Ruthenium-based oxidations

This very hindered secondary alcohol—located in a complex molecule—can be efficiently oxidized to the corresponding ketone in a biphasic system, using Ru04 generated from RuC13 and excess of NaI04. An additional oxidation of a cyclic ether to a lactone occurs under the reaction conditions. 

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A variety of methods have been described to solve the task in solution.16 Common oxidative agents for this transformation include various heavy-metal reagents such as chromium-or ruthenium-based oxides, pyri-dine-S03, and dimethylsulfoxide (DMSO) in combination with acetic anhydride, carbodiimide, or oxalyl chloride for activation. One of the most prominent methods for the reliable conversion of sensitive compounds is the Dess-Martin reagent or its nonacetylated equivalent, 1-hydroxy-(17/)-benzo-l,2-iodoxol-3-one-l-oxide (2-iodoxybenzoic acid, IBX). 

Miscellaneous. Ruthenium dioxide-based thick-film resistors have been used as secondary thermometers below I K (92). Ruthenium dioxide-coated anodes ate the most widely used anode for chlorine production (93). Ruthenium(IV) oxide and other compounds ate used in the electronics industry as resistor material in apphcations where thick-film technology is used to print electrical circuits (94) (see Electronic materials). Ruthenium electroplate has similar properties to those of rhodium, but is much less expensive. Electrolytes used for mthenium electroplating (95) include [Ru2Clg(OH2)2N] Na2[Ru(N02)4(N0)0H] [13859-66-0] and (NH 2P uds(NO)] [13820-58-1], Several photocatalytic cycles that generate... 

Ruthenium (IV) oxide [12036-10-1] M 133.1, d 6.97. Freed from nitrates by boiling in distilled water and filtering. A more complete purification is based on fusion in a KOH-KNO3 mix to form the soluble ruthenate and perruthenate salts. The melt is dissolved in water, and filtered, then acetone is added to reduce the ruthenates to the insoluble hydrate oxide which, after making a slurry with paper pulp, is filtered and ignited in air to form the anhydrous oxide [Campbell, Ortner and Anderson Anal Chem 33 58 1961]. 

For Cl2 or 02 evolution the stability of ruthenium based electrodes is not sufficient on a technical scale. Therefore the possibility of stabilizing the ruthenium oxide without losing too much of its outstanding catalytic performance was investigated by many groups. For the Cl2 process, mixed oxides with valve metals like Ti or Ta were found to exhibit enhanced stability (see Section 3.1), while in the case of the 02 evolution process in solid polymer electrolyte cells for H2 production a mixed Ru/Ir oxide proved to be the best candidate [68, 80]. 

The electroactive units in the dendrimers that we are going to discuss are the metal-based moieties. An important requirement for any kind of application is the chemical redox reversibility of such moieties. The most common metal complexes able to exhibit a chemically reversible redox behavior are ferrocene and its derivatives and the iron, ruthenium and osmium complexes of polypyridine ligands. Therefore it is not surprising that most of the investigated dendrimers contain such metal-based moieties. In the electrochemical window accessible in the usual solvents (around +2/-2V) ferrocene-type complexes undergo only one redox process, whereas iron, ruthenium and osmium polypyridine complexes undergo a metal-based oxidation process and at least three ligand-based reduction processes. 

C. Oxidation with Ruthenium Tetraoxide and Chromate Based Oxidants... 

Molybdenum-based catalysts are highly active initiators, however, monomers with functionalities with acid hydrogen, such as alcohols, acids, or thiols jeopardize the activity. In contrast, ruthenium-based systems exhibit a higher stability towards these functionalities (19). An example for a molybdenum-based catalyst is (20) MoOCl2(t-BuO)2, where t-BuO is the tert-butyl oxide radical. The complex can be prepared by reacting M0OCI4 with potassium tert-butoxide, i.e., the potassium salt of terf-butanol. 

This is considerably different from the recombination reaction with, for example, typical ruthenium dyes. This slow re-reduction of the dyad is explained by the low redox potential of the osmium center, the value of 0.66 V (vs. SCE) observed, points to a small driving force for the redox process. This observation is important for the design of dyes for solar cell applications. Osmium compounds have very attractive absorption features, which cover a large part of the solar spectrum. However, their much less positive metal-based oxidation potentials will result in a less effective re-reduction of the dyes based on that metal and this will seriously affect the efficiency of solar cells. In addition, for many ruthenium-based dyes, the presence of low energy absorptions, desirable for spectral coverage, is often connected with low metal-based redox potentials. This intrinsically hinders the search for dyes which have a more complete coverage of the solar spectrum. Since electronic and electrochemical properties are very much related, a lowering of the LUMO-HOMO distance also leads to a less positive oxidation potential. 

Other radical-based transformations are ruthenium-catalyzed oxidative dimerizations of phenols [263] and reductive dimerizations [264], The isomerization of chiral c/s-epoxides to tram-epoxides catalyzed by 2-10 mol% TpRu(py)2Cl proceeds at 100 °C in 95-98% yields with inversion of configuration [265], A radical or SN2 mechanism was discussed for this process. 

Photocatalytic enantioselective oxidative arylic coupling reactions have been investigated by two different groups. Both studies involved the use of ruthenium-based photocatalysts [142, 143]. In 1993, Hamada and co-workers introduced a photostable chiral ruthenium tris(bipyridine)-type complex (A-[Ru(menbpy)3]2+) 210 possessing high redox ability [143]. The catalytic cycle also employed Co(acac)3 211 to assist in the generation of the active (A-[Ru(menbpy)3]3+) species 212. The authors suggested that the enantioselection observed upon binaphthol formation was the result of a faster formation of the (R)-enantiomer from the intermediate 213 (second oxidation and/or proton loss), albeit only to a rather low extent (ee 16 %) (Scheme 54). 

These protocols can be regarded as promising because of their simplicity and broad scope. The conventional method for the production of aldehydes from alkenes consists of ozonolysis followed by workup under reducing conditions [72]. The ruthenium-based method, using either oxone or NaI04 as the oxidant, is an interesting alternative. The high selectivity to aldehyde versus carboxylic acid was reached by manipulation of the amount of terminal oxidant (2 Eq.) and the reaction time. 

Other ruthenium-based catalysts for the aerobic oxidation of alcohols have been described where it is not clear if they involve oxidative dehydrogenation by low-valent ruthenium, to give hydridoruthenium intermediates, or by high-valent oxoruthenium. Masutani et al. [107] described (nitrosyl)Ru(salen) complexes, which can be activated by illumination to release the NO ligand. These complexes demonstrated selectivity for oxidation of the alcoholic group versus epoxidation, which was regarded as evidence for the intermediacy of Ru-oxo moieties. Their excellent alcohol coordination properties led to a good enantiomer differentation in the aerobic oxidation of racemic secondary alcohols (Fig. 19) and to a selective oxidation of primary alcohols in the presence of secondary alcohols [108]. 

In 2000, Yamaguchi et al. [116] synthesized a ruthenium-based hydroxyapatite catalyst, with the formula (RuCl)10(PO4)6(OH)2. This catalyst could also be recycled and displayed a reasonable substrate scope in the aerobic alcohol oxidations (Eq. 30). TOFs reported in this case were generally somewhat lower, on the order of 1 h 1 for 2-octanol to 12 h 1 for benzyl alcohol. The fact that distinct Ru-Cl species are present at the surface points in the direction of a hydridometal mechanism. 

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