Having determined the molecular composition of the key dealkylation transition state, we turned to computational modeling to elucidate the nature of catalyst-substrate interactions leading to rate acceleration and enantioinduction. A systematic search of transition-state conformers led to the identification of low-energy diastereomeric structures leading to the major (R) and minor (S) enantiomers of 3a, with relative energies in excellent agreement with the experimentally observed enantioselectivities. In both transition structures, the catalyst engages in a network of stabilizing noncovalent interactions with both cationic and anionic components of the phosphonium ion pair (Fig. ), in agreement with previous studies on thiourea-arylpyrrolidine scaffolds . These interactions appear to stabilize both charged components of the ion pair, consistent with our original design hypothesis. Close analysis of the computational models reveals specific stabilizing interactions that might be responsible for enantioinduction. The diastereomeric transition state structures leading to the minor (S) and major (R) product enantiomers display almost identical catalyst geometries but are related by a 120-degree rotation of the phosphonium ions within the catalyst active site. TS cat,S positions the phosphonium P-Ph group below the 2-phenanthryl group of the catalyst, while splaying its benzyl groups (highlighted green). The lower energy TS cat,R , in contrast, positions the phosphonium P-Ph group into solvent and stacks the two benzyloxy groups, with one residing below the 2-phenanthryl group of the catalyst. While TS cat,S and TS cat,R both possess several noncovalent attractive interactions in common, the following interactions are likely enantiodifferentiating: TS cat,S possesses one additional H-bonding interaction between a benzylic C-H and the amide oxygen (Fig. . orange box) while TS cat,R incorporates two benzylic C-H-π interactions in (Fig. , blue box). The net energetic benefit of losing one benzylic C-H hydrogen bonding interaction and gaining two benzylic C-H-π interactions may be sufficient to account for the sense and magnitude of enantioinduction . High enantioselectivity in Arbuzov-type reactions promoted by 1g demands that the catalyzed pathway is faster than the uncatalyzed racemic background reaction, a phenomenon that was confirmed in kinetic experiments (Fig. ). Given that spectroscopic and kinetic studies establish the resting state in the catalytic reaction as 1g⋅10a (Fig. ), the phosphonium chloride ion pair must be more reactive when bound to HBD 1g than in the free state. This experimental observation would seem to contradict the established understanding that SN2 reactions involving ionic nucleophile-electrophile pairs are strongly inhibited by hydrogen bonding interactions such as those that arise in protic media . In such systems, hydrogen bonding is understood to give rise to rate deceleration by several orders of magnitude due to preferential stabilization of the fully charged resting state over the partially charged transition state (Fig. ). In attempting to understand how HBD catalyst 1g might achieve rate acceleration in the phosphonium dealkylation step, we noted that the computationally modeled transition-state structure (Fig. ) predicts that 1g binds simultaneously to both cationic and anionic components of the phosphonium ion pair. We hypothesized that the catalytic activity of 1g might arise from this synergistic, geometrically well-defined ion-pair binding mode that would not be possible in simple hydrogen bonding environments such as protic solvent.