Lithium n-Butylborohydride as a Selective Reducing Agent for the Reduction of Enones, Cyclic Ketones, and Selected Carbonyl Compounds

Article abstract of DOI:10.1021/jo00138a021

Lithium n-butylborohydride, the "ate" complex generated from n-butyllithium and borane-dimethyl sulfide complex in an equimolar ratio either in toluene-n-hexane or in tetrahydrofuran-n-hexane was reacted with enones, cyclic and bicyclic ketones, and selected carbonyl compounds in order to explore the reducing properties and to determine the synthetic utility of this reagent.Lithium n-butylborohydride in toluene-n-hexane is very effective for selective 1,2-reduction of acyclic enones and conjugated cyclohexenones and is solvent sensitive.The reduction of conjugatedcyclopentenones is more prone to 1,4-reduction than that of conjugated cyclohexenones.The reagent in tetrahydrofuran-n-hexane reduces the unhindered cyclic ketones 3-methyl-, 4-methyl-, and 4-tert-butylcyclohexanone to the corresponding thermodynamically more stable isomers (equatorial OH) with stereoselectivities of 92percent, 94percent, and 98percent, respectively.The stereoselectivities obtained with this reagent for such reductions are better than those reported with simple hydride reagents.Esters and lactones are rapidly and quantitatively reduced to the corresponding alcohols at 0 deg C in toluene-n-hexane, whereas they are inert to this reagent at -78 deg C, which permits the selective reduction of the ketones in the presence of the esters at the latter temperature.Acid chlorides are rapidly and quantitatively reduced to the corresponding alcohols.Acid anhydrides are reduced to an equimolar mixture of the acid and alcohol even at -78 deg C.Carboxylic acids and primary and secondary amides are not reduced by the reagent at room temperature and are recovered unchanged.Tertiary amides are resistant to reduction and are only slowly convertedto the corresponding amines and alcohols.