Calculated coordinates for 2 and 3 (scheme 1) are designated as "ylide preceding endo-TS1" or "ylide preceding exo-TS1" respectively in the supporting information 9 and they in fact represent rotamers around the C-O bond (Figure ). Species 2 (endo) has a C-O torsion angle of ~10° and species 3 (exo) ~165° These represent the two lowest energy conformations, whilst two further minima are higher in energy (Figure ) and are not used here for calculation of KIE. It is noteworthy that the rotation about the S-O bond is coupled in a gear-like manner to the C-O rotation, so that both occur in synchrony. In the original study, 2 was used as the reactant for computing KIE for endo-TS1 and in our replication work, we followed the same procedure.