Allenylboronic Acid Pinacol Ester: A Selective Partner for [4 + 2] Cycloadditions.

We have studied the reaction of allenylboronic acid pinacol ester with cyclopentadiene with experimental and computational methods. The reaction occurred efficiently with complete Diels-Alder periselectivity and regioselectivity at the proximal double bond. The concerted mechanism for the observed transformation was computed to be favored over competitive addition to the distal double bond, [3,3]-sigmatropic rearrangements, and stepwise radical mechanism. This unprecedented Diels-Alder reaction enables the construction of synthetically versatile boron-substituted cycloadducts.