Monthly Archives: March 2024

Intramolecular CH‐Hydrogen Bonding During the Dissociation of the Oxaphosphetane Intermediate Facilitates Z/E‐Selectivity in Wittig Olefination

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Intramolecular CH-Hydrogen Bonding During the Dissociation of the Oxaphosphetane Intermediate Facilitates Z/E-Selectivity in Wittig Olefination

The Z-selective synthesis of novel nitrostilbenes has been reported. An intrinsic role of intramolecular hydrogen bonding during the dissociation of OPA-intermediate is critical for the stereoselective Wittig olefination. DFT calculations and X-ray measured intramolecular CH hydrogen bonding distances strongly support the observed phenomenon. Further, the current methodology has been utilized to synthesize medicinally potential salicylate-methyl-ester-based 2-arylindole derivatives.


Abstract

Herein, DFT studies corroborating experimental results revealed that the shortest intramolecular hydrogen bonding distance of cis/trans-oxaphosphetane (OPA) oxygen with the CH-hydrogen of a triphenylphosphine phenyl ring provides good evidence for the attained olefin Z/E-selectivity in Wittig olefination of the studied examples. 2-Nitrobenzaldehyde, 3-nitrobenzaldehyde, 2-nitro-3-bromobenzaldehyde, 2-nitro-5-bromobenzaldehyde and 2-nitro-5-arylbenzaldehydes provided Z-nitrostilbenes with (2-chloro-4-hydroxy-3-methoxy-5-(methoxycarbonyl)benzyl) triphenylphosphonium chloride as the major products. However, 4-nitrobenzaldehyde and 2-nitro-6-bromobenzaldehydes furnished E-nitrostilbenes as the major products in high yields. Furthermore, the DFT computed intramolecular CH1/CH2-hydrogen bond distances with Cl/NO2 of selected stilbene derivatives were in good agreement with intramolecular hydrogen bond distances measured from single-crystal X-ray diffraction measurements.

Bacterial hydrazine biosynthetic pathways featuring cupin/methionyl tRNA synthetase‐like enzymes

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Nitrogen-Nitrogen (N–N) bond-containing functional groups in natural products and synthetic drugs play significant roles in exerting biological activities. The mechanisms of N–N bond formation in natural organic molecules have garnered increasing attention over the decades. Recent advances have illuminated various enzymatic and nonenzymatic strategies, and our understanding of natural N–N bond construction is rapidly expanding. A group of didomain proteins with zinc-binding cupin/methionyl-tRNA synthetase (MetRS)-like domains, also known as hydrazine synthetases, generates amino acid-based hydrazines, which serve as key biosynthetic precursors of diverse N–N bond-containing functionalities such as hydrazone, diazo, triazene, pyrazole, and pyridazinone groups. In this review, we summarize the current knowledge on hydrazine synthetase mechanisms and the various pathways employing this unique bond-forming machinery.

Carbazole Based Smaragdyrins: Synthesis, Aromaticity Switching, and Formation of a Spiro‐Dimer

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Carbazole-incorporated smaragdyrin BF2-complex 3 was synthesized by SNAr reaction of 3,5-dibromo-8-mesityl-BODIPY 1 with 3,6-di(tert-butyl)-1,8-di(pyrrol-2-yl)carbazole 2 as a nucleophile. Demetalation of 3 with ZrCl4 gave the corresponding smaragdyrin free base 4 in a good yield. Oxidations of 3 and 4 with MnO2 gave smaragdyrins 5 and 6, respectively, both followed by aromaticity switching, since the oxidized products showed a moderate paratropic ring current owing to their 20π-electronic circuits. Further, treatment of 4 with [RhCl(CO)2]2 in the presence of NaOAc gave RhI complex 7, and oxidation of 3 with RuCl3 in the presence of triethylamine led to the formation of a spiro dimer product, 8.

Design, Synthesis, and Characterization of Novel Styryl Dyes as Fluorescent Probes for Tau Aggregate Detection in Vitro and in Cells

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Design, Synthesis, and Characterization of Novel Styryl Dyes as Fluorescent Probes for Tau Aggregate Detection in Vitro and in Cells

Novel styryl dye derivatives incorporating indolium and quinolinium cores are synthesized to examine their binding capability with tau aggregates. These dyes possess enhanced fluorescence in viscous environments. Among them, Dye 4, containing a quinolinium moiety, shows the most outstanding increase in fluorescence upon binding to tau aggregates in vitro and in cells. These probes exhibit potential for Alzheimer's disease detection.


Abstract

A series of novel styryl dye derivatives incorporating indolium and quinolinium core structures were successfully synthesized to explore their interacting and binding capabilities with tau aggregates in vitro and in cells. The synthesized dyes exhibited enhanced fluorescence emission in viscous environments due to the rotatable bond confinement in the core structure. Dye 4, containing a quinolinium moeity and featuring two cationic sites, demonstrated a 28-fold increase in fluorescence emission upon binding to tau aggregates. This dye could also stain tau aggregates in living cells, confirmed by cell imaging using confocal fluorescence microscopy. A molecular docking study was conducted to provide additional visualization and support for binding interactions. This work offers novel and non-cytotoxic fluorescent probes with desirable photophysical properties, which could potentially be used for studying tau aggregates in living cells, prompting further development of new fluorescent probes for early Alzheimer's disease detection.

Sodium‐Mediated Reductive C−C Bond Cleavage Assisted by Boryl Groups

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Sodium-Mediated Reductive C−C Bond Cleavage Assisted by Boryl Groups

Boryl-group-assisted reductive C−C bond cleavage of 1,2-diaryl-1,2-diborylethanes is described. The substrates, 1,2-diaryl-1,2-diborylethanes, are synthesized by reductive diboration of stilbenes. The combination of these reactions, reductive diboration and reductive cleavage provides a new strategy for reductive C=C double bond cleavage.


Abstract

In contrast to the well-established oxidative C=C double bond cleavage to give the corresponding carbonyl compounds, little is known about reductive C=C double bond cleavage. Here we report that C−C single bond cleavage in 1,2-diaryl-1,2-diborylethanes proceeds by reduction with sodium metal to yield α-boryl benzylsodium species. In combination with our previous reductive diboration of stilbenes, the overall transformation represents reductive cleavage of the C=C double bonds of stilbene to yield α-boryl-α-sodiated toluenes. This reductive two-step C=C double bond cleavage is applicable to ring-opening or ring-expansion reactions of polycyclic aromatic hydrocarbons.

Colloidal Plasmonic Metasurfaces for the Enhancement of Non‐Linear Optical Processes and Molecular Spectroscopies

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Colloidal Plasmonic Metasurfaces for the Enhancement of Non-Linear Optical Processes and Molecular Spectroscopies

Recent developments in colloidal-based plasmonic metasurfaces unlocked new opportunities for the application of these highly dynamic systems to non-linear optical phenomena and molecular spectroscopies. This Minireview summarizes potential advantages and current challenges of colloidal systems for the enhancement of second and third order light-matter interactions and offers a critical perspective on their potential impact on the next generation of photonics metamaterials.


Abstract

Colloidal metasurfaces are emerging as promising candidates for the development of functional chemical metamaterials, combining the undisputed control over crystallography and surface chemistry achieved by synthetic nanochemistry with the scalability and versatility of colloidal self-assembly strategies. In light of recent reports of colloidal plasmonic materials displaying high-performing optical cavities, this Minireview discusses the use of this type of metamaterials in the specific context of non-linear optical phenomena and non-linear molecular spectroscopies. Our attention is focused on the opportunities and advantages that colloidal nanoparticles and self-assembled plasmonic metasurfaces can bring to the table compared to more traditional nanofabrication strategies. Specifically, we believe that future work in this direction will express the full potential of non-linear molecular spectroscopies to explore the chemical space, with a deeper understanding of plasmon-molecule dynamics, plasmon-mediated processes, and surface-enhanced chemistry.

Hexagonal Boron Nitride Spacers for Fluorescence Imaging of Biomolecules

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Hexagonal Boron Nitride Spacers for Fluorescence Imaging of Biomolecules

We employ few-layer hexagonal boron nitride (hBN) as a precisely tailorable fluorescence spacer between rhodamine-labelled phosphatidylethanolamine lipid (Rh-PE) membranes and graphene substrates on SiO2/Si substrates. The pre-determined hBN thicknesses can be employed to control the non-radiative energy transfer properties of graphene, with fluorescence quenching following a d −4 distance-dependent behaviour.


Abstract

Fluorescence imaging is an invaluable tool to investigate biomolecular dynamics, mechanics, and interactions in aqueous environments. Two-dimensional materials offer large-area, atomically smooth surfaces for wide-field biomolecule imaging. Despite the success of graphene for on-chip biosensing and biomolecule manipulation, its strong fluorescence-quenching properties pose a challenge for biomolecular investigations that are based on direct optical readouts. Here, we employ few-layer hexagonal boron nitride (hBN) as a precisely tailorable fluorescence spacer between labelled lipid membranes and graphene substrates. By stacking high-quality hBN crystals in the 10–20 nm thickness range on monolayer graphene, we observe distance-dependent fluorescence intensity variations. Remarkably, with hBN spacers as thin as 20 nm, the fluorescence intensity is comparable to bare SiO2/Si substrates, while the intensity was reduced to 60 % and 80 % with ~10 nm and ~16 nm hBN thicknesses respectively. We confirm that pre-determined hBN thicknesses can be employed to control the non-radiative energy transfer properties of graphene, with fluorescence quenching following a d −4 distance-dependent behaviour. This seamless integration of electronically active and dielectric van der Waals materials into vertical heterostructures enables multifunctional platforms addressing the manipulation, localization, and visualization of biomolecules for fundamental biophysics and biosensing applications.

Recent Understanding of Water‐Assisted CO2 Hydrogenation to Alcohols

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Recent Understanding of Water-Assisted CO2 Hydrogenation to Alcohols

Continuously removing water is desired to alleviate thermodynamic barriers during CO2 hydrogenation. However, at low concentrations, water can enhance alcohol productivity. Here, we discuss the main findings that led to an atomic-level understanding of water promotional effects in CO2 hydrogenation to alcohols.


Abstract

Alcohol production from CO2 hydrogenation is a cutting-edge process in sustainable chemistry that holds vast promise for addressing climate change by recycling and repurposing emissions. Many strategies have been proposed to improve the process efficiency. In-situ generated, and trace amounts of water added to the feed stream have recently proved to be determinant to promote key reaction steps, increasing alcohol selectivity and yield. Here, we discuss the main findings that led to an atomic-level understanding of water promotional effects in CO2 hydrogenation to alcohols. H2O and the products resultant from its dissociation (OH and O) can act in different ways, stabilizing intermediates and active sites or participating in the hydrogen transfer mechanisms during the reaction. Gaining insights into the mechanisms underlying water promotion offers a cost-effective strategy for enhancing alcohol production efficiency.

Pushing the Efficiency of the Selective and Base‐free Air‐Oxidation of HMF by Varying the Properties of Carbon‐based Supports

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Pushing the Efficiency of the Selective and Base-free Air-Oxidation of HMF by Varying the Properties of Carbon-based Supports

Support for base-free: Pt supported on various activated carbon and carbon black materials was prepared and tested for the base-free, selective air-oxidation of 5-(Hydroxymethyl)furfural to 2,5-Furandicarboxylic acid. Depending on the chemical properties (graphitization degree, O-containing functional groups) of the supports as determined by comprehensive characterization, highly active and selective catalysts were found due to the active role of the support.


Abstract

The selective oxidation of 5-(Hydroxymethyl)furfural (HMF) to 2,5-Furandicarboxylic acid (FDCA) is highly attractive for the production of renewable monomers as substitute for fossil-based monomers. To achieve a sustainable synthesis, we report on advances for a base-free approach, reducing waste from the process, using air as oxidant and heterogeneous catalysts. Various Carbon-based supports, which can be bio-sourced and cost-efficient, for Pt particles were investigated as they allow for an easy reuse and at the end-of-life Pt can be recycled to enable a closed cycle. Commercially available supports with varying properties, which might replace the base, were studied with Pt particles of similar size and loading. Significant differences in the catalytic activity were observed, which were correlated with the O-functionalities and graphitization degree of the supports derived from Raman spectroscopy, temperature-programmed desorption, and X-ray photoelectron spectroscopy. An activated carbon (Norit ROX) rich in quinone/pyrone-type groups and a carbon black-based catalyst with graphene-layers pushed the efficiency with enhanced FDCA-yields enabling the complete substitution of the homogeneous base. This allows to circumvent the base in this process which together with high selectivity, air as oxidant, a reusable catalyst, and the use of bio-based feedstock contributes to the sustainability of the production of renewable monomers.