14-electron ruthenium catalysts

Scheme 4.5 Synthetic route for the preparation of 14-electron ruthenium catalysts bearing the CAAC ligand.

The unique power of Hoveyda s recyclable ruthenium catalyst D in RCM with electron-deficient and sterically demanding substrates is illustrated in Honda s total synthesis of the simple marine lactone (-)-malyngolide (54), which contains a chiral quaternary carbon center (Scheme 10) [35]. Attempted RCM of diene 52 with 5 mol% of NHC catalyst C for 15 h produced the desired... 

These transition-metal catalysts contain electronically coupled hydridic and acidic hydrogen atoms that are transferred to a polar unsaturated species under mild conditions. The first such catalyst was Shvo s diruthenium hydride complex reported in the mid 1980s [41 14], Noyori and Ikatiya developed chiral ruthenium catalysts showing excellent enantioselectivity in the hydrogenation of ketones [45,46]. 

An important aspect of hydrogen transfer equilibrium reactions is their application to a variety of oxidative transformations of alcohols to aldehydes and ketones using ruthenium catalysts.72 An extension of these studies is the aerobic oxidation of alcohols performed with a catalytic amount of hydrogen acceptor under 02 atmosphere by a multistep electron-transfer process.132-134... 

Besides the electronic spectral studies noted above, we have also carried out in situ studies of the acidic ruthenium catalyst using nmr and infrared spectral techniques. A key set of observations derive from the and 13C nmr spectra of an operating catalyst at 90° and Pco 1 atm which indicate the presence of only one major ruthenium species. The proton spectrum shows a sharp singlet at 24.0 T which remains such when the solution is cooled to room temperature, although the slow formation of other species was observed over a period of hours at the latter conditions. The 1H-decoupled 13C spectrum of the... 

It is assumed that the mechanism proceeds via activation of the imine by the ruthenium catalyst (structure 169), followed by reaction with ethyl diazoacetate to generate a metal-bound ylide intermediate. Intramolecular ruthenium- assisted attack of the carbanion 170 onto the iminium ion provides the corresponding aziridine with moderate to high // selectivity. Imines bearing electron-donating groups (R2) showed significant rate enhancement. 

In the transition metal-catalyzed reactions described above, the addition of a small quantity of base dramatically increases the reaction rate [17-21]. A more elegant approach is to include a basic site into the catalysts, as is depicted in Scheme 20.13. Noyori and others proposed a mechanism for reactions catalyzed with these 16-electron ruthenium complexes (30) that involves a six-membered transition state (31) [48-50]. The basic nitrogen atom of the ligand abstracts the hydroxyl proton from the hydrogen donor (16) and, in a concerted manner, a hydride shift takes place from the a-position of the alcohol to ruthenium (a), re-... 

PEM fuel cells operate at relatively low temperatures, around 80°C. Low temperature operation allows them to start quickly (less warm-up time) and results in less wear on system components, resulting in better durability. However, they require that a noble-metal catalyst (typically platinum) be used to separate the hydrogen s electrons and protons, adding to system cost. The platinum catalyst is also extremely sensitive to CO poisoning, making it necessary to employ an additional reactor to reduce CO in the fuel gas if the hydrogen is derived from an alcohol or hydrocarbon fuel. This also adds cost. Developers are currently exploring platinum/ruthenium catalysts that are more resistant to CO. 

Ruthenium catalysts, coordinated with an N-heterocyclic carbene allowed for the ROMP of low-strain cyclopentene and substituted cyclopentenes (10,23). Suitable ruthenium and osmium carbene compounds may be synthesized using diazo compounds, by neutral electron donor ligand exchange, by cross metathesis, using acetylene, cumulated olefins, and in an one-pot method using diazo compounds and neutral electron donors (24). The route via diazo compounds is shown in Figure 1.7. 

Intramolecular addition of a hydroxy group to the terminal sp-carbon of pent-4-yn-l-ols, leading to the corresponding cycloisomerization dihydropyrans, has been successfully achieved with a similar ruthenium catalyst precursor containing the electron-deficient tris(p-fluorophenyl)phosphine ligand, excess phosphine, and sodium N-hydroxysuccinimide as additives (Scheme 9) [20]. 

Scheme 9. Cycloisomerization of pent-4-yn-l-ols catalyzed by ruthenium catalysts bearing electron deficient phosphine ligands.

Ruthenium catalysts found many applications in C-C bond formation reactions (selected reviews [157-161]). Ruthenium occurs mostly in oxidation states +2 and +3, but lower as well as higher oxidation states can easily be reached. Thus ruthenium compounds are frequently used in oxidative transformations proceeding by either single or two electron transfer pathways (selected reviews [162-164]). It has long been known that ruthenium complexes can be used for the photoactivation of organic molecules (selected reviews [165, 166]). Ruthenium complexes are applied as catalysts in controlled or living radical polymerizations [167-169]. 

Functionalized dienes can be obtained by C-C bond formation between 1,3-dienes and alkenes via oxidative coupling with electron-rich ruthenium catalysts but also via insertion into Ru-H and then Ru-C bonds. For example, Ru(COD)(COT) catalyzed the selective codimerization of 1,3-dienes with acrylic compounds to give 3,5-dienoic acid derivatives [18] (Eq. 13). -coordination of 1,3-diene to a hydridoruthenium leads to a 7r-allylruthenium species to selectively give, after coupling with the C=C bond and isomerization, the functionalized conjugated 1,3-dienes. 

Ruthenium catalysts can participate in electron-transfer processes. Thus, a variety of radical reactions of organic halides have been catalyzed by ruthenium complexes, as in the following example [ 126] (Eq. 95). 

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