Preparation of Dihydroborole Derivatives by a Simple 1,1-Carboboration Route

2,3-Dihydroboroles are readily formed by treatment of dicyclopropylacetylene with the strongly electrophilic borane B(C6F5)(3). The reaction involves a sequence of 1,1-carboboration reactions and proceeds via dienylborane intermediates. Consistent with this, the reaction of the conjugated enyne 15c with B(C6F5)(3) leads to the formation of a dihydroborole.