Addition of Bromine to Ketenes and Bisketenes: Electrophilic Attack at Carbonyl Carbon and Neighboring Group Participation

Addition of bromine to bisketene (Me₃SiC=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 CD₃CN compared to CDCl₃, consistent with an ionization pathway for the rearrangement. The bromination of 1 in CH₂ClCH₂Cl followed second-order kinetics with a rate constant (2.1 ± 0.1) × 10⁴ M^-1 s^-1 at 25°C. The first-order dependence of bromine addition to 1 on [Br₂] is attributed to intramolecular nucleophilic assistance by the second ketenyl moiety in an initial complex of 1 and Br₂ to give E-7. A transition structure for this process has been calculated by ab initio methods. By contrast PhMe₂SiCH=C=O 11 and γ-oxoketene 16 underwent bromination by third-order kinetics, second order in [Br₂], indicating the absence of neighboring group participation in the rate-limiting step. The bisketene Me₂Si-(CH=C=O)₂ (13) underwent bromination by mixed kinetics with both first- and second-order terms in [Br₂].