B-Allyl-10-Ph-9-borabicyclo[3.3.2]decanes:  Strategically Designed for the Asymmetric Allylboration of Ketones

The simple and efficient syntheses of B-allyl-10-(phenyl)-9-borabicyclo[3.3.2]decane (1) in both enantiomeric forms are reported. The remarkable enantioselectivity (81−99% ee) of these reagents in the allylboration process at −78 °C is only modestly diminished when the process is conducted at 0 °C, a phenomenon attributable to its rigid bicyclic structure. In addition to providing the homoallylic alcohols 6 efficiently (70−92%), the procedure also permits the efficient recovery of the chiral boron moiety (67−82%) as an air-stable crystalline N-methylpseudoephedrine complex 4 for the direct regeneration of 1 with allylmagnesium bromide in ether (98%). The reagent gives predictable stereochemistry, providing a strategically designed “chiral pocket” which is particularly receptive to leading methyl groups (e.g., methyl ethyl ketone, 87% ee).