Addition Compounds of Alkali Metal Hydrides. 28. Preparation of Potassium Dialkoxymonoalkylborohydrides from Cyclic Boronic Esters. A New Class of Reducing Agents

The reaction of cyclic boronic esters possessing a wide range of steric requirements with excess potassium hydride to form the corresponding potassium dialkoxymonoalkylborohydrides was explored. In cases involving a less hindered diol such as ethylene glycol, 2,3-butanediol, or 1,3-propanediol, the reaction is slightly exothermic and quite facile, being complete in less than 1 h at 25 °C. In cases involving a highly hindered diol such as pinacol, the reaction is very sluggish, even at 65 °C. The stability of the potassium dialkoxymonoalkylborohydrides is strongly dependent upon the steric bulkiness of the alkyl groups of the boronic ester. Thus, for R = n-hexyl, 3-hexyl, tert-butyl, or thexyl, the addition product is quite stable to disproportionation. However, for R = methyl, the corresponding borohydride is unstable, undergoing rapid redistribution to form a white precipitate. The stable potassium dialkoxymonoalkylborohydrides thus formed reduce 2-methylcyclohexanone with moderate stereoselectivity, giving the cis isomer preferentially, with selectivities of 73–84%. © 1986, American Chemical Society. All rights reserved.