31P NMR spectra (proton-coupled) of the of the time course of reaction of tetrakis-tributylammonium pyrophosphate, 0.16 M, in DMF-d7 with 13C-labeled CDI. Panel A: 1.1 mol CDI per mol pyrophosphate; from top to bottom: 5, 12 and 33 min after CDI addition. Panel B: 3 mol CDI per mol pyrophosphate, from top to bottom: 5, 25 and 240 min after CDI addition. “*” Designates the condensation by-products (tetraphosphates).