Hydride formation mechanisms

B. Vigeholm, K. Jensen, B. Larsen, A.S. Pedersen, Elements of hydride formation mechanisms in nearly spherical magnesium powder particles, J. Less-Comm. Metals 131 (1987) 133-141. 

Water as an impurity is known to promote the breakaway corrosion of a number of metals in addition to iron in CO2 the effect has been reported for magnesium (hydrocarbons have more effect on the oxidation of this metal), beryllium, zirconium and sodium. In the latter case water is known to convert the oxide to deliquescent NaOH but acceleration of beryllium oxidation probably results from hydride formation and mechanical damage to the oxide. 

The theoretical hmit of 5.4% (NaAlH4+2 mol% TiN) for the two subsequent decomposition reactions is in both cases only observed in the first cycle. The reason for the decrease in capacity is stiU unknown and litde is known about the mechanism of alanate activation via titanium dopants in the sohd state. Certainly, the ease of titanium hydride formation and decomposition plays a key role in this process, but whether titanium substitution in the alanate or the formation of a titanium aluminum alloys, i.e., finely dispersed titanium species in the decomposition products is crucial, is stiU under debate [41]. 

The variation of the substituent pattern of the introduced silane provides further insight into the reaction mechanism of the CO activation process of scheme 2 (Table 1) The yield of ju-carbyne-complex (O-attack of the silane) compared to silyl hydride formation (Mn-attack of the silane) is a function of the Lewis-acidity of the silane. However, even with the strongly acidic HSiCl3 as reagent, the product ratio 12/13 is still 1 9. 

It now appears that at least two mechanisms exist for the base-promoted homogeneous water gas shift reaction, differing in the method of hydride formation. The "associative mechanism", first proposed by Pettit and co-workers (1,4), involves nucleophilic attack on a carbonyl ligand and it has two variations. 

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 formation was a fundamental step in the mechanism, and indeed PtHCl(PPh3)2 species were isolated from the reactant solutions. However, their formation was not the rate-determining step, since the same rate of hydrogenation was observed with either PtCl2(PPli3)2 or PtHCl(PPh3)2 complexes. 

Y. Chen, J.S. WiUiams, Formation of metal hydrides by mechanical alloying, J. Alloys Compd. Ill (1995) 181-184. 

The first mechanism appears to be the better basis for describing most of the results referred to by Cramer (56). It will, however, be noted that the addition-elimination mechanism requires that the metal catalyst be supplied as a metal hydride. Where the catalyst has not been supplied in this form, the reaction has usually been carried out in the presence of reagents known to convert transition metal compounds to hydrides (e.g. protonic acids, alcohols or hydrogen). These substances are known as co-catalysts and, where they have been used, induction periods have been encountered which are consistent with hydride formation as required in mechanism (a), but which would not be expected for (b). 

Thymidine-specific depyrimidination of DNA by this and other Ru(lV) 0x0 complexes, e.g. electrocatalytically by [Ru(0)(py)(bpy)2] Vaq. formate buffer was studied and related to their Ru(IV)/Ru(ll) redox potentiis [664]. Oxidation of formate and of formic acid to CO by stoich. aT-[Ru(0)(py)(bpy)2] Vwater was studied kinetically, and a two-electron hydride transfer mechanism proposed [665]. 

The formation of formate esters in the hydroformylation reaction (90, 64) may be explained by a CO-alkoxide insertion reaction as well as by the CO-hydride insertion mechanism mentioned above. Aldehydes formed in the hydroformylation reaction can be reduced by cobalt hydrocarbonyl (27) presumably by way of an addition of the hydride to the carbonyl group (90, 2). If the intermediate in the reduction is an alkoxycobalt carbonyl, carbon monoxide insertion followed by hydrogenation would give formate esters (90, 64). 

D Ulivo, A. 2004. Chemical vapor generation by tetrahydroborate (III) and other borane complexes in aqueous media—A critical discussion of fundamental processes and mechanisms involved in reagent decomposition and hydride formation. Spectrochim. Acta B 59 793-825. 

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... 

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