Catalytic Generation and Chemoselective Transfer of Nucleophilic Hydrides from Dihydrogen

Copper(I)–N-heterocyclic-carbene (NHC) complexes enabled the catalytic generation of nucleophilic hydrides from dihydrogen (H₂) and their subsequent transfer to allylic chlorides. The highly chemoselective catalyst displayed no concomitant hydrogenation reactivity; in fact, the terminal double bond formed in the hydride transfer remained intact. Switching to deuterium gas (D₂) allowed for regioselective monodeuteration with excellent isotope incorporation.     

Quantum mechanical scattering cross sections and permeability coefficients for ions crossing the human red cell and resting squid axon membranes

Quantum mechanical calculations of elastic scattering cross sections for some permeant ions crossing the human red blood cell and resting axolemma squid axon membranes have been carried out using the three-dimensional spherically symmetric square potential well. Making the assumption that the permeability coefficient is inversely proportional to scattering cross section, we obtain the order of membrane selectivity for the ions as well as values for the permeability coefficients. Despite the relatively simple method used, good agreement between calculated permeability coefficients and data available in the literature is obtained. We suggest that elastic scattering cross section measurements for ions in various membranes would be valuable not only because they give a precise idea about the permeability ratios between ions but they also determine the form of the potential the ions are moving in.     

A short proof of the Approximation Conjecture for amenable groups

We give a short proof of the Approximation Conjecture with complex coefficients for amenable groups [G. Elek, The Strong Approximation Conjecture holds for amenable groups, J. Funct. Anal. 239 (1) (2006) 345–355].     

The synthesis of azadirachtin: a potent insect antifeedant

We describe in full the first synthesis of the potent insect antifeedant azadirachtin through a highly convergent approach. An O-alkylation reaction is used to unite decalin ketone and propargylic mesylate fragments, after which a Claisen rearrangement constructs the central C8-C14 bond in a stereoselective fashion. The allene which results from this sequence then enables a second critical carbon-carbon bond forming event whereby the [3.2.1] bicyclic system, present in the natural product, is generated via a 5-exo-radical cyclisation process. Finally, using knowledge gained through our early studies into the reactivity of the natural product, a series of carefully designed steps completes the synthesis of this challenging molecule.     

Strong intramolecular Si-N interactions in the chlorosilanes Cl3-nHnSiOCH2CH2NMe2 (n = 1-3)

The compounds Cl 3SiOCH 2CH 2NMe 2 ( 1) and Cl 2HSiOCH 2CH 2NMe 2 ( 2) were prepared by reactions of lithium 2-(dimethylamino)ethanolate with SiCl 4 and HSiCl 3. The analogous reaction with H 2SiCl 2 gave ClH 2SiOCH 2CH 2NMe 2 ( 3), but only in a mixture with Cl 2HSiOCH 2CH 2NMe 2 ( 2), from which it could not be separated. All compounds were characterized by IR and NMR ( (1)H, (13)C, (29)Si) spectroscopy, 1 and 2 by elemental analyses and by determination of their crystal structures. Cl 3SiOCH 2CH 2NMe 2 ( 1) and Cl 2HSiOCH 2CH 2NMe 2 ( 2) crystallize as monomeric ring compounds with pentacoordinate silicon atoms participating in intramolecular Si-N bonds [2.060(2) A ( 1), 2.037(2) A ( 2)]. The dative bonds in 1 and 2 between the silicon and nitrogen atoms could also be proven to exist at low temperatures in solution in (1)H, (29)Si-HMBC-NMR experiments by detection of the scalar coupling between the (29)Si and the protons of the NCH 2 and NCH 3 groups. A function describing the chemical shift delta exp (29)Si dependent on the chemical shifts of the individual equilibrium components, the temperature, and the free enthalpy of reaction was worked out and fitted to the experimental VT-NMR data of 1 and 2. This provided values of the free reaction enthalpies of Delta G = -28.8 +/- 3.9 kJ x mol (-1) for 1 and Delta G = -22.3 +/- 0.4 kJ x mol (-1) for 2 and estimates for the chemical shifts of open-chain (index o) and ring conformers (index r) for 1 of delta r = -94 +/- 2 ppm and delta o = -36 +/- 5 ppm and for 2 of delta r = -82 +/- 1 ppm and delta o = -33 +/- 4 ppm. The value of delta r for 1 is very close to that obtained from a solid-state (29)Si MAS NMR spectrum. Quantumchemical calculations (up to MP2/TZVPP) gave largely differing geometries for 1 (with a Si...N distance of 3.072 A), but well reproduced the geometry of 2. These differences are due to Cl...H and Cl...C repulsions and solid state effects, which can be modeled by conductor-like screening model calculations and also rationalized in terms of the topology of the electron density, which was analyzed in terms of the quantum theory of atoms in molecules.     

Helical complexes containing diamide-bridged benzene-o-dithiolato/catecholato ligands

The benzene-o-dithiol/catechol ligands H4-2 and H4-3 react with [TiO(acac)2] to give the dinuclear, double-stranded anionic complexes [Ti2(L)2(mu-OCH3)2](2-) ([22](2-), L=2(4-); [23](2-), L=3(4-)). NMR spectroscopic investigations reveal that the complex anion [Ti2(2)2(mu-OCH3)(2)](2-) is formed as a mixture of three of four possible isomers/pairs of enantiomers, whereas only one isomer of the complex anion [Ti2(3)2(mu-OCH3)(2)](2-) is obtained. The crystal structure analysis of (PNP)2[Ti2(3)2(mu-OCH3)2] shows a parallel orientation of the ligand strands, whereas the structure determination for (AsPh4)2[Ti2(2)2(mu-OCH3)2] does not yield conclusive results about the orientation of the ligand strands due the presence of different isomers in solution, the possible co-crystallisation of different isomers and severe disorder in the crystal. NMR spectroscopy shows that ligand H4-3 reacts at elevated temperature with [TiO(acac)2] to give the triple-stranded helicate (PNP)4[Ti2(3)3] ((PNP)4[24]) as a mixture of two isomers, one with a parallel orientation of the ligand strands and one with an antiparallel orientation. Exclusively the triple-stranded helicates [Ti2(L)(3)](4-) ([25](4-), L=1(4-); [26](2-), L=4(4-)) are formed in the reaction of ligands H4-1 and H4-4 with [TiO(acac)2]. The molecular structures of Na(PNP)3[Ti2(1)3]CH(3)OHH(2)OEt(2)O (Na(PNP)3[25]CH(3)OHH(2)OEt(2)O) and Na(1.5)(PNP)(6.5)[Ti2(4)3]2.3 DMF (Na(1.5)(PNP)(6.5)[26]2.3 DMF) reveal a parallel orientation of the ligand strands in both complexes, which is retained in solution. The sodium cations present in the crystal structures lead to two different kinds of aggregation in the solid state. Na-[25]-Na-[25]-Na polymeric chains are formed from compound Na(PNP)3[25], with the sodium cations coordinated by the carbonyl groups of two ligand strands from two different [Ti2(1)3](4-) ions in addition to solvent molecules. In contrast to this, two [Ti2(4)3](4-) ions are connected by a sodium cation that is coordinated by the three meta oxygen atoms of the catecholato groups of each complex tetraanion to form a central {NaO6} octahedron in the anionic pentanuclear complex {[26]-Na-[26]}(7-).     

Oxygenation of simple zinc alkyls: surprising dependence of product distributions on the alkyl substituents and the presence of water

Oxygenation reactions of dialkylzinc solutions have been investigated. Me2Zn reacts with oxygen in the absence of water to give the bis(heterocubane) [(MeZn)6Zn(OMe)8], whereas Et2Zn and iPr2Zn afford the mono(heterocubanes) [(RZn)4(OR)4]. In the presence of small amounts of water (added during or after the oxygenation) the product types are reversed for Me2Zn and Et2Zn giving [(MeZn)4(OMe)4] and [(EtZn)6Zn(OEt)8], while being retained for iPr2Zn (giving [(iPrZn)4(OiPr)4]). Full characterization of all products by NMR spectroscopy, mass spectrometry, and elemental analyses is provided, and crystal structures of [(EtZn)6Zn(OEt)8] and [(iPrZn)4(OiPr)4] are reported. A rationalization of the different reactivities is attempted on the basis of DFT-calculated energies of some key reactants.     

Metallosupramolecular chemistry with bis(benzene-o-dithiolato) ligands

The bis(benzene-o-dithiol) ligands H(4)-1, H(4)-2, and H(4)-3 react with [Ti(OC(2)H(5))(4)] to give dinuclear triple-stranded helicates [Ti(2)L(3)](4)(-) (L = 1(4)(-), 2(4)(-), 3(4)(-)). NMR spectroscopic investigations revealed that the complex anions possess C(3) symmetry in solution. A crystal structure analysis for (PNP)(4)[Ti(2)(2)(3)] ((PNP)(4)[14]) confirmed the C(3) symmetry for the complex anion in the solid state. The complex anion in Li(PNP)(3)[Ti(2)(1)(3)] (Li(PNP)(3)[13]) does not exhibit C(3) symmetry in the solid state due to the formation of polymeric chains of lithium bridged complex anions. Complexes [13](4)(-) and [14](4)(-) were obtained as racemic mixtures of the Delta,Delta and Lambda,Lambda isomers. In contrast to that, complex (PNP)(4)[Ti(2)(3)(3)] ((PNP)(4)[15]) with the enantiomerically pure chiral ligand 3(4)(-) shows a strong Cotton effect in the CD spectrum, indicating that the chirality of the ligands leads to the formation of chiral metal centers. The o-phenylene diamine bridged bis(benzene-o-dithiol) ligand H(4)-4 reacts with Ti(4+) to give the dinuclear double-stranded complex Li(2)[Ti(2)(4)(2)(mu-OCH(3))(2)] containing two bridging methoxy ligands between the metal centers. The crystal structure analysis and the (1)H NMR spectrum of (Ph(4)As)(2)[Ti(2)(4)(2)(mu-OCH(3))(2)] ((Ph(4)As)(2)[(16]) reveal C(2) symmetry for the anion [Ti(2)(4)(2)(mu-OCH(3))(2)](2)(-). For a comparative study the dicatechol ligand H(4)-5, containing the same o-phenylene diamine bridging group as the bis(benzene-o-dithiol) ligands H(4)-4, was prepared and reacted with [TiO(acac)(2)] to give the dinuclear complex anion [Ti(2)(5)(2)(mu-OCH(3))(2)](2)(-). The molecular structure of (PNP)(2)[Ti(2)(5)(2)(mu-OCH(3))(2)] ((PNP)(2)[17]) contains a complex anion which is similar to [16](2)(-), with the exception that strong N-H...O hydrogen bonds are formed in complex anion [17](2)(-), while N-H...S hydrogen bonds are absent in complex anion [16](2)(-).     

X-ray submicrometer phase contrast imaging with a Fresnel zone plate and a two dimensional grating interferometer

The application of a two dimensional (2D) grating interferometer-Fresnel zone plate combination for quantitative submicron phase contrast imaging is reported. The combination of the two optical elements allows quick recovery of the phase shift introduced by a sample in a hard X-ray beam, avoiding artifacts observed when using the one dimensional (1D) interferometer for a sample with features oriented in the unsensitive direction of the interferometer. The setup provides submicron resolution due to the optics magnification ratio and a fine sensitivity in both transverse orientations due to the 2D analysis gratings. The method opens up possibilities for sub-micro phase contrast tomography of microscopic objects made of light and/or homogeneous materials with randomly oriented features.