Selective Reductions. 26. Lithium Triethylborohydride as an Exceptionally Powerful and Selective Reducing Agent in Organic Synthesis. Exploration of the Reaction with Selected Organic Compounds Containing Representative Functional Groups.

Article abstract of DOI:10.1021/jo01289a001

The approximate rates, stoichiometry, and products of the reaction of lithium triethylborohydride (LiEt3BH) with selected organic compounds containing representative functional groups under standard conditions (tetrahydrofurane, 0 deg C) were examined in order to explore the reducing characteristics of this reagent and to establish the utility of the reagent as a selective reducing agent.Primary and secondary alcohols, phenols, and thiols evolve hydrogen rapidly and quantitatively, whereas the reaction with 3-ethyl-3-pentanol is slow. n-Hexylamine is inert to this reagent.Aldehydes and ketones are reduced rapidly and quantitatively to thecorresponding alcohols.Even the highly hindered ketone 2,2,4,4-tetramethyl-3-pentanone is reduced within 30 min.The stereoselectivities achieved with this reagent in the reduction of mono- and bicyclic ketones are better than those realized with lithium aluminium hydride, lithium alkoxyaluminohydrides and lithium borohydride; thus norcamphor is reduced to 1percent exo- and 99percent endo-2-norbornanol.The reagent rapidly reduces cinnamaldehyde to the cinnamyl alcohol stage, with further addition to the double bond being sluggish.Anthraquinone is cleanly reduced to 9,10-dihydro-9,10-dihydroxyanthracene.The diol was isolated in 77percent yield.Carboxylic acids evolve hydrogen rapidly and quantitatively (1 equiv);further reduction is very slow.Acyclic anhydrides utilize 2 equiv of hydride to give an equimolar mixture of acid and alcohol after hydrolysis.Utilizing this procedure, we converted phthalic anhydride to phthalide in 90percent yield.Acid chlorides, esters, and lactones are rapidly and quantitatively reduced to the corresponding carbinols.Epoxides undergo rapid reduction with the uptake of 1 equiv of hydride.In the case of unsymmetrical epoxides, exclusive Markovnikov ring opening was observed.Acetals, ketals, and ortho esters are inert to this reagent.Primary amides evolve 1 equiv of hydrogen rapidly.Further reduction of caproamide is slow, whereas benzamide is not reduced.Tertiary amides are rapidly and quantitatively reduced by LiEt3BH exclusively to the corresponding alcohols.Such a clean transformation has not been observed with any other hydride reagent currently available.Benzonitrile rapidly utilizes 2 equiv of hydride to go to the amine stage, whereas capronitrile takes only 1 equiv.Hydrolysis of the latter reaction mixture did not give the expected caproaldehyde,but n-hexylamine and the starting materials were obtained in equal amounts.It appears possible to selectively reduce tertiary amides and aromatic nitriles to aldehydes in excellent yields by utilizing stoichiometric quantities of the reagent. 1-Nitropropane utilizes only 1 equiv of hydride for hydrogen evolution without any reduction.Nitrobenzene,azobenzene, and azoxybenzene are rapidly reduced.Cyclohexanone oxime rapidly evolves hydrogen but no reduction is observed.Phenyl isocyanate readily consumes 1 equiv of hydride in going to the formanilide stage.Pyridine is rapidly reduced to the tetrahydropyridine stage, followed by ...