Oxo-ruthenium

In contrast the oxo-ruthenium complex c ,c -[ (bpy)2Runl(0H2) 2(//-0)]4+ and some of its derivatives are known to be active catalysts for the chemical or electrochemical oxidation of water to dioxygen.464-472 Many studies have been reported473 181 on the redox and structural chemistry of this complex for understanding the mechanism of water oxidation. Based on the results of pH-dependent electrochemical measurements, the basic structural unit is retained in the successive oxidation states from Rum-0 Ru111 to Ruv O Ruv.466... 

Among the metal complexes used in electrocatalytic oxidation of organic compounds, polypyridyl oxo-ruthenium complexes have attracted special attention,494"508 especially [RuIV(terpy)(bpy)0]2+.495 197,499,500,502,504 This high oxidation state is reached from the corresponding Run-aqua complex by sequential oxidation and proton loss (Equations (75) and (76)). 

A number of mechanistic pathways have been identified for the oxidation, such as O-atom transfer to sulfides, electrophilic attack on phenols, hydride transfer from alcohols, and proton-coupled electron transfer from hydroquinone. Some kinetic studies indicate that the rate-determining step involves preassociation of the substrate with the catalyst.507,508 The electrocatalytic properties of polypyridyl oxo-ruthenium complexes have been also applied with success to DNA cleavage509,5 and sugar oxidation.511... 

DR. THOMAS MEYER (University of North Carolina) Wagner de Giovanni, in my lab, is studying the reaction between an oxo ruthenium compound and N0 ... 

In contrast to the previously mentioned reactions, which involve either oxo-ruthenium or ruthenium hydride species as intermediates, free-radical reactions can also be promoted by ruthenium. The aerobic oxidation of alcohols proceeds smoothly at room temperature in the presence of 4 eq. of an aldehyde, for example, acetaldehyde, and a catalyst comprising a 1 1 mixture of RuC13 nH20 and Co(OAc)2, in ethyl acetate (Eq. 31) [122]. 

The results were rationalized by assuming that the corresponding percar-boxylic acid is formed by cobalt-mediated free-radical autoxidation of the aldehyde. Subsequent reaction of ruthenium(III) with the peracid affords oxo-ruthenium(V) carboxylate, which is the active oxidant. Compared with the aerobic oxidations discussed earlier the method suffers from the drawback that 1 eq of a carboxylic acid is formed as a coproduct. 

Many of these oxo ruthenium (and similar osmium) species can be involved in reversible redox reactions in which OH or H20 participate, for example,... 

The binding of the trinuclear J,3-oxo-ruthenium acetate clusters to the metaflo-porphyrin ring can also improve the activity of the metalloporphyrin catalyst, by incorporating new electron-withdrawing/donor-, and/or ET properties. Because of... 

Epoxidation of styrene or stilbene with the ruthenium [RuCl2(cod)L] complex of bis[(45)-(l-methylethyl)oxazolin-2-yl]methane afforded only racemic epoxide, suggesting that the reaction is not metal centered. In fact, mechanistic studies of this reaction indicate that the metal acts as a promoter for the production of i-PrOsH and that it is this species that carries out the epoxidation, either directly or by the formation of an intermediate oxo-ruthenium species. 

As shown in Scheme 3.8, the catalytic oxidation reactions can be rationalized by assuming the formation of oxo-ruthenium species by the reaction of low-valent ruthenium complexes with peroxides. The C-H activation at the a-position of amines and the subsequent electron transfer gives iminium ion ruthenium complex 55. Trapping 55 with f-BuOOH would afford the corresponding a-ferf-butylhydroxy-amines, water, and low-valent ruthenium complex to complete the catalytic cyde. 

The reaction can be rationalized by assuming the mechanism which involves 0x0-ruthenium complex (Scheme 3.9). Hydrogen abstraction with oxo-ruthenium species gives phenoxyl radical 73, which undergoes fast electron transfer to the ruthenium to give a cationic intermediate 74. Nucleophilic reaction with the second molecule of t-BuOOH gives the product 72. 

This chapter highlights the ruthenium-catalyzed dehydrogenative oxidation and oxygenation reactions. Dehydrogenative oxidation is especially useful for the oxidation of alcohols, and a variety of products such as ketones, aldehydes, and esters can be obtained. Oxygenation with oxo-ruthenium species derived from ruthenium and peroxides or molecular oxygen has resulted in the discovery of new types of biomi-metic catalytic oxidation reactions of amines, amides, y3-lactams, alcohols, phenols, and even nonactivated hydrocarbons tmder extremely mild conditions. These catalytic oxidations are both practical and useful, and ruthenium-catalyzed oxidations will clearly provide a variety of futrue processes. 

Araki, K., S. Dovidauskas, H. Winnischofer, A.D.P. Alexiou, and H.E. Toma (2001). A new highly efficient tetra-electronic catalyst based on a cobalt porphyrin bound to four mu3-oxo-ruthenium acetate clusters. J. Electroanal. Chem. 498, 152-160. 

As shown in Scheme 7.4, the catalytic oxidation reactions can be rationalized by assuming the formation of oxo-ruthenium species by the reaction of low-valent... 

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