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2.2.4.1 Diastereoselective Reduction of Ketones

DOI: 10.1055/sos-SD-202-00017

Zaidlewicz, M.; Pakulski, M. M.Science of Synthesis: Stereoselective Synthesis, (20112110.

Reductions of carbonyl groups with stoichiometric amounts of nucleophilic borohydrides, hydridoaluminates, and electrophilic boranes and aluminum hydrides have been thoroughly studied and are described in monographs and reviews.[‌212‌,‌221‌,‌340‌‌344‌] Extensive studies on the stereochemistry of the reduction of cyclohexanones with sodium borohydride and lithium aluminum hydride, considered as relatively small-sized hydride donors, reveal their preference for axial attack on the carbonyl group, leading to the formation of an equatorial alcohol. The preference has been attributed to favorable electronic interactions in the transition state.[‌345‌] Alternatively, the torsional strain developing in the formation of an axial alcohol is considered to be a major factor favoring axial attack.[‌346‌,‌347‌] Bulky hydride donors favor attack from the least-hindered equatorial direction, called steric approach control,[‌348‌] producing an axial alcohol. These observations have led to the development of sterically demanding, bulky trialkylborohydrides, achieving excellent selectivity in the reduction of ketones (Table 1). Steric approach control may operate in the reduction of certain hindered cyclic ketones even with small-sized hydride donors. In such cases, sodium borohydridecerium(III) chloride tends to produce the opposite stereoisomer.[‌349‌]

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