Bis(pyridine)-Based Bromonium Ions. Molecular Structures of Bis(2,4,6-collidine)bromonium Perchlorate and Bis(pyridine)bromonium Triflate and the Mechanism of the Reactions of 1,2-Bis(2'-pyridylethynyl)benzenebrominum Triflate and Bis(pyridine)bromonium Triflate with Acceptor Olefins

1,2-Bis(2'-pyridylethynyl)benzenebromonium triflate (4) and bis(pyridine)bromonium triflate (5) have been prepared and the mechanism of their reaction with various acceptors including eight alkenes of various structure, collidine, and Br(-) are reported. The reaction of 4 with neutral acceptors is second-order overall and involves a preequilibrium dissociation of the bidentate-bound Br(+) to form an unstable monodentate open form (4-op), which reacts with all neutral acceptors at or near the diffusion limit. Br(-) reacts with 4 by a different mechanism involving a direct nucleophilic attack on the Br(+). The reaction of 5 with acceptors proceeds by a dissociative mechanism to reversibly form an unstable intermediate (pyr-Br(+)), which reacts with 4-penten-1-ol, 4-pentenoic acid adamantylidineadamantane and cyclohexene with nearly the same selectivity. The crystal and molecular structures of bis(2,4,6-collidine)bromonium perchlorate (2-ClO(4)) and 5 were determined by X-ray crystallography.