Hydrogenation of a,3-unsaturated aldehydes

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

Scheme 15.2 Distribution of products in the hydrogenation of a,/ -unsaturated aldehydes.

The most essential conclusions of the investigations are summarized below. Note that all considerations discussed in connection with the hydrogenation of a,/)-unsaturated aldehydes are also relevant to the hydrogenation of unsaturated ketones ... 

Scheme 3.6 Reaction, separation, and recycling processes in hydrogenation of a, / -unsaturated aldehydes with Ru complex catalyst under gas (H2, C02)-liquid (PEG) biphasic conditions (Reproduced from Ref. [39] with permission from The Royal Society of Chemistry)...

We thought to start with the metal-free biomimetic transfer hydrogenation of a,(-)-unsaturated aldehydes as a model reaction which has been earlier discovered in our laboratory and independently in that of MacMillan et al. (Scheme 24). We have prepared a large number of ammonium salts as crystalline solids by mixing different primary and secondary amines with a chiral phosphoric acid. In particular, the ammonium salts of sterically hindered chiral phosphoric acids could catalyze... 

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. 

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... 

The hydrogenation of a, -unsaturated aldehydes to give allylic alcohols is of considerable interest, since these reactions are an important step in the industrial symthesis of fine chemicals, particularly, of pharmaceuticals and cosmetics [98,99,249,433-440,446]. A simplified reaction scheme is shown in Fig. 6.18. 

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-... 

Although common examples in the literature mention acrolein and crotonalde-hyde hydrogenation, it is probably best to consider research on cinnamaldehyde hydrogenation for determining the best conditions to apply to the hydrogenation of a,/ -unsaturated aldehydes. 

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. 

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