Lithium n-butylborohydride as a selective reducing agent for the reduction of enones, cyclic ketones, and selected carbonyl compounds (original) (raw)
1982, The Journal of Organic Chemistry
Lithium n-butylborohydride, the "ate" complex generated from n-butyllithium and borane-dimethyl sulfide complex in an equimolar ratio either in toluenen-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 toluenen-hexane is very effective for selective 1,Z-reduction of acyclic enones and conjugated cyclohexenones and is solvent sensitive. The reduction of conjugated cyclopentenones is more prone to 1,Creduction 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 stereoselecivities of 92%, 94%, and 98%, 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 "C in toluene-n-hexane, whereas they are inert to this reagent at -78 "C, which permits the selective reduction of the ketones in the presence of the esters at the latter temperature.
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