Mechanism of the mild functionalization of arenes by diboron reagents catalyzed by iridium complexes. Intermediacy and chemistry of bipyridine-ligated iridium trisboryl complexes

This paper describes mechanistic studies on the functionalization of arenes with the diboron reagent B(2)pin(2) (bis-pinacolato diborane(4)) catalyzed by the combination of 4,4'-di-tert-butylbipyridine (dtbpy) and olefin-ligated iridium halide or olefin-ligated iridium alkoxide complexes. This work identifies the catalyst resting state as [Ir(dtbpy)(COE)(Bpin)(3)] (COE = cyclooctene, Bpin = 4,4,5,5-tetramethyl-1,3,2-dioxaborolanyl). [Ir(dtbpy)(COE)(Bpin)(3)] was prepared by independent synthesis in high yield from [Ir(COD)(OMe)](2), dtbpy, COE, and HBpin. This complex is formed in low yield from [Ir(COD)(OMe)](2), dtbpy, COE, and B(2)pin(2). Kinetic studies show that this complex reacts with arenes after reversible dissociation of COE. An alternative mechanism in which the arene reacts with the Ir(I) complex [Ir(dtbpy)Bpin] after dissociation of COE and reductive elimination of B(2)pin(2) does not occur to a measurable extent. The reaction of [Ir(dtbpy)(COE)(Bpin)(3)] with arenes and the catalytic reaction of B(2)pin(2) with arenes catalyzed by [Ir(COD)(OMe)](2) and dtbpy occur faster with electron-poor arenes than with electron-rich arenes. However, both the stoichiometric and catalytic reactions also occur faster with the electron-rich heteroarenes thiophene and furan than with arenes, perhaps because eta(2)-heteroarene complexes are more stable than the eta(2)-arene complexes and the eta(2)-heteroarene or arene complexes are intermediates that precede oxidative addition. Kinetic studies on the catalytic reaction show that [Ir(dtbpy)(COE)(Bpin)(3)] enters the catalytic cycle by dissociation of COE, and a comparison of the kinetic isotope effects of the catalytic and stoichiometric reactions shows that the reactive intermediate [Ir(dtbpy)(Bpin)(3)] cleaves the arene C-H bond. The barriers for ligand exchange and C-H activation allow an experimental assessment of several conclusions drawn from computational work. Most generally, our results corroborate the conclusion that C-H bond cleavage is turnover-limiting, but the experimental barrier for this bond cleavage is much lower than the calculated barrier.     

Ammonium ion mediated resorcarene capsules: ESI-FTICRMS study on gas-phase structure and ammonium ion affinity of tetraethyl resorcarene and its per-methylated derivative

The ammonium ion binding affinities of tetraethyl resorcarene (1) and its per-methylated derivative (2) were studied by electrospray ionization Fourier transform ion cyclotron resonance (ESI-FTICR) mass spectrometry. Ten different ammonium ions were tested as guests for the resorcarenes. A strong tendency for complex formation was observed with all ammonium ions of size and charge distribution suitable for noncovalent interactions with the cavities of the resorcarene hosts 1 and 2. Although differences in ammonium ion affinities were observed between 1 and 2 due to the dissimilar conformations, the overall tendency was that increase in the degree of substitution and the length of carbon chain of the ammonium cation facilitated the complex formation until the sterical hindrance impeded the complexation. Dimeric as well as monomeric ammonium ion complexes were formed with resorcarene 1, but resorcarene 2 was unable to form the dimeric capsules because of the lack of H-bond donor possibilities. The nature of binding of the guest was further investigated with ion-molecule reactions and by determination of the single crystal X-ray structure of host 1 complexed with tetramethyl ammonium bromide.     

A Self‐Assembled Bicontinuous Cubic Phase with a Single‐Diamond Network

The first single‐diamond cubic phase in a liquid crystal is reported. This skeletal structure with the space group is formed by self‐assembly of bolaamphiphiles with swallow‐tailed lateral chains. It consists of bundles of π‐conjugated p‐terphenyl rods fused into an infinite network by hydrogen‐bonded spheres at tetrahedral four‐way junctions. We also present a quantitative model relating molecular architecture to the space‐filling requirements of six possible bicontinuous cubic phases, that is, the single‐ and double‐network versions of gyroid, diamond, and “plumber′s nightmare”.     

Light-induced coumarin cyclopentannelation

[reaction: see text] Whereas cyclopentenylcarbenes resulting from photocycloaddition of 4-alk-1-ynylcoumarins to 2,3-dimethylbut-2-ene undergo tandem cyclization to hitherto unknown tetracyclic (4-hetera)cyclopent[b,c]acenaphthylenes, the corresponding cyclopentenylnitrenes stemming from 4-cyanocoumarins and the same alkene are converted into tricyclic imines via H-abstraction.     

Multiple steady states in a CSTR with total condenser: Comparison of equilibrium and nonequilibrium models

Comparison of equilibrium and nonequilibrium models of a CSTR with total condenser focused on the multiple steady states and dynamic behaviour was carried out. The steady-state behaviour of the model system, MTBE synthesis from methanol and isobutene in a reactive distillation column, was studied in terms of the input parameters, i. e. feed flow rate of methanol or butenes, reflux ratio, and mass of catalyst. The dynamic behaviour of the system during the start-up was investigated and perturbations of manipulated variables were found to cause transitions between the parallel steady states. Presented at the 33rd International Conference of the Slovak Society of Chemical Engineering, Tatranské Matliare, 22–26 May 2006.     

Novel approach to the synthesis of 3-acyl substituted indolizines. The synthesis of 3-(indolizinyl-2)alanine and 4-(indolizinyl-3) homoalanine derivatives

A novel approach to the synthesis of 3-acylindolizines and the transformations of some acids into tryptophane analogues are described. Reaction of ethyl 2-pyridinylacetate and methyl 2-quinolinylacetate with N-trifluoroacetyl-5-bromo-4-oxonorvaline methylester led to N-trifluoroacetyl-3-(1-ethoxycarbonylindolizinyl-2) alanine methyl ester and N-trifluoroacetyl-3-(3-methoxycarbonylpyrrolo [1,2-a]quinolinyl-2) alanine methyl ester, respectively. Treatment of ethyl 2-pyridinylacetate and 2-pyridinylacetonitrile, first with N,N-dimethylformamide dimethyl acetal (DMFDMA), followed by reaction with phenacyl bromide, gave the corresponding 3-benzoylindolizines, while the reaction of ethyl 2-pyridinylacetate and 2-pyridinylacetonitrile with DMFDMA, followed by treatment with (S)-N-trifluoroacetyl-5-bromo-4-oxonorvaline methyl ester, gave the corresponding N-trifluoroacetyl-4-oxo-4-(indolizinyl-3)homoalanine methyl esters.     

Preparation and Characterisation of Nano-Structured WO<Subscript>3</Subscript>-TiO<Subscript>2</Subscript> Layers for Photoelectrochromic Devices

Nano-structured WO₃-TiO₂ layers were prepared by the sol-gel route. To obtain transparent, porous and crack free layers up to 0.8 μ m with a single dipping cycle a templating strategy was used. As a template three-dimensionally network based on organically modified silane was introduced to the WO₃ and TiO₂ sols. The WO₃ layers were dip-coated onto the conductive glass substrate (TCO) and the TiO₂ layers on the top of the WO₃ layer. The morphology and the structure of the layers were determined by Scanning Electron Microscopy (SEM), High Resolution Transmission Electron Microscopy (HR-TEM), Energy Dispersive X-Ray Spectroscopy (EDXS), Auger and Infrared spectroscopy. SEM image of the WO₃-TiO₂ layer confirmed the nano-porosity of the layers and give the size of the particles of about 10 nm for TiO₂ and 30 nm for WO₃ layer. Further analysis indicated that the titanium sol penetrates the WO₃ layer. Particles in the WO₃ layer consist of a crystalline monoclinic WO₃ core surrounded by a 5–10 nm amorphous phase consisting of WO₃, TiO₂ and SiO₂. The WO₃-TiO₂ layers were used to assemble all solid state photoelectrochromic (PE) devices. Under 1 sun irradiation (1000 W/m²) the visible transmittance of the PE device changes from 62% to 1.6%. The colouring and bleaching processes last about 10 minutes.     

Near-Infrared Spectrum of the First Excited State of Au2+

Au₂⁺ is a simple but crucial model system for understanding the diverse catalytic activity of gold. While the Au₂⁺ ground state (X²Σ_g⁺) is understood reasonably well from mass spectrometry and computations, no spectroscopic information is available for its first excited state (A²Σ_u⁺). Herein, we present the vibrationally resolved electronic spectrum of this state for cold Ar-tagged Au₂⁺ cations. This exceptionally low-lying and well isolated A²Σ_(u)⁺←X²Σ_(g)⁺ transition occurs in the near-infrared range. The observed band origin (5738 cm⁻¹, 1742.9 nm, 0.711 eV) and harmonic Au−Au and Au−Ar stretch frequencies (201 and 133 cm⁻¹) agree surprisingly well with those predicted by standard time-dependent density functional theory calculations. The linearly bonded Ar tag has little impact on either the geometric or electronic structure of Au₂⁺, because the Au₂⁺⋅⋅⋅Ar bond (∼0.4 eV) is much weaker than the Au−Au bond (∼2 eV). As a result of 6 s←5d excitation of an electron from the antibonding σ_u^* orbital (HOMO-1) into the bonding σ_g orbital (SOMO), the Au−Au bond contracts substantially (by 0.1 Å).