Past exam of the chemistry B course of the University of Cambridge / 2021 / Paper 1 / B2 / a / ii / Solution

For an amine-first pathway, use triethylamine as a nucleophile in a reversible conjugate addition. Its lone pair attacks C3 of the enone; electron shifts through C2 and the carbonyl give a zwitterionic enolate with a C3–$\mathrm{NE}{\mathrm{t}}_3^{+}$ bond.

The C2 nucleophilic site of that enolate attacks $\mathrm{B}{\mathrm{r}}_2$, installing C2–Br and releasing bromide. Removal of C2–H by base then reforms C2=C3 while the C3–N bond breaks back to neutral triethylamine. This addition–bromination–elimination sequence gives the same C2-brominated enone and regenerates the nucleophilic amine, with another base equivalent accepting the eliminated proton.

These are alternative formal sequences requested by the question, not a kinetic determination that one must dominate under unspecified conditions. In particular, the amine-first sequence does not require triethylamine to generate a high concentration of a vinylic carbanion by direct deprotonation.