Efficient Access to Substituted Silafluorenes by Nickel‐Catalyzed Reactions of Biphenylenes with Et2SiH2

The reaction of biphenylene ( 1 ) with Et₂SiH₂ in the presence of [Ni(PPhMe₂)₄] results in the formation of a mixture of 2‐diethylhydrosilylbiphenyl [ 2 (Et₂HSi)] and 9,9,‐diethyl‐9‐silafluorene ( 3 ). Silafluorene 3 was isolated in 37.5 % and 2 (Et₂HSi) in 36.9 % yield. The underlying reaction mechanism was elucidated by DFT calculations. 4‐Methyl‐9,9‐diethyl‐9‐silafluorene ( 7 ) was obtained selectively from the [Ni(PPhMe₂)₄]‐catalyzed reaction of Et₂SiH₂ and 1‐methylbiphenylene. By contrast, no selectivity could be found in the Ni‐catalyzed reaction between Et₂SiH₂ and the biphenylene derivative that bears tBu substituents in the 2‐ and 7‐positions. Therefore, two pairs of isomers of tBu‐substituted silafluorenes and of the related diethylhydrosilylbiphenyls were formed in this reaction. However, a subsequent dehydrogenation of the diethylhydrosilylbiphenyls with Wilkinson’s catalyst yielded a mixture of 2,7‐di‐tert‐butyl‐9,9‐diethyl‐9‐silafluorene ( 8 ) and 3,6‐di‐tert‐butyl‐9,9‐diethyl‐9‐silafluorene ( 9 ). Silafluorenes 8 and 9 were separated by column chromatography.     

Determination of iron by Z-GFAAS and the influence of short-term precision and long-term precision

A detailed method validation of graphite-furnace atomic absorption spectrometry (GFAAS) with Zeeman background correction was performed. The aim is to perform a detailed investigation of short-term precision as opposed to long-term precision. It was suggested that release of graphite flakes into the light path during measurement significantly influenced the performance of the method. It was found that significant deviations with respect to the certified values were frequent and an estimate of reliable uncertainties was obtained only after a high number of repetitions. Uncertainty of Interlaboratory testing was evaluated as a method to estimate uncertainties that are comparable to uncertainties that were obtained by Interlaboratory testing and to uncertainties predicted by the Horwitz curve. To a large extent, the uncertainty in measurement that was predicted by pooled calibrations corresponded to the uncertainties that were obtained from multiple determinations of unknowns. It was thus proposed that a large proportion of the difference in uncertainty in measurement between laboratories could be explained by properties of the different detectors. In order to support accuracy, it is suggested that a higher level of uncertainty should be accepted in analytical investigations.     

Biohybrid photosynthetic charge accumulation detected by flavin semiquinone formation in ferredoxin-NADP+ reductase

Flavin chemistry is ubiquitous in biological systems with flavoproteins engaged in important redox reactions. In photosynthesis, flavin cofactors are used as electron donors/acceptors to facilitate charge transfer and accumulation for ultimate use in carbon fixation. Following light-induced charge separation in the photosynthetic transmembrane reaction center photosystem I (PSI), an electron is transferred to one of two small soluble shuttle proteins, a ferredoxin (Fd) or a flavodoxin (Fld) (the latter in the condition of Fe-deficiency), followed by electron transfer to the ferredoxin-NADP⁺ reductase (FNR) enzyme. FNR accepts two of these sequential one electron transfers, with its flavin adenine dinucleotide (FAD) cofactor becoming doubly reduced, forming a hydride which is then passed onto the substrate NADP⁺ to form NADPH. The two one-electron potentials (oxidized/semiquinone and semiquinone/hydroquinone) are similar to each other with the FNR protein stabilizing the hydroquinone, making spectroscopic detection of the intermediate semiquinone state difficult. We employed a new biohybrid-based strategy that involved truncating the native three-protein electron transfer cascade PSI → Fd → FNR to a two-protein cascade by replacing PSI with a molecular Ru(ii) photosensitizer (RuPS) which is covalently bound to Fd and Fld to form biohybrid complexes that successfully mimic PSI in light-driven NADPH formation. RuFd → FNR and RuFld → FNR electron transfer experiments revealed a notable distinction in photosynthetic charge accumulation that we attribute to the different protein cofactors [2Fe2S] and flavin. After freeze quenching the two-protein systems under illumination, an intermediate semiquinone state of FNR was readily observed with cw X-band EPR spectroscopy. The increased spectral resolution from selective deuteration allowed EPR detection of inter-flavoprotein electron transfer. This work establishes a biohybrid experimental approach for further studies of photosynthetic light-driven electron transfer chain that culminates at FNR and highlights nature''s mechanisms that couple single electron transfer chemistry to charge accumulation, providing important insight for the development of photon-to-fuel schemes.

One electron at a time, photosynthetic biohybrids enable charge accumulation via the flavin semiquinone of ferredoxin-NADP⁺ reductase.     

Electrochemistry of complex molecular and biomolecular scale entities

Structural mapping of intermediate size and large molecules and biomolecules at ultra-high resolution using single-crystal electrodes and in situ scanning tunnelling microscopy continues to disclose surprising findings. In situ scanning tunnelling spectroscopy has also recently disclosed new electrochemical conductivity features at the level of the single molecule. We overview briefly elements of this development over the last few years, with focus on three recent discoveries: (1) a new packing mode of a core monolayer target thiol, the amino acid cysteine (Cys) on Au(100)-electrodes, quite different from Cys packing on Au(111)- and Au(110)-electrodes; (2) transition of a core ferrocene in situ scanning tunnelling spectroscopy probe from stochastic single-molecule to macroscopic behaviour, a concept at the heart of nanoscience; and (3) unexpected behaviour of the large molybdenum enzyme sulfite oxidase, when going from macroscopic to single-molecule electrochemistry. We compare these studies with other recent discoveries of single-molecule protein conductivity and molecular scale inorganic nanostructures.     

Synthesis,resolution and absolute configuration of 4-amino-3-phenylpiperidine

Racemic cis-4-amino-1-benzyl-3-phenylpiperidine was prepared by reductive amination of the respective 4-piperidone via its oxime. The resolution of the racemate was accomplished by crystallization as the mandelate. The enantiomeric purity of this material was checked by NMR after derivatization to the corresponding camphorsulfonamide to be 97% ee. The absolute configuration of one enantiomer was confirmed by X-ray single crystal diffraction of the para-bromobenzenesulfonamide derivative.     

Lower bounds for randomized direct search with isotropic sampling

Randomized direct-search methods for the optimization of a function f:Rⁿ→R that is given by a black box for f-evaluations are investigated. These iterative methods generate new candidate solutions by adding isotropically distributed vectors to the current candidate solution. Lower bounds on the number of f-evaluations necessary for reducing the approximation error in the search space are proved.     

Subwavelength ripple formation induced by tightly focused femtosecond laser radiation

Subwavelength ripples (<λ/4) are obtained by scanning a tightly focused beam (∼1 μm) of femtosecond laser radiation (λ = 800 nm, t_p = 100 fs) over the surface of either bulk fused silica and silicon and Er:BaTiO₃. The ripple pattern extends coherently over many overlapping laser pulses parallel and perpendicular to the polarisation. Investigated are the dependence of the ripple spacing on the spacing of successive pulses, the direction of polarisation and the material. The evolution of the ripples is investigated by applying pulse bursts with N = 1 to 20 pulses. The conditions under which these phenomena occur are specified, and some possible mechanisms of ripple growth are discussed. Potential applications are presented.     

Finiteness Theorems for Graphs and Posets Obtained by Compositions

We investigate classes of graphs and posets that admit decompositions to obtain or disprove finiteness results for obstruction sets. To do so we develop a theory of minimal infinite antichains that allows us to characterize such antichains by means of the set of elements below it.In particular we show that the following classes have infinite antichains with respect to the induced subgraph/poset relation: interval graphs and orders, N-free orders, orders with bounded decomposition width. On the other hand for orders with bounded decomposition diameter finiteness of all antichains is shown. As a consequence those classes with infinite antichains have undecidable hereditary properties whereas those with finite antichains have fast algorithms for all such properties.