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

Abstract

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.

Halogen Bonding in Solution

“…In the acetonitrile solution of [bis(acetonitrile)iodine] + salts, the iodine-bound acetonitrile is readily displaced by the stronger electron donor pyridine to give the corresponding [bis(pyridine)iodine] + complex. Analogous ligand exchange has also been observed by other investigators [85,89]. Brown et al reported a tenfold increase of exchange rate for the ligand replacement upon changing the solvent from dichloromethane to the more polar acetonitrile [89].…”

“…Analogous ligand exchange has also been observed by other investigators [85,89]. Brown et al reported a tenfold increase of exchange rate for the ligand replacement upon changing the solvent from dichloromethane to the more polar acetonitrile [89].…”

Total syntheses of the squalene-derived halogenated polyethers ent -dioxepandehydrothyrsiferol and armatol A via bromonium- and Lewis acid-initiated epoxide-opening cascades

“…Br(coll) 2 ClO 4 was prepared according to the previously reported procedure, 41 and kept at 0 °C in the absence of light.…”

Catalytic, Asymmetric Halofunctionalization of Alkenes—A Critical Perspective

“…Simple consideration of the lack of available orbitals suggests that this predissociation equilibrium is likely common to all doubly ligated halogen(I) sources, as is the end-on approach dictated by the NÀBr s* orbital geometry. [24] These considerations have obvious implications for the depiction of reasonable reaction mechanisms.…”