Past exam of the chemistry course of the year 4 of the natural sciences course of the University of Cambridge / 2022 / Paper 1 / 9 / a / ii / Solution

The nonnucleophilic magnesium amide metalates the triazole carbon between the two ring nitrogen atoms, conventionally C5 of 1-propyl-1,2,4-triazole. That proton is more readily removed because the resulting heteroaryl anion is stabilized by the neighboring nitrogen atoms; the reagent therefore acts as a base rather than adding a tetramethylpiperidinyl group.

The heteroaryl organomagnesium intermediate attacks the allylic bromide, forming a new C–C bond. The product is 5-(cyclohex-2-en-1-yl)-1-propyl-1,2,4-triazole. Ordinary allylic substitution and an $S_{\mathrm{N}}{2}^{\prime }$ description give the same unlabelled cyclohexenyl connectivity for this unsubstituted allylic ring, so the structural assignment does not distinguish those trajectories.

The molecular formula checks the assignment: N-propyltriazole is ${\mathrm{C}}_5{\mathrm{H}}_9{\mathrm{N}}_3$; replacing a ring H by ${\mathrm{C}}_6{\mathrm{H}}_9$ gives ${\mathrm{C}}_{11}{\mathrm{H}}_{17}{\mathrm{N}}_3$, as printed.