Cyclohexanones axial alcohols from

A useful method for the synthesis of axial alcohols from unhindered cyclohexanones is by hydrogenation over rhodium in THF-HCl, Reduction... 

These mechanistic interpretations can also be applied to the hydrogenation of cyclohexanones. In acid, the carbonium ion (19) is formed and adsorbed on the catalyst from the least hindered side. Hydride ion transfer from the catalyst gives the axial alcohol (20). " In base, the enolate anion (21) is also adsorbed from the least hindered side. Hydride ion transfer from the catalyst followed by protonation from the solution gives the equatorial alcohol (22). 

The addition of hydrogen to hindered ketones usually occurs preferentially from a less hindered side and the stereochemistry is less influenced by the nature of solvents than in the cases of unhindered ketones, as generalized in ASB rules 1 and 2. According to these rules, the axial alcohols are formed predominantly in the hydrogenation of hindered cyclohexanones under both acidic and neutral (or alkaline) conditions. The ABS rule, however, is oversimplified, as pointed out by Dauben et al.,180 since the addition of hydrogen from a less hindered side does not always give axial alcohols as in the cases of 5a-cholestan-l-one (28)181 and the 12-oxo steroid 29 (hecogenin),182 where... 

The other group of moderately hindered cyclohexanones which has been examined in detailed is a group of substituted 1-decalones, which gave results similar to those of the 12-keto steroids on Li-NHs reduction. These decalones were all derived from protected diketone (51) and included two different 2-monosubstituted derivatives (52) and (53). These decalones were reduced both in the presence of a proton donor (NH4CI) and under anhydrous conditions in yields of 69-99%. The axial alcohol was the major product in the presence of a proton donor, while the equatorial alcohol was predominant in its ab-... 

Hydrogenations of cyclohexanones in basic media leads to the formation of the more stable, equatorial alcohol, 18, as the primary product (Eqns. 18.18-19). With basic Raney nickel catalysts, the predominance of equatorial alcohol has been shown to arise from the isomerization of the initially produced axial alcohol under hydrogenation conditions. The use of platinum oxide with its alkaline impurity also leads to extensive equatorial alcohol formation unless acetic acid is used as the solvent. This basic impurity was shown to be... 

Reduction of cyclohexanones. Henbest and Mitchell2 have described in detail their method for reduction of substituted cyclohexanones predominantly to axial alcohols using a soluble iridium-phosphite catalyst prepared in situ from iridium tetrachloride and phosphorous acid (or an easily hydrolyzed ester of this acid). Often 96% or more of the axial alcohol is obtained in this way. 

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