Anionic metal hydrides

Table 7.3 Rate constants for reaction of anionic metal hydrides with n-BuBr (THF solvent at 26°C) [25].

Unsaturated organic compounds are reduced by various neutral and anionic metal hydrides with or without transition metal catalysts. Certain chiral transition metal complexes, when used as catalysts, exhibit unique regio- and stereoselectivity. 

Reaction of Hydrides. The donor semi-bridging interaction of the carbonyls in 1 might render them more susceptible to nucleophilic attack by such reagents as hydrides. We reasoned that the formation of n2-formyl species, e.g., IV, or alternatively, anionic metal hydrides, e.g., V, might result. The actual reaction of l with Et3BH is consistently more complex than anticipated (-3.31. 

Although Hg hydrides are claimed, most studies relate to Zn and Cd, where polymeric binary compounds, (MHj), are isolated as well as anionic metal hydrides, e.g.. 

The anionic metal hydride takes up a proton from the aqueous medium to yield a dihydride (step ) a known example is H2Fe(CO)4 [5 a, 12]. In the final step , hydrogen elimination through replacement by CO closes the catalytic cycle, thus furnishing the water-gas shift reaction of eq. (2). 

M(C0)5X , and H2 in the presence of a general base to provide anionic metal hydrides. This process was shown to be first-order in both metal complex and dihydrogen and was not inhibited by addition of carbon monoxide. Consistent with the rds in catalysis being formation of the metal hydride intermediate, the metal catalyzed reaction of RX/CO2/H2 to provide HCOOR is not inhibited by CO. The well-established formation of metalloformate, M(C0)s02CH", from M(C0)5H and CO2 is followed by a less facile process involving the reaction of the metalloformate with RX. This latter reaction is first-order in both metal complex and alkyl halide and is inhibited by carbon monoxide. 

Stoichiometric Hydride Tranter Reactivity of Anionic Metal Hydride Complexes 57... 

These anionic metal hydrides also readily react with acyl chlorides at 25 °C, converting them to aldehydes [28]. An advantage is that in many cases further reduchon of the aldehyde to alcohols, which is a problem with some borohydride reagents, is avoided with these metal hydrides. The hydride in (CO)5CrH or (C0)5WH readily exchanges with deuterated alcohols, so use of the hydride in the presence of CH3OD provides a means of converting acyl chlorides or alkyl haUdes to deuterated products, as shown in Equahon 3.5 [29]. 

Catalytic Hydrogenation of Ketones with Anionic Metal Hydrides... 

Grey RA, Pez GP, Wallo A (1981) Anionic metal hydride catalysts.2. Application to the hydrogenation of ketones, aldehydes, carboxylic-acid esters, and nitriles. J Am Chem Soc 103 7536-7542... 

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