 Solution

The carbon–phosphorus coupling in the ${}^{13}$C spectrum is consistent with deprotonation of the methyl group rather than loss of that group:Addition to the nitrile initially gives $\mathrm{P}{\mathrm{h}}_2\mathrm{P}(\mathrm{O})\mathrm{C}{\mathrm{H}}_2\mathrm{C}(\mathrm{tBu})=\mathrm{NLi}$. An intramolecular proton transfer then converts the imidoyl anion into a conjugated lithiated enamide:The phosphorus–carbon bond is retained. The rearrangement is a hydrogen migration, not a migration of the phosphoryl group to nitrogen. Addition of half an equivalent of TMEDA per Li gives the hemisolvated dimerThe tetrameric and dimeric nuclearities are established for this reaction system; the spectra in the question are consistent with them, but spectra alone would not uniquely prove every detail of the aggregates.^[ref]