Ruthenium hydride catalyst

Ruthenium hydride catalysts can also initiate a variety of cycloisomerizations of 1,5- and 1,6-enynes as well as dienes, as exemplified by the RuClH(CO)(PPh3)3-catalyzed reactions shown in Scheme 64.249... 

Ruthenium and iridium are commonly used in catalysts for this reaction. A selectively deuterated hydroxycyclopentadienyl ruthenium hydride catalyst has been employed to probe the mechanism 48 The relative rates of different steps determine whether the process is stereospecific (typically trans) or stereorandom. 

C-H activation pathway cannot be ruled out for this transformation, however. They subsequently observed C-H insertion of a cis substituent by means of isotope studies [32], The cycloheptene product was observed as the major product when cis- or tri-substituted enyne and acetylenic ester termini were present. Ruthenium hydride catalysts reported by Mori and Dixneuf can also initiate the cycloisomerization of 1,5- and 1,6-enynes and dienes [33, 34], The vinylruthenium hydride can be obtained from RuClH(CO)(PPh3)3 or in the presence of acetic acid or ethanol. 

Cycloisomerization of allyl propargyl ethers takes place under mild conditions when performed with a ruthenium hydride catalyst obtained from Cp RuCl(COD) in the presence of acetic acid or ethanol. 3,4-Dialkylidenete-trahydrofurans were produced in good yields [ 71 ] (Eq. 53). It is noteworthy that the C-H and C-C bonds formed are always syn. This is the result of the cis addition of the Ru-H bond to the C=C bond. 

The catalytic activation of allylic carbonates for the alkylation of soft car-bonucleophiles was first carried out with ruthenium hydride catalysts such as RuH2(PPh3)4 [108] and Ru(COD)(COT) [109]. The efficiency of the cyclopen-tadienyl ruthenium complexes CpRu(COD)Cl [110] and Cp Ru(amidinate) [111] was recently shown. An important catalyst, [Ru(MeCN)3Cp ]PF6, was revealed to favor the nucleophilic substitution of optically active allycarbonates at the most substituted allyl carbon atom and the reaction took place with retention of configuration [112] (Eq. 85). The introduction of an optically pure chelating cyclopentadienylphosphine ligand with planar chirality leads to the creation of the new C-C bond with very high enantioselectivity from symmetrical carbonates and sodiomalonates [113]. 

Another way to synthesize disilacycles has been investigated by Mise et al. <1998CC699, 2005JOM3451>. Compounds having two vinyldimethylsilyl groups on adjacent carbons have been successfully cyclized by ruthenium hydride catalysts via a metathetical reaction (Equations 20 and 21). 

Keywords silylative coupling, dlvlnyl sllyl ethers, l,l-bis(sllyl)ethenes, slloxylene-alkylene-vinylene oligomers, ruthenium hydride catalyst... 

The silylative coupling reaction of l,2-bis(dimethyIvinylsiloxy)ethane was effectively catalyzed by 1 mol% of ruthenium hydride catalyst, and the divinyl compound was completely consumed within 1 h at 80 °C. The reaction successfully proceeded without the solvent under air, but toluene could also be employed without affecting either the activity of the catalyst or the selectivity of this process. Application of this catalytic system for silylative coupling cyclization of l,2-bis(dimethyl-vinylsiloxy)ethane gave exclusively a cyclic product (Isolated yield 85%) with the exo-methylene bond between two silicon atoms in the molecule (2,2,4,4-tetramethyl-3-methylene-l,5-dioxa-2,4-dlsilacycloheptane) accompanied only by trace amounts of oligomers. 

Chelate-Assisted Oxidative Coupling Reaction of Arylamides and Unactivated AUcenes Mechanistic Evidence for Vinyl C-H Bond Activation Promoted by an Electrophilic Ruthenium Hydride Catalyst... 

The induction period of the catalysis involves the replacement of an acetonitrile ligand by silicon hydride followed by o-bonding metathesis reaction, which makes the H-atom exchange of silicon hydride for the Cl-atom of Ru-complex to form [RuH(NCMe)4]+ with the elimination of chlorosilane. The formation of ruthenium hydride catalyst (4) (1) that generates the product is a... 

Scheme 7.15 Calculated mechanisms for olefin Isomerization (a) hydride-transfer mechanism with a ruthenium hydride catalyst, adapted from Ref [65, 66] and (b) methylldene Insertion Into an allyllc C-H bond, adapted from Ref [63]. All energies are Gibbs free energies In kcal mor. ...

HCI -ketone Scheme 18.8 Formation of ruthenium hydride catalysts in the presence of a base (L = PPh3) [21]. 

Ruthenium hydride complexes, e.g., the dimer 34, have been used by Hofmann et al. for the preparation of ruthenium carbene complexes [19]. Reaction of 34 with two equivalents of propargyl chloride 35 gives carbene complex 36 with a chelating diphosphane ligand (Eq. 3). Complex 36 is a remarkable example because its phosphine ligands are, in contrast to the other ruthenium carbene complexes described so far, arranged in a fixed cis stereochemistry. Although 36 was found to be less active than conventional metathesis catalysts, it catalyzes the ROMP of norbornene or cyclopentene. 

The cross metathesis of vinylsilanes is catalyzed by the first-generation ruthenium catalyst 9. This transformation has been extensively investigated from both preparative and mechanistic points of view by Marciniec et al. [86]. Interestingly, the same vinylsilanes obtained from cross metathesis may also result from a ruthenium-hydride-catalyzed silylative coupling and there might be some interference of metathesis and nonmetathesis mechanisms [87]. 

A mixture of catalyst 110 and vinyl trimethylsilyl enolether 115 has been used in cycloisomerisation of (V-allyl-o-vinylanilines 114 and (V.A-diallyl-p-toluenesulfonamide 115 to afford the corresponding products 118 and 119, respectively (Scheme 5.30) [34]. It is believed that the active catalyst species is the ruthenium hydride NHC complex 117. 

Scheme 3.7 Generation of the active hydride catalyst by hydrogen transfer from formic acid or iso-propanol via /5-hydride elimination from formate or alkoxide intermediates. The square represents a vacant site on ruthenium.

Noyori and coworkers reported well-defined ruthenium(II) catalyst systems of the type RuH( 76-arene)(NH2CHPhCHPhNTs) for the asymmetric transfer hydrogenation of ketones and imines [94]. These also act via an outer-sphere hydride transfer mechanism shown in Scheme 3.12. The hydride transfer from ruthenium and proton transfer from the amino group to the C=0 bond of a ketone or C=N bond of an imine produces the alcohol or amine product, respectively. The amido complex that is produced is unreactive to H2 (except at high pressures), but readily reacts with iPrOH or formate to regenerate the hydride catalyst. 

Hydride ion transfer from formic acid and its salts finds widespread application in the reduction of organic substrates, but limited use has been made of the procedure under phase-transfer catalytic conditions. However in the presence of a ruthenium complex catalyst, it is possible to selectively reduce the C=C bonds of conjugated ketones with sodium formate [11], The rate of reduction is fastest with tetrahexyl-ammonium hydrogensulphate and Aliquat the complete reduction of chalcone being effected within one hour, whereas with benzyltriethylammonium chloride only ca. 15% reduction is observed after two hours under similar conditions. 

In the early syntheses of alkenyl alkylidene-mthenium catalysts, the first generation of Grubbs catalyst, it was observed that propargyl chloride could be a convenient source of the vinylcarbene initiator [53] with respect to the previous one arising from activation of cyclopropene [4] (Equation 8.3). In this synthesis the alkylidene hydrogen atom arises from the ruthenium hydride. 

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). 

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... 

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