Regioselective alkenylation of imidazoles by nickel/Lewis acid catalysis

Nickel/Lewis acid binary catalysis is found effective to direct regioselective alkenylation of imidazoles through C-H bond activation and stereoselective insertion of alkynes. Use of P(t-Bu)₃ as a ligand allows exclusive regioselective C(2)-alkenylation, while PCyp₃ is found effective for C(5)-alkenylation of C(2)-substituted imidazoles. The reaction demonstrates a broad scope of imidazoles and internal alkynes to give trisubstituted ethenes highly regio- and stereoselectively in modest to good yields.     

Oxidative cyclization of 2-arylphenols to dibenzofurans under Pd(II)/peroxybenzoate catalysis

2-Arylphenols undergo intramolecular C-H bond activation/C-O bond formation to afford dibenzofuran derivatives under palladium catalysis in the presence of tert-butyl peroxybenzoate as an oxidant. Kinetic isotope effect experiments indicated that C-H bond cleavage is the rate-limiting step of the reaction.     

Difluoromethylation of some C-H acids with chlorodifluoromethane under conditions of phase transfer catalysis (PTC)

Selected C-H acids react with difluorocarbene generated from chlorodifluoromethane with concentrated aqueous solution of sodium hydroxide, and a catalyst, benzyltriethylammonium chloride (TEBAC) in benzene or THF affording C-difluoromethyl substituted derivatives. This process is restricted to C-H acids of pK_a ≅ 16.3-19.1. The observed facts are rationalized.     

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.     

Variational RRKM theory calculation of thermal rate constant for carbon—hydrogen bond fission reaction of nitro benzene

The present work provides quantitative results for the rate of unimolecular carbon-hydrogen bond fission reaction of benzene and nitro benzene at elevated temperatures up to 2000 K. The potential energy surface for each C-H (in the ortho, meta, and para sites) bond fission reaction of nitro benzene was investigated by ab initio calculations. The geometry and vibrational frequencies of the species involved in this process were optimized at the MP2 level of theory, using the cc-pvdz basis set. Since C-H bond fission channel is barrier less reaction, we have used variational RRKM theory to predict rate constants. By means of calculated rate constant at the different temperatures, the activation energy and exponential factor were determined. The Arrhenius expression for C-H bond fission reaction of nitro benzene on the ortho, meta and para sites are k(T) = 2.1 × 10¹⁷exp(?56575.98/T), k(T) = 2.1 × 10¹⁷exp(?57587.45/T), and k(T) = 3.3 × 10¹⁶exp(?57594.79/T) respectively. The Arrhenius expression for C-H bond fission reaction of benzene is k(T) = 2 × 10¹⁸exp(?59343.48.18/T). The effect of NO₂ group, location of hydrogen atoms on the substituted benzene ring, reaction degeneracy, benzene ring resonance and tunneling effect on the rate expression have been discussed.     

Highly stereoselective construction of functionalized cyclopropanes from the reaction between acetylenic esters and C-H acids in the presence of triphenylarsine

A one-pot triphenylarsine-catalyzed synthesis of trans-cyclopropane derivatives is achieved by means of the reaction between acetylenic esters and C-H acids in the presence of triphenylarsine. This procedure is simple and proceeds under mild reaction conditions. Its success depends on the choice of solvent, temperature and C-H acid used.     

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).     

Predictable and site-selective functionalization of poly(hetero)arene compounds by palladium catalysis

The challenge of achieving selective and predictable functionalizations at C-H bonds with complex poly(hetero)aromatic substrates was addressed by two different approaches. Site-selectivity can be obtained by applying various reaction conditions that are (hetero)arene specific to substrates that contain indoles, pyridine N-oxide, and polyfluorinated benzenes. An experimental classification of electron-rich heteroarenes based on their reactivity toward palladium-catalyzed C-H functionalization was established, the result of which correlated well with the order of reactivity predicted by the DFT-calculated concerted metalation-deprotonation (CMD) pathway. Model substrates containing two reactive heteroarenes were then reacted under general reaction conditions to demonstrate the applicability this reactivity chart in predicting the regioselectivity of the palladium-catalyzed direct arylation and benzylation reactions.     

Decarbonylative C-h coupling of azoles and aryl esters: unprecedented nickel catalysis and application to the synthesis of muscoride a

A nickel-catalyzed decarbonylative C-H biaryl coupling of azoles and aryl esters is described. The newly developed catalytic system does not require the use of expensive metal catalysts or silver- or copper-based stoichiometric oxidants. We have successfully applied this new C-H arylation reaction to a convergent formal synthesis of muscoride A.     

Distribution of photo-cross-linked products from 3-aryl-3-trifluoromethyldiazirines and alcohols

The photolysis of N-[4-(3-trifluoromethyl-3H-diazirin-3-yl)benzoyl]-2,2′-ethylenedioxybis(ethylamine) (1) in EtOH, PrOH, i-PrOH, BuOH, and i-BuOH was carried out in both solution (at room temperature) and solid phase (at −196 °C), and the resultant C-H and O-H insertion products were analyzed semi-quantitatively by LC/ESI-MS/MS. The carbene insertion reactions at low temperature produced all possible C-H and O-H insertion products in a relatively uniform distribution, which could not be accomplished by the solution-phase reaction.     

Direct C-H bond arylation of 2-hydroxybenzaldehydes with arylboronic acids via ligand-free palladium catalysis

A mild and efficient ligand-free palladium-catalyzed direct C-H bond arylation reaction was developed to afford 2-hydroxybenzophenones in good to excellent yields from easily available 2-hydroxybenzaldehydes and arylboronic acids. The given reaction provided one of the easiest pathways for accessing 2-hydroxybenzophenones, and a variety of functional groups could be tolerated in this process.     

Mechanistic Study of Palladium-Catalyzed Oxidative C-H/C-H Coupling of Polyfluoroarenes with Simple Arenes

Pd-catalyzed oxidative C-H/C-H coupling reaction is an emerging type of C-H acti-vation reaction, which attracts great interests because both reaction partners do not re-quire pre-functionalization. In the present study, we employed DFT methods to investigatethe mechanism of Pd(OAc)₂-catalyzed oxidative C-H/C-H coupling of pentafluoroben-zene with benzene. Four possible pathways were examined in the C-H activation part: path A benzene-pentafluorobenzene mechanism (C-H activation of benzene occurs before the C-H activation of pentafluorobenzene), path B pentafluorobenzene-benzene mechanism (C-H activation of benzene occurs after the C-H activation of pentafluorobenzene), path C benzene-pentafluorophenylsilver mechanism (C-H activation of benzene and subsequenttransmetalation with pentafluorophenyl silver complex), path D pentafluorophenylsilver-benzene mechanism (transmetalation with pentafluorophenyl silver complex and subsequent C-H activation of benzene). Based on the calculations, the sequences of two C-H activation steps are found to be different in the oxidative couplings of same substrates (i.e. pentaflu-orobenzene and benzene) in different catalytic systems, where the additive Ag salts played a determinant role. In the absence of Ag salts, the energetically favored pathway is path B (i.e. the C-H activation of pentafluorobenzene takes place before the C-H cleavage of benzene). In contrast, with the aid of Ag salts, the coordination of pentafluorophenylsilver to Pd center could occur easily with a subsequent C-H activation of benzene in the second step, and the second step significantly raises the whole reaction barrier. Alternatively, in thepresence of Ag salts, the kinetically preferred mechanism is path C (i.e. the C-H activation of benzene takes place in the first step followed by transmetalation with pentafluorophenyl-silver complex), which is similar to path A. The calculations are consistent with the H/D exchange experiment and kinetic isotope effects. Thus the present study not only offers a deeper understanding of oxidative C-H/C-H coupling reaction, but also provides helpful insights to further development of more efficient and selective oxidative C-H/C-H coupling reactions.     

Oxidative C-H/C-H Coupling of Dipyrromethanes with Azines by TiO2-Based Photocatalytic System. Synthesis of New BODIPY Dyes and Their Photophysical and Electrochemical Properties

Oxidative C-H/C-H coupling reactions of dipyrromethanes with azines in the presence of a heterophase oxidative photocatalytic system (O₂/TiO₂/visible light irradiation) were carried out. As a result of cyclization of obtained compounds with boron trifluoride etherate, new hetaryl-containing derivatives of 4,4-difluoro-4-boron-3a,4a-diaza-s-indacene were synthesized. For the obtained compounds, absorption and luminescence spectra, quantum yields of luminescence as well as cyclic volt-amperograms were measured.     

Rhodium catalyzed synthesis of isoindolinones via C-H activation of N-benzoylsulfonamides

An efficient approach to a wide range of isoindolinones, including 3-monosubstituted and 3,3-disubstituted isoindolinones, from the annulation of N-benzoylsulfonamides with olefins and diazoacetate has been developed. The transformation is broadly compatible with both terminal and internal olefins. Moreover, diazoacetate is for the first time incorporated into an amide-directed C-H functionalization reaction. Specifically, the rhodium complex [{RhCl₂Cp*}₂] enables the in situ dimerization of diazoacetate in addition to its role in catalyzing C-H functionalization/cross-coupling.     

C-H activations on a 1H-1,4-benzodiazepin-2(3H)-one template

Air stable benzodiazepine containing palladacycles were synthesized by a C-H activation reaction and studied by mass spectrometry and X-ray crystallography. Catalytic C-H functionalizations of 1-methyl-5-phenyl-1H-1,4-benzodiazepin-2(3H)-one with diphenyliodonium hexafluorophosphate led to a mixture, which included the starting material and the expected product 1-methyl-5-(2′-biphenyl)-1H-1,4-benzodiazepin-2(3H)-one.     

Pt-Catalyzed sp C-H bond activation of o-alkyl substituted aromatic carboxylic acid derivatives for the formation of aryl lactones

Synthesis of aryl lactones from ortho-alkyl substituted aromatic carboxylic acids is described on the basis of sp³ C-H bond activation using either palladium or platinum catalysts. Kinetic isotope studies reveal that the reaction takes place presumably by the chelation assistance of metal catalyst to the carboxylic group followed by the C-H bond activation.     

Steam Reforming Reactions of Methanol Over Nickel and Copper Catalysts

Steam-reforming reactions of methanol over NiO/Al₂O₃ and CuO/ZnO have been investigated. Over the nickel catalyst, the reaction rate is of zero kinetic order with respect to either methanol or steam, and the activation energy is 12.4 kJmol^?1, whereas with copper catalyst, the rate is expressed according to the literature as kP_a/(1 + K_aP_a + K_b+P_b) in which “a” and “b” are methanol and steam, respectively. The rate-controlling step of the reaction is assigned to the dissociation of O-H bond with dehydrogenation of C-H bond proceed rapidly to form carbon oxides. With copper catalyst the intrinsic participation of a water molecule during the dehydrogenation of C-H bond leads to the formation of carbon dioxide. With nickel catalyst, the dehydrogenation proceed more rapidly than the migration of a water molecule from an alumina site to a nickel site and causes almost exclusively the formation of carbon monoxide and hydrogen at a lower reaction temperature.     

The variation of coordination modes of aromatic imines in iron carbonyl complexes: Is there a correlation between bond lengths in organometallic model compounds and the reactivity of the ligands in catalytic C-C bond formation reactions?

The reaction of aromatic imines with Fe₂(CO)₉ proceeds via a two-step reaction sequence. A C-H activation reaction in ortho-position with respect to the exocyclic imine function is followed by an intramolecular hydrogen transfer reaction towards the former imine carbon atom. The resulting dinuclear iron carbonyl complexes show an aza-ferra-cyclopentadiene ligand which is apically coordinated by the second iron tricarbonyl moiety. Comparing the bond lengths of 43 different compounds, which were synthesized and structurally characterized in our group shows that the iron iron bond length correlates with one of the iron carbon bond lengths. The longer the iron carbon bond between the apically coordinated iron atom and the carbon atom next to the former imine carbon atom is, the shorter is the iron iron bond. The same ligands may be used as the substrates in ruthenium catalyzed C-C bond formation reactions. Whereas most of the imines react via the formal insertion of CO and/or ethylene into the C-H bond in ortho-position to the imine function, the ligands that show the longest iron carbon bond lengths in the model compounds under the same reaction conditions produce different types of isoindolones.     

Palladium(II)-catalyzed oxidative Heck coupling of thiazole-4-carboxylates

Oxidative Heck coupling of thiazole-4-carboxylates via palladium(II)-catalyzed C-H bond activation has been achieved in moderate to good yields. No ligand, and no acidic additive were used in the reaction. The results showed that this protocol tolerated a series of substitutions on the thiazole ring. A preliminary attempt of direct arylation with p-xylene via Pd(II)-catalyzed C-H bond activation has also been done.