Structure, magnetism and optical properties of achiral and chiral two-dimensional oxalate-bridged anionic networks with symmetric and asymmetric ammonium cations

A series of two-dimensional (2D) oxalate-based compounds, namely [N(n-C4H9)4][M(II)Cr(III)(ox)3] (M(II) = Mn, Fe; ox = C2O4(2-)) and [N(C2H5)(n-C3H7)(n-C4H9)(n-C5H11)][M(II)M(III)(ox)(3)] ((M(II), M(III)) =(Mn, Cr), (Fe, Cr), (Mn, Fe)) were synthesised starting from racemic tris(oxalato)metalate: rac-[M(III)(ox)3]3- (M(III) = Cr, Fe). For Cr(III), the synthesis has been undertaken starting from resolved (Delta)- or (Lambda)-[Cr(III)(ox)3]3-. The natural circular dichroism measurements assess the enantioselectivity of the synthesis. X-Ray powder diffraction analysis has revealed that, when racemic reagents are used to synthesise Mn(II) containing compounds, a R3c achiral space group is found. In contrast a P6(3) chiral space group is found when starting from (Delta)- or (Lambda)-[Cr(III)(ox)3]3-. Surprisingly, whatever the optical purity of the starting building block, all Fe(II) containing compounds crystallise in the P6(3) chiral space group. The magnetic properties of the synthesised compounds confirm that these compounds are ferromagnets for M(III)= Cr. For M(II)= Mn, Theta ranges between 9 and 11 K and T(c) equals 6 K. For M(II)= Fe, Theta ranges between 14 and 16 K and Tc between 11 and 12 K. [N(C2H5)(n-C3H7)(n-C4H9)(n-C5H11)][Mn(II)Fe(III)(ox)3] is an antiferromagnet with Theta = - 107 K and T(N) = 29 K.     

2,2‐Dimethyl‐5‐nitroso‐1,3‐dioxan‐5‐yl benzoate, 2,2‐dimethyl‐5‐nitroso‐1,3‐dioxan‐5‐yl 4‐chlorobenzoate and 5‐nitroso‐1,3‐dioxan‐5‐yl 4‐chlorobenzoate

The crystal structures of 2,2‐dimethyl‐5‐nitroso‐1,3‐dioxan‐5‐yl benzoate, C₁₃H₁₅NO₅, (I), 2,2‐dimethyl‐5‐nitroso‐1,3‐dioxan‐5‐yl 4‐chlorobenzoate, C₁₃H₁₄ClNO₅, (II), and 5‐nitroso‐1,3‐dioxan‐5‐yl 4‐chlorobenzoate, C₁₁H₁₁NO₅, (III), have been determined in order to gain insight into the conformational preference of α‐benzoyloxynitroso. Unfavourable 1,3‐diaxial interactions force (I) and (II) to crystallize in the 2,5 twist‐boat conformation, whereas compound (III), lacking this destabilizing interaction, crystallizes in the chair conformation.     

Lewis acid-promoted hetero Diels-Alder cycloaddition of alpha-acetoxynitroso dienophiles

[reaction: see text] alpha-Acetoxynitroso compound 3 has been prepared as a new stable, isolable, and reactive dienophile in nitroso Diels-Alder reactions. The yield of the [4 + 2] cycloaddition of alpha-acetoxynitroso dienophile with 1,3-dienes could be enhanced in the presence of 20 mol % Lewis acid. An unexpected retro hetero-Michael reaction from 26 was observed, leading to the cleavage of the N-O bond of the cycloadduct. This tandem nitroso Diels-Alder/retro hetero-Michael sequence has been used with cyclic and acyclic 1,3-dienes.     

Efficient cleavage of the N-O bond of 3,6-dihydro-1,2-oxazines mediated by some alpha-hetero substituted carbonyl compounds in mild conditions

The efficient cleavage of the N-O bond of some nitroso Diels-Alder cycloadducts has been achieved in mild conditions, mediated either by 2,2-dimethyl-1,3-dioxan-5-one or 1,3-dithiolane-2-carboxaldehyde. These new and purely organic conditions allow an excellent tolerance with respect to many functional groups that would have been affected by previous reductive cleavage conditions.     

Characterization of two series of nitrogen-containing dendrimers by natural abundance 15N NMR

Two series of small generation dendrimers built with phosphorus atoms at each branching point and various types of nitrogen atoms at natural abundance of (15)N within the branches are characterized by a gradient enhanced GHNMQC (gradient hydrogen-nitrogen multiple quantum coherence) (1)H-(15)N NMR technique. The first series contains two types of nitrogen atoms, included in phosphorhydrazone linkages (CH=NNMe-P(S)), whereas the second series contains four types of nitrogen atoms included in azobenzene linkages (Ar-N=N-Ar') in addition to the phosphorhydrazone. The influence of the trans/cis isomerization of the azo bond on the (15)N NMR has also been studied. Despite the low solubility of the azobenzene-containing dendrimers, which renders the detection of some signals difficult, (15)N NMR appears as a very sensitive tool to detect chemical changes in the dendritic structure.     

Does catalysis of reductive dechlorination of tetra- and trichloroethylenes by vitamin B12 and corrinoid-based dehalogenases follow an electron transfer mechanism?

Knowing the mechanism by which dangerous organic chloride pollutants, such as tetra- or trichloroethylene, are reductively cleaved is an important task for the establishment of remediation strategies and for a better comprehension of bacterial dehalorespiration by corrinoid-based dehalogenases. On the basis of electrochemical and thermodynamic data, application of outersphere and dissociative electron transfer theories allows the prediction of the pertinent activation/driving force relationships characterizing the electron transfer mechanism. They are validated by application of the redox catalysis method to the reaction with two typical outersphere electron donors. The kinetic gap is more than 11 and 7 orders of magnitude for the dehalogenase and for cobalamin, respectively, showing that the electron transfer mechanism is not operative. Multistep mechanisms in which the chloroethylene molecule enters the cobalt coordination sphere are preferred.     

Bioinspired Oxidative Cyclization of the Geissoschizine Skeleton for the Total Synthesis of (−)‐17‐nor‐Excelsinidine

We report the first total synthesis of (?)‐17‐nor‐excelsinidine, a zwitterionic monoterpene indole alkaloid that displays an unusual N4?C16 connection. Inspired by the postulated biosynthesis, we explored an oxidative coupling approach from the geissoschizine framework to forge the key ammonium–acetate connection. Two strategies allowed us to achieve this goal, namely an intramolecular nucleophilic substitution on a 16‐chlorolactam with the N4 nitrogen atom or a direct I₂‐mediated N4?C16 oxidative coupling from the enolate of geissoschizine.     

An Oxygen‐Tolerant PET‐RAFT Polymerization for Screening Structure–Activity Relationships

The complexity of polymer–protein interactions makes rational design of the best polymer architecture for any given biointerface extremely challenging, and the high throughput synthesis and screening of polymers has emerged as an attractive alternative. A porphyrin‐catalysed photoinduced electron/energy transfer–reversible addition‐fragmentation chain‐transfer (PET‐RAFT) polymerisation was adapted to enable high throughput synthesis of complex polymer architectures in dimethyl sulfoxide (DMSO) on low‐volume well plates in the presence of air. The polymerisation system shows remarkable oxygen tolerance, and excellent control of functional 3‐ and 4‐arm star polymers. We then apply this method to investigate the effect of polymer structure on protein binding, in this case to the lectin concanavalin A (ConA). Such an approach could be applied to screen the structure–activity relationships for any number of polymer–protein interactions.