Reduction of Ketones in Preference to Aldehydes

Hydrostannation of carbonyl compounds with tributyltin hydride is promoted by radical initiation and Lewis or protic acid catalysis.The activation of the carbonyl group by the acidic species allows the weakly nucleophilic tin hydride to react via a polar mechanism. Silica gel was a suitable catalyst allowing chemoselective reduction of carbonyl groups under conditions that left many functional groups unchanged. Tributyltin triflate generated in situ from the tin hydride and triflic acid was a particularly efficient catalyst for the reduction of aldehydes and ketones with tributyltin hydride in benzene or 1,2-di-chloromethane at room temperature. Esters and ketals were not affected under these conditions and certain aldehydes were reduced selectively in preference to ketones. 

Much emphasis has been placed on the selectivity of quaternary ammonium borohydrides in their reduction of aldehydes and ketones [18-20]. Predictably, steric factors are important, as are mesomeric electronic effects in the case of 4-substituted benzaldehydes. However, comparison of the relative merits of the use of tetraethyl-ammonium, or tetra-n-butylammonium borohydride in dichloromethane, and of sodium borohydride in isopropanol, has shown that, in the competitive reduction of benzaldehyde and acetophenone, each system preferentially reduces the aldehyde and that the ratio of benzyl alcohol to 1-phenylethanol is invariably ca. 4 1 [18-20], Thus, the only advantage in the use of the ammonium salts would appear to facilitate the use of non-hydroxylic solvents. In all reductions, the use of the more lipophilic tetra-n-butylammonium salt is to be preferred and the only advantage in using the tetraethylammonium salt is its ready removal from the reaction mixture by dissolution in water. 

An aldehyde is reduced in preference to a ketone using sodium borohydride in a solution containing a thiol for instance reduction of an equimolar mixture of nonanal and nonan-2-one using sodium borohydride and t-butylthiol gave 94% nonan-l-ol and 6% nonan-2-ol. ... 

G. Sekar of the Indian Institute of Technology Madras, in Chennai, established (Tetrahedron Lett. 2008, 49, 2457) a convenient procedure for oxidizing primary alcohols such as 26 to the acid 27. Secondary alcohols were oxidized to ketones. AUyhc and benzylic alcohols could be oxidized in preference to saturated alcohols. Tobin J. Marks ofNorthwestem University devised (Organic Lett. 2008,10, 317) a La catalyst for the oxidative amination of aldehydes. In its present incarnation, excess aldehyde served as the reductant. If a less expensive reductant could be found, this would be a very useful procedure, avoiding the carboxyhc acid activation usually required for amide formation. 

An alternate method of producing the 21-hydroxy-20-ketone consists in lithium aluminum hydride reduction of the dimethyl acetal, hydrolysis to the 20-hydroxy-21-aldehyde and rearrangement, preferably via the bisulfite addition product... 

Besides direct reduction, a one-pot reductive amination of aldehydes and ketones with a-picoline-borane in methanol, in water, and in neat conditions gives the corresponding amine products (Scheme 8.2).40 The synthesis of primary amines can be performed via the reductive amination of the corresponding carbonyl compounds with aqueous ammonia with soluble Rh-catalyst (Eq. 8.17).41 Up to an 86% yield and a 97% selectivity for benzylamines were obtained for the reaction of various benzaldehydes. The use of a bimetallic catalyst based on Rh/Ir is preferable for aliphatic aldehydes. 

In situ production of phosphine-free CuH from CuCl or CuOAc (0.3-1.0 equivalents), in the presence of an excess of PhMe2SiH in DM I at room temperature, displays a remarkable preference for reductions of aryl ketones (e.g., 15) over aliphatic ones such as 16 (Eq. 5.23) [46]. Reactions require a day or more to reach completion, concentrations of 0.5 M notwithstanding, but yields have been uniformly good (77-88%) for the few cases examined. Aldehydes, however, show no such selectivity and are reduced to the corresponding primary alcohols, albeit in high yields. 

Largely, the same principles apply for water treatment. Consequently, activated carbon is suitable for organic molecules that are nonpolar and of high molecular weight. Trichloroethylene, benzene, ethylbenzene, toluene, and xylene are easily adsorbed in the gas phase when activated carbon, for instance, is used. On the other hand, adsorption is not preferably selected in applications in relation to aldehydes, ketones, and alcohols. In a successful application, reduction in emissions from 400-2000 ppm to under 50 ppm can be achieved (EPA, 1999), especially for VOCs with boiling points between 20 -and 175 °C. 

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