Ruthenium hydride intermediates

Olefin cross-metathesis followed by intramolecular oxa-Michael addition, catalysed by the second-generation Hoveyda-Gmbbs catalyst, has been developed as a cascade, one-pot procedure for 5-hydroxy olefins (147) and a,/ -unsaturated carbonyl compounds (148) (trani-crotonaldehyde or A-acryloyl-2,5-dimethylpyrrole, etc.) as the reacting partners. In the presence of a Brpnsted acid in CH2CI2 at 25-35 °C, the resulting 2,6-cA-substituted tetrahydropyrans (149) were obtained with excellent diastereoselectivity. The role of ruthenium hydride intermediates has been investigated in detail. 

Silene-transition metal complexes were proposed by Pannell121 for some iron and tungsten systems, and such species were observed spectroscopically by Wrighton.122,123 Thus intermediates such as 33 have been proposed in the preparation of carbosilane polymers from hydrosilanes,124 both as intermediates in the isotope scrambling observed to occur in similar ruthenium hydride systems125 126 and in the 5N2 addition of alkyllithium species to chlorovinylsilanes.47... 

Knowledge of the active site allows for speculation on the mechanism of H2-D20 exchange which these Fe4 systems catalyze 473,483). Ruthe-nium(III) systems catalyze such an exchange via a ruthenium(III) hydride intermediate (7, p. 73 Section II,A), as exemplified in reactions (82) and (83), and iron hydrides must be involved in the hydrogenase systems. Ruthenium(III) also catalyzes the H2 reduction of ruthenium(IV) via reaction (82), followed by reaction (84) (3), and using these ruthenium systems as models, a very tentative scheme has been proposed 473) for... 

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]. 

The proposed mechanism involves either path a in which initially formed ruthenium vinylidene undergoes nonpolar pericyclic reaction or path b in which a polar transition state was formed (Scheme 6.9). According to Merlic s mechanism, the cyclization is followed by aromatization of the ruthenium cyclohexadienylidene intermediate, and reductive elimination of phenylruthenium hydride to form the arene derivatives (path c). A direct transformation of ruthenium cyclohexadienylidene into benzene product (path d) is more likely to occnir through a 1,2-hydride shift of a ruthenium alkylidene intermediate. A similar catalytic transformation was later reported by Iwasawa using W(CO)5THF catalyst [14]. 

Another possible reason that ethylene glycol is not produced by this system could be that the hydroxymethyl complex of (51) and (52) may undergo preferential reductive elimination to methanol, (52), rather than CO insertion, (51). However, CO insertion appears to take place in the formation of methyl formate, (53), where a similar insertion-reductive elimination branch appears to be involved. Insertion of CO should be much more favorable for the hydroxymethyl complex than for the methoxy complex (67, 83). Further, ruthenium carbonyl complexes are known to hydro-formylate olefins under conditions similar to those used in these CO hydrogenation reactions (183, 184). Based on the studies of equilibrium (46) previously described, a mononuclear catalyst and ruthenium hydride alkyl intermediate analogous to the hydroxymethyl complex of (51) seem probable. In such reactions, hydroformylation is achieved by CO insertion, and olefin hydrogenation is the result of competitive reductive elimination. The results reported for these reactions show that olefin hydroformylation predominates over hydrogenation, indicating that the CO insertion process of (51) should be quite competitive with the reductive elimination reaction of (52). 

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. ... 

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]. 

What is especially intriguing is the reverse behavior exhibited by the use of ruthenium carbonyl as the metal carbonyl. This reaction, which is catalytic in both Ru3(CO)2 and quaternary ammonium halide (which accelerates the rate of formation of the hydride intermediate), occurs in much higher yield under a carbon monoxide than a nitrogen atmosphere (22). The reaction conditions used for the Ru3(CO)12-catalyzed reaction are much milder than those reported using the water gas shift reaction [100°C, 500 psi] (25). 

In an example illustrating an approach featuring the intermediacy of a ruthenium hydride reagent, initial isomerization of the starting compound 168 generated the intermediate 169, which was thereafter annulated to the target indole 170 (Scheme 19) <2006JOC4255>. 

It is noteworthy that computational and experimental studies have shown that the formation of ruthenium-vinylidenes from terminal alkynes and ruthenium hydride complexes also proceeds via the formation of t -vinyl intermediate (Scheme 8.4) [14]. Thus, in this case the vinylidene ligand is not formed directly from the alkyne, and its /3-hydrogen atom arises from the hydrido ligand. 

Allylic alcohols are isomerized via direct interaction of the ruthenium atom with alcohol. /3-Elimination of ruthenium hydride from metal alkoxide yields a ruthe-nium-enone species C which undergoes insertion of the olefinic moiety into the Ru-H to form an oxyallylic intermediate D. As a result, the hydrogen atom shifts from the a- to y-position of the allylalcohol. Protonolysis of the oxyallylic species leads to a saturated carbonyl compound and cationic unsaturated species, [CpRu(PPh3)2] A. 

In a subsequent study, Ugo and co-workers (7) started from the monomeric species [( / -C5H5)Ru(CO)2X] (X = Cl, Br, 1) in order to generate more easily the hydride intermediate [( / -C5H5)Ru(CO)2H], employing a tertiary amine to remove HX [Eq. (4)]. The activity of the ruthenium... 

By contrast, some ruthenium hydride clusters reported by Siiss-Eink are thought to hydrogenate arenes through r -bondcd intermediates, as represented in Fig. 

A review of homogeneous hydrogenation by Ru catalysts was published in 1970 and the synthesis and properties of ruthenium-hydride complexes known prior to 1977 have been reviewed . An up to date review of ruthenium-hydride complexes appeared in 1984 . We concern ourselves here with hydrogenation reactions that involve Ru(II) species as catalyst precursors or catalytic intermediates. 

The first steps in the proposed mechanism are essentially the same as those established for monometal Rh/PPhj catalysts, except that the proposed addition of H2 oxidises two metal centres. In monometal systems, the final steps are the addition of H2 to a rhodium(i) to produce a rhodium(iii) dihydride species that can then eliminate aldehyde product. The ruthenium-rhodium intermediate avoids this problem by having a proximate Ru-H moiety, which can intra-molecularly transfer a hydride to facilitate the aldehyde elimination. Thus the final steps of the mechanism are H and CO bridge formation between the... 

Hybrid catalysts derived from cocondensation of Group 8 metal-chloro complexes with Si(OEt)4 via a sol-gel process were highly active for the synthesis of A/,iV-dimethyIformamide from CO2, H2 and dimethylamine under supercritical conditions, affording turnover numbers up to 100800 at 100% selectivity [61]. The activity of the catalysts, containing methylphosphine ligands, decreased in the order Ru>Ir>Pt,Pd>Rh. It seemed that the high activity of silica matrix stabilized ruthenium complexes was due to the formation of an active hydride intermediate by hydrogenolysis of the Ru-Cl bond. 

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