Ronny Neumann

Handbook of phase transfer catalysis

Nucleophilic aliphatic and aromatic substitutions in phase transfer catalysis mechanism and synthetic applications kinetics and mechanism in phase transfer catalysis synthesis and properties of quaternary ammonium phase transfer catalysts phase transfer reactions under basic conditions applications of phase transfer catalysis in industrial organic chemistry PTC in polymer chemistry PTC in carbohydrate chemistry PTC in heterocyclic chemistry phase transfer catalysis in oxidation processes organometallic reactions under phase-transfer conditions sonochemical and microwave activation in phase transfer catalysis analytical applications of phase transfer catalysis triphase catalysis chiral phase transfer chemical modification of polymers via phase transfer catalysis phase transfer catalysis of uncharged species.

A ruthenium-substituted polyoxometalate as an inorganic dioxygenase for activation of molecular oxygen

The development of a procedure for the epoxidation of alkenes with molecular oxygen is an important industrial and synthetic goal. Non-radical activation of dioxygen by monooxygenase enzymes such as the iron-porphyrin-based cytochrome P-450 involves the insertion of one oxygen atom of O2 into the organic substrate while the other oxygen is reduced to water in the presence of an electron donor. Dioxygenase enzymes, on the other hand, catalyse insertion of both oxygen atoms without reducing agents. Oxidation of hydrocarbons with dioxygen catalysed by transition-metal compounds invariably proceeds by free-radical pathways rather than by dioxygenase-type reactions, and so do not allow the epoxidation of alkenes with molecular oxygen. The sterically hindered ruthenium tetramesityl porphyrin complex has been shown previously to activate dioxygen in a dioxygenase mode. We describe here the use of the polyoxometalate {[WZnRu2(OH)(H2O)](ZnW9O34)2}11−(ref. 2) as a catalyst for non-radical molecular oxygen activation and alkene epoxidation. The polyoxometalate can be considered to act as an inorganic dioxygenase catalyst. The advantages of using inorganic catalysts such as polyoxometalates, as opposed to organometallic complexes, is their well documented stability against decomposition by self-oxidation.

Photoreduction of Carbon Dioxide to Carbon Monoxide with Hydrogen Catalyzed by a Rhenium(I) Phenanthroline−Polyoxometalate Hybrid Complex

A phenanthroline ligand decorated at the 5,6-position with a 15-crown-5 ether was used to prepare a metalorganic-polyoxometalate hybrid complex Re(I)(L)(CO)(3)CH(3)CN-MHPW(12)O(40) (L = 15-crown-5-phenanthroline, M = Na(+), H(3)O(+)). X-ray diffraction, (1)H and (13)C NMR, ESI-MS, IR, and elemental analysis were used to characterize this complex. In the presence of Pt/C, the polyoxometalate moiety in Re(I)(L)(CO)(3)CH(3)CN-MHPW(12)O(40) can oxidize H(2) to two protons and two electrons which in the presence of visible light can catalyze the photoreduction of CO(2) to CO with H(2) as the reducing agent instead of the universally used amines as sacrificial reducing agents. An EPR spectrum of a stable intermediate species under reaction conditions shows characteristics of a PW(V)W(VI)(11)O(40) and a Re(0) species with a tentative assignment of the intermediate as Re(0)(L)(CO)(3)(S)-MH(3)PW(V)W(VI)(11)O(40).

Activation of molecular oxygen, polyoxometalates, and liquid-phase catalytic oxidation.

In this Forum Article, we discuss the use of dioxygen (O(2)) in oxidations catalyzed by polyoxometalates. One- and two-electron-transfer oxidation of organic substrates is catalyzed by H(5)PV(2)Mo(10)O(40) and often occurs via an outer-sphere mechanism. The reduced polyoxometalate is reoxidized in a separate step by O(2) with the formation of water. H(5)PV(2)Mo(10)O(40) also catalyzes electron transfer-oxygen transfer reactions. Here, in contrast to the paradigm that high-valent oxo species are often stronger oxygen-transfer species than lower-valent species, the opposite occurs. Thus, oxygen transfer from the catalyst is preceded by electron transfer from the organic substrate. The monooxygenase-type reduction of O(2) with polyoxometalates is also discussed based on the formation of a stable iron(III) hydroperoxide compound that may have implications for the oxidation of other lower-valent polyoxometalates such as vanadium(IV)- and ruthenium(II)-substituted polyoxometalates. Finally, the formation of hybrid compounds through the attachment of electron-accepting polyoxometalates to coordination compounds can modify the reactivity of the latter by making higher-valent oxidation states more accessible.

Compliance and Results of Treatment for Amblyopia in Children More Than 8 Years Old

Our prospective study of 350 amblyopic children divided them into three age groups: 2 to 5 1/2 years, 5 1/2 to 8 years, and 8 years and older. Treatment consisted of occlusion of the good eye. Compliance with treatment was analyzed by age group for the whole study population, but visual acuity results were evaluated only in children who complied fully with treatment and who cooperated at the initial visual acuity test with a Snellen chart. Younger children were significantly more compliant than older ones. This was probably the primary reason for the high incidence of treatment failure in older children. Children older than 8 years who complied with treatment showed a marked improvement in visual acuity--one almost as good as that in the younger children. A better initial visual acuity can be taken as a good prognostic sign, especially for this age group. In each group, most of the improvement occurred during the first three months of treatment. Improvement after this period was marginal.

Photochemical Reduction of Carbon Dioxide Catalyzed by a Ruthenium- Substituted Polyoxometalate

A polyoxometalate of the Keggin structure substituted with Ru(III), (6)Q(5)[Ru(III)(H(2)O)SiW(11)O(39)] in which (6)Q=(C(6)H(13))(4)N(+), catalyzed the photoreduction of CO(2) to CO with tertiary amines, preferentially Et(3)N, as reducing agents. A study of the coordination of CO(2) to (6)Q(5)[Ru(III)(H(2)O)SiW(11)O(39)] showed that 1) upon addition of CO(2) the UV/Vis spectrum changed, 2) a rhombic signal was obtained in the EPR spectrum (g(x)=2.146, g(y)=2.100, and g(z)=1.935), and 3) the (13)C NMR spectrum had a broadened peak of bound CO(2) at 105.78 ppm (Delta(1/2)=122 Hz). It was concluded that CO(2) coordinates to the Ru(III) active site in both the presence and absence of Et(3)N to yield (6)Q(5)[Ru(III)(CO(2))SiW(11)O(39)]. Electrochemical measurements showed the reduction of Ru(III) to Ru(II) in (6)Q(5)[Ru(III)(CO(2))SiW(11)O(39)] at -0.31 V versus SCE, but no such reduction was observed for (6)Q(5)[Ru(III)(H(2)O)SiW(11)O(39)]. DFT-calculated geometries optimized at the M06/PC1//PBE/AUG-PC1//PBE/PC1-DF level of theory showed that CO(2) is preferably coordinated in a side-on manner to Ru(III) in the polyoxometalate through formation of a Ru-O bond, further stabilized by the interaction of the electrophilic carbon atom of CO(2) to an oxygen atom of the polyoxometalate. The end-on CO(2) bonding to Ru(III) is energetically less favorable but CO(2) is considerably bent, thus favoring nucleophilic attack at the carbon atom and thereby stabilizing the carbon sp(2) hybridization state. Formation of a O(2)C-NMe(3) zwitterion, in turn, causes bending of CO(2) and enhances the carbon sp(2) hybridization. The synergetic effect of these two interactions stabilizes both Ru-O and C-N interactions and probably determines the promotional effect of an amine on the activation of CO(2) by [Ru(III)(H(2)O)SiW(11)O(39)](5-). Electronic structure analysis showed that the polyoxometalate takes part in the activation of both CO(2) and Et(3)N. A mechanistic pathway for photoreduction of CO(2) is suggested based on the experimental and computed results.

Phenanthroline decorated by a crown ether as a module for metallorganic-polyoxometalate hybrid catalysts: the Wacker type oxidation of alkenes with nitrous oxide as terminal oxidant.

A 1,10-phenanthroline ligand decorated at the 5,6-position by a 15-crown-5 ether moiety was prepared. Ligation of Pd(II) at the nitrogen atom positions followed by complexation at the crown ether group of a redox active H(5)PV(2)Mo(10)O(40) polyoxometalate yielded a hybrid metallorganic-polyoxometalate complex, Pd(II)(15-crown-5-phen)Cl(2)-H(5)PV(2)Mo(10)O(40). This complex was characterized by IR, UV-vis, ESI-MS, and NMR spectroscopy and elemental analysis that all confirmed the hybrid nature of the complex. Pd(II)(15-crown-5-phen)Cl(2)-H(5)PV(2)Mo(10)O(40) was used as a catalyst for the Wacker type oxidation of 1-alkenes to yield the corresponding methylketones in essentially quantitative yields using nitrous oxide as the terminal oxidant.

MICROFABRICATION OF AN ELECTROLUMINESCENT POLYMER LIGHT EMITTING DIODE PIXEL ARRAY

We describe a method to microfabricate a light emitting diode array with pixels based on conjugated electroluminescent polymers sandwiched between appropriate electrodes. This method, based on direct photoablation with the 193 nm emission of an excimer laser, maintains the properties of these unique polymers. The technique as described here has already achieved an array of 20 μm×20 μm pixels with enhanced electroluminescence (EL) from these pixels and possible spectral tuning of the EL by the application of varying external field. This method can be extended to achieve nanometer dimensionalities using near‐field nanolithography.

Vanadium silicate xerogels in hydrogen peroxide catalyzed oxidations

Abstract Vanadium silicate xerogels (V 2 O 5 SiO 2 ) were prepared by the sol-gel method by hydrolysis of vanadium and silicon alkoxides. The use of these xerogels as catalysts for oxidation of alkenes, alcohols and phenol was studied using 30% aqueous hydrogen peroxide as oxidant. It was found that the manner of xerogel preparation strongly influenced the catalytic activity of V 2 O 5 SiO 2 . Alcohols were the preferred solvents for the reaction and did not leach vanadium oxide into solution. For alkenes, epoxidation was the dominant oxidation reaction the yield and selectivity depending on the nucleophilicity and oxidizability of the substrate. For alkenes of intermediate nucleophilicity allylic oxidation was significant. Secondary alcohols were oxidized in low to fair yields whereas primary alcohols were inert. Phenol was oxidized selectively to a 2:1 mixture of hydroquinone/catechol. UV-vis and electron spin resonance spectra of V 2 O 5 SiO 2 treated with hydrogen peroxide conclusively showed formation of vanadium-peroxo species with polarization of the O O bond as seen by formation of vanadium (IV) electron spin resonance spectra leading to oxidation by both heterolytic and homolytic cleavage.

Electron Transfer−Oxygen Transfer Oxygenation of Sulfides Catalyzed by the H5PV2Mo10O40 Polyoxometalate

The oxygenation of sulfides to the corresponding sulfoxides catalyzed by H(5)PV(2)Mo(10)O(40) and other acidic vanadomolybdates has been shown to proceed by a low-temperature electron transfer-oxygen transfer (ET-OT) mechanism. First, a sulfide reacts with H(5)PV(2)Mo(10)O(40) to yield a cation radical-reduced polyoxometalate ion pair, R(2)(+*),H(5)PV(IV)V(V)Mo(10)O(40), that was identified by UV-vis spectroscopy (absorptions at 650 and 887 nm for PhSMe(+*) and H(5)PV(IV)V(V)Mo(10)O(40)) and EPR spectroscopy (quintet at g = 2.0079, A = 1.34 G for the thianthrene cation radical and the typical eight-line spectrum for V(IV)). Next, a precipitate is formed that shows by IR the incipient formation of the sulfoxide and by EPR a VO(2+) moiety supported on the polyoxometalate. Dissolution of this precipitate releases the sulfoxide product. ET-OT oxidation of diethylsulfide yielded crystals containing [V(O)(OSEt(2))(x)(solv)(5-x)](2+) cations and polyoxometalate anions. Under aerobic conditions, catalytic cycles can be realized with formation of mostly sulfoxide (90%) but also some disulfide (10%) via carbon-sulfide bond cleavage.