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Oximes derivatives have been vastly used in organic synthesis. In this review, C(sp3)−H bond functionalization of oximes derivatives via iminyl radical induced 1,5−hydrogen atom transfer was discussed. According to the different type of products, this review was divided into three parts: (1) C(sp3)−H bond functionalization for C−C bond formation. (2) C(sp3)−H bond functionalization for C−N bond formation. (3) C(sp3)−H bond functionalization for C−S, C−F bond formation.

Direct photocatalyzed hydrogen atom transfer (d-HAT) can be considered a method of choice for the elaboration of aliphatic C–H bonds. In this manifold, a photocatalyst (PCHAT) exploits the energy of a photon to trigger the homolytic cleavage of such bonds in organic compounds. Selective C–H bond elaboration may be achieved by a judicious choice of the hydrogen abstractor (key parameters are the electronic character and the molecular structure), as well as reaction additives. Different are the classes of PCsHAT available, including aromatic ketones, xanthene dyes (Eosin Y), polyoxometalates, uranyl salts, a metal-oxo porphyrin and a tris(amino)cyclopropenium radical dication. The processes (mainly C–C bond formation) are in most cases carried out under mild conditions with the help of visible light. The aim of this review is to offer a comprehensive survey of the synthetic applications of photocatalyzed d-HAT.

The explanation of spectroscopic data was one of the first great victories of quantum theory. In modern science and technology, the mastery of atomic physics is responsible for decisive progress ranging from laser technology to the exploration of the cosmos.

The Breslow intermediate is key to several NHC-organocatalyzed reactions. However, accessing the Breslow intermediate via a direct intramolecular hydrogen atom transfer (HAT) is unlikely and can on...

The ·OH-initiated reaction mechanism and kinetics of sulfoxaflor were investigated by using electronic structure calculations. The possible hydrogen atom and cyano group abstraction reaction pathways were studied, and the calculated thermochemical parameters show that the hydrogen atom abstraction from the C7 carbon atom is the more favorable reaction pathway. The subsequent reactions for the favorable intermediate (I4) with other atmospheric reactive species, such as O2, H2O, HO2·, and NOx· (x = 1, 2), were studied in detail. The products identified from the subsequent reactions could contribute to secondary organic aerosol (SOA) formation in the atmosphere. The intermediates and products formed from the initial and subsequent reactions are equally as toxic as the parent sulfoxaflor. At 298 K, the rate constant calculated for the formation of the favorable intermediate I4 is 2.54 × 10-12 cm3 molecule-1 s-1, which shows that the lifetime of sulfoxaflor is 54 h. The excited-state calculation performed through time-dependent density functional theory shows that the photolysis of the title molecule is unlikely in the atmosphere. The global warming potentials (GWPs) for different time horizons, photochemical ozone creation potential (POCP), and ecotoxicity analysis were also studied for the insecticide sulfoxaflor.

Hydrated aluminium cations have been investigated as a photochemical model system with up to ten water molecules by UV action spectroscopy in a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. Intense photodissociation was observed starting at 4.5 eV for two to eight water molecules with loss of atomic hydrogen, molecular hydrogen and water molecules. Quantum chemical calculations for n=2 reveal that solvation shifts the intense 3s-3p excitations of Al+ into the investigated photon energy range below 5.5 eV. During the photochemical relaxation, internal conversion from S1 to T2 takes place, and photochemical hydrogen formation starts on the T2 surface, which passes through a conical intersection, changing to T1 . On this triplet surface, the electron that was excited to the Al 3p orbital is transferred to a coordinated water molecule, which dissociates into a hydroxide ion and a hydrogen atom. If the system remains in the triplet state, this hydrogen radical is lost directly. If the system returns to singlet multiplicity, the reaction may be reversed, with recombination with the hydroxide moiety and electron transfer back to aluminium, resulting in water evaporation. Alternatively, the hydrogen radical can attack the intact water molecule, forming molecular hydrogen and aluminium dihydroxide. Photodissociation is observed for up to n=8. Clusters with n=9 or 10 occur exclusively as HAlOH+ (H2 O)n-1 and are transparent in the investigated energy range. For n=4-8, a mixture of Al+ (H2 O)n and HAlOH+ (H2 O)n-1 is present in the experiment.

In this study, we investigate the reaction mechanisms of precursor adsorption during the atomic layer deposition (ALD) using density functional theory (DFT) calculations and ideal-gas methods; herein, we considered adsorption of tris(dimethylamino)silane (TDMAS) on a hydroxylated SiO2 surface to be the example for our investigation. When the reaction free energy is calculated, the DFT results obtained at 0 K suggest that the dissociation of a hydrogen atom from TDMAS is favorable upon TDMAS adsorption, which is inconsistent with the experimental results where one dimethylamino group is released. The experimental results can be accurately predicted when enthalpy and entropy changes are considered at elevated temperatures, thereby indicating the significance of finite-temperature effects in free-energy changes for solid-gas reactions. We analyze the changes in enthalpy and entropy and find that dimethylamine is a more favorable gaseous product than H2 owing to its larger translational and rotational entropy.

Cyclobutanols are privileged cyclic skeletons in natural products and synthetic building blocks. C(sp3)−H functionalization is a prolonged challenge in organic synthesis. The synthesis of cyclobutanols through double C(sp3)-H bond functionalization remains elusive. Here we report the efficient synthesis of cyclobutanols through intermolecular radical [3 + 1] cascade cyclization, involving the functionalization of two C − H bonds through sequential hydrogen atom transfer. The copper complex reduces the iodomethylsilyl alcohols efficiently under blue-light irradiation to initiate the tandem transformation. The mild reaction tolerates a broad range of functional groups and allows for the facile generation of elaborate polycyclic structures., Cyclobutanols are difficult structures to access given their inherent ring strain. Here, the authors present a method to form substituted cyclobutanols from a photocatalytic cascade reaction, proceeding via a 1,5- or 1,6-hydrogen atom transfer and subsequent ring closure.

Bicyclo[1.1.1]pentanes (BCPs) are important in drug design as sp3-rich bioisosteres of arenes and tert-butyl groups; however, the preparation of BCPs with adjacent quaternary carbons is barely known. We report a facile synthesis of α-quaternary BCPs using organophotoredox and hydrogen atom transfer catalysis in which α-keto radicals, generated through oxidation of β-ketocarbonyls, undergo efficient addition to [1.1.1]propellane. The BCP products can be transformed into a variety of useful derivatives, including enantioenriched BCPs featuring α-quaternary stereocenters.

The relationship between the lactim-lactam tautomerization and the free-radical scavenging reaction in vitamin B9 [folic acid (FA)] was investigated by density functional theory calculations. 6-Methylpterin was also adopted for the detailed analyses of various reaction paths. For pterin, the transition state of the tautomerization with two water molecules (n = 2) was calculated to be of the lowest activation energy. The proton-transfer circuit of n = 2 is retained (not broken) even with the addition of outer water molecules, n = 2 + 2, 2 + 4, 2 + 8, and 2 + 14. At the oxidation of the system composed of 6-methylpterin + (H2O)2 + HO•, the radical character of HO• is directly transmitted to the pterin ring along with the C-O → H → O → H → O → H → OH proton transfer. The patterns of the electron transfer (pterin ring → OX•) and the concomitant proton transfer via the water dimer were commonly obtained for the oxidant (OX•) = HO•, Cl3C-O2•, N3•, or SO4-•. The hydrogen atom transfer mechanism was ruled out. Two conformations of the puckered form with the -C(═O)-OH···N intramolecular hydrogen bonds of FA were found to have the stability similar to that of the linear conformer. Both the tautomerization and the oxidation were calculated to occur competitively in the three conformers.

The study of two complexes of fluorenonophane with CHCl3 and CHBr3 molecules has revealed that they differ mainly by the halogen bonds between host and guest molecules. The experimental and theoretical quantum chemical study has shown that the strength of a halogen bond depends on the nature of a halogen atom as well as its orientation to the π-system. The more positive electrostatic potential was revealed at the bromine atom indicating the stronger halogen bond with its participation that was confirmed by the interaction energies calculated for corresponding dimers and the evaluation of the true energy of a halogen bond. The orientation of the chlorine atom at the carbon aromatic atom instead of the center of the benzene ring leads to the shortest Hal…C distance that points out the stronger interaction according to the geometrical characteristics. The EDA analysis of the fluorenonophane complexes with CHCl3 and CHBr3 and their analogs with one halogen atom replaced by the hydrogen atom allows us to presume that the nature of halogen bonding is rather dispersive than electrostatic.

In this O-alkyl­ated sulfonyl­pyrazolone, the sulfur atom lies 0.558 (1) Å out of the pyrazole ring plane. The NH2 group is involved in an intra­molecular hydrogen bond to a sulfonyl oxygen atom and in a three-centre system with the two oxygen atoms of the side chain at C3, forming a ribbon structure., In the title compound, C18H17N3O4S, the pyrazole ring is planar, with the sulfur atom lying 0.558 (1) Å out of the ring plane. The NH2 group is involved in an intra­molecular hydrogen bond to a sulfonyl oxygen atom; its other hydrogen atom forms an asymmetric three-centre hydrogen bond to the two oxygen atoms of the —O—CH2—C=O— grouping, via the 21 screw axis, forming a ribbon structure parallel to the b axis. Translationally adjacent, coplanar ribbons form a layer parallel to (10).

The concerted transfer of protons and electrons enables the activation of small-molecule substrates by bypassing energetically costly intermediates. Here, we present the synthesis and characterization of several hydrogenated forms of an organofunctionalized vanadium oxide assembly, [V6O13(TRIOLNO2)2]2-, and their ability to facilitate the concerted transfer of protons and electrons to O2. Electrochemical analysis reveals that the fully reduced cluster is capable of mediating 2e-/2H+ transfer reactions from surface hydroxide ligands, with an average bond dissociation free energy (BDFE) of 61.6 kcal/mol. Complementary stoichiometric experiments with hydrogen-atom-accepting reagents of established bond strengths confirm that the electrochemically established BDFE predicts the 2H+/2e- transfer reactivity of the assembly. Finally, the reactivity of the reduced polyoxovanadate toward O2 reduction is summarized; our results indicate a stepwise reduction of the substrate, proceeding through H2O2 en route to the formation of H2O. Kinetic isotope effect experiments confirm the participation of hydrogen transfer in the rate-determining step of both the reduction of O2 and H2O2. This work constitutes the first example of hydrogen atom transfer for small-molecule activation with reduced polyoxometalates, where both electron and proton originate from the cluster.

Rate coefficients for the reaction of NO 3 radicals with a series of aromatic aldehydes were measured in a 7300 L simulation chamber at ambient temperature and pressure by relative and absolute methods. The rate coefficients for benzaldehyde (BA), ortho-tolualdehyde (O-TA), meta-tolualdehyde (M-TA), para-tolualdehyde (P-TA), 2,4-dimethyl benzaldehyde (2,4-DMBA), 2,5-dimethyl benzaldehyde (2,5-DMBA) and 3,5-dimethyl benzaldehyde (3,5-DMBA) were k1= 2.6 ± 0.3, k2= 8.7 ± 0.8, k3= 4.9 ± 0.5, k4= 4.9 ± 0.4, k5= 15.1 ± 1.3, k6= 12.8 ± 1.2 and k7= 6.2 ± 0.6, respectively, in the units of 10 −15 cm 3 molec. −1 s −1 at 298 ± 2 K. The rate coefficient k13 for the reaction of the NO 3 radical with deuterated benzaldehyde (benzaldehyde-d 1) was found to be half that of k1 . The end product of the reaction in an excess of NO 2 was measured to be C 6 H 5 C(O)O 2 NO 2 . Theoretical calculations of aldehydic bond energies and reaction pathways indicate that the NO 3 radical reacts primarily with aromatic aldehydes through the abstraction of an aldehydic hydrogen atom. The atmospheric implications of the measured rate coefficients are briefly discussed.

The purpose of this study is to investigate thermodynamic and kinetic properties on the hydrogen-atom-donating ability of 4-substituted Hantzsch ester radical cations (XRH•+), which are excellent NADH coenzyme models. Gibbs free energy changes and activation free energies of 17 XRH•+ releasing H• [denoted as ΔGHDo(XRH•+) and ΔGHD≠(XRH•+)] were calculated using density functional theory (DFT) and compared with that of Hantzsch ester (HEH2) and NADH. ΔGHDo(XRH•+) range from 19.35 to 31.25 kcal/mol, significantly lower than that of common antioxidants (such as ascorbic acid, BHT, the NADH coenzyme, and so forth). ΔGHD≠(XRH•+) range from 29.81 to 39.00 kcal/mol, indicating that XRH•+ spontaneously releasing H• are extremely slow unless catalysts or active intermediate radicals exist. According to the computed data, it can be inferred that the Gibbs free energies and activation free energies of the core 1,4-dihydropyridine radical cation structure (DPH•+) releasing H• [ΔGHDo(DPH•+) and ΔGHD≠(DPH•+)] should be 19–32 kcal/mol and 29–39 kcal/mol in acetonitrile, respectively. The correlations between the thermodynamic driving force [ΔGHDo(XRH•+)] and the activation free energy [ΔGHD≠(XRH•+)] are also explored. Gibbs free energy is the important and decisive parameter, and ΔGHD≠(XRH•+) increases in company with the increase of ΔGHDo(XRH•+), but no simple linear correlations are found. Even though all XRH•+ are judged as excellent antioxidants from the thermodynamic view, the computed data indicate that whether XRH•+ is an excellent antioxidant in reaction is decided by the R substituents in 4-position. XRH•+ with nonaromatic substituents tend to release R• instead of H• to quench radicals. XRH•+ with aromatic substituents tend to release H• and be used as antioxidants, but not all aromatic substituted Hantzsch esters are excellent antioxidants.

A kinetic analysis of the hydrogen atom transfer (HAT) reactions from a series of organic compounds to the iron(IV)-oxo complex [(N4Py)FeIV(O)]2+ and to the phthalimide N-oxyl radical (PINO) has been carried out. The results indicate that a higher activating effect of α-heteroatoms toward the HAT from C-H bonds is observed with the more electrophilic PINO radical. When the N-hydroxy precursor of PINO, N-hydroxyphthalimide (NHPI), is used as a HAT mediator in the oxidation promoted by [(N4Py)FeIV(O)]2+, significant differences in terms of selectivity have been found. Product studies of the competitive oxidations of primary and secondary aliphatic alcohols (1-decanol, cyclopentanol, and cyclohexanol) with alkylaromatics (ethylbenzene and diphenylmethane) demonstrated that it is possible to modify the selectivity of the oxidations promoted by [(N4Py)FeIV(O)]2+ in the presence of NHPI. In fact, alkylaromatic substrates are more reactive in the absence of the mediator while alcohols are preferably oxidized in the presence of NHPI.

Hydrogen atom transfer (HAT) process is a powerful and effective strategy for activating C-H bonds followed by further functionalization. Intramolecular 1,n (n = 5 or 6)-HATs are common and frequently encountered in organic synthesis. However, intramolecular 1,n (n = 2 or 3)-HAT is very challenging due to slow kinetics. Compared to proton-shuttle process, which is well established for organic synthesis, hydrogen radical-shuttle (HRS) is unexplored. In this work, a HRS-enabled decarboxylative annulation of carbonyl compounds via photoredox catalysis for the synthesis of indanones is developed. This protocol features broad substrate scope, excellent functional group tolerance, internal hydrogen radical transfer, atom- and step-economy. Critical to the success of this process is the introduction of water, acting as both HRS and hydrogen source, which was demonstrated by mechanistic experiments and density functional theory (DFT) calculations. Importantly, this mechanistically distinctive HAT provides a complement to that of typical proton-shuttle-promoted, representing a breakthrough in hydrogen radical transfer, especially in the inherently challenging 1,2- or 1,3-HAT., Although hydrogen atom transfer is widely observed in synthetic organic chemistry, intramolecular hydrogen atom transfer between atoms separated by fewer than four bonds is kinetically slow. Here the authors show a method to form indanones, with hydrogen atoms shuttled across short distances by water.

The quenching of fluorescence (FL) of uranyl ion $${\text{UO}}_{2}^{{2 + }}$$ and tryptophan Trp during their selective intracomplex photoexcitation in aqueous and methanol solutions has been studied. The Stern–Volmer constants have been determined for all studied systems. A static FL quenching mechanism has been established. It has been found that only intracomplex photoinduced electron transfer (PET) from Trp to $${\text{UO}}_{2}^{{2 + }}$$ is observed in aqueous solutions, regardless of the type of photoexcitation. In the case of excitation of the Trp⋅⋅⋅ $${\text{UO}}_{2}^{{2 + }}$$ complex in CH3OH, a competing intermolecular photoinduced transfer of a hydrogen atom from methanol to the electronically excited uranyl ion is observed along with the PET.

The M06/6-31+G(d) method has been used to study the reactivity of the two forms of curcumin (enol and keto). The energies needed for the three thermodynamic mechanisms: HAT, SET-PT and SPLET, have been calculated, in different solvents, to determine the most probable hydrogen atom transfer mechanism. The solvent effect is evaluated using an implicit solvation model (IEF-PCM). The results show the existence of an intramolecular hydrogen bond strength, in the enol form, which prevents the dissociation of hydrogen atom. In nonpolar solvent, the value of BDFE is lower than PA and IP; this means that HAT is the most favorable mechanism of the two forms of curcumin, while the SPLET mechanism is thermodynamically preferred in polar solvent. The UV/Vis spectra have been determined by the time-dependent density functional theory (TD-DFT) to show the maximum absorption wavelength value of curcumin and the nature of the excited states.

Photocatalytic hydrogen atom transfer is a very powerful strategy for the regioselective C(sp3)-H functionalization of organic molecules. Herein, we report on the unprecedented combination of decatungstate hydrogen atom transfer photocatalysis with the oxidative radical-polar crossover concept to access the direct net-oxidative C(sp3)-H heteroarylation. The present methodology demonstrates a high functional group tolerance (40 examples) and is scalable when using continuous-flow reactor technology. The developed protocol is also amenable to the late-stage functionalization of biologically relevant molecules such as stanozolol, (-)-ambroxide, podophyllotoxin, and dideoxyribose.

Summary Hydrogen-atom transfer (HAT) provides straightforward methods to generate open-shell radical intermediates from R-H (R = C, Si, etc.) bonds and offers unique opportunities for green and sustainable synthesis. Traditional HAT protocols required harsh conditions and relied on the use of harmful reagents such as Cl2 and peroxides. An emerging strategy is photoinduced intermolecular HAT, in which transformations can be driven by photocatalysis under mild conditions. In recent years, photoinduced intermolecular HAT reactions have seen substantial development of their versatility, efficiency, and selectivity. This review summarizes recent advances (up to December 2020) in this rapidly expanding research area. The representative examples provided are classified according to the active species responsible for hydrogen atom abstraction. The reactivity, selectivity, and established transformations for each type of active species are briefly summarized. This review aims to provide guidance for the application of photoinduced HAT in R-H functionalization reactions and to inspire further progress in this research area.

The nature of the bonding in the bifluoride anion, FHF-, has long been a topic of discussion with little resolution. A recent (2021) spectroscopic-theoretical study concluded that the bonds in this molecule represent a "crossover from hydrogen to chemical bonding." Spin-coupled generalized valence bond (SCGVB) theory is an advanced orbital theory that describes a broad range of molecules and molecular processes, and its application has provided valuable insights in the electronic structure of many molecules with "unusual" bonding motifs. SCGVB calculations on the FHF- anion indicate that the bonding in this molecule cannot be attributed to a traditional hydrogen bond or a traditional covalent bond. Instead, the bonds in FHF- represent a new bonding motif-two polarized, delocalized F- anions held together by a positively charged hydrogen atom, i.e., the bonding resembles that for a proton-bound anion pair.

The microwave, rotational spectrum between 5.6 and 19.7 GHz of the gas-phase heterodimer formed between acetylene and (E)-1-chloro-1,2-difluoroethylene is obtained using both broadband, chirped-pulse and narrow band, Balle-Flygare Fourier transform microwave spectrometers. The structure of the complex is determined from the rotational constants obtained via the analysis of the spectra for the normal isotopologue of the complex and three isotopically substituted species: the singly substituted 37Cl isotopologue, obtained in natural abundance, and two isotopologues singly substituted with 13C, obtained using an isotopically enriched HC13CH sample. The acetylene forms a hydrogen bond with the fluorine atom on singly halogenated carbon and a secondary interaction with the hydrogen atom on that same carbon. The angle strain induced in forming the secondary interaction is offset by the favorable electrostatics of the hydrogen bond to fluorine. Comparisons with acetylene complexes of 1,1,2-trifluoroethylene and cis-1,2-difluoroethylene show the effects of halogen substitution at the remote carbon on this bonding motif.

The C(sp3)-H functionalization of O-pentafluorobenzoyl ketone oximes was implemented under visible light irradiation with copper complexes as catalysts. The reactions involve iminyl-radical-mediated intramolecular hydrogen atom transfer as the key step, with the iminyl radicals being generated via copper-effected N-O cleavage. The reaction afforded 3,4-dihydro-2H-pyrroles under the conditions of [Cu(DPEphos)(bcp)]PF6 and DABCO, while γ-pentafluorobenzoyloxy ketones were produced predominantly when [Cu(dpp)2]PF6 and InCl3·4H2O were used as catalysts.

The photochemical deracemization of 2,4-disubstituted 2,3-butadienamides (allene amides) was investigated both experimentally and theoretically. The reaction was catalyzed by a thioxanthone which is covalently linked to a chiral 1,5,7-trimethyl-3-azabicyclo[3.3.1]nonan-2-one skeleton providing a U-shaped arrangement of the sensitizing unit relative to a potential hydrogen-bonding site. Upon irradiation at λ = 420 nm in the presence of the sensitizer (2.5 mol %), the amides reached at -10 °C a photostationary state in which one enantiomer prevailed. The enantioenriched allene amides (70-93% ee) were isolated in 74% to quantitative yield (19 examples). Based on luminescence data and DFT calculations, energy transfer from the thioxanthone to the allene amides is thermodynamically feasible, and the achiral triplet allene intermediate was structurally characterized. Hydrogen bonding of the amide enantiomers to the sensitizer was monitored by NMR titration. The experimental association constants (Ka) were similar (59.8 vs 25.7 L·mol-1). DFT calculations, however, revealed a significant difference in the binding properties of the two enantiomers. The major product enantiomer exhibits a noncovalent dispersion interaction of its arylmethyl group to the external benzene ring of the thioxanthone, thus moving away the allene from the carbonyl chromophore. The minor enantiomer displays a CH-π interaction of the hydrogen atom at the terminal allene carbon atom to the same benzene ring, thus forcing the allene into close proximity to the chromophore. The binding behavior explains the observed enantioselectivity which, as corroborated by additional calculations, is due to a rapid triplet energy transfer within the substrate-catalyst complex of the minor enantiomer.

To examine the effect of 5s2 lone electron pair of antimony atom on the reaction of antimony trifluoride and triethanolamine in the presence of sodium methylate, the crystal structure of the reaction product -2-fluoro-6-(2-hydroxyethyl)-1,3-dioxa-6-aza-2-stibacy-cylooctane (1-fluoro-2-hydrostibatrane) FSb (OCH2CH2)2NCH2CH2OH) was confirmed. In the compound structure, the hydrogen atom of the 2-hydtoxyethyl group of each molecule forms an intermolecular hydrogen bond with the oxygen atom of one of the five-membered SbOCH2CH2N half-cycles in a neighbouring molecule. A geometry of both five-member N-C-C-O-Sb heterocycles, end-capped by transannular N→Sb bond in the 1-fluoro-2-hydrostibatrane molecule, is almost identical. C-O, C-C, N-C interatomic distances and valence angles in two endocyclic units (NCCOSb) are comparable to those observed in RSi(OCH2CH2)3N silatranes. A coordination polyhedron of the Sb atom can be represented as a transition from a bisphenoid to Sb(O3)N trigonal pyramid, with a nitrogen atom at the apex and three oxygen atoms in the base. The N→Sb transannular coordinate bond length is 2.402(4) Å, which is 0.40 Å greater than the Sb-N covalent bond standard length. The Sb-F bond (1.997(4) Å) is 0.12 Å longer than that in the SbF3 molecule, and insignificantly shorter than that of the Sb-Fax (2.028(3) Å) in the SbF3Gly crystalline complex. The fluorine atom substantially strays from the N→Sb axis to the direction of O(1) and O(2) atoms. The oxygen atom of the 2-hydroxyethyl group lies at a distance of 2.899(3) A from that of Sb, intermediate between the valence bond length and the sum of the Van der Waals radii of these atoms. Combined with the F atom position, one can assume the 1-fluoro-2-hydrostibatrane crystal structure as a “frozen” state of the SNi(Sb) type nucleophilic attack of the oxygen atom, uncompleted because of its repulsion by the 5s2 lone electronic pair of antimony atom.

Phthalimide N-oxyl (PINO) is a potent hydrogen atom transfer (HAT) catalyst that can be generated electrochemically from N-hydroxyphthalimide (NHPI). However, catalyst decomposition has limited its application. This paper details mechanistic studies of the generation and decomposition of PINO under electrochemical conditions. Voltammetric data, observations from bulk electrolysis, and computational studies suggest two primary aspects. First, base-promoted formation of PINO from NHPI occurs via multiple-site concerted proton-electron transfer (MS-CPET). Second, PINO decomposition occurs by at least two second-order paths, one of which is greatly enhanced by base. Optimal catalytic efficiency in PINO-catalyzed oxidations occurs in the presence of bases whose corresponding conjugate acids have pKa's in the range of ∼11-15, which strikes a balance between promoting PINO formation and minimizing its decay.

Fe, Co, and Mn hydride-initiated radical olefin additions have enjoyed great success in modern synthesis, yet the extension of other hydrogen radicalophiles instead of olefins remains largely elusive. Herein, we report an efficient Fe-catalyzed intramolecular isonitrile–olefin coupling reaction delivering 3-substituted indoles, in which isonitrile was firstly applied as the hydrogen atom acceptor in the radical generation step by MHAT. The protocol features low catalyst loading, mild reaction conditions, and excellent functional group tolerance., A mild and efficient method has been developed to synthesize 3-substituted indoles via an Fe-catalyzed radical isonitrile–olefin coupling reaction initiated by MHAT to isonitriles.

Avenanthramides (AVAs), unique phenolic compounds in oats, have attracted increasing interest due to their health benefits. Eight representative AVAs were studied using the density functional theory (DFT) method to elucidate their antioxidant activity and mechanism. Preference of different mechanisms was evaluated based on thermodynamic descriptors involved in double (2H+/2e-) free radical scavenging reactions. It was found that the hydrogen atom transfer (HAT) mechanism is more favorable in the gas and benzene phases, while sequential proton loss electron transfer (SPLET) is preferred in polar media. The results suggest the feasibility of the double HAT and double SPLET mechanisms for 2s and c-series AVAs. The sequential triple proton loss double electron transfer (StPLdET) mechanism represents the dominant pathway in aqueous solution at physiological pH. In addition, the sequential proton loss hydrogen atom transfer (SPLHAT) mechanism provides an alternative pathway to trap free radicals. Results also revealed the important role of the catechol, guaiacyl, and carboxyl moieties.

The antioxidative activity of natural product compound amurensin H (1) and its three synthetic compounds 5-(6-hydroxy-2-(4-hydroxyphenyl)benzofuran-3-yl)benzene-1,3-diol (2), (E)-2-(4-hydroxyphenyl)-4-(4-hydroxystyryl)benzofuran-6-ol (3), and (E)-5-(6-hydroxy-4-(4-hydroxystyryl)benzofuran-3-yl)benzene-1,3-diol (4) are extensively studied by DFT (density functional theory) method. At the theoretical B3LYP/6-311G(d,p) level, the HAT (hydrogen atom transfer) mechanism is assignable to compounds 1-4 in gas, but the SPL-ET (sequential proton loss-electron transfer) mechanism is the main way in water and methanol. The antioxidative actions of the studied compounds are mainly based on OH bond breakage, especially OH groups directly substituted at benzofuran nucleus. Amurensin H (1) is better than the remaining compounds due to 6-OH group. 1-6-OH induces the low bond BDE (bond dissociation enthalpy) and PA (proton affinity) values in thermal analysis. Importantly, the kinetic reaction of 1-6-OH + HOO• in methanol exerts the lowest ΔG# value of 5.6 kcal/mol and the highest K value of 1.983 x 1010 L/mol.s. At the B3LYP/6-311G(d,p)/LANL2DZ level, the successful formation of complex [Zn(compound 1)2(H2O)2] at 6-OH is confirmed by molecular orbital and UV-Vis spectroscopic analyses.

Photoredox-catalyzed C-O bond formation reactions are reported. The decarboxylative esterification reaction allows the conversion of a variety of arylacetic acids into the corresponding benzyl carboxylates. Furthermore, the use of (diacetoxyiodo)benzene allows the conversion of the benzylic C-H bond through hydrogen atom transfer. The reactions were applied to the divergent transformation of pharmaceuticals via decarboxylative or C-H esterification reactions.

The chemical dynamics of the elementary reaction of ground state atomic silicon (Si; 3 P) with germane (GeH4 ; X1 A1 ) were unraveled in the gas phase under single collision condition at a collision energy of 11.8±0.3 kJ mol-1 exploiting the crossed molecular beams technique contemplated with electronic structure calculations. The reaction follows indirect scattering dynamics and is initiated through an initial barrierless insertion of the silicon atom into one of the four chemically equivalent germanium-hydrogen bonds forming a triplet collision complex (HSiGeH3 ; 3 i1). This intermediate underwent facile intersystem crossing (ISC) to the singlet surface (HSiGeH3 ; 1 i1). The latter isomerized via at least three hydrogen atom migrations involving exotic, hydrogen bridged reaction intermediates eventually leading to the H3 SiGeH isomer i5. This intermediate could undergo unimolecular decomposition yielding the dibridged butterfly-structured isomer 1 p1 (Si(μ-H2 )Ge) plus molecular hydrogen through a tight exit transition state. Alternatively, up to two subsequent hydrogen shifts to i6 and i7, followed by fragmentation of each of these intermediates, could also form 1 p1 (Si(μ-H2 )Ge) along with molecular hydrogen. The overall non-adiabatic reaction dynamics provide evidence on the existence of exotic dinuclear hydrides of main group XIV elements, whose carbon analog structures do not exist.

Hydrogen-atom-transfer (HAT) is an efficient way for direct C H functionalization of inert C H bonds, therefore it has attracted great interests in recent years. So far, various HAT catalysts have been developed. Among them, quinuclidine and its derivatives show different characters toward other HAT catalysts as they tend to abstract electron-rich and hydridic hydrogens in the presence of weak and neutral C H bonds. These features enable direct C H functionalization of compounds with various groups which are unable or difficult by other methods. This review summarizes recent advance of photoinduced reactions with quinuclidine and its derivatives as HAT catalysts and exhibits powerful synthetic potential by using quinuclidine and its derivatives as HAT catalysts.

Hydrogen peroxide (H2O2) has recently received much attention as a safe and clean energy carrier for hydrogen molecules. In this study, based on direct ab initio molecular dynamics (AIMD) calculations, we demonstrated that H2O2 is directly formed via the photoelectron detachment of O-(H2O)n (n = 1-6) (water clusters of an oxygen radical anion). Three electronic states of oxygen atoms were examined in the calculations: O(X)(H2O)n (X = 3P, 1D, and 1S states). After the photoelectron detachment of O-(H2O)n (n = 1) to the 1S state, a complex comprising O(1S) and H2O, O(1S)-OH2, was formed. A hydrogen atom of H2O immediately transferred to O(1S) during an intracluster reaction to form H2O2 as the final product. Simulations were run to obtain a total of 33 trajectories for n = 1 that all led to the formation of H2O2. The average reaction time of H2O2 formation was calculated to be 57.7 fs in the case of n = 1, indicating that the reaction was completed within 100 fs of electron detachment. All the reaction systems O(1S)(H2O)n (n = 1-6) indicated the formation of H2O2 by the same mechanism. The reaction times for n = 2-6 were calculated to range between 80 and 180 fs, indicating that the reaction for n = 1 is faster than that of the larger clusters, that is, the larger the cluster size, the slower the reaction is. The reaction dynamics of the triplet O(3P) and singlet O(1D) potential energy surfaces were calculated for comparison. All calculations yielded the dissociation product O(X)(H2O)n → O(X) + (H2O)n (X = 3P and 1D), indicating that the O(1S) state contributes to the formation of H2O2. The reaction mechanism was discussed based on the theoretical results.

DFT calculations on the photoisomerization of hydrazones of 1,2,4-oxadiazole derivatives to 1,2,5-triazoles have been performed showing that the reaction occurred through the first excited singlet state. The Z isomer gave the reaction through a hydrogen atom transfer of the hydrazonic nitrogen atom to the nitrogen atom in four position on the oxadiazole ring. In this case, the isomerization was a concerted reaction. The E isomer could undergo the same reaction. However, it could not be a concerted reaction but required the presence of a ring opening intermediate.

Quantum chemical methods are used to calculate the electron density of molecules and positions of absorption bands of the N–H group of pyrrole, indole, carbazole in the presence and in the absence of H-complexes with an acetone molecule. It is established that the effectiveness of H-complex formation decreases together with the positive charge on the hydrogen atom of the N–H bond. The efficiency of H-bond formation is estimated by the low-frequency shift of the absorption peak corresponding to the N–H vibration of pyrrole derivatives. The obtained results suggest that the Coulomb component is not prevalent in the formation of H-complexes of pyrrole derivatives with acetone while the covalent component makes a more significant contribution.

The first electrochemical synthesis of diarylphosphole oxides (DPOs) was achieved under mild conditions. The practical protocol employs commercially available and inexpensive DABCO as a hydrogen atom transfer (HAT) mediator, leading to various DPOs in moderate to good yields. This procedure can also be applied to the synthesis of six-membered phosphacycles, such as phenophosphazine derivatives. Mechanistic studies suggested that the reaction proceeds via an electro-generated phosphinyl radical.

Olefins are prevalent substrates and functionalities. The synthesis of olefins from readily available starting materials such as alcohols, amines and carboxylic acids is of great significance to address the sustainability concerns in organic synthesis. Metallaphotoredox-catalyzed defunctionalizations were reported to achieve such transformations under mild conditions. However, all these valuable strategies require a transition metal catalyst, a ligand or an expensive photocatalyst, with the challenges of controlling the region- and stereoselectivities remaining. Herein, we present a fundamentally distinct strategy enabled by electron donor–acceptor (EDA) complexes, for the selective synthesis of olefins from these simple and easily available starting materials. The conversions took place via photoactivation of the EDA complexes of the activated substrates with alkali salts, followed by hydrogen atom elimination from in situ generated alkyl radicals. This method is operationally simple and straightforward and free of photocatalysts and transition-metals, and shows high regio- and stereoselectivities., A visible-light-induced defunctionalization strategy for the synthesis of olefins by using easily available alcohols, amines and carboxylic acids as starting materials is demonstrated.

Minisci-type reactions constitute one of the most powerful methods for building up complexity around basic heteroarenes. The most desirable variants involve formal oxidative coupling of a C-H bond on each partner, leading back to the simplest possible starting materials. We herein disclose a method that enables such a coupling of linear amides and heteroarenes with full control of enantioselectivity at the newly formed stereocenter as well as site selectivity on both the heteroarene and the amide. This is achieved by the use of a chiral phosphoric acid catalyst in conjunction with diacetyl as a combined hydrogen atom transfer reagent and oxidant. Diacetyl is directly photoexcitable, and thus, no extraneous photocatalyst is required: an added feature that contributes to the simplicity and practicality of the protocol.

Asymmetric functionalization of inert C(sp3)-H bonds is a straightforward approach to realize versatile bond-forming events, allowing the precise assembly of molecular complexity with minimal functional manipulations. Here, we describe an asymmetric photocatalytic C(sp3)-H bond addition to α-substituted acrylates by using tetrabutylammonium decatungstate (TBADT) as a hydrogen atom transfer (HAT) photocatalyst and chiral phosphoric acid as a chiral proton-transfer shuttle. This protocol is supposed to occur via a radical/ionic relay process, including a TBADT-mediated HAT to cleave the inert C(sp3)-H bond, a 1,4-radical addition, a back hydrogen abstraction, and an enantioselective protonation. A variety of inert C-H bond patterns and α-substituted acrylates are well tolerated to enable the rapid synthesis of enantioenriched α-stereogenic esters from simple raw materials.

The catalytic application of the bis(amino)cyclopropenium ion has been investigated in conjugate addition reactions. The hydrogen atom, which is attached to the cyclopropene ring of bis(amino)cyclopropenium salts, is moderately acidic and can potentially serve as a hydrogen-bond donor catalyst in some organic transformations. This hypothesis has been successfully realized in the 1,6-conjugate addition reactions of p-quinone methides with various nucleophiles such as indole, 2-naphthol, thiols, phenols, and so forth. The spectroscopic studies (NMR and UV-vis) as well as the deuterium isotope labeling studies clearly revealed that the hydrogen atom (C-H) that is present in the cyclopropene ring of the catalyst is indeed solely responsible for catalyzing these transformations. In addition, these studies also strongly indicate that the C-H hydrogen of the cyclopropene ring activates the carbonyl group of the p-quinone methide through hydrogen bonding.

In this study, solvent-assisted co-grinding method is used to form the cocrystal of 8-hydroxy quinoline-5-sulfonic acid (HQS) and 5-chloro-8-hydroxyquinoline (CHQ). In order to determine spectroscopic and electronic properties, the theoretical characterization has been carried out. Charge delocalization patterns and second-order perturbation energies of the most interacting orbitals have also been computed and predicted. Geometrical parameters are in agreement with reported values. Molecular electrostatic potential plot predicts the reactive sites and electropositive potential region is around the hydrogen atom bonded through the nitrogen atoms, negative potentials on oxygen atoms and phenyl rings. Molecular docking of the study of the HQS-CHQ molecule has been performed for the receptors, 3C52, 4IIT, 3QYD and 4FGY.

The densities, ρ, speeds of sound, u, of binary piperidine or 1-methylpiperidine (1) + 2-pyrrolidinone or pyridine (2), and densities, ρ123, speeds of sound, u123, of ternary piperidine or 1-methylpiperidine (1) + 2-pyrrolidinone (2) + pyridine (3) mixtures and molar heat capacities, CP, for the aforesaid binary mixtures have been measured over the entire composition range at temperatures ranging from (293.15–308.15) K with an increment of 5 K. The excess molar volumes $$V^{\text{E}}$$ , $$V_{123}^{\text{E}}$$ , excess isentropic compressibilities, $$\kappa_{S}^{\text{E}}$$ , $$(\kappa_{S}^{\text{E}} )_{123}$$ for the binary and ternary mixtures and excess heat capacities, $$C_{P}^{\text{E}}$$ , for investigated binary mixtures (determined by using experimental data) were fitted with the Redlich–Kister equations, and adjustable binary as well as ternary parameters along with the standard deviations have been estimated. The analyses of $$V^{\text{E}}$$ , $$\kappa_{S}^{\text{E}}$$ , $$C_{P}^{\text{E}}$$ data, quantum mechanical calculations and IR spectra of binary mixtures have revealed that piperidine (1) + 2-pyrrolidinone (2) mixtures are characterized by interactions between nitrogen and hydrogen atoms of piperidine with the hydrogen and oxygen atoms of 2-pyrrolidinone; 1-methylpiperidine (1) + 2-pyrrolidinone (2) mixtures are characterized by interactions between nitrogen and electron deficient carbon atom of 1-methylpiperidine with hydrogen and oxygen atoms of 2-pyrrolidinone; piperidine (1) + pyridine (2) mixtures are characterized by interactions between the hydrogen atom of piperidine with the nitrogen atom of pyridine; 1-methylpiperdine (1) + pyridine (2) mixtures are characterized by the interaction of the nitrogen atom of 1-methylpiperidine with the Π-electron cloud of pyridine’s aromatic ring.

In combustion, enols can undergo keto-enol tautomerizations, which are intermediate steps in the formation of pollutant species. In this work, we performed a theoretical kinetic study of the step-wise propen-2-ol tautomerization catalyzed by hydrogen and hydroperoxyl radicals. Ab initio calculations at the CCSD(T)/aug-cc-pVTZ//M06-2X/cc-pVTZ level were run, and rate constants were calculated using the multistructural torsional variational transition state theory with small-curvature tunneling corrections. Hydrogen and hydroperoxyl radicals can induce a step-wise mechanism toward keto formation with a lower barrier than that of unimolecular tautomerization. The potential energy surface comprising these reactions is complex, involving different intermediates that are connected by different types of pathways. The hydrogen-assisted tautomerization consists of two steps where the formation of an intermediate radical takes place as a result of the addition of the hydrogen atom to the double bond of propen-2-ol. The high-pressure limit rate constants of the reactions of this intermediate radical toward propen-2-ol and acetone exhibit an Arrhenius behavior, in agreement with previous works. In the hydroperoxyl-assisted tautomerization, the acetone formation has two routes involving an overall of four steps. The route with the highest energy barrier becomes prominent above 800 K due to multistructural anharmonicity effects, which must be included for an accurate kinetic description of the titled reactions. Calculations of pressure-dependent rate constants showed that the original system-specific quantum Rice-Ramsperger-Kassel theory, together with the modified strong collision model (SS-QRRK/MSC), significantly underpredict the bimolecular stabilization rate constants for the hydrogen-assisted tautomerization above 1200 K by factors of up to three orders of magnitude when compared with the benchmark Rice-Ramsperger-Kassel-Markus/master equation method. To solve this problem, we tested two alternative definitions of the collision efficiency parameter by using an improved implementation of the SS-QRRK/MSC approach developed by us for chemically activated reactions. One of these definitions, provided by Gilbert et al. (1983), corrected the bimolecular stabilization rate constant behavior and yielded a maximum deviation factor of only 4.5 at 2000 K and 100 atm. For the hydroperoxyl-assisted tautomerization, pressure effects are negligible because the stabilization of the energized adduct cannot compete with the reaction leading to the final product for most of the physical conditions studied. Our calculated rate constants can be used to perform more accurate kinetic modeling of alcohols. Besides, the implementation of the SS-QRRK theory with the collision efficiency of Gilbert et al. (1983) proposed in this work is useful for computing pressure-dependent rate constants of chemically activated reactions, including all possible refinements (multi-dimensional tunneling, multistructural anharmonicity, etc.) considered in high-pressure limit calculations.

In recent years, advancements in photocatalysis have allowed for a plethora of chemical transformations under milder conditions. Many of these photochemical reactions utilize hydrogen atom transfer processes to obtain desired products. Hydrogen atom transfer processes can follow one of two unique pathways: the first, a direct path and the second, an indirect path. In this paper, we highlight the ability of eosin Y to act as a direct hydrogen atom transfer catalyst from both synthetic and computational chemistry perspectives.

Acetic acid CH3C(O)OH attracts significant attention in interstellar chemistry because it is considered to be a potential precursor for the formation of amino acids, such as glycine and alanine. Th...

We present an efficient and versatile visible light-driven methodology to transform aryl aldehydes and ketones chemoselectively either to alcohols or to pinacol products with CdSe/CdS core/shell quantum dots as photocatalysts. Thiophenols were used as proton and hydrogen atom donors and as hole traps for the excited quantum dots (QDs) in these reactions. The two products can be switched from one to the other simply by changing the amount of thiophenol in the reaction system. The core/shell QD catalysts are highly efficient with a turn over number (TON) larger than 4 X 10(4) and 4 X 10(5) for the reduction to alcohol and pinacol formation, respectively, and are very stable so that they can be recycled for at least 10 times in the reactions without significant loss of catalytic activity. The additional advantages of this method include good functional group tolerance, mild reaction conditions, the allowance of selectively reducing aldehydes in the presence of ketones, and easiness for large scale reactions. Reaction mechanisms were studied by quenching experiments and a radical capture experiment, and the reasons for the switchover of the reaction pathways upon the change of reaction conditions are provided.

Cyclic guanidines are found in many biologically active compounds and natural products. Further, the for-mation of the atypical 7-membered ring of cyclic guanidine remains challenging due to a lack of efficient preparation strategies and low yield. Herein, a catalytic synthetic method for cyclic guanidines was developed via transition-metal hydrogen atom transfer and radical-polar crossover. This mild and functional-group tolerant process enabled the cycliza-tion of an alkenyl guanidines bearing common protective groups, such as Cbz and Boc groups. This powerful method not only provided typical 5- and 6-membered rings but also the atypical 7-membered ring. The derivatization of the products afforded various heterocycles. We also investigated the selective cyclization of mono-protected or hetero-protected (TFA and Boc) alkenyl guanidines and their further derivatizations.