Abstract
A mechanistic study of the enantioselective asymmetric allylboration of ketones with allyldiisopropoxyborane catalyzed by chiral biphenols resulted in the development of improved reaction process. In a ligand exchange process involving the chiral biphenol and the boronate to liberate isopropanol as the key step, addition of isopropanol to the reaction was found to increase the overall rate and enantioselectivity. In the design of an improved reaction, a boronate possessing a tethered alcohol would more readily liberate catalyst at the end of a reaction. The use of allyldioxaborinane with 2 mol% (S)-3,3′-Br2-BINOL and 2 equivalents t-BuOH relative to ketone at room temperature results in high yields and enantioselectivities. Insight gathered from the mechanistic investigation resulted in the development of a reaction process that uses less catalyst (from 15 mol% to 2 mol%) at warmer temperatures (from -35 °C to room temperature).