To determine the minimum energy pathway for the formation of five-membered rings 4 and 6, DFT calculations were carried out (see SI for details) using the xylyl group instead of mesityl for the calculations as a simplified model. A recent computational study by Zhang and Su exploring the cycloaddition of structurally related azophosphines with cyclooctyne found a concerted reaction pathway, with an activation barrier of 22.0 kcal.mol -1 . Cummins also calculated the reaction of their MesN2PA with cyclooctyne to be thermodynamically favourable via a concerted pathway, with subsequent loss of the anthracene unit. Our calculations showed a stepwise mechanism to instead be preferred for the cycloadditions of azophosphines 1 and 3 with diethyl acetylenedicarboxylate (Scheme 4). Focusing on the pathway for 3 to 6, initial attack of the phosphorus lone pair (HOMO) of 3 to the alkyne yields the zwitterionic intermediate I1, with a corresponding activation barrier of 20.4 kcal.mol -1 (TS1) corresponding to P-C bond formation. I1 adopts a partially allenic, partially bent structure, best described as somewhere between the two forms I1A and I1B. This is supported by a C1-C2 bond length of 1.329 Å, consistent with a C=C double bond, and an elongated C2-C3 bond length of 1.401 Å, with a C1-C2-C3 bond angle of 138.3°. The HOMO of I1 has contributions from both the anionic carbon centre (I1A) and anionic oxygen (I1B), while the LUMO is mainly centred on the N=N π* orbital. Subsequent HOMO to LUMO ring-closing proceeds over a small activation barrier of 3.3 kcal.mol -1 (TS2). Scheme 4. Computed pathways (Gibbs free energy, kcal.mol -1 ) for the cycloadditions of 1 and 3 with (C(CO2Et))2 to form 4 and 6 at the ωB97XD/def2TZVP//ωB97XD(toluene)//def2QZVP level of theory using the xylyl group instead of mesityl for a slightly simplified model. For full computational details see SI.