We have calculated the intermediate states and TS of the most probable pathway in Figure using various methods and approaches (details are given in ESI). To confirm, that the mechanism indeed proceeds only through two TS, we have performed the intristic reaction coordinate (IRC) scans for both of these TS. The results at the r 2 SCAN-3c level of theory in toluene is shown in Figure , and more IRC scans, including the animations, are given in the ESI. As we can see, the reaction indeed proceeds upon a formation of the intermediate cyclic adduct, which then converts into the isocyanate in a single step. To ensure that we are not missing an intermediate structure in this complex motion, we performed an additional relaxed internal rotation scan for the internal rotation of two rings was made at the ωB97x-D3(BJ)/def2-TZVPPD level of theory in both gas phase and benzene. No stable conformer of the intermediate B was found (results are shown in ESI).