Axial alcohols, preparative methods

The method is useful in the preparation of other axial alcohols. Henbest6 has reported the reductions of 3-bbutylcyclohexanone, 3,3,5-trimethylcyclohexanone, and cholestanone to the axial alcohols by this procedure, although for the preparation of 3 a-cholestanol the procedure of Edward7 is preferred by the checkers. Recently8 2,4,4-trimethylcyclohexanone has been reduced to the pure axial alcohol by this method in 90% yield. 

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. 

A Co(salophen)/zeolite catalyst was prepared by the template synthesis method. This catalyst proved to be active in the ruthenium catalyzed oxidation of benzyl alcohol. The heteroge-nized Co(salophen), having the same amount of complex produced a higher rate in the oxidation reactions than the free complex. It can be explained by the sites isolation theory. In the case of the heterogenized catalyst it was not necessary to use an extra axial ligand such as triphenylphosphine. It was also found that in the case of Co(salophen)/zeolite catalyst the choice of the solvent was not so critical, as in the case of the free complex. 

As noted earlier in this chapter, the enantioselective hydrosilylation of olefins could be a useful method to prepare chiral, non-racemic alcohols. A.lthough the scope of highly enantioselective hydrosilylations is limited, high enantioselectivities have been obtained for the asymmetric hydrosilylation of alkenes and vinylarenes. A majority of the most selective chemistry has been conducted using a palladium catalyst containing an axially chiral monophosphine ligand. 

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