Cyclohexanones equatorial alcohols from

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). 

As observed with cyclohexanones, the diastereoselectivity of the addition reaction of trimeth-ylaluminum to 2-methylcyclopentanone depends on the stoichiometry of the reactants. Thus, addition of one equivalent of trimcthylaluminum proceeds via preferential tram attack whereas, due to the "compression effect , addition of an excess of the reagent leads to the formation of the equatorial alcohol via predominant attack from the cis side (Table 3)6. In contrast to the addition reactions with trimethylaluniinum, no reversal of the diastereoselectivity upon change of reagent stoichiometry was observed in the addition of triphenylaluminum to 2-methylcyclopentanone6. Even with an excess of the aluminum reagent trans attack predominates. However, the diastereoselectivity is lower than with the use of an equimolar amount of the reactants. 

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... 

In cyclohexanones, the pyramidalisation is similarly greater in the radical anion 7.155 than it was in the ketone 5.138 (see p. 230), thus explaining the high degree of synthetically useful selectivity for the formation of the equatorial alcohol trans-1.157 from the ketone 7.156, as a result of axial protonation.1068... 

The extreme buUdness of these aluminum reagents is able to control the addition reaction of carbonyl compounds by complexation which will or can lead to unexpected stereoselectivity. When MAD was mixed with the carbonyl compound of 4-tert-butylcyclohexanone, a stable 1 1 complex was formed. This complex was treated with methyllithium at low temperature to yield an equatorial alcohol almost exclusively. The observed stereochemical outcome was opposite that of the product from reaction of cyclohexanone with methyllithium. The equatorial selectivity achieved with MAD was found to be almost perfect [11, 12], Furthermore, if the same reaction conditions are applied to chiral aldehydes, a fi-Felkin selectivities are observed [12] (Scheme 5). 

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