title: Addition of Bromine to Ketenes and Bisketenes: Electrophilic Attack at Carbonyl Carbon and Neighboring Group Participation.
abstract: Addition of bromine to bisketene (Me(3)SiC=C=O)(2) (1) gave the fumaryl dibromide E-7, whose stereochemistry was proven by X-ray structure determination. Upon warming, E-7 rearranged to the furanone 8, and this process was faster in the more polar CD(3)CN compared to CDCl(3), consistent with an ionization pathway for the rearrangement. The bromination of 1 in CH(2)ClCH(2)Cl followed second-order kinetics with a rate constant (2.1 +/- 0.1) x 10(4) M(-)(1) s(-)(1) at 25 degrees C. The first-order dependence of bromine addition to 1 on [Br(2)] is attributed to intramolecular nucleophilic assistance by the second ketenyl moiety in an initial complex of 1 and Br(2) to give E-7. A transition structure for this process has been calculated by ab initio methods. By contrast PhMe(2)SiCH=C=O 11 and gamma-oxoketene 16 underwent bromination by third-order kinetics, second order in [Br(2)], indicating the absence of neighboring group participation in the rate-limiting step. The bisketene Me(2)Si(CH=C=O)(2) (13) underwent bromination by mixed kinetics with both first- and second-order terms in [Br(2)].
authors.affiliation: Department of Chemistry, University of Toronto, Toronto, Ontario, Canada M5S 3H6, Department of Chemistry, Queen's University, Kingston, Ontario, Canada K7L 3N6, Institut für Organische Chemie, Universität Würzburg, D-97074 Würzburg, Germany, and Max-Planck-Institut für Festkörperforschung, D-70569 Stuttgart, Germany.
authors.forename: R. S.
authors.initials: RS
authors.lastname: Brown
authors.forename: Manfred
authors.initials: M
authors.lastname: Christl
authors.forename: Alan J.
authors.initials: AJ
authors.lastname: Lough
authors.forename: Jihai
authors.initials: J
authors.lastname: Ma
authors.forename: Eva-Maria
authors.initials: EM
authors.lastname: Peters
authors.forename: Karl
authors.initials: K
authors.lastname: Peters
authors.forename: Frank
authors.initials: F
authors.lastname: Sammtleben
authors.forename: Henryka
authors.initials: H
authors.lastname: Slebocka-Tilk
authors.forename: Kuangsen
authors.initials: K
authors.lastname: Sung
authors.forename: Thomas T.
authors.initials: TT
authors.lastname: Tidwell