Ruthenium nitrobenzene reduction

Grignard additions, 9, 59, 9, 64 indium-mediated allylation, 9, 687 in nickel complexes, 8, 150 ruthenium carbonyl reactions, 7, 142 ruthenium half-sandwiches, 6, 478 and selenium electrophiles, 9, W11 4( > 2 in vanadocene reactions, 5, 39 Nitrites, with trinuclear Os clusters, 6, 733 Nitroalkenes, Grignard additions, 9, 59-60 Nitroarenes, and Grignard reactivity, 9, 70 Nitrobenzenes, reductive aminocarbonylation, 11, 543... 

The cluster Ru3(CO)i2 is known to react in the presence of bases to afford, under certain experimental conditions, [HRu3(CO)u] [42]. Since the corresponding iron cluster is known to react with nitrobenzene to yield an hydrido imido cluster [HFe3(CO)9(NPh)] , which can be protonated and afford small amounts of aniline [43], a catalytic cycle was proposed, and supported by some model reactions, for the ruthenium-catalysed reduction of nitroarenes by CO/H2O, which includes the intermediate formation of the trinuclear hydrido cluster [44-46]. However, one of us has recently shown that [HFe3(CO)n] and the corresponding imido complex play no role in the Fe3(CO)i2-promoted reduction of nitrobenzene [6, 47] and the proposal of an active role of [HRu3(CO)ii] appears to be questionable. 

Similar effects were demonstrated by them for palladium and ruthenium in the reduction of pyridine, and for ruthenium and platinum in the reduction of nitriles and of nitrobenzenes. Since they show similar although not as large synergistic effects when the two metals are introduced on separate carbon particles, one may be sure that the enhancement is not an intrinsic property of some chemical combination or contact between the different metals. As the authors point out, the various metals are known to have differing catalytic effectiveness for the reduction of different functional groups. Thus, when reduction takes place via more than one chemical step, such as in the case... 

A more detailed picture of electron transfer processes involving Ru(bipy) 3 was provided by a study of the quenching of this excited species by a series of nitrobenzene derivatives (245). The dependence of the quenching rate constant on the reduction potential of the quencher is shown in Fig. 11. The observed dependence is in excellent agreement with that expected for quenching which involves oxidation of the excited ruthenium complex. 

Future trends in reduction of substituted nitrobenzenes will probably be based on novel catalysts. Homogenous transition metal (ruthenium and rhodium) catalysts offer routes to chemospecific reduction of aromatic nitro groups16. Novel catalytic methods involving combinatorial chemistry may offer pathways to new industrial hydrogenation processes, where selective reduction is desired. A number of solution- and solid-phase C /Mo0 redox couple reductions of substituted nitroarenes to the corresponding anilines have been proposed17. 

The reduction of nitrobenzene to aniline by CO and Hj is more effectively carried out with ruthenium than with iron or osmium . 

Some model reactions on related trinuclear ruthenium clusters, especially related to the reduction reactions of nitrobenzene to aniline, have also been reported by Bhaduri and coworkers and several papers have also been published by different groups on related reactions of ruthenium and osmium clusters containing imido. amido or isocyanate fragments, although the latter were not intended as models for catalytic reactions. 

Each of the catalysts listed earlier in the hydroformylation reaction are found to be catalysts for the reduction of nitrobenzene to aniline the rhodium, osmium, and ruthenium species are particularly eflFective although with osmium higher temperatures must be used. (A. F. M. Igbal previously has reported that several derivatives of rhodium, including Rh6(CO)i6 can act as catalysts for the reduction of nitrobenzene to aniline using CO + H2O (19).) The catalysts listed are also much sturdier than was found in the case of iron carbonyl, and in the experiments listed in Table III the total amount of nitrobenzene and catalyst in a 1000 molar ratio was added to the reaction at the outset. In no case was there observed any precipitate of metal oxides or carbonates, and presumably much higher catalytic turnover numbers could be realized if the reaction were run in a continuous-type reactor. 

The intermediate (B) is expected to be relatively stable and sufficiently long-lived to permit rotation about the C-C bond. Since the stereochemistry of the olefin is retained in this reaction, the latter is the mechanism favoured. Dodecacarbonyltriruthenium, pentacarbonylruthenium, and tris-(acetylacetonato)ruthenium(iii) have been shown to promote the homogeneous reduction of nitrobenzene to aniline in the presence of carbon... 

The ruthenium compounds Ru3(CO)i2, Ru(CO)s, and Ru(acac)3 were found to catalyse the homogeneous reduction of nitrobenzene to aniline by CO and H2 [20]. At 140-160 °C, Pco = 80-100 atm, Ph2 = 80-100 atm, and substrate/cat. = 200-400, ca. 66.5 % aniline was obtained. Iron derivatives were much less effective. With a CO/H2 ratio higher than 1, diphenylurea was also a product and its yield increased gradually with the CO/H2 ratio. 

It is not clear why the ruthenium catalyst is not able to induce the heterocyclisation from the presumably intermediate nitrene complex (Scheme 6). The benzamides 32 could be formed via the insertion of an intermediate isocyanate in the aromatic C-H bond of the solvent. This reaction has some precedents, where benzanilides were obtained by reaction of PhNCO with benzene, or directly from nitrobenzene, carbon monoxide and benzene, catalysed by rhodium carbonyl clusters [56-58], However, the reluctance of Ru3(CO)i2 to catalyse the reduction of nitrobenzene to phenylisocyanate in solvents such as benzene [22, 23] does not support this hypothesis. 

Some model reactions on related trinuclear ruthenium clusters, especially related to the reduction reactions of nitrobenzene to aniline, have also been reported by Bhaduri and co-workers [159-162]. 

The addition of OMe to cluster-coordinated GO has been used in the electrospray spectrometry of metal carbonyl complexes.Heterometallic clusters such as Gp Fe(GO)(/x-GO)2Ru2(GO)4(//-H) /x-MeG(GOO)Gl have been obtained upon carbonyl substitution with MeG(Gl)HGOOH on the tetra-heterometallic precursor. An efficient synthetic method for Ru3(GO)i2 starting from ruthenium dioxide hydrate has been reported in the presence of formic or acetic acid, carboxylate complexes are obtained. These favor the reductive carbonylation of nitrobenzene to aniline and A,iV -diphenylurea. 

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