P-Amino-cyclopentadienylidene-phosphoranes versus P-cyclopentadienyl-iminophosphoranes--tautomeric protic forms of a new bidentate CpPN ligand system

The Staudinger reaction of cyclopentadienyl-phosphanes C5H5-PMe2 (P1), C5H5-PPh2 (P2), and C5Me4H-PMe2 (P3) with azides Me3SiN3, 1-AdN3 (Ad = 1-adamantyl) and DipN3 (Dip = 2,6-diisopropylphenyl) has been studied. The nature of the products depends on the substituents at the C5 ring, at the phosphorus and nitrogen atoms: A series of new P-amino-cyclopentadienylidene-phosphoranes 2-6 was synthesized by reactions of P1-P3 with 1-AdN3 (2-4) and DipN3 (5,6). In contrast, P-cyclopentadienyl-iminophosphoranes 1 and 7 were obtained as predominant tautomers by reaction of less CH-acidic P3 with less N-basic Me3SiN3 and DipN3. Both tautomers are protonated forms of ambident, potentially chelating constrained-geometry ligands. The molecular structures of three members of this "CpPN" ligand family, 3, 4 and 6, are characterized by X-ray crystallography.     

{7-[Bis(carboxymethyl)amino]coumarin-4-yl}methoxycarbonyl derivatives for photorelease of carboxylic acids, alcohols/phenols, thioalcohols/thiophenols, and amines

Light-induced release of biomolecules from inactive precursor molecules represents a powerful method to study cellular processes with high temporal and spatial resolution. Here we report the synthesis and photochemistry of a series of {7-[bis(carboxymethyl)amino]coumarin-4-yl}methyl carboxylates, carbonates, carbamates, and thiocarbonates as potential phototriggers for compounds with COOH, OH, NH(2), and SH functions. The compounds are soluble in aqueous buffer, show low fluorescence, and are efficiently photolysed by irradiation with UV/Vis or IR light to release carboxylates, alcohols, phenols, amines, thioalcohols, or thiophenols.     

Thiourea-enhanced flavin photooxidation of benzyl alcohol

Upon irradiation, flavin oxidises 4-methoxybenzyl alcohol to the corresponding aldehyde using aerial O(2) as the terminal oxidant. We have observed that this reaction is significantly accelerated by the presence of thiourea. A series of thiourea-functionalised flavins has been prepared from flavin isothiocyanates and their photocatalytic efficiencies have been monitored by NMR. The alcohol photooxidation proceeds rapidly and cleanly with high turnover numbers of up to 580, exceeding previously reported performances. A likely mechanistic rationale for the more than 30-fold acceleration of the photo-redox reaction by thiourea has been derived from spectroscopic, electrochemical, and kinetic studies. Thus, thiourea acts as an electron-transfer mediator for the initial photooxidation of 4-methoxybenzyl alcohol by the excited flavins. This mechanism has similarities to electron-relay mechanisms in flavoenzymes, for which cysteine sulfenic acid intermediates are proposed. The observation that thiourea mediates flavin photo-redox processes is valuable for the design of more sophisticated photocatalysts based on Nature's best redox chromophore.     

Utilizing reversible copper(II) peptide coordination in a sequence-selective luminescent receptor

Although vast information about the coordination ability of amino acids and peptides to metal ions is available, little use of this has been made in the rational design of selective peptide receptors. We have combined a copper(II) nitrilotriacetato (NTA) complex with an ammonium-ion-sensitive and luminescent benzocrown ether. This compound revealed micromolar affinities and selectivities for glycine- and histidine-containing sequences, which closely resembles those of copper(II) ion peptide binding: the two free coordination sites of the copper(II) NTA complex bind to imidazole and amido nitrogen atoms, replicating the initial coordination steps of non-complexed copper(II) ions. The benzocrown ether recognizes the N-terminal amino moiety intramolecularly, and the significantly increased emission intensity signals the binding event, because only if prior coordination of the peptide has taken place is the intramolecular ammonium ion-benzocrown ether interaction of sufficient strength in water to trigger an emission signal. Intermolecular ammonium ion-benzocrown ether binding is not observed. Isothermal titration calorimetry confirmed the binding constants derived from emission titrations. Thus, as deduced from peptide coordination studies, the combination of a truncated copper(II) coordination sphere and a luminescent benzocrown ether allows for the more rational design of sequence-selective peptide receptors.     

Controllable Isomerization of Alkenes by Dual Visible‐Light‐Cobalt Catalysis

We report herein that thermodynamic and kinetic isomerization of alkenes can be accomplished by the combination of visible light with Co catalysis. Utilizing Xantphos as the ligand, the most stable isomers are obtained, while isomerizing terminal alkenes over one position can be selectively controlled by using DPEphos as the ligand. The presence of the donor–acceptor dye 4CzIPN accelerates the reaction further. Transformation of exocyclic alkenes into the corresponding endocyclic products could be efficiently realized by using 4CzIPN and Co(acac)₂ in the absence of any additional ligands. Spectroscopic and spectroelectrochemical investigations indicate Co^I being involved in the generation of a Co hydride, which subsequently adds to alkenes initiating the isomerization.     

Site-Selective,Remote sp3 C−H Carboxylation Enabled by the Merger of Photoredox and Nickel Catalysis

A photoinduced carboxylation of alkyl halides with CO₂ at remote sp³ C−H sites enabled by the merger of photoredox and Ni catalysis is described. This protocol features a predictable reactivity and site selectivity that can be modulated by the ligand backbone. Preliminary studies reinforce a rationale based on a dynamic displacement of the catalyst throughout the alkyl side chain.     

Exploiting the chemical shift displacement effect in the detection of glutamate and glutamine (Glx) with PRESS

A PRESS (Point RESolved Spectroscopy) sequence for the improved detection of the C2 protons of Glx (glutamate and glutamine) at approximately 3.75ppm is presented in this work. It is shown that for spins like the C2 protons of Glx which are involved solely in weak coupling interactions, the chemical shift displacement effect can be turned to advantage by exploiting PRESS refocusing pulses with bandwidths less than the chemical shift difference between the target spins and the spins to which they are weakly coupled. The narrow-bandwidth PRESS sequence allows refocusing of the J-coupling evolution of the target protons in the voxel of interest independently of echo time yielding signal equivalent to that which can be obtained with a one-pulse acquire sequence (assuming ideal pulses and ignoring T2 relaxation). The total echo time of PRESS was set long enough for the decay of macromolecule signal and the two echo times were empirically optimized so that the Glx signal at 3.75ppm suffered minimal contamination from myo-inositol. The efficacy of the method was verified on phantom solutions of Glx and on brain in vivo.     

Pentane and tetra­hydro­furan solvates of trans‐di­chloro­bis­[1,2‐ethane­diyl­bis­(di­phenyl­phos­phine)‐P,P′]­molybdenum(II)

trans‐[MoCl₂(dppe)₂] [dppe is 1,2‐ethane­diyl­bis­(di­phenyl­phos­phine), C₂₆H₂₄P₂] was obtained as a side product from the reaction of trans‐[Mo(dppe)₂(N₂)₂] with Cp*GeCl to give the germyl­yne complex trans‐[Cl(dppe)₂Mo[triple‐bond]Ge(η¹‐Cp*)]. The crystal structures of the hemi­pentane (0.5C₅H₁₂) and di­tetra­hydro­furan (2C₄H₈O) solvates of trans‐[MoCl₂(dppe)₂], (IIIa) and (IIIb), respectively, have been determined.     

Mathematics and Neural Networks –A Glance at some Basic Connections

In the following paper, we present a brief and easily accessible introduction to the theory of neural networks under special emphasis on the rôle of pure and applied mathematics in this interesting field of research. In order to allow a quick and direct approach even for nonspecialists, we only consider three-layer feedforward networks with sigmoidal transfer functions and do not cover general multi-layer, recursive or radial-basis-function networks. Moreover, we focus our attention on density and complexity results while construction problems based on operator techniques are not discussed in detail. Especially, in connection with complexity results, we show that neural networks in general have the power to approximate certain function spaces with a minimal number of free parameters. In other words, under this specific point of view neural networks represent one of the best possible approximation devices available. Besides pointing out this remarkable fact, the main motivation for presenting this paper is to give some more mathematicians an idea of what is going on in the theory of neural networks and, perhaps, to encourage, at least a few of them, to start working in this highly interdisciplinary and promising field, too.