One-step Aldol Condensations to Ketones

Lower temperature processes and catalysts exist for so-called one-step aldol condensations to produce ketones by condensation of two ketones or of a ketone 

The alcoholic dehydrogenation reaction requires a transition metal component unless it is possible to operate above 470 K, where moderately basic catalysts also become active for this reaction. An example catalyst would be a MgAlOx hydrotalcite of low A1 content. These catalysts can activate H2/D2 exchange at these temperatures. The alcohol adsorbs dissociatively to surface alkoxide and hydroxy, a step requiring proximity of a basic and a Lewis acid site. There follows a hydride abstraction, mediated by another Lewis acid group, and heterolytic association to give H2. 

When a ketone and an aldehyde are condensed in a cross-aldol reaction, excess ketone must be used to avoid aldehyde self-condensation, and even so the surface is essentially saturated with aldehyde at low temperature. Exceptions are reactions of easily enolizable ketones at ambient and lower temperatures. Therefore increasing the temperature often increases selectivity as well, a situation reminiscent of decarboxylative condensation, by allowing for greater ketone adsorption. 

An enolate formed from an unsymmetric ketone can be of two types the less electronegative (usually more substituted) side is deprotonated preferentially by many homogeneous transition metal complexes with N-donor or halide ligands, in agreement with local hard-soft acid-base concepts. But steric hindrance plays a greater role with heterogeneous catalysts, to such an extent that the less substituted side is often deprotonated preferentially.  

Some examples of one-step processes for methylketone production are as follows. 

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