New selective reducing agent

Abstract

The ate complex generated from n-butyllithium and borane-dimethyl sulfide complex in equimolar ratio in both toluene-n-hexane and tetrahydrofuran-n-hexane, designated herein lithium n-butylborohydride, 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. This reagent in toluene-n-hexane is very effective for selective 1,2 reduction of acyclic enones and conjugated cyclohexenones. The reduction of conjugated cyclopentenones is more prone to 1,4 reduction than that of cyclohexenones. The enone reduction with this reagent is sensitive to the nature of solvent. This reagent in tetrahydrofuran-n-hexane reduces the unhindered cyclic ketones, 3-methyl-, 4-methyl-, and 4-tert-butylcyclohexanone to the corresponding more stable isomer in 91.5, 94.4, 97.5\% stereoselectivity, respectively. The stereoselectivities obtained with this reagent in the reduction of unhindered cyclic ketones are better than those reported with simple borohydrides. Esters and lactones are rapidly and quantitatively reduced to corresponding alcohols at $0^\circ C$ in toluene-n-hexane, whereas they are inert to this reagent at $-78^\circ C$. Selective reduction of the ketone in the presence of the ester by using this reagent can be accomplished at $-78^\circ C$. This reagent fails to reduce carboxylic acids and the original acids are recovered. Acid chlorides are rapidly and quantitatively reduced to the corresponding alcohols. This reagent reduces acid anhydrides to an equimolar mixture of acid and alcohol at even $-78^\circ C$. Primary and seconadary amides are not reduced with this reagent at room temperature after 24 hours and the original amides are recovered. Whereas N,N-di-n-butylcaproamide is not reduced, N,N-di-n-butylbenzamide is slowly reduced to the corresponding amine as a major product.