Regio- and (E)-Stereoselective Triborylation of Propargylic Carbonates

An efficient synthesis of (E)-alken-1,2,3-triboronates form readily available propargylic carbonates is described. The reaction enjoys excellent regio- and E-selectivity and many synthetically useful functional groups can be tolerated. Based on mechanistic studies, a two-step mechanism via 1,2-allenyl boronate intermediate is proposed.