Addition Compounds of Alkali Metal Hydrides. 27. A General Method for Preparation of the Potassium 9-Alkoxy-9-boratabicyclo[3.3.1\nonanes. A New Class of Stereoselective Reducing Agents

Abstract

The reaction in tetrahydrofuran of potassium hydride with representative B-alkoxy-9-borabicyclo[3.3.1\nonanes (B-OR-9-BBN) containing alkoxy groups with increasing steric requirements was examined in detail to establish the generality of this synthesis of the corresponding potassium 9-alkoxy-9-boratabicyclo[3.3.1\nonanes (K9OR-9-BBNH) and the stereoselectivities of these new reagents for the reduction of cyclic ketones. For R = Me and n-Bu, the reactions with potassium hydride are very fast, almost instantaneous, even at 0 °C. However, the products are unstable and rapidly undergo redistribution, even in the presence of excess potassium hydride. Moderately hindered alkoxy derivatives, R = 2-Pr and 2-Bu, react somewhat slower (1 h at 0 °C and 25 °C, respectively) and the products are stable to redistribution. More hindered alkoxy derivatives, R = t-Bu, t-Am, Thx, require 24 h at 25 °C. Even more hindered alkoxy groups, R = 3-ethyl-3-pentyl and 2,4-dimethyl-2-pentyl, require even longer reaction times and higher temperatures. All reagents show high stereoselectivities in the reduction of cyclic ketones, with the stereoselectivities generally increasing with increasing steric requirements of the alkoxy substituent. The thexyl derivative appears especially favorable, with the byproducts of the reaction readily removed from the reaction mixture. © 1985, American Chemical Society. All rights reserved.