Selective Reductions. 36. Reaction of Lithium 9-Boratabicyclononane with Selected Organic Compounds Containing Representative Functional Groups

Article abstract of DOI:10.1021/jo00191a010

The approximate rates, stoichiometry, and products of the reaction of lithium 9-boratabicyclo<3.3.1>nonane with selected organic compounds containing representative functional groups were examined under standard conditions (tetrahydrofuran, room temperature) in order to explore the reducing characteristics of this reagent and to establish the utility of this reagent as a selective reducing agent.Primary alcohols, phenols, and thiols evolve hydrogen rapidly and quantitatively.However, the reaction of 3-hexanol and 3-ethyl-3-pentanol is very slow. n-Hexylamine is inert to this reagent.Aldehydes and ketones are reduced rapidly and quantitatively to the corresponding alcohols.Even the highly hindered ketone, 2,2,4,4-tetramethyl-3-pentanone, is reduced within 30min.Reduction of camphor gives 91percent isoborneol and 9percent borneol, respectively.Cinnamaldehyde is rapidly reduced to the cinnamyl alcohol quantitatively without attacking the double bond.Carboxylic acids liberate hydrogen rapidly and quantitatively, but further reduction is very slow.Anhydrides consume 2 equiv of hydride without further hydride uptake, corresponding to reduction to an equimolar mixture of carboxylic acid and alcohol.Acid chlorides, esters, and lactones are rapidly reduced to the corresponding alcohols.Epoxides utilize 1 equiv of hydride at a moderate rate.In the case of unsymmetrical epoxides, the Markovnikov ring opening is predominant.Acetal, ketal, and ortho esters are inert to this reagent.Primary amides liberate hydrogen slowly.Caproamide undergoes slow reduction, but benzamide is not reduced.Tertiary amides consume 2 equiv of hydride slowly, undergoing reduction to the corresponding amines.Benzonitrile is reduced to the amine stage within 12 h; however, an aliphatic nitrile, capronitrile, is reduced only sluggishly. 1-Nitropropane rapidly liberates 1 equiv of hydrogen, but further reduction is very slow.Nitrobenzene utilizes 2.5 equiv of hydride, 1 for hydrogen evolution and 1.5 for reduction.Azobenzene is inert and azoxybenzene is reduced very sluggishly.Cyclohexanone oxime rapidly evolves 1 equiv of hydrogen, but no reduction is observed.Phenyl isocyanate consumes only 1 equiv of hydride to proceed to the formanilide stage.Pyridine is reduced very slowly.However, pyridine N-oxide undergoes rapid reduction with this reagent.Disulfides are rapidly reduced to the thiol stage, whereas, sulfoxides, sulfones, sulfonic acids, and sulfides are inert to this reagent.Cyclohexyl tosylate is also inert, but n-octyl tosylate undergoes reduction within 3.0 h.This hydride is inert to a typical n-alkyl chloride but reacts moderately with n-alkyl bromide and rapidly with an n-alkyl iodide.A secondary alkyl bromide is almost inert to this hydride.