Department of Chemistry

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Dimeric indole alkaloids represent a structurally unique class of natural products having interesting biological activities. Recently, we reported the first total synthesis of flinderoles B and C, structurally unique and potent antimalarial natural products. Central to the design of the approach and by virtue of a one-pot, acid-catalyzed dimerization reaction, the route also provided total synthesis of the borreverine class of natural products. This full account details the progress of efforts that culminated in the protecting-group-free, six-step total synthesis of all of the flindersia alkaloids: dimethylisoborreverine, isoborreverine, flinderoles A-C, and their analogues. A biomimetic approach featuring a scalable and catalytic formal [3 + 2] cycloaddition and Diels-Alder reaction is outlined in detail. On the basis of the experimental observations, a detailed mechanism has been proposed for the dimerization of tertiary alcohol 28. © 2013 American Chemical Society.

World University root node

Journal of Organic Chemistry

Biomimetic total syntheses of borreverine and flinderole alkaloids

Nowadays, specifying the quality of a transparent conducting electrode (TCE) using a figure of merit (FoM) is considered nearly mandatory. However, not much attention is paid to the local variations in the FoM itself across the large area of the TCE. This calls for the definition of a local FoM (LFoM), particularly relevant with regard to several new generation TCEs which have been and are being proposed recently. A LFoM based on local measurements of transmission and sheet resistance, pixel by pixel, would be a Herculean task. The present article addresses this central issue by defining a LFoM based on the diffraction efficiency (DE) of a calibrated high-resolution transmission grating overlaid with a given TCE. The DE value, which critically depends on the periodic nature of the grating material, is shown to be highly sensitive to the various nonuniformities in the TCE overlaid on the grating, with length scales comparable to the grating period. The effectiveness of the so-defined LFoM was demonstrated using a pointer laser and a photodiode in combination with a transmission grating with ∼μm periodicity by taking ITO/glass and ITO/PET as case examples. A metal grating pattern of Cu deposited on seed Pd grating lines was fabricated as an example of new generation TCE and examined for FoM and LFoM, however, without the aid of the external grating. The LFoM based on DE presented here should serve as an excellent screening method for both conventional and emerging TCEs. © 2012 American Chemical Society.

ACS Applied Materials and Interfaces

Holistic method for evaluating large area transparent conducting electrodes

Dalton Transactions

Lipophilic bismuth phosphates: a molecular tetradecanuclear cage and a 1D-coordination polymer. Synthesis, structure and conversion to BiPO 4

Radicals and anions: The TEMPO-mediated oxidation of magnesiated iodoarenes A provides highly regioregular poly(m-phenylenes) by strictly alternating anionic/radical cross-over chain-growth polymerization. Poly(m-phenylenes) with mean molecular weight (Mn) of up to 20 000 g mol-1 can be prepared under mild conditions by this method. The approach is a new concept for transition-metal-free polyarene synthesis. TEMPO=2,2,6,6-tetramethylpiperidine- N-oxyl radical. Copyright © 2012 WILEY-VCH Verlag GmbH &amp; Co. KGaA, Weinheim.

Angewandte Chemie - International Edition

Radical/anionic SRN1-type polymerization for preparation of oligoarenes

A Pd grating of periodicity of 1.5 μm comprising of 1 μm wide nanocrystalline Pd lines has been obtained by a direct micromolding method to serve as Hydrogen sensor element in an optical diffraction set up. The device uses a low power diode laser and a photodetector and works with sensitivity of ∼20%. The hydrogen sensing action is based on monitoring the changes in the diffraction efficiency (DE) which is defined as the ratio of the first and the zeroth order diffracted beam intensities. The diffraction efficiency undergoes large and sudden changes as the nanocrystalline grating becomes disordered due to PdH x formation, as monitored using in-situ microscopy and optical profilometric measurements. This is truly a low cost, portable hydrogen sensor meant for large installations. © 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

International Journal of Hydrogen Energy

A low cost optical hydrogen sensing device using nanocrystalline Pd grating

Direct dynamics trajectory simulations were performed for two examples of the thiolate-disulfide exchange reaction, that is, HS- + HSSH and CH3S- + CH3SSCH3. The trajectories were computed for the PBE0/6-31+G(d) potential energy surface using both classical microcanonical sampling at the ion-dipole complex and quasi-classical Boltzmann sampling (T = 300 K) at the central transition state. The potential energy surface for these reactions involves a hypercoordinate sulfur intermediate. Despite the fact that the intermediate resides in a shallow well (less than 5 kcal/mol), very few trajectories follow a direct substitution path (the SN2 pathway). Rather, the mechanism is addition-elimination, with several trajectories sampling the intermediate for long times, up to 15 ps or longer. © 2012 American Chemical Society.

Journal of Physical Chemistry A

Mechanism of thiolate-disulfide exchange: Addition-elimination or effectively SN2? Effect of a shallow intermediate in gas-phase direct dynamics simulations

Bismuth–ferrocene carboxylates: synthesis and structure

The application of nitroxides for the development of new synthetic methods and their implementation in polymer chemistry, material science and beyond is one of the major research topics in our laboratory in the institute of organic chemistry at the WWU Münster. This short review focuses on our recent progress towards nitroxidebased transition-metal-free oxidative coupling reactions. The demand for organic surrogates for transition metals in such transformations is in our eyes unquestionable, since environmental and economic issues have become progressively more important in recent years. For this purpose, the 2,2,6,6-tetramethylpiperidine-N-oxyl radical (TEMPO) is shown to be a highly efficient oxidant for the homo- and cross-coupling of Grignard reagents. This powerful C-C bond forming strategy allows the generation of conjugated polymers from bifunctional Grignard reagents. Moreover, cross-coupling of alkynyl Grignard compounds and nitrones can be accomplished under aerobic atmosphere with catalytic amounts of TEMPO. It is also shown that TEMPO-derived N-oxoammonium salts can act as suitable oxidants for formation of C-N bonds between non-preactivated benzoxazoles and secondary amines under metal-free conditions. © Schweizerische Chemische Gesellschaft.

Fulltext

Chimia

Transition-metal-free oxidative coupling reactions for the formation of C-C and C-N bonds mediated by TEMPO and its derivatives

The biosynthesis of abietadiene is the first biosynthetically relevant process shown to involve a potential energy surface with a bifurcating reaction pathway. Herein, we use gas-phase, enzyme-free direct dynamics simulations to study the behavior of the key reaction (bifurcating) step, which is conversion of the C 20 pimaradienyl cation to the abietadienyl cation. In a previous study (J. Am. Chem. Soc.2011, 133, 8335), a truncated C 10 model was used to investigate this reaction. The current work finds that the complete C 20 pimaradienyl cation gives reaction dynamics similar to that reported for the truncated C 10 model. We find that in the absence of the enzyme, the C 20 abietadienyl cation is generated in almost equal quantity (1.3:1) as an unobserved (in nature) seven-membered ring product. These simulations allude to a need for abietadiene synthase to steer the reaction to avoid generation of the seven-membered ring product. The methodology of post-transition state chemical dynamics simulations is also considered. The trajectories are initiated at the rate-controlling transition state (TS) separating the pimaradienyl and abietadienyl cations. Accurate results are expected for the short-time direct motion from this TS toward the abietadienyl cation. However, the dynamics may be less accurate for describing the unimolecular reactions that occur in moving toward the pimaradienyl cation, due to the unphysical flow of zero-point energy. © 2012 American Chemical Society.

Journal of Chemical Theory and Computation

Gas-phase chemical dynamics simulations on the bifurcating pathway of the pimaradienyl cation rearrangement: Role of enzymatic steering in abietic acid biosynthesis

Pd grating patterns have been fabricated using the process of micromolding in capillary employing a Pd alkanethiolate precursor, which could be converted to metal in situ by thermolysis. Thus generated Pd grating were uniform in width (∼950 nm) and spacing (∼450 nm) over millimeter square areas on glass substrates. Importantly, the pattern when used as an optical grating produced a diffraction pattern with a high resolution (&gt;2000); the intensities of widely separated (diffraction angle, ∼26·8°) diffracted spots could be measured using a simple photodiode. By varying the concentration of Pd precursor (2mMto 25 mM), thickness of the resulting gratings could be adjusted in the range of ∼15-115 nm. By adjusting the grating parameters optimally, a maximum diffraction efficiency of 36% has been achieved. Thus fabricated Pd grating was used as seed catalyst to deposit Cu by electroless plating. The Cu deposition process has also been monitored by employing AFM, SEM and EDS analysis. The diffraction efficiency values corroborate well with the changes in the grating thickness due to Cu deposition. The grating structures presented can be reproducibly fabricated for rapidly emerging optical diffraction based sensing applications. This has been demonstrated in the case of aqueous Cu 2+ by depositing the latter electrolessly on Pd. © Indian Academy of Sciences.