Kinetic resolution of P-stereogenic phosphine boranes via deprotonation using s-butyllithium/(−)-sparteine

The attempted kinetic resolution of racemic secondary phosphine boranes [t-BuPhP(BH₃)H and t-BuMeP(BH₃)H] by P–H deprotonation using 0.5 equiv of s-BuLi and (?)-sparteine was unsuccessful and generated racemic benzyl bromide-trapped adducts in 42–49% yield. In contrast, an efficient kinetic resolution was observed with racemic tertiary phosphine boranes [t-BuPhP(BH₃)Me and t-BuEtP(BH₃)Me] by C–H deprotonation on the P–Me group using 0.5 or 0.6 equiv of s-BuLi and (?)-sparteine. For example, the use of 0.6 equiv of s-BuLi/(?)-sparteine with t-BuEtP(BH₃)Me and trapping with DMF gave the (R)-aldehyde adduct in 37% yield and 83:17 er together with recovered (R)-t-BuEtP(BH₃)Me in 44% yield and 74:26 er. These are the first examples of kinetic resolution of P-stereogenic phosphine boranes via deprotonation using s-BuLi/(?)-sparteine.     

On the two-ligand catalytic asymmetric deprotonation of N-boc pyrrolidine: probing the effect of the stoichiometric ligand

To map out the stoichiometric ligand requirements in the two-ligand catalytic asymmetric deprotonation of N-Boc pyrrolidine, 24 different ligands have been evaluated; the highest enantioselectivity (90:10 er) was obtained by using s-BuLi in the presence of 0.3 equiv of (-)-sparteine and 1.3 equiv of a cyclohexanediamine-derived ligand.     

Enantioselective construction of all-carbon quaternary spirocenters through a Pd-catalyzed asymmetric intramolecular ipso-Friedel–Crafts allylic alkylation of phenols

A novel catalytic asymmetric synthetic method for making spirocyclohexadienones with an all-carbon quaternary spirocenter was developed based on the Pd-catalyzed intramolecular ipso-Friedel–Crafts allylic alkylation of phenols. When 5 mol % of the Pd catalyst and 12 mol % of (?)-9-NapBN (?)-3e were used, the spirocyclic adduct was obtained with up to 93% ee, albeit with low chemical yield. On the other hand, when using 6 mol% of the Trost ligand (R,R)-3k, the spirocyclic adducts were obtained in good yields with up to 89% ee (diastereoselectivity = 9.2:1).     

DFT study of group 8 catalysts for the hydrophenylation of ethylene: Influence of ancillary ligands and metal identity

Computational methods are used to investigate catalytic hydrophenylation of ethylene using complexes of the type [(Y)M(L)(CH₃)(NCMe)]ⁿ⁺ [Y = Mp, n = 1; Y = Tp, n = 0; M = Ru or Os; L = PMe₃, PF₃, or CO; Mp = tris(pyrazolyl)methane; Tp = hydrido-tris(pyrazolyl)borate]. The conversion of ethylene and benzene to ethylbenzene with [(Y)M(L)(Ph)]ⁿ⁺ as catalyst involves four steps: (1) ethylene coordination, (2) ethylene insertion into the M–Ph bond, (3) benzene coordination, and (4) benzene C–H activation. DFT calculations form the basis to compare stoichiometric benzene C–H activation by [(Y)M(L)(CH₃)(NCMe)]ⁿ⁺ complexes to yield methane and [(Y)M(L)(Ph)(NCMe)]ⁿ⁺. In addition, starting from the 16-electron species [(Y)M(L)(Ph)]ⁿ⁺, potential energy surfaces for the formation of ethylbenzene are calculated to reveal the impact of modifications to the scorpionate ligand (Mp or Tp), co-ligand (L) and metal center (M).     

Application of a laser produced plasma: Experimental Stark widths of single ionized lead lines

The optical emission spectroscopy from laser produced plasma generated by a 10,640 Å radiation, with an irradiance of 1.4 × 10¹⁰ W cm^− 2 on several lead targets, in vacuum and in an atmosphere of argon, was recorded and analyzed between 1900 and 7000 Å. The Local Thermodynamic Equilibrium conditions and plasma homogeneity have been checked. Stark widths for 31 lines of Pb II have been measured. These lines measured in this work correspond to the transitions 7s → 6p, n(n = 8, 9, 10)s → 7p, n(n = 7, 8)p → 7s, n(n = 7, 8)p → 6p², n(n = 7, 8)d → 7p, n(n = 5, 6)f → 6d, n(n = 5, 6)f → 6p². The population level distribution and the corresponding temperatures were obtained using Boltzmann plots. The plasma electron densities were determined using well-known Stark broadening parameters of spectral lines. Special attention was dedicated to the possible self-absorption of the different transitions. Temporal evolution of the plasma parameters was studied between 0.1 and 9 μs. Stark broadening parameters of the spectral lines were measured at 2.5 μs where the electron temperature was close to 11,300 K and the electron density to 0.8 × 10¹⁶ cm^− 3. The experimental results obtained have been compared with the experimental and theoretical values given by other authors. A systematic trend of this parameter versus temperature of 4244.9 Å Pb II line has been presented.     

Asymmetric ortho-lithiation of 1,n-dioxa[n]paracyclophane derivatives for the generation of planar chirality

The asymmetric induction of planar chirality in 1,n-dioxa[n]paracyclophane derivatives via asymmetric ortho-lithiation is described. Enantioselective ortho-lithiation of unflippable 1,n-dioxa[n]paracyclophanes (n≤11) using sec-BuLi-(?)-sparteine at ?78 °C and subsequent treatment with electrophiles gave the corresponding planar-chiral monosubstituted paracyclophanes with excellent ee. Further lithiation of these compounds and treatment with electrophiles gave planar-chiral paracyclophanes with two different substituents. Dilithiation of unflippable 1,n-dioxa[n]paracyclophanes gave the corresponding C₂-symmetrical disubstituted products with almost perfect ee. In the case of flippable 1,n-dioxa[n]paracyclophanes (n≥12), a stepwise reaction was required for the highly enantioselective formation of disubstituted products.     

Correlation of alkylamine nucleophilicities with their basicities

The relationship of the nucleophilicity of alkylamines to their basicity is explored with emphasis on steric hindrance to solvation. The equation n = 1.43(?Σσ¹ + δE_s) + 6.35, where n is the Swain-Scott nucleophilic value, σ¹ is the Taft polarity value, and δE_s is the Taft steric value, correlates the nucleophilic constants of 17 common alkylamines and ammonia over two powers of 10 with a correlation coefficient of 0.98. The equation n ? 1.42 δE_s = 0.44(pK_a + S) + 0.17, where S is the solvation constant, correlates the nucleophilicities of these amines and ammonia with their pK_a values over 3 powers of 10 with a correlation coefficient of 0.99. Excessive steric hindrance and nearby functional groups cause deviations from these equations.     

Sterically encumbered chiral amino alcohols for the titanium catalyzed asymmetric alkylation of benzaldehyde

Sterically encumbered chiral l-amino alcohols with secondary amines and tertiary alcohols catalyze the enantioselective alkylation of benzaldehyde with diethyl zinc. Using 2 mol % of amino alcohol catalyst predominantly gave (R)-1-phenylpropanol with enantiomeric excesses of up to 61%. Using the in situ prepared titanium complex at 2 mol % as catalyst also favored the (R)-enantiomer with enantiomeric excesses of up to 73%. In almost all cases, the addition catalyzed by the titanium complex exhibited higher enantioselectivity than that of the amino alcohol ligand alone.     

A novel design of a temperature-controlled FT-ICR cell for low-temperature black-body infrared radiative dissociation (BIRD) studies of hydrated ions

A novel design for a temperature-controlled ICR cell is described for use in black-body infrared radiative dissociation (BIRD) studies of weakly bound systems like water clusters. Due to several improved design features, it provides a very uniform black-body radiation environment, and at the same time maintains efficient pumping for a low collision rate on the order of 10^?2 s^?1. At the lowest temperatures reached, nominally 89 K cell plate temperature, water evaporation effectively ceases, while intracluster reactions in V⁺(H₂O)_n with a small activation energy are still observed. BIRD rate constants for Ag⁺(H₂O)_n, n = 4–6, are shown in the temperature range T = 160–320 K. For n = 6, a linear Arrhenius plot with R² = 0.9943 is obtained without any calibration, confirming the suitability of the cell for quantitative BIRD studies.     

Development of a selective fluorimetric technique for rapid trace determination of zinc using 3-hydroxyflavone

A sensitive and a selective spectrofluorimetric method have been developed for the rapid determination of trace levels of zinc. The method is based on complex formation between zinc and 3-hydroxyflavone (3HF), which displays an intense emission signal around 478 nm. The analytical performance of the method was examined by considering the factors that affect the complex formation such as pH, mole ratio of the metal and solvent type. The optimum conditions for the complex formation were metal to ligand stoichiometric ratio of 1:1 at pH 7.5 with 0.1 M Tris buffer. Under these conditions the detection limit attained was 1.5 ppb. The method was appropriately validated and yielded relative standard deviations of less than 2% (n = 5), which was considered acceptable. It was successfully applied to the trace determination of zinc in drinking water, hair shampoo and pharmaceutical samples.     

Densities,speeds of sound,and refractive indices of the ternary mixtures (toluene + methyl acetate + butyl acetate) and (toluene + methyl acetate + methyl heptanoate) at 298.15 K

Density, ρ, speed of sound, u, and refractive index, n_D, at 298.15 K and atmospheric pressure have been measured over the entire composition range for (toluene + methyl acetate + butyl acetate) and (toluene + methyl acetate + methyl heptanoate) systems. Excess molar volumes, V^E, isentropic compressibility, κ_s, isentropic compressibility deviations, Δκ_s, and changes of refractive index on mixing, Δn_D, for the above systems, have been calculated from experimental data and fitted to Cibulka, Singh et al., and Nagata and Sakura equations, standard deviations from the regression lines are shown. Geometrical solution models, Tsao and Smith, Kholer, Jacob and Fitzner, Rastogi et al. were also applied to predict ternary properties from binary contributions.     

Oscillator strength measurements of the 5s5p 3P1 → 5snd 3D2 Rydberg transitions of cadmium

We report oscillator strengths distribution in the bound region of cadmium corresponding to the 5s5p ³P₁ → 5snd ³D₂ (21 ≤ n ≤ 52) Rydberg transitions. The experiment was performed using two frequency doubled dye lasers simultaneously pumped by a common Nd:YAG laser in conjunction with an atomic beam set-up. The absolute photoionization cross section of the 5s5p ³P₁ intermediate state at the 5s ²S_1/2 ionization threshold has been measured as 20 (4) Mb using the saturation technique. The measured value of the photoionization cross section at threshold is used to extract the f-values of the above mentioned Rydberg transitions.     

Synthesis and spectroscopic characterization of new tetradentate Schiff base and its coordination compounds of NOON donor atoms and their antibacterial and antifungal activity

New Schiff base (H₂L) ligand is prepared via condensation of o-phthaldehyde and 2-aminobenzoic acid in 1:2 ratio. Metal complexes are prepared and characterized using elemental analyses, IR, solid reflectance, magnetic moment, molar conductance, ¹H NMR, ESR and thermal analysis (TGA). From the elemental analyses data, the complexes were proposed to have the general formulae [MCl(L)(H₂O)]·2H₂O (where M = Cr(III) and Fe(III)); [M(L)]·yH₂O (where M = Mn(II), Ni(II), Cu(II) and Zn(II), y = 1–2) and [M(L)(H₂O)_n]·yH₂O (where M = Co(II) (n = y = 2), Co(II) (n = y = 1), Ni(II) (n = 2, y = 1). The molar conductance data reveal that all the metal chelates were non-electrolytes. IR spectra show that H₂L is coordinated to the metal ions in a bi-negative tetradentate manner with NOON donor sites of the azomethine-N and carboxylate-O. The ¹H NMR spectral data indicate that the two carboxylate protons are also displaced during complexation. From the magnetic and solid reflectance spectra, it was found that the geometrical structure of these complexes are octahedral (Cr(III), Fe(III), Co(II) and Ni(II)), square planar (Cu(II)), trigonal bipyramidal (Co(II)) and tetrahedral (Mn(II), Ni(II) and Zn(II)). The thermal behaviour of these chelates showed that the hydrated complexes losses water molecules of hydration in the first step followed immediately by decomposition of the ligand molecule in the subsequent steps. The biological activity data show that the metal complexes to be more potent/antibacterial than the parent Shciff base ligand against one or more bacterial species.     

Enantioselective addition of dialkylzinc reagents to N-(diphenylphosphinoyl)imines catalyzed by β-aminoalcohols with the prolinol skeleton

Several β-aminoalcohols with the prolinol framework are shown to be very efficient catalysts for the enantioselective addition of dialkylzinc reagents to N-(diphenylphosphinoyl)imines. The use of 0.5 equiv of the catalyst leads to the expected addition products in good yields and with ee up to 94% in a reaction time of only 4 h at room temperature. This ee is the highest value reported so far using 0.5 equiv of an aminoalcohol as a promoter. High enantioselectivities are obtained in the addition of dialkylzincs to both aromatic and aliphatic imines. The amount of the catalyst can be reduced to 0.25 equiv with a slight decrease in the ee. A very interesting effect of the addition rate and temperature on the enantioselectivity was also observed.     

N-Benzyl-l-prolinol: an efficient catalyst for the enantioselective addition of dialkylzinc reagents to N-(diphenylphosphinoyl)imines

Commercially available N-benzyl-l-prolinol has shown to be a very efficient catalyst for the enantioselective addition of dialkylzinc reagents to N-(diphenylphosphinoyl)imines. The use of 0.5 equiv of this β-aminoalcohol as a catalyst leads to the expected addition products in good yields and with ees up to 92% in a reaction time of only 4 h at room temperature. This ee is almost equal to the highest value reported so far using 0.5 equiv of an aminoalcohol as promoter, although the reaction time is much shorter in our case. The amount of the catalyst can be reduced to 0.25 equiv with a slight decrease in the ee. An interesting effect of the addition rate and temperature on the enantioselectivity has been observed.     

Enantioselective kinetic resolution of 3-phenyl-2-ketones using Baeyer–Villiger monooxygenases

The enantioselective kinetic resolution of two 3-phenyl-2-ketones using four different Baeyer–Villiger monooxygenases (BVMO) expressed recombinantly in Escherichia coli was studied. The highest enantioselectivity (E = 82) was achieved for 3-phenyl-2-butanone using a BVMO originating from Pseudomonas fluorescens. A BVMO from Pseudomonas putida showed an opposite (R)-enantiopreference and E = 12.     

Noise and ac impedance analysis of ion transfer kinetics at the micro liquid/liquid interface

Fluctuation analysis was utilized to determine the TEA ion transfer kinetics across the water/1,2-dichloroethane interface. The obtained data were compared with those derived from electrochemical impedance spectroscopy experiments using the same electrolytic cell. The apparent standard rate constants k_s determined by these two techniques have a similar value. The average value k_s = 0.37 cm s^− 1 is comparable with the previously reported value k_s = 0.2 cm s^− 1. The experimental approach utilizing a thick wall glass micro-capillary to fix the interface exhibits a very small stray capacitance value, proving this system to be suitable for determining the kinetics of the fast ion transfer across a liquid/liquid interface. Application of a method employing a small perturbation signal prevents polarization of the inner capillary surface by current flowing through the cell. The induced polarization of the capillary can affect ion concentration at the interface due to electroosmosis and thus make the kinetic data evaluation difficult or erroneous.     

Gas chromatography–mass spectrometry study of hydrogen–deuterium exchange reactions of volatile hydrides of As,Sb, Bi,Ge and Sn in aqueous media

The H–D exchange processes in MH_n or MD_n hydrides (M = As, Sb, Bi, n = 3; M = Ge, Sn, n = 4) taking place when they are in contact with H₂O or D₂O solution at different pH or pD values (interval of pH = [0,13]) have been investigated using gas chromatography–mass spectrometry (GC-MS). MH_n or MD_n compounds were injected into the headspace of reaction vials (4–12 ml) containing 1–2 ml of buffered solution maintained under stirring or shaking conditions. The isotopic composition of the gaseous phase hydrides/deuterides was determined at regular intervals in the range of time 0–15 min. The MH_n or MD_n compounds were synthesized in separate vials and their purity was checked separately before injection into the reaction vials. The mass spectra were deconvoluted in order to estimate the relative abundance of each species formed following the H–D exchange process (AsH_nD_3−n , SbH_nD_3−n, BiH_nD_3−n, n = 0–3; GeH_nD_4−n, SnH_nD_4−n, n = 0–4) and the relative abundance of H and D. In the investigated pH (or pD) interval arsanes and stibanes undergo H–D exchange in alkaline media for pH > 7. No H–D exchange was detected for the other hydrides, where the prevailing process is their decomposition in the aqueous phase. A reaction model, based on the formation of protonated or deprotonated intermediates is proposed for H–D exchange of MH_n or MD_n compounds placed in contact with H₂O or D₂O at different pH or pD values. The H–D exchange in the already formed hydrides can be source of the interference in mechanistic studies on hydride formation performed using labeled reagents; no H–D exchange was detected within the following pH intervals that can be considered free from interference: arsanes pH = [0,7), stibanes pH = [0,7), bismuthanes, germanes and stannanes pH = [0,13].     

Oxidative homo-coupling of potassium aryltrifluoroborates catalyzed by gold nanocluster under aerobic conditions

Gold(0) nanoclusters, stabilized by poly(N-vinyl-2-pyrrolidone) (Au:PVP-n), catalyzed the oxidative homo-coupling reaction of potassium aryltrifluoroborate in water under air. Catalytic activity was dependent on the size of clusters. The smallest cluster Au:PVP-1 (d_av = 1.3 ± 0.3 nm) gave the highest activity, while Au:PVP-7 (d_av = 9.5 ± 1.0 nm) did not catalyze the homo-coupling. The catalyst was reusable for several times. Positively charged surface on the Au cluster, generated by the adsorption of molecular oxygen, would be the active site of the catalysis.     

Phosphanes with bulky oligosilyl substituents

By reaction of dichloroheptasilane [(SiMe₃)₂MeSi]₂SiCl₂ with lithiumphosphanides LiPHR, the silylphosphanes [(SiMe₃)₂MeSi]₂SiClPHR with R = 2, 4, 6-tri-tert-butylphenyl ( = supermesityl, Mes1) (1) and Si(SiMe₃)₃ ( = hypersilyl, Hyp) (2) were prepared. Both compounds were characterized with X-ray diffraction, multinuclear NMR spectroscopy and elemental analysis. Compound 1 did not react with n-BuLi, but only with a large excess of tert-BuLi. Phosphasilene [(SiMe₃)₂MeSi]₂SiPMes¹ could be identified by a ³¹P NMR signal at +346 ppm. All attempts to separate it from the reaction mixture failed due to many by-products which had formed through SiSi and SiP bond cleavage. Lithiation of 2 was possible with 4.2 equiv. of tert-BuLi, and crystals of the lithiumphosphanide [(SiMe₃)₂MeSi]₂SiClPLiHyp (3) could be obtained from THF, albeit in a quality not sufficient for X-ray diffraction. All attempts to achieve LiCl elimination and formation of the phosphasilene [(SiMe₃)₂MeSi]₂SiPSi(SiMe₃)₃ failed due to the unusual stability of the lithiumphosphanide. Prolongued refluxing in toluene (110 °C) only led to complete loss of coordinated THF, and ³¹P⁷Li spin spin coupling could be observed in the ³¹P NMR spectrum (¹J_PLi = 84 Hz).Reaction of potassium phosphanide [(SiMe₃)₃Si]SiMe₃PK with SiCl₄ led to the formation of [(SiMe₃)₃Si](SiMe₃)P(SiCl₃) (4), which could be successfully characterized with X-ray diffraction and multinuclear NMR spectroscopy. SiP bond lengths vary between 218 pm (SiCl₃) and 230 pm (hypersilyl). Despite these differences, ³¹P²⁹Si coupling constants are nearly identical (92.4 Hz and 85.5 Hz, respectively).