Stereochemical Effects Research Articles

Revealing the Critical Role of the Lone Pair Electrons of Bi3+ in Electrocatalytic Oxygen Reduction Reaction on Mn‐Based BiMn2O5 Mullite Surface

AbstractThere is a research gap regarding the role of lone pair electrons of post‐transition metal cations in catalysis field, yet such studies hold significant implications for expanding their applications. Herein, the role of Bi3+ ion lone pair electrons in electrocatalytic oxygen reduction reaction (ORR) is elucidated using ternary mullite oxides as prototype catalysts. Among the three synthesized mullites ((Sm/Y/Bi)Mn2O5), BiMn2O5 exhibits notably superior ORR catalytic activity. Through advanced characterization techniques, including in situ Raman, in situ infrared, and X‐ray absorption spectroscopy, combined with theoretical calculations, it is determined that the superior electrocatalytic performance of BiMn2O5 originates from the lone pair electrons that activate catalytic sites through stereochemical effects. Specifically, the Bi3+ lone pair electrons in the 6s orbital near Fermi level interact with O 2p orbitals, causing a shift of the oxygen ligands, and significant variance in A─O bond lengths. This structural distortion of BiO8 coordination unit directly results in a large Mn─O bond angle at the active center of BiMn2O5 and strong covalent interactions, facilitating charge transfer and refining the adsorption behavior of oxygen intermediates to achieve the overpotential of 0.25 V vs. RHE. This study provides insights into developing next‐generation catalysts by harnessing the lone pair electrons of post‐transition metal cations.

Stereochemical influence of 4ʹ-methyl substitutions on truncated 4ʹ-thioadenosine derivatives: Impact on A3 adenosine receptor binding and antagonism

Herein, we investigated the stereochemical effects of 4ʹ-methyl substitution on A3 adenosine receptor (A3AR) ligands by synthesizing and evaluating a series of truncated 4ʹ-thioadenosine derivatives featuring 4ʹ-α-methyl, 4ʹ-β-methyl, and 4ʹ,4ʹ-dimethyl substitutions. We successfully synthesized these derivatives, using the stereoselective addition of an organometallic reagent, KSAc-mediated sulfur cyclization, and Vorbrüggen condensation. Binding assays demonstrated that the 4ʹ-β-methyl substitution conferred the highest affinity for A3AR, with compound 1 h exhibiting a Ki = 3.5 nM, followed by the 4ʹ,4ʹ-dimethyl and 4ʹ-α-methyl substitutions. Notably, despite the absence of the 5ʹ-OH group, compound 1 h unexpectedly displayed partial agonism. Computational docking studies indicated that compound 1 h, the β-methyl derivative, adopted a South conformation and maintained strong interactions within the receptor, including a critical interaction with Thr94, a residue known to be notable for agonistic effects. Conversely, compound 2 h, the α-methyl derivative, also adopted a South conformation but resulted in a flattened structure that hindered interactions with Thr94 and Asn250. The dimethyl derivative 3 h exhibited steric clashes with Thr94, contributing to a reduction in binding affinity. However, the docking results for 3 h indicated a North conformation, suggesting that the change in sugar conformation due to the additional 4ʹ-methyl group altered the angle between the α-methyl group and the sugar plane, enabling binding despite the increased steric bulk. These findings suggest that not only do the substituents and their stereochemistry influence receptor–ligand interactions, but the conformation and the resulting spatial orientation of the substituents also play a crucial role in modulating receptor–ligand interaction. This stereochemical insight offers a valuable framework for the design of new, selective, and potent A3AR ligands, potentially facilitating the development of novel therapeutics for A3AR-related diseases such as glaucoma, inflammation, and cancer.

Unveiling the bifunctional roles of Cetyltrimethylammonium bromide in construction of Nb2CTx@MoSe2 heterojunction for fast potassium storage

Exploring robust electrode materials which could permit fast and reversible insertion/extraction of large K+ is a crucial challenge for potassium-ion batteries (PIBs). Smart interfacial design could facilitate electron/ion transport as well as assure the integrity of electrode. Herein, Cetyltrimethylammonium bromide (CTAB) was found to play bifunctional roles in construction of Nb2CTx@MoSe2 heterostructure. Firstly, functionalization of CTAB on the surface of Nb2CTx could influence the subsequent growth of MoSe2 by electrostatic effect, stereochemical effect and the synergetic Lewis acid-base interaction, leading to the formation of Nb2CTx@MoSe2 with tiled heterostructure. Secondly, the interlayer spacing of Nb2CTx was expanded from 0.77 to 1.21 nm owing to the pillar effect of CTAB. As excepted, the capacity retention was 80 % from 100 mA g−1 (406 mA h g−1) to 1000 mA g−1 concerning rate capability and the specific capacity maintained at 240 mA h g−1 (at 2000 mA g−1) over 300 cycles. The calculated DK values from Galvanostatic intermittent titration technique (GITT) measurement of the titled C-T-Nb2CTx@MoSe2@C electrode is two orders of magnitude larger than the traditional T-Nb2CTx@MoSe2@C electrode, further confirming intimate interface between MoSe2 and Nb2CTx could provide convenient potassium-ion transport channels and fast diffusion kinetics. Finally, ex-situ characterizations at different charging and discharging voltage stages, including ex-situ XRD/Raman/HRTEM/XPS have been carried out to reveal the potassium storage mechanism. This work provides a facile strategy for the regulation of interface engineering by the assist of CTAB which could extend to other MXenes-TMDs (Transition metal dichalcogenides) hybrid electrodes.

An unexpected stereochemical effect of thio-substituted Asp in native chemical ligation

This study presents an unexpected finding that the cis isomer of β-thio-Asp exhibits higher ligation activity than the trans isomer. This discovery sheds light on the intricate nature of native chemical ligation and highlights the importance of factors beyond the steric effects of the side chain in modulating ligation activity.

Phosphorus Chemistry at the Roots of Bioenergetics: Ligand Permutation as the Molecular Basis of the Mechanism of ATP Synthesis/Hydrolysis by FOF1-ATP Synthase.

The integration of phosphorus chemistry with the mechanism of ATP synthesis/hydrolysis requires dynamical information during ATP turnover and catalysis. Oxygen exchange reactions occurring at β-catalytic sites of the FOF1-ATP synthase/F1-ATPase imprint a unique record of molecular events during the catalytic cycle of ATP synthesis/hydrolysis. They have been shown to provide valuable time-resolved information on enzyme catalysis during ATP synthesis and ATP hydrolysis. The present work conducts new experiments on oxygen exchange catalyzed by submitochondrial particles designed to (i) measure the relative rates of Pi-ATP, Pi-HOH, and ATP-HOH isotope exchanges; (ii) probe the effect of ADP removal on the extent of inhibition of the exchanges, and (iii) test their uncoupler sensitivity/resistance. The objectives have been realized based on new experiments on submitochondrial particles, which show that both the Pi-HOH and ATP-HOH exchanges occur at a considerably higher rate relative to the Pi-ATP exchange, an observation that cannot be explained by previous mechanisms. A unifying explanation of the kinetic data that rationalizes these observations is given. The experimental results in (ii) show that ADP removal does not inhibit the intermediate Pi-HOH exchange when ATP and submitochondrial particles are incubated, and that the nucleotide requirement of the intermediate Pi-HOH exchange is adequately met by ATP, but not by ADP. These results contradicts the central postulate in Boyer's binding change mechanism of reversible catalysis at a F1 catalytic site with Keq~1 that predicts an absolute requirement of ADP for the occurrence of the Pi-HOH exchange. The prominent intermediate Pi-HOH exchange occurring under hydrolytic conditions is shown to be best explained by Nath's torsional mechanism of energy transduction and ATP synthesis/hydrolysis, which postulates an essentially irreversible cleavage of ATP by mitochondria/particles, independent from a reversible formation of ATP from ADP and Pi. The explanation within the torsional mechanism is also shown to rationalize the relative insensitivity of the intermediate Pi-HOH exchange to uncouplers observed in the experiments in (iii) compared to the Pi-ATP and ATP-HOH exchanges. This is shown to lead to new concepts and perspectives based on ligand displacement/substitution and ligand permutation for the elucidation of the oxygen exchange reactions within the framework of fundamental phosphorus chemistry. Fast mechanisms that realize the rotation/twist, tilt, permutation and switch of ligands, as well as inversion at the γ-phosphorus synchronously and simultaneously and in a concerted manner, have been proposed, and their stereochemical consequences have been analyzed. These considerations take us beyond the binding change mechanism of ATP synthesis/hydrolysis in bioenergetics.

Deoxygenation of Oxiranes by λ3 σ3 -Phosphorus Reagents: A Computational, Mechanistic, and Stereochemical Study.

The deoxygenation of parent and substituted oxiranes by λ3 σ3 -phosphorus reagents has been explored in detail, therefore unveiling mechanistic aspects as well as regio- and stereochemical consequences. Attack to a ring C atom is almost always preferred over one-step deoxygenation by direct P-to-O attack. In most cases a carbene transfer occurs as first step, leading to a phosphorane and a carbonyl unit that thereafter react in the usual Wittig fashion via the corresponding λ5 σ5 -1,2-oxaphosphetane intermediate. Betaines rarely constitute true minima after the first C-attack to oxiranes, at least in the gas-phase. Use of the heavier derivatives AsMe3 and SbMe3 as oxirane deoxygenating reagents was also mechanistically studied. The thermodynamic tendency of λ3 σ3 -phosphorus reagents to act as oxygen (O-attack) or carbene acceptors (C-attack) was theoretically studied by means of the thermodynamic oxygen-transfer potential (TOP) and the newly defined thermodynamic carbene-transfer potential (TCP) parameters, that were explored in a wider context together with many other acceptor centres.

Manipulating Lone-Pair-Driven Luminescence in 0D Tin Halides by Pressure-Tuned Stereochemical Activity from Static to Dynamic.

The excellent luminescence properties and structural dynamics driven by the stereoactivity of the lone pair in a variety of low-dimensional ns2 metal halides have attracted growing investigations for optoelectronic applications. However, the structural and photophysical aspects of the excited state associated with the lone pair expression are currently open questions. Herein, zero-dimensional Sn-based halides with static stereoactive 5 s2 lone pairs are selected as a model system to understand the correlations between the distinctive lone pair expression and the excited-state structural relaxation and charge carrier dynamics by continuous lattice manipulation. Lattice compression drives 5 s2 lone pair active switching and self-trapped exciton (STE) redistribution by suppressing excited-state structural deformation of the isolated SnBr4 2- units. Our results demonstrate that the static expression of the 5 s2 lone pair results in a red broadband triplet STE emission with a large Stokes shift, while its dynamic expression creates a sky-blue narrowband emission dominated by the radiative recombination of singlet STEs. Our findings and the photophysical mechanism proposed highlight the stereochemical effects of lone pair expression in controlling light emission properties and offer constructive guidelines for tuning the optoelectronic properties in diverse ns2 metal halides.

Manipulating Lone‐Pair‐Driven Luminescence in 0D Tin Halides by Pressure‐Tuned Stereochemical Activity from Static to Dynamic

AbstractThe excellent luminescence properties and structural dynamics driven by the stereoactivity of the lone pair in a variety of low‐dimensional ns2 metal halides have attracted growing investigations for optoelectronic applications. However, the structural and photophysical aspects of the excited state associated with the lone pair expression are currently open questions. Herein, zero‐dimensional Sn‐based halides with static stereoactive 5 s2 lone pairs are selected as a model system to understand the correlations between the distinctive lone pair expression and the excited‐state structural relaxation and charge carrier dynamics by continuous lattice manipulation. Lattice compression drives 5 s2 lone pair active switching and self‐trapped exciton (STE) redistribution by suppressing excited‐state structural deformation of the isolated SnBr42− units. Our results demonstrate that the static expression of the 5 s2 lone pair results in a red broadband triplet STE emission with a large Stokes shift, while its dynamic expression creates a sky‐blue narrowband emission dominated by the radiative recombination of singlet STEs. Our findings and the photophysical mechanism proposed highlight the stereochemical effects of lone pair expression in controlling light emission properties and offer constructive guidelines for tuning the optoelectronic properties in diverse ns2 metal halides.

Subtle Stereochemical Effects Influence Binding and Purification Abilities of an FeII4L4 Cage.

A tetrahedral FeII4L4 cage assembled from the coordination of triangular chiral, face-capping ligands to iron(II). This cage exists as two diastereomers in solution, which differ in the stereochemistry of their metal vertices, but share the same point chirality of the ligand. The equilibrium between these cage diastereomers was subtly perturbed by guest binding. This perturbation from equilibrium correlated with the size and shape fit of the guest within the host; insight as to the interplay between stereochemistry and fit was provided by atomistic well-tempered metadynamics simulations. The understanding thus gained as to the stereochemical impact on guest binding enabled the design of a straightforward process for the resolution of the enantiomers of a racemic guest.

Identification of heterochirality-mediated stereochemical interactions in peptide architectures

In this work, we illustrate a strategy for constructing heterochiral peptide architectures with distinct structural, mechanical and thermal characteristics. A series of nanotube structures based on diphenylalanine (FF) and its chiral derivatives were examined. Pronounced effects relating to heterochirality on mechanostability and thermal stability can be identified. The homochiral peptide FF and its enantiomer ff formed nanotubes with high thermal and mechanical stabilities (Young’s modulus: 20.3 ± 5.9 GPa for FF and 21.2 ± 4.7 GPa for ff). In contrast, heterochiral nanotubes formed by Ff and fF manifest superstructures along the axial direction with differed thermal and mechanical strength (Young’s modulus: 7.3 ± 2.4 GPa for Ff and 8.3 ± 2.1 GPa for fF). Combining their single-crystal XRD structure and in silico results, it was demonstrated that the spatial orientations of aromatic moieties were subtly changed by heterochirality of peptide building blocks, which led to intramolecular face-to-face interactions. As the result, both intermolecular axial and interchannel interactions in heterochiral nanotubes were weakened as reflected in the strikingly deteriorated mechanical and thermal stabilities. Conversely, two aromatic side chains of the homochiral peptides were staggered and formed interdigitated steric zippers, which served as strong glues that secured the robustness of nanotubes in both axial and radial orientation. Furthermore, the generality of the heterochiral-mediated stereochemical effects was demonstrated in other “FF class” dipeptides, including fluorinated Ff, FW and FL. Our results unequivocally revealed the relationship between amino acid chirality, peptide molecule packing, and physical stabilities of “FF class” dipeptide self-assembled materials and provide valuable molecular insights into chirality-mediated stereochemical interactions in determining the properties of peptide architectures.

Do Stereochemical Effects Overcome a Charge-Induced Perturbation in Isolated Protonated Cyclo(Tyr-Tyr)?

Two diastereomers of the protonated diketopiperazine (DKP) dipeptide cyclo(Tyr-Tyr), namely, cyclo(LTyr-LTyr)H+ and cyclo(LTyr-DTyr)H+, are studied in a cryogenic ion trap by means of IR photodissociation spectroscopy combined with quantum chemical calculations. The two diastereomers have similar structures in which one of the rings is folded over the DKP ring and the other one is extended in a trans geometry, allowing a strong OH+···π interaction to take place. This contrasts to the observation of a stacked geometry for neutral cyclo(LTyr-LTyr) only under supersonic expansion conditions that do not exist for cyclo(LTyr-DTyr). In the protonated form, the strength of the OH+···π interaction is different for the two diastereomers, resulting in a ∼110 cm-1 difference in the ν(OH+) frequency and a smaller but clearly identifiable difference in the protonated amide ν(NH) frequency. Stereochemical effects are therefore still evidenced despite the strong perturbation due to the excess charge.

Novel Sterically Crowded and Conformationally Constrained α-Aminophosphonates with a Near-Neutral pKa as Highly Accurate 31P NMR pH Probes. Application to Subtle pH Gradients Determination in Dictyostelium discoideum Cells.

In order to discover new 31P NMR markers for probing subtle pH changes (<0.2 pH unit) in biological environments, fifteen new conformationally constrained or sterically hindered α-aminophosphonates derived from diethyl(2-methylpyrrolidin-2-yl)phosphonate were synthesized and tested for their pH reporting and cytotoxic properties in vitro. All compounds showed near-neutral pKas (ranging 6.28–6.97), chemical shifts not overlapping those of phosphorus metabolites, and spectroscopic sensitivities (i.e., chemical shifts variation Δδab between the acidic and basic forms) ranging from 9.2–10.7 ppm, being fourfold larger than conventional endogenous markers such as inorganic phosphate. X-ray crystallographic studies combined with predictive empirical relationships and ab initio calculations addressed the inductive and stereochemical effects of substituents linked to the protonated amine function. Satisfactory correlations were established between pKas and both the 2D structure and pyramidalization at phosphorus, showing that steric crowding around the phosphorus is crucial for modulating Δδab. Finally, the hit 31P NMR pH probe 1b bearing an unsubstituted 1,3,2-dioxaphosphorinane ring, which is moderately lipophilic, nontoxic on A549 and NHLF cells, and showing pKa = 6.45 with Δδab = 10.64 ppm, allowed the first clear-cut evidence of trans-sarcolemmal pH gradients in normoxic Dictyostelium discoideum cells with an accuracy of <0.05 pH units.

High‐pressure synthesis, crystal structure, and properties of a novel quadruple perovskite CeMn3Rh4O12

AbstractA novel transition metal oxide CeMn3Rh4O12 has been synthesized using high‐pressure and high‐temperature conditions of 12 GPa and 1573 K. CeMn3Rh4O12 crystallizes in the AA'3B4O12‐type quadruple perovskite structure with A‐site ordering in a cubic space group of Im in the high‐temperature phase at above 310 K, whereas CeMn3Rh4O12 undergoes a structural phase transition with symmetry lowering down to probable triclinic symmetry on cooling below this temperature. X‐ray absorption spectroscopy reveals that all the constituent ions are trivalent at room temperature, suggesting the valence state of Ce3+Mn3+3Rh3+4O12. Magnetization study demonstrates an antiferromagnetic transition attributed to A'‐site Mn3+ ions at TN = 35 K, whereas a Curie–Weiss analysis finds an additional paramagnetic component attributed to the Ce3+ 4f1 ions. We discuss the differences in structural and magnetic properties between CeMn3Rh4O12 and LaMn3Rh4O12, eventually proposing a possible stereochemical effect of Ce3+ on crystal structure in the quadruple perovskite.

Detailed Structural Features of the Perovskite-Related Halide RbPbI3 for Solar Cell Applications.

All-inorganic lead halide perovskites like CsPbBr3, CsPbI3, or RbPbI3 are good replacements for the classical hybrid organic–inorganic perovskites like CH3NH3PbI3, susceptible to fast degradation in the presence of humid air. They also exhibit outstanding light absorption properties suitable for solar energy applications. Here, we describe the synthesis of RbPbI3 by mechanochemical procedures with green credentials, avoiding toxic or expensive organic solvents; this specimen exhibits excellent crystallinity. We report neutron powder diffraction data, essential to revisit some subtle structural features around room temperature (200–400 K). In all these regimes, the orthorhombic Pnma crystal structure is characterized by the presence along the b direction of the crystal of double rows of edge-sharing PbI6 octahedra. The lone electron pairs of Pb2+ ions have a strong stereochemical effect on the PbI6 octahedral distortion. The relative covalency of Rb–I versus Pb–I bonds shows that the Pb–I-related motions are more rigid than Rb–I-related vibrations, as seen in the Debye temperatures from the evolution of the anisotropic displacements. The optical gap, measured by diffuse reflectance UV–vis spectroscopy, is ∼2.51 eV and agrees well with abinitio calculations. The thermoelectric Seebeck coefficient is 3 orders of magnitude larger than that of other halide perovskites, with a value of ∼117,000 μV·K–1 at 460 K.

Stereochemistry-Induced Discrimination in Reaction Kinetics of Photo-RAFT Initialization

Photoinduced reversible addition–fragmentation chain transfer (photo-RAFT) polymerization combines conventional RAFT polymerization with photochemistry, in which the carbon radicals (R radicals) are generated from RAFT agents (except the cases using external radical photoinitiators) by direct photolysis or photoredox catalysis. As one of the key components, RAFT agent is responsible for living control of photo-RAFT polymerization and various reaction kinetics in the processes of activation/deactivation, propagation, and termination. Despite considerable efforts having been devoted to investigating structural effect of RAFT agents on polymerization kinetics, the impact of stereochemistry in RAFT agents is still underexplored. In this article, we presented a significant stereochemical effect of initial RAFT agents on the kinetics of a photo-RAFT polymerization during the initialization period (the period of consumption of initial RAFT agent). Specifically, two diastereomeric RAFT agents were synthesized and employed for photo-RAFT polymerization. Under a condition of low monomer/RAFT agent ratio, we observed distinct consumption rates of two RAFT agents in separated polymerization systems, with one diastereomer faster than the other. A similar result was observed in the mixed polymerization system (a 1:1 mixture of two diastereomeric RAFT agents), demonstrating a discrimination in RAFT agent consumption in spite of the presence of RAFT chain transfer. Additionally, this discrimination was also present in thermally initiated RAFT polymerization, whereas an opposite trend was observed in the separated systems. The kinetics of initialization processes was analyzed with the aid of density functional theory calculations, which provided plausible explanation of such discrimination. The reported stereochemical effect applies to long dormant RAFT polymer chains beyond the initialization period, in which different stereochemical R groups are always present over the course of RAFT polymerization.

Enantiopure, luminescent, cyclometalated Ir(III) complexes with N-heterocyclic carbene-naphthalimide chromophore: design, vibrational circular dichroism and TD-DFT calculations.

A series of chiral cyclometalated iridium complexes of the type [Ir(C^N)2(C^C:)], {(C^N) = ppy (2); dfppy (3)} featuring a naphthalimide N-heterocyclic carbene ligand (C^C:) = (Naphthalimide-NHC) are described and fully characterized. The racemic complexes rac-2 and rac-3 were resolved via chiral column chromatography techniques into their corresponding enantiopure species Δ-2, Λ-2, Δ-3, Λ-3 as confirmed by their CD curves. This unique class of molecules containing organic and inorganic chromophores might be used as a platform to probe the stereochemical effect on the photophysical properties. Vibrational circular dichroism (VCD) was used as an important tool to assign successfully the stereochemistry of the enantiomers. TD-DFT calculations are also advanced to support the experimental studies and to rationalize the observed results.

Waltzing around the stereochemistry of membrane crosslinkers for precise molecular sieving in organic solvents

Crosslinking of polymeric membranes induces changes in both membrane stability and separation performance. Numerous membrane-crosslinking methods have been developed with the objective of obtaining improved membranes. However, none of these methods systemically investigated the stereochemical effects of the crosslinker in the pursuit of better stability and performance. Herein, we address this knowledge gap by presenting a systematic investigation of the stereochemistry of crosslinkers. The intrinsically microporous poly(ether-ether-ketone)-containing Tröger's base (iPEEK-TB) was synthesized and employed in the fabrication of organic solvent nanofiltration (OSN) membranes. Crosslinkers were carefully selected based on the stereochemical position of the two benzyl bromide functional groups, separated by distances of 4.3, 8.2, 8.5, and 12.4 Å and significant effects arising from crosslinking on membrane physical properties, morphology, and OSN performance were investigated. Crosslinked membranes showed excellent solvent resistance, mechanical flexibility, and thermal stability. As a function of crosslinking distance, the molecular weight cutoff (MWCO) values of the membranes varied in the range of 575–750 g mol−1. The para isomer of the crosslinkers resulted in higher permeance relative to membranes crosslinked with their counterpart ortho isomers, and vice versa, the ortho substitution resulted in higher solute rejection values compared with para isomers. An increase of 50% and 12% in acetonitrile permeance relative to the annealed benchmark membrane was observed upon the treatment using iPEEK-TB with 4,4′-bis(bromomethyl)biphenyl (p-BBMBP) and 2,2′-bis(bromomethyl)-1,1′-biphenyl (o-BBMBP), respectively, whereas a permeance decrease of approximately 23% and 32% was noted upon treatment with α,α′-Dibromo-p-xylene (p-DBX) and α,α′-Dibromo-o-xylene (o-DBX), respectively. The corresponding MWCO changes were found to decrease for all crosslinked membranes within the range of 12%–40%. The crosslinked membranes demonstrated stable performance in polar aprotic solvents such as N,N-dimethylformamide and N-methyl-2-pyrrolidone. The Molecular dynamic (MD) simulations supported the obtained performance results through the variations in the fractional free volume (FFV). This work demonstrates the importance of crosslinker selection for OSN membrane performance and solvent resistivity and opens new avenues for fine-tuning membrane stability and OSN performance.

Structure and collision-induced dissociation of the protonated cyclo His-Phe dipeptide: mechanistic studies and stereochemical effects

The role of stereochemical factors on the structure and the fragmentation paths of the protonated cyclic dipeptide cyclo histidine–phenylalanine is studied under ion traps conditions by combining tandem mass spectrometry, laser spectroscopy, quantum chemical calculations and chemical dynamics simulations. Vibrational spectroscopy obtained by Infrared Multiple Photon Dissociation (IRMPD) reveals a small difference between the two diastereomers, c- $$\hbox {LLH}^{+}$$ and c- $$\hbox {LDH}^{+}$$ , arising mainly from ancillary CH... $$\uppi $$ interactions. In contrast, there is a strong influence of the residues chirality on the collision-induced dissociation (CID) processes. Chemical dynamics simulations rationalize this effect and evidence that proton mobility takes place, allowing isomerization to intermediate cyclic structures that are different for c- $$\hbox {LLH}^{+}$$ and c- $$\hbox {LDH}^{+}$$ , resulting in different barriers to proton mobility. This effect is related to the protonation of the imidazole ring. It contrasts with the minute stereochemical effects observed for other cyclic dipeptides in which the proton is borne by an amide CO.

Understanding the Stereochemical Effect on the Properties of Emerging Thermosets: Sustainable Polybenzoxazines

Exploration of the properties of monomers by modulating their stereochemistry is not well reported in polybenzoxazines (PBzs). Isosorbide contains a rigid bicyclic (endo–exo-configuration) core that has shown promise in high-performance thermoplastic polymers. In this report, new chiral PBzs were prepared from industrially available feedstocks, carbohydrate-derived (isosorbide), and lignin- or cashew nut waste-sourced renewable phenols (cardanol and guaiacol). A fully biobased benzoxazine monomer based on a one-step condensation reaction of isosorbide diamine (isa) with phenols was carried out for 15 min under microwave and solvent- and catalyst-free conditions. Structural elucidation of monomers was achieved by two-dimensional (2D) NMR spectroscopy [heteronuclear single quantum correlation (HSQC) and COSY (correlated spectroscopy)]. The “isa”-based monomer revealed a comparable ring-opening polymerization temperature and a good processing window for the widely available bisphenol-A/aniline-based monomer. The resultant polymers showed a higher Tg, better thermomechanical properties, and adhesive strength compared to other classical petrobased PBzs. The current work extends the importance of the major component of the sugar-derived isohexide family member, isosorbide, in polybenzoxazine chemistry and suggests that stereocontrol in the monomer is another approach to affect the thermomechanical properties of this upcoming class of high-performance phenolic polymer.

Cobalt-Catalyzed Hartung-Mukaiyama Cyclization of γ-Hydroxy Olefins: Stereocontrolled Synthesis of the Tetrahydrofuran Moiety of Amphidinolide N.

Cobalt-catalyzed Mukaiyama-type cyclization of γ-hydroxy olefins is known as an atom- and step-economical means for stereoselective synthesis of 2,5-trans-substituted tetrahydrofuran derivatives. In this study, we investigated the synthesis of a series of 2,5-substituted tetrahydrofuran derivatives by means of a cobalt-catalyzed Hartung-Mukaiyama cyclization. The stereochemical consequence of the reaction was found to be largely dependent on the substitution pattern and relative configuration of γ-hydroxy olefins. 2,5-cis-Substituted tetrahydrofuran derivatives could be obtained diastereoselectively from appropriately substituted γ-hydroxy olefins. Additionally, relatively bulky olefin substituents and unprotected hydroxy groups at non-interfering positions (e.g., α and δ) were well tolerated in the reaction. Finally, the synthetic versatility of the Hartung-Mukaiyama cyclization was demonstrated through a stereocontrolled synthesis of the tetrahydrofuran moiety of amphidinolide N, a potent cytotoxic macrolide of marine origin. This study expands the capacity of Mukaiyama-type cyclization in that it can be used in convergent assembly of complex tetrahydrofuran motifs from internal olefins.