Hydrogenation, of aldehydes and ketones

Hydrogenation of the carbonyl function is an important synthetic transformation and can be catalyzed by complexes of several transition metals including -among others- Co, Rh, Ru, Ir, and Os. In aqueous organometallic catalysis the first examples were given by the hydrogenation of water-soluble 2-oxo-carboxylic acids, 1,3-dihydroxyacetone and fmctose [47-54], later the same substrates were also used for testing new catalysts [29], 

Several catalysts have been found to show considerable activity in aqueous systems for hydrogenation of aliphatic and aromatic aldehydes, 

One of the major challenges is the selective hydrogenation of unsaturated aldehydes to unsaturated alcohols which attracted much interest [182], The highly selective hydrogenation of 3-methyl-2-butenal (prenal) to 3-methyl-2- 

It is also seen from Table 3.8, that with the various Ru-phosphine complexes as catalysts allowing high conversions of cinnamaldehyde at 35-120 °C under 20-30 bar H2, in many cases water/toluene or water/benzene mixtures were used as solvent. Here the interesting point is in that in the absence of excess phosphine, arenes react the following way  

It is to be mentioned that water-soluble phosphine complexes of rhodium(I), such as [RhCl(TPPMS)3], [RhCl(TPPTS)3], [RhCl(PTA)3], 

---

The most obvious way to reduce an aldehyde or a ketone to an alcohol is by hydro genation of the carbon-oxygen double bond Like the hydrogenation of alkenes the reac tion IS exothermic but exceedingly slow m the absence of a catalyst Finely divided metals such as platinum palladium nickel and ruthenium are effective catalysts for the hydrogenation of aldehydes and ketones Aldehydes yield primary alcohols... 

Reduction of Aldehydes and Ketones to Hydrocarbons. Deep hydrogenation of aldehydes and ketones removes the oxygen functionahty and produces the parent hydrocarbons. 

The mechanism of catalytic hydrogenation of aldehydes and ketones is probably similar to that of Reaction 15-11, though not much is known about it. " ... 

Molybdenum and tungsten carbonyl hydride complexes were shown (Eqs. (16), (17), (22), (23), (24) see Schemes 7.5 and 7.7) to function as hydride donors in the presence of acids. Tungsten dihydrides are capable of carrying out stoichiometric ionic hydrogenation of aldehydes and ketones (Eq. (28)). These stoichiometric reactions provided evidence that the proton and hydride transfer steps necessary for a catalytic cycle were viable, but closing of the cycle requires that the metal hydride bonds be regenerated from reaction with H2. 

Table 38.3 Hydrogenation of aldehydes and ketones in aqueous-organic two-phase systems. 

Hydrogenation of aldehydes and ketones Hydrogenation of miscellaneous organic substrates... 

Acidity of a-hydrogens of aldehydes and ketones The aldehydes and ketones undergo a number of reactions due to the acidic nature of a-hydrogen. 

Cathodic surfaces of finely divided platinum, palladium and nickel have a low hydrogen overvoltage and the dominant electrochemical reaction is the generation of a layer of hydrogen atoms. The electrocatalytic hydrogenation of aldehydes and ketones can be achieved at these surfaces. Cathodes of platinum or palladium black operate in both acid solution [203] and in methanol containing sodium methoxide [204], The carbonyl compound is converted to the alcohol. Reduction of 4-tert-butylcyclohexanone is not stereoselective, however, 1,2-diphenylpropan-l-one is converted to the / reo-alcohol. 

MIGNONAC REACTION. Formation of amines by catalytic hydrogenation of aldehydes and ketones in liquid ammonia and absolute ethanol in [he presence of a nickel catalyst. 

The advantage over most other kinds of reduction is that usually the product can be obtained simply by filtration from the catalyst, then distillation. The common catalysts are nickel, palladium, copper chromite, or platinum activated with ferrous iron. Hydrogenation of aldehyde and ketone carbonyl groups is much slower than of carbon-carbon double bonds so more strenuous conditions are required. This is not surprising, because hydrogenation of carbonyl groups is calculated to be less exothermic than that of carbon-carbon double bonds ... 

Reduction of carbonyl compounds (Section 10-11) a. Catalytic hydrogenation of aldehydes and ketones... 

---