Selective Hydrogenation of a,-Unsaturated Aldehydes

Carbonyl hydrogenation is generally less facile than olefin hydrogenation, making selective hydrogenation of a, -unsaturated aldehydes to the allyl alcohol a special challenge. Substitution of the carbon atom attached to the carbonyl (i. e. from the aldehyde to the ketone), substantially increases the steric hindrance to carbonyl adsorption, hence the lack of reports in the literature of selective unsaturated ke- 

The subject has been reviewed by Gallezot and Richard, who give special attention to mechanism, steric, and electronic effects as investigated by surface-science techniques [5]. 

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Selective hydrogenation of a-/ unsaturated aldehydes Rh-TPPTS or Ru-TPPTS G-L-L/water soluble catalysis [11]... 

Reduction.—Selective hydrogenation of a, -unsaturated aldehydes to the corresponding unsaturated alcohols has been accomplished using a catalytic amount... 

Selective Reactions on Modified Metal Surfaces the Ligand Effect Hydrogenation of a- Unsaturated Aldehydes... 

Rh and Ir complexes stabilized by tertiary (chiral) phosphorus ligands are the most active and the most versatile catalysts. Although standard hydrogenations of olefins, ketones and reductive aminations are best performed using heterogeneous catalysts (see above), homogeneous catalysis becomes the method of choice once selectivity is called for. An example is the chemoselective hydrogenation of a,/ -unsaturated aldehydes which is a severe test for the selectivity of catalysts. 

Chromium compounds increase the activity of platinum catalysts by increasing the electron densities of the active sites. jhe addition of ferrous sulfate, which promotes the hydrogenation of carbonyl groups, and zinc acetate, which inhibits the hydrogenation of double bonds, to platinum gives a catalyst system capable of effecting the selective hydrogenation of an unsaturated aldehyde to an unsaturated alcohoP - (Eqn. 11.12). ... 

Examples in which transition metal clusters have been used as selective catalysts in the hydrogenation of complicated structures are rare, but there are reports on chemoselective and on regioselective applications. Ru3(CO),2 in the presence of phosphorous cocatalysts catalyzes the hydrogenation of a,/ -unsaturated aldehydes... 

Reduced osmium on carbon is an excellent catalyst for selective hydrogenation of aj3-unsaturated aldehydes to unsaturated alcohols.1 Cinnamaldehyde — cinnamyl alcohol (95% yield). Reduced rates are observed with alumina as the support. This selective reduction is not applicable to a,/3-unsaturated ketones thus hydrogenation of mesityl oxide afforded methyl isobutyl ketone. 

Several adatoms effects were observed, with various reactions. This effect can usually be depicted as poisoning of undesirable sites, resulting simultaneously in a decrease of the global catalytic activity and in a significant increase of the selectivities for the desired products. We describe here three examples, the hydrogenation of a, -unsaturated aldehydes (the same reaction as above but now in the presence of a very small amount of tin), the isomerization of 3-carene into 2-carene, and the dehydrogenation of butan-2-ol into methyl ethyl ketone. 

Y.Onal,M. Lucas, P. Claus, Application of a capillary microreactor for selective hydrogenation of oqfl-unsaturated aldehydes in aqueous multiphase catalysis, Chem. Eng. Tech. 2005, 28, Tn-TlS. 

As shown above, intermetallic compounds have also been used as catalysts for the selective hydrogenation of a, -vmsaturated aldehydes to the unsaturated alcohols. Here, a large amovmt of work in conventional catalyst development... 

Selective hydrogenation of a -unsaturated carbonyl compounds can be carried out by reduction with iron pentacarbonyl and a small amount of base in moist solvents. The method is applicable to oc -unsaturated aldehydes, ketones, esters, and lactones with negligible over-reduction of the carbonyl group and is susceptible to the steric environment of the olefin. Spectroscopic evidence suggests that solutions of equimolecular amounts of iodine and thiocyanogen contain an appreciable concentration of iodine thiocyanate. Addition of alkenes results in tran.r-addition to yield jS-iodo-thiocyanates which in base suffer rapid hydrolysis of the thiocyanate followed by ring closure to the episulphide (516). As a synthetic procedure this does not appear to be applicable to acyclic olefins. ... 

The selective hydrogenation of a,P-unsaturated aldehydes to allylic alcohols (desired products) and/or saturated aldehydes is of commercial relevance, as mentioned in the... 

Thus, [HRh(C0)(TPPTS)3]/H20/silica (TPPTS = sodium salt of tri(m-sulfophenyl)phopshine) catalyzes the hydroformylation of heavy and functionalized olefins,118-122 the selective hydrogenation of a,/3-unsaturated aldehydes,84 and the asymmetric hydrogenation of 2-(6 -methoxy-2 -naphthyl)acrylic add (a precursor of naproxen).123,124 More recently, this methodology was tested for the palladium-catalyzed Trost Tsuji (allylic substitution) and Heck (olefin arylation) reactions.125-127... 

Choppin GR (2003) Actinide speciation in the environment. Radiochim Acta 91 645-649 Claus P (1998) Selective hydrogenation of a,P-unsaturated aldehydes and other C = O and C = C bonds containing compounds. Top Catal 5 51-62 Colon D, Weber EJ, Anderson JL, Winget P, Suarez LA (2006) Reduction of nitrobenzenes and N-hydroxylanilines by Fe(II) species Elucidation of the reaction mechanism. Environ Sci Technol 40 4449-4454... 

The selective hydrogenation of a,/3-unsaturated aldehydes to give the corresponding unsaturated alcohols [Eq. (9)] was investigated with the ruthenium complex catalysts, initially present as [Ru(H)(Cl)(tppts)3] or [Ru(H)2(tppts)4] (91). 

Unsaturated aldehydes.4 A nickel boride catalyst similar to P-2 nickel boride is obtained by reaction of NiCl2 and excess NaBH4 in C2H5OH. It effects selective hydrogenation of a,P-alkynal acetals to the (Z)-a,p-alkenal acetals. 

Selectivity in the hydrogenation of ,/3-unsaturated aldehydes has become an important topic in heterogeneous catalysis (refs. 1-4). Unsaturated alcohols, important in the synthesis of fine chemicals, may be produced selectively over certain supported group VUI-metal catalysts (refs. 5,6), but the general problem of the selective intramolecular hydrogenation of carbonyl groups remains a challenging task. 

The hydrogenation of a,/ -unsaturatcd aldehydes to saturated aldehydes is readily achieved over most platinum metal catalysts under mild conditions. The selective synthesis of unsaturated alcohols is, however, much more difficult to perform and several attempts have been made to develop a suitable catalytic system. 

On the basis of the above conclusions, the selective hydrogenation of a,/J-unsaturated aldehydes to unsaturated alcohols can be achieved by taking into account the following considerations and factors ... 

Iridium,204,205 together with osmium, has been not widely used in catalytic hydrogenation. Recently, however, iridium or iridium-based catalysts have been shown to be effective in various hydrogenations, such as in selective hydrogenation of a,P-unsaturated aldehydes to allylic alcohols (Section 5.2), of aromatic nitro compounds to the corresponding hydroxylamines (Section 9.3.6), of halonitrobenzenes to haloanilines without loss of halogen (Section 9.3.2), and in the stereoselective hydrogenation of carbon to carbon double bonds (see, e.g., eqs. 3.25-3.27 and Table... 

TABLE 5.1 Selective Hydrogenation of a,p-Unsaturated Aldehydes to Allylic Alcohols over Ir-C Catalyst0 ... 

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