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The ligand 2,7-bis(6-methyl-2-pyridyl)-1,8-naphthyridine (

The readily prepared and vinylated β-carboline

Metal-metal cooperativity is emerging as an important strategy in catalysis. This requires appropriate ligand scaffolds that can support two metals in close proximity. Here we report nickel-promoted formation of a dinucleating planar macrocyclic ligand that can support bimetallic dinickel(II) and dinickel(I) complexes. Reaction outcomes can be tuned by variation of the substituents and reaction conditions to favour dinucleating macrocyclic, mononucleating macrocyclic or conventional pincer architectures.

Diradical generation followed by radical-radical cross-coupling is a powerful synthetic tool, but its detailed mechanism has yet to be established. Herein, we proposed and confirmed a new model named relayed proton-coupled electron transfer (relayed-PCET) for diradical generation, which could open a door for new radical-radical cross-coupling reactions. Quantum mechanics calculations were performed on a selected carbene-mediated diradical cross-coupling reaction model and a designed model, and the exact electronic structural changes during the radical processes have been observed for the first time.

Computational studies show that the activity of ATRP catalysts can be dramatically improved using quinuclidine bridgehead ligands.

Using quantum-chemical calculations and kinetic modelling, we reveal the dominant reaction pathway in polymer autoxidation, (i) whether oxygen promotes or hamper degradation and (ii) how defects in the polymer backbone affect degradation.

A phosphine-catalyzed approach to pyrrolines has been developed that involves two mechanistically unlinked catalytic processes. The first involves the redox isomerization of amino crotonates to provide access to aliphatic tosyl imines, which then engage in a (3+2) annulation with various allenoates. The reaction shows generality, with 24 examples established, along with a low yielding and moderately enantioselective variant. Mechanistic studies indicate that the viability of the process is linked to the selection of catalysts with similar propensity to add to the two coupling partners.

Luciferin is one of Nature's most widespread luminophores, and enzymes that catalyze luciferin luminescence are the basis of successful commercial "glow" assays for gene expression and metabolic ATP formation. Herein we report an electrochemical method to promote firefly's luciferin luminescence in the absence of its natural biocatalyst-luciferase. We have gained experimental and computational insights on the mechanism of the enzyme-free luciferin electrochemiluminescence, demonstrated its spectral tuning from green to red by means of electrolyte engineering, proven that the colour change does not require, as still debated, a keto/enol isomerization of the light emitter, and gained evidence of the electrostatic-assisted stabilization of the charge-transfer excited state by double layer electric fields. Luciferin's electrochemiluminescence, as well as the in situ generation of fluorescent oxyluciferin, are applied towards an optical measurement of diffusion coefficients.

Benzamidinium compounds have found widespread use in both medicinal and supramolecular chemistry. In this work, we show that benzamidiniums hydrolyse at room temperature in aqueous base to give the corresponding primary amide. This reaction has a half-life of 300 days for unsubstituted benzamidinium at pH 9, but is relatively rapid at higher pHs (e.g. t1/2 = 6 days at pH 11 and 15 hours at pH 13). Quantum chemistry combined with first principles kinetic modelling can reproduce these trends and explain them in terms of the dominant pathway being initiated by attack of HO– on benzamidine. Incorporation of the amidinium motif into a hydrogen bonded framework offers a substantial protective effect against hydrolysis.

The full catalytic cycle of the proline-catalyzed intermolecular aldol reaction of acetone and p-nitrobenzaldehyde in acetone solvent has been investigated by quantum chemistry at the G3(MP2,CC)//M062X/6–31+G(d)/SMD level of theory, and the results used to develop an ab initio kinetic model. Proline catalyzes the aldol reaction according to the enamine mechanism. The initial reaction between proline and acetone was reinvestigated, and a revised mechanism for enamine formation is proposed in which a second proline assists the process contributing to the enamine formation. Using various initial concentrations of proline while keeping the experimental ratio of water, aldehyde and acetone constant, we find that the enamine formation from the first-order to proline pathway dominates when the concentration of proline is low ( 1.0 M) excess water is expected to decrease %ee. Side reactivity, including aldol condensation, acetone self-aldolization, oxazolidinone formation and azomethine and 1-oxapyrrolizidine formation were all considered in our kinetic model but shown to have a negligible effect (

Radical polymerization with reversible addition‐fragmentation chain transfer (RAFT polymerization) has been successfully applied to generate polymers of well‐defined architecture. For RAFT polymerization a source of radicals is required. Recent work has demonstrated that for minimal side‐reactions and high spatio‐temporal control these should be formed directly from the RAFT agent or macroRAFT agent (usually carbonothiosulfanyl compounds) thermally, photochemically or by electrochemical reduction. In this work, we investigated low‐energy electron attachment to a common RAFT agent (cyanomethyl benzodithioate), and, for comparison, a simple carbonothioylsulfanyl compound (dimethyl trithiocarbonate, DMTTC) in the gas phase by means of mass spectrometry as well as quantum chemical calculations. We observe for both compounds that specific cleavage of the C−S bond is induced upon low‐energy electron attachment at electron energies close to zero eV. This applies even in the case of a poor homolytic leaving group (.CH3 in DMTTC). All other dissociation reactions found at higher electron energies are much less abundant. The present results show a high control of the chemical reactions induced by electron attachment., Electron attachment leads to the efficient dissociation of carbonothioylsulfanyl molecules which are relevant for Reversible Addition‐Fragmentation chain Transfer (RAFT) polymerization. We show that the preferred C−S bond cleavage can be initiated by an electron with almost no kinetic energy, in contrast to other dissociation reactions. This feature leads to a pronounced control of bond dissociation upon electron attachment.

A simple nonphotochemical procedure is reported for Cu(I)-catalyzed C-N coupling of aliphatic halides with amines and amides. The process is loosely based on the Goldberg reaction but takes place readily at room temperature. It uses Cu(I)Br, a commonly used and inexpensive atom transfer radical polymerization precatalyst, along with the cheap ligand N,N,N',N″,N″-pentamethyldiethylenetriamine, to activate the R-X bond of the substrate via inner-sphere electron transfer. The procedure brings about productive C-N bond formation between a range of alkyl halide substrates with heterocyclic aromatic amines and amides. The mechanism of the coupling step, which was elucidated through application of computational methods, proceeds via a unique Cu(I) → Cu(II) → Cu(III) → Cu(I) catalytic cycle, involving (a) inner-sphere electron transfer from Cu(I) to the alkyl halide to generate the alkyl radical; (b) successive coordination of the N-nucleophile and the radical to Cu(II); and finally reductive elimination. In the absence of a nucleophile, debrominative homocoupling of the alkyl halide occurs. Control experiments rule out SN-type mechanisms for C-N bond formation.

Density functional theory (DFT) has been employed in predicting the enantioselectivity of the aldol reaction between acetone and p-nitrobenzaldehyde catalyzed by proline and its derivatives Me2bdc-Pro (bdc = 1,4-benzenedicarboxylate) and Me2bpdc-Pro (bpdc = 4,4'-biphenyldicarboxylate). For each catalyst, our computationally predicted values at the M062X/6-31+G(d) level of theory with the SMD solvent model are in excellent agreement with experimental results reported in the literature. Electron-donating and electron-withdrawing groups (viz., SO3-, NMe2, SO3H, and NMe3+) were installed at the C4 position of the proline-based catalysts to study the impact of electrostatic effects on stereoselectivity. The electron-donating groups decrease and even invert the enantioselectivity, while the electron-withdrawing ones increase it. Enantiomeric excesses in the range of 49-71 and 59-68% are predicted for Me2bdc-Pro and Me2bpdc-Pro catalysts with the electron-withdrawing SO3H and NMe3+ installed respectively, values much higher than those of the corresponding unmodified catalysts. More interestingly, enantiomeric excesses decrease and, in the case of SO3-, are even inverted in favor of the other enantiomer when the electron-donating groups are installed. These results highlight the importance of electrostatic effects, and polar effects more generally, in optimal organocatalyst design for stereoselective C-C bond-forming reactions.

Quantum chemistry is used to investigate the nature of protonated N-heterocyclic carbene (NHC·H+) catalysed decarboxylation recently reported by Zhang et al. (ACS Catal., 2021, 11, 3443–3454). Our results show that there are strong electrostatic effects within the NHC·H+ catalysed decarboxylation, and these dominate hydrogen bonding. At the same time, energy decomposition analyses and comparison between the original NHC·H+ catalyst and a truncated form reveal that stabilizing dispersion interactions are also critical, as is induction. We also show that the electrostatic effects and their associated catalytic effects can be further enhanced using charged functional groups.

An expanded pincer ligand tBu-PONNOP (2,7-bis(di-tert-butylphosphinito)-1,8-naphthyridine) has been synthesised and its coordination to coinage metals has been studied. Bimetallic complexes were produced with metal halide salts of the type [M2X2(tBu-PONNOP)] (X = Cl, M = Au, Ag, Cu; X = I, M = Cu) with a varying degree of interaction with the naphthyridyl backbone in the order Au < Ag < Cu. The salts [Ag2(tBu-PONNOP)2][BArF4]2 (ArF = 3,5-C6H3(CF3)2) and [Ag2(NCMe)2(tBu-PONNOP)]X2 (X = BArF4, PF6) were prepared, which may serve as a source of tBu-PONNOP via transmetallation.

The ability to reverse controlled radical polymer ization and regenerate the monomer would be highly beneficial for both fundamental research and applications, yet this has remained very challenging to achieve. Herein, we report a near-quantitative (up to 92%) and catalyst-free depolymerization of various linear, bulky, cross-linked, and functional polymethacrylates made by reversible addition−fragmentation chain-transfer (RAFT) poly merization. Key to our approach is to exploit the high end-group fidelity of RAFT polymers to generate chain-end radicals at 120 °C. These radicals trigger a rapid unzipping of both conventional (e.g., poly(methyl methacrylate)) and bulky (e.g., poly(oligo- (ethylene glycol) methyl ether methacrylate)) polymers. Impor tantly, the depolymerization product can be utilized to either reconstruct the linear polymer or create an entirely new insoluble gel that can also be subjected to depolymerization. This work expands the potential of polymers made by controlled radical polymerization, pushes the boundaries of depolymerization, offers intriguing mechanistic aspects, and enables new applications., Journal of the American Chemical Society, 144 (10), ISSN:0002-7863, ISSN:1520-5126

cis-1,2-Dihydrocatechols 5 (X = Me and Cl), which are available in the homochiral form through the whole-cell biotransformation of toluene and chlorobenzene, respectively, undergo Diels–Alder cyclo...

Computational chemistry at the G3(MP2)-RAD//M06-2X/6-31+G(d,p)//SMD level of theory was used to study the oxidation of a test set of methyl adducts of nitroxide radicals and methyl adducts of Blatter's radical, a Kuhn verdazyl and two oxo-verdazyls. The barriers and the reaction energies of the S

Photoactivation of tetrazoles to form nitrile imines primed for 1,3-dipolar cycloaddition reactions is of widespread utility in chemistry. In contrast, the corresponding thermal reactions usually possess prohibitively high barriers and have garnered significantly less attention. Here, computational chemistry at the M06-2X/6-31+G(d,p) level of theory with SMD solvent corrections is used to show that these thermal activation barriers can be significantly reduced through the use of nonconjugated charged functional groups (CFGs). For 2,5-dimethyl-tetrazole, a positive CFG on the

International audience; Time-dependent density functional theory calculations have been performed to study the photocleavage reactions of chromophore-functionalized alkoxyamines in nitroxide-mediated photo-polymerization. Two case studies were considered: azaphenalene derivatives and benzophenone-based alkoxyamines. For the azaphenalenes, we show that the expected homolysis pathway is actually inaccessible. Instead, these alkoxyamines exhibit low-lying excited states that exhibit an electronic structure about the nitroxide moiety similar to that of the formally oxidized radical cation. As a result, the cleavage of these alkoxyamines can be described as mesolytic-like, rather than homolytic. As with formally oxidized species, mesolytic cleavage can result in the production of either carbon-centered radicals or carbocations, with only the former resulting in radical polymerization. Here, the cleavage products are found to be dependent on the respective radical/cation stabilities of the monomer units of choice (styrene, ethyl propanoate, and ethyl isobutyrate). In contrast to the azaphenalenes, in the benzophenone-based alkoxyamines, conjugation between the nitroxide and chromophore moieties appears to facilitate homolysis, due to the ideal alignment of singlet and triplet states of different symmetries.

Theoretical calculations have been performed in order to investigate the impact of different substitution patterns on predicted photoreactivity of alkoxyamines fused to an anthraquinone chromophore. Amino and hydroxy groups (similar to those which have been previously synthesized) are introduced and their effect on excited state energies and charge transfer is assessed. Analogous to formally oxidized alkoxyamines, the charge-separated nNπ* state can undergo mesolytic cleavage or bimolecular or SN2 reactions with nucleophiles, according to the substitution patterns and other reagents present. While homolytic cleavage is in principle promoted by triplet ππ* states, the accessible ππ* triplet states in this system are centered on the chromophore and unreactive. We show that the reactive nNπ* state, which bears a negative charge, is stabilized by hydroxy substitution while amino substitution will destabilize it. After mesolysis to a carbon centred radical, the nitroxide radical re-forms; however, when carbocations are produced the remaining open-shell singlet is stable and unable to undergo coupling with the carbocation.

Controlled degradation of polypropylene (PP) is used industrially to improve the properties of crude PP. While this degradation is traditionally initiated by organic peroxides, N-acyloxyamines are ...

Oriented electrostatic fields can exert catalytic effects upon both the kinetics and the thermodynamics of chemical reactions; however, the vast majority of studies thus far have focused upon groun...

Atom transfer radical polymerization (ATRP) is the most commonly utilized technique in controlled radical polymerization. However, the identification of more active catalysts could further increase its scope, both for polymerization and small-molecule synthesis more generally. To this end, a series of novel ligands were designed on the basis of two strategies: replacing nitrogen-based ligands with their phosphorus equivalents and rigidifying the ligand cap of nitrogen-based ligands so as to enforce short Cu-cap distances. Each ligand was assessed using accurate computational chemistry, which was used to compute the thermodynamics and, in selected cases, kinetics of an ATRP reaction with a model methyl methacrylate propagating radical. In principle, the use of phosphorus ligand caps was found to be a powerful strategy for increasing catalyst activity. Unfortunately, in practice, speciation issues sacrificed much of their advantage. In contrast, cap rigidification increases the activity of nitrogen-based ligands, well beyond existing ATRP ligands such as TPMANMe2. The effectiveness of these ligands was further demonstrated for hard-to-activate initiating systems based on ethylene, vinyl chloride, and vinyl acetate polymerization. Several of these improved ligands are synthetically accessible, with rigid piperidine or quinuclidine analogues of TPMANMe2 possessing improved thermodynamic and kinetic activity by 2 to 3 orders of magnitude.

Alkoxyamines, which are connected with a phenol moiety by a (substituted) methylene bridge undergo homolytic cleavage upon chemical oxidation or a photo-induced hydrogen transfer. This selectively triggered reaction yields a nitroxide radical. In the presence of an excess of lead dioxide as the oxidant in tert-butylbenzene as solvent, spontaneous, instantaneous and almost quantitative generations of nitroxides from various alkoxyamines are observed at room temperature, which support activation energies for the cleavage lower than 100 kJ mol−1. The rate and the amount of released nitroxide depend on the amount of “catalyst” and the structure of alkoxyamines.

Herein we demonstrate that ionic liquids can form long-lived double layers, generating electric fields detectable by straightforward open circuit potential (OCP) measurements. In imidazolium-based ionic liquids an external negative voltage pulse leads to an exceedingly stable near-surface dipolar layer, whose field manifests as long-lived (∼1-100 h) discrete plateaus in OCP versus time traces. These plateaus occur within an ionic liquid-specific and sharp potential window, defining a simple experimental method to probe the onset of interfacial ordering phenomena, such as overscreening and crowding. Molecular dynamics modeling reveals that the OCP arises from the alignment of the individual ion dipoles to the external electric field pulse, with the magnitude of the resulting OCP correlating with the product of the projected dipole moment of the cation and the ratio between the cation diffusion coefficient and its volume. Our findings also reveal that a stable overscreened structure is more likely to form if the interface is first forced through crowding, possibly accounting for the scattered literature data on relaxation kinetics of near-surface structures in ionic liquids.

A simple nonphotochemical procedure is reported for Cu(I)-catalyzed C-N coupling of aliphatic halides with amines and amides. The process is loosely based on the Goldberg reaction but takes place readily at room temperature. It uses Cu(I)Br, a commonly used and inexpensive atom transfer radical polymerization precatalyst, along with the cheap ligand

Pijning et al report the novel observation that the platelet aIIbβ3 integrin receptor occurs in 2 different covalent forms. Onethird of the molecules lack 1 disulfide bond, thereby changing the distribution of the receptor in the platelet membrane, its signaling properties, and its rate of uptake into clathrin-coated pits., Key Points The platelet αIIbβ3 integrin receptor is produced in various covalent forms that have different functions.One-third of αIIbβ3 molecules are missing an αIIb disulfide bond that changes distribution and function of the receptor., Visual Abstract, The αIIbβ3 integrin receptor coordinates platelet adhesion, activation, and mechanosensing in thrombosis and hemostasis. Using differential cysteine alkylation and mass spectrometry, we have identified a disulfide bond in the αIIb subunit linking cysteines 490 and 545 that is missing in ∼1 in 3 integrin molecules on the resting and activated human platelet surface. This alternate covalent form of αIIbβ3 is predetermined as it is also produced by human megakaryoblasts and baby hamster kidney fibroblasts transfected with recombinant integrin. From coimmunoprecipitation experiments, the alternate form selectively partitions into focal adhesions on the activated platelet surface. Its function was evaluated in baby hamster kidney fibroblast cells expressing a mutant integrin with an ablated C490-C545 disulfide bond. The disulfide mutant integrin has functional outside-in signaling but extended residency time in focal adhesions due to a reduced rate of clathrin-mediated integrin internalization and recycling, which is associated with enhanced affinity of the αIIb subunit for clathrin adaptor protein 2. Molecular dynamics simulations indicate that the alternate covalent form of αIIb requires higher forces to transition from bent to open conformational states that is in accordance with reduced affinity for fibrinogen and activation by manganese ions. These findings indicate that the αIIbβ3 integrin receptor is produced in various covalent forms that have different cell surface distribution and function. The C490, C545 cysteine pair is conserved across all 18 integrin α subunits, and the disulfide bond in the αV and α2 subunits in cultured cells is similarly missing, suggesting that the alternate integrin form and function are also conserved.

Bench- and air-stable 1-methoxy-2,2,6,6-tetramethylpiperidine (TEMPO-Me) is relatively unreactive at ambient temperature in the absence of an electrochemical stimulus. In this report, we demonstrate that the one-electron electrochemical oxidation of TEMPO-Me produces a powerful electrophilic methylating agent in situ. Our computational and experimental studies are consistent with methylation proceeding via a SN2 mechanism, with a strength comparable to the trimethyloxonium cation. A protocol is developed for the electrochemical methylation of aromatic acids using TEMPO-Me.

Here we report molecular films terminated with diazonium salts moieties at both ends which enables single-molecule contacts between gold and silicon electrodes at open circuit via a radical reaction. We show that the kinetics of film grafting is crystal-facet dependent, being more favorable on ⟨111⟩ than on ⟨100⟩, a finding that adds control over surface chemistry during the device fabrication. The impact of this spontaneous chemistry in single-molecule electronics is demonstrated using STM-break junction approaches by forming metal-single-molecule-semiconductor junctions between silicon and gold source and drain, electrodes. Au-C and Si-C molecule-electrode contacts result in single-molecule wires that are mechanically stable, with an average lifetime at room temperature of 1.1 s, which is 30-400% higher than that reported for conventional molecular junctions formed between gold electrodes using thiol and amine contact groups. The high stability enabled measuring current-voltage properties during the lifetime of the molecular junction. We show that current rectification, which is intrinsic to metal-semiconductor junctions, can be controlled when a single-molecule bridges the gap in the junction. The system changes from being a current rectifier in the absence of a molecular bridge to an ohmic contact when a single molecule is covalently bonded to both silicon and gold electrodes. This study paves the way for the merging of the fields of single-molecule and silicon electronics.

Many approaches have been used to improve the accuracy of implicit solvent models including solute cavity scaling, introducing explicit solvent molecules, and changing the level of theory for the s...