The Bond Dissociation Energy of the N-O Bond

Catalytic Asymmetric Access to Structurally Diverse N-Alkoxy Amines via a Kinetic Resolution Strategy

Chiral N-alkoxy amines are increasingly vital substrates in bioscience. However, asymmetric synthetic strategies for these compounds remain scarce. Catalytic kinetic resolution represents an attractive approach to prepare structurally diverse enantiopure N-alkoxy amines, which has remained elusive due to the notably reduced nucleophilicity of the nitrogen atom together with the low bond dissociation energies of labile NO-C and N-O bonds. We here report a general kinetic resolution of N-alkoxy amines through chemo- and enantioselective oxygenation. The mild and green titanium-catalyzed approach features broad substrate scope (55 examples), noteworthy functional group compatibility, high catalyst turnover number (up to 5200), excellent selectivity factor (s > 150), and scalability.

Copper-catalysed dehydrogenation or lactonization of C(sp3)-H bonds

Cytochrome P450 enzymes are known to catalyse bimodal oxidation of aliphatic acids via radical intermediates, which partition between pathways of hydroxylation and desaturation1,2. Developing analogous catalytic systems for remote C-H functionalization remains a significant challenge3-5. Here, we report the development of Cu(I)-catalysed bimodal dehydrogenation/lactonization reactions of synthetically common N-methoxyamides through radical abstractions of the γ-aliphatic C-H bonds. The feasibility of switching from dehydrogenation to lactonization is also demonstrated by altering reaction conditions. The use of a readily available amide as both radical precursor and internal oxidant allows for the development of redox-neutral C-H functionalization reactions with methanol as the sole side product. These C-H functionalization reactions using a Cu(I) catalyst with loading as low as 0.5 mol.% is applied to the diversification of a wide range of aliphatic acids including drug molecules and natural products. The exceptional compatibility of this catalytic system with a wide range of oxidatively sensitive functionality demonstrates the unique advantage of using a simple amide substrate as a mild internal oxidant.

Mechanism of the Sharpless Epoxidation Reaction: A DFT Study

The Sharpless reaction is an enantioselective epoxidation of prochiral allylic alcohols that employs a Ti(IV) catalyst formed from titanium tetra(isopropoxide), Ti(O-i-Pr)4, diethyl tartrate (DET), and the oxidizing agent tert-butyl hydroperoxide. The M06-2X DFT functional with the 6-311+G(d,p) basis set has been employed to model the structures and energetics of the Sharpless epoxidation reaction. The monomeric tetracoordinate titanium(IV) diethyltartrate is thermodynamically strongly favored to dimerize, producing a pentacoordinate catalyst, [Ti(DET)(O-i-Pr)2]2, that is a more reactive chiral epoxidation catalyst. The rapid ligand exchange reactions needed to generate the "loaded" catalyst and to repeat the overall catalytic cycle have been examined and are found to have activation energies that are much lower than the epoxidation barriers. The transition structures and activation energies for the enantioselective epoxidation of allyl alcohol, trans-methyl-allyl alcohol and trans-tert-butyl-allyl alcohol with the "loaded" Sharpless catalyst, [Ti2(DET)2(O-i-Pr)2-(OAllyl)-(OOt-Bu)], are presented. The effect of the C═O···Ti interactions on the activation energies and the significance of the O-C-C═C dihedral angle on the enantioselectivity of the epoxidation reaction are discussed.

A physical organic strategy to predict and interpret stabilities of chemical bonds in energetic compounds for the discovery of thermal-resistant properties

CONTEXT

The in-depth understanding about the stability of chemical bonds in energetic compounds plays a central role for molecular design and safety-related evaluations. Most energetic compounds contain nitro as explosophores, and nitro cleavage is fundamental for thermal and mechanical stability. However, the quantum chemistry approach to accurately predict energy and temperature properties related to bond stability is challenging, due to the tradeoff between computational costs and deviations. Herein, the bond orders are proposed as accurate and computational-cost efficient descriptors for predicting the chemical bond stability and thermal-resistant properties. The intrinsic bond strength index (IBSI) demonstrates the best prediction for experimental homolytic bond dissociation energies (R2 > 0.996), which is on par with the results from high-precision quantum chemistry methods. The effects from bond connectivity and steric hindrance hierarchy were analyzed to reveal underlying mechanisms. Additionally, the IBSI descriptors are successfully applied to predict the thermal decomposition temperatures of 24 heat-resistant energetic compounds (R2 = 0.995), thus validating the effectiveness for the prediction and interpretation of chemical bond stability in energetic compounds via a physical organic approach.

METHODS

All DFT calculations were performed with Gaussian 09 software. To investigate the dependence of the method on functionals and basis sets, 9 DFT methods were considered (B3LYP/6-31G(d,p), B3LYP/6-311G(d,p), B3LYP/def2-TZVP, M062X/6-31G(d,p), M062X/6-311G(d,p), M062X/def2-TZVP, ωB97XD/6-31G(d,p), ωB97XD/6-311G(d,p), and ωB97XD/def2-TZVP). The bond order descriptors LBO and IBSI are obtained through the bond order analysis module in the Multiwfn software.

Mechanisms for radical reactions initiating from N-hydroxyphthalimide esters

Due to their ease of preparation, stability, and diverse reactivity, N-hydroxyphthalimide (NHPI) esters have found many applications as radical precursors. Mechanistically, NHPI esters undergo a reductive decarboxylative fragmentation to provide a substrate radical capable of engaging in diverse transformations. Their reduction via single-electron transfer (SET) can occur under thermal, photochemical, or electrochemical conditions and can be influenced by a number of factors, including the nature of the electron donor, the use of Brønsted and Lewis acids, and the possibility of forming charge-transfer complexes. Such versatility creates many opportunities to influence the reaction conditions, providing a number of parameters with which to control reactivity. In this perspective, we provide an overview of the different mechanisms for radical reactions involving NHPI esters, with an emphasis on recent applications in radical additions, cyclizations and decarboxylative cross-coupling reactions. Within these reaction classes, we discuss the utility of the NHPI esters, with an eye towards their continued development in complexity-generating transformations.

Discovery of a Hydroxylamine-Based Brain-Penetrant EGFR Inhibitor for Metastatic Non-Small-Cell Lung Cancer

Metastases to the brain remain a significant problem in lung cancer, as treatment by most small-molecule targeted therapies is severely limited by efflux transporters at the blood-brain barrier (BBB). Here, we report the discovery of a selective, orally bioavailable, epidermal growth factor receptor (EGFR) inhibitor, 9, that exhibits high brain penetration and potent activity in osimertinib-resistant cell lines bearing L858R/C797S and exon19del/C797S EGFR resistance mutations. In vivo, 9 induced tumor regression in an intracranial patient-derived xenograft (PDX) murine model suggesting it as a potential lead for the treatment of localized and metastatic non-small-cell lung cancer (NSCLC) driven by activating mutant bearing EGFR. Overall, we demonstrate that an underrepresented functional group in medicinal chemistry, the trisubstituted hydroxylamine moiety, can be incorporated into a drug scaffold without the toxicity commonly surmised to accompany these units, all while maintaining potent biological activity and without the molecular weight creep common to drug optimization campaigns.

Enzymatic Nitrogen Incorporation Using Hydroxylamine

Hydroxylamine-derived reagents have enabled versatile nitrene transfer reactions for introducing nitrogen-containing functionalities in small-molecule catalysis, as well as biocatalysis. These reagents, however, result in a poor atom economy and stoichiometric organic waste. Activating hydroxylamine (NH2OH) for nitrene transfer offers a low-cost and sustainable route to amine synthesis, since water is the sole byproduct. Despite its presence in nature, hydroxylamine is not known to be used for enzymatic nitrogen incorporation in biosynthesis. Here, we report an engineered heme enzyme that can utilize hydroxylammonium chloride, an inexpensive commodity chemical, for nitrene transfer. Directed evolution of Pyrobaculum arsenaticum protoglobin generated efficient enzymes for benzylic C-H primary amination and styrene aminohydroxylation. Mechanistic studies supported a stepwise radical pathway involving rate-limiting hydrogen atom transfer. This unprecedented activity is a useful addition to the "nitrene transferase" repertoire and hints at possible future discovery of natural enzymes that use hydroxylamine for amination chemistry.

N-Oxides as Control Element for the Direction of a Sigmatropic Rearrangement: Application as a Switch for Fluorescence

An exceptional level of control over the direction of the (2,3)-sigmatropic rearrangement between N-oxides and alkoxylamine is achieved by simply changing the solvent in which they are dissolved. Protic solvents like water, methanol and hexafluoroisopropanol favour the N-oxide form, while other solvents like acetone, acetonitrile and benzene favour the alkoxylamine. The reaction temperature and nature of the substituents on the alkene affect the rate of rearrangement. A N-oxide fragment was attached to two fluorescent molecules and acted as an on/off switch for their fluorescence. The conversion of alkoxylamines into the corresponding N-oxides has not previously been described and is here termed the 'Reverse Meisenheimer Rearrangement'.

Chemoselective Oxidation of Thiols with Oxoammonium Cations

The oxidation of various aryl and aliphatic thiols with the commercially available and environmentally benign reagent Bobbitt's salt (1) has been investigated. The reaction affords the corresponding disulfide products in good to excellent yields (71-99%) and can be accomplished in water, methanol, or acetonitrile solvent. Moreover, the process is highly chemoselective, tolerating traditionally oxidation-labile groups such as free amines and alcohols. Combined experimental and computational studies reveal that the oxidation takes place via a polar two-electron process with concomitant and unexpected deoxygenation of the oxoammonium cation through homolysis of the weak N-O bond, differing from prototypical radical-based thiol couplings. This unusual consumption of the oxidant has significant implications for the development of new nitroxide-based radical traps for probing S-centered radicals, the advancement of new electrochemical or catalytic processes involving nitroxide/oxoammonium salt redox couples, and applications to biological systems.

Bond dissociation energies of ethyl valerate and tripropionin

CONTEXT

Due to the expected decrease in the availability of conventional oils, numerous studies are currently underway to find complementary sources of energy. Among the explored avenue is that of biofuels. Ethyl valerate (ETV) and tripropionin (TPP) are two biofuels whose thermal decomposition has not received the attention it deserves. Herein, we have evaluated the bond dissociation enthalpies (BDHs) to predict how easy it is to break some bonds in these compounds, and subsequently contribute to revealing the initiation step in their combustion reactions. Our computations consistently predict C4-C5 and C1-C2 bonds in ETV and TPP as the weakest bonds, likely to break first and initiate the thermal decomposition of these two compounds, respectively. The conformational changes in ETV and TPP have only a small influence on the BDHs of 1 kcal/mol at M06-2X/6-311 + G(3df,2p). B3LYP and ωB97XD appear to be the most affordable methods for estimating BDHs at 6-31G(d,p) as they give good results for ETV (RMSD: 2.94 kcal/mol and 3.22 kcal/mol) and performed better than CBS-QB3 (RMSD: 3.64 kcal/mol). Using a larger basis set, the M06-2X (RMSD: 3.61 kcal/mol) and ωB97XD (RMSD: 3.51 kcal/mol) functionals are found to provide the most accurate predictions at 6-311 + G(3df,2p) as compared to G4MP2.

METHODS

BDHs of ETV and TPP are computed using density functional theory (DFT) and quantum chemistry composite methods at 6-31G(d,p) and 6-311 + G(3df,2p) levels. Because of its reliability and accuracy in thermochemical calculations, the G4MP2 theory is used as a reference to gauge the performance of DFT methods. All the calculations were carried out using the Gaussian 09 program.

Effect of Substituents on Molecular Reactivity during Lignin Oxidation by Chlorine Dioxide: A Density Functional Theory Study

Lignin is a polymer with a complex structure. It is widely present in lignocellulosic biomass, and it has a variety of functional group substituents and linkage forms. Especially during the oxidation reaction, the positioning effect of the different substituents of the benzene ring leads to differences in lignin reactivity. The position of the benzene ring branched chain with respect to methoxy is important. The study of the effect of benzene substituents on the oxidation reaction's activity is still an unfinished task. In this study, density functional theory (DFT) and the m062x/6-311+g (d) basis set were used. Differences in the processes of phenolic oxygen intermediates formed by phenolic lignin structures (with different substituents) with chlorine dioxide during the chlorine dioxide reaction were investigated. Six phenolic lignin model species with different structures were selected. Bond energies, electrostatic potentials, atomic charges, Fukui functions and double descriptors of lignin model substances and reaction energy barriers are compared. The effects of benzene ring branched chains and methoxy on the mechanism of chlorine dioxide oxidation of lignin were revealed systematically. The results showed that the substituents with shorter branched chains and strong electron-absorbing ability were more stable. Lignin is not easily susceptible to the effects of chlorine dioxide. The substituents with longer branched chains have a significant effect on the flow of electron clouds. The results demonstrate that chlorine dioxide can affect the electron arrangement around the molecule, which directly affects the electrophilic activity of the molecule. The electron-absorbing effect of methoxy leads to a low dissociation energy of the phenolic hydroxyl group. Electrophilic reagents are more likely to attack this reaction site. In addition, the stabilizing effect of methoxy on the molecular structure of lignin was also found.

Prediction of gaseous medium insulation strength based on electrostatic potential on real space function isosurface

CONTEXT

SF₆ is widely used in electrical equipment due to its chemical stability and insulation strength, but it is a strong greenhouse gas and its use has been restricted internationally. In order to reduce the SF₆ usage, it is needed to find a replacement gas for SF₆. Electrical breakdown test is always adopted to select potential substitutes, but it is resource and time intensive. Thus, a structure-activity relationship model is needed to effectively predict the gas insulation strength. In this work, we calculated the isosurface electrostatic potential of 68 gas molecules in case of electron probability density, Laplacian of electron density, electron localization function, and localized orbital function. The distribution characteristics of these four real space functions were analyzed. Furthermore, correlation between the electrostatic potential parameters and insulation strength was presented. Finally, a prediction model for insulation strength of gaseous medium was established. Using the electrostatic potential parameter on the localized orbital locator function with a threshold of 0.05 a.u., the prediction model achieved the best performance with a coefficient of determination of 0.860 and a mean squared error of 0.0663.

METHODS

The quantization calculation tool used in this work is the GAUSSIAN 16 software. The M06-2X method with the 6-311G++(d, p) basis set is used to optimize the molecular structure and output stable wavefunction files. Then the wavefunction analysis software Multiwfn is used to plot the contour map of the gas molecules and calculate the radial distribution patterns.

Formation and reactivity of NHC-boryl radicals: insight into substituent effect from theoretical calculations

Substituent modification effects of N-heterocyclic carbene (NHC) boranes on their hydrogen atom abstraction (HAA) reactions and the chemical reactivities of corresponding NHC-boryl radicals have been investigated by density functional theory calculations. The substituent modification of NHC-boranes may notably affect the HAA reaction, both kinetically and thermodynamically, and shows remarkable substitution position dependence. The multi-site-modification of NHC-boranes is proved to be more effective for reduction of the B-H bond dissociation energy (BDE), promotion of the HAA reaction, and the reactivity regulation of their corresponding NHC-boryl radicals. Computational screening reveals that the spin density and the charge population of the radical boron center have good correlation with the B-H BDEs of NHC-boranes and the chemical reactivities of NHC-boryl radicals, and they can be considered as property and reactivity descriptors of these boron-based systems. The present results and established scaling relationships are beneficial to promote the advancement of design of NHC-boryl radical catalysis.

Recent Advances in the Synthesis of Di- and Trisubstituted Hydroxylamines

As an underrepresented functional group in bioorganic and medicinal chemistry, the hydroxylamine unit has historically received little attention from the synthetic community. Recent developments, however, suggest that hydroxylamines may have broader applications such that a review covering recent developments in the synthesis of this functional group is timely. With this in mind, this review primarily covers developments in the past 15 years in the preparation of di- and trisubstituted hydroxylamines. The mechanism of the reactions and key features and shortcomings are discussed throughout the review.

Copper-catalyzed [1,3]-nitrogen rearrangement of O-aryl ketoximes via oxidative addition of N–O bond in inverse electron flow†

The [1,3]-nitrogen rearrangement reactions of O-aryl ketoximes were promoted by N-heterocyclic carbene (NHC)-copper catalysts and BF₃·OEt₂ as an additive, affording ortho-aminophenol derivatives in good yields. The reaction of substrates with electron-withdrawing substituents on the phenol moiety are accelerated by adding silver salt and modifying the substituent at the nitrogen atom. Density functional theory calculations suggest that the rate-determining step of this reaction is the oxidative addition of the N–O bond of the substrate to the copper catalyst. The negative ρ values of the substituent at both the oxime carbon and phenoxy group indicate that the donation of electrons by the oxygen and nitrogen atoms accelerates the oxidative addition.

[1,3]-Nitrogen rearrangement reactions of O-aryl ketoximes was catalytically promoted by IPrCuBr and BF₃·OEt₂. The oxidative addition of the N–O bond to the Cu catalyst is accelerated by donation of electrons from both nitrogen and oxygen atoms.

Interactions of Nitric Oxide Molecules with Pure and Oxidized Silver ClustersAgn{plus minus}/AgnO{plus minus} (n=11-13). A Computational Study

In this work we studied, within DFT, the interaction of NO with pure and oxidized Agn, both anionic and cationic, composed from11 to 13 Ag atoms. In that size interval, shell closing effects are not expected, and structural and electronic odd-even effects will determine the strength of interaction. We obtained that species Agn{plus minus} and AgnO{plus minus} with odd number of electrons (n=12) adsorb NO with higher energy than their neighbours. This result agrees with the facts observed in recent mass spectroscopy measurements, which were performed at finite temperature. The adsorption energy is about twice for oxidized clusters compared to pure ones, and higher for anions than for cations. The adsorption of another NO molecule on AgnNO{plus minus} forms Agn(NO)2{plus minus}, with the dimer (NO)2 in cis configuration, and binding the two N atoms with two neigbour Ag atoms. The n=12 show the higher adsorption energy again. In absence of reaction barriers, Agn(NO)2{plus minus} dissociate spontaneously into AgnO{plus minus} and N2O, except the n= 12 anion. The máximum high barrier along the dissociation path of Ag13(NO)2- is about 0.7 eV. Further analysis of PDOS for Ag11-13 (NO)x{plus minus} (x=0,1,2) molecules shows that bonding between NO and Agn mainly occurs in the range between -3.0 eV and 3.0 eV. The overlap between 4 d of Ag and 2 p of N and O is larger for Ag12(NO)2{plus minus} than for neighbour sizes. For n=12, the d bands are close to the (NO)2 2π orbital, leading to extra back-donation charge from the 4 d of Ag to the closer 2π orbital of (NO)2.

Calculating bond dissociation energies of X-H (X=C, N, O, S) bonds of aromatic systems via density functional theory: a detailed comparison of methods

In this study, the performance of 17 different density functional theory functionals was compared for the calculation of the bond dissociation energy (BDE) values of X-H (X=C, N, O, S) bonds of aromatic compounds. The effect of the size of the basis set (expansions of 6-31(G)) was also assessed for the initial geometry and zero-point energy calculations, followed by the single-point BDE calculations with different model chemistries with the 6-311 + (3df,2p) basis set. It was found that the size of the basis set for geometry optimization has a much smaller effect on the accuracy of BDE than the choice of functional for the following single-point calculations. The M06-2X, M05-2X and M08-HX functionals yielded highly accurate BDE values compared to experimental data (with the average mean unsigned error MUE = 1.2-1.5 kcal mol^-1), performing better than any of the other functionals. The results suggest that geometry optimization may be performed with B3LYP functional and a small basis set, whereas the M06-2X, M05-2X and M08-HX density functionals with a suitably large basis set offer the best method for calculating BDEs of ArX-H (X=C, N, O, S) bonds.

New insight into degradation of chloramphenicol using a nanoporous Pd/Co3O4cathode: characterization and pathways analysis

The growing chloramphenicol (CAP) in wastewater brought a serious threat to the activity of activated sludge and the spread of antibiotics resistance bacteria. In this study, a highly ordered nanoporous Co₃O₄layer on Co foil through anodization was prepared as cathode for nitro-group reduction and electrodeposited with Pd particles for dechlorination to reduce CAP completely. After 3 h treatment, almost 100% of CAP was reduced. Co²⁺ions in Co₃O₄served as catalytic sites for electrons transfer to CAP through a redox circle Co²⁺-Co³⁺-Co²⁺, which triggered nitro-group reduction at first. With the presence of Pd particles, more atomic H* were generated for dechlorination, which increased 22% of reduction efficiency after 3 h treatment. Therefore, a better capacity was achieved by Pd/Co₃O₄cathode (K = 0.0245 min^-1,Kis reaction constant) than by other cathodes such as Fe/Co₃O₄(K = 0.0182 min^-1), Cu/Co₃O₄(K = 0.0164 min^-1), and pure Co₃O₄(K = 0.0106 min^-1). From the proposed reaction pathway, the ultimate product was carbonyl-reduced AM (dechlorinated aromatic amine product of CAP) without antibacterial activity, which demonstrated this cathodic technology was a feasible way for wastewater pre-treatment.

Unusual C-C bond cleavage of an α-trifloxy Sialic acid hemiacetal under Lattrell-Dax conditions

We describe the novel oxidative fragmentation of methyl (5-acetamido-4,7,8,9-tetra-O-acetyl-5-deoxy-3-O-trifluoromethanesulfonyl-β-D-erythro-L-gluco-2-nonulopyranos)onate 2 on stirring with sodium nitrite in DMF to give the novel 3-acetamido-2,5,6,7-tetra-O-acetyl-d-glycero-d-galacto-heptono-1,4-lactone 3 in excellent yield. Stirring of the same triflate with sodium carbonate on the other hand affords the novel methyl (5-acetamido-7,8,9-tri-O-acetyl-3,6-anhydro-5-deoxy-d-manno-3-ene-2-nonulos)onate 19 also in excellent yield. Reduction of the heptono lactone with sodium borohydride followed by acetylation gives a peracetylated aminodeoxyheptitol 6 that adopts the zig zag conformation of its carbon backbone.

Mechanism of Orbital Interactions in the Sharpless Epoxidation with Ti(IV) Peroxides: A DFT Study

The M06-2X DFT functional has been employed to examine monomeric titanium(IV) hydroperoxo catalysts that model the individual steps in the dimeric titanium(IV)-catalyzed Sharpless reaction. This is the first example of a transition structure for titanium(IV) tert-butyl hydroperoxide-catalyzed epoxidation that describes the molecular motion required for oxygen atom transfer. These epoxidation catalysts have been examined for both bimolecular reactions with E-2-butene and the intramolecular epoxidation of allyl alcohol. The transition structure for the bimolecular peroxyacetic acid epoxidation of E-2-butene has been shown to be spiro in nature, and likewise, the intramolecular epoxidation of allyl alcohol is also nearly spiro. The significance of the O-C-C═C dihedral angle of allyl alcohol is examined for the Ti(IV) tert-butyl hydroperoxide epoxidation mechanism. Evidence is presented that supports a hexacoordinate titanium peroxo environment that exists in the dimeric form of the Sharpless catalyst. The mechanism for a 1,3-rearrangement of the alkoxide ligand in a titanium hydroperoxide to the Ti center in concert with oxygen atom transfer of the proximal oxygen to the C═C bond of the substrate is presented. The dimerization of Ti(IV)-(R,R)-diethyl tartrate-diisopropoxide and its hydrolysis have been calculated. The mechanism for rapid ligand exchange with alkyl hydroperoxides involving the Ti(O-i-Pr)₄ precursor is examined to show how the active epoxidation catalyst is produced.