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

摘要

The reaction of cyclic boron 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 less hindered diol such as pinacol, the reaction is very sluggish, even at 65 C. The stability of the potasium 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 F = methyl, the corresponding borohydride is unstable, undergoing rapid redistribution to form a white precipitate. The stable potassium dialkoxymonalkylborohydrides thus formed redice 2-, methylcyclohexanone with moderate stereoselectivity, giving the cis isomer preferentially, with selectivities of 73-84%.