Selective reductions. 33. potassium triisopropoxyborohydride as a selective reducing agent in organic synthesis. reaction with selected organic compounds containing representative functional groups

Abstract

The approximate rate and stoichiometry of the reaction of excess pure potassium triisopropoxyborohydride, KIPBH, with 56 selected compounds containing representative functional groups under stand ardized conditions (tetrahydrofuran, 0 °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 reagentare very slow. Aldehydes and ketones 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, ds-2-methylcyclohexanol, in a high ratio (91% cis isomer). Cinnamaldehyde is rapidly reduced to cinnamyl alcohol, 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 partiallyreduced after 48 h, while azobenzene and azoxybenzene are inert. Partial reduction of cyclohexanone oxime is observed, while phenyl isocyanate, pyridine, and pyridine N-oxide are inertunder these conditions. Di-n-butyl disulfide and diphenyl disulfide are reduced rapidly and quantitativelyto the corresponding mercaptans with partial hydrogen evolution. Other sulfur compounds studied, such as p-tolyl methyl sulfide, diphenyl sulfone, methanesulfonic acid, and p-toluenesulfonic acid, are inert toward the reagent. Only partial reduction of cyclohexyltosylate isobserved. 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 mixedtrialkylcarbinols 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 a's-vinylboronic esters, providinga convenient synthetic route to these derivatives. © 1984, American Chemical Society. All rights reserved.