Complex hydrides reducing agents

There has been an increasing number of reports of the utilization of zinc hydride complexes as precursors in organic transformations and as reducing agents. Zinc hydrides have been less well investigated than the other group 13 hydrides due to the formation of polymeric and oligomeric species that are difficult to characterize. 

A. Hajos, Reduction with Inorganic Reducing Agents - Metal Hydrides and Complex Hydrides, in Methoden Org. Chem. (Houben-Weyl) 4th ed., 1952-, Reduction Part II (H. Kropf, Ed.), Bd. 4/ld, 1, Georg Thieme Verlag, Stuttgart, 1981. 

Common reducing agents are hydrogen in the presence of metallic or complex catalysts (e.g. Ni, Pd, Pt, Ru, Rh), hydrides (e.g. alanes, boranes, LIAIH, NaBHJ, reducing metals (e.g. Li, Na, Mg, Ca, Zn), and low-valent compounds of nitrogen (e.g. NjHj, NjHJ, phosphorus (e.g. triethyl phosphite, triphenyiphosphine), and sulfur (e.g. HO-CHj-SOjNa = SFS, sodium dithionite = Na S O. ... 

Hajos, A. 1979, Complex Hydrides and Related Reducing Agents in Organic Synthesis, Elsevier Amsterdam New York... 

A noteworthy development is the use of KH for complexing alkylboranes and alkoxyboranes to form various boron hydrides used as reducing agents in the pharmaceutical industry. Potassium tri-j -butylborohydride [54575-50-7] KB(CH(CH2)C2H )2H, and potassium trisiamylborohydride [67966-25-0] KB(CH(CH2)CH(CH2)2)3H, are usefiil for the stereoselective reduction of ketones (66) and for the conjugate reduction and alkylation of a,P-unsaturated ketones (67). 

The alkali metal halides, particularly NaCl and KCl, find extensive application in industry (pp. 71 and 73). The hydrides are frequently used as reducing agents, the product being a hydride or complex metal hydride depending on the conditions used, or the free element if the hydride is unstable. Illustrative examples using NaH are ... 

Although estrone and estradiol (26) have both been isolated from human urine, it has recently been shown that it is the latter that is the active compound that binds to the so-called estrogen receptor protein. Reduction of estrone with any of a large number of reducing agents (for example, any of the complex metal hydrides) leads cleanly to estradiol. This high degree of stereoselectivity to afford the product of attack at the alpha side of the molecule is characteristic of many reactions of steroids. 

E.R.H. Walker, The Functional Group Selectivity of Complex Hydride Reducing Agents, Soc. Reviews 5, 23 (1976). 

Then we established that borane and its derivatives are acidic reducing agents (31-34), in contrast to the basic reducing agents, the complex hydrides. 

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