Figure 4. Enantioinduction is achieved by the thiourea catalysts through a selective, attractive cation-π interaction between the extended aromatic residue on the catalyst and the acidic a-protons in the episulfonium ion.

a. Correlation between rate and enantioselectivity of reactions catalysed by thioureas 3a-3g. Each data point represents the average rate determined from two individual kinetic experiments, with the error bar showing the range of the measurements. The rate constants for the major and the minor pathways (k_asym,major and k_asym,minor, respectively) are calculated on the basis of the rate equations (eqs. 3 & 4) and the following equations: (a) r_asym = r_asym,major + r_asym,minor, (b) er = r_asym,major/r_asym,minor. Lines represent least-squares fits. b. ¹H NMR binding study of thiourea and 5 in d₈-toluene, showing attractive interactions between the aromatic group in 3e and the a-protons in 5. The resonances of the benzylic protons and the methyl protons in 5 are labled with blue and green dots, respectively. In the two bottom spectra, the methyl resonance overlaps with the solvent peak, but can be identified when zoomed-in c. Electrostatic potential maps for fully optimized structures (B3LYP/6-31G(d)) of the episulfonium ion derived from 1a and the sulfonium ion in 5, revealing a similar distribution of positive charge over the benzylic protons. Negative potentials are shown in red and positive potentials in blue.