To rationalize these observations, we once again turned to DFT calculations, using the solvation parameters for toluene. Having already established that the rate-limiting step is the initial addition, the reactants and transition states for the different substituted examples were obtained. These revealed that whereas the computed barrier for 3 is 27.3, the barriers for 1, 48, and 50 are 30.7, 27.5, and 29.2 kcal/mol, respectively. In general, a deactivating effect by larger groups is observed. Comparing the transition state for 3 (52) with that for 1 (53) reveals a significant difference between the two (Figure ). In this process, the double bond shifts along one carbon atom and in the transition state there are therefore two adjacent partial double bonds that would ideally have the three atoms involved and all of their substituents coplanar (substructure highlighted in blue in Figure ). For the reaction of 52, this can be tolerated and the C(Me)-C-C-Cl dihedral angle is 163°. However, co-planarity in the reaction of 1 would entail the Ph group clashing with the quinoline. To avoid this clash, the C(Ph)-C-C-Cl dihedral angle must reduce to 147° in transition state 53, and this species corresponds to a higher barrier than for 52.