Addition of Bromine to Ketenes and Bisketenes: Electrophilic Attack at Carbonyl Carbon and Neighboring Group Participation.
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)].
Brown
R. S.
RS
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.
Christl
Manfred
M
Lough
Alan J.
AJ
Ma
Jihai
J
Peters
Eva-Maria
EM
Peters
Karl
K
Sammtleben
Frank
F
Slebocka-Tilk
Henryka
H
Sung
Kuangsen
K
Tidwell
Thomas T.
TT