THE BROMINATION OF VINYLBORONIC ACIDS ON ALUMINA

The reaction of a series of vinylboronic acids with Br-2 on alumina ha s been examined. A mixture of (E)- and (Z)-vinyl bromides was formed i n every case by competing mechanisms: (1) S-E(2) reaction of the surfa ce-complexed vinylboronic acid with Br-2 to form the E isomer and (2) anti-elimination of B(OH)(3) and Br from the surface-complexed adduct arising by the reaction of vinylboronic acid and Br-2 in solution, whi ch affords the Z isomer. As was the case with the comparable iodinatio n reactions, the bromination of the vinylboronic acids synthesized fro m phenylacetylene and 3,3-dimethyl-1-butyne gave inordinately large am ounts of (E)-vinyl halides. Because of steric factors in these example s the anti elimination described above is retarded and a syn eliminati on occurs preferentially. Experiments were performed to test the mecha nisms.