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) 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 PhMe2SiCH=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 Me2Si-(CH=C=O)(2) (13) underwent bromination by mixed kinetics with both first- and second-order terms in [Br-2].