Similar effects are responsible for the increased enantioselectivity offered by catalysts 10-12 in comparison to 3 (e.g., Figure for comparison of 3 vs. 11). Catalyst-transition state complex 64 adopts a preferred conformation where the C-Cl bond is anti with respect to the catalyst P=O (Figure ) and this is not possible for the pro-S transition state due to the 3,3'-substituents: this is a key contributor to enantioselectivity. In the alternative conformation that must be adopted when the catalyst has a 3,3'-substituent, the aniline unit adopts an anti-orientation that places it between two of the 2,6-alkyl substituents: these are then key to enantioselectivity in the rate-determining proton transfer event. As the substituent bulk is increased from i-Pr (catalyst 3) to cycloalkyl (catalysts 10-12), transition state-stabilizing contacts increase between the alkyl group at the 4-position and the aniline unit in the favored transition state. This serves to strengthen preference for the observed enantiomer. In contrast, moving from i-Pr to larger cycloalkyl groups increasingly destabilizes the transition state that leads to the disfavored enantiomer by compressing the gap that the aniline occupiesthe aniline is effectively extruded and must rotate away. Thus, good shape complementarity with the 4-alkyl in the pro-R transition state and space-filling by the 2,6-alkyl groups in the pro-S transition state lead to the observed improvement in enantioselectivity with the cycloalkyl catalysts.