Addition Compounds of Alkali Metal Hydrides. 29. Preparation and Properties of Chiral Dialkylmonoalkoxyborohydrides. A New Class of Asymmetric Reducing Agents

Article abstract of DOI:10.1021/jo00367a006

A series of chiral 9-alkoxy-9-borabicyclo<3.3.1>nonane derivatives were synthesized by the reaction of 9-borabicyclo<3.3.1>nonane (9-BBN) with several readily available chiral alcohols, such as (-)-isopinocampheol, (+)-menthol, (-)-4-isocaranol, (+)-trans-2-methylcyclopentanol, and (-)-1,2:5,6-di-O-isopropylidene-α-D-glucofuranose.A chiral borinic ester possessing a cyclic chiral dialkylboryl moiety, (+)-2-(cyclohexyloxy)-4,8-dimethyl-2-borabicyclo<3.3.1>nonane, was also synthesized.With one exception, all of these chiral borinic esters were readily converted into the corresponding chiral dialkylmonoalkoxy borohydrides by treatment with excess potassium hydride in THF at 25 deg C.The addition of potassium hydride to (+)-9-(menthyloxy)-9-borabicyclo<3.3.1>nonane (9-O-Men-9-BBN) was very slow, requiring 15 days at 65 deg C (refluxing THF).The chiral dialkylmonoalkoxyborohydrides thus formed are all stable at 25 deg C and can be stored for several months.They were tested against acetophenone and 3-methyl-2-butanone as representative prochiral ketones.These reagents reduce acetophenone with up to 78percent ee and 3-methyl-2-butanone with up to 61percent ee at -78 deg C.