Ruthenium hydride alkynes

Since activation of the N-H bond of PhNHj by Ru3(CO)i2 has been reported to take place under similar conditions [306], it has been proposed that the reaction mechanism involves (i) generation of an anUido ruthenium hydride, (ii) coordination of the alkyne, (iii) intramolecular nucleophilic attack of the nitrogen lone pair on the coordinated triple bond, and (iv) reductive ehmination of the enamine with regeneration of the active Ru(0) center [305]. 

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. 

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. 

A mechanistic study of the ruthenium-catalysed hydroamidation of terminal alkynes HC=CR with amides R CONHR, which included deuterium labelling and the in situ infrared (IR), NMR, and ESI-MS experiments, suggests that the reaction proceeds via an oxidative addition of the amide, followed by insertion of the tt-coordinated alkyne into a ruthenium-hydride bond to produce fra 5-R CON(R )-Ru(L )-CH=CHR, rearrangement to the vinylidene species... 

Mechanistic studies on the Ru-catalyzed hydroamidation of terminal alkynes support the involvement of ruthenium hydride and ruthenium vinylidene species as the key intermediates [120]. The proposed pathway starts with the initial oxidative addition of the amide N-H bond to the ruthenium complex to afford ruthenium-hydride I followed by 1,2-insertion of a Jt-coordinated alkyne to vinyl-ruthenium species II. Rearrangement to the vinylidene species HI followed by nucleophilic attack of the amide and subsequent reductive elimination would give rise to the product (Scheme 20). 

Ruthenium hydride pincer complex [Ru(PNP)(H)2(H2)j [PNP=l,3-bis(di-terf-butyl-phosphinomethyl)pyridine] and its borane analog [Ru(PNP)(H)2(HBpin)] (HBpin=pinacolborane) catalyze the hydroboration of terminal alkynes to give selectively Z-vinylboronates in high yields (Scheme 32) [146]. Mechanistic studies... 

Scheme 32 Hydroboration of terminal alkynes to Z-vinylboronates using a ruthenium hydride pincer complex as catalyst...

A mechanistic pathway is proposed based upon the observed regioselectivities and other results that were obtained during the exploration of the scope and limitations of the Alder-ene reaction.38 Initially, coordination of the alkene and alkyne to the ruthenium catalyst takes place (Scheme 5). Next, oxidative addition affords the metallocycles 42 and 43. It is postulated that /3-hydride elimination is slow and that the oxidative addition step is reversible. Thus, the product ratio is determined by the rate at which 42 and 43 undergo /3-hydride elimination. 

Ruthenium complexes do not have an extensive history as alkyne hydrosilylation catalysts. Oro noted that a ruthenium(n) hydride (Scheme 11, A) will perform stepwise alkyne insertion, and that the resulting vinylruthenium will undergo transmetallation upon treatment with triethylsilane to regenerate the ruthenium(n) hydride and produce the (E)-f3-vinylsilane in a stoichiometric reaction. However, when the same complex is used to catalyze the hydrosilylation reaction, exclusive formation of the (Z)-/3-vinylsilane is observed.55 In the catalytic case, the active ruthenium species is likely not the hydride A but the Ru-Si species B. This leads to a monohydride silylmetallation mechanism (see Scheme 1). More recently, small changes in catalyst structure have been shown to provide remarkable changes in stereoselectivity (Scheme ll).56... 

The proposed mechanism involves the formation of ruthenium vinylidene 97 from an active ruthenium complex and alkyne, which upon nucleophilic attack of acetic acid at the ruthenium vinylidene carbon affords the vinylruthenium species 98. A subsequent intramolecular aldol condensation gives acylruthenium hydride 99, which is expected to give the observed cyclopentene products through a sequential decarbonylation and reductive elimination reactions. 

The same ethylidene ruthenium complex, as well as its iron congener, is alternatively obtained through direct protonation of the dimetallacycles 64a (M = Fe) and 64b (M = Ru) (64). In this case, the carbonyl alkyne carbon-carbon bond is broken irreversibly to give the cationic /x, 17s-vinyl complexes 65a and 65b, which undergo nucleophilic attack by hydride (NaBFLi) to produce complexes of methylcarbene (63a,b) (Scheme 21a). Deuterium-labeling experiments prove that the final compounds arise from initial hydride addition to the /3-vinylic carbon of 65. However, isolation of small amounts of the 7j2-ethylene complex 66 indicates that hydride attack can also occur at the a-vinylic carbon (64). 

Heterometal alkoxide precursors, for ceramics, 12, 60-61 Heterometal chalcogenides, synthesis, 12, 62 Heterometal cubanes, as metal-organic precursor, 12, 39 Heterometallic alkenes, with platinum, 8, 639 Heterometallic alkynes, with platinum, models, 8, 650 Heterometallic clusters as heterogeneous catalyst precursors, 12, 767 in homogeneous catalysis, 12, 761 with Ni—M and Ni-C cr-bonded complexes, 8, 115 Heterometallic complexes with arene chromium carbonyls, 5, 259 bridged chromium isonitriles, 5, 274 with cyclopentadienyl hydride niobium moieties, 5, 72 with ruthenium—osmium, overview, 6, 1045—1116 with tungsten carbonyls, 5, 702 Heterometallic dimers, palladium complexes, 8, 210 Heterometallic iron-containing compounds cluster compounds, 6, 331 dinuclear compounds, 6, 319 overview, 6, 319-352... 

A ruthenium based catalytic system was developed by Trost and coworkers and used for the intermolecular Alder-ene reaction of unactivated alkynes and alkenes [30]. In initial attempts to develop an intramolecular version it was found that CpRu(COD)Cl catalyzed 1,6-enyne cycloisomerizations only if the olefins were monosubstituted. They recently discovered that if the cationic ruthenium catalyst CpRu(CH3CN)3+PF6 is used the reaction can tolerate 1,2-di- or tri-substituted alkenes and enables the cycloisomerization of 1,6- and 1,7-enynes [31]. The formation of metallacyclopentene and a /3-hydride elimination mechanism was proposed and the cycloisomerization product was formed in favor of the 1,4-diene. A... 

One of the most reported pathways for C=C and C=C bonds coupling involves the oxidative coupling and the ruthenacyde intermediate formation. The first ruthenium-catalyzed Unear codimerization of disubstituted alkynes and alkenes involved acrylates or acrylamides and selectively produced 1,3-dienes [33] (Eq. 23). The proposed mechanism involves a ruthenacyclopentene via oxidative coupUng on the Ru(0) catalyst Ru(COD)(COT). The formation of 1,3-di-ene results from intracyclic /1-hydride eUmination, this process taking place only when a favored exocyclic /1-elimination is not possible. 

One of the first examples of ruthenium-catalyzed C-C bond formation afforded the synthesis of cyclobutenes, from norbornene derivatives with dimethyl acetylenedicarboxylate, and was reported by Mitsudo and coworkers [45, 46] by using various catalysts such as RuH2(CO)[P(p-C6H4F)3]3 or RuH2(PPh3)4. More recently, the complex Cp RuCl(COD) has shown to be an excellent catalyst for the [2+2] cycloaddition of norbornenes with various internal alkynes [45] (Eq. 33) and with a variety of substituted norbornenes and norbornadienes [47]. The ruthenacycle intermediate, formed by oxidative coupling, cannot undergo /1-hydride elimination and leads to cyclobutene via a reductive elimination. 

Reaction between alkynes with electron-withdrawing groups and ruthenium alkyl or hydride complexes results in complexes containing alkenyl or... 

The reactivity of osmium with terminal alkynes can be used to prepare bimetallic complexes. When an osmium hydride reacts with a terminal aUcyne that is part of a ruthenium complex, the product is a Ru-Os bimetallic complex with a hydrocarbon bridge (Scheme 16). Another bimetallic complex, [Cp(PPh3 )20s=C=C=CH-C=C-0s(PPh3)2Cp]+, resulted from the reaction between [CpOs(PPh3)2]+ with HC=CCH(OH)C=CH followed by alumina. The unsaturated C5 bridging group is V-shaped. 

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