Selective Reductions: 33. Potassium Triisopropoxyborohydride as a Selective Reducing Agent in Organic Synthesis. Reaction with Selected Organic Compounds Containing Representative Functional Groups

Article abstract of DOI:10.1021/jo00179a026

The approximate rate and stoichiometry of the reaction of excess pure potassium triisopropoxyborohydride, KIPBH, with 56 selected compounds containing representative functional groups under standardized conditions (tetrahydrofuran, 0 deg C) was examined in order to define the characteristics of the reagent for selective reductions.Primary, secondary, and tertiary alcohols evolve hydrogen partially, even after a long period of time.Phenol also generates partial hydrogen, and the reactions of those amines and thiols studied with the reagent are very slow.Aldehydes andketones are reduced rapidly and quantitatively to give the corresponding alcohols.Unlike sodium and potassium borohydrides, KIPBH is very stereoselective. 2-Methylcyclohexanone can be reduced to the correspondingly less stable isomer, cis-2-methylcyclohexanol, in a high ratio (91percent cis isomer).Cinnamaldehyde is rapidly reduced to cinnamylalcohol, and further reduction is very slow under these conditions.Anthraquinone is cleanly reduced to 9,10-dihydro-9,10-anthracenediol.Carboxylic acids liberate hydrogen only partially, and further reduction is very slow.Acid chlorides consume 1 equiv of hydride rapidly, but the corresponding aldehydes do not form.Esters are almost inert toward the reagent. γ-Butyrolactone and phthalide are reduced only slowly.Epoxides are inert toward the reagent.Primary aliphatic amides evolve hydrogen slowly and primary aromatic amides evolve 1 equiv of hydrogen, but no significant reduction occurs.Tertiary amides and nitriles are inert toward the reagent.Of the nitrogen compounds studied, nitrobenzene is partially reduced after 48h, while azobenzene and azoxybenzene are inert.Partial reduction ofcyclohexanone oxime is observed, while phenyl isocyanate, pyridine, and pyridine N-oxide are inert under these conditions.Di n-butyl disulfide and diphenyl disulfide are reduced rapidly and quantitatively to the corresponding mercaptans with partial hydrogen evolution.Other sulfur compounds studied, such as p-tolyl methyl sulfide, diphenylsulfone, methanesulfonic acid, and p-toluenesulfonic acid, are inert toward the reagent.Only partial reduction of cyclohexyl tosylate is observed.Potassium triisopropoxyborohydride is a valuable reagent in boron chemistry.Thus, it transfers 1 equiv of hydride to dialkylhaloboranes, and the resulting dialkylborane can be transformed to a mixed trialkylborane, providing a potential route to mixed trialkylcarbinols or unsymmetrical ketones.The reagent rapidly transfers hydride to even severely hindered trialkylboranes, providing a simple synthetic route to these useful reagents.Finally, it readily converts 2-bromo-trans-vinylboronic esters to the cis-vinylboronic esters, providing a convenient synthetic route to these derivatives.