Analysis of the reactivities of organic compounds in hydrogen atom abstraction from their C-H bonds by the sulfate radical anion SO 4 ·−

An experimental rate constant data array (>50 values) for the reactions of the sulfate radical anion with the C-H bonds of various organic compounds is analyzed by Denisov’s intersecting parabolas method. The data array is divided into the following four groups according to the type of the C-H bond attacked and the br _e value: compounds with primary C-H bonds (br _e = 13.14 ± 0.17), compounds with secondary C-H bonds remote from the polar groups (br _e = 12.64 ± 0.34), compounds with secondary α-C-H bonds (br _e = 13.28 ± 0.24), and compounds with tertiary C-H bonds. It is demonstrated by DFT calculations that the rate constant of the reaction SO ₄ ^·? + RH is determined by nonspecific solvation and by the ion-dipole interaction in the transition state of the reaction.     

Metal-free oxidative C-C bond formation of active methylenic sp C-H bonds with benzylic sp C-H and allylic sp C-H bonds mediated by DDQ

An efficient and simple method for the oxidative coupling of benzylic and allylic sp³ C-H bonds with active methylenic sp³ C-H bonds under metal-free conditions was developed by employing 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) as an oxidant. The reaction was shown to proceed smoothly for various 1,3-dicarbonyl compounds with a range of benzylic and allylic substrates in good to excellent yields.     

Reaction of propionitrile with methylidyne: A theoretical study

The results of a CCSD(T)-F12/cc-pVTZ-f12//ωB97XD/cc-pVTZ quantum-chemical study of the potential energy surface (PES) for the reaction of propionitrile with methylidyne are combined with Rice-Ramsperger-Kassel-Marcus (RRKM) calculations of the reaction rate constants and product branching ratios in the deep space conditions corresponding to the zero-pressure limit at various collision energies. The most energetically favorable reaction pathways have been identified. The reaction outcome has been shown to strongly depend on the branching in the entrance reaction channel, between CH additions and insertions into various C-H and C-C bonds. For instance, CH addition to the N atom predominantly leads to 3H-pyrrole + H (p9), with CH₂NC + C₂H₄ (p2) also being a significant product. CH addition to the triple C≡N bond mostly results in 2-methylene-2H-azirine + CH₃ (p13), whereas CH insertions into C-H bonds in the CH₃ and CH₂ groups of propionitrile form CH₂CN + C₂H₄ (p1) and CH₂CHCN + CH₃ (p7) respectively. Less likely CH insertions into single C-C bonds yield CH₃CHCHCN + H (p5) and CH₂CHCH₂CN + H (p8). The results indicate that the methylidyne + propionitrile reaction may represent a critical step toward the formation of heterocyclic N-containing molecules in the interstellar medium and in planetary atmospheres.     

A ring-like coordination structure constructed by Cu(II) and bis(2-(pyridin-2-ylthio)ethyl)ether ligand

The reaction of bis(2-(pyridin-2-ylthio)ethyl)ether (L) with CuCl₂ in CH₃OH/CHCl₃ solution gives a novel 24-membered ring-like coordination structure which has been characterized by elemental analysis, IR, and X-ray single-crystal diffraction. The X-ray diffraction analysis reveals that [Cu₂L₂Cl₄] crystallizes in the triclinic, space group $P\bar 1$ . The ring-like structure is consisted of two ligands with two copper ions coordinated to the pyridyl nitrogen atoms from two ligands respectively. The largest diameters of the planarshaped dinuclear ring are about 12.7 Å. The adjacent ring-like units are linked via C-H…O hydrogen bonds to generate an infinite linear chain, which are further linked by C-H…Cl hydrogen bonds and weak intermolecular interactions to form a three-dimensional network.     

Photoinduced carbamoylation of ethereal C-H bonds using pentafluorophenyl isocyanate

An intermolecular carbamoylation of ethers has been achieved via photoinduced functionalization of ethereal C-H bonds in the presence of benzophenone at ambient temperature. The carbamoyl group, a one-carbon unit with a high oxidation state, derived from pentafluorophenyl isocyanate (C₆F₅NCO) is chemoselectively introduced at the position geminal to the oxygen functionality in a single step. The present reaction system effectively activates the tertiary C-H bond at the sterically hindered site of fused bicyclic systems, and enables introduction of the carbamoyl group, which is then available for further synthetic elaborations.     

Stretching vibrations of CH bonds in spectra of dicarbaclosododecaboranes (p-, m-, and o-carboranes)

Parameters of bands corresponding to stretching vibrations of C-H bonds in spectra of o-, m-, and p-isomers of the icosahedral carborane B₁₀H₁₀C₂H₂ have been measured with high accuracy. The stretching vibrations of C-H bonds in spectra of o- and m-carborane (12), molecules which have C_2v point group symmetry, are demonstrated to appear the same as in the spectrum of the p-isomer with D_5d symmetry. The frequencies of Raman and IR lines practically coincide. This indicates the absence of interaction of the stretching vibrations of the two C-H bonds even in the o-carborane molecule.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 10, pp. 2263–2267, October, 1989.     

Calculation of the magnetic properties of C-H bonds and their contribution to the magnetic susceptibility of saturated hydrocarbons

The magnetic susceptibility tensors of the C-H bonds in the CH₄, C₂H₆, C₃H₈, n-C₅H₁₂, iso-C₅H₁₂, and neo-C₅H₁₂ molecules have been calculated by the method of varying the vector potential with the use of a multiparameteric gradient-transformation function, which was constructed in the form of a polynomial in spherical coordinates and takes into account the electron correlation. The influence of the electron correlation on the magnetic properties of C-H bonds is greater than that in homonuclear molecules (H₂) and bonds (C-C) and increases strongly with enhancement of the electron correlation in the wave function not perturbed by an external magnetic field. In contrast to the previously established identity of the magnetic properties of C-C bonds, the magnetic properties of C-H bonds depend both on the structure and geometry of the molecule as a whole and on the location of the bond itself in the molecule. The values of the mean susceptibility of the molecules considered calculated in the additive approximation are in good agreement with the experimental values.     

The energy and geometric characteristics of the transition state in reactions of RO<Stack><Subscript>2</Subscript><Superscript>•</Superscript></Stack> with carbonyl compound C-H bonds

The energy and geometry of the transition state in reactions of the ethyl peroxyl radical with ethane, ethanol (its α and β C-H bonds), acetone, butanone-2, and acetaldehyde were calculated by the density functional theory method. In all these reactions (except EtO_2/? + ethanol α C-H bond), the C…H…O reaction center has an almost linear configuration (φ = 176° ± 2°); polar interaction only influences the r ^≠ (C…O) interatomic bond. In the reaction of EtO_2/? with the ethanol α C-H bond, it is the O-H…O H-bond formed in the transition state that determines the configuration of the reaction center with the angle φ(C…H…O) = 160°. The results were used to estimate the r ^≠ (C…H) and r ^≠ (O…H) interatomic bonds in the transition state by the method of intersecting parabolas and the contribution of polar interaction to the activation energy of reactions between peroxyl radicals and aldehydes and ketones.     

Ligand properties of aromatic azines: C-H activation, metal induced disproportionation and catalytic C-C coupling reactions

The reaction of aromatic azines with Fe₂(CO)₉ yields dinuclear iron carbonyl cluster compounds as the main products. The formation of these compounds may be rationalized by a C-H activation reaction at the aromatic substituent in ortho position with respect to the exocyclic C-N double bond followed by an intramolecular shift of the corresponding hydrogen atom toward the former imine carbon atom. The second imine function of the ligand does not react. Additional products arise from the metal induced disproportionation of the azine into a primary imine and a nitrile. So also one of the imine C-H bonds may be activated during the reaction. Depending on the aromatic substituent of the azine ligands iron carbonyl complexes of the disproportionation products are isolated and characterized by X-ray crystallography. C-C coupling reactions catalyzed by Ru₃(CO)₁₂ result in the formation of ortho-substituted azines. In addition, ortho-substituted nitriles are identified as side-products showing that the metal induced disproportionation reaction also takes place under catalytic conditions.     

Theoretical investigation on cyclohexane dehydrogenation catalyzed by V<Subscript>2</Subscript><Superscript>+</Superscript> in gas-phase

The dehydrogenation reaction mechanism of cyclohexane catalyzed by dimer transition metal cluster V₂⁺ has been investigated at the B3LYP/6-31G (d, p) level of density functional theory. Density of states (DOS) graph is used to understand more deeply the roles of the front molecular orbital of the initial complexes. After the first molecular dehydrogenation, the reaction mainly consists of two competition mechanisms. First, the C-H bonds of cyclohexane can be effectively activated by the V₂⁺ cation, yielding the same-face dehydrogenation products. Second, the C-C bonds are activated, forming the different-face dehydrogenation products. Our calculations indicate that the reaction takes place more easily along the low-spin potential energy surface on the same-face and is a low-barrier or even barrier-free transformation. Carbon-carbon single bonds are nonpolar and generally far less reactive. A comparison of the reaction mechanism of V₂⁺ and congener Ti₂⁺ with cyclohexane has been presented. The bond dissociation energies (BDEs) of V₂⁺ are greater than that of Ti₂⁺, leading to difficulties in forming sandwich complexes in the different-face dehydrogenation of cyclohexane, and the same-face dehydrogenation is an important reaction channel.     

Intermolecular interactions in the acyclic tetraazagold(III) metal complexes

The packings of the crystal structures of the gold(III) iodide, perchlorate, and hexafluorophosphate complexes with the iminate-amine ligand, [Au(C₉H₁₉N₄)]Y₂ (Y = I (I), ClO₄ (II), and PF₆ (III)), are studied in comparison. The packings of structures I–III are similar. They are determined and stabilized by a vast network of intermolecular interactions (hydrogen bonds N-H…A, C-H…A, C-H…Au, and C-H…π and short contacts Au…A (A = I, O, F)). Structures I–III are characterized by cation-cationic and cationanionic infinite chains and closed rings formed by classical and nonclassical hydrogen bonds and contacts. The structures include multicentered hydrogen bonds. The replacement of the anion does not basically change the packing structures.     

Oxygen transfer catalysis by chemical analogs of monooxygenases

New chemical analogs of monooxygenases and new reactions of alkane oxidation mediated by metal complexes are reviewed. Mechanisms of dioxygen and alkane activation and those of oxygen transfer from O₂ and active oxygen donors to C-H bonds via ferryl and other intermediates are discussed.Submitted as an invited lecture at the VIIIth International Symposium on Homogeneous Catalysis held on August 2–7, 1992 (Amsterdam, the Netherlands).Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 272–279, February, 1993.     

Reactivity of C-H bonds in cyclohexanone and 1-tert-butylperoxycyclohexanol toward the tert-butylperoxyl radical

The kinetics of oxygen uptake and the composition of the cyclohexanone oxidation products in the azobisisobutyronitrile-initiated oxidation of cyclohexanone in the presence of tert-butyl hydroperoxide have been investigated by the Howard-Ingold method. The partial rate constants of the reaction of the tert-butylperoxyl radical with the C-H bonds of cyclohexanol and 1-tert-butylperoxycyclohexanole at 333 K have been determined. The carbonyl group of cyclohexanone activates the C-H bonds in the 2- and 6-positions (α) and deactivates the C-H bonds in the 3- and 5-positions (β) compared to the C-H bonds in the 4-position (γ), whose reactivity is similar to that of the methylenic C-H bonds in cyclohexane. Evaluation of the joint effect of the hydroxyl and tert-butylperoxyl groups in 1-tert-butylperoxycyclohexanol suggests a considerable deactivation of the C-H bonds in the 2- and 6-positions (β) and, to a lesser extent, in the 3- and 5-positions (γ).     

The versatile role of norbornene in C-H functionalization processes: concise synthesis of tetracyclic fused pyrroles via a threefold domino reaction

The synthesis of novel tetracyclic fused pyrroles from 1-(2-iodophenyl)-1H-pyrrole and various bromoalkyl-aryl alkynes via a palladium(0)-catalyzed and norbornene-mediated threefold domino reaction is reported. PdCl₂ and tri-2-furylphosphine (TFP) in the presence of norbornene and Cs₂CO₃ in CH₃CN at 90 °C gave a variety of tetracyclic fused pyrroles in usually high yields. In the described reaction sequence two of the three carbon-carbon bonds are formed by functionalization of an unactivated aryl C-H bond.     

Utilisation of 1,3-dialkylimidazolium-2-carboxylates as CO2-carriers in the presence of Na and K: application in the synthesis of carboxylates, monomethylcarbonate anions and halogen-free ionic liquids

1,3-Dialkylimidazolium-2-carboxylate compounds have recently been fully characterised. Here we describe the utilisation of 1,3-dimethyl (1a) and 1-butyl-3-methyl-imidazolium-2-carboxylate (1b) in a carboxylation reaction with transfer of the CO₂ moiety to benzoylacetone and methanol for the synthesis, in high yield, of benzoylacetate and monoalkylcarbonate anions, respectively. Conversely, when compounds 1a and b are reacted with carbonylic substrates lacking C-H active bonds, a product resulting from nucleophilic attack of the 1,3-dialkylimidazol-2-ylidene species on the carbonyl moiety is obtained. The reported reactions can find applications in organic synthesis and in the synthesis of halogen-free ionic liquids.     

Structures and reactions of radical cations of some bicycloalkanes and their related alkenes as studied by 4 k matrix esr

The σ radical cations of most typical bicycloalkanes such as norbornane and bicyclo[2,2,2]octane are radiolytically produced at 4 K in halogenocarbon matrices and are studied by ESR spectroscopy. Their electronic and geometrical structures as well as their dynamical behaviors have been elucidated from the hyperfine structures and their temperature changes. The semi occupied molecular orbital (SOMO) of the former cation is 4a₂, in which the unpaired electron delocalizes over the four exo C-H bonds giving large hyperfine coupling. The latter is a Jahn-Teller active species and exhibits static distortion from D_3h to C_2v at 4 K in CFCl₃, and the SOMO is likely to be 6b₂, in which the unpaired electron delocalizes over the four endo C-H bonds giving large proton coupling, although a dynamically averaged structure with 12 equivalent methylene protons is observed in C-C₆F₁₂ as well as in CFCl₂CF₂Cl matrices at 77 K. The unpaired electron distribution in bicycloalkane radical cations is similar to that in cycloalkane radical cations previously studied. Upon warming both the cations undergo deprotonation to give 2-yl alkyl radicals from the exo or endo C-H bond, at which the higher unpaired electron density is populated. In addition to these radical cations, the structures and reactions of the radical cations of the related bicycloalkenes such as norbornadiene, quadricyclane, and bicyclo[2,2,2]octene have also been studied. The hydride ion transfer to an olefinic radical cation to form an alkyl radical is observed for the bicyclo[2,2,2]octene radical cation as the first example observed by ESR.     

Chemical bond inside endohedral complexes H@C59B and H@C59P

The geometries, electronic structures, and spin densities on the atoms of paramagnetic heterofullerenes C₅₉X (X = B, P) and their endohedral derivatives H@C₅₉X were obtained from B3LYP/6-31G* density functional calculations. The encapsulated hydrogen atom can form a C-H bond inside the fullerene sphere. The energies of the C-H(endo) bonds are 40–50 kcal mol⁻¹ lower than those of the corresponding exo-bonds. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1239–1243, July, 2007.     

Intramolecular oxidative addition of C-F and C-H bonds to [Pt(dba)2]. Crystal structure of [PtCl{Me2NCH2CH2NCH(2,4,5-C6HF3)}]

The reactions of compound [Pt(dba)₂] with ligands RCHNCH₂CH₂NMe₂ (1a-1f) in which R is a fluorinated aryl ring produced activation of C-F bonds when two fluorine atoms are present in the ortho positions of the aryl ring or activation of C-H bonds for ligands containing only one fluoro substituent in ortho. Both C-F and C-H bond activation are favoured by an increase of the degree of fluorination of the ring. Further reaction with lithium halides produced cyclometallated platinum (II) compounds [PtX(Me₂NCH₂CH₂NCHR)] (X = Br, Cl) (2) containing a terdentate [C,N,N′] ligand. The obtained compounds were fully characterized including a structure determination for [PtCl{Me₂NCH₂CH₂NCH(2,4,5-C₆HF₃)}] (2d′).     

Amide-directed arylation of sp C-H bonds using Pd(II) and Pd(0) catalysts

Protocols to effect β-arylation of sp³ C-H bonds via Pd(II)/(IV) and Pd(0)/(II) catalytic cycles have been achieved using a newly developed monodentate CONHC₆F₅ directing group. These reactions provide an unprecedented means to functionalize sp³ C-H bonds in aliphatic carboxylic acid-derived substrates.     

Synthesis and crystal structures of two new Schiff base cobalt(II) and nickel(II) complexes

The Schiff base ligand (HL) obtained from phenylmethanamine and 5-methoxysalicylaldehyde are used as ligands for Co(II) and Ni(II) resulting in complexes [Co(L)₂] (I) and [Ni(L)₂] (II), and their solid state structures were determined by X-ray crystallography. In both complexes, weak interactions play an important role in the molecular self-assembly. Complex I was stacked up to the 2D layers by C-H…O hydrogen bonds and C-H…π interactions. In contrast, complex II was extended into 2D sheet by C-H…O hydrogen bonds, the C-H…π interactions, and edge-to-face interactions.