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)].

Competing kinetic pathways in the bromine addition to allylic ethers in 1,2‐dichloroethane: Opposite temperature effects

“…Another case concerns the bromination of a bisketene in DCE, which follows a full second‐order kinetic pathway 8. This result has been attributed to the nucleophilic assistance of a ketenyl moiety during the addition of the molecular bromine to the other ketenyl group.…”

Cationic ring‐opening polymerization of six‐membered cyclic carbonates with ester groups

“…In this case, neighboring group participation is employed to stabilize the propagating polymer end, resulting in a new class of living polymerization. Generally, neighboring group participation plays an important role not only in selective chemical syntheses such as oligosaccharides9 but also in other fields of synthetic chemistry, including the regiochemical control of the ring opening of oxirane by nucleophiles,10 the nucleophilic substitution reaction of N ‐sulfonyloxyamide under basic conditions,11 the solvolysis of neophyl‐like systems,12 and the addition of bromine to ketenes 13. In the field of polymer chemistry, the control of the polymerization by neighboring group participation has been recently reported in the cationic ring‐opening polymerization of ester‐substituted oxiranes14 and imide‐substituted oxetanes 15.…”