Burkhard Ziemer

A Dinuclear Nickel(I) Dinitrogen Complex and its Reduction in Single‐Electron Steps

Electron by electron: Beta-diketiminato nickel(I) complex fragments are capable of activating N(2) through coordination. The resulting complex can be reduced in two single-electron steps, which further activates the N-N bond. The picture shows the structure of the singly reduced complex with mu-eta(1):eta(1)-bound N(2).

Insight into the mechanism of direct catalytic aldol addition mediated by ambifunctional titanium complexes

Abstract X-Ray structure analysis of a titanium(IV) isopropoxide/mandelic acid complex provides an insight into the mechanism of this novel direct enantioselective aldol addition. The catalytic cycle mediated by the titanium(IV) alkoxide/mandelic acid complex is presented.

Stereoselective synthesis of the C18–C28 fragment of apoptolidin

An efficient, stereocontrolled synthesis of the C18–C28 segment of apoptolidin has been achieved. Key steps are a stannous triflate-mediated aldol reaction, the acylation of a Weinreb amide with an E-alkenyl lithium reagent and the dihydroxylation of a C19–C20 double bond.

Tetranuclear BINOL−Titanium Complex in Selective Direct Aldol Additions

The extremely robust and water-stable tetranuclear complex Ti(4)(micro-BINOLato)(6)(micro(3)-OH)(4) (1) catalyzes the direct aldol addition with high degrees of regioselectivity at the sterically more encumbered alpha-side of unsymmetrical ketones. The formation of quaternary stereocenters is described. Oxygen-containing ene components can also be used as starting material in these reactions. When used with aliphatic aldehydes, acetals 18 or acetals of aldol adducts 20 were observed. As few as 0.2 mol % loadings with this catalyst 1 were enough to complete the reactions. Mechanistical aspects of the reactions are discussed.

Structural Studies of Novel Siloxysilsesquioxanes

Five silsesquioxanes (RSiO3/2)2n with R = siloxy and n = 4, 5 have been investigated by X-ray crystallography, MALDI-TOF mass spectrometry, and differential scanning calorimetry (DSC).

Synthesis and biological evaluation of integrin antagonists containing trans- and cis-2,5-disubstituted THF rings.

The synthesis of a series of RGD mimetics is described. All compounds consist of a central 2,5-disubstituted tetrahydrofuran core, a variable linker to a guanidino group, and a beta-amino alanine unit to mimic the carboxylic acid. Three types of linkers were investigated: a simple four-atom methylene chain (type A, compounds 14, 15, 16, and 17), a four-atom methylene chain with an additional chiral center, and a nitrogen substituent (type B, compounds 38, 39, and 40), and an amide linker of different length with an additional chiral center (type C, compounds 59, 60, 61, and 62). A variety of compounds were tested as potential integrin antagonists in a receptor binding assay (alphaIIbbeta3, alphavbeta3, and alphavbeta5). The relative and absolute configuration of the chiral centers at the THF ring had a pronounced effect on the binding activity and selectivity. Compound 14 proved to be a selective inhibitor of alphaIIbbeta3 (IC50=20nM), whereas compound 40 exhibited high activity for binding of alphaIIbbeta3 (IC50=67nM) and alphavbeta3 (IC50=52nM).

What is behind ‘tungsten blue oxides’?

Abstract According to the proposed quantitative chemical and X-ray analyses, the composition of the ‘tungsten blue oxides’ (TBO) investigated varies over a very broad spectrum, depending on the temperature, atmosphere, and speed and also on the actual manufacturing process of the ammonium paratungstate tetrahydrate (APT·4H 2 O) reduction. In addition to crystalline compounds {hexagonal tunsten bronze (HTB), WO 3 , W 20 O 58 , and W 18 O 49 } the industrially produced TBO contains 30–55% of X-ray-amorphous fraction. APT·4H 2 O changes in a stream of dry hydrogen at 130°C into the phase APT·∼2H 2 O, observed for the first time. The intermediate X-ray-amorphous phases ranging from 250 to 350°C are the starting pool not only for HTB but also for the crystalline tungsten oxides. The HTB-containing TBO powders display a considerable potassium incorporation via the cation exchange of H + being replaced by K + . The solid-state high-resolution 1 H NMR investigation of TBO allows a distinction to be made between different proton-containing species.

Four‐Coordinate Trispyrazolylboratomanganese and ‐iron Complexes with a Pyrazolato Co‐ligand: Syntheses and Properties as Oxidation Catalysts

A series of complexes of the type [(Tp(R1,R2))M(X)] (Tp = trispyrazolylborato) with R(1)/R(2) combinations Me/tBu, Ph/Me, iPr/iPr, Me/Me and for M = Mn or Fe coordinating [Pz(Me,tBu)](-) (Pz = pyrazolato) or Cl(-) as co-ligand X has been synthesised. Although the chloride complexes were very unreactive and stable in air, the pyrazolato series was far more reactive in contact with oxidants like O(2) and tBuOOH. The [(Tp(R1,R2))M(Pz(Me,tBu))] complexes proved to be active pre-catalysts for the oxidation of cyclohexene with tBuOOH, reaching turnover frequencies (TOFs) ranging between moderate and good in comparison to other manganese catalysts. Cyclohexene-3-one and cyclohexene-3-ol were always found to represent the main products, with cyclohexene oxide occasionally formed as a side product. The ratios of the different oxidation products varied with the reaction conditions: in the case of a peroxide/alkene ratio of 4:1, considerably more ketone than alcohol was obtained and cyclohexene oxide formation was almost negligible, whereas a ratio of 1:10 led to a significant increase of the alcohol proportion and to the formation of at least small amounts of the epoxide. Pre-treatment of the dissolved [(Tp(R1,R2))M(Pz(Me,tBu))] pre-catalysts with O(2) led to product distributions and TOFs that were very similar to those found in the absence of O(2), so that it may be argued that tBuOOH and O(2) both lead to the same active species. The results of EPR spectroscopy and ESI-MS suggest that the initial product of the reaction of [(Tp(Me,Me))Mn(Pz(Me,tBu))] with O(2) contains a Mn(III)(O)(2)Mn(IV) core. Prolonged exposure to O(2) leads to a different dinuclear complex containing three O-bridges and resulting in different TOFs/product distributions. Analogous findings were made for other complexes and formation of these overoxidised products may explain the deviation of the catalytic performances if the reactions are carried out in an O(2) atmosphere.

Synthesis of Chiral 4‐(ω‐Hydroxyalkyl)pyrazolidin‐3‐ones by Ring‐Chain Transformation of α‐Alkylidenelactones with Hydrazines

The configuration and/or formulae of the following compounds need to be corrected. (1′R,E)-3-(2-Benzyloxy-3-hydroxypropylidene)dihydrofuran-2-one(7c): Correct formula: (4S,5R,4′S)-5-(2,2-Dimethyl[1,3]dioxolan-4-yl)-4-(2-hydroxyethyl)pyrazolidin-3-one (11a): Original formula correct (4R,5S,1′R)-5-(1-Benzyloxy-2-hydroxymethyl)-4-(2-hydroxyethyl)pyrazolidin-3-one (11g): Correct formula:

On the mechanism of directed, TiCl4-mediated aldol addition—an easy access to substituted 2.4-furandiols

Abstract Different results of directed TiCl 4 -mediated aldol additions in the presence of bases as well as in the absence of bases followed by an α-hydroxylation were obtained.