Ru-based catalyst

In 2002, Hoveyda et al. reported the synthesis, structure and reactivity of a chiral bidentate Ru-based catalyst 65, bearing a binaphthyl moiety, for olefin metathesis [33]. Preference for a bidentate chiral imidazolinylidene was based on the hypothesis that such a ligand would induce chirality more efficiently. This catalyst was designed by analogy with similar achiral complexes 66 that... 

The different classes of Ru-based catalysts, including crystalline Chevrel-phase chalcogenides, nanostructured Ru, and Ru-Se clusters, and also Ru-N chelate compounds (RuNj), have been reviewed recently by Lee and Popov [29] in terms of the activity and selectivity toward the four-electron oxygen reduction to water. The conclusion was drawn that selenium is a critical element controlling the catalytic properties of Ru clusters as it directly modifies the electronic structure of the catalytic reaction center and increases the resistance to electrochemical oxidation of interfacial Ru atoms in acidic environments. 

Alkali-promoted Ru-based catalysts are expected to become the second generation NHs synthesis catalysts [1]. In 1992 the 600 ton/day Ocelot Ammonia Plant started to produce NH3 with promoted Ru catalysts supported on carbon based on the Kellogg Advanced Ammonia Process (KAAP) [2]. The Ru-based catalysts permit milder operating conditions compared with the magnetite-based systems, such as low synthesis pressure (70 -105 bars compared with 150 - 300 bars) and lower synthesis temperatures, while maintaining higher conversion than a conventional system [3]. 

The outstanding performances of five-membered NHC ligands in organometallic chemistry and catalysis prompted Grubbs and co-workers to develop a novel stable four-membered NHC [64]. Following their interest in developing new ruthenium olefin metathesis catalysts, they synthesised and fully characterised complex 51 to study the impact of the architecturally unique NHC ligand on the activity of the Ru-based catalyst [65] (Fig. 3.20). In the RCM of 1 at 40°C in CH Cl with 51 (5 mol% catalyst), the reaction reached completion within 20 min, whereas less than 10 min are required for standard catalysts 14 and 16. It should be noted that catalysts 14 and 16 are able to complete the RCM of 1 with only 1 mol% catalyst at 30°C. 

CM has, in most cases, a good to excellent ii-selectivity. This is primarily due to steric reasons in the metallacycle intermediate of the metathesis. The high ii-selectivity of the CM makes it an ideal method for the stereocontrolled synthesis of stilbenes [152], while there is still no highly Z-selective Ru-based catalyst known [153]. 

A useful application in the manufacture of ion-exchange resins may well be possible which avoids the use of carcinogenic chloromethyl ether. Here, a polymer of p-methyl styrene is chlorinated on the side chain with aqueous NaOCl and a phase-transfer catalyst. Sasson et al. (1986) have shown how stubborn . substituted aromatics like nitro/chlorotoluenes can be oxidized to the corresponding acids by using aqueous NaOCl containing Ru based catalyst. 

This is also a reversible, exothermic reaction carried out in various types of fixed-bed reactors, involving different arrangements for flow (axial or radial), and temperature adjustment see Figure 11.5. The traditional catalyst is promoted Fe, but more active Ru-based catalysts are finding use, despite the added... 

The skeletal rearrangements are cycloisomerization processes which involve carbon-carbon bond cleavage. These reactions have witnessed a tremendous development in the last decade, and this chemistry has been recently reviewed.283 This section will be devoted to 7T-Lewis acid-catalyzed processes and will not deal, for instance, with genuine enyne metathesis processes involving carbene complex-catalyzed processes pioneered by Katz284 and intensely used nowadays with Ru-based catalysts.285 By the catalysis of 7r-Lewis acids, all these reactions generally start with a metal-promoted electrophilic activation of the alkyne moiety, a process well known for organoplatinum... 

The groups of Marinetti and Genet have shown that several bisphosphetane-derived ligands (58, 59, 61) form effective Ru-based catalysts for the hydrogena-... 

It is interesting to note that the same effect of water, both on activity and C5 + and methane selectivity, is noted for a supported Ru catalyst.52 The same rate equation as was used for Fe and Co-based catalysts, including a term for water, was also valid for the Ru-based catalyst (eqn (4)). 

A Recent Approach to Permanent Immobilization of a Ru-based Catalyst... 

Amongst Ru complexes the tetroxide is the best able to effect C-H bond activation, though some other Ru-containing reagents will do this. Some of the oxidations with Ru-based catalysts have been with adamantane as a substrate its common oxidation products are shown in Fig. 4.2. 

There has recently been much work in this area using Ru-based catalysts, particularly with porphyrin-based catalysts, following the work by Sharpless et al. on asynunetric epoxidation of allylic alcohols by a titanium-based tartrate system. There are reviews on asymmetric epoxidations catalysed by chiral Ru porphyrins [5, 18]. 

In bimetallic catalysts, Cu-Ru is an important system. Combinations of the Group Ib metal (Cu) and Group VIII metal (Ru)-based catalysts are, for example, used for the dehydrogenation of cyclohexane to aromatic compounds and in ethane hydrogenolysis involving the rupture of C-C bonds and the formation of C-H bonds (Sinfelt 1985). Here we elucidate the structural characteristics of supported model Cu-Ru systems by EM methods, including in situ ETEM. 

Keywords Asymmetric synthesis, Chiral catalysis, Mo-based catalysts, Natural product synthesis, Olefin metathesis, Recyclable catalysts, Ru-based catalysts, Supported chiral catalysts... 

Scheme 21. Air-stable chiral Ru-based catalyst for olefin metathesis can be used for highly effective and selective AROM/CM reactions...

The chiral Mo-based catalysts discussed herein are more sensitive to moisture and air than the related Ru-based catalysts [ 1 ]. However, these complexes, remain the most effective and general asymmetric metathesis catalysts and are significantly more robust than the original hexafluoro-Mo complex 1. It should... 

The chiral Mo-based catalysts discussed herein are more senstive to moisture and air than the related Ru-based catalysts [1], However, these complexes, remain the most effective and general asymmetric metathesis catalysts and are significantly more robust than the original hexafluoro-Mo complex 1. It should be noted that chiral Mo-based catalysts 4,11, 25, 34 and 77 can be easily handled on a large scale. In the majority of cases, reactions proceed readily to completion in the presence of only 1 mol% catalyst and, in certain cases, optically pure materials can be accessed within minutes or hours in the absence of solvents little or no waste products need to be dealt with upon obtaining optically pure materials. Chiral catalyst 4a is commercially available from Strem, Inc. (both antipodes and racemic). The advent of the protocols for in situ preparation of chiral Mo catalyst 77, the supported and recyclable complex 82 and the debut of a chiral Ru catalyst (83) augur well for future development of practical chiral metathesis catalysts. The above attributes collectively render the chiral catalysts discussed above extremely attractive for future applications in efficient, catalytic, enan-tioselective and environmentally conscious protocols in organic synthesis. 

To illustrate the utility of the metal salen complexes, several reactions are outlined in Scheme 1. They include the asymmetric epoxidation of unfimctionalized cw-disubstituted and trisubstituted olefins, which are promoted by (salen)Mn complexes." In the case of trani-disubstituted olefins, the simple (salen)Mn complexes do not exhibit the same levels of enantioselectivity as they do with the cis- and trisubstituted derivatives. Promising alternatives include more elaborate (salen)Mn complexes based on the binaphthyl imit, (salen)Cr complexes,and (salen)Ru-based catalysts. Catalysts based on (salen)Co moiety have exhibited amazing levels of selectivity in the hydrolytic kinetic resolution (HKR) of terminal epoxides. The HKR allows access to terminal epoxides and diols with very high enantioselectivities. 

Up to the mid-1980s the field of enantioselective hydrogenation had been dominated by the Rh-based Wilkinson-type catalysts. Then, Noyori et al. introduced a new family of Ru-based catalysts, which showed a wider applicability than the Rh catalysts. a, ff-Unsaturated acids other than dehydroamino acids became substrates which could be hydrogenated with high enantioselectivity... 

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