摘要：

The generality of the synthesis of potassium triisopropoxyborohydride, stabilized toward disproportionation by storing over excess potassium hydride, was examined with seven additional organoborates of varying steric requirements. The reaction of trimethoxy- and triethoxyborane with potassium hydride proceeded readily at room temperature, but the products could not be stabilized by the presence of excess potassium hydride. Triphenoxyborane reacted readily, even at -10°C, and stabilization was achieved. Tri-sec-butoxy- and tricyclopentoxyborane required refluxing in THF for 12-24 h, and the products were stabilized over potassium hydride. Finally, the reactions of tris(2-methylcyclohexoxy)- and tri-tert-butoxyborane were even slower, requiring a number of days for completion. Both products were stabilized toward disproportionation over potassium hydride. Indeed, potassium tri-tert-butoxyborohydride was quite stable toward disproportionation without excess potassium hydride. The stereoselectivities of these reagents in the reduction of representative cyclic ketones were examined. The stereoselectivities varied in an erratic manner with the steric requirements of the alkoxy group and did not approach the stereoselectivities previously achieved with lithium tri-sec-butylborohydride and lithium trisiamylborohydride.

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