Direct Conversion of Aldehydes and Ketones to Allylic Halides by a NbX(5-)[3,3] Rearrangement.

Sequential addition of vinylmagnesium bromide and NbCl(5), or NbBr(5), to a series of aldehydes and ketones directly provides homologated, allylic halides. Transposition of the intermediate vinyl alkoxide is envisaged through a metalla-halo-[3,3] rearrangement with concomitant delivery of the halogen to the terminal carbon. The [3,3] rearrangement is equally effective for the conversion of a propargyllic alcohol to the corresponding allenyl bromide.