DOTTADs--readily made novel metal ligands with multivariant functionality

The interaction of Hantzsch pyridinecarboxylic acids with dialkylformamides and POCl3, followed by treatment with NH4OH yields 1,8-dioxo-1,2,7,8-tetrahydro-2,7,10-triazaanthracenes (DOTTADs), which have great potential as useful ligands for Group I and II metals and some transition metals. The corresponding Hantszch esters similarly for DOTTADs or their bis-imines by way of isolable intermediates, which are then treated with an amine RNH2. DOTTAD-imines are also available from DOTTADs with amines and this reaction is also effective with 1,8-diamino-3,6-dioxaoctane to give macrocyclic analogues, as well as with chiral amines. DOTTAD-imines can be reduced with diethylsilane and Wilkinson's catalyst to the corresponding amines, which can also be formed from DOTTADs by reductive amination with amines and Na(AcO)3BH. Mono-aldols of DOTTADs are easily formed by treatment of DOTTADs with acetone.     

Enrico Fermi in Pisa

I discuss the early work of Enrico Fermi (1901-1954) as a student in Pisa at the Scuola Normale Superiore and at the University of Pisa (1918-1922), paying particular attention to the four papers he published during those years and to his licenza and doctoral theses.     

Random packings of frictionless particles

We conduct numerical simulations of random packings of frictionless particles at T = 0. The packing fraction where the pressure becomes nonzero is the same as the jamming threshold, where the static shear modulus becomes nonzero. The distribution of threshold packing fractions narrows, and its peak approaches random close packing as the system size increases. For packing fractions within the peak, there is no self-averaging, leading to exponential decay of the interparticle force distribution.     

High-performance AlGaAs-based VCSELs emitting in the 760 nm wavelength range

Vertical-cavity surface-emitting lasers (VCSELs) showing continuous-wave operation with emission wavelengths ranging between 717 and 790 nm have been fabricated. The samples are grown by solid source molecular beam epitaxy on GaAs substrates and are entirely based on the AlGaAs material system. A maximum single-mode output power of 2.0 mW is obtained at 754 nm wavelength by using the inverted surface relief technique.     

Exploring the first steps in core-shell electrocatalyst preparation: in situ characterization of the underpotential deposition of Cu on supported Au nanoparticles

The underpotential deposition (upd) of a Cu shell on a non-Pt nanoparticle core followed by galvanic displacement of the Cu template shell to form core-shell electrocatalyst materials is one means by which the Pt-based mass activity targets required for commercialization of PEM fuel cells may be reached. In situ EXAFS measurements were conducted at both the Au L(3) and the Cu K absorption edges during deposition of Cu onto a carbon-supported Au electrocatalyst to study the initial stages of formation of such a core-shell electrocatalyst. The Au L(3) EXAFS data obtained in 0.5 mol dm(-3) H(2)SO(4) show that the shape of the Au core is potential dependent, from a flattened to a round spherical shape as the Cu upd potential is approached. Following the addition of 2 mmol dm(-3) Cu, the structure was also measured as a function of the applied potential. At +0.2 V vs Hg/Hg(2)SO(4), the Cu(2+) species was found to be a hydrated octahedron. As the potential was made more negative, single-crystal studies predict an ordered bilayer of sulfate anions and partially discharged Cu ions, followed by a complete/uniform layer of Cu atoms. In contrast, the model obtained by fitting the Au L(3) and Cu K EXAFS data corresponds first to partially discharged Cu ions deposited at the defect sites in the outer shell of the Au nanoparticles at -0.42 V, followed by the growth of clusters of Cu atoms at -0.51 V. The absence of a uniform/complete Cu shell, even at the most negative potentials investigated, has implications for the structure, and the activity and/or stability, of the core-shell catalyst that would be subsequently formed following galvanic displacement of the Cu shell.     

Glycosylidene Carbenes. Part 15. Synthesis of disaccharides from allopyranose-derived vicinal 1,2-diols. Evidence for the protonation by a H-bonded hydroxy group in the σ-plane of the intermediate carbene,followed by attack on the oxycarbenium ion in the π-plane

The α-D -allo-diol 9 possesses an intramolecular H-bond (HO? C(3) to O? C(1)) in solution and in the solid state (Fig. 2). In solution, it exists as a mixture of the tautomers 9a and 9b (Fig. 3), which possess a bifurcated H-bond, connecting HO? C(2) with both O? C(1) and O? C(3). In addition, 9a possesses the same intramolecular H-bond as in the solid state, while 9b is characterized by an intramolecular H-bond between HO? C(3) and O? C(4). In solution, the β-D -anomer 12 is also a mixture of tautomers, 12a and presumably a dimer. The H-bonding in 9 and 12 is evidenced by their IR and ¹H-NMR spectra and by a comparison with those of 3–8, 10 , and 11 . The expected regioselectivity of glycosidation of 9 and 12 by the diazirine 1 or the trichloroacetimidate 2 is discussed on the basis of the relative degree of acidity/nucleophilicity of individual OH groups, as governed by H-bonding. Additional factors determining the regioselectivity of glycosidation by 1 are the direction of carbene approach/proton transfer by H-bonded OH groups, and the stereoelectronic control of both the proton transfer to the alkoxy-alkyl carbene (in the σ-plane) and the combination of the thereby formed ions (π-plane of the oxycarbenium ion). Glycosidation of 9 by the diazirine 1 or the trichloroacetimidate 2 proceeded in good yields (75–94%) and with high regioselectivity. Glycosidation of 9 and 12 by 1 or 2 gave mixtures of the disaccharides 14–17 and 18–21 , respectively (Scheme 2). As expected, glycosidation of 12 by 1 or by 2 gave a nearly 1:1 mixture of regioisomers and a slight preference for the β-D -anomers (Table 4). Glycosidation of the α-D -anomer 9 gave mostly the 1,3-linked disaccharides 16 and 17 (α-D β-D ) along with the 1,2-linked disaccharides 14 and 15 (α-D < β-D , 1,2-/1,3-linked glycosides ca. 1:4), except in THF and at low temperature, where the β-D -configurated 1,2-linked disaccharide 15 is predominantly formed. Similarly, glycosidation of 9 with 2 yielded mainly the 1,3-linked disaccharides (1,2-/1,3-linked products ca. 1:3 and α-D /β-D ca. 1:4). Yields and selectivity depend upon the solvent and the temperature. The regioselectivity and the unexpected stereoselectivity of the glycosidation of 9 by 1 evidences the combined effect of the above mentioned factors, which also explain the lack of regio-complementarity in the glycosidation of 9 by 1 and by 2 (Scheme 3). THF solvates the intermediate oxycarbenium ion, as evidenced by the strong influence of this solvent on the regio- and stereoselectivity, particularly at low temperatures, where kinetic control leads to a stereoelectronically preferred axial attack of THF on the oxycarbenium ion.     

Synthesis of natural products from the Indian neem tree Azadirachta indica

The synthesis of five natural products (3, 6, 7, 10, and 14), isolated from the Indian neem tree Azadirachta indica, is reported from a common intermediate (2). The judicious choice of transacetalization conditions allows efficient access to both the azadirachtinin (9 and 10) and the azadirachtin (3, 6, 7, and 14) skeletons.     

Asymmetric Synthesis of α-Aminophosphonic Acids by Cycloaddition of N-Glycosyl-C-dialkoxyphosphonoylnitrones

Addition of dialkyl phosphites to the nitrone 6 , formed in situ from the oxime 5 and formaldehyde gave the hydroxylamines 7 (86%) and 8 (88%), which reacted with p-benzoquinone in the presence of ethylene via the C-dialkoxyphosphonoylnitrones 9 and 10 to yield the cycloaddition products 11 – 14 (80–85%) with a diastereoselectivity of about 50%. The cycloaddition products were transformed into the monoisopropylidene derivatives 15 – 18 and the diacetates 19 – 22 . Comparison of the NMR spectra and the specific rotations of the compounds 19 – 22 with those of the corresponding α-ammo-acid derivatives 23 – 26 of known configuration indicated preferential formation of the L -isomers. The cycloaddition products were transformed in good yield into the L -α-aminophosphonic acids 29 , 30 , 36 , and 39 .     

Kupplungsreaktionen von Diazoniumbetainen mit 1,6-Methano[10]-annulenderivaten und ihre Cyclisierungsreaktionen zu Heterocyclen

Summary Coupling reactions of the diazoniumbetaines2, 4, 6a, 6b, 6c with derivatives of 1,6-methano[10]annulene1a–c and/or -naphthol7 yield the dyes3a–c, 5a, 5c, 8a–c, 9, 11. The syntheses of different triazine derivatives12a–c, 13a–c, 15, 16, and17a, 17c are described.

Herrn Kollegen Gerhard Schwenker mit den besten Wünschen zum 65. Geburtstag gewidmet