Palladium-catalyzed cycloaddition of alkynyl aryl ethers with internal alkynes via selective ortho C-H activation

Alkynyl aryl ethers react with internal alkynes through selective ortho C-H activation by a palladium(0) catalyst to give substituted 2-methylidene-2H-chromenes. The alkynoxy group acts as a directing group to promote ortho C-H functionalization. Deuterium-labeling experiments indicated that the arylpalladium hydride complex is a key intermediate via oxidative addition. Various functional groups tolerate the present transformation to give the corresponding products.     

Cobalt-catalyzed, room-temperature addition of aromatic imines to alkynes via directed C-H bond activation

A quaternary catalytic system consisting of a cobalt salt, a triarylphosphine ligand, a Grignard reagent, and pyridine has been developed for chelation-assisted C-H bond activation of an aromatic imine, followed by insertion of an unactivated internal alkyne that occurs at ambient temperature. The reaction not only tolerates potentially senstitive functional groups (e.g., Cl, Br, CN, and tertiary amide), but also displays a unique regioselectivity. Thus, the presence of substituents such as methoxy, halogen, and cyano groups at the meta-position of the imino group led to selective C-C bond formation at the more sterically hindered ortho positions. Under acidic conditions, the hydroarylation products of dialkyl- and alkylarylacetylenes underwent cyclization to afford benzofulvene derivatives, while those of diarylacetylenes afforded the corresponding ketones in moderate to good yields. A mechanistic investigation into the reaction with the aid of deuterium-labeling experiments and kinetic analysis has indicated that oxidative addition of the ortho C-H bond is the rate-limiting step of the reaction. The kinetic analysis has also shed light on the complexity of the quaternary catalytic system.     

Ruthenium-catalyzed C-H/CO/Olefin coupling reaction of N-arylpyrazoles. Extraordinary reactivity of N-arylpyrazoles toward carbonylation at C-H bonds

The reaction of 1-arylpyrazoles with CO and ethylene in the presence of Ru(3)(CO)(12) resulted in regioselective carbonylation at the ortho C-H bonds. While it is found that the pyrazole ring also functions as the directing group for C-H bond cleavage, the efficiency of the reaction depends on the position of the pyrazole ring.     

Specific ortho orientation in the vicarious substitution of hydrogen in aromatic nitro compounds with carbanion of chloromethyl phenyl sulfone

Vicarious nucleophilic substitution of hydrogen atoms in nitroarenes with chloromethylphenyl sulfone proceeds selectively ortho to the nitro group when carried out in t-BuOK/THF base/solvent system. In the majority of 3-substituted nitrobenzene derivatives substitution occurs at the most hindered position 2. These conditions offer an efficient method of synthesis of 2,6 and 2,3-disubstituted nitrobenzene derivatives.     

Ab initio reaction path analysis of benzene hydrogenation to cyclohexane on Pt(111)

First-principles density functional theory calculations were performed to obtain detailed insight into the mechanism of benzene hydrogenation over Pt(111). The results indicate that benzene hydrogenation follows a Horiuti-Polanyi scheme which involves the consecutive addition of hydrogen adatoms. A first-principles-based reaction path analysis indicates the presence of a dominant reaction path. Hydrogenation occurs preferentially in the meta position of a methylene group. Cyclohexadiene and cyclohexene are expected to be at best minor products, since they are not formed along the dominant reaction path. The only product that can desorb is cyclohexane. Along the dominant reaction path, two categories of activation energies are found: lower barriers at approximately 75 kJ/mol for the first three hydrogenation steps, and higher barriers of approximately 88 kJ/mol for steps four and six, where hydrogen can only add in the ortho position of two methylene groups. The highest barrier at 104 kJ/mol is calculated for the fifth hydrogenation step, which may potentially be the rate-determining step. The high barrier for this step is likely the result of a rather strong C-H...Pt interaction in the adsorbed reactant state (1,2,3,5-tetrahydrobenzene) which increases the barrier by approximately 15 kJ/mol. Benzene and hydrogen are thought to be the most-abundant reaction intermediates.     

Catalytic addition of C-H bonds to multiple bonds with the aid of ruthenium complexes

Ruthenium complexes, e.g., RuH2(CO)(PPh3)3, have been found to catalyze the direct addition of ortho carbon-hydrogen bonds of aromatic ketones to olefins and acetylenes with high efficiency and selectivity. The C-H/olefin coupling reaction is applicable to not only C-H bonds in aromatic ketones but also to those in a,b-enones and aro-matic esters. Catalytic addition of ortho carbon-hydrogen bonds of aromatic imines to olefins is found to be catalyzed by Ru3(CO)12.     

New bis(aryloxy)-Ti(iv) complexes and their use for the selective dimerization of ethylene to 1-butene

New titanium complexes of general formula [(ArO)(n)Ti(Oi-Pr)((4-n))] were synthesized and used as pre-catalysts for the selective dimerization of ethylene to 1-butene. The complexes were prepared in cyclohexane using [Ti(Oi-Pr)(4)] and one or two equivalents of the corresponding phenols (ArOH) at room temperature. In this work, both monodentate and chelating phenols were evaluated. For alkyl-substituted phenols, it was demonstrated that large steric hindrance at both ortho and ortho' positions selectively yielded the mono-substituted complexes [(ArO)Ti(Oi-Pr)(3)]. Substitution at only one of the ortho positions allowed both the mono- and the di-substituted Ti complexes to be isolated. When a heteroatom was introduced on the phenol backbone, di-substitution systematically occurred except with phenols presenting a hemilabile -CH(2)NR(2) group at the ortho position. Upon activation with 3 equiv. of AlEt(3) at 20 bar and 60 °C, all the complexes selectively dimerized ethylene to 1-butene (>86% of butenes among which 99% of 1-butene). An increase of the steric bulk at the ortho position of the ligand or the introduction of a functional group led to decreased activity compared to [Ti(Oi-Pr)(4)].     

Theoretical study of the decomposition reactions in substituted nitrobenzenes

The influence of substituent nature and position on the unimolecular decomposition of nitroaromatic compounds was investigated using the density functional theory at a PBE0/6-31+G(d,p) level. As the starting point, the two main reaction paths for the decomposition of nitrobenzene were analyzed: the direct carbon nitrogen dissociation (C6H5 + NO2) and a two step mechanism leading to the formation of phenoxyl and nitro radicals (C6H5O + NO). The dissociation energy of the former reaction was calculated to be 7.5 kcal/mol lower than the activation energy of the second reaction. Then the Gibbs free energies were computed for 15 nitrobenzene derivatives characterized by different substituents (nitro, methyl, amino, carboxylic acid, and hydroxyl) in the ortho, meta, and para positions. In meta position, no significant changes appeared in the reaction energy profiles whereas ortho and para substitutions led to significant deviations in energies on the decomposition mechanisms due to the resonance effect of the nitro group without changing the competition between these mechanisms. In the case of para and meta substitutions, the carbon-nitro bond dissociation energy has been directly related to the Hammett constant as an indicator of the electron donor-acceptor effect of substituents.     

Neutron and X-ray diffraction and spectroscopic investigations of intramolecular [C-H...F-C] contacts in post-metallocene polyolefin catalysts: modeling weak attractive polymer-ligand interactions

A family of Group 4 post-metallocene catalysts, supported by fluorine-functionalized tridentate ligands with the fluorine substituent in the locality of the metal center, is described. For the first time, the contentious C-H...F-C interaction has been characterized by a neutron diffraction study, which has allowed the position of the hydrogen atoms to be accurately determined. The nature of the weak intramolecular C-H...F-C contacts in these complexes in solution and the solid state was probed by using multinuclear NMR spectroscopy in tandem with neutron and X-ray crystallography. Evidence is presented to demonstrate that the spectroscopic C-H...F-C coupling occurs "through-space" rather than "through-bond" or by MF coordination. The titanium catalysts exhibit excellent activities and high co-monomer incorporation in olefin polymerization. The observed intramolecular C-H...F-C interactions are important with regards to potential applications in polyolefin catalysis because they substantiate the proposed ortho-F...H(beta) ligand-(polymer chain) contacts derived from DFT calculations for the remarkable fluorinated phenoxyimine Group 4 catalysts. Compared with agostic and co-catalyst...metal contacts, weak attractive noncovalent interactions between a polymer chain and a judiciously designed "active" ligand is a new concept in polyolefin catalysis.     

Selective ortho C-h activation of haloarenes by an Ir(I) system

The cationic PNP-Ir(I)(cyclooctene) complex 1 (PNP = 2,6-bis-(di-tert-butyl phosphino methyl)pyridine) reacts with benzene at 25 degrees C to quantitatively yield the crystallographically characterized, square pyramidal, iridium phenyl hydride complex cis-(PNP)Ir(Ph)(H), 2, in which the hydride is trans to the vacant coordination site. The cationic complex 2 is stable to heating at 100 degrees C, in sharp contrast to the previously reported unstable neutral, isoelectronic (PCP)Ir(H)(Ph) (PCP = eta(3)-2,6-((t)()Bu(2)PCH(2))(2)C(6)H(3)). Heating of 2 at 50 degrees C with other arenes results in arene exchange. Complex 1 activates C-H bonds of chloro- and bromobenzene with no C-halide oxidative addition being observed. Selective ortho C-H activation takes place, the process being directed by halogen coordination and being thermodynamically and kinetically favorable. The meta- and para-C-H activation products are formed at a slower rate than the ortho isomer and are converted to it. NMR data and an X-ray crystallographic study of the ortho-activated chlorobenzene complex, which was obtained as the only product upon heating of 1 with chlorobenzene at 60 degrees C, show that the chloro substituent is coordinated to the metal center.     

Temperature-dependent transitions between normal and inverse equilibrium isotope effects for coordination and oxidative addition of C-H and H-H bonds to a transition metal center

The temperature dependence of the equilibrium isotope effects (EIEs) for coordination and oxidative addition of C-H and H-H bonds to the tungstenocene species {[H2Si(C5H4)2]W} has been determined with the aid of DFT (B3LYP) calculations. The EIE for coordination of CH4 and CD4 does not exhibit typical van't Hoff type behavior in which there is a monotonic variation of EIE with temperature; rather, the temperature dependence of the EIE exhibits a maximum, with inverse values (<1) at low temperature and normal values (>1) at high temperatures. The temperature dependence of the EIE for oxidative addition of CH4 and CD4 differs significantly from that for coordination, with the EIE being normal at all temperatures and approaching infinity at 0 K. In contrast to oxidative addition of methane which is normal at all temperatures, the EIE for oxidative addition of H2 and D2 exhibits a transition from inverse to normal upon raising the temperature. The existence of inverse EIEs in these systems at low temperatures is a result of the zero point energy changes for the products upon isotopic substitution being greater than those for the reactants (H2 or CH4).     

Highly selective mild stepwise allylation of N-methoxybenzamides with allenes

An efficient Rh(III)-catalyzed stepwise ortho allylation of N-methoxybenzamides 1 with polysubstituted allenes is reported. This C-H functionalization involving allenes is conducted under very mild conditions (-20 °C or room temperature) and compatible with ambient air and moisture, and it can be applied to terminal or internal allenes with different synthetically attractive functional groups. Highly efficient axial chirality transfer has been realized, yielding optically active lactones.     

Chelation-assisted Rh(I)-catalyzed ortho-alkylation of aromatic ketimines or ketones with olefins

Described herein is the Rh(I)-catalyzed ortho-alkylation of aromatic ketimines or ketones with olefins. This method showed high reactivity and selectivity to monoalkylation for a variety of olefins including 1-alkenes with an allylic proton, alpha,omega-dienes, and internal olefins. For a mechanistic study, H/D exchange experiments were carried out, which demonstrated that the ortho C-H bond could be easily cleaved even at the low temperature of 45 degrees C. The key step of this reaction is the formation of a stable five-membered metallacycle by a chelation-assisted ortho C-H bond activation. Furthermore, the direct ortho-alkylation of aromatic ketones with the Rh(I) complex was successfully achieved by adding 50 mol % of benzylamine as a chelation-assistant tool.     

Pd(II)-catalyzed ortho trifluoromethylation of arenes and insights into the coordination mode of acidic amide directing groups

A Pd(II)-catalyzed trifluoromethylation of ortho C-H bonds with an array of N-arylbenzamides derived from benzoic acids is reported. N-Methylformamide has been identified as a crucial promoter of C-CF(3) bond formation from the Pd center. X-ray characterization of the C-H insertion intermediate has revealed a rare coordination mode of acidic amides as directing groups and the origin of their capacity in directing C-H activation.     

Regioselective orthopalladation of (Z)-2-aryl-4-arylidene-5(4H)-oxazolones: scope, kinetico-mechanistic, and density functional theory studies of the C-H bond activation

Orthopalladated complexes derived from (Z)-2-aryl-4-arylidene-5(4H)-oxazolones have been prepared by reaction of the oxazolone with palladium acetate in acidic medium. The reaction is regioselective, only the ortho C-H bond of the arylidene ring being activated, producing a six-membered ring. The scope and reaction conditions of the orthopalladation are dependent on the acidity of the solvent. In CF(3)CO(2)H a large number of oxazolones can be metalated under mild conditions. As acidity decreases a lesser number of oxazolones can be efficiently palladated and harsher conditions must be used to achieve similar yields. The C-H bond activation in acidic medium agrees with an ambiphilic mechanism, as determined from kinetic measurements at variable temperature and pressure for different oxazolones substituted at the arylidene ring. The mechanism has been confirmed by density functional theory (DFT) calculations, where the formation of the six-membered ring is shown to be favored from both a kinetic and a thermodynamic perspective. In addition, the dependence of the reaction rate on the acidity of the medium has also been accounted for via a fine-tuning between the C-H agostic precoordination and the proton abstraction reaction in the overall process occurring on coordinatively saturated [Pd(κ(N)-oxazolone)(RCO(2)H)(3)](2+).     

Effects of fluorination on iridium(III) complex phosphorescence: magnetic circular dichroism and relativistic time-dependent density functional theory

We use a combination of low temperature, high field magnetic circular dichroism, absorption, and emission spectroscopy with relativistic time-dependent density functional calculations to reveal a subtle interplay between the effects of chemical substitution and spin-orbit coupling (SOC) in a family of iridium(III) complexes. Fluorination at the ortho and para positions of the phenyl group of fac-tris(1-methyl-5-phenyl-3-n-propyl-[1,2,4]triazolyl)iridium(III) cause changes that are independent of whether the other position is fluorinated or protonated. This is demonstrated by a simple linear relationship found for a range of measured and calculated properties of these complexes. Further, we show that the phosphorescent radiative rate, k(r), is determined by the degree to which SOC is able to hybridize T(1) to S(3) and that k(r) is proportional to the inverse fourth power of the energy gap between these excitations. We show that fluorination in the para position leads to a much larger increase of the energy gap than fluorination at the ortho position. Theory is used to trace this back to the fact that fluorination at the para position increases the difference in electron density between the phenyl and triazolyl groups, which distorts the complex further from octahedral symmetry, and increases the energy separation between the highest occupied molecular orbital (HOMO) and the HOMO-1. This provides a new design criterion for phosphorescent iridium(III) complexes for organic optoelectronic applications. In contrast, the nonradiative rate is greatly enhanced by fluorination at the ortho position. This may be connected to a significant redistribution of spectral weight. We also show that the lowest energy excitation, 1A, has almost no oscillator strength; therefore, the second lowest excitation, 2E, is the dominant emissive state at room temperature. Nevertheless the mirror image rule between absorption and emission is obeyed, as 2E is responsible for both absorption and emission at all but very low (<10 K) temperatures.     

Overtone spectra of aniline derivatives

Overtone spectra of 2-ethylaniline, N-methylaniline, N-ethylaniline, N,N-dimethylaniline and N,N-diethylaniline have been studied in 2500-15000 cm(-1) region. Vibrational frequency and anharmonicity constants for aryl/alkyl C-H stretch and N-H stretch vibrations have been determined. The effect of substitution of C(2)H(5) group on the ortho position in the ring and CH(3)/C(2)H(5) at the positions of the H-atom in NH(2) group has been studied in these molecules. It is noted that the aryl C-H stretching frequency and the N-H stretching frequency is appreciably increased due to the replacement of H in NH(2) group by CH(3)/C(2)H(5). These experimental observations are well supported by theoretical calculations for charge density on N-atom using molecular orbital AM(1) method.     

Ortho-selective direct cross-coupling reaction of 2-aryloxazolines and 2-arylimidazolines with aryl and alkenyl halides catalyzed by ruthenium complexes

The ortho position of the aromatic ring in 2-aryloxazolines and 2-arylimidazolines is selectively arylated and alkenylated with organic halides in the presence of a ruthenium(II)-phosphine complex. In the case of unsubstituted and para-substituted phenyloxazolines, 1:2 coupled products were obtained preferentially, while 1:1 coupled products were obtained in the case of meta-substituted phenyloxazolines and N-acylarylimidazolines. The reaction is proposed to proceed via the generation of an organoruthenium intermediate, formed by oxidative addition of the organic halide, and ortho-ruthenation directed by the coordination of the 2-oxazolinyl or 2-imidazolinyl group to the ruthenium center.     

Revisiting acido-basicity of the MgO surface by periodic density functional theory calculations: role of surface topology and ion coordination on water dissociation

Low-coordinated (LC) ions at the MgO surface (noted Mg2+LC and O2-LC with L = 1-5), located on monatomic and diatomic steps, corners, step divacancies, and kinks, have been modeled thanks to periodic density functional theory (DFT) calculations (VASP). Ions of lowest coordination induce the strongest surface geometry relaxation and the highest surface energies. The hydration energies of these sites and thermodynamic stabilities of the resulting surfaces were studied. The factors controlling the interaction strength between water and the surface are the possibility for the hydroxyl group to adopt a bridging geometry between two Mg2+ cations in concave areas of the surface, such as the bottom of the monatomic step, and at second order the surface atomic coordination, and especially the presence of three-coordinated ions. The Lewis basicity and acidity of O2-LC and Mg2+LC, respectively, increase as their coordination number decreases, which implies the same trend for the Br?nsted basicity of the Mg2+-O2- pair toward water. However, this trend can be changed if pairs leading to the formation of bridging OH groups are involved, typically on monatomic steps or in step divacancies where O2C-H and O3C-H are obtained, respectively, instead of the expected O1C-H. Thanks to thermodynamic calculations, the state of the surface as a function of temperature can be determined at a given pressure, unraveling the roles of surface topology and ions coordination.     

Ruthenium-catalysed ortho alkylation of hydroxyacetophenones; the functionalisation of ring C aromatic diterpenoids

Ortho C-H bond coupling of some 2-alkoxyacetophenones with olefins catalysed by ruthenium complexes results in a high yield of the ortho alkylated product, providing that a suitable protecting group is employed. No such protection was required for a para-alkoxy group; an activating effect was also observed. Bicyclic and tricyclic analogues react similarly.