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

Article abstract of DOI:10.1021/jo00353a012

The reaction of cyclic boronic esters possesing a wide range of steric requirements with excess potassium hydride to form the corresponding potassium dialkoxymonoalkylborohydrides was explored.In case 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 deg C.In the case involving a highly hindered diol such as pinacol, the reaction is very sluggish, even at 65 deg 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 73-84percent