Hydrogenation of Cyclohexanones to Axial Alcohols

As generalized in the ASB rule 1, many axial alcohols have been obtained by hydrogenating substituted cyclohexanones over platinum catalysts in the presence of hydrochloric acid. However, this method has the following disadvantages  

The hydrogenations over platinum may be accompanied by hydrogenolysis to give hydrocarbons, especially with unhindered ketones in acidic media. 

The stereoselectivities are not always very high, particularly in the cases of unhindered ketones.154-158 For example, only 78% of the cis (axial) isomer was formed in the hydrogenation of 4-t-butylcyclohexanone over platinum oxide in AcOH-HCl.156 

When acetic acid, in which better stereoselectivities are usually obtained, is used as the solvent in such strongly acidic condition, acetylated products may be formed and neutralization of the solvent and hydrolysis of the products are often required to isolate the alcohols produced. 

TABLE 5.8 Stereoselective Hydrogenation of Unhindered Cyclohexanones to Axial Alcohols over Rhodium Catalyst  

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