Computational modelling assisted in rationalizing the reactivity and stereoselectivity SAR observed during catalyst screening reported in Table (Figure ). Free energy barriers for the catalytic cycle are lower for catalysts possessing the 4-(i-Pr) group (3 and 6) than for those without (5) or for the catalyst with 3,5-substituents (7, 8; Table S8, S10). The overall barrier arises from a combination of how effectively the catalyst stabilizes the ground state (the catalyst-product complex (65, Figure )) and the reaction transition state (64, Figure ). The inclusion of a 4-(i-Pr) group to catalyst 5 gives catalyst 6. This increases substrate-catalyst interactions, decreasing ground state stabilization more than the corresponding effect on the transition state, leading to a reduction in barrier. In contrast, adding i-Pr groups at positions 3 and 5 (catalyst 7) causes an even larger destabilization of the ground state; however, this is exceeded by the effect on the transition state, leading to an overall increase in energy barrier. For 6, the disruption of aromatic stacking in the product complex 65 is outweighed by added van der Waals-type contacts in the transition state 64 (vide infra) while this does not happen for 7.