Ruthenium hydroquinones

The sequence has been applied to the synthesis of 1,4-cyclohexanedione from hydroquinone 10), using W-7 Raney nickel as prepared by Billica and Adkins 6), except that the catalyst was stored under water. The use of water as solvent permitted, after hltration of the catalyst, direct oxidation of the reaction mixture with ruthenium trichloride and sodium hypochlorite via ruthenium tetroxide 78). Hydroquinone can be reduced to the diol over /o Rh-on-C at ambient conditions quantitatively (20). 

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... 

Fluorescent redox switches based on compounds with electron acceptors and fluorophores have been also reported. For instance, by making use of the quinone/ hydroquinone redox couple a redox-responsive fluorescence switch can be established with molecule 19 containing a ruthenium tris(bpy) (bpy = 2,2 -bipyridine) complex.29 Within molecule 19, the excited state of the ruthenium center, that is, the triplet metal-to-ligand charge transfer (MLCT) state, is effectively quenched by electron transfer to the quinone group. When the quinone is reduced to the hydroquinone either chemically or electrochemically, luminescence is emitted from the ruthenium center in molecule 19. Similarly, molecule 20, a ruthenium (II) complex withhydroquinone-functionalized 2,2 6, 2"-terpyridine (tpy) and (4 -phenylethynyl-2,2 6, 2"- terpyridine) as ligands, also works as a redox fluorescence switch.30... 

The anthraquinone exhibits a similar redox behavior as benzoquinone. Thus, redox luminescence switch can also be constructed with fluorophore linked to anthraquinone. For example, the luminescence of molecule 22, a ruthenium complex with an appended anthraquinone moiety, can be reversibly tuned through the interconversion between the anthraquinone and the corresponding hydroquinone.32... 

Oxidation of Tetramethylethylene. Tetramethylethylene, TME, was an excellent model olefin since it was rapidly and selectively oxidized in the presence of many transition metal complexes (12). Oxidation of TME in the presence of the group VIII metal complexes [MCI(CO)-(Ph3P)2] (M = Rh, Ir) at 50°C gave two major products 2,3-dimethyl-2,3-epoxybutane, I, and 2,3-dimethyl-3-hydroxy-l-butene, II (Reaction 5). Reaction mixtures were homogeneous with no observable deposits of insoluble materials. Little oxidation occurred under these conditions in the absence of the metal complexes, but low yields of I and II were obtained in the presence of a radical initiator (Table I). Reactions were severely inhibited by hydroquinone. The ruthenium (II) complex, [RuCl2(Ph3P)3]2, also promoted efficient oxidation of TME yielding I... 

Development modifiers alter the rate of development (development accelerators or inhibitors) or may render silver halide grains developable. Certain complexes of ruthenium, in particular [Ru(NH3)6]Cl3, are accelerators of development with hydroquinones and ascorbic acid.46 Several complex ions of cobalt, for example [Co(NH3)5OH2]3+, [CoC1(NH3)5]2+ and [Co(en)3]3+, are development accelerators.47... 

Mitsudo et al. [32] found that hydroquinones can be obtained by the reaction of internal alkynes with norbornene and CO using N-methylpiperidine as a solvent in the presence of Ru3(CO)12 (Eq. 16). The reaction is proposed to proceed via the maleoyl ruthenium complex 15, which is generated from an alkyne, two molecules of CO, and ruthenium. Norbornene is inserted into this complex to give the quinone, which undergoes reduction to the final product under the re-... 

Ruthenium compounds are widely used as catalysts for hydrogen-transfer reactions. These systems can be readily adapted to the aerobic oxidation of alcohols by employing dioxygen, in combination with a hydrogen acceptor as a cocatalyst, in a multistep process. For example, Backvall and coworkers [85] used low-valent ruthenium complexes in combination with a benzoquinone and a cobalt Schiff s base complex. The proposed mechanism is shown in Fig. 14. A low-valent ruthenium complex reacts with the alcohol to afford the aldehyde or ketone product and a ruthenium dihydride. The latter undergoes hydrogen transfer to the benzoquinone to give hydroquinone with concomitant... 

Recently Yamaguchi and Mizuno[ 113] reported ruthenium on alumina to be a powerful and recyclable catalyst for selective alcohol oxidation. This method displayed a large substrate scope (Eq. 29, Table 4) and tolerates the presence of sulfur and nitrogen groups. Only primary aliphatic alcohols required the addition of hydroquinone. TOFs in the range from 4 h 1 (for secondary allylic alcohols) to 18 h 1 (for 2-octanol) were obtained in trifluorotoluene, while in the solvent-free oxidation at 150 °C a TOF of 300 h 1 was observed for 2-octanol. 

The present hydrogen transfer reaction is extended to the aerobic oxidation of alcohols. Thus, the oxidation of alcohols can be carried out with a catalytic amount of hydrogen acceptor under an O2 atmosphere by a multistep electron-transfer process. As shown in Scheme 3.4, the ruthenium dihydrides formed during the hydrogen transfer can be regenerated by a multistep electron-transfer process including hydroquinone, ruthenium complex, and molecular oxygen. 

Considerable effort has been devoted to achieving the intermolecular catalytic Pauson-Khand reaction. The mthenium complex-catalyzed reaction of an alkyne with an alkene such as ethylene or 2-norbornene under CO gave hydroquinone derivatives [79], with CO (2 mol) being introduced into the products (Eq. 11.36). This reaction is the first example of the preparation of hydroquinone derivatives by the reaction of alkynes and alkenes with CO, while hydroquinone is synthesized by the ruthenium-catalyzed reaction of 2 mol acetylene with 2 mol CO (Eq. 11.37) [80]. 

The reactions were conducted under nitrogen even though oxygen had no effect on the reaction. However, the addition of AIBN (2,2 -azobisisobutyronitrile), a fi ee radical initiator, accelerated the rate of reaction, while the addition of hydroquinone, a radical trap, completely inhibited the reaction. These results are consistent with a radical type mechanism which may involve a ruthenium oxo species. Further mechanistic studies are in progress. 

Under comparable reaction conditions, the carbonylation of alkynes can be steered in another direction by varying the catalyst metals. For instance, using iron pentacarbonyl [25] or ruthenium carbonyl [26] as catalysts, the principal product is hydroquinone (eq. (11)). 

Ruthenium carbonyl, Ru3(CO)12, has been reported to catalyze the cyclohydrocarbonylation of acetylene to give hydroquinone ... 

HC=CH -t- 2CO -f- 2HaO the synthesis of hydroquinone in the presence of iron and ruthenium catalysts ... 

Whereas no reaction occurred in the absence of catalyst, ruthenium catalysts were able to remove most of the Total Organic Carbon (TOC) of p-HBZ acid aqueous solutions at relatively mild conditions after 7h. The most important intermediates detected were phenol, hydroquinone and maleic acid. The results related to the study of the effects of the acid used for the preparation of aerogel supports (TiOa, ZrOa) and the nature of ruthenium precursor on the catalytic properties of the samples are gathered in the Tables 4 and 5. As shown in Table 4, the use of nitric acid during the synthesis of the Ti02 support and Ru(N0)(N03)3 as the metal precursor leads to the more efficient Ru/TiOa catalyst (63.8% TOC abatment). 

Although it has been described that ruthenium is an efficient catalyst for the reduction of the aromatic nucleus at raised temperatures and pressures, its superiority is most useful when a nitrogen group is substituted on the benzene ring (16-18). Rhodium catalysts are, however, more efficient, even at raised pressures and temperatures, for the hydrogenation of compounds like benzene, hydroquinone, and /3-naphthol. At a hydrogen... 

Lehn has recently reported a two-component system, 3, in which a quinone subunit is covalently linked to the photo-active ruthenium(II) tris-bipyridine complex, [Ru kbpy)3]. The quinone fragment, RO2/ undergoes a reversible two-electron redox change, in acidic solution, to give the hydroquinone derivative RO2 + 2e +... 

Hydroquinone was beneficial to the reaction outcome, its effect being attributed to the suppression of radical side-reactions. The proposed mechanism consists of iniHal electrophilic attack of the metal onto the C—H bond of the arene to give an arylruthenium species with concomitant proton release, followed by alkene insertion into the Ru—C bond, -hydride elimination to liberate the cinnamate and a ruthenium hydride, and finally regeneration of the active catalyst either by insertion of another alkene into the Ru-H bond protonation of the alkylruthenium complex and elimination of methyl propionate (under an... 

Starting from the same substrates, even hydroquinones can be prepared by insertion of two molecules of CO. In 1998, Mitsudo and coworkers [21a] demonstrated that hydroquinones could be achieved in a ruthenium-catalyzed cyclocarbonylation by using alkynes and 2-norbornenes. UnsymmetricaUy substituted hydroquinones were obtained in high yields by this novel ruthenium-catalyzed transformation. For the preparation of higher substituted hydroquinones, functionalized alkenes could... 

Scheme 1.8 Ruthenium-catalyzed carbonylative synthesis of hydroquinones.

Electrochemical properties such as redox potential can also serve as inputs to molecular YES logic gates since redox indicators also have a rather long history. " Complex 2 is poorly emissive at 610 nm (when excited at 453 nm) as a result of PET from the ruthenium-based lumophore to the benzoquinone moiety across the dimethylene spacer. If a potential of —0.6 V (vs standard calomel electrode SCE) is applied to it in wet acetonitrile, the benzoquinone unit is fully reduced to the hydroquinone form once two coulombs are passed per equivalent of 2. This succeeds because the reduction potential of 2 is —0.44 V. Upon reduction, a luminescence enhancement factor of 6 is seen. The system is stable in both the benzoquinone and hydroqninone forms in the absence of applied potentials, which means that these cases show evidence of memory unlike ion-driven cases such as 1. However, the hydroquinone form can be reset to 2 by application of two coulombs at a potential of -1-1.1 V. 

Hydroquinone has thus been obtained with a 73% yield at 250 C under 200 bar of carbon monoxide using a ruthenium catalyst [138]. 

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