Metal hydride species

Proton electroreduction catalyzed by metal complexes is different from reduction at a metal electrode. It definitely involves the formation of metal hydride species through protonation of electroreduced, low-oxidation-state metal complexes that function as Bronsted base (Equation (5)). From protonated... 

Apparently an ion-exchange resin will allow the absorption of the metal hydride species onto its surface by protonation and subsequent elimination of hydrogen (Equation 2.11). This hydrogen elimination is a reversible reaction. The metal species remains as a labile species that can be desorbed by hydrogen in a fluid-stripping medium. 

If some metal hydride species catalyzes the hydrogenation of benzaldehyde ... 

The use of amines allows much higher nucleophile concentrations than those achievable with Bronsted bases. We have used solutions as concentrated as 6 M Me N. This vast difference in available nucleophile concentration partially explains the huge increase in rate afforded by NMe3 over the rate with Bronsted bases. Very large concentrations of hydroxide may promote the base attack step but can decrease the rate of the WGSR due to inhibition of the protonation of the metal hydride species. 

This section has dealt with the oxidation of CO to C02, especially as it enters into the water-gas shift reaction (26a). A reasonable view of the homogeneous catalysis of this reaction, whether in basic or acidic media, is emerging in which CO formation proceeds from nucleophilic attack of water or OH" on an activated carbonyl followed by either reductive decarboxylation or hetero-atom -elimination yielding, respectively, a reduced metal or a metal hydride species. 

Elimination of alkenes has been already dealt with in Section II,F (oxidative addition-elimination), while other reactions involving elimination of dihydrogen may be found in that section and in Section II,E this latter section includes many cases in which metal hydride species are believed to act as intermediates. 

Whichever catalyst system is used to prepare an a-amino acid derivative at scale, the cost of the ligand can play a major economical role, often more than the metal. The cost of the catalyst can be offset by large substrate-to-catalysts ratios that can be improved by recycles and fast reactions. However, because metal hydride species are invariably involved in the catalytic cycle, recycles usually mean reuse in a short time span. The conclusion is that expensive ligands must be extremely good at the desired reduction. This is particularly true with amino acid derivatives because almost all are crystalline and offer the possibility of enantioenrichment during the purification process. Thus, high ee s may not be required in the reduction itself. In many cases, the synthesis of the substrate is the difficult part of the synthesis. This problem has been highlighted in the synthesis of [3-amino acids (see Section 2.6). 

Metal hydride species can undergo an insertion reaction with an olefinic double bond, resulting in the formation of a metal alkyl species which then generates the requisite carbenoids by a-hydride abstraction [110-112] ... 

In search of model systems for iron hydrogenases, Sellmann et al. (67) investigated the interaction of I e(hdt)2 2 with H+, H2, and H . Formation of H2 was observed in the reaction with H+. The reaction mechanism was proposed to follow a step-wise protonation, forming a thiol-hydride complex and H2 is proposed to form via heterolytic elimination from the metal hydride species (Scheme 6). Theoretical calculations suggest that concerted H2 elimination from a dithiol species is thermally forbidden (67). 

Metal hydride species exhibit a wide range of reactivity, depending on the nature of the M-H bond. In those cases where the bond is very hydridic in nature, alcohols normally react to form alkoxides, with liberation of H2 ... 

This reaction frequently accompanies reactions catalyzed by metal hydride species, e.g., hydrides of Co, Rh, or Ni. 

Release of the unsaturated chain end of a polyolefin can occur by fi-H transfer to the metal or to a monomer molecule (see Appendix 1 for backgound material). A metal-alkyl species, i.e. the starting unit for a new polymer chain, arises from the metal-hydride species formed in the first case by insertion of an olefin, or it can be formed directly by f-H transfer to a monomer (Figure 20). While the results are thus identical, the two reaction paths differ in their respective kinetics In the first case, the rate-limiting p-H transfer is independent of the olefin concentration, while the rate of p-H transfer to a monomer requires the formation of an olefin-containing reaction complex and will thus increase linearly with olefin concentration. 

Product formation can be visualized as occurring via a nucleophilic attack on the carbonyl carbon of the acyl -metal species with, in the case of a nucleophile of the H u (HjO. HOR. H2NR etc.) type, concurrent regeneration of a metal-hydride species... 

Re inserts in pure hydrocarbons and in C H bonds near an aromatic or triene group. The intermediate metal hydride species react further in order to fill their coordination shell ... 

The reaction of alkyl-substituted tungsten-carbene complexes of the type (88b) have been reported by Macomber to react with alkynes to give dienes of the type (319). One mechanism that has been proposed to account for this product is a 3-hydride elimination from the metallacyclobutene intermediate (320) and subsequent reductive elimination in the metal hydride species (321). An additional example of this type of reaction has been reported by Rudler, also for an alkyl tungsten carbene complex. Chromium complexes have not been observed to give diene products of this type the reaction of the analogous chromium complex (88a) with diphenylacetylene gives a cyclobutenone as the only reported product (see Scheme 31). Acyclic products are observed for both tungsten and chromium complexes in their reactions with ynamines. These reactions produce amino-stablized carbene complexes that are the result of the formal insertion of the ynamine into the metal-carbene bond. ... 

The two established pathways for transition metal-catalyzed alkene isomerization are the jr-allyl metal hydride and the metal hydride addition-elimination mechanisms. The metal hydride addition-elimination mechanism is the more common pathway for transition metal-catalyzed isomerization. In this mechanism, free alkene coordinates to a metal hydride species. Subsequent insertion into the metal-hydride bond yields a metal alkyl. Formation of a secondary metal alkyl followed by y3-elimination yields isomerized alkene and regenerates the metal hydride. The jr-allylhydride mechanism is the less commonly found pathway for alkene isomerization. Oxidative addition of an activated allylic C-H bond to the metal yields a jr-allyl metal hydride. Transfer of the coordinated hydride to the opposite end of the allyl group yields isomerized alkene. 

It was proposed that the catalytic cycle is initiated by c-bond metathesis between Cp 2YMe(THF) and PhSiFI3, producing a catalytically active metal hydride species Cp 2YH . In the next step, the catalyst inserts preferentially at the alkyne sites. This intermediate undergoes cyclization via an intramolecular olefin insertion and produces a second intermediate alkylyttrium species. In the reaction with silane this intermediate undergoes a subsequent cr-bond metathesis to generate the cyclized product.1029... 

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