Diastereoselective Synthesis of Axially Chiral Xylose-Derived 1,3-Disubstituted Alkoxyallenes: Scope, Structure, and Mechanism.

The deprotonation of differently substituted propargyl xylosides with s-BuLi/TMEDA followed by protonation with t-butanol at -115 °C provided a range of new axially chiral 1,3-disubstituted alkoxyallenes in a diastereoselective way. Numerous reaction parameters such as solvent, temperature, or protonating agent were examined as well as protecting groups on the xyloside moiety and the influence of the substituents on the alkyne part. The configuration of the main diastereoisomer of 3-methyl-1-xyloside-allene was determined for the first time by single-crystal X-ray diffraction analysis and nOe NMR experiments. Furthermore, DFT calculations on the propargyl/allenyl lithium intermediates formed in the course of the deprotonation reaction provided new structural insights of these complexes. The subsequent protonation process with alcohols was investigated by theoretical surface exploration, revealing the importance of the approach of the alcohol toward the lithium compounds on the reaction outcome.

Xylose Incorporated in Unsymmetrical Gold(I) N-Heterocyclic Carbene Complexes: Synthesis, Characterization and Antibacterial Activity.

A series of Au(I) complexes containing unsymmetrical N-heterocyclic carbene (imidazolylidene and benzimidazolylidene) functionalized with a xyloside group and an alkyl moiety (methyl and mesityl) was prepared using efficient procedures from D-xylose. Their characterization was carried out in solution by multinuclear NMR, HR-MS spectrometry and cyclic voltammetry, as well as in the solid state by means of single crystal X-ray diffraction analysis for two of them. Evaluation of their ability to inhibit bacterial growth showed a preference for a Gram-positive strain, Staphylococcus aureus, over a Gram-negative strain, Pseudomonas aeruginosa.

Click reactions as a key step for an efficient and selective synthesis of D-xylose-based ILs.

D-Xylose-based ionic liquids have been prepared from D-xylose following a five steps reaction sequence, the key step being a click cycloaddition. These ionic liquids (ILs) have been characterized through classical analytical methods (IR, NMR, mass spectroscopy, elemental analysis) and their stability constants, Tg and Tdec, were also determined. Considering their properties and their hydrophilicity, these compounds could be alternative solvents for chemical applications under mild conditions.

Bolaamphiphiles derived from alkenyl L-rhamnosides and alkenyl D-xylosides: importance of the hydrophilic head.

The two step synthesis of a new bolaamphiphile derived from alkenyl L-rhamnosides was described. The general synthetic strategy of bolaamphiphiles derived from L-rhamnose was based on a previous work describing the synthesis of bolaamphiphiles derived from D-xylose. The conformational properties of this new compound were investigated by FTIR spectroscopy in an aqueous film in order to obtain a reference for further studies about the membrane-interacting properties. Moreover, the surface activity of this new bolaamphiphile was analyzed by Langmuir balance technology and was compared with that of the analogous bolaamphiphile derived from alkenyl D-xylosides. The findings indicate that the rhamnoside-based bolaform has an increased surface activity and a better ability to form aggregates than xyloside-based one.

Olefin metathesis in nano-sized systems.

The interplay between olefin metathesis and dendrimers and other nano systems is addressed in this mini review mostly based on the authors' own contributions over the last decade. Two subjects are presented and discussed: (i) The catalysis of olefin metathesis by dendritic nano-catalysts via either covalent attachment (ROMP) or, more usefully, dendrimer encapsulation - ring closing metathesis (RCM), cross metathesis (CM), enyne metathesis reactions (EYM) - for reactions in water without a co-solvent and (ii) construction and functionalization of dendrimers by CM reactions.

Electroactive benzothiazole hydrazones and their [Mo6O19]2- derivatives: promising building blocks for conducting molecular materials.

The electroactive benzothiazole hydrazone AMBTH-H(2), a new member of the 2,2'-azino-bis(N-alkylbenzothiazole) family, was synthesised in a five-step procedure and characterised by using X-ray diffraction along with two intermediates and the N-methylbenzothiazole hydrazone MBTH-H(2). Both AMBTH-H(2) and MBTH-H(2) were coupled to [Mo(6)O(19)](2-) in acetonitrile in the presence of dicyclohexylcarbodiimide and dimethylaminopyridine to give two new diazoalkane-hexamolybdates, which were isolated as tetrabutylammonium salts and characterised by using IR, UV/Vis and NMR spectroscopies, cyclic voltammetry and, for one of them, X-ray diffraction. The packing arrangement molecules in crystals of AMBTH-H(2), the redox features of the AMBTH-hexamolybdate hybrid together with a good electronic communication between the organic pi system and the molybdenum centres make these compounds very promising blocks for the synthesis of conducting molecular materials.

"Click" glycodendrimers containing 27, 81, and 243 modified xylopyranoside termini.

A series of large glycodendrimers containing 27, 81, and 243 terminal modified xylose branches from the first (G(1)-27) to the third generation (G(3)-243) were synthesized from 2'-azidoethyl 2,3,4-tri-O-acetyl-beta-d-xylopyranoside and alkynyl-terminated dendrimers by "click" chemistry that is confirmed to be an excellent method to obtain large glycodendrimers exemplified by the use of modified xylose. The dendrimers were first characterized by (1)H NMR, (13)C{(1)H} NMR, elemental analysis, and IR spectroscopy. The size progression in the series was also demonstrated using both DOSY NMR and size exclusion chromatography (SEC), the latter technique showing the good polydispersity of all the dendrimers. The size measured by dynamic light scattering (DLS) for the dendrimer G(3)-243 is close to that obtained by the DOSY NMR method.

Formation of (PNP)Rh complexes containing covalent rhodium-zinc bonds in studies of potential Rh-catalysed Negishi coupling.

While investigating rhodium-catalyzed Negishi coupling, it was observed that the (PNP)Rh fragment readily inserted into zinc-carbon bonds to form isolable molecules with covalent rhodium-zinc bonds.

Sodium borohydride stabilizes very active gold nanoparticle catalysts.

Long-term stable 3 nm gold nanoparticles are prepared by a simple reaction between HAuCl4 and sodium borohydride in water under ambient conditions which very efficiently catalyze 4-nitrophenol reduction to 4-nitroaniline.

Gold nanoparticles as electron reservoir redox catalysts for 4-nitrophenol reduction: a strong stereoelectronic ligand influence.

The stereoelectronic properties of the stabilizing ligands of gold nanoparticles (AuNPs) are shown to play a considerable role in their catalytic efficiency for 4-nitrophenol reduction by NaBH4, consistent with a mechanism involving restructuration of the AuNP surface that behaves as an "electron reservoir".

C-H and C-O oxidative addition in reactions of aryl carboxylates with a PNP pincer-ligated Rh(I) fragment.

Reactions of a series of phenyl esters with a (PNP)Rh fragment have been studied. PhO(2)CPh only underwent C-H oxidative addition (OA). PhO(2)CCF(3) chiefly underwent acyl-oxygen OA. PhO(2)CBu(t) and PhO(2)CNEt(2) initially underwent OA of an ortho-C-H bond of the phenyl group but continued thermolysis led to the phenyl-oxygen OA products.

Carbon-halide oxidative addition and carbon-carbon reductive elimination at a (PNP)Rh center.

An unsaturated (PNP)Rh fragment can be generated by means of C-C reductive elimination from (PNP)Rh(Me)(Ar) or (PNP)Rh(Ar)(Ar). This fragment undergoes carbon-halogen oxidative addition with aryl chlorides, bromides, and iodides at room temperature. The C-H oxidative addition products in reactions with haloarenes are not observed, and evidence is presented that carbon-halogen oxidative addition is thermodynamically preferred. C-C reductive elimination from (PNP)Rh(Me)(Ar) and (PNP)Rh(Ar)(Ar) proceeds near quantitatively as a clean, first-order reaction.