Hydrogenation of unsaturated aldehydes

Reduction of unsaturated carbonyl compounds to the saturated carbonyl is achieved readily and in high yield. Over palladium the reduction will come to a near halt except under vigorous conditions (73). If an aryl carbonyl compound, or a vinylogous aryl carbonyl, such as in cinnamaldehyde is employed, some reduction of the carbonyl may occur as well. Carbonyl reduction can be diminished or stopped completely by addition of small amounts of potassium acetate (i5) to palladium catalysts. Other effective inhibitors are ferrous salts, such asferroussulfate, at a level of about one atom of iron per atom of palladium. The ferrous salt can be simply added to the hydrogenation solution (94). Homogeneous catalysts are not very effective in hydrogenation of unsaturated aldehydes because of the tendencies of these catalysts to promote decarbonylation. 

IrCl(PPh3)3 or IrCl(CO)(PPh3)2 or phenylacetic acids Hydrogenation of unsaturated aldehydes to 714... 

A bimetallic catalyst can be obtained by the reaction of tetrabutyltin with Rh/Si02 catalyst. The partial hydrogenolysis leads to the Rhs[Sn(n-C4H9)2]/ Si02 surface organometallic complexes, which proved to be fully selective in the hydrogenation of unsaturated aldehydes into the corresponding unsaturated alcohols.318... 

Scheme 15.7 Hydrogenation of unsaturated aldehydes using Noyori s system.

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 to be mentioned that water-soluble phosphine complexes of rhodium(I), such as [RhCl(TPPMS)3], [RhCl(TPPTS)3], [RhCl(PTA)3], either preformed, or prepared in situ, catalyze the hydrogenation of unsaturated aldehydes at the C=C bond [187, 204, 205]. As an example, at 80 °C and 20 bar H2, in 0.3-3 h cinnamaldehyde and crotonaldehyde were hydrogenated to the corresponding saturated aldehydes with 93 % and 90 % conversion, accompanied with 95.7 % and 95 % selectivity, respectively. Using a water/toluene mixture as reaction medium allowed recycling of the catalyst in the aqueous phase with no loss of activity. 

SELECTIVE HYDROGENATION OF UNSATURATED ALDEHYDES OVER ZEOLITE-SUPPORTED METALS... 

In fine chemical manufacturing, the application of promoted platinum catalysts is less known. Maxted and Akhar have reported that the addition of stannous, manganous, ceric and ferric chloride to platinum oxide (Adams catalyst) facilitates the hydrogenation of aldehydes, ketones and olefins (ref. 1). The selective hydrogenation of unsaturated aldehydes or ketones to unsaturated alcohols has been achieved by the addition of ferrous sulfate and zinc acetate to platinum catalysts (ref. 2). 

From a thermodynamic point of view, the hydrogenation of the C=C bond is more favorable than that of the C=0 group. Since there are numerous other important processes to synthesize saturated aldehydes and ketones, the main objective of recent research efforts is to increase the selectivity of hydrogenation of unsaturated oxo compounds into unsaturated alcohols. The results summarized below clearly indicate the significant success achieved in the selective hydrogenation of unsaturated aldehydes. The hydrogenation of unsaturated ketones, in turn, cannot be accomplished yet with similar selectivities. 

To account for the ready hydrogenation of unsaturated aldehydes and ketones, Goetz and Orchin proposed the formation of 7r-oxapropenyl derivatives. 

Liu R, Cheng H, Wang Q et al (2008) Selective hydrogenation of unsaturated aldehydes in a poly(ethylene glycol)/compressed carbon dioxide biphasic system. Green Chem 10(10) 1082-1086... 

Table 9.3 Hydrogenation of unsaturated aldehydes and ketones (S is the selectivity to unsaturated alcohol). 

Prereduced rhenium heptoxide catalyst,65 especially the catalyst poisoned with pyridine, has been found to give high yields of unsaturated alcohols in the hydrogenation of unsaturated aldehydes (Table 5.2).66 A typical hydrogenation with the rhenium catalyst is shown in eq. 5.27. In the vapor phase hydrogenation of acrolein to allyl alcohol, the selectivity of rhenium catalysts has been found to be improved by poisoning with CO and CS2.67... 

TABLE 5.2 Hydrogenation of Unsaturated Aldehydes over Prereduced Rhenium Heptoxide Poisoned with Pyridine1 ... 

The lactam 143 was treated with m-chloroperbenzoic acid, followed by treatment with acid anhydride to yield the aldehyde 144. Catalytic hydrogenation of unsaturated aldehyde 144 gave saturated aldehyde 145, and reduction of 145 with lithium aluminum hydride provided racemic lupinine 4 and racemic epilupinine 139. 

While some vapor phase hydrogenations of unsaturated aldehydes have shown good initial selectivities, in most cases the reaction selectivity decreases significantly with prolonged reaction time. The vapor phase hydrogenation of crotonaldehyde over a Pt/Ti02 catalyst in the SMSI state, however, showed reasonable selectivities that remained relatively constant over the entire time of the reaction. 0°... 

The importance of controlling the pH in a two-phase reaction was demonstrated by Jod and co-workers [8, 9]. During their investigation of the hydrogenation of unsaturated aldehydes (Scheme 2) with Ru/TPPMS complexes, they observed a remarkable switch in selectivity on changing the pH. 

---