ChemRxiv
Not a member yet
27047 research outputs found
Sort by
Reductive amination of carbonyl C–C bonds enables formal nitrogen insertion
No full textGiven its ubiquity in various biological and physical processes1–3, the reductive amination of ketones and aldehydes is one of the oldest and most widely used methods for amine synthesis4. As a cornerstone of synthetic chemistry, it has largely remained unchanged since its discovery over a century ago5. Herein, we report the mechanistically-driven development of a complementary reaction, which reductively aminates the C–C σ-bond attached to carbonyls, not the carbonyl C–O π-bond, generating value-added linear and cyclic 3° amines in a modular fashion. Critical to the success of this endeavor were mechanistic insights that enabled us to modulate the resting state of a borane catalyst, minimize deleterious disproportionation of a hydroxylamine nitrogen source, and control the migratory selectivity of a key nitrenoid reactive intermediate. Experimental evidence support the reaction occurring through a reductive amination/stepwise reductive Stieglitz cascade, via a ketonitrone, which can be interrupted under catalyst-control to generate valuable N,N-disubstituted hydroxylamines. The method reported herein enables various net transformations that would otherwise require lengthy synthetic sequences using pre-existing technologies. This is highlighted by its application to a two-step protocol for the formal insertion of a single nitrogen atom into the core framework of abundant hydrocarbon feedstocks, the site-selective late-stage C–C amination of complex molecules, diversity-oriented synthesis of isomeric amines from a single precursor, and transposition of nitrogen to different positions within a heterocycle
Finding non-fluorinated alternatives to fluorinated gases used as refrigerants
No full textHydrofluorocarbons (HFCs) and so-called hydrofluoroolefins (HFOs) are used as refrigerants in air conditioning, refrigeration, chillers, heat pumps and devices for dehumidification and drying. However, many HFCs, including R-134a and R-125, have a high global warming potential and some of the HFCs and HFOs degrade atmospherically and form persistent degradation products. Thus, there is an urgent need to replace fluorinated refrigerants with non-fluorinated working fluids to avoid direct emissions due to leakage, incorrect loading or removal. It is important, however, also to select refrigerants with high efficiencies to avoid indirect CO2 emissions due to a (too) high energy consumption during the use phase. The present study investigates the available non-fluorinated alternatives to fluorinated refrigerants and shows that a transition to non-fluorinated refrigerants, in general, is possible and has happened in many sectors already. Technically, there are only slight barriers to overcome to replace fluorinated refrigerants in almost all newly developed systems conforming to existing standards. Additionally, we show that alternatives are available even for some use cases for which derogations have been proposed in the PFAS restriction proposal and suggest making these derogations more specific to support bringing the use of non-fluorinated refrigerants into practice
Non-concerted Alkyne Insertion in Au(I) Acetylides: Influence of the Nuclearity
No full textThe reaction between NHC-supported (NHC = N-heterocyclic carbene) gold(I) trimethylsilylacetylide complexes with NHC gold(I) hydroxide species renders different symmetrical homobimetallic Au complexes. These compounds readily undergo migratory insertion of DMAD (dimethyl acetylenedicarboxylate) at 25 °C to give the corresponding bimetallic enyne products. On the contrary, monometallic analogues require much more forcing reaction conditions (excess of DMAD and temperature ≥ 110 °C) to give the same transformation. Experimental and computational studies reveal that the second metal fragment is responsible for the enhanced nucleophilicity of the reactive carbon atom of the acetylide C≡C bond, which initially leads to a more favorable interaction with DMAD in the rate-determining step of an unprecedented, non-concerted mechanism where the lability of the Au−C bonds plays an instrumental role
17O NMR spectroscopy reveals CO2 speciation and dynamics in hydroxide-based carbon capture materials.
No full textCarbon dioxide capture technologies are set to play a vital role in mitigating the current climate crisis. Solid-state 17O NMR spectroscopy can provide key mechanistic insights that are crucial to effective sorbent design and development. In this work, we present the fundamental aspects and complexities for the study of hydroxide-based CO2 capture systems by 17O NMR spectroscopy. We perform static DFT NMR calculations to assign peaks for general hydroxide CO2 capture products, finding that 17O NMR can readily distinguish between bicarbonate, carbonate and water species. However, in application to CO2 binding in two test case hydroxide-functionalised metal-organic frameworks – MFU-4l and KHCO3-CD-MOF, we find that a dynamic treatment is necessary to obtain agreement between computational and experimental spectra. We therefore introduce a workflow that leverages machine-learning force fields to capture dynamic effects across multiple chemical exchange regimes, providing a significant improvement on static DFT predictions. In MFU-4l, we parameterise, in a pre-determined fashion, a two-component dynamic motion of the bicarbonate motif involving a rapid carbonyl seesaw motion and intermediate hydroxyl proton hopping. For KHCO3-CD-MOF, we combined experimental and modelling approaches to propose a new mixed carbonate-bicarbonate binding mechanism and thus, we open new avenues for the study and modelling of hydroxide-based CO2 capture materials by 17O NMR spectroscopy
A Generalized Theory of the Chemical Kinetics from Statistical Mechanics
No full textIn this contribution, I derive the transition state theory through a probabilistic model embedded in statistical mechanics. This leads to a formulation equivalent to the Eyring equation proving that the transfer coefficient must always be one. This derivation proves that the transition between two systems that are themselves in thermal equilibrium, but separated by a bidirectional transition of any kind, can be described by the Eyring equation. The herein presented derivation does not only lead to the prediction of chemical kinetics, but also provides another proof of the second law of thermodynamics demonstrating intuitively how chemical kinetics, energy, and entropy are all fundamentally statistical phenomena
Two-step Tandem Electrochemical Conversion of Oxalic Acid and Nitrate to Glycine
No full textThis study presents a facile tandem strategy for improving the efficiency of glycine electrosynthesis from oxalic acid and nitrate. In this tandem electrocatalytic process, oxalic acid is first reduced to glyoxylic acid, while nitrate is reduced to hydroxylamine. Subsequent coupling of these two precursors results in the formation of a C-N bond, producing the intermediate glyoxylic acid oxime, which is further reduced in situ to glycine. Here we show, using only a simple Pb foil electrode, that maximizing the yield of the first step of the transformation (i.e. the reduction of oxalic acid to glyoxylic acid) prior to the coupling step allows for an unprecedented selectivity and conversion for glycine electrosynthesis to be achieved. Overall, a maximum glycine faradaic efficiency (FE) of 59 % is achieved at -300 mA cm-2 and a high glycine partial current density of -232 mA cm-2 and a glycine production rate of 0.82 mmol h-1 cm-2 are attained at 400 mA cm-2, thereby paving the way for an energy and economically efficient electrochemical synthesis of glycine
Activation energy measurements to determine metabolic bottlenecks of a bioelectrochemical system studied using microfluidics and the Arrhenius equation
No full textTo target development of bioelectrochemical systems, we developed an advanced microfluidic method to identify reaction bottlenecks in the metabolic activity of a pure-culture Geobacter sulfurreducens electroactive biofilm (EAB). The microfluidic system was devised to include perpendicular flow orientation for improved boundary layer uniformity and was combined with an embedded 3-electrode system to accurately apply a constant potential during the entire experimental duration. A 3-sensor temperature control system provided the basis of accurate temperature pulsing, which modified the EAB metabolic activity over short time intervals relative to the bacterial doubling rate. The system, together with the unique ability to control hydrodynamic, electrochemical, and thermal conditions, was used as the basis for an Arrhenius approach to obtain activation energy barrier values at different growth times, acetate concentrations, and flow rates. The results indicated that bottlenecks in the overall metabolic activity after 1 month of growth time were related to electron transfer through extracellular cytochrome c. After the EAB further matured to 4 months old, the bottleneck appeared to switch to enzyme-driven acetate oxidation. Based on this hypothesis, we observed after 4-months, that strong increases in effective enzyme concentration were primarily obtained by increasing flow rate, and secondarily by increasing acetate concentration
Diffusion power spectra as a window into dynamic materials architecture
No full textUnderstanding molecular dynamics in heterogeneous environments is a foundational step in tuning macromolecular reactivity. This is especially important in the chemical recycling of commodity and specialty polymers, which is often undertaken in aqueous media using molecular or enzymatic catalysts. Yet, it remains a challenge study dynamic materials architectures without accurate discernment of the behavior of water in confining media to capture the complexity of the operative transport processes. Here, we develop experimental and analytical methodologies describing the complete set of diffusive eigenmodes that exist within time-varying, non-Euclidean boundary conditions—a situation commonly found in the reactive deconstruction of polymers. Diffusion power spectra, as discerned by an NMR-based method, yield frequency-domain velocity autocorrelation functions that are analyzed in the context of physical models parameterized with fractal mathematics. The results connect time-evolving local motion in polymers to chemical reactivity during acid-catalyzed deconstruction of elastomers. The fundamental understanding provided herein offers practical tools for engineering materials with tailored properties and behaviors, with particular attention to the design of reactive polymers that advance circular materials economies and sustainable chemistry practices
Microplastic particles contain ice nucleation sites that can be inhibited by atmospheric aging
No full textRecent research has shown that microplastics are widespread in the atmosphere. However, we know little about their ability to nucleate ice and their impact on ice formation in clouds. Ice nucleation by microplastics could also limit their long-range transport and global distribution. The present study explores the heterogeneous ice-nucleating ability of seven microplastic samples in the immersion-freezing mode. Two polypropylene samples and one polyethylene terephthalate sample froze heterogeneously with median freezing temperatures of -20.9°C, -23.2°C and -21.9°C, respectively. The number of ice nucleation sites per surface area, n_s (T), ranged from 10^-1 to 10^4 cm^-2 in a temperature interval of -15 to -25°C, which is comparable to that of volcanic ash and fungal spores. After exposure to ozone or a combination of UV light and ozone, simulating atmospheric aging, the ice nucleation activity decreased in some cases and remained unchanged in others. Our freezing data suggest that microplastics may promote ice formation in cloud droplets. In addition, based on a comparison of our freezing results and previous simulations using a global transport model, ice nucleation by microplastics will impact their long-range transport to faraway locations and global distribution
Bioorthogonal chemical engineering of rAAV capsid: Advancing gene therapy targeting using proteins
No full textWe report the chemical conjugation of recombinant Adeno Associated Virus (rAAV) capsid with various functionalities, including proteins, using a bioorthogonal strategy. rAAVs were azido-coated or DBCO-coated by chemically modifying lysine or tyrosine residues. Lysine residues were modified using a phenyl isothiocyanate anchor, and tyrosine residues using either an aryldiazonium salt or a N-methyl luminol derivative. We demonstrate anchor- dependent labelling levels, as observed with biochemical assays and mass spectrometry. Strain-promoted azide-alkyne cycloaddition (SPAAC) was then implemented and evaluated on the rAAV to append functionalities such as fluorescein, biotin and carbohydrates to the azido- coated capsids. We confirmed the efficiency of the bioorthogonal reaction and observed a stronger reactivity with dibenzylcyclooctyne (DBCO) compared to bicyclononyne (BCN). The optimized SPAAC reaction was finally used to label the viral vectors with two relevant nanobodies targeting specific immune cell receptors (CD62L and CD45). In vitro transduction assays conducted with one rAAV-nanobody conjugate demonstrated the promising targeting properties of these chemically modified vectors. Thus, we anticipate that this strategy will positively impact the field of rAAV capsid engineering and contribute in tissue-specific targeting for the optimisation of gene therapy treatments