Nucleophiles complex metal hydrides

A less powerful complex metal hydride is Na BH4 which will reduce aldehydes and ketones only, and does not attack carboxylic acid derivatives nor does it—as Li AlH4 does—attack NO2 or C=N present in the same compound. It has the great advantage of being usable in hydroxylic solvents. A wide variety of other reagents of the MH4 , MH3OR , MHjfORlj type have been developed their relative effectiveness is related to both the nucleophilicity and size of MH4 , etc. 

A number of complex metal hydrides such as lithium aluminium hydride (LiAlH4, abbreviated to LAH) and sodium borohydride (NaBHj) are able to deliver hydride in such a manner that it appears to act as a nucleophile. We shall look at the nature of these reagents later under the reactions of carbonyl compounds (see Section 7.5), where we shall see that the complex metal hydride never actually produces hydride as a nucleophile, but the aluminium hydride anion has the ability to effect transfer of hydride. Hydride itself, e.g. from sodium hydride, never acts as a nucleophile owing to its small size and high charge density it always acts as a base. Nevertheless, for the purposes of understanding the transformations. 

Whilst the complex metal hydride is conveniently regarded as a source of hydride, it never actually produces hydride as a nucleophile, and it is the aluminium hydride anion that is responsible for... 

The reverse reaction, i.e. reduction, is also indicated in the scheme, and may be compared with the chemical reduction process using complex metal hydrides, e.g. LiAlPLj or NaBH4, namely nucleophilic addition of hydride and subsequent protonation (see Section 7.5). The reduced forms NADH and NADPH are conveniently regarded as hydride-donating reducing agents (see Box 7.6). We also noted that there were stereochemical features associated with these coenzymes (see Box 3.14). During a reduction... 

The reactions of complex metal hydrides occur by an attack of the nucleophilic hydride ion on an electrophilic center.1 Aromatic nitrogen heterocycles in which the nitrogen has contributed only one electron to the -system (1) are electrophilic as compared with benzene, and have been shown to undergo reduction by the active reducing agent, lithium aluminum hydride. The nitrogen heterocycles in which the heteroatom has contributed two electrons to the 77-system (2) are electron-rich as compared with benzene and usually do not undergo reaction by reduction with complex metal hydrides.2 A combination of these two structural features, as in oxazoles (3), usually induces sufficient electrophilicity to allow attack by the hydride ion and reduction. 

Reactions in which complex metallic hydrides (LiAlH4, etc.) act as sources of nucleophilic hydride are of considerable synthetic value (Gaylord, 1956 Wiberg and Amberger, 1971). In the case of nitro-aromatics, some interesting hydride displacements [e.g. (38)] have... 

An attack of a nucleophilic transition metal hydride on coordinated CO has also been observed by Labinger and Wong, who treated carbonyls of Ct, Mo, W, Mn, Fe, Ku and Co with Cpi Nbllj. which is of a lower hydridic nature than the main group hydrides or group IV hydridic complexes (54]. Small amounts of C C3 hydrocarbons are formed. However, when the reaction of CpjNbH with Cr(CO)(( is carried out under H], only ethane t formed. Carbene intermediates are proposed to account for this selectivity (Scheme 14). 

Alanes and boranes are initially electrophilic rather than nucleophilic, and coordinate onto free electron pairs in the molecule prior to hydride delivery. The chemistry of metal hydrides has been extensively reviewed, as has the reduction of nitrogen heterocycles with complex metal hydrides. Generally, NBH reductions, carried out in alcohols, produce tetrahydropyridines, whilst LAH generates mainly di-hydropyridines. 

The metal hydrides exhibit different reducing properties. The complex metal hydrides (LiAlH4, NaBH4) are nucleophilic in character hydride is transported from the complex anion to the electron-deficient centers of the functional groups. Another group of metal hydrides (boranes, alanes) are strong Lewis acids they interact with centers that are relatively richer in electrons. The selectivity of the reduction can be improved and the scope of its application can be extended by the joint use of these two types (mixed hydrides). 

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