Palladium lithium metal reduction

Common catalyst compositions contain oxides or ionic forms of platinum, nickel, copper, cobalt, or palladium which are often present as mixtures of more than one metal. Metal hydrides, such as lithium aluminum hydride [16853-85-3] or sodium borohydride [16940-66-2] can also be used to reduce aldehydes. Depending on additional functionahties that may be present in the aldehyde molecule, specialized reducing reagents such as trimethoxyalurninum hydride or alkylboranes (less reactive and more selective) may be used. Other less industrially significant reduction procedures such as the Clemmensen reduction or the modified Wolff-Kishner reduction exist as well. 

By reduction of aldehydes and ketones Aldehydes and ketones are reduced to the corresponding alcohols by addition of hydrogen in the presence of catalysts (catalytic hydrogenation). The usual catalyst is a finely divided metal such as platinum, palladium or nickel. It is also prepared by treating aldehydes and ketones with sodium borohydride (NaBH4) or lithium aluminium hydride (LLAIH4). Aldehydes yield primary alcohols whereas ketones give secondary alcohols. 

The thermal treatment of the lithiated alumina aimed to decompose unreacted butyl lithium resulted in reduction of palladium (see catalysts N°3 and 4). In this case undesired reduction of palladium was attributed to the presence of highly dispersed metallic lithium or lithiumhydride formed from C HgLi during the thermal treatment. 

Methods (i) and (ii) require palladium(II) salts as reactants. Either palladium acetate, palladium chloride or lithium tetrachloropalladate(II) usually are used. These salts may also be used as catalysts in method (iii) but need to be reduced in situ to become active. The reduction usually occurs spontaneously in reactions carried out at 100 °C but may be slow or inefficient at lower temperatures. In these cases, zero valent complexes such as bis(dibenzylideneacetone)palladium(0) or tetrakis(triphenylphos-phine)palladium(O) may be used, or a reducing agent such as sodium borohydride, formic acid or hydrazine may be added to reaction mixtures containing palladium(II) salts to initiate the reactions. Triarylphosphines are usually added to the palladium catalysts in method (iii), but not in methods (i) or (ii). Normally, 2 equiv. of triphenylphosphine, or better, tri-o-tolylphosphine, are added per mol of the palladium compound. Larger amounts may be necessary in reactions where palladium metal tends to precipitate prematurely from the reaction mixtures. Large concentrations of phosphines are to be avoided, however, since they usually inhibit the reactions. 

Catalytic reductions over platinum or palladium, which are usually quantitative methods for the identification of organic peroxides, are problematic. Little of the expected 1,2-diol is obtained because the dioxetane fragments into its carbonyl products due to metal catalysis." However, lithium aluminium hydride reduction under subambient conditions affords the expected 1,2-diol quantitatively. Again, the sterically hindered dioxetane (9) is an exception. Here zinc in acetic acid proved successful. ... 

Addition of a toluene solution of the cuprate, Cu2Li2(p-tolyl)4-2Et20 (1), to a solution of palladium(II) acetate in toluene leads to the in situ formation of a thermally unstable organocopper complex in which the lithium atom of 1 has been replaced by the more electronegative precious metal. Subsequent reductive elimination of the organic tolyl group from the unstable bimetallic complex in the presence of silica affords supported bimetallic particles, which without further treatment are an active catalytic system. (Equation 3)... 

Metal hydrides such as lithium tetrahydridoaluminate have been used for reduction of aliphatic and alicyclic nitro to amino compounds in general, 2 moles of this aluminate are applied per mole of nitro compound. However, aromatic nitro compounds are usually reduced thereby only to the azo stage. Indeed, apart from special cases, the reduction of nitro compounds by lithium tetrahydridoaluminate has only slight practical importance.27 Sodium tetra-hydridoborate had been recommended for reduction of o-nitrophenol in the presence of a palladium-charcoal catalyst.73... 

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