Isomerization-Free Matteson Homologations of Substituted Allylboronic Esters.

Substituted allyl derivatives with an internal double bond can be homologated under standard conditions via a 1,2-shift, whereas allylboronic esters with terminal double bonds can also be homologated via a S N ' reaction. This allyl isomerization is catalyzed by ZnCl 2 and can be suppressed by omitting ZnCl 2 during the formation of α-chloroboronic ester. However, in the second step with Grignard reagents, ZnCl 2 was added to suppress side reactions of the product formed.