Ruthenium hydride species

Scheme 10 Ruthenium carbene complexes from ruthenium hydride species prepared in situ [18]...

An olefin metathesis/double bond isomerization sequence can be promoted by the catalysis of in situ generated ruthenium hydride species from ruthenium complex 1 (Scheme 41 ).68... 

The proposed mechanism involves coordination of allene and ce,/j-unsaturated ketone to the cationic cydopentadienylruthenium species 137. Subsequent formation of the ruthenacyde 139, followed by /3-hydride elimination, generates the ruthenium hydride species 140. Finally, reductive elimination closes the cycle and regenerates the ruthenium intermediate 137 (Scheme 14.33) [68, 71]. 

A detailed review of the mechanisms of the hydrogenation of polar double bonds by ruthenium hydride species have been published by Clapham et alP The article examines the properties of over 100 catalyst systems for transfer and... 

Alkaline solutions of Ru(CO)t2 (KOH in aqueous ethoxyethanol) have also been found to catalytically decompose formic acid (5 7,5S). Presumably this occurs by way of anionic ruthenium hydride derivatives [e.g., HRu3(CO)7,] reacting with HCOOH to provide a ruthenium formate derivative and H2. Subsequent / -elimination of hydride from the ruthenium formate led to regenerating the anionic ruthenium hydride species and carbon dioxide. We have recently synthesized and fully characterized a possible ruthenium formato intermediate for this process, Ru3(CO),0-(02CH) (9) (59). Indeed this species in part extrudes C02 in the presence of CO with concomitant production of Ru3(CO),, H. ... 

Conjugated dienes were thus selectively obtained by hydrovinylation of alkynes catalyzed by a cationic ruthenium alkylidene complex [43] (Eq. 31). This reaction is thought to be promoted by the ruthenium hydride species resulting from the deprotonation of the <5-methyl group of the metallic precursor, followed by the sequential insertion of alkyne and ethylene into the metal-hydride and metal-vinyl bonds. 

In contrast to the previously mentioned reactions, which involve either oxo-ruthenium or ruthenium hydride species as intermediates, free-radical reactions can also be promoted by ruthenium. The aerobic oxidation of alcohols proceeds smoothly at room temperature in the presence of 4 eq. of an aldehyde, for example, acetaldehyde, and a catalyst comprising a 1 1 mixture of RuC13 nH20 and Co(OAc)2, in ethyl acetate (Eq. 31) [122]. 

The proposed mechanism for the oxidation reaction is presented in Scheme 14.44. The formation of Ru-alcoholate (species 11) by the reaction of catalyst I with benzyl alcohol is considered to be the first step of the catalytic cycle, followed by P-hydride elimination to produce the corresponding carbonyl compound and probably a ruthenium hydride species. Subsequent reaction of complex 111 with may afford the complex Ru-hydroperoxide IV. The uptake of alcohol again completes the cycle with the formation of and H O. 

When a similar reaction is carried out using ethylene, the expected methylene complex is not observed. It appears as though the methylene complex is not stable in water and decomposes rapidly to a mixture of ruthenium hydride species. 

A very efficient group of catalysts are the 2-pyridylethanyl substituted ruthenium carbene complexes 9 and 10. Also this new class of catalyst can be easily prepared, either via the reaction of Grubbs benzylidene catalyst with a 2-(3-butenyl)pyridine or directly via a one-pot procedure for the synthesis of ruthenium carbenes starting from [RuCl2(l,5-cyclooctadiene)] via a ruthenium hydride species, see Scheme 5b. 

Alkene isomerization/migration is an undesired side reaction in olefin metathesis. This isomerization has been attributed to ruthenium hydride species formed upon catalyst decomposition [63, 64]. Several computational investigations of ruthenium hydride-catalyzed olefin isomerization have been reported, in which the isomerization was found to occur via olefin insertion into the ruthenium hydride, rotation about the Ru-C a bond, and P-hydride elimination (Scheme 7.15a) [65, 66]. These processes typically require only relatively low activation barriers. Once the ruthenium hydride is formed, olefin isomerization... 

A variety of additives have been developed to suppress the migration or isomerization of olefins during an olefin metathesis process. In Ru-catalyzed olefin metathesis, it has been proposed that ruthenium hydride species, formed in situ during a metathesis reaction due to catalyst decomposition, are largely responsible for the isomerization process. For example, Grubbs and coworkers [57] have isolated the decomposition product 100, and have shown that it can promote olefin isomerization (Scheme 12.30). 

A number of structural motifi are therefore potentially detrimental to the performance of metathesis reactions some of these appear innocent, and can be hard to identify in densely functionalized molecules. Processes that can lead to ruthenium hydride complexes are particularly problematic, as these ruthenium hydride species can bring about unwanted alkene isomerization in substrates and/or products. 

Isomerization can also be observed with the Grubbs ruthenium catalysts, both before and after metathesis. The isomerization reaction is believed to be catalysed by ruthenium hydride species, such as 8.460, generated by decomposition of the Grubbs catalyst. ... 

Sheldon and Arends found that the combination RuCl2(PPh3)3-TEMPO affords an efficient catalytic system for the aerobic oxidation of a broad range of primary and secondary alcohols at 100 °C, giving the corresponding aldehydes and ketones, respectively, in >99% selectivity in all cases [86]. The reoxidation of the ruthenium hydride species with TEMPO was proposed in the latter system [86c[. Allylic alcohols can be converted into a,(3-unsaturated aldehydes with 1 atm of molecular oxygen in the presence of RUO2 catalyst [87]. 

Heteroatom-directed C(sp )-H bond fimctionalizalion with stoichiometric transition metals was first disclosed in 1984 [1]. In 2002, Sames and coworkers developed an efficient route to constmct the teleocidin B4 core via the activation of C(sp )-H bond to prepare two diastereomeric paUadacycle key intermediates [2]. As a follow-up work, Ru3(CO)i2-catalyzed arylation of various C(sp )-H bonds with arylboronate esters using pyridine, pyrimidine, and amidine as directing groups was reported (Scheme 1.1) [3]. The use of ketones as solvent was necessary for a successfid arylation, mainly due to the trapping effect of the ruthenium hydride species. Despite of its efficiency, this transformation needs elevated temperatures (150 °C). Further, pyridine-directed a-C(sp )-H arylation of piperidines with arylboronate esters was developed with alcohols as solvent [4]. 

On the other hand, treatment of norbornadiene with a catalytic amount of Ru(cod)(cot) (cod = cyclo-l,5-octadiene, cot = cyclooctatetraene) and an electron-deficient olefin caused a unique dimerization reaction to afford cage compound 18 (Scheme 7.6) [8]. Although the precise reaction mechanism is unclear, it is proposed that the reaction proceeds through the alkylruthenium intermediate 13. It adds intramolecularly to the alkene moiety to form 14, which further adds to the remaining alkene moiety. Subsequent oxidative addition of a C-C bond located in proximity to the ruthenium center affords 15. Reductive elimination ensues to give the alkylruthenium intermediate 16. Subsequent P-carbon elimination breaks the strained norbornane skeleton to furnish alkylruthenium 17. P-Hydride elimination produces the cage molecule 18 along with a ruthenium hydride species. 

SCHEME 2.140 True catalyst, ruthenium hydride species as reactive intermediate and proposed six-membered cyclic transition state. 

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