Past exam of the chemistry B course of the University of Cambridge / 2025 / Paper 1 / B1 / b / i

The first stage is a Stork enamine reaction. Pyrrolidine attacks the ketone carbonyl, and proton transfers give a carbinolamine. Loss of water gives an iminium ion; removal of the benzylic alpha proton produces the enamine conjugated with the aromatic ring. Removal of water and prolonged heating favour this condensation. That conjugated enamine is preferred over the alternative with its double bond on the other side of the original carbonyl.

The nucleophilic enamine carbon attacks the methylene carbon of bromoacetonitrile by an SN2 reaction, displacing bromide and making the carbon–carbon bond. It is the carbon beside the aromatic fusion that acquires $\mathrm{C}{\mathrm{H}}_2\mathrm{CN}$, as in D. Finally, water adds to the alkylated iminium carbon; proton transfers and cleavage of the C–N bond regenerate the ketone and release pyrrolidine. This explains carbon alkylation, its position, and restoration of the carbonyl rather than treating the amine as a permanent substituent.