Showing papers by "Ileana Dragutan published in 2007"

Modulation of oxidative damage by nitroxide free radicals.

Abstract: Piperidine nitroxides like 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) are persistent free radicals in non-acidic aqueous solutions and organic solvents that may have value as therapeutic agents in medicine. In biological environments, they undergo mostly reduction to stable hydroxylamines but can also undergo oxidation to reactive oxoammonium compounds. Reactions of the oxoammonium derivatives could have adverse consequences including chemical modification of vital macromolecules and deleterious effects on cell signaling. An examination of their reactivity in aqueous solution has shown that oxoammonium compounds can oxidize almost any organic as well as many inorganic molecules found in biological systems. Many of these reactions appear to be one-electron transfers that reduce the oxoammonium to the corresponding nitroxide species, in contrast to a prevalence of two-electron reductions of oxoammonium in organic solvents. Amino acids, alcohols, aldehydes, phospholipids, hydrogen peroxide, other nitroxides, hydroxylamines, phenols and certain transition metal ions and their complexes are among reductants of oxoammonium, causing conversion of this species to the paramagnetic nitroxide. On the other hand, thiols and oxoammonium yield products that cannot be detected by ESR even under conditions that would oxidize hydroxylamines to nitroxides. These products may include hindered secondary amines, sulfoxamides and sulfonamides. Thiol oxidation products other than disulfides cannot be restored to thiols by common enzymatic reduction pathways. Such products may also play a role in cell signaling events related to oxidative stress. Adverse consequences of the reactions of oxoammonium compounds may partially offset the putative beneficial effects of nitroxides in some therapeutic settings.

Towards new generations of metathesis metal-carbene pre-catalysts

Abstract: A short general introduction combined with some historical milestones in the field of olefin metathesis is presented followed by an overview of recent repre- contributions and overwhelming work invested in intelligent design and innovative synthesis in this area. Despites of recent advances there is still a great interest in the generation of new, better performing, and more environment friendly metathesis.

A Resourceful New Strategy in Organic Synthesis: Tandem and Stepwise Metathesis/Non‐Metathesis Catalytic Processes.

Abstract: Highlighting the impressive potential of tandem and stepwise metathesis/non-metathesis reactions in synthesis, the present review extends the scope of the newly gained renown of metathesis as a progressive policy for advanced, elegant and economical organic synthesis. Background is provided for the most encountered to date applications where fundamental non-metathetical synthetic transformations (hydrogenation, oxidation, isomerization, allylation, cyclopropanation, etc.) and a variety of name reactions (Diels–Alder, Claisen, Heck, Ugi, Pauson–Khand, Kharasch addition, etc.) are occurring in tandem, as concurrent or sequential processes, with every known type of metathetical reactions catalyzed by ruthenium or molybdenum complexes.

Synthetic approach to ruthenium vinylidene complexes and their applications in metathesis catalysis

Abstract: Convenient routes for synthesis of ruthenium vinylidene complexes have been described, the majority of which use alkynes along with a ruthenium source as the main starting materials. Their successful application as efficient precatalysts in synthesis of heterocyclic and carbocyclic compounds by ring-closing metathesis of alpha,omega-dienes and in the synthesis of polymers by ring-opening metathesis polymerization of cyclooctene, norbornene, 5-substituted norbornene and dicyclopentadiene are fully illustrated. Relevant aspects concerning the activity and stereoselectivity in metathesis reactions of this type of Ru complexes are highlighted

Synthesis of New Elastomers with Controlled Structures Based on Alkylene Oxides, Grafted by ROMP

Abstract: Poly(propylene oxide-ran-allyl glycidyl ether) (PPO-AGE) copolymers have been prepared using as initiating system an alkyl aluminum compound, con- trolled amounts of water and different organic compounds that act as cocatalysts, e.g. ethers, diols, phosphines, salicylic acid derivatives, and organozinc compounds. Among the investigated catalysts the most productive systems showed to be the ternary systems derived from triisobutylaluminum (TIBA) and water having Zn(acac)2 as a third component. These copolymers have subsequently been grafted by ring- opening metathesis polymerization (ROMP) with cyclooctene in the presence of Grubbs first-generation ruthenium (Ru) catalyst to produce new comblike copolymers.

N-heterocyclic carbene ruthenium complexes: Synthesis and catalytic properties

Abstract: Synthesis of imidazol(in)-2-ylidene ruthenium complexes relevant for metathesis and related catalytic processes is described. As an attractive alternative of importance for practical applications, the in situ generation of new NHC ruthenium precatalysts, starting from the easily accessible imidazolium and imidazolinium salts, ruthenium dimer [RuCl2(p-cymene)]2 and a base, is reported in detail. The new NHC family of ruthenium complexes offers great promise as metathesis precatalysts enjoying a wide application profile in organic and polymer synthesis.