= Solution
Let $L^-=\mathrm{Ph_2P(O)CH=C(tBu)NH^-}$, with its O and N donors available for <lithium> coordination. A plausible symmetric tetramer has a \b[Li$_4$O$_4$ cubane-type core], with alternating Li and phosphoryl O vertices and an additional enamide N donor attached to each Li. The O,N-chelating <Ligand in coordination chemistry> can be arranged equivalently around the aggregate, consistent with one $^7$Li environment. This is a connectivity model, not a uniquely reconstructed diffraction structure.
The initially formed nitrile-addition product has an acidic $\mathrm{CH_2}$ group next to the <phosphine oxide>. Transfer of one of these <protons> to the nitrile-derived N gives a delocalized <lithiated enamide>, reinforced by O,N coordination and <organolithium aggregation>. The organic framework becomes $\mathrm{P(O)-CH=C(tBu)-NHLi}$.
Assign \b[$\delta_H=4.82$ to the vinylic CH and $\delta_H=3.25$ to NH]. Each integrates to one <proton>, as required after hydrogen migration. Their singlet appearance does not mean that the original carbon has become a freely rotating, equivalent-proton methylene group.
\Image[/past-exam-of-the-chemistry-course-of-the-year-4-of-the-natural-sciences-course-of-the-university-of-cambridge/2022/paper-1-enamide-aggregates.png]
{title=Enamide formation and plausible lithium–oxygen aggregation connectivities}
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