Meta Carbons Research Articles

Effects of Ligand Composition on the Oxidative Carbonylation of Toluene to Toluic Acid Catalyzed by Rh(III) Complexes

Experimental and theoretical studies were conducted to investigate the influence of anionic ligands (e.g., CF(3)COO(-), CH(3)SO(3)(-)) on the catalytic activity and selectivity of Rh(III) in the oxidative carbonylation of toluene to toluic acid. The catalyst activity was found to pass through a maximum as the pK(a) of the conjugate Brønsted acid decreases from 4.63 to -2.00, with the maximum activity occurring at pK(a) = 0.35, corresponding to CClF(2)COOH. The theoretical analysis showed that the strength of toluene coordination increases with decreasing basicity of the anion (i.e., decreasing pK(a) of the corresponding acid). In contrast, the activation barrier for C-H activation increases with decreasing ligand pK(a). The experimentally observed effect of anion composition on catalyst activity can be explained using calculations of the apparent rate coefficient for toluene activation based on density functional theory and transition state theory. The ratio of p- to m-toluic acid formed increases with decreasing acid pK(a) and passes through a maximum for pK(a) = 0, corresponding to CF(3)COOH. The effect of anion composition on the isomer selectivity is attributed to changes in the charge density on the Rh(III) cation, which in turn affect the distribution of charge on the para and meta carbon atoms of the aromatic ring.

DFT Study of the Group Interactions in 1,3,5‐Triamino‐2,4,6‐Trinitrobenzene and 1,3,5‐Triamino‐2,4,6‐Tridifluoroaminobenzene

AbstractDensity functional calculations on isodesmic disproportionation reactions of 1,3,5‐triamino‐2,4,6‐trinitrobenzene (TATB) and 1,3,5‐triamino‐2,4,6‐tridifluoroaminobenzene (TATDB) indicate that the interaction between nitro groups on meta carbons of TATB, which brings about unstability to the molecule, is surprisingly larger than that between difluroamino groups in TATDB. The electron‐withdrawing and electron‐donating groups generate large positive and very small negative values of Edisproportion, respectively. When both electron‐withdrawing and electron‐donating groups are attached to the benzene skeleton at the same time, large negative disproportionation energy is produced, which stabilizes the derivatives. The values of Edisproportion for TATB and TATDB are predicted to be ‐48.03 kJ/mol and ‐63.54 kJ/mol, respectively, indicating that the total interaction among groups with stabilization effects in TATDB is larger than that in TATB. The large difference of the Edisproportion values between TATB and TATDB is derived from the large difference between the interactions of the meta‐nitro group and those of meta‐difluoroamino groups. The energy barriers for the C‐N internal rotation of NO2 group and NF2 groups are 74.7 kJ/mol and 185.8 kJ/mol for TATB and TATDB, respectively. The large energy barrier for the rotation of the NF2 group is caused by its stabilization interaction with neighbor amino groups, instead of steric effects. When the number of pairs of amino‐nitro or amino‐difluoroamino groups increases, there are more negative charges on the NO2/NF2 groups and on the O/F atoms.

Isotope Effects, Dynamics, and the Mechanism of Solvolysis of Aryldiazonium Cations in Water

The mechanism of the heterolytic solvolysis of p-tolyldiazonium cation in water was studied by a combination of kinetic isotope effects, theoretical calculations, and dynamics trajectories. Significant (13)C kinetic isotope effects were observed at the ipso (k(12)C/k(13)C = 1.024), ortho (1.017), and meta (1.013) carbons, indicative of substantial weakening of the C(2)-C(3) and C(5)-C(6) bonds at the transition state. This is qualitatively consistent with a transition state forming an aryl cation, but on a quantitative basis, simple S(N)1 heterolysis does not account best for the isotope effects. Theoretical S(N)2Ar transition structures for concerted displacement of N(2) by a single water molecule lead to poor predictions of the experimental isotope effects. The best predictions of the (13)C isotope effects arose from transition structures for the heterolytic process solvated by clusters of water molecules. These structures, formally saddle points for concerted displacements on the potential energy surface, may be described as transition structures for solvent reorganization around the aryl cation. Quasiclassical dynamics trajectories starting from these transition structures afforded products very slowly, compared to a similar S(N)2 displacement, and the trajectories often afforded long-lived aryl cation intermediates. Critical prior evidence for aryl cation intermediates is reconsidered with the aid of DFT calculations. Overall, the nucleophilic displacement process for aryldiazonium ions in water is at the boundary between S(N)2Ar and S(N)1 mechanisms, and an accurate view of the reaction mechanism requires consideration of dynamic effects.

Conformational instability of a proximally-difunctionalized calix[4]-diquinone

We have recently described the electrochemical oxidation of 5,11,17,23-tetra- tert-butyl-25, 26-bis(diphenylphosphinoylmethoxy)-27,28-dihydroxycalix[4]arene into the corresponding diquinone 1. The solid state structure of 1 has now been determined by a single crystal X-ray diffraction study. Diquinone 1 crystallizes in the monoclinic space group with a=10.3507(12), b=25.219(4), c=20.2315(14) Å, β=101.166(8)°, V=5181.1(10) Å 3, Z=4, and D x =1.273 g cm −3. The calixarene skeleton adopts a partial cone conformation in which one quinone ring is anti-oriented with respect to the other three rings of the calixarene core. A variable temperature NMR study shows that 1 is dynamic in solution, each quinone unit undergoing fast oscillation about the axis that passes through the two meta carbon atoms bonded to the adjacent methylene groups. The motion of the quinone rings was confirmed by 2D NMR experiments.

Revisiting the Regiospecificity of Burkholderia xenovorans LB400 Biphenyl Dioxygenase toward 2,2′-Dichlorobiphenyl and 2,3,2′,3′-Tetrachlorobiphenyl

2,2'-Dichlorobiphenyl (CB) is transformed by the biphenyl dioxygenase of Burkholderia xenovorans LB400 (LB400 BPDO) into two metabolites (1 and 2). The most abundant metabolite, 1, was previously identified as 2,3-dihydroxy-2'-chlorobiphenyl and was presumed to originate from the initial attack by the oxygenase on the chlorine-bearing ortho carbon and on its adjacent meta carbon of one phenyl ring. 2,3,2',3'-Tetrachlorobiphenyl is transformed by LB400 BPDO into two metabolites that had never been fully characterized structurally. We determined the precise identity of the metabolites produced by LB400 BPDO from 2,2'-CB and 2,3,2',3'-CB, thus providing new insights on the mechanism by which 2,2'-CB is dehalogenated to generate 2,3-dihydroxy-2'-chlorobiphenyl. We reacted 2,2'-CB with the BPDO variant p4, which produces a larger proportion of metabolite 2. The structure of this compound was determined as cis-3,4-dihydro-3,4-dihydroxy-2,2'-dichlorobiphenyl by NMR. Metabolite 1 obtained from 2,2'-CB-d(8) was determined to be a dihydroxychlorobiphenyl-d(7) by gas chromatographic-mass spectrometric analysis, and the observed loss of only one deuterium clearly shows that the oxygenase attack occurs on carbons 2 and 3. An alternative attack at the 5 and 6 carbons followed by a rearrangement leading to the loss of the ortho chlorine would have caused the loss of more than one deuterium. The major metabolite produced from catalytic oxygenation of 2,3,2',3'-CB by LB400 BPDO was identified by NMR as cis-4,5-dihydro-4,5-dihydroxy-2,3,2',3'-tetrachlorobiphenyl. These findings show that LB400 BPDO oxygenates 2,2'-CB principally on carbons 2 and 3 and that BPDO regiospecificity toward 2,2'-CB and 2,3,2,',3'-CB disfavors the dioxygenation of the chlorine-free ortho-meta carbons 5 and 6 for both congeners.

Structural Diversity of Pentafluorophenylcopper Complexes. First Evidence of π-Coordination of Unsupported Arenes to Organocopper Aggregates

The binding behavior of arenes toward pentafluorophenylcopper was investigated. Crystallization of pentafluorophenylcopper from a mixture of 1,2-dichloroethane and cyclohexane affords the homoleptic tetrameric complex [Cu(C6F5)]4, whereas a π-complex, [Cu(C6F5)]4(η2-toluene)2, is obtained from toluene solution. The unique coordination of two toluene molecules in [Cu(C6F5)]4(η2-toluene)2 leads to major structural changes as the regular square planar tetramer is distorted toward a butterfly structure with one short and one long diagonal Cu···Cu distance of 2.5935(3) and 3.955(1) A, respectively. The toluene molecules are bound in an unsymmetrical η2-coordination mode with the shortest contacts observed between copper and the meta carbon atoms (d(Cu−C) = 2.271(2), 2.298(2) A) and slightly longer distances to the para positions (d(Cu−C) = 2.339(2), 2.455(4) A). Multinuclear NMR studies show that complexation to the intact tetrameric cluster species also occurs in neat solution of the aromatic species. Strongl...

A 13C NMR study of the structure of four cinnamic acids and their methyl esters

The 13C NMR spectra, both in DMSO solution and in the solid state of four cinnamic acids (p-methoxy, p-hydroxy, p-methyl, p-chloro) and their corresponding methyl esters have been recorded. The two main results in the solid state are: (i) the only significant difference between acids and esters chemical shifts concerns the CO group which, on average, appears at 173ppm in the acids and 168ppm in the esters; (ii) the signals of the ortho and meta carbons both in the acids and the esters are splitted. The two ‘anomalies’ disappear in DMSO solution. These observations can be rationalized using simple GIAO/B3LYP/6-31G∗ calculations.

A complete model for the prediction of 1H- and 13C-NMR chemical shifts and torsional angles in phenyl-substituted pyrazoles

1H- and 13C-NMR spectra of a number of N-phenyl- and C-phenylpyrazole derivatives have been obtained. The parameter most susceptible to changes in the dihedral angle θ is the difference δ meta-C – δ ortho-C . Values for this parameter have been determined and its usefulness for conformational studies of phenyl-substituted pyrazoles has been demonstrated. A correlation between torsional angles calculated by molecular mechanics and differences in 13C chemical shifts of the ortho and meta carbon atoms of the phenyl groups in 29 N-phenyl-substituted pyrazole derivatives and 11 C-phenyl-substituted pyrazole derivatives has been found. For the N-phenyl-substituted derivatives a correlation between torsional angles and δ meta-H – δ ortho-H values has also been demonstrated. In all cases good correlations between angles and differences in chemical shifts were observed.

Topological Analysis of the Electron Localization Function (ELF) Applied to the Electrophilic Aromatic Substitution

The topological analysis of the electron localization function ELF provides a partition of the molecular space into basins of attractors which have a clear chemical signification. The hierarchy of these basins is given by the bifurcation of the localization domains. In the case of π-donor substituents (OH, NH2, F, CH3, C6H5, Cl), the aromatic domain is first opened close to the substituted carbon and then in the vicinity of the meta carbon; whereas for attractor substituents (CN, CHO, NO2, CF3 and CCl3), it is first opened in the ortho and para positions. The orienting effects of the electrophilic substitutions are correlated with these bifurcations. The experimental favored positions always correspond to the locally electronegative carbons (i.e., those which keep their shell structure at the higher ELF values). This suggests that the local Pauli repulsion plays a noticeable role in the orienting effects which are complementary to the charge transfer effect involved in standard quantum chemical pictures.

Regiospecificity of dioxygenation of di- to pentachlorobiphenyls and their degradation to chlorobenzoates by the bph-encoded catabolic pathway of Burkholderia sp. strain LB400.

Burkholderia sp. strain LB400 is one of the most potent aerobic polychlorobiphenyl (PCB)-degrading microorganisms that have been characterized. Its PCB-dioxygenating activity originates predominantly or exclusively from the biphenyl dioxygenase encoded by its bph gene cluster. Analysis of the dioxygenation products of several di- to pentachlorinated biphenyls formed by this enzyme revealed a complex dependence of the regiospecificity and the yield of dioxygenation on the substitution patterns of both the oxidized and the nonoxidized rings. No dioxygenolytic attack involving chlorinated meta or para carbons was observed. Therefore, the ability of the enzyme to hydroxylate chlorinated carbons appears to be limited to the ortho position. However, it is not limited to monochlorinated rings, as evidenced by dioxygenation of the 2, 4-disubstituted ring at carbons 2 and 3. This site of attack is strikingly different from that of the 2,5-dichlorinated ring, which has been shown to be dihydroxylated at positions 3 and 4 (J. D. Haddock, J. R. Horton, and D. T. Gibson, J. Bacteriol. 177:20-26, 1995). These results demonstrate that a second substituent of ortho-chlorinated rings crucially influences the site of dioxygenation at this ring and thereby determines whether or not the initial chlorobiphenyl oxidation product is further metabolized through the bph-encoded pathway. The 2,4-dichlorinated ring can alternatively be attacked at carbons 5 and 6. The preferred site crucially depends on the substitution pattern of the other ring. The formation of more than a single dioxygenation product was found predominantly with congeners that contain two chlorinated rings, both of which are similarly prone to dioxygenation or one is substituted only at carbon 3.

Absolute proton affinities of biphenyl and its derivatives

The spatial structure of biphenyl 1 is studied by the semiempirical AM1 and ab initio HF/6-31G* and MP2(fc)/6-31G* theoretical models. The resulting bond distances are in good agreement with the X-ray structure. The calculated dihedral angle is in accordance with the value observed by the electron diffraction technique (ϕ = 45°). Its large value is a compromise between the steric hindrance effect and the π-electron conjugation. The estimated barrier heights for the internal rotation are very low however. Theoretical values are in accordance with the available experimental evidence. The calculated proton affinity (PA) obtained by the scaled (AM1)sc model and by using the MP2 level of theory compares very well with the experimental value. It is some 13 kcal mol–1 higher than the reference PA value of benzene, because of the strong resonance interaction between the two phenyl rings. The increase in the π-electron conjugation energy triggered by protonation overcomes the steric repulsion between H atoms thus decreasing the twist angle by some 20°. The apical carbon atom, placed para to the coannular CC bond, is most susceptible to the proton attack. On the other hand, the PA values for ipso and meta carbons are substantially lower since an amplification of the inter-ring conjugation interaction is then precluded. The PA increments for the CH3 group and F atom monosubstituted biphenyls are determined by using the (AM1)sc approach. They are employed in estimating proton affinities of a number of polysubstituted biphenyls applying a very simple additivity formula based on the independent substituent approximation (ISA). It is shown that the performance of the additivity rule is very good. Variation in the PA of substituted biphenyls is rationalized in terms of the conjugation effect and repulsion between hydrogen atoms or substituents attached to the face-to-face ortho positions of the neighbouring rings (steric effect).Finally, the proton affinity of fluorene possessing a “frozen” planar biphenyl moiety is calculated and compared with that of the paradigmatic Mills–Nixon (MN) system—indan. It is found that PA values of the former compound are determined by the MN and resonance effects, the latter being predominant. The most basic site in fluorene is the C(4) atom where both effects act in a synergistic way.

Complexation Studies with a Calix[4]arene-Derived Phosphinite − Divergent Arrays of Cavities Linked by MCl2 Fragments (M = Pd, Pt)

A cone-shaped calix[4]arene-derived monophosphinite, 25,27-di(ethoxycarbonylmethoxy)-26-hydroxy-28-(diphenylphosphinooxy)calix[4]arene (L1), has been prepared in two steps from calix[4]arene. Treatment of L1 with [AuCl(tetrahydrothiophene)] in tetrahydrofuran gave the phosphinite complex [AuClL1] (1). Reaction of [PdCl2(PhCN)2] with two equivalents of L1 gave selectively trans-[PdCl2L12] (2). Complex 2 · 2 CH2Cl2 crystallizes in the triclinic space group P1¯. The molecule is centrosymmetric, leading to a divergent array of the two calixarene moieties. The palladium atom resides outside both cones. As a result of steric interactions, the meta and para carbon atoms of the PPh2-bearing aryl ring are significantly pushed to the interior of the cavity. The related platinum complex trans-[PtCl2L122] (3), was obtained similarly from [PtCl2(PhCN)2] and presents in the solid state a structure which is isomorphous to that of 2. The diphosphinite 25,27-di(ethoxycarbonylmethoxy)-26,28-bis(diphenylphosphinoxy)calix[4]arene (cone) (L2) has also been prepared and reacts with [PtCl2(PhCN)2] to yield oligomeric material.

13C NMR Relaxation Studies of Pyridine and Pentafluoropyridine Liquids Confined to Nanopores of Porous Silica Glasses

A dynamic nuclear magnetic resonance (NMR) study of the polar liquids pyridine and pentafluoropyridine (PFP) confined to porous silica sol−gel glasses is reported. The 13C NMR spin−lattice relaxation times, T1, of ortho (C2), meta (C3), and para (C4) carbons were measured in glasses with pore radii ranging from 30 to 94 Å, over a temperature range of −22 to +20 °C. The experimental data were analyzed in terms of the two-state, fast exchange model, and the surface-layer relaxation times, T1s, were calculated. On the basis of surface enhancement factors, T1b/T1s, where T1b is the relaxation time of the bulk liquid, we conclude that fluorination considerably weakens the interaction of pyridine with the glass surface. This is consistent with pyridine serving as a hydrogen bond acceptor for the surface. By analyzing the relaxation differences in C2, C3, and C4, we find that the reorientation of confined pyridine is highly anisotropic, due to directional hydrogen bonding between the N atoms of pyridine and the OH groups on the glass surface. In contrast, we see no confinement effect on the rotational anisotropy of PFP.

13C−13C Spin−Spin Coupling Tensors in Benzene As Determined Experimentally by Liquid Crystal NMR and Theoretically by ab Initio Calculations

This study reports experimentally and theoretically (ab initio) determined indirect CC spin−spin coupling tensors nJCC in benzene. The CC spin−spin coupling constants nJCC between the ortho, meta, and para (n = 1, 2, and 3) positioned carbons were experimentally determined in two ways: firstly by utilizing the 2H/1H isotope effect on the carbon shieldings in neat monodeuteriobenzene and recording the 13C satellite spectrum in a 1H-decoupled 13C NMR spectrum, and secondly by recording the 13C NMR spectrum of fully 13C-enriched benzene (13C6H6) and carrying out its complete analysis. The anisotropies of the corresponding coupling tensors, ΔnJCC, were resolved experimentally by liquid crystal 1H and 13C NMR using dipolar couplings corrected for both harmonic vibrations and deformations. The results obtained in three thermotropic liquid crystal solvents are in good mutual agreement, indicating the reliability of the determinations. The anisotropy of the ortho, meta, and para CC indirect couplings are ca. +17...

Conversion of chlorobiphenyls into phenylhexadienoates and benzoates by the enzymes of the upper pathway for polychlorobiphenyl degradation encoded by the bph locus of Pseudomonas sp. strain LB400.

Metabolism of 21 chlorobiphenyls by the enzymes of the upper biphenyl catabolic pathway encoded by the bph locus of Pseudomonas sp. strain LB400 was investigated by using recombinant strains harboring gene cassettes containing bphABC or bphABCD. The enzymes of the upper pathway were generally able to metabolize mono- and dichlorinated biphenyls but only partially transform most trichlorinated congeners investigated: 14 of 15 mono- and dichlorinated and 2 of 6 trichlorinated congeners were converted into benzoates. All mono- and at least 8 of 12 dichlorinated congeners were attacked by the bphA-encoded biphenyl dioxygenase virtually exclusively at ortho and meta carbons. This enzyme exhibited a high degree of selectivity for the aromatic ring to be attacked, with the order of ring preference being non- > ortho- > meta- > para-substituted for mono- and dichlorinated congeners. The influence of the chlorine substitution pattern of the metabolized ring on benzoate formation resembled its influence on the reactivity of initial dioxygenation, suggesting that the rate of benzoate formation may frequently be determined by the rate of initial attack. The absorption spectra of phenylhexadienoates formed correlated with the presence or absence of a chlorine substituent at an ortho position.

Observations on selectivity reversal during chiral auxiliary-directed asymmetric nucleophile additions to arene–manganese tricarbonyl complexes

A study of hydride nucleophile reactions with η6-(2R, 5R)-dimethylpyrrolidinylbenzene–manganese tricarbonyl hexafluorophosphate 1 reveals an unexpected reversal of selectivity during addition of hydride to the diastereotopic meta carbons, which is explained on the basis of changes in transition state location as the reactivity of the nucleophile is varied.

Synthesis of some heterocyclic derivatives of (η-arene)(η-cyclopentadienyl) iron(II) hexafluorophosphates including estimation of dihedral angles in both free and complexed ligands

The preparations are described of a series of mono and disubstituted derivatives of [(η-arene) (η-cyclopentadienyl)Fe[[PF 6] complexes in which a range of indole-related substituents is bonded to the arene moiety via nitrogen. The preparations involved the S NAR displacement of chlorine from the corresponding chlorobenzene iron sandwich complex by nitrogen-centred anions derived by reaction of the heterocycle with 1BuOK in DMSO or 80% DMSO- tBuOH. The 1H and 13C NMR data are presented and discussed. Analysis of the 13C data using substituent chemical shifts of the ortho and meta carbons of the arene moiety has allowed estimation of the dihedral angle between the arene and the heterocyclic components in both the free and complexed N-phenyl heterocyclic systems.

Comparison between alkylrhodoximes and alkylcobaloximes. Solution studies of some alkylrhodoximes, pyRh(DH)2R, and crystal structure of the complexes with RCH2CF3, CH2Cl, n-Pr

Syntheses, kinetics and NMR measurements of pyRh(DH)2R complexes, where py=pyridine and DH=monoanion of dimethylglyoxime, with various R alkyl groups are described. X-ray crystal structures of the compounds with RCH2CF3 (I), CH2Cl (II), n-Pr (III) are reported. Comparison of the geometry of the Rh(DH)2 and Co(DH)2 moieties shows significant differences in the MN, NO and O···O distances, with a decrease of the approximate symmetry from D2h in cobaloximes to C2h in rhodoximes. The axial Rh-C and Rh-N distances lengthen with increasing bulk and the σ-donating ability of R, respectively; the Rh-C distances are 2.059(5) Å in I and 2.069(5) Å in II, and the Rh-N(axial) distances are 2.145(3) (I), 2.178(3) (II) and 2.188(5) (III) Å. The chemical shifts of the pyridine meta and para carbons decrease with the increase of the alkyl group σ-donating power; the linear relationship found between the δ values of the pyridine para carbon in rhodoximes and the EP values, derived from NMR data of cobaloximes, shows that the transmission of electronic charge from R to the neutral ligand through rhodium occurs in a way similar to that through cobalt. The rate constants for the pyridine dissociation increase with the σ-donating ability of the alkyl group, as measured by EP parameters. These results show that, as already reported for cobaloximes, trans-influence and trans-effect have the same trend in rhodoximes too.

Low-Temperature Two-Dimensional Heteronuclear Shift Correlation Spectroscopy of A 1, 4-Benzodiazepine

Abstract The unsubstituted phenyl ring in ketazolam, a 1,4-benzodiazepine derivative, has a two-fold rotational barrier about the aryl to tert-alkyl bond connecting this phenyl group to the rest of the molecule. At 50.3 MHz, all twenty of the 13C resonances of ketazolam are resolved below -40°C, including two distinct resonances for the ortho carbons and two for the meta carbons. Twelve of the twenty 13C nuclei resonate between 138 and 125 ppm, necessitating assignment via a combination of one-bond and long-range heteronuclear shift correlation experiments.

Free-electron theory of substituent effects on molecular orbital coefficients

Rules for subsLiLuent effects on MO coefficients are developed by perturbational freeelectron theory. Theπ-MOs of substituted polyenes are regarded as waves shifted toward an electronegative atom in the conjugated system or shifted away from an electron-donating group. The concepts are extended to substituted ben7enes to show the relative magnitudes of the HOMO coefficients at ortho/para and meta carbons.