Non-covalent interaction (NCI) plots visually indicate favourable interactions within the system, with both diastereomeric transition states showing stabilising (green) interactions with the isothiouronium intermediate (inset in Figure ). Both electron rich motifs form stabilising interactions with the N-acylated catalyst; however, by considering the stabilisation of the complex relative to the three isolated components of TS1 (see SI), we can quantify the interaction for each recognition motif. O heteroatom recognition in TS1major is favoured by 11.4 kcal/mol relative to the aryl recognition in TS1minor which is a strong contributor towards the selectivity. This arises from the preferential orientation of the N-C=O carbonyl donor motif over the isothiouronium ion that can be observed from the electrostatic potential surface in Figure .