Iodine(III)‐Catalyzed Rearrangements of Imides: A Versatile Route to α,α‐Dialkylated α‐Hydroxy Carboxylamides

A tertiary hydroxy group α to a carboxyl moiety comprises a key structural motif in many bioactive substances. With the herein presented metal‐free rearrangement of imides triggered by hypervalent λ³‐iodane, an easy and selective way to gain access to such a compound class, namely α,α‐disubstituted‐α‐hydroxy carboxylamides, was established. Their additional methylene bromide side chain constitutes a useful handle for rapid diversification, as demonstrated by a series of further functionalizations. Moreover, the in situ formation of an iodine(III) species under the reaction conditions was proven. Our findings clearly corroborate that hypervalent λ³‐benziodoxolones are involved in these organocatalytic reactions.     

Nonlinear frequency response analysis: a recent review and perspectives

The nonlinear frequency response analysis (NFRA) can be seen as an extension of electrochemical impedance spectroscopy. NFRA gives a full and detailed representation of the system response and can establish a connection between model parameters and the experimentally observed phenomena. In this article, different theoretical NFRA approaches and the most recent application examples are discussed. A simple electrochemical example is used to showcase the benefits and disadvantages of analyzing the system response by using different approaches. In addition, it was shown how to extract experimental harmonic values and analyze them.     

Synthesis,characterization and properties of methylaminocellulose

Homogeneously prepared tosylcelluloses (TC) with degrees of substitution (DS) of DS_Tos 0.1–1.8 were used as intermediates for the synthesis of methylaminocelluloses (MAC) by nucleophilic substitution with methylamine. TC with DS_Tos up to 1.1 were shown to be valuable intermediates for selective synthesis of MAC with DS_MA varying from 0.1 to approximately 1. No nucleophilic substitution was observed at higher DS_Tos. At the chosen reaction conditions (60 °C, 48 h) residual tosyl moieties remained unchanged and little hydrolysis took place. The samples obtained were characterized by means of elemental analysis, FTIR and ¹³C CP/MAS NMR spectroscopy. ¹³C CP/MAS NMR spectroscopy was found to be an efficient tool for quantification of DS_MA. Furthermore, the swelling behaviour in water was investigated and preliminary tests concerning the bilirubin adsorption capacity of MAC were carried out.     

Methylene-azaphosphirane as a reactive intermediate

Reaction of the transient phosphinidene complexes R-P=W(CO)5 with N-substituted-diphenylketenimines leads unexpectedly to the novel 2-aminophosphindoles, as confirmed by an X-ray crystal structure determined for one of the derivatives. Experimental evidence for a methylene-azaphosphirane intermediate was found by using the iron-complexed phosphinidene iPr2N-P=Fe(CO)4, which affords the 2-aminophosphindole together with the novel methylene-2,3-dihydro-1H-benzo[1,3]azaphosphole. Analysis of the reaction pathways with DFT indicates that the initially formed methylene-azaphosphirane yields both phosphorus heterocycles by way of a [1,5]- or [1,3]-sigmatropic shift, respectively, followed by a H-shift. Strain underlies both rearrangements, which causes these remarkably selective conversions that can be tuned by changing the substituents.     

Some reactions of 1-alkyl-1,2,3,4-tetrahydropyridines with organic azides. Synthesis of 1-alkylpiperidylidene-2-sulfonamides

The 1,3-dipolar cycloaddition reaction of 1-substituted-1,2,3,4-tetrahydropyridines 4a-d with organic azides 5 afforded the respective 1-substituted-piperidylidene-2-sulfonamides 6 . In contrast, the reaction of (E)-1-(1-propenyl)-1,2,3,4-tetrahydropyridine ( 4e ) with 4-chlorobenzenesulfonyl azide yielded 6m as well as 1-(4-chlorophenyl)sulfonimine-1,2,3,4-tetrahydropyridine ( 7 ) arising from addition of the azide to the (E)-1-(1-propenyl) substituent of 4e .     

Characterization of the monovalent ion position and hydrogen-bond network in guanine quartets by DFT calculations of NMR parameters

Conformational stability of G-quartets found in telomeric DNA quadruplex structures requires the coordination of monovalent ions. Here, an extensive Hartree-Fock and density functional theory analysis of the energetically favored position of Li+, Na+, and K+ ions is presented. The calculations show that at quartet-quartet distances observed in DNA quadruplex structures (3.3 A), the Li+ and Na+ ions favor positions of 0.55 and 0.95 A outside the plane of the G-quartet, respectively. The larger K+ ion prefers a central position between successive G-quartets. The energy barrier separating the minima in the quartet-ion-quartet model are much smaller for the Li+ and Na+ ions compared with the K+ ion; this suggests that K+ ions will not move as freely through the central channel of the DNA quadruplex. Spin-spin coupling constants and isotropic chemical shifts in G-quartets extracted from crystal structures of K+- and Na+-coordinated DNA quadruplexes were calculated with B3LYP/6-311G(d). The results show that the sizes of the trans-hydrogen-bond couplings are influenced primarily by the hydrogen bond geometry and only slightly by the presence of the ion. The calculations show that the R(N2N7) distance of the N2-H2...N7 hydrogen bond is characterized by strong correlations to both the chemical shifts of the donor group atoms and the (h2)J(N2N7) couplings. In contrast, weaker correlations between the (h3)J(N1C6') couplings and single geometric factors related to the N1-H1...O6=C6 hydrogen bond are observed. As such, deriving geometric information on the hydrogen bond through the use of trans-hydrogen-bond couplings and chemical shifts is more complex for the N1-H1...O6=C6 hydrogen bond than for the N2-H2...N7 moiety. The computed trans-hydrogen-bond couplings are shown to correlate with the experimentally determined couplings. However, the experimental values do not show such strong geometric dependencies.     

Interference between the hydrogen bonds to the two rings of nicotine

The biochemical transport and binding of nicotine depends on the hydrogen bonding between water and binding site residues to the pyridine ring and the protonated pyrrolidinium ring. To test the independence of these two moderately separated hydrogen-bonding sites, we have calculated the structures of clusters of protonated nicotine with water and a bicarbonate anion, benzene, indole, or a second water molecule. Unprotonated nicotine-water clusters have also been studied for contrast. The potential energy surfaces are first explored with an intermolecular anisotropic atom-atom model potential. Full geometry optimizations are then carried out using density functional theory to include nonadditive terms in the interaction energies. The presence of the charge on the pyrrolidine nitrogen removes the conventional hydrogen-bonding site on the pyridine ring. The hydrogen-bond ability of this site is nearly recovered when the protonated pyrrolidinium ring is bound to a bicarbonate anion, whereas its interaction with benzene shows a much smaller effect. Indole appears to partially restore the hydrogen-bond ability of the pyridine nitrogen, although indole and benzene both pi-bond to the pyrrolidinium ring. A second hydrogen-bonding water produces a significant conformational distortion of the nicotine. This demonstrates the limitations of the conventional qualitative predictions of hydrogen bonding based on the independence of molecular fragments. It also provides benchmarks for the development of atomistic modeling of biochemical systems.     

Fluorescent, Siderophore-Based Chelators. Design and Synthesis of a Trispyrenyl Trishydroxamate Ligand, an Intramolecular Excimer-Forming Sensing Molecule Which Responds to Iron(III) and Gallium(III) Metal Cations

Two pyrene-labeled hydroxylamines, 5-Bn and 5-Bz, O-protected with the benzyl and the benzoyl group, respectively, have been prepared for the generation of siderophore-based new chelators incorporating both the pyrene chromophore and the hydroxamic acid functionality. 5-Bz formed the starting point toward the synthesis of the tripod-shaped trishydroxamate, 1. That trichromophoric ligand displays remarkable fluorescence emission properties (dual emission: "monomer" and excimer type) which are markedly and selectively modified by binding Fe(NO(3))(3) and Ga(NO(3))(3). Ferric ions induce a quasi total quenching of the pyrene fluorescence, whereas the nonquenching Ga(III) cations are observed to affect the value of the excimer-to-monomer fluorescence intensity ratio. Ethylenediaminetetraacetic acid (EDTA) competition reactions yielded an estimated value of 3.8 for log K of the complex LFe in methanol/water (80/20 v/v), where K = ([LFe][H(+)](3))/([LH(3)][Fe(3+)]) and L is the ligand in its totally deprotonated form. Compound 1 is the prototype of a new class of photoresponsive molecular systems which could act as sensitive probes for metal cation detection and recognition.     

Über Oxotantalate der Lanthanide des Formeltyps MTaO4 (M = La – Nd,Sm – Lu)

About Lanthanide Oxotantalates with the Formula MTaO₄ (M = La – Nd, Sm – Lu) Besides being a by‐product of solid state syntheses in tantalum ampoules the lanthanide(III) oxotantalates of the formula MTaO₄ can be easily prepared by sintering lanthanide sesquioxide M₂O₃ and tantalum(V) oxide Ta₂O₅ with sodium chloride as flux. Under these conditions two structure types emerge depending upon the M³⁺ cationic radius. For M = La – Pr the MTaO₄‐type tantalates crystallize in the space group P2₁/c with lattice constants of a = 762(±1), b = 553(±4), c = 777(±4) pm, β = 101(±1)° and four formula units per unit cell. With M = Nd, Sm – Lu, the monoclinic cell dimensions (space group P2/c) shrink to the lattice constants like a = 516(±9), b = 551(±9), c = 534(±9) pm, β = 96.5(±0.3)° and there are only two formula units present. Both structures show a coordination sphere of eight oxygen atoms for the lanthanide trications shaped as distorted square antiprism for the structure with the larger lanthanides (in the following referred to as A‐type) and as trigonal dodecahedron for the structure with the smaller ones (called as B‐type in the following). The coordination environment about the Ta⁵⁺ cations can be described as a slightly distorted octahedron (CN = 6) for the A‐type structure of MTaO₄ and a heavily distorted one (CN = 6) for the B‐type. The difference between the two types results from the interconnection of these [TaO₆]^7? octahedra. Whereas they are connected via four vertices to form corrugated layers according to parallel the bc‐plane in the A‐type, the octahedra of the B‐type MTaO₄ structure share edges to built up zig‐zag chains along the c axis.