Highly (E)-Selective BF3•Et2O-Promoted Allylboration of Chiral Nonracemic α-Substituted Allylboronates and Analysis of the Origin of Stereocontrol

delta-Methyl-substituted homoallylic alcohols 2 were prepared in 71-88% yield, E:Z>30:1 and 93% to >95% ee via BF3 center dot Et2O-promoted allylboration with alpha-Me-allylboronate 1. The origin of high (E)-selectivity is proposed.