Low Electrophilicity Research Articles

Overlooked electrolyte destabilization by manganese (II) in lithium-ion batteries

Transition-metal dissolution from cathode materials, manganese in particular, has been held responsible for severe capacity fading in lithium-ion batteries, with the deposition of the transition-metal cations on anode surface, in elemental form or as chelated-complexes, as the main contributor for such degradations. In this work we demonstrate with diverse experiments and calculations that, besides interfacial manganese species on anode, manganese(II) in bulk electrolyte also significantly destabilizes electrolyte components with its unique solvation-sheath structure, where the decompositions of carbonate molecules and hexafluorophosphate anion are catalyzed via their interactions with manganese(II). The manganese(II)-species eventually deposited on anode surface resists reduction to its elemental form because of its lower electrophilicity than carbonate molecule or anion, whose destabilization leads to sustained consumption. The reveal understanding of the once-overlooked role of manganese-dissolution in electrolytes provides fresh insight into the failure mechanism of manganese-based cathode chemistries, which serves as better guideline to electrolyte design for future batteries.

Base Metal Catalysts for Deoxygenative Reduction of Amides to Amines

The development of efficient methodologies for production of amines attracts significant attention from synthetic chemists, because amines serve as essential building blocks in the synthesis of many pharmaceuticals, natural products, and agrochemicals. In this regard, deoxygenative reduction of amides to amines by means of transition-metal-catalyzed hydrogenation, hydrosilylation, and hydroboration reactions represents an attractive alternative to conventional wasteful techniques based on stoichiometric reductions of the corresponding amides and imines, and reductive amination of aldehydes with metal hydride reagents. The relatively low electrophilicity of the amide carbonyl group makes this transformation more challenging compared to reduction of other carbonyl compounds, and the majority of the reported catalytic systems employ precious metals such as platinum, rhodium, iridium, and ruthenium. Despite the application of more abundant and environmentally benign base metal (Mn, Fe, Co, and Ni) complexes for deoxygenative reduction of amides have been developed to a lesser extent, such catalytic systems are of great importance. This review is focused on the current achievements in the base-metal-catalyzed deoxygenative hydrogenation, hydrosilylation, and hydroboration of amides to amines. Special attention is paid to the design of base metal catalysts and the mechanisms of such catalytic transformations.

Identification of Structural, Spectroscopic, and Electronic Analysis of Synthesized 4-(5-Phenyl-1,3,4-Oxadiazol-2-Ylthio)-3-Methylbenzene-1,2-Diol: A Theoretical Approach

Structural, spectroscopic and electronic analysis of 4-(5-phenyl-1,3,4-oxadiazol-2-ylthio)-3-methylbenzene-1,2-diol (POTMBD) were identified by comparison of experimental spectroscopic methods and theoretical levels. Computational studies of POTMBD were carried out using density functional theory DFT//B3LYP/M06 levels of theory with 6–31++G(d,p) basis set combination to optimized geometrical parameters. A pleasant correlation was found between experimental SC-XRD structure and DFT optimized geometries. Electronic properties including natural bond orbital (NBO) analysis, frontier molecular orbitals (FMOs) analysis, nonlinear optical (NLO) parameters, and molecular electrostatic potential (MEP) were calculated using same levels of theory. Stability of POTMBD arising from hyperconjugative interactions, charge delocalizations from donor to acceptor unit that were analyzed by using NBO analysis. POTMBD with small band gap, high electronic chemical potential (µ) (∼0.2 eV) and high chemical softness (σ) (∼12.5 eV) displayed high reactivity. Also, low electrophilicity (ω) (0.001) and high nuleophilicity especially on N8 and N9 introduced POTMBD as a good candidate in drug delivery. Hence, molecular docking study was performed with binding pocket of peroxisome proliferator activated receptor-gamma (PPAR gamma receptor, PDB code: 1FM9) to clarify the binding energy of POTMBD target to introduce of favored anti-diabetic agents. The ultraviolet–visible absorption spectrum of POTMBD was observed in the range of 200–400 nm in time dependent DFT method was used to obtain more electronic properties. Moreover, the spectral features of POTMBD including FT-IR, Raman, 13C NMR, and 1H NMR were calculated and compared with experimental data in this research.

Solvent Effects on Intra-/Intermolecular Charge Transfer in Indoloquinoxaline-Based Dyes.

In this work, kinetics and dynamics of the functionality of indoloquinoxaline-based dye-sensitized solar cells (DSSCs), QX22-QX25, were investigated in gas and solvent media. Quantum chemistry properties of the dyes at the excited states show that each moiety of the (D)2-A-π-A system has a specific effect on the photovoltaic properties. Solvent effect analysis shows that among ethanol, toluene, tetrahydrofuran, and methylene dichloride, toluene is the preferred medium for intra-/intermolecular charge transfer, dynamically and kinetically. Moreover, the behavior of the light harvesting efficiency (LHE) and incident photon-to-current efficiency (IPCE) are not similar, due to a strong effect of the Gibbs energy of electron injection on the energy conversion efficiency. Finally, the dye composed of -COOH as the anchoring group and thiophene as the π-spacer is the best candidate to be applied in DSSC due to its better efficiency originated from a lower electrophilicity and electronic chemical potential.

Structural properties and vibrational analysis of Potassium 5-Br-2-isonicotinoyltrifluoroborate salt. Effect of Br on the isonicotinoyl ring

In this work, the potassium 5-Br-2-isonicotinoyltrifluoroborate salt (B-ITFB) have been experimentally characterized by using FT-IR, FT-Raman and Ultravioleta-visible spectra while Ab-initio calculations in gas and in aqueous solution phases were employed to predict their structural and vibrational properties. Here, the results were compared with those reported for the potassium 2-isonicotinoyltrifluorborate salt (ITFB) in order to know the impact of Br on the 2-isonicotinoyl ring and its effects on the properties of B-ITFB. Potential energy surface using B3LYP/6-311++G** calculations has revealed two conformers for B-ITFB with C1 symmetries where only one of them presents the minimum energy. The incorporation of Br in the 5 position of isonicotinoyl ring generates: (i) a change in the symmetry from CS in ITFB to C1 in B-ITFB, (ii) a decreasing in the solvation energy due to higher size of Br, (iii) notable reduction in the Mulliken charges on all the C atoms of ring and, especially, on the C11 atom of trifluoroborate group in both media, (v) smaller bond order values, (vi) an important inductive acceptor effect, (vii) a decreasing in the electron density distribution, specifically in solution and (viii) a smaller reactivity and low electrophilicity and nucleophilicity indexes, as compared with ITFB. On the other hand, for B-ITFB the force fields in both media, the complete assignments of 45 normal vibration modes and the force constants are reported for first time.

Recent Advances in the Metal-Catalyzed Activation of Amide Bonds.

The amide functional group is commonly found in peptides, proteins, pharmaceutical compounds, natural products, and polymers. The synthesis of amides is typically performed by using classical approaches that involve the reaction between a carboxylic acid and an amine in the presence of an activator. Amides are thought to be an inert functional group, because they are unsusceptible to nucleophile attack, owing to their low electrophilicity. The reason for this resistance is clear: the resonance stability of the amide bond. However, transition metal catalysis can circumvent this stability by selectively rupturing the N-C bond of the amide, thereby facilitating further cross-coupling or other reactions. In this Focus Review, we discuss the recent advances in this area and present a summary of methods that have been developed for activating the amide N-C bond by using precious and non-precious metals.

Influence of substitution of various functional groups on inhibition efficiency of TEMPO analogues on styrene polymerization

Vinyl monomers such as styrene may undergo undesired polymerization reactions during the industrial processes in which they are synthesized and purified. To avoid this undesired phenomenon, polymerization inhibitors such as (2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl (TEMPO) and its analogues can be used. An inhibitor and stability screening setup built in-house was used to compare the effectiveness of thirteen TEMPO analogues in inhibiting the polymerization of styrene in toluene at 130 °C. The data show that the substituent in the 4′-position in the cyclic structure of the TEMPO molecules strongly influenced their effectiveness in inhibiting polymerization. Computational methods based on the density functional theory (DFT) approach have been utilized to understand the reactivity of the TEMPO analogues. The global reactivity descriptors derived from the DFT calculations indicate that the TEMPO analogues with high global electrophilicity are more effective than those with low global electrophilicity in inhibiting the polymerization. The data suggest that the solubility of the TEMPO analogues in toluene and the keto–enol tautomerism of certain molecules may also affect their ability to inhibit styrene polymerization.

Evaluation of the structural properties of powerful pesticide dieldrin in different media and their complete vibrational assignment

Dieldrin was characterized by using Fourier Transform infrared (FT-IR) and Raman (FT-Raman), Ultraviolet–Visible (UV–Visible) spectroscopies. The structural and vibrational properties for dieldrin in gas phase and in aqueous solution were computed combining those experimental spectra with hybrids B3LYP and WB97XD calculations by using the 6-31G* and 6-311++G** basis sets. Here, the experimental available Hydrogen and Carbon Nuclear Magnetic Resonance (1H and 13C NMR) for dieldrin were also used and compared with those predicted by calculations. The B3LYP/6-311++G** method generates the most stable structures while the results have demonstrated certain dependence of the volume and dipole moment values with the method, size of the basis set and, with the studied media. The lower solvation energy for dieldrin (−32.94 kJ/mol) is observed for the higher contraction volume (−2.4 Å3) by using the B3LYP/6-31G* method. The NBO studies suggest a high stability of dieldrin in gas phase by using the WB97XD/6-31G* method due to the n→π* and n*→π* interactions while the AIM analyses support this high stability by the C18⋯H26 and C14⋯O7 contacts. The different topological properties observed in the R5 ring suggest that probably this ring plays a very important role in the toxics properties of dieldrin. The frontier orbitals show that when dieldrin is compared with other toxics substances the reactivity increases in the following order: CO < STX < dieldrin < C6Cl6 <TCAB < TCAOB < CN− where evidently, the presence of five rings and six Cl atoms decrease the reactivity of dieldrin, as compared with hexachlorobencene. The WB97XD method and the two basis sets predicted for dieldrin in both media low reactivities, higher nucleophilicity and, low electrophilicity. All the bands observed in the IR and Raman spectra were completely assigned to the 75 vibration normal modes and their harmonic force fields and force constants for first time are reported for dieldrin. The predicted FTIR, FT-Raman, UV–Visible and 1H and 13C NMR spectra for dieldrin show a reasonable concordance with the experimental ones.

Role of Lewis‐Base‐Coordinated Halogen(I) Intermediates in Organic Synthesis: The Journey from Unstable Intermediates to Versatile Reagents

The widespread use of halogen‐based reagents in organic synthesis is well known. Hypervalent halogens (oxidation states III and V) have been explored to a significant extent. Among them, hypervalent iodine reagents have found major use and applications. In contrast, understanding of the reactivity of halogen(I) species in the presence of Lewis bases, and their reaction mechanisms, are very limited. This microreview sheds light on the importance of halogen(I) species and their applications in organic synthesis. In commercially available halogen(I) precursors such as N‐halosuccinimides, the halogen atom is attached to the nitrogen atom through a covalent bond with low electrophilicity and thus low reactivity. Interestingly, the reactivity increases if the electrophilic halogen(I) atom is attached to a Lewis base through a polarized covalent bond. The halogen‐bonding interaction between the Lewis base and the halonium ion (or halonium ion equivalent) results in highly reactive and thermodynamically stable halogen(I) intermediates that act as versatile reagents. This microreview unravels the evolution of the unstable halogen(I) intermediates to become versatile reagents.

Metal-Free C-H Alkyliminylation and Acylation of Alkenes with Secondary Amides.

Carbon–carbon bond formation by metal-free cross-coupling of two reactants with low reactivity represents a challenge in organic synthesis. Secondary amides and alkenes are two classes of bench-stable compounds. The low electrophilicity of the former and low nucleophilicity of the latter make the direct coupling of these two partners challenging yet highly desirable. We report herein an unprecedented intermolecular reaction of secondary amides with alkenes to afford α,β-unsaturated ketimines or enones, which are versatile intermediates for organic synthesis and are prevalent in bioactive compounds and functional materials. Our strategy relies on the chemoselective activation of the secondary amide with trifluoromethanesulfonic anhydride (Tf2O)/2-fluoropyridine to generate a highly reactive nitrilium intermediate, which reacts efficiently with alkenes. This metal-free synthesis is characterized by its mild reaction conditions, excellent functional group tolerance and chemoselectivity, allowing the preparation of multi-functionalized compounds without using protecting groups.

New Stable and Persistent Acyclic Diaminocarbenes.

The portfolio of acyclic diaminocarbenes (ADACs) has been substantially expanded, owing to the synthesis of eleven new formamidinium salts, mostly of the type [(iPr2N)CH(NRR')][PF6], for use as immediate carbene precursors. The corresponding ADACs (iPr2N)C(NRR') were sufficiently stable for isolation in the case of NRR' = 2-methylpiperidino (13), 3-methylpiperidino (14), 4-methylpiperidino (15), morpholino (17) and NiPrPh (20), but had to be trapped in situ in the case of NRR' = 2,2,6,6-tetramethylpiperidino (12) and NiPrMe (19). The tetraaryl-substituted ADACs (Ph2N)2C (22) and (Ph2N)C[N(C6F5)2] (24) also could only be generated and trapped in situ. Trapping with elemental selenium was particularly efficient, affording the corresponding selenourea derivative in all cases, whereas trapping with [{Rh(μ-Cl)(cod)}2] did not work for 12 and 24. The (77)Se NMR chemical shifts, δ((77)Se), of the selenourea compounds derived from the new ADACs lie in the range 450-760 ppm, which indicates a much higher electrophilicity and π-accepting capability of ADACs in comparison with NHCs, which typically exhibit δ((77)Se)<200 ppm. The extreme low-field shift of 758 ppm observed for 12Se can be rationalised by the results of DFT calculations, which revealed that ADAC 12 has a minimum energy conformation with the 2,2,6,6-tetramethylpiperidino unit perpendicular to the N2C plane, which suppresses the π donation of this amino group and causes an unusually low LUMO energy and high electrophilicity.

Expanding the Scope of the Tail-to-Tail Dimerization of Vinyl Compounds Catalyzed by N-Heterocyclic Carbene

Abstract The tail-to-tail dimerization of vinyl compounds catalyzed by an N-heterocyclic carbene was found to show a very broad substrate scope and tolerance to various functional groups. The optimization of the dimerizations of 2-vinylpyridine and n-butyl acrylate, as low and high electrophilicity model substrates, indicates that high reaction temperatures around 150 °C are required for both substrates. In addition to methacrylates and methacrylonitrile, the applicable substrates range from an activated styrene and vinyl pyridines to highly reactive acrylonitrile, dimethyl acrylamide, and various functionalized acrylates. The alcohol additives significantly increase the catalytic turnover number to 18 for the dimerization of acrylates. Deuterium labeling experiments indicate that the alcohols promote the intermolecular proton transfers.

Computational Study of Octasilacubane Structural Properties with Density Functional Theory Method

In the present study, the density functional theory method with various basis sets for optimizing and computing the structural and energetic properties of octasilacubane were performed at 298.15 K and 1 atmosphere. The data from the calculations on this compound show us the internal angles in the structure are 90 degrees. For this reason, the molecule has serious angular pressure. This angular strain is tolerated by a higher percentage of p-orbitals of silicon atoms in the Si-Si σ-bonds formation. According to calculation, σ(Si−Si) bonds in the octasilacubane system are formed from an sp3.44d0.02 hybrid. Also, IR and NMR spectra of the structure were simulated. The electronic chemical potential, the hardness and electrophilicity power of the molecule were obtained from frontier orbitals energies. The data show the structure has low electrophilicity. Heats of formation and detonation parameters were then calculated at studied levels of B3LYP (Becke, 3 parameter, Lee-Yang-Parr) theory. The simulation results revealed that this compound because for its high molecular weight is not a viable candidate of high energy density materials (HEDMs).

Di- and triarylmethylium ions as probes for the ambident reactivities of carbanions derived from 5-benzylated Meldrum's acid.

The kinetics of the reactions of carbocations with carbanions 1 derived from 5-benzyl-substituted Meldrum's acids 1-H (Meldrum's acid = 2,2-dimethyl-1,3-dioxane-4,6-dione) were investigated by UV/Vis spectroscopic methods. Benzhydryl cations Ar2CH(+) added exclusively to C-5 of the Meldrum's acid moiety. As the second-order rate constants (kC) of these reactions in DMSO followed the linear free-energy relationship lg k = sN (N+E), the nucleophile-specific reactivity parameters N and sN for the carbanions 1 could be determined. In contrast, trityl cations Ar3C(+) reacted differently. While tritylium ions of low electrophilicity (E<-2) reacted with 1 through rate-determining β-hydride abstraction, more Lewis acidic tritylium ions initially reacted at the carbonyl oxygen of 1 to form trityl enolates, which subsequently reionized and eventually yielded triarylmethanes and 5-benzylidene Meldrum's acids by hydride transfer.

Discovery of a Statistically Significant and Interpretable Relationship Between Redox Reactivity and Lethality of Drugs

A statistically significant and interpretable relationship between electrophilicity as a redox reactivity indicator and LD50 as a lethality indicator of drugs was discovered, and this relationship could be interpreted by the action of the cytochrome P450. The drugs chosen in this study were Topoisomerase II inhibitor anticancer drugs, and the electrophilicity of drugs was obtained by quantum chemical calculation. Since the P450 detoxification mechanism is the catalytic oxidation of drug molecules, it may infer that the drug molecules being easily oxidized (low electrophilicity) will be weak in lethality in general. In addition, this relationship revealed two structural scaffolds for the anthracycline-based topoisomerase II inhibitors, and their lethality mechanisms are not totally the same. Such relationship can assist in designing new drugs that candidates possessing low electrophilicity are recommended for lowering of lethality, and moieties providing a large inductive effect can reduce the electrophilicity of the anthracycline-based topoisomerase II inhibitors.

Covalent inhibition of SUMO and ubiquitin-specific cysteine proteases by an in situ thiol–alkyne addition

Posttranslational modification of proteins with ubiquitin and ubiquitin-like modifiers such as SUMO can be reverted by specific proteases, also referred to as deubiquitinases and isopeptidases, most of which are cysteine-dependent. We have found that the replacement of the conserved C-terminal glycine with propargylamine converts SUMO and ubiquitin to highly efficient covalent inhibitors of their cognate cysteine proteases. Attack of the catalytic cysteine onto the terminal alkyne results in the formation of a vinyl sulfide linkage. Although this reaction is reminiscent of the inhibitory mechanism of the isosteric nitrile inhibitors it was unexpected due to the low electrophilicity of the alkyne group. We show that a precise location of the functional group in the active site of the protease is crucial for the reaction, which was not inhibited by the presence of a radical scavenger. Furthermore, a mutational study of key catalytic residues in the SUMO-protease Senp1, that is H533A and D550A of the catalytic triad and Q597A as part of the oxyanion hole, revealed that these residues are not required for the observed covalent adduct formation. We therefore propose that the reaction is an in situ thiol–alkyne addition. Due to the high chemical inertness of the alkyne moiety the respective protease inhibitors should be well-suited for cellular and therapeutic applications. In keeping with this idea, selective labeling with propargylated SUMO and Ub probes was observed in lysates of cell lines expressing the cognate proteases after transient transfection.

(Dimethylamino)borylene and Related Complexes of Electron-Rich Metal Fragments: Generation of Nucleophile-Resistant Cations by Spontaneous Halide Ejection

Spontaneous halide ejection from a three-coordinate Lewis acid has been shown to offer a remarkable new route to cationic metal complexes featuring a linear, multiply bonded boron-donor ligand. The exploitation of electron-rich [CpM(PR3)2] fragments within boryl systems of the type LnMB(hal)NR2 leads to the spontaneous formation in polar solvents of chemically robust borylene complexes, [LnM(BNR2)]+, with exceptionally low electrophilicity and short M–B bonds. This is reflected by M–B distances (ca. 1.80 Å for FeB systems) which are more akin to alkyl-/aryl-substituted borylene complexes and, perhaps most strikingly, by the very low exothermicity associated with the binding of pyridine to the two-coordinate boron center (ΔH = −7.4 kcal mol–1, cf. −40.7 kcal mol–1 for BCl3). Despite the strong π electron release from the metal fragment implied by this suppressed reactivity and by such short M–B bonds, the barrier to rotation about the Fe═B bond in the unsymmetrical variant [CpFe(dmpe)(BN{C6H4OMe-4}Me)]+ is found to be very small (ca. 2.9 kcal mol–1). This apparent contradiction is rationalized by the orthogonal orientations of the HOMO and HOMO-2 orbitals of the [CpML2]+ fragment, which mean that the M–B π interaction does not fall to zero even in the highest energy conformation.

Catalytic asymmetric intermolecular Stetter reactions of enolizable aldehydes with nitrostyrenes: computational study provides insight into the success of the catalyst.

Over the past decade, N-heterocyclic carbenes (NHCs) have been used as catalysts in a variety of C-C bond forming reactions.[1] Our group has been interested in the development of chiral NHCs as catalysts for the asymmetric intra-molecular Stetter reaction[2,3] and more recently the inter-molecular variant.[4,5] We recently reported that hetaryl aldehydes and enals react efficiently with nitroalkenes in the Stetter reaction, leading to β-nitro ketones with high enantioselectivity.[4d] Crucial to the success of this method was the development of a fluorinated triazolium salt pre-catalyst that provides significantly enhanced enantioselectivity over des-fluoro analogues.[6] Although this new catalyst system greatly expands the scope of this method, these conditions are not amenable to the use of unactivated aliphatic aldehydes. Due to their lower electrophilicity than aryl aldehydes, aliphatic aldehydes have rarely been used successfully in the asymmetric inter-molecular Stetter reaction.[7,8]

First principles simulation of reaction steps in the atomic layer deposition of titania: dependence of growth on Lewis acidity of titanocene precursor

It is a common finding that titanocene-derived precursors do not yield TiO(2) films in atomic layer deposition (ALD) with water. For instance, ALD with Ti(OMe)(4) and water gives 0.5 Å/cycle, while TiCp*(OMe)(3) does not show any growth (Me = CH(3), Cp* = C(5)(CH(3))(5)). From mass spectrometry we found that Ti(OMe)(4) occurs in the gas phase practically exclusively as a monomer. We then used first principles density functional theory (DFT) to model the ALD reaction sequence and find the reason for the difference in growth behaviour. Both precursors adsorb initially via hydrogen-bonding. The simulations reveal that the Cp* ligand of TiCp*(OMe)(3) lowers the Lewis acidity of the Ti centre and prevents its coordination to surface O ('densification') during both of the ALD pulses. The effect of Cp* on Ti seems to be both steric (full coordination sphere) and electronic (lower electrophilicity). This crucial step in the sequence of ALD reactions is therefore not possible in the case of TiCp*(OMe)(3) + H(2)O, which means that there is no deposition of TiO(2) films.

Cubyl cations: A quantum-chemical study

By the quantum-chemical method PBE0/6-311G (3d5f7,p) equilibrium structural parameters of cubyl cation C8H 7 + , adduct C8H 7 + ·MeOH and methoxycubane molecule C8H7OMe were determined. In the free cation the bond lengths are as follows: C1Cα 1.489, CαCβ 1,569, CβCγ 1.556, and CH 1.085–1.087 A; angles at C1 are 98.6°; charge on the C1 atom is +0.39 au (NPA). In the methoxycubane the cubyl-O bond is shorter, while in the cationic adduct is longer than the O-Me bond: 1.368 1.469 A. A relatively low electrophilicity of the cubyl cation compared with CH 3 + is concluded and the possibility of its obtaining by radiochemical methods is assumed.