Metal hydride transfer

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. 

Finally, dienes may be converted into monoenes under transition metal hydride transfer conditions. Acidic organic alcohols, e.g., catechol, are effective in these reactions, and 1,5-cyclooctadiene is reduced to cyclooctene Hydrogen transfer from amines such as 1,3-propanediamine is also efficient. Thus, when 1,5-cyclooctadiene and 1,3-propanediamine are stirred at 140°C for 2 h in the presence of Pd black, cyclooctene is obtained in 85% yield together with 24 ° ... 

Such reactions presumably involve the rapid, reversible protonation of the substrate olefin or ketone, followed by hydride transfer from the metal (or the Si of the silane) (eq 1). Several studies have confirmed that metal hydrides transfer H in a single step, not by sequential transfer of H- and e [78]. As we would expect, the rates of these reactions increase with acidity [73, 74,75] and are first-order in substrate [75]. When pivaldehyde is used as a substrate with TsOH and CpRe(NO)(PPh3)H no rearrangement occurs, implying that transfer of hydride to the protonated substrate is efficient [75]. 

In the mechanism mentioned above, the metal hydride transfers an FI atom to an organic radical. This reaction is useful, even in stoichiometric (non-chain) reactions, because it allows to easily generate an organometallic or inorganic radical. It is thermodynamically favorable, because the C-H bond is stronger than the M-H bond (see the table of E-H bond energies. Chap. 5). 

Figure 1.9 Examples of functionally important intrinsic metal atoms in proteins, (a) The di-iron center of the enzyme ribonucleotide reductase. Two iron atoms form a redox center that produces a free radical in a nearby tyrosine side chain. The iron atoms are bridged by a glutamic acid residue and a negatively charged oxygen atom called a p-oxo bridge. The coordination of the iron atoms is completed by histidine, aspartic acid, and glutamic acid side chains as well as water molecules, (b) The catalytically active zinc atom in the enzyme alcohol dehydrogenase. The zinc atom is coordinated to the protein by one histidine and two cysteine side chains. During catalysis zinc binds an alcohol molecule in a suitable position for hydride transfer to the coenzyme moiety, a nicotinamide, [(a) Adapted from P. Nordlund et al., Nature 345 593-598, 1990.)...

Alcohols react with metal hydrides, MH, and with hydrogen halides, HX, but in very different ways. Proton transfer is involved in both reactions, but different molecules act as the proton donor and acceptor. 

A number of metals have the ability to absorb hydrogen, which may be taken into solid solution or form a metallic hydride, and this absorption can provide an alternative reaction path to the desorption of H,. as gas. In the case of iron and iron alloys, both hydrogen adsorption and absorption occur simultaneously, and the latter thus gives rise to another equilibrium involving the transfer of H,<,s across the interface to form interstitial H atoms just beneath the surface ... 

The racemization mechanism of sec-alcohols has been widely studied [16,17]. Metal complexes of the main groups of the periodic table react through a direct transfer of hydrogen (concerted process), such as aluminum complexes in Meerwein-Ponn-dorf-Verley-Oppenauer reaction. However, racemization catalyzed by transition metal complexes occurs via hydrogen transfer processes through metal hydrides or metal dihydrides intermediates (Figure 4.5) [18]. 

The mechanism of homogeneous hydrogenation catalyzed by RhCl(Ph3P)3 ° involves reaction of the catalyst with hydrogen to form a metal hydride (PPh3)2RhH2Cl (43), which rapidly transfers two hydrogen atoms to the alkene. 

At this time we do not have a firm nnderstanding of how CrCl2 and VCI3 catalyze the double bond isomerization and why other metal chlorides are less effective. We propose that CrCh" or VCh" anion plays a role in hydride transfer, facilitating donble bond isomerization. CnCh is less effective and both lactic acid and pyruvaldehyde are formed. FeCh" and MnCh" anions are ineffective in the transformation and only pyruvaldehyde is formed. The fact that only a small amount of 1,3-dihydroxyacetone is formed is consistent with the NMR observation that the compounds exist as hemiacetal dimers in ionic hquids and not as monomers. Otherwise 1,3-dihydroxyacetone would be expected as a major product (16). 

Metal hahdes in imidazolium ionic hquids offer unique enviromnents able to facihtate dehydration reactions. Under such conditions certain metal halides are able to catalyze formal hydride transfer reactions that otherwise do not occur in the ionic liquid media. We have now discovered two systems in which this transformation has been observed. The initial system involves the conversion of glucose to fractose followed by dehydration the second system involves the dehydration of glycedraldehyde dimer followed by isomerization to lactide. CrCls" anion is the only catalyst that has been effective for both systems. VCI3" is effective for the glyceraldehyde dimer system but not for glucose. 

The mechanism of such reactions using unsaturated carboxylic acids and Ru(BINAP)(02CCH3)2 is consistent with the idea that coordination of the carboxy group establishes the geometry at the metal ion.26 The configuration of the new stereocenter is then established by the hydride transfer. In this particular mechanism, the second hydrogen is introduced by protonolysis, but in other cases a second hydride transfer step occurs. 

The suggested catalytic cycle for the diamine catalysts indicates that the NH group of the diamine plays a direct role in the hydride transfer through a six-membered TS.53 A feature of this mechanism is the absence of direct contact between the ketone and the metal. Rather, the reaction is pictured as a nucleophilic delivery of hydride from ruthenium, concerted with a proton transfer from nitrogen. 

Several factors affect the reactivity of the boron and aluminum hydrides, including the metal cation present and the ligands, in addition to hydride, in the complex hydride. Some of these effects can be illustrated by considering the reactivity of ketones and aldehydes toward various hydride transfer reagents. Comparison of LiAlH4 and NaAlH4 has shown the former to be more reactive,63 which is attributed to the greater... 

Kaim, W. Thermal and Light Induced Electron Transfer Reactions of Main Group Metal Hydrides and Organometallics. 169, 231-252 (1994). 

More recently homogeneous hydrogenation catalysts, such as RhCl(Ph3P)3, have been developed which are soluble in the reaction medium. These are believed to transfer H to an alkene via a metal hydride intermediate they, too, lead to a considerable degree of SYN stereoselectivity in hydrogen addition. 

Molecular hydrogen is rather unreactive at ambient conditions, but many transition and lanthanide metal ions are able to bind and therefore activate H2, which results in transformation into H (hydride) 11 (hydrogen radical) or H+ (proton), and subsequent transfer of these forms of hydrogen to the substrate.7,8 In this context, not only metal hydride but also dihydrogen complexes of transition metal ions, play a key role,9 10 especially since the first structural characterization of one of these species in 1984 by Kubas.11... 

In this latter hydridic route for hydrogen transfer from alcohols to ketones, two additional possibilities can be considered one involving a metal hydride arising purely from a C—11 (path 2a), and another in which it may originate from both the O—11 and C—I I (path 2b) in this case any of the hydrides on the metal may add to the carbonyl carbon. 

Rhin(bpy)3]3+ and its derivatives are able to reduce selectively NAD+ to 1,4-NADH in aqueous buffer.48-50 It is likely that a rhodium-hydride intermediate, e.g., [Rhni(bpy)2(H20)(H)]2+, acts as a hydride transfer agent in this catalytic process. This system has been coupled internally to the enzymatic reduction of carbonyl compounds using an alcohol dehydrogenase (HLADH) as an NADH-dependent enzyme (Scheme 4). The [Rhin(bpy)3]3+ derivative containing 2,2 -bipyridine-5-sulfonic acid as ligand gave the best results in terms of turnover number (46 turnovers for the metal catalyst, 101 for the cofactor), but was handicapped by slow reaction kinetics, with a maximum of five turnovers per day.50... 

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