Mechanism, metal hydride mixed

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. 

The mechanisms of these reactions are probably complex but radical processes seem likely. The final products of insertion into the metal metal bond are actually trinuclear species. Insertion into terminal metal hydride or metal alkyl bonds could, in principle, generate mixed-metal dimers. In practice, aside from insertion into the metal halogen bond, such as that shown in equation (65), few such reactions have been reported. 

Mechanical mixing via high-energy ball-milling is a sustainable experimental technique, which is generally used for the preparation of metal hydrides and other organometallic materials.Recently, it has also been used in organic synthesis and some important transformations are reviewed in this section. 

The ASi used in this route can be prepared by either conventional high temperature routes or via low temperature reaction of the element. Si, with the metal hydrides [44]. The reaction to prepare clathrates with hydrogen encapsulation was performed either in solution (dioctyl ether) at 250 °C or as a solid state reaction at 200 °C under vacuum. The solid state reaction is prepared by mechanical mixing (or ball-milling) of the two solids, ASi and NH4X, pressing them into a pellet which is then placed into a furnace and heated under vacuum to 200 °C. The solution reaction uses the same precursors, but they are dispersed in a solvent and... 

To solve this problem, the composite hydrogen storing material is obtained by carrying out a mechanical crushing and mixing of NH3BH3 and a metal hydride [149]. Novel alkali-metal... 

The C—H bond activation of benzene leading to equimolar amounts of metal hydride and metal phenyl products resembles the oxidative addition mechanism by late transition metal complexes, in which a single, mixed metal hydride phenyl product forms. The intriguing reactivity and its generality for rare-earth metals prompted a detailed mechanistic study. Inspired by previous reports of inverse-sandwich rare-earth metal arene com-plexes and taking into account the capacity ofbenzene to accept elec-... 

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