Diverging Pathways in the Activation of Allenes with Lewis Acids and Bases: Addition, 1,2-Carboboration, and Cyclization

The reactions of allenes with frustrated (or cooperative) Lewis acid/base pairs result in the 1,4-addition of the base pair to the allene. The reactions of allenyl ketones and esters just in the presence of the strong Lewis acid B­(C6F5)3 afford the selective formation of the 1,2-carboboration products. In both cases the Lewis acid activates the allene to either a C6F5 migration or nucleophilic attack by the Lewis base. In addition to the 1,2-carboboration pathway, which can be viewed as being triggered by activation of the ketone (σ-activation), in the case of allenyl esters the corresponding cyclization products are observed in the presence of water.