Solvent sensitivity of ortho substituent effect on 13C NMR chemical shift of the carboxyl carbon (δco) in benzoic acid

A reverse ortho effect is observed for the (13)C NMR chemical shifts of the carboxyl carbon (δ(co)) in benzoic acids measured in aprotic solvents of varying polarity. The ortho effect on δ(co) is best described by a combination of the reverse field and steric accelerating effects of the substituent in an 80: 20 pattern in apolar aprotic solvents and a 60: 40 pattern in dipolar aprotic ones. Interestingly, no good enough correlation was found between δ(co) and log k(1) of the acids measured in similar solvents. A critical analysis of the results clearly indicates the use of an apolar aprotic solvent and not a dipolar aprotic one as the solvent of choice for investigating intrinsic substituent effects on δ(c) in an aromatic system.     

From C1 to C3: Copper-Catalyzed gem-Bis(trifluoromethyl)olefination of α-Diazo Esters with TMSCF3

A Cu-catalyzed gem-bis(trifluoromethyl)olefination of α-diazo esters, using TMSCF₃ as the only fluorocarbon source, has been developed and provides an exquisite method to access gem-bis(trifluoromethyl)alkenes. This unprecedented olefination process involves a carbene migratory insertion into “CuCF₃” to generate the α-CF₃-substituted organocopper species, which then undergoes β-fluoride elimination and two consecutive addition-elimination processes to give the desired products. The key to this efficient one-pot C₁-to-C₃ synthetic protocol lies in the controllable double (over single and triple) trifluoromethylations of the gem-difluoroalkene intermediates.     

Mechanistic studies on selectivity in the B(C6F5)3-catalyzed allylstannation of aldehydes: is hypercoordination at boron responsible?

[structure: see text] The selective, B(C6F5)3-catalyzed allylstannation of aldehydes with proximal donor groups is shown to proceed via borane abstraction of the allyl group from the tin reagent and activation of the substrate by "Bu3Sn+". This is supported by a number of 19F NMR experiments. The selectivity of the reaction is not attributable to hypercoordinate boron as proposed by the discoverers of this highly selective reaction but likely involves chelation at tin.     

Triethanolamine zinc phosphite, (C6H13NO3)Zn2(HPO3): a templated network or a network of clusters?

(C6H13NO3)Zn2(HPO3) (I) displays an extended hybrid organic/inorganic structure in which the triethanolamine organic species acts as an anionic tetradentate ligand, rather than a typical protonated cationic template. Crystal data for I: Mr = 357.89, monoclinic, P2(1)/c (no. 14), a = 8.4216(4) angstroms, b = 9.9262(5) angstroms, c = 12.8494(6) angstroms, beta = 91.824(1) degrees, V = 1073.6(1) angstroms3, Z = 4.     

Six-coordinate Co(2+) with H(2)O and NH(3) ligands: which spin state is more stable?

Octahedral, six-coordinate Co(2+) can exist in two spin states. For biological ligands, H(2)O and NH(3), the most stable spin state is high spin (S = (3)/(2)). The difference in energy between high and low spin is dependent upon the ligand mix and coordination stereochemistry. High spin optimized geometries for these model compounds give structures close to octahedral symmetry. Low spin permits significant Jahn-Teller distortion. H(2)O ligands preferentially assume axial positions. Continuum solvent has a greater effect on low spin Co(2+), and it reduces the energy difference between the two spin states. For some ligand combinations optimized in the presence of solvent, there is no significant difference in energy between spin states.     

Structure of water (H2O) 6 − and ammonium (NH3) 13 − clusters with an excess electron: Quantum chemical studies

The B3LYP method within DFT and the ab initio MP2 method with an extended 6-311++G(3df,3pd) basis set are employed to calculate the adiabatic bound state of an excess electron in (H₂O) ₆ ^? water and (NH₃) ₁₃ ^? . ammonium clusters. Adiabatic electron affinity of (H₂O)₆ and (NH₃)₁₃ clusters is 0.03–0.18 eV and 0.18 eV respectively. The calculated vertical binding energies of the excess electron in anionic clusters ((H₂O) ₆ ^? 0.37÷0.66 eV and (NH₃) ₁₃ ^? 0.26 eV) agree well with the experimental values of 0.50 eV and 0.22 eV obtained from photoelectron spectra. A cavity model of solvated electrons in water and ammonium is considered.     

Observing the enantiomeric 1H chemical shift non-equivalence of several α-amino ester signals using tris[3-(trifluoromethylhydroxymethylene)-(+)-camphorato]samarium(III): a chiral lanthanide shift reagent that causes minimal line broadening

The chiral lanthanide shift reagent, tris[3-(trifluoromethylhydroxymethylene)-(+)-camphorato]samarium(III) [Sm(tfc)₃], was shown to resolve the ¹H NMR signals of the enantiomers of α-amino esters without causing serious line broadening. This distinctive feature of Sm(tfc)₃ made it possible to examine the enantiomeric chemical shift non-equivalence of several protons in ester substrates, increasing the reliability of the empirical assignment of the absolute configuration as compared to earlier techniques.     

Selective Hydrogenation of CO2 Dictated by Isomers in Au28(SR)20 Nanoclusters: Which One is Better?

It is a challenge to make clear how isomerism in a heterogeneous catalyst induces distinct differences in catalytic properties, as attainment of the structural isomerism in a conventional catalyst is difficult. By successfully identifying the isomerism in the atomically precise Au nanoclusters, an exciting opportunity for unravelling catalysis of isomeric catalysts is opened up. Herein, we report that the isomerism in the Au₂₈(SR)₂₀ nanoclusters with different surface atom arrangements can indeed render different catalytic behaviors in the selective hydrogenation of CO₂. We anticipate that our studies will serve as a starting point for fundamental investigations about how to control the catalytic activity and selectivity by the isomerism-induced catalysis.     

Regioselective anion generation and alkylation at carbon α to nitrogen in (CO)5WC[N(CH3)2]C6H4-p-CH3

Treatment of (CO)₅WC[N(CH₃)₂]C₆H₄-p-CH₃ (1) with lithium diisopropylamide (LDA) in THF at −78°C followed by quenching with D₂O leads to incorporation of deuterium into the (E)-N-methyl group only. Reaction of the anion of 1 with benzyl bromide at −78°C followed by quenching with water gave the E-isomer of (CO)₅WC[N(CH₃)CH₂CH₂C₆H₅]C₆H₄-p-CH₃ (2E, 26%) and recovered 1. When a mixture of the anion of 1 and benzyl bromide was warmed from −78°C to ambient temperature, a mixture of the E-isomer of the dibenzylated product (CO)₅WC[N(CH₃)CH(CH₂C₆H₅)₂]C₆H₄-p-CH was obtained. Reaction of the anion of 1 with allyl bromide gave (CO)₅WC[N(CH₃)CH₂CH₂CHCH₂]C₆H₄-p-CH₃ (5, 38%) and with methyl iodide gave a mixture of (CO)₅WC[N(CH₃)CH₂CH₃]C₆H₄-p-CH₃ (6, 7%) and (CO)₅W C[N(CH₃)CH(CH₃)₂]C₆H₄-p-CH₃ (7, 16%).     

B(C6F5)3-Catalyzed Conjugate Addition Reactions: Addition of C–H Aromatics as Nucleophiles to Nitroalkenes

Russian Journal of Organic Chemistry - A mild and efficient B(C6F5)3-catalyzed conjugate addition of N, N-dialkylanilines to aromatic nitroalkenes is reported. The catalyst shows excellent...     

Anions or cations: who is in charge of inhibiting the nickel(II) promoted B- to Z-DNA transition?

Various weakly binding cations and anions were studied at a concentration of 10 mM to ascertain their interaction with the nickel(II) promoted B- to Z-DNA transition of poly d(GC). These salts were ranked according to the decreasing amounts of nickel needed for the B- to Z-DNA transition and provided the following order: NaCl approximately Me4NCl > LiCl > MgCl2 > no salt > NaBF4 approximately NaNO3 approximately NaClO4. Remarkably, it was found that going from sodium nitrate to sodium chloride increased the necessary amount of nickel to induce the transition to the left-handed helix of poly d(GC) by a factor of 10. This dramatic effect cannot be explained by the binding constant of nickel(II) to chloride to form the monocationic complex. We believe that this is the first reported example of the role of chloride anions, which appear to modulate the interaction of nickel(II) ions with the polyanionic DNA.     

Insertion reaction of propene into Rh?H bond in HRh(CO)(PH3)2(C3H6) compound: A density functional study

Quantum mechanical calculations at the MP4 (SDQ) level using the BP86‐optimized geometries were carried out to investigate the energies and reaction mechanism for the propene (CH₃ C¹H CH$^{\mathrm{2}}_{\mathrm{2}}$) insertion reaction into the Rh H bond, using the cis‐HRh(CO)(PH₃)₂ compound as a model catalytic species. Since the reaction may occur on the branched carbon 1 or in the normal carbon 2 , which leads to branched and normal Rh(alkyl) compounds, respectively, we investigated these two mechanisms. The results show that the insertion in the branched carbon has an activation energy of 16.2 kcal/mol, and the activation energy for the reaction to take place at the normal carbon is 14.3 kcal/mol. These activation energies, together with the calculated relative energy of the metal–alkyl compounds formed after the insertion considering these two pathways, were used to access the regioselectivity on this reaction. We found a ratio of normal‐ and iso‐products, n:iso, of (96:4), which is in excellent agreement with the experimental regioselectity of (95:5). © 2000 John Wiley & Sons, Inc. Int J Quant Chem 78: 42–51, 2000     

Solvation-induced σ-complex structure formation in the gas phase: a revisit to the infrared spectroscopy of [C6H6-(CH3OH)2]+

Structures of the [C(6)H(6)-(CH(3)OH)(2)](+) cluster cation are investigated with infrared (IR) spectroscopy. While the noncovalent type structure has been confirmed for the n = 1 cluster of [C(6)H(6)-(CH(3)OH)(n)](+), only contradictory interpretations have been given for the spectra of n = 2, in which significant changes have been observed with the Ar tagging. In the present study, we revisit IR spectroscopy of the n = 2 cluster from the viewpoint of the σ-complex structure, which includes a covalent bond formation between the benzene and methanol moieties. The observed spectral range is extended to the lower-frequency region, and the spectrum is measured with and without Ar and N(2) tagging. A strongly hydrogen-bonded OH stretch band, which is characteristic to the σ-complex structure, is newly found with the tagging. The remarkable spectral changes with the tagging are interpreted by the competition between the σ-complex and noncovalent complex structures in the [C(6)H(6)-(CH(3)OH)(2)](+) system. This result shows that the microsolvation only with one methanol molecule can induce the σ-complex structure formation.     

New multicomponent organic semiconductors based on ET with polymeric Anions: α″-(ET)2N(CN)2·2H2O and α‴-(ET)6(NO3)3·2C2H5O2N3

New multicomponent radical cation salts derived from bis(ethylenedithio)tetrathiafulvalene (ET) were prepared: bis(ethylenedithio)tetrathiafulvalene dicyanamide dihydrate α″-(ET)₂N(CN)₂·2H₂O and bis-(ethylenedithio)tetrathiafulvalene nitrate α?-(ET)₆(NO₃)₃·2C₂H₅O₂N₃ containing two biuret molecules (C₂H₅O₂N₃). The crystal structures of the compounds were determined, and their conducting properties were examined. Both salts have layered structures in which radical cation layers alternate with nonconducting anionic layers. The radical cation layers in the salts α″-(ET)₂N(CN)₂·2H₂O and α?-(ET)₆(NO₃)₃·2C₂H₅O₂N₃ are packed in the α″ and α? fashion, respectively. Anionic layers consist of polymeric chains formed by hydrogen bonding between [N(CN)₂]? anions and water molecules in α″-(ET)₂N(CN)₂·2H₂O or between NO?₃ anions and biuret molecules in α?-(ET)₆(NO₃)₃·2C₂H₅O₂N₃. Both salts show semiconductor conductivity.     

Intramolecular Mobility in the Complexes 2SbCl3· p-C6H4(CH3)2and SbCl3· C6H5COCH3according to 35Cl NQR Data

Temperature dependences of the ³⁵Cl and ¹²¹Sb spin-lattice relaxation time ₁in antimony trichloride complexes with p-xylene and acetophenone were studied using NQR spectroscopy. The activation energies of the retarded motion (similar to that discovered in some v complexes of SbCl₃) of chlorine atoms were determined from the ³⁵Cl ₁(T) function. New data on the shape of the ¹²¹Sb ₁(T) function are discussed.     

Ge(14)[Ge(SiMe(3))(3)](5)Li(3)(THF)(6): the largest metalloid cluster compound of germanium: on the way to fullerene-like compounds?

The reaction of GeBr with LiGe(SiMe(3))(3) yields the largest metalloid cluster compound of germanium Ge(14)[Ge(SiMe(3))(3)](5)Li(3)(THF)(6), in which 14 germanium atoms are arranged as a hollow sphere in the cluster core, showing that in the case of germanium also fullerene-like compounds might be present in the borderland between the molecular and solid states.     

Stereocontrolled Synthesis of α-3-Deoxy-d-manno-oct-2-ulosonic Acid (α-Kdo) Glycosides Using C3-p-Tolylthio-Substituted Kdo Donors: Access to Highly Branched Kdo Oligosaccharides

3-Deoxy-d -manno-oct-2-ulosonic acid (Kdo) is an eight-carbon monosaccharide found widely in bacterial lipopolysaccharides (LPSs) and capsule polysaccharides (CPSs). We developed an indirect method for the stereoselective synthesis of α-Kdo glycosides with a C3-p-tolylthio-substituted Kdo phosphite donor. The presence of the p-tolylthio group enhanced the reactivity, suppressed the formation of elimination by-products (2,3-enes), and provided complete α-stereocontrol. A variety of Kdo α-glycosides were synthesized by our method in excellent yields (up to 98 %). After glycosylation, the p-tolylthio group can be efficiently removed by free-radical reduction. Subsequently, the orthogonality of the phosphite donor and thioglycoside donor was demonstrated by the one-pot synthesis of a trisaccharide in Helicobacter pylori and Neisseria meningitidis LPS. Moreover, an efficient total synthesis route to the challenging 4,5-branched Kdo trisaccharide in LPSs from several A. baumannii strains was highlighted. To demonstrate the high reactivity of our approach further, the highly crowded 4,5,7,8-branched Kdo pentasaccharide was synthesized as a model molecule for the first time. Additionally, the reaction mechanism was investigated by DFT calculations.     

How reliable are GIAO calculations of 1H and 13C NMR chemical shifts? A statistical analysis and empirical corrections at DFT (PBE/3z) level

Reliability of calculated (1)H and (13)C NMR chemical shifts for various classes of organic compounds obtained with gauge-invariant atomic orbital (GIAO) approach has been studied at the PBE/3ζ level (as implemented in PRIRODA code) using linear regression analysis with experimental data. Empirical corrections for the calculated chemical shifts δ(H,calc) = δ(PBE/3ζ) - 0.08 ppm (RMS 0.18 ppm, MAD 0.66 ppm) and δ(C,calc) = δ(PBE/) (3) (ζ) - 6.35 ppm (RMS 3.09 ppm, MAD 9.42 ppm) have been developed using the sets of 263 and 308 experimental values for (1)H and (13)C chemical shifts, respectively. The confidence intervals of NMR chemical shifts at 95% confidence probability are δ(H,calc) ± 0.35 ppm for (1)H and δC,calc) ± 6.05 ppm for (13)C.     

Intermolecular interactions in two (ferrocenylmethyl)benzimidazoles incorporating the 4-MeOC6H4 and 3,4-(MeO)2C6H3 groups: analysis of MeO—C—C distortions from ideal 120° geometry

The title compounds, 1-ferrocenyl­methyl-2-(4-methoxy­phenyl)-1H-benz­imidazole, [Fe(C₅H₅)(C₂₀H₁₇N₂O)], (I), and 2-(3,4-di­methoxy­phenyl)-1-ferrocenyl­methyl-1H-benz­imid­az­ole, [Fe(C₅H₅)(C₂₁H₁₉N₂O₂)], (II), are model electroactive compounds for anion sensor and antimalarial applications. Distortions from the ideal 120° angle about the MeO—C—C groups are evident, with angles of 115.1 (2) and 125.0 (2)° in (I), and 115.9 (2) and 124.6 (2)°, and 115.7 (2) and 125.1 (2)° in (II). The main intermolecular hydrogen bonds in (I) comprise C—H⋯N and C—H⋯π(C₅H₅) interactions, while in (II), only weak C—H⋯π(imidazole) and C—H⋯π(arene) interactions are present.