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.