First, the reactant (PhCNO) forms a pre-intermediate A with the catalyst (SO 2 ). Then, it converts into the intermediate B by a [3+2]-cycloaddition through the transition state TS1. Then, the intermediate B converts into products through the TS2. In this case, the newly formed five-membered ring rotates by 90 • with respect to the phenyl ring, and the nitrogen moves towards the phenyl group. Then, the post-intermediate C is produced, which is a complex of the reaction product (PhNCO) with the catalyst, and in the end, they dissociate into separate species.