Bi- and trinuclear oxalamidinate complexes of palladium as catalysts in the copper-free Sonogashira reaction and in the Negishi reaction

The binuclear complex [(acac)Pd(oxam)Pd(acac)] 1 (oxam: tetraphenyl oxalic amidinate) has been prepared from H₂oxam and Pd(acac)₂ in excellent yield. The complex was characterized by elemental analyses, mass spectroscopy, ¹H NMR, ¹³C NMR spectroscopy and in the solid state by X-ray single crystal diffraction analyses. 1 consists of a bimetallic centrosymmetric unit in which the planar oxam ligand acts in a bis-chelating fashion. Each palladium center is in a planar environment.The complex 1 acts as highly selective pre-catalyst in the copper-free Sonogashira reaction between 4-bromoacetophenone and phenylacetylene. Its long-time catalytic activity is higher than that of the related binuclear complex 2 (oxam: tetra-p-tolyl oxalic amidinate) or that of the trinuclear compound [(acac)Pd(oxam)Zn(oxam)Pd(acac)] (3), the solid-state structure of which was also determined by an X-ray structural analysis of single crystals. In addition, 2 is an active and extremely selective pre-catalyst for the Negishi reaction between 3,5,6,8-tetrabromophenanthroline and R-CC-ZnCl (R: Ph, (ⁱprop)₃Si) to form tetra-alkyne-substituted derivatives.     

Phase stability in the systems AeAl(2-x)Mgx (Ae = Ca, Sr, Ba): electron concentration and size controlled variations on the laves phase structural theme

The systems AeAl(2-x)Mgx (Ae = Ca, Sr, Ba) display electron concentration induced Laves phase structural changes. However, the complete sequence MgCu2 --> MgNi2 --> MgZn2 with increasing x (decreasing electron count) is only observed for Ae = Ca. Compounds SrAl(2-x)Mgx (0 < x < or = 2) and BaAl(2-x)Mgx (x = 0.85 and 2.0) were synthesized and structurally characterized by X-ray diffraction experiments. For the Sr system the structural sequence CeCu2 --> MgNi2 --> MgZn2 occurs with increasing Mg content x. Thus, larger Sr does not allow the realization of the MgCu2 structure at low x. For Ae = Ba a binary compound BaAl2 does not exist, but more Ba-rich Ba7Al13 forms. The reinvestigation of the crystal structure of Ba7Al13 by selected area and convergent beam electron diffraction in a transmission electron microscope revealed a superstructure, which subsequently could be refined from single X-ray diffraction data. The formula unit of the superstructure is Ba21Al40 (space group P31m, Z = 1, a = 10.568(1) angstroms, c = 17.205(6) angstroms). In Ba21Al40 a size match problem between Ba and Al present in Ba7Al13 is resolved. The structure of Ba7Al13 (Ba21Al40) can be considered as a Ba excess variant of the hexagonal MgNi2 Laves phase type structure. An incommensurately modulated variant of the MgNi2 structure is obtained for phases BaAl(2-x)Mgx with x = 0.8-1. At even higher Mg concentrations a structural change to the proper MgZn2 type structure takes place.     

Dibenzotetraaza crown ethers: a new family of crown ethers based on o-phenylenediamine

Dibenzotetraaza (DBTA) crown ethers possess two o-phenylenediamine moieties. They are homologues of dibenzo crown ether phase-transfer catalysts and were prepared from the condensation of benzimidazoles with oligo(ethyleneglycol) dichlorides and oligo(ethyleneglycol) ditosylates. Compounds with ring sizes ranging from 18-crown-6 to 42-crown-14 were prepared. In addition, various altered benzimidizoles were used to produce DBTA crown ethers with modified substituents and ether bridges, as well as benzimidazolidine crown ethers. The synthetic approach presented here proved to be a convenient route to a new family of crown ethers with overall yields of up to 48% based on the benzimidazole. Yields for the ring-closing step were generally high, ranging from 51% to 94%, without the need for high-dilution conditions. Reaction of the DBTA crown ethers with alkyl and benzyl halides was found to be a facile way to obtain the corresponding tetra(N-organyl) compounds. Picrate extraction studies were carried out to determine phase-transfer catalytic capabilities. Extraction efficiencies for alkali-metal ions were lower than those for dibenzo-18-crown-6. Efficiencies were higher for other metal ions, with some selectivity for Pb(2+). Tetra(N-methyl) DBTA-18-crown-6 generally exhibited higher extraction efficiencies than its N-H analogue, but the selectivity was lower.     

The use of bis(diphenylphosphino)amines with N-aryl functionalities in selective ethylene tri- and tetramerisation

A systematic study was conducted on the Cr catalysed tri- and tetramerisation of ethylene using bis(diphenylphosphino)amine ligands with N-aryl functionality. This study revealed that the oligomerisation reaction product selectivity is primarily dependent on the structure and size of the N-aryl groups.

Addition of sufficient steric bulk to the N-phenyl group via ortho-alkyl substitution increased the combined 1-hexene and 1-octene selectivity (overall alpha selectivity) to above 82% at an overall 1-octene selectivity of 56%. The introduction of a single carbon spacer between the N-atom and the aryl-moiety, as well as the addition of branching on this carbon, resulted in further selectivity improvements, achieving an overall 1-octene selectivity of 64% and an overall alpha selectivity of 84%. This was obtained at catalyst productivities in excess of 1,000,000 g/g Cr/h.     

Characterization of a putative sensory [FeFe]-hydrogenase provides new insight into the role of the active site architecture

[FeFe]-hydrogenases are known for their high rates of hydrogen turnover, and are intensively studied in the context of biotechnological applications. Evolution has generated a plethora of different subclasses with widely different characteristics. The M2e subclass is phylogenetically distinct from previously characterized members of this enzyme family and its biological role is unknown. It features significant differences in domain- and active site architecture, and is most closely related to the putative sensory [FeFe]-hydrogenases. Here we report the first comprehensive biochemical and spectroscopical characterization of an M2e enzyme, derived from Thermoanaerobacter mathranii. As compared to other [FeFe]-hydrogenases characterized to-date, this enzyme displays an increased H₂ affinity, higher activation enthalpies for H⁺/H₂ interconversion, and unusual reactivity towards known hydrogenase inhibitors. These properties are related to differences in active site architecture between the M2e [FeFe]-hydrogenase and “prototypical” [FeFe]-hydrogenases. Thus, this study provides new insight into the role of this subclass in hydrogen metabolism and the influence of the active site pocket on the chemistry of the H-cluster.

Characterization of a group D putative sensory [FeFe]-hydrogenase reveals how the active site can be tuned to decrease CO inhibition and increase stability of a reduced H-cluster while retaining the ability to catalyze H⁺/H₂ interconversion.     

Phase Transition and Crystal Structure of the Monomeric Europium(II) Thiolate Eu(SC36H49)2

Single crystal X‐ray diffraction of Eu(SC₃₆H₄₉)₂ has revealed a first order phase transition at a temperature of 119 K, that involves a reduction of the specific volume by 1 %. The transition corresponds to a doubling of the unit cell, as it is the result from reorientations of isopropyl groups that appear at peripheral sites of the organic ligands. It is argued that a denser packing is achieved at the expense of a less favourable conformation of one of the two crystallographically‐independent complexes in the low‐temperature phase. The Bond‐Valence method is used to show that interactions of equal strengths are present between europium and both sulphur atoms and both coordinating phenyl rings.     

Rearrangement of 2-aryl-3,3-dichloroazetidines: intermediacy of 2-azetines

An easy synthesis of 2-aryl-3,3-dichloroazetidines, a rather unexplored class of azaheterocycles, is described. The title compounds were easily obtained by reduction of the corresponding 4-aryl-3,3-dichloro-2-azetidinones with monochloroalane, which in turn were synthesized by a ketene-imine [2 + 2] cycloaddition. The reactivity of 3,3-dichloroazetidines with bases was investigated, yielding 2-[dimethoxy(aryl)methyl]aziridines by ring contraction when treated with sodium methoxide. Furthermore, reacting the 3,3-dichloroazetidines with sodium hydride in DMSO, followed by aqueous workup, afforded 1-alkyl-2-aroylaziridines, by hydrolysis of the intermediate 2-azetines and ring closure of the transient 3-amino-2-chloro-1-phenyl-1-propanone derivatives. Monitoring this reaction in an NMR tube, using sodium hydride in DMSO-d(6), allowed the characterization of the intermediate strained heterocyclic enamines, i.e., 2-azetines, by (1)H and (13)C NMR.     

The Reactivity of Cr‐Te Superlattice Reactants and of Co‐deposited Cr‐Te Films: Studies with in‐situ X‐ray Diffractometry

The reactivity of a Cr‐Te multilayer film consisting of 326 Cr/Te double‐layers with about 88(3) at% Te was investigated with in‐situ X‐ray diffraction. The occurrence of a superstructure reflection caused by the multilayer system is observed. During the annealing procedure between 80 and 90 °C the layer‐by‐layer arrangement is destroyed and elemental Te crystallizes. After 6 h isothermal annealing at 140 °C CrTe₃ starts to crystallize. The amount and the crystal size of CrTe₃ increase with raising temperature and Te is consumed during the growth process. At 220 °C the formation of crystalline Cr₂Te₃ is observed and between 220 and 260 °C CrTe₃ is completely decomposed into Cr₂Te₃. The two Cr tellurides grow as highly textured materials. The size of CrTe₃ crystals is significantly larger than that of Cr₂Te₃. For a co‐deposited Cr‐Te film with composition 77(3) at% Te the tri‐telluride and elemental Te crystallize simultaneously. The texture of CrTe₃ is less pronounced and different reflections of the tri‐telluride occur besides the (h00)‐reflections. Up to 160 °C a pronounced increase of the crystallite size is observed and the crystals are significantly larger than those obtained with the multifilm sample. At 130 °C the crystallization kinetics seem to be controlled by nucleation.     

The simplest coronoids: Hollow hexagons

Hollow hexagons form a subclass of primitive coronoid systems. The macrocyclic aromatic hydrocarbon kekulene corresponds to a hollow hexagon. The hollow hexagons for given numbers of hexagonal units (h) were enumerated by computer aid, but also an analytical solution for the numbers of hollow hexagons was achieved. For the Kekulé structure counts (k) of a hollow hexagon a general formula is reported. Also the maximum and minimumK numbers are considered.This article is Part VI of the series Enumeration and Classification of Coronoid Hydrocarbons. For Part V, see ref. [36].