Decarboxylative Condensation, Aldehydes and Alcohols

Aldehydes and alcohols can be oxidized to their corresponding carboxylic acids on certain oxide surfaces. Carboxylate formation can also take place by the breakdown of certain adsorbates, such as an ester to carboxylate and alkoxy, or by hydride transfer from one adsorbate to another, as takes place in the Cannizzaro disproportionation reaction of aldehydes to alkoxy and carboxylate. The oxidation reactions are less likely on hydroxylated surfaces, because the surface is less nucleophilic. Hydrogenation of an acid to an aldehyde can take place on oxides of intermediate M-O bond strength at temperatures of 623-723 K, and at lower temperatures on the Group VIII metals with low M-O bond strength. Obviously, once the carboxylates are formed, condensation to ketones could occur according to one of the mechanisms given above. 

When a good hydrogenation metal (e.g., Pt, Pd, Rh) is added to the typical MOx, ketonization and surface acetate formation is suppressed in the absence of 02. A shift to decarboxylation plus hydrogenation occurs, giving alkanes, CO, CO2, and H2O. This is in part due to hydrogenation of the acids to aldehydes, which are then more reactive in aldol condensations. Ketonization is suppressed to a lesser degree or not at all when the poorer (in terms of ability to hydrogenate) d-type transition metals were used (e.g., Cu).  

Surfaces with basic sites form enolates from both the aldehydes and ketones, leading to multiple aldol condensations and Michael additions. Candidate molecules must be enolizable, i.e., contain an a-hydrogen atom. Aldol condensation / Michael addition products cover the range from a,p-unsaturated aldehydes, saturated aldehydes, hydrogenated products (alcohols), and the heavier aromatics resulting from multiple condensations. The presence of coordina- 

Another competing reaction that can occur on strongly acidic or basic surfaces, mostly at lower temperatures, is the Tischenko cross-esterification. This reaction can be written as follows for aldehydes  

On metals with stable lower oxidation states, but not completely reduced, dehydration/dehydrogenation reactions become preferred. An example reaction is reductive coupling (the McMurry reaction) °  

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