniline.

Reconciling Electrostatic and n→π* Orbital Contributions in Carbonyl Interactions

Interactions between carbonyl groups are prevalent in protein structures. Earlier investigations identified dominant electrostatic dipolar interactions, while others implicated lone pair n→π* orbital delocalisation. Here these observations are reconciled. A combined experimental and computational approach confirmed the dominance of electrostatic interactions in a new series of synthetic molecular balances, while also highlighting the distance-dependent observation of inductive polarisation manifested by n→π* orbital delocalisation. Computational fiSAPT energy decomposition and natural bonding orbital analyses correlated with experimental data to reveal the contexts in which short-range inductive polarisation augment electrostatic dipolar interactions. Thus, we provide a framework for reconciling the context dependency of the dominance of electrostatic interactions and the occurrence of n→π* orbital delocalisation in C=O⋅⋅⋅C=O interactions.

Supramolecular assembly through intermolecular n → π* interactions through a coordinated perrhenate formed via superoxidation of Re(i) to Re(vii) in the formation of substituted Re(CO)3 complexes bearing Diimine ligands

Two Re(i) tricarbonyl complexes bearing Ph₄TAP and dafone ligands and having a coordinated perrhenate group obtained via in situ superoxidation of Re(i) to Re(vii) have been characterized.

Internal catalysis for dynamic covalent chemistry applications and polymer science

In this review, we provide a concise analysis of internal catalysis as an attractive design principle to combine chemical robustness with reactivity in dynamic covalent chemistry applications and a material context.

Synergistic repulsive interactions trigger pathway complexity

We demonstrate the impact of synergistic repulsive interactions on pathway complexity in aqueous media.