Development of the first well-defined tungsten oxo alkyl derivatives supported on silica by SOMC: towards a model of WO3/SiO2 olefin metathesis catalyst

A well-defined, silica-supported tungsten oxo alkyl species prepared by the surface organometallic chemistry approach displays high and sustained activity in propene metathesis. Remarkably, its catalytic performances outpace those of the parent imido derivative, underlining the importance of the oxo ligand in the design of robust catalysts.     

Highly Fluorinated Tris(indazolyl)borate Silylamido Complexes of the Heavier Alkaline Earth Metals: Synthesis,Characterization, and Efficient Catalytic Intramolecular Hydroamination

Heteroleptic silylamido complexes of the heavier alkaline earth elements calcium and strontium containing the highly fluorinated 3‐phenyl hydrotris(indazolyl)borate {F₁₂‐Tp^{4Bo, 3Ph}}^? ligand have been synthesized by using salt metathesis reactions. The homoleptic precursors [Ae{N(SiMe₃)₂}₂] (Ae=Ca, Sr) were treated with [Tl(F₁₂‐Tp^{4Bo, 3Ph})] in pentane to form the corresponding heteroleptic complexes [(F₁₂‐Tp^{4Bo, 3Ph})Ae{N(SiMe₃)₂}] (Ae=Ca ( 1 ); Sr ( 3 )). Compounds 1 and 3 are inert towards intermolecular redistribution. The molecular structures of 1 and 3 have been determined by using X‐ray diffraction. Compound 3 exhibits a Sr ??? MeSi agostic distortion. The synthesis of the homoleptic THF‐free compound [Ca{N(SiMe₂H)₂}₂] ( 4 ) by transamination reaction between [Ca{N(SiMe₃)₂}₂] and HN(SiMe₂H)₂ is also reported. This precursor constitutes a convenient starting material for the subsequent preparation of the THF‐free complex [(F₁₂‐Tp^{4Bo, 3Ph})Ca{N(SiMe₂H)₂}] ( 5 ). Compound 5 is stabilized in the solid state by a Ca???β‐Si?H agostic interaction. Complexes 1 and 3 have been used as precatalysts for the intramolecular hydroamination of 2,2‐dimethylpent‐4‐en‐1‐amine. Compound 1 is highly active, converting completely 200 equivalents of aminoalkene in 16 min with 0.50 mol % catalyst loading at 25 °C.     

Tridentate Complexes of Palladium(II) and Platinum(II) Bearing bis‐Aryloxide Triazole Ligands: A Joint Experimental and Theoretical Investigation

A novel class of palladium(II) and platinum(II) complexes bearing tridentate bis‐aryloxide triazole ligands was prepared by using straightforward and high‐yielding synthetic routes. The complexes were fully characterized and the molecular structures of four derivatives were unambigously determined by single‐crystal X‐ray diffractometric analyses. For the most promising luminescent Pt^II derivatives, further experimental investigations were carried out to characterize their photophysical features and to ascertain the nature of the emitting excited state by means of electronic absorption, steady‐state, and time‐resolved emission techniques in different conditions. In degassed fluid solution the complexes displayed broad and featureless photoluminescence with λ_em=522–585 nm, excited‐state lifetime up to few microseconds and quantum yield (PLQY) up to 17 %, depending on the nature of both ancillary ligand and substituent on the tridentate ligand. Computational investigation using density functional theory and time‐dependent DFT were performed to gain insight into the electronic processes responsible for optical transitions and structure–photoluminescence relationship. Jointly, experimental and theoretical characterization indicated that the radiative transition arises from an excited state with admixed triplet‐manifold metal‐to‐ligand charge transfer and ligand‐centered (³MLCT/³LC) character. We elucidated the modulation of the photophysical properties upon variation of substituents for this new family of complexes.     

Polymeric prodrugs of mitomycin C

Poly-[N-(2-hydroxyethyl)-L-glutamine] (PHEG) prodrugs of the cytotoxic agent Mitomycin C were synthesized using peptidyl spacers to link the drug to the polymeric carrier. The influence on the length and detailed structure of the oligopeptide on the rate of drug release was investigated in buffer, in the presence of lysosomal enzymes (tritosomes, cathepsin B and D) and metalloprotease type IV collagenase. It was observed that tetra- and hexapeptide based conjugates generally release Mitomycin C (MMC) more effectively than tripeptide derivatives. The gly-phe-ala-leu conjugate released MMC very rapidly both in presence of lysosomal enzymes and collagenase IV. Only in the presence of the aspartic protease cathepsin D, the gly-phe-leu-gly-phe-leu derivative turned out to be a better substrate. In vivo studies against C26 solid tumour bearing mice suggest that PHEG-spacer-MMC conjugates act as prodrugs of MMC: antitumour efficacy of the macromolecular prodrugs was better than free MMC both in inhibition of tumour growth and increasing survival.     

NR transfer reactivity of azo-compound I of P450. How does the nitrogen substituent tune the reactivity of the species toward C-H and C=C activation?

We studied electronic structures and reactivity patterns of azo-compound I species (RN-Cpd I) by comparison to O-Cpd I of, e.g., cytochrome P450. The study shows that the RN-Cpd I species are capable of C=C aziridination and C-H amidation, in a two-state mechanism similar to that of O-Cpd I. However, unlike O-Cpd I, here the nitrogen substituent (R) exerts a major impact on structure and reactivity. Thus, it is demonstrated that Fe=NR bonds of RN-Cpd I will generally be substantially longer than Fe=O bonds; electron-withdrawing R groups will generate a very long Fe=N bond, whereas electron-releasing R groups should have the opposite effect and hence a shorter Fe=N bond. The R substituent controls also the reactivity of RN-Cpd I toward C=C and C-H bonds by exerting steric and electronic effects. Our analysis shows that an electron-releasing substituent will lower the barriers for both bond activation reactions, since the electronic factor makes the reactions highly exothermic, while an electron-withdrawing one should raise both barriers. The steric bulk of the substituent is predicted to inhibit more strongly the aziridination reactions. It is predicted that electron-releasing substituents with small bulk will create powerful aziridination reagents, whereas electron-withdrawing substituents like MeSO(2) will prefer C-H bond activation with preference that increases with steric bulk. Finally, the study predicts (i) that the reactions of RN-Cpd I will be less stereospecific than those of O-Cpd I and (ii) that aziridination will be more stereoselective than amidation.     

Phenyl Silicates with Substituted Catecholate Ligands: Synthesis,Structural Studies and Reactivity

While the generation of aryl radicals by photoredox catalysis under reductive conditions is well documented, it has remained challenging under an oxidative pathway. Because of the easy photo-oxidation of alkyl bis-catecholato silicates, a general study of phenyl silicates bearing substituted catecholate ligands has been achieved. The newly synthesized phenyl silicates have been fully characterized, and their reactivity has been explored. It was found that, thanks to the substitution of the catecholate moiety, notably with the 4-cyanocatecholato ligand, the phenyl radical could be generated and trapped. Computational studies provided a rationale for these findings.     

Biodegradable polyphosphazenes for drug delivery

Polyphosphazene derivatives having amino acid ester side groups were prepared by reaction of poly(dichlorophosphazene) with ethyl esters of amino acids. The in vitro degradation studies demonstrated that the rate of degradation depends on the nature of the amino acids. Introducing small amounts of hydrolytically sensitive groups such as depsipeptide ester or hydrolysis-catalysing moieties, such as histidine ethyl ester co-substituents, resulted in an increase of the degradation. The rate of hydrolytic degradation of the polyphosphazene material could be controlled by the content of the hydrolytically sensitive side groups or by blending hydrolysis-sensitive polymers with more stable derivatives. The results obtained from the in vivo implantation of biodegradable polyphosphazenes in mice indicate that the materials are very well tolerated by the animal body. Biodegradable polyphosphazenes have been used as matrix for the design of drug delivery systems. The rate of the in vitro release of mitomycin C from biodegradable polyphosphazenes can be controlled by changing the chemical composition of the polymer or by blending polymers of different chemical compositions.     

Carboxymethyl glycoside lactone (CMGL) synthons: Scope of the method and preliminary results on step growth polymerization of α-azide-ω-alkyne glycomonomers

Carboxymethyl glycoside lactones (CMGLs) are bicyclic synthons which open readily for accessing new types of pseudo-glycoconjugates, such as sugar-amino acid hybrids, neoglycolipids, pseudodisaccharides, and membrane imaging systems. After lactone opening, free OH-2 is available for further functionalization, leading to 1,2-bisfunctionalized derivatives. This strategy is illustrated herein with new polymerizable systems of the AB type bearing both azide and alkyne functions prepared from α or β gluco-CMGL synthons. After the reaction of lactones with propargylamine, an azido group was introduced by two different sequences leading to either the 2-manno-azido or the 6-gluco-azido products. The capability of these AB monomers to undergo step growth polymerization through copper(I) catalyzed alkyne-azide cycloaddition (CuAAC) and generate glycopolytriazoles was evidenced.     

Site selective generation of sol-gel deposits in layered bimetallic macroporous electrode architectures

The elaboration of an original composite bimetallic macroporous electrode containing a site-selective sol-gel deposit is reported. Regular colloidal crystals, obtained by a modified Langmuir-Blodgett approach, are used as templates for the electrogeneration of the desired metals in the form of a well-defined layered bimetallic porous electrode. This porous matrix shows a spatially modulated electroactivity which is subsequently used as a strategy for targeted electrogeneration of a sol-gel deposit, exclusively in one predefined part of the porous electrode.     

High selectivity production of propylene from 2-butene: non-degenerate pathways to convert symmetric olefins via olefin metathesis

The first example of propylene production from 2-butene in promising yield is described by reacting trans-2-butene over tungsten hydrides precursor W-H/Al(2)O(3) at 150 °C and different pressures in a continuous flow reactor. The tungsten carbene-hydride active site operates as a "bi-functional catalyst" through the disfavoured 2-butene isomerisation on W-hydride and 2-butenes/1-butene cross-metathesis on W-carbene.