Reductions with Organic Compounds

T. L. Ho, Reduction of organic compounds with low-valent species of Group IV B, V B and VIB metals. Synthesis 1979, 1. 

J.M. Lalancette et al., Reduction of Functional Groups with Sulfurated Borohydrides, Synthesis 1972, 526. J. Malek u. M. Cerny, Reduction of Organic Compounds by Alkoxyaluminohydrides, Synthesis 1972, 217. S.-C. Chen, Molecular Rearrangements in Lithium Aluminium Hydride Reduction, Synthesis 1974, 691. 

C. F. Lane, Reductions of Organic Compounds with Dihorane, Chem. Reviews 76, 773 (1976). 

Most of the CL6 found in nature is a result of domestic and industrial emissions (Steven et al. 1976). Interaction of +6 chromic oxide, dichromate, or chromate compounds with organic compounds can result in reduction to the comparatively less toxic trivalent form (Taylor and Parr 1978). 

Electrochemical reduction of organic compounds in aprotic solvents is a very suitable way for preparing reactive anionic intermediates of great synthetic value [148]. Carbanions electrogenerated from organic halides readily react with various electrophiles. 

Iron was one of the first metals employed for the reduction of organic compounds over 130 years ago. It is used in the form of filings. Best results are obtained with 80 mesh grain [765]. Although some reductions are carried out in dilute or concentrated acetic acid the majority are performed in water in the presence of small amounts of hydrochloric acid, acetic acid or salts such as ferric chloride, sodium chloride (as little as 1.5-3%) [765], ferrous sulfate [766] and others. Under these conditions iron is converted to iron oxide, Fe304. Methanol or ethanol are used to increase the solubility of the organic material in the aqueous medium [766] (Procedure 34, p. 213). 

Divalent chromium salts show very strong reducing properties. They are prepared by reduction of chromium(III) compounds with zinc [187] or a zinc-copper couple and form dark blue solutions extremely sensitive to air. Most frequently used salts are chromous chloride [7SS], chromous sulfate [189], and less often chromous acetate. Reductions of organic compounds are carried out in homogeneous solutions in aqueous methanol [190], acetone [191], acetic acid [192], dimethylformamide [193] or tetrahydrofuran [194] (Procedure 37, p. 214). 

The astronomical number of reductions of organic compounds described in the literature makes an exhaustive survey impossible The most complete treatment is published in Volumes 4/lc and 4/ld of the Houben-Weyl compendium Methoden der Organischen Chemie. Other monographs dealing mainly with sections of this topic are listed in the bibliography (p.257). 

A metal ion (Lewis acid) has an influence on the reduction of organic compounds in aprotic solvents. Namely, the metal ion, M+, forms ion-pairs with Q and Q2 and shifts the first and second waves, to greater or lesser extents, to positive po-... 

The main methods of reducing ketones to alcohols are (a) use of complex metal hydrides (b) use of alkali metals in alcohols or liquid ammonia or amines 221 (c) catalytic hydrogenation 14,217 (d) Meerwein-Ponndorf reduction.169,249 The reduction of organic compounds by complex metal hydrides, first reported in 1947,174 is a widely used technique. This chapter reviews first the main metal hydride reagents, their reactivities towards various functional groups and the conditions under which they are used to reduce ketones. The reduction of ketones by hydrides is then discussed under the headings of mechanism and stereochemistry, reduction of unsaturated ketones, and stereochemistry and selectivity of reduction of steroidal ketones. Finally reductions with the mixed hydride reagent of lithium aluminum hydride and aluminum chloride, with diborane and with iridium complexes, are briefly described. 

Carbon brings about reduction of arsenious oxide at a temperature below red heat,11 while in carbon monoxide reduction begins at 60° C.12 The numerous reactions of arsenious oxide with organic compounds are described in Vol. XI, Part II, of this Series. Silicon tetrachloride heated for 30 hours at 270° to 280° C. with the oxide yields arsenic trichloride,13 whilst silicochloroform when heated with the oxide in the presence of aqueous sodium hydroxide-or sodium hydrogen carbonate... 

These results indicate that Cr(VI) is reduced only in the presence of Fe(II) or sulfide as reductants. Reduction by organic compounds probably does not occur fast enough under the conditions of the lake. However, calculations indicate that a slow removal process, with a half-time of 100-230 days, removes Cr(VI) from the water column in both oxic and suboxic waters (83). It is not known whether this process is reduction, possibly by organic material, or a weak sorptive reaction. 

Baker s yeast reduction of organic compounds, especially carbonyl compounds, is an extremely useful method of obtaining chiral products255-257. Recently, much effort has been expended to improve the ee obtained in this process. In one very useful example, l-acetoxy-2-alkanones have been reduced enantioselectively into (5 )-l-acetoxy-2-alkanols in 60-90% yields and with 95-99% ee258. The reaction readily occurs in a variety of solvents, both aqueous and nonaqueous. The reduction is fairly selective and so may be brought about in the presence of a-amide, ether, ester and other acid functional groups, in reasonable yields and with excellent ee (equation 65)259 -261. Thus, in the synthesis of the C-13 side chain of taxol, the key step was the reduction of a w-ketoester to the corresponding alcohol in 72% overall yield (equation 66)262. 

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