Enantioselective syntheses of (E)-γ,δ-disubstituted homoallylic alcohols via BF3•OEt2-catalyzed aldehyde allylboration and analysis of the origin of E-selectivity: A1,2 allylic strain vs. syn-pentane interaction

Enantioselective allylboration of aldehydes with alpha-substituted beta-methyl allylboronate was reported. By using BF3 center dot OEt2 as the catalyst, gamma,delta-disubstituted homoallylic alcohols were obtained in good yields with high E-selectivities and enantioselectivities. Transition state analysis revealed that the disfavored transition state suffers from a syn-pentane interaction between the BF3 catalyst and axially oriented alpha-substituent of the allylboron reagent. Such a syn-pentane interaction is severe enough to overcome the A(1,2) allylic strain between the beta-methyl group and the alpha-substituent of the boron reagent that is present in the favored competing transition state. Consequently, the reaction proceeded with equatorial placement of the alpha-substituent to furnish gamma-methyl substituted homoallylic alcohols with high E-selectivity. Published by Elsevier Ltd.