Structure–Solubility Relationship of 1,4-Dioxane Complexes of Di(hydrocarbyl)magnesium

Systematic variation of the 1,4-dioxane (dx) concentration during the precipitation of sparingly soluble [MgBr₂(dx)₂] from ethereal Grignard solutions of RMgBr has allowed the structural investigation of crystallized [R₂Mg(dx)_n] (n=1, 1.5, 2, and 3), which form during this dioxane method, depending on the bulkiness of R. The numbering of the complexes explored in this study is based on the number n of dioxane molecules per magnesium atom, followed by the substituent R; an apostrophe denotes coordination polymers. The following derivatives were studied by X-ray crystal-structure determination and NMR spectroscopy: n=1: [Me₂Mg(μ-dx)]_∞ ( 1′-Me ) and [nPr₂Mg(μ-dx)]_∞ ( 1′-nPr ); n=1.5: [{iPr₂Mg(dx)}₂(μ-dx)] ( 1.5-iPr ), [{oTol₂Mg(dx)}₂(μ-dx)] ( 1.5-oTol ), and [(Me₃Si-C≡C)₂Mg(dx)_1.5]_∞ ( 1.5′-C₂SiMe₃ ); n=2: [tBu₂Mg(dx)₂] ( 2-tBu ) and [oTol₂Mg(dx)₂] ( 2-oTol ); n=3: [Ph₂Mg(dx)₃] ( 3-Ph ). In the structure types 1′ , 1.5 , and 2 , the magnesium atom exhibits the coordination number 4, whereas pentacoordinate metal atoms are observed in types 3 and 1.5′ . The structure type 2′ is realized for [(Ph-C≡C)₂Mg(dx)₂]_∞ ( 2′-C₂Ph ), [MgCl₂(dx)₂]_∞ ( 2′-Cl ), and [MgBr₂(dx)₂]_∞ ( 2′-Br ) with hexacoordinate metal atoms. The solubility of the dioxane adducts in common organic solvents strongly depends on the degree of aggregation with the solubility decreasing from molecular to strand to layer structures.     

Toward Transition-Metal-Templated Construction of Arylated B4 Chains by Dihydroborane Dehydrocoupling

The reactivity of a diruthenium tetrahydride complex towards three selected dihydroboranes was investigated. The use of [DurBH₂] (Dur=2,3,5,6-Me₄C₆H) and [(Me₃Si)₂NBH₂] led to the formation of bridging borylene complexes of the form [(Cp*RuH)₂BR] (Cp*=C₅Me₅; 1 a : R=Dur; 1 b : R=N(SiMe₃)₂) through oxidative addition of the B−H bonds with concomitant hydrogen liberation. Employing the more electron-deficient dihydroborane [3,5-(CF₃)₂-C₆H₃BH₂] led to the formation of an anionic complex bearing a tetraarylated chain of four boron atoms, namely Li(THF)₄[(Cp*Ru)₂B₄H₅(3,5-(CF₃)₂C₆H₃)₄] ( 4 ), through an unusual, incomplete threefold dehydrocoupling process. A comparative theoretical investigation of the bonding in a simplified model of 4 and the analogous complex nido-[1,2(Cp*Ru)₂(μ-H)B₄H₉] ( I ) indicates that there appear to be no classical σ-bonds between the boron atoms in complex I , whereas in the case of 4 the B₄ chain better resembles a network of three B−B σ bonds, the central bond being significantly weaker than the other two.     

Chemoselectivity of Tertiary Azides in Strain-Promoted Alkyne-Azide Cycloadditions

The strain-promoted alkyne-azide cycloaddition (SPAAC) is the most commonly employed bioorthogonal reaction with applications in a broad range of fields. Over the years, several different cyclooctyne derivatives have been developed and investigated in regard to their reactivity in SPAAC reactions with azides. However, only a few studies examined the influence of structurally diverse azides on reaction kinetics. Herein, we report our investigations of the reactivity of primary, secondary, and tertiary azides with the cyclooctynes BCN and ADIBO applying experimental and computational methods. All azides show similar reaction rates with the sterically non-demanding cyclooctyne BCN. However, due to the increased steric demand of the dibenzocyclooctyne ADIBO, the reactivity of tertiary azides drops by several orders of magnitude in comparison to primary and secondary azides. We show that this chemoselective behavior of tertiary azides can be exploited to achieve semiorthogonal dual-labeling without the need for any catalyst using SPAAC exclusively.     

Sulfilimines as Versatile Nitrene Transfer Reagents: Facile Access to Diverse Aza‐Heterocycles

We herein report the unprecedented synthesis of diverse biologically important aza‐heterocycles by employing sulfilimines as nitrene transfer reagents. This class of sulfur‐based aza‐ylides had not been successfully used for gold nitrene transfer before. This work contains an efficient generation of α‐imino gold carbenes by N?S cleavage of sulfilimines. These gold carbenes undergo C?H insertion, cyclopropanation, and nucleophilic attack to form indoles (44 examples), 3‐azabicyclo[3.1.0]hexan‐2‐imines (24 examples), and imidazoles (3 examples). Our study represents a unique gold‐catalyzed reaction between alkynes and sulfur ylides, and also includes the first aza‐heterocycle synthesis that proceeds by intermolecular nitrene transfer followed by cyclopropanation of the α‐imino gold carbenes. Moreover, an unexpected synthesis of 4‐acylquinolines (3 examples) from 2‐acylphenyl sulfilimines and propargylic silyl ether derivatives by a 1,2‐hydride shift onto the α‐imino gold carbene and a subsequent Mukaiyama aldol cyclization was discovered.     

Cytotoxic flavonoids from two Lonchocarpus species

A new isoflavone, 4′-prenyloxyvigvexin A (1) and a new pterocarpan, (6aR,11aR)-3,8-dimethoxybitucarpin B (2) were isolated from the leaves of Lonchocarpus bussei and the stem bark of Lonchocarpus eriocalyx, respectively. The extract of L. bussei also gave four known isoflavones, maximaisoflavone H, 7,2′-dimethoxy-3′,4′-methylenedioxyisoflavone, 6,7,3′-trimethoxy-4′,5′-methylenedioxyisoflavone, durmillone; a chalcone, 4-hydroxylonchocarpin; a geranylated phenylpropanol, colenemol; and two known pterocarpans, (6aR,11aR)-maackiain and (6aR,11aR)-edunol. (6aR,11aR)-Edunol was also isolated from the stem bark of L. eriocalyx. The structures of the isolated compounds were elucidated by spectroscopy. The cytotoxicity of the compounds was tested by resazurin assay using drug-sensitive and multidrug-resistant cancer cell lines. Significant antiproliferative effects with IC₅₀ values below 10 μM were observed for the isoflavones 6,7,3′-trimethoxy-4′,5′-methylenedioxyisoflavone and durmillone against leukemia CCRF-CEM cells; for the chalcone, 4-hydroxylonchocarpin and durmillone against its resistant counterpart CEM/ADR5000 cells; as well as for durmillone against the resistant breast adenocarcinoma MDA-MB231/BCRP cells and resistant gliobastoma U87MG.ΔEGFR cells.     

Polyelectrolyte-compression forces between spherical DNA brushes

Optical tweezers are employed to measure the forces of interaction within a single pair of DNA-grafted colloids, dependent on the molecular weight of the DNA chains, and the concentration and valence of the surrounding ionic medium. The resulting forces are short range and set in as the surface-to-surface distance between the colloidal cores reaches the value of the brush height. The measured force-distance relation is analyzed by means of a theoretical treatment that quantitatively describes the effects of compression of the chains on the surface of the opposite-lying colloid. Quantitative agreement with the experiment is obtained for all parameter combinations.     

Nonadiabatic two-parameter charge and spin pumping in a quantum dot

We study dc charge and spin transport through a weakly coupled quantum dot, driven by a nonadiabatic periodic change of system parameters. We generalize the model of Tien and Gordon to simultaneously oscillating voltages and tunnel couplings. When applying our general result to the two-parameter charge pumping in quantum dots, we find interference effects between the oscillations of the voltage and tunnel couplings. We show that these interference effects may explain recent measurements in metallic islands. Furthermore, we discuss the possibility to electrically pump a spin current in presence of a static magnetic field.     

Sources of electrical conductivity in SnO2

SnO2 is widely used as a transparent conductor and sensor material. Better understanding and control of its conductivity would enhance its performance in existing applications and enable new ones, such as in light emitters. Using density functional theory, we show that the conventional attribution of n-type conductivity to intrinsic point defects is incorrect. Unintentional incorporation of hydrogen provides a consistent explanation of experimental observations. Most importantly, we find that SnO2 offers excellent prospects for p-type doping by incorporation of acceptors on the Sn site. Specific strategies for optimizing acceptor incorporation are presented.