Ruthenium pyridines

These spherical nano-particles about 55 nm in diameter have a fluorescent material of ruthenium pyridine inside, and the shell of silicon dioxide, as shown in Fig. 36. The excitation wavelength of the ruthenium pyridine is 480 nm and the emission wavelength is 592 nm [81]. In order to get a clear image of nano-particles, the mass concentration of the fluorescent particles should be limited to a very low level. 

The transition metal-mediated [2 -i- 2 -i- 2] cyclocotrimerization of two alkynes and a nitrile is a powerful and straightforward route to substituted pyridines [9]. In particular, catalytic cyclocotrimerization is undoubtedly desirable as a metal-atom economically and environmentally benign process. Effective catalysis, however, has been confined to cobalt [40], although a variety of transition metals (Ti [41], Zr/Ni [42], Ta [43], Co [44], and Rh [45]) have been found to mediate the stoichiometric cyclocotrimerization. With respect to ruthenium, pyridine formation from acetoni-... 

A group in Switzerland has developed a photovoltaic cell that can function as a window for a building.376 In one example, a ruthenium pyridine complex photosensitizer is attached to the titanium dioxide semiconductor by a phos-phonate. An iodine-based electrolyte (Kl3 dissolved in 50 50 ethylene carbonate/propylene carbonate) is between the panes. All the films are so thin that they are transparent. The efficiency is 10-11%. Variants on such cells have included fullerenes377 and polypyrrole.378 The use of solid electrolytes will avoid problems that might occur if a seal on a liquid electrolyte leaked. 

X-ray crystallography, 3, 623 Ruscodibenzofuran synthesis, 4, 698, 709 Ruthenacyclobutane, 3-cyano-synthesis, 1, 667 Ruthenium complexes with pyridines, 2, 124 triazenido, 5, 675 Rutin... 

Thus, Mathis et al. [1, 2] investigated oxidation reactions with 4-nitroperbenzoic acid, sodium hypobromite, osmium tetroxide and ruthenium tetroxide. Hamann et al. [3] employed phosphorus oxychloride in pyridine for dehydration. However, this method is accompanied by the disadvantages that the volume applied is increased because reagent has been added and that water is sometimes produced in the reaction and has to be removed before the chromatographic separation. 

Imidazole is characterized mainly by the T) (N) coordination mode, where N is the nitrogen atom of the pyridine type. The rare coordination modes are T) - (jt-) realized in the ruthenium complexes, I-ti (C,N)- in organoruthenium and organoosmium chemistry. Imidazolium salts and stable 1,3-disubsti-tuted imidazol-2-ylidenes give a vast group of mono-, bis-, and tris-carbene complexes characterized by stability and prominent catalytic activity. Benzimidazole follows the same trends. Biimidazoles and bibenzimidazoles are ligands as the neutral molecules, mono- and dianions. A variety of the coordination situations is, therefore, broad, but there are practically no deviations from the expected classical trends for the mono-, di-, and polynuclear A -complexes. 

Rapoport s findings have been confirmed in the authors laboratory where the actions of carbon-supported catalysts (5% metal) derived from ruthenium, rhodium, palladium, osmium, iridium, and platinum, on pyridine, have been examined. At atmospheric pressure, at the boiling point of pyridine, and at a pyridine-to-catalyst ratio of 8 1, only palladium was active in bringing about the formation of 2,2 -bipyridine. It w as also found that different preparations of palladium-on-carbon varied widely in efficiency (yield 0.05-0.39 gm of 2,2 -bipyridine per gram of catalyst), but the factors responsible for this variation are not knowm. Palladium-on-alumina was found to be inferior to the carbon-supported preparations and gave only traces of bipyridine,... 

Rhodium-on-carbon has also been found to bring about the formation of 2,2 -biquinoline from quinoline, the yield and the percentage conversion being similar to that obtained with palladium-on-carbon. On the other hand, rhodium-on-carbon failed to produce 2,2 -bipyridine from pyridine, and it has not yet been tried with other bases. Experiments with carbon-supported catalysts prepared from ruthenium, osmium, iridium, and platinum have shown that none of these metals is capable of bringing about the formation of 2,2 -biquinoline from quinoline under the conditions used with palladium and rhodium. ... 

Reductive alkylation by alcohol solvents may occur as an unwanted side reaction 22,39), and it is to avoid this reaction that Freifelder (20) recom mends ruthenium instead of nickel in pyridine hydrogenation. Alkylation by alcohols may occur with surprising ease 67). Reduction of 18 in ethanol over 10% palladium-on carbon to an amino acid, followed bycyclization with /V,/V-dicyclohexylcarbodiimide gave a mixture of 19 and 20 wiih the major product being the /V-ethyl derivative 49,50). By carrying out the reduction in acetic acid, 20 was obtained as the sole cyclized product 40). 

Both amine oxides related to pyridines and aliphatic amine oxides (/25) are easily reduced, the former the more so. Pyridine N-oxide has been reduced over palladium, platinum, rhodium, and ruthenium. The most active was rhodium, but it was nonselective, reducing the ring as well. Palladium is usually the preferred catalyst for this type of reduction and is used by most workers 16,23,84 158) platinum is also effective 100,166,169). Katritzky and Monrol - ) examined carefully the selectivity of reduction over palladium of a... 

By monitoring the intensity of the carbonyl absorption it was observed that oxidation of methyl 4,6-0-benzylidene-2-deoxy-a-D-Zt/ ro-hexopyrano-side with chromium trioxide-pyridine at room temperature gave initially the hexopyranosid-3-ulose (2) in low concentration, but attempts to increase this yield resulted in elimination of methanol to give compound 3. However, when methyl 4,6-0-benzylidene-2-deoxy-a-D-Zt/ ro-hexo-pyranoside is oxidized by ruthenium tetroxide in either carbon tetrachloride or methylene dichloride it affords compound 2 without concomitant elimination. When compound 2 was heated for 30 minutes in pyridine which was 0.1 M in either perchloric acid or hydrochloric acid it afforded compound 3, but in pyridine alone it was recoverable unchanged (2). Another example of this type of elimination, leading to the introduction of unsaturation into a glycopyranoid ring, was observed... 

Asymmetric epoxidation of olefins with ruthenium catalysts based either on chiral porphyrins or on pyridine-2,6-bisoxazoline (pybox) ligands has been reported (Scheme 6.21). Berkessel et al. reported that catalysts 27 and 28 were efficient catalysts for the enantioselective epoxidation of aryl-substituted olefins (Table 6.10) [139]. Enantioselectivities of up to 83% were obtained in the epoxidation of 1,2-dihydronaphthalene with catalyst 28 and 2,6-DCPNO. Simple olefins such as oct-l-ene reacted poorly and gave epoxides with low enantioselectivity. The use of pybox ligands in ruthenium-catalyzed asymmetric epoxidations was first reported by Nishiyama et al., who used catalyst 30 in combination with iodosyl benzene, bisacetoxyiodo benzene [PhI(OAc)2], or TBHP for the oxidation of trons-stilbene [140], In their best result, with PhI(OAc)2 as oxidant, they obtained trons-stilbene oxide in 80% yield and with 63% ee. More recently, Beller and coworkers have reexamined this catalytic system, finding that asymmetric epoxidations could be perfonned with ruthenium catalysts 29 and 30 and 30% aqueous hydrogen peroxide (Table 6.11) [141]. Development of the pybox ligand provided ruthenium complex 31, which turned out to be the most efficient catalyst for asymmetric... 

An alternative approach to phosphine-free ruthenium precatalysts is based on pyridine complex 70 [48], which has been established by Grubbs et al. as a valuable precursor for other mixed NHC-phosphine complexes (cf. Scheme 15). Complex 70 is only moderately active in the cross metathesis of allylbenzene... 

Novel ruthenium-amidinate complexes of the type (j -CgHsRlRufamidina-te)X (R = Me, OMe, F X = Cl, Br, OTf) and [Ru(amidinate)(MeCN)4][PF6] have been synthesized by photochemical displacement of the benzene ligand in (j -CgHglRufamidinatelX by substituted arenes or MeCN. The acetonitrile ligands of [Ru(amidinate)(MeCN)4][PF6] are easily replaceable by other cr-donor ligands (L) such as pyridines, phosphines, and isocyanides to afford the corresponding derivatives [Ru(amidinate)(MeCN) (L)4 ][PF6] n — 1, 2). These reactions are summarized in Scheme 142. ... 

Associated to copper(II) pre-catalysts, bis(oxazolines) also allowed the asymmetric Diels-Alder and hetero Diels-Alder transformations to be achieved in nearly quantitative yield and high diastereo- and enantioselectivities. Optically active sulfoximines, with their nitrogen-coordinating site located at close proximity to the stereogenic sulfur atom, have also proven their efficiency as copper ligands for these asymmetric cycloadditions. Other precursors for this Lewis acid-catalyzed transformation have been described (e.g., zinc salts, ruthenium derivatives, or rare earth complexes) which, when associated to bis(oxazolines), pyridine-oxazolines or pyridine-bis(oxazolines), led to efficient catalysts. 

Zhang et al. [49] prepared a chiral ruthenium complex coordinated by a pyridine-bis(imine) ligand (structure 43 in Scheme 21). 

Cornejo et al. [65] reported the first immobihzation of pyridine-bis(oxa-zoline) chiral hgands and the use of the corresponding solid ruthenium complex in the model cyclopropanation test. They synthesized vinyl-PyBOx, the vinyl functionahty being introduced in the fourth position of the pyridine ring. This monomer was further homo- or copolymerized in the presence of styrene and divinylbenzene. The corresponding ruthenium catalysts proved... 

Third generation initiators are based on the NHC system of second generation initiators, but do not contain any phosphine ligand. Instead, one or two pyridine ligands are weakly bound to the ruthenium centre (c/. Fig. 3.28, complexes 73 and 74c). Pyridine dissociates very easily and hardly competes with the olefin for the coordination site. As a result, complete initiation and fast propagation are enabled, therefore living polymerisation is rendered possible. 

In 2004, ruthenium-catalysed asymmetric cyclopropanations of styrene derivatives with diazoesters were also performed by Masson et al., using chiral 2,6-bis(thiazolines)pyridines. These ligands were prepared from dithioesters and commercially available enantiopure 2-aminoalcohols. When the cyclopropanation of styrene with diazoethylacetate was performed with these ligands in the presence of ruthenium, enantioselectivities of up to 85% ee were obtained (Scheme 6.6). The scope of this methodology was extended to various styrene derivatives and to isopropyl diazomethylphosphonate with good yields and enantioselectivities. The comparative evaluation of enantiocontrol for cyclopropanation of styrene with chiral ruthenium-bis(oxazolines), Ru-Pybox, and chiral ruthenium-bis(thiazolines), Ru-thia-Pybox, have shown many similarities with, in some cases, good enantiomeric excesses. The modification... 

Imamura (11,20,21) synthesized several similar perpendicular dimers exploiting axial coordination of the 4-pyridyl free-base porphyrin to Ru(II)CO (3) and Os(II)CO (4) porphyrins (Fig. 1). The pyridine-ruthenium and pyridine-osmium interactions are much stronger than the pyridine-zinc interaction, and the complexes are perfectly stable in solution and can be isolated by precipitation. One of the ruthenium dimers was characterized by FAB-MS (11). Complexation is accompanied by characteristic changes in JH NMR chemical shift, indicating... 

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