Efficient two-step synthesis of 3-halo-3-enals or 2-halo-2-alkenyl ketones from propargylic bromides via a unique cationic 1,2-aryl or proton shift in electrophilic addition reaction of 2,3-allenols with X+

The reaction of readily available 1-substituted 2,3-allenols with Br-2, NBS, or I-2 afforded the not-easily-available but synthetically useful 3-halo-3-alkenals or 2-halo-2-alkenyl ketones in good yields via a sequential electrophilic interaction of X+ with the allene moiety, a 1,2-aryl or proton shift, and a H+-elimination process; the structures of the products were established by X-ray diffraction study.