Product 6 is favoured by 50.1 kcal.mol -1 relative to the starting materials. An identical pathway was found for the formation of 4 from azophosphine 1, with similar activation barriers calculated. The preference for a stepwise pathway in our system compared to the previously computed concerted pathways may be rationalised by the stabilisation of the zwitterionic intermediate I1 by the electron-withdrawing ester groups, and the fact that our study is on P,P-dialkyl azophosphines that are more nucleophilic than the previously studied P,P-diphenyl systems. A preference for stepwise over concerted reaction pathways has previously been shown for cycloaddition reactions of the analogous alkyne dimethyl acetylenedicarboxylate with dienylisobenzofurans. We also sought to determine the minimum energy pathway for the formation of seven-membered ring 7, and rationalise the why the analogous heterocycle with R = t Bu substituents did not form. Our calculations suggest that the first step, corresponding to nucleophilic attack by the dicoordinate nitrogen centre of 4/6 to the alkyne (TS3), is the rate-limiting step of the reaction (Scheme 5). This proceeds over an energy barrier of 18.1 kcal.mol -1 for 4 (R = i Pr), but the corresponding energy barrier for 6 (R = t Bu) is significantly higher, at 24.7 kcal.mol -1 . This barrier is accessible for 4 at room temperature, but was inaccessible for 6 in our experiments. Attempts to overcome this barrier by heating the reaction led to an intractable mixture of products. Seven-membered rings 7 and 8 form at room temperature but decompose above 50 °C, which may indicate that if the energy barrier for R = t Bu subsituents can be overcome at elevated temperatures, the product subsequently decomposes and cannot be isolated. After this first step, the subsequent intermediate, I2, adopts a partially allenic and partially bent structure (analogous to I1A and I1B, Scheme 4), although in this case the bent structure is stabilised to a greater extent as the carbocation has a weak interaction with the proximal electron-poor phosphorus centre. We were also able to locate a further intermediate (I3), which was only present for R = i Pr substituents, in which the P-N interatomic distance is significantly longer (2.145 in I3 vs. 1.756 in I2) and P-C bond shorter (1.842 in I3 vs 2.012 in I2). After extensive searching, we were unable to locate any transition states between I2 and the products, although relaxed potential energy surface scans indicated that these steps would be <5 kcal.mol -1 for R = i Pr and <7 kcal.mol -1 for R = t Bu and are thus clearly not ratelimiting (see SI for further details). Scheme 5. Computed pathways (Gibbs free energy, kcal.mol -1 ) for the reaction of 4 and 6 with (C(CO2Et))2 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.