Mechanism, metal hydride compounds

The plausible mechanism of this ruthenium-catalyzed isomerization of allylic alcohols is shown in Scheme 15. This reaction proceeds via dehydrogenation of an allylic alcohol to the corresponding unsaturated carbonyl compound followed by re-addition of the metal hydride to the double bond. This mechanism involves dissociation of one phosphine ligand. Indeed, the replacement of two triphenylphosphines by various bidentate ligands led to a significant decrease in the reactivity.37... 

Some experimental evidences are in agreement with this proposed mechanism. For example, coordinating solvents like diethyl ether show a deactivating effect certainly due to competition with a Lewis base (149). For the same reason, poor reactivity has been observed for the substrates carrying heteroatoms when an aluminum-based Lewis acid is used. Less efficient hydrovinylation of electron-deficient vinylarenes can be explained by their weaker coordination to the nickel hydride 144, hence metal hydride addition to form key intermediate 146. Isomerization of the final product can be catalyzed by metal hydride through sequential addition/elimination, affording the more stable compound. Finally, chelating phosphines inhibit the hydrovinylation reaction. 

A reaction which is rather new and not mentioned in older textbooks is the so-called o-bond metathesis. It is a concerted 2+2 reaction immediately followed by its retrograde reaction giving metathesis. Both late and early transition metal alkyls are prone to this reaction, but for d° early transition metals there is no other mechanism than o-bond metathesis at hand. Many similar reactions such as the reaction of metal alkyls with other HX compounds could be described as if they would follow this pathway, but the use of the term o-bond metathesis is restricted to those reactions in which one reacting species is a metal hydrocarbyl or metal hydride and the other reactant is a hydrocarbon or dihydrogen. In Figure 2.30 the reaction has been depicted. 

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). 

Metal Hydrides. The simplest reactions in this group are the various catalytic reduction reactions of carbon monoxide. Methane or higher hydrocarbons, methanol or higher alcohols, and a variety of other oxygenated organic compounds may be formed, depending upon the catalyst and reaction conditions (23). There is little evidence about the mechanism of these reactions, but the initial step in every example is probably a carbon monoxide insertion into a metal hydride, followed by reduction reactions. 

Metal Hydrides. Metal hydrides generally react readily with acetylenes, often by an insertion mechanism. Cobalt hydrocarbonyl gives complicated mixtures of compounds with acetylenes. The only products which have been identified so far are dicobalt hexacarbonyl acetylene complexes (34). Greenfield reports that, under conditions of the hydroformy lation reaction, acetylenes give only small yields of saturated monoaldehydes (30), probably formed by first hydrogenating the acetylene and then reacting with the olefin. Other workers have identified a variety of products from acetylene, carbon monoxide, and an alcohol with a cobalt catalyst, probably cobalt hydrocarbonyl. The major products observed were succinate esters (74,19) and succinate half ester acetals (19). 

Of course, many other nontransition metal hydrides which reduce carbonyl compounds are known but there is little conclusive evidence on the mechanism of these reactions. 

The main methods of reducing ketones to alcohols are (a) use of complex metal hydrides (b) use of alkali metals in alcohols or liquid ammonia or amines 221 (c) catalytic hydrogenation 14,217 (d) Meerwein-Ponndorf reduction.169,249 The reduction of organic compounds by complex metal hydrides, first reported in 1947,174 is a widely used technique. This chapter reviews first the main metal hydride reagents, their reactivities towards various functional groups and the conditions under which they are used to reduce ketones. The reduction of ketones by hydrides is then discussed under the headings of mechanism and stereochemistry, reduction of unsaturated ketones, and stereochemistry and selectivity of reduction of steroidal ketones. Finally reductions with the mixed hydride reagent of lithium aluminum hydride and aluminum chloride, with diborane and with iridium complexes, are briefly described. 

Golubeva, A.V. (1982) Synthesis of hydride compounds of aluminum using binary hydrides of alkaline metals in conditions of mechanical activation and their utilization for synthesis of non-solvated aluminum hydride. Ph.D. Thesis in the field of chemistry, Moscow, 162 p. 

One of new methods of magnesium, transition and rare-earth metals hydrides and their compounds obtaining is mechanical-chemical method. Numerous quantity of works are devoted to the improving kinetic, sorption properties of hydrides which were treated mechanically or were obtained by this method in hydrogen medium under pressure [1-7]. Great consideration is given to the influence of dispersity on phase equilibrium. At the same time the investigation of mechanical... 

The mechanism of hydride formation of ZrMn and TiMn, is related to segregation and preferential oxidation of the Mn on the surface and precipitation of metallic Zr and Ti at the subsurface below the oxidized Mn. The Zr and Ti metallic precipitates catalyze the Hj 2 H reaction. In addition, the Zr and Ti precipitates themselves may form hydrides and spill over atomic H to the intermetallic compound lying below. 

---