Chiral Synthesis via Organoboranes. 44. Racemic and Diastereo- and Enantioselective Homoallenylboration Using Dialkyl 2,3-Butadien-1-ylboronate Reagents. Another Novel Application of the Tandem Homologation−Allylboration Strategy

Abstract

A highly general and efficient racemic and diastereo- and enantioselective homoallenylboration has been achieved with a novel boron reagent, dialkyl 2,3-butadien-1-ylboronate (dialkyl homoallenylboronate). The starting diisopropyl 2,3-butadien-1-ylboronate is prepared from allenylmagnesium bromide and diisopropyl (halomethyl)boronate. This β,γ-unsaturated boronate reagent reacts readily with aldehydes via the usual allylic rearrangement to give the alkyl(1,3-butadien-2-yl)methanols in excellent yield. Among the solvents examined, toluene favored enhanced reaction rates. The reaction is relatively sensitive to steric effects, so that sterically hindered aldehydes react significantly slower. Generally, the reactivities are moderately lower than those of the corresponding simple allylboronates, possibly due to the reduced HOMO electron density of the internal double bond. Also, the reagent exhibits a unique anti diastereoselectivity in reaction with α-chiral aldehydes in contrast to the syn selectivity obser...