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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.

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