Hydrogen-Bond Catalysis of Imine Exchange in Dynamic Covalent Systems

Self-Healing Transparent Poly(dimethyl)siloxane with Tunable Mechanical Properties: Toward Enhanced Aging Materials for Space Applications

When exposed to the geostationary orbit, polymeric materials tend to degrade on their surface due to the appearance of cracks. Implementing the self-healing concept in polymers going to space is a new approach to enhancing the lifespan of materials that cannot be replaced once launched. In this study, the elaboration of autonomous self-healing transparent poly(dimethylsiloxane) (PDMS) materials resistant to proton particles is presented. The PDMS materials are functionalized with various compositions of urea and imine moieties, forming dynamic covalent and/or supramolecular networks. The hydrogen bonds induced by the urea ensure the formation of a supramolecular network, while the dynamic covalent imine bonds are capable of undergoing exchange reactions. Materials with a broad range of mechanical properties were obtained depending on the composition and structure of the PDMS networks. As coating applications in a spatial environment were mainly targeted, the surface properties of the polymer are essential. Thus, percentages of scratch recovery were determined by AFM. From these data, self-healing kinetics were extracted and rationalized based on the polymer structures. The obtained data were in good agreement with the relaxation times determined by rheology in stress relaxation experiments. Moreover, the accelerated aging of materials under proton irradiation, simulating a major part of the geostationary environment, revealed a strong limitation or disappearance of cracks while keeping the transparency of the PDMS. These promising results open routes to prepare new flexible autonomous polymeric materials for space applications.

DFT-Guided Mechanistic Insights into Chemodivergence: A Mixed Explicit-Implicit Solvent Description to Dictate the Chemoselectivity

Herein we report a density functional theory (DFT)-guided mechanistic investigation of the nitrile reduction reaction, which exhibits a solvent-dependent chemodivergence. This study reveals an interesting mechanistic picture, highlighting the exact role of a protic solvent, isopropanol, in regulating the reaction outcome. The explicit solvent effect involving polar protic isopropanol favors imine metathesis by proton hopping through stepwise addition and elimination steps and thus produces a secondary amine as the final product. In contrast, the nonpolar solvent n-hexane is incapable of facilitating the proton migration and stops the solvent-assisted imine metathesis. As a result, only primary amines are obtained as the final product. This DFT study provides a recipe for the choice of solvents that can dictate chemoselectivity in product formation.

DFT Study of Imine-Exchange Reactions in Iron(II)-Coordinated Pincers

The imine bond is among the most applied motifs in dynamic covalent chemistry. Although its uses are varied and often involve coordination to a transition metal for stability, mechanistic studies on imine exchange reactions so far have not included metal coordination. Herein, we investigated the condensation and transimination reactions of an Fe2+ -coordinated diimine pyridine pincer, employing wB97XD/6-311G(2d,2p) DFT calculations in acetonitrile. We first experimentally confirmed that Fe2+ is strongly coordinated by these pincers, and is thus a justified model ion. When considering a four-membered ring-shaped transition state for proton transfers, the required activation energies for condensation and transimination reaction exceeded the values expected for reactions known to be spontaneous at room temperature. The nature of the incoming and exiting amines and the substituents on the para-position of the pincer had no effect on this. Replacing Fe2+ with Zn2+ or removing it altogether did not reduce it either. However, the addition of two ethylamine molecules lowered the energy barriers to be compatible with experiment (19.4 and 23.2 kcal/mol for condensation and transimination, respectively). Lastly, the energy barrier of condensation of a non-coordinated pincer was significantly higher than found for Fe2+ -coordinating pincers, underlining the catalyzing effect of metal coordination on imine exchange reactions.

Antitumor Activity of PEGylated and TEGylated Phenothiazine Derivatives: Structure–Activity Relationship

The paper aims to investigate the antitumor activity of a series of phenothiazine derivatives in order to establish a structure–antitumor activity relationship. To this end, PEGylated and TEGylated phenothiazine have been functionalized with formyl units and further with sulfonamide units via dynamic imine bonds. Their antitumor activity was monitored in vitro against seven human tumors cell lines and a mouse one compared to a human normal cell line by MTS assay. In order to find the potential influence of different building blocks on antitumor activity, the antioxidant activity, the ability to inhibit farnesyltransferase and the capacity to bind amino acids relevant for tumor cell growth were investigated as well. It was established that different building blocks conferred different functionalities, inducing specific antitumor activity against the tumor cells.

A Twisted Chiral Cavitand with 5-Fold Symmetry and Its Length-Selective Binding Properties

Controlling bottom-up syntheses from chiral seeds to construct architectures with specific chiralities is currently challenging. Herein, a twisted chiral cavitand with 5-fold symmetry was constructed by bottom-up synthesis using corannulene as the chiral seed and pillar[5]arene as the chiral wall. After docking between the seed and the wall, their dynamic chiralities (M and P) are fixed. Moreover, the formed hedges also exhibit M and P chirality. Through dynamic covalent bonding, the thermodynamically stable product is obtained selectively as a pair of enantiomers (MMM and PPP), where all three subcomponents, i.e., the corannulene, hedges, and pillar[5]arene, are tilted in the same direction. Furthermore, the twisted cavitand exhibits length-selective binding to alkylene dibromides, with three maximum binding constants being unexpectedly observed.

Anion-assisted amidinium exchange and metathesis

Dynamic covalent chemistry has become an invaluable tool for the design and preparation of adaptable yet robust molecular systems. Herein we explore the scope of a largely overlooked dynamic covalent reaction - amidinium exchange - and report on conditions that allow formal amidinium metathesis 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.

Configurational and Constitutional Dynamics of Enamine Molecular Switches

Dual configurational and constitutional dynamics in systems based on enamine molecular switches has been systematically studied. pH-responsive moieties, such as 2-pyridyl and 2-quinolinyl units, were required on the "stator" part, also providing enamine stability through intramolecular hydrogen-bonding (IMHB) effects. Upon protonation or deprotonation, forward and backward switching could be rapidly achieved. Extension of the stator π-system in the 2-quinolinyl derivative provided a higher E-isomeric equilibrium ratio under neutral conditions, pointing to a means to achieve quantitative forward/backward isomerization processes. The "rotor" part of the enamine switches exhibited constitutional exchange ability with primary amines. Interestingly, considerably higher exchange rates were observed with amines containing ester groups, indicating potential stabilization of the transition state through IMHB. Acids, particularly BiIII , were found to efficiently catalyze the constitutional dynamic processes. In contrast, the enamine and the formed dynamic enamine system showed excellent stability under basic conditions. This coupled configurational and constitutional dynamics expands the scope of dynamic C-C and C-N bonds and potentiates further studies and applications in the fields of molecular machinery and systems chemistry.