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.