Reducing simple metal hydrides

Although methods both for reduction and for oxidation are weU developed, there is always room for improvement. While ketones are usually reduced using metal hydrides, hydrogen gas is much less expensive on scale. Charles P. Casey of the University of Wisconsin has devised J. Am. Chem. Soc. 2007, 129, 5816) an Fe-based catalyst that effects the transformation of 1 to 2. Note that the usually very reactive monosubstituted alkene is not reduced and does not migrate. Takeshi Oriyama of Ibaraki University has developed a catalyst, also Fe-based (Chemistry Lett. 2007, 38) for reducing aldehydes to ethers. Using this approach, an alcohol such as 3 can be converted into a variety of substituted benzyl ethers, including 5. Simple aliphatic aldehydes and alcohols also work well. 

Increasing effort has been applied to develope asymmetric transfer hydrogenations for reducing ketones to alcohols because the reaction is simple to perform and does not require the use of reactive metal hydrides or hydrogen. Ruthenium-catalyzed hydrogen transfer from 2-propanol to ketones is an efficient method for the preparation of secondary alcohols. 

Pyrimidine and simple alkyl derivatives are not reduced by NaBH4. Lithium aluminum hydride converts pyrimidines to di- or tetra-hydro derivatives. In general, electron-withdrawing substituents promote reduction of the ring, while electron-releasing substituents have the opposite effect. The metal hydride may act as a base and abstract a proton from the a-position in a substituent, in which case the anionic substrate may resist reduction in the ring. 

Today, metal hydrides can be synthesised in purchasable equipment. Principally a solution which contains AsCl3 or SbCl3 is pre-reduced, with for example KI and ascorbic acid, and then reduced with sodium borohydride to arsine or stibine. This is done in a closed system and the method is simple (Fig. 13). The determination of As via arsine in iron ores is now available as draft proposal ISO/TCI 02. The method described for the concentration range of 0.0001—0.05 mass% is as follows. 

Because so many of the recent developments in reduction methodology have involved metal hydride reducing agents, this chapter inevitably has an old-fashioned flavor. Most of the methods described below originated many years ago, and many of them do not figure prominently in the current literature. However, this is not to say that they have no value. In many cases they are economical and suitable for large scale use, and it is quite likely that the best way to make substantial amounts of a simple aldehyde (especially an aromatic one) will be found in this chapter. Moreover, there are some cases where recent improvements, which may not be widely appreciated, have given a classical method a new lease of life. The reader is directed particularly to the amine-modified Rosenmund described in Section 1.12.3. 

Oxidation and hydrolysis reactions are two common methods of producing oxide films. In this class of reaction, the metal precursor contains metal atoms in a reduced valence state, usually a metal hydride or metal halide. The oxidizing agent is an oxygen containing gas such as O2, O3, CO2, H2O, or N2O. The reaction is a simple oxidation scheme. Examples of such reactions are ... 

Diphenyltin dihydride reduces aldehydes and simple ketones to alcohols in yields of about 80%, and the reaction differs from reduction with a metal hydride in that a >C=0 + (CsHjljSnHj ------------------------- CHOH + CsHsljSn... 

Brown and his co-workers have compared representative simple and complex metal hydrides in order to assess their capabilities for the hydrogenolysis of alkyl halides. Of the reducing agents studied, lithium triethylborohydride is the most powerful, and is the reagent of choice for the hydrogenolysis of alkyl... 

Hydrides are compounds that contain hydrogen (qv) in a reduced or electron-rich state. Hydrides may be either simple binary compounds or complex ones. In the former, the negative hydrogen is bonded ionicaHy or covalendy to a metal, or is present as a soHd solution in the metal lattice. In the latter, which comprise a large group of chemical compounds, complex hydridic anions such as BH, A1H, and derivatives of these, exist. 

Hydrides — True hydrides (i.e., those in which the hydrogen is in its anionic or most reduced form) are salt-like compounds in which the hydrogen is combined with alkali metals, either alone as simple hydrides or in association with other elements as complex hydrides. Hydrides react with water to release hydrogen. 

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