In addition to the aforementioned DFT studies, we also sought to gain experimental evidence for the proposed reaction pathway. We speculated that the replacement of the reactive -CO2Et functionality on 1 with an unreactive -CH3 group would prevent ringclosing, and thus stop the reaction proceeding past I3. We thus synthesised phosphorus/nitrogen-containing Scheme 3. Computed pathways (Gibbs free energy, kcal.mol -1 ) for the reactions of 1 and 2 with H2O to form the corresponding pyrazolylphosphine oxides, at the ωB97XD/def2TZVP//ωB97XD(toluene)//def2QZVP level of theory. For full computational details see ESI.