Selective Hydrogenation of Unsaturated Ketones

Solodar studied the reduction of piperitenone (8), a compound containing two different olefinic bonds and a prochiral carbon-oxygen double bond. He found that mainly the sterically less hindered C=C bond was favored in the hydrogenation reaction, and thus pulegone was the major product, but also other compounds were formed in minor amounts. 

BAUNT HEIL. lASZLO MARKO, and SZILARD TdROS 

Reducing a,/S-unsaturated ketones with catalysts prepared from [Rh(diene)Cl]2 and PR3 in situ again the C=C bond is saturated preferentially, but performing the reaction in primary alcohols hydrogen transfer as a side reaction may take place, and the aldehyde thus formed is decar-bonylated and the catalyst deactivated/  

In contrast to the results discussed above, hydrosilylation of unsaturated ketones with rhodiumphosphine catalysts makes the selective saturation of either the C=C or the C=0 bond possible. Using monohydrosilanes, 1,4-addition takes place and saturated ketones are formed, while dihydrosilanes prefer the 1,2-addition and unsaturated alcohols are the products (9)  

Accordingly a-ionone(a) and citral(d) (a,j8-unsaturated carbonyl compounds containing an additional isolated double bond) could be transformed with 96-97% selectivity into dihydroionone(c) and citronellal(f) in the presence of RhCKPPhajia and EtaSiH (10,11)  

---

A number of examples are known where palladium catalysts were applied to the selective hydrogenation of unsaturated ketones to saturated ketones. Hydrogenation of 4-acetylcyclohexene to acetylcyclohexane took place rapidly with a small amount of... 

---