We investigated the catalytic performance of the transient directing group using visual comparison methods developed by Blackmond (RPKA) and Burés (VTNA) Same-excess kinetic experiments can determine if product inhibition or catalyst deactivation occur during a reaction. Pleasingly, deactivation of the amino acid catalyst was not occurring. Instead, an inhibitory effect of the sulfonyl aldehyde product was observed, which slowed reaction progress. Studying the imine formation by 1 H NMR, in the absence of copper, indicated a preference for balanine to form an imine with the starting aldehyde in competition with the sulfonylated product aldehyde (41:7 imine of 1:3a). The simple imine formation was unlikely to be the cause of inhibition, therefore, we propose that the inhibitory effect arises from the formation of a copper ligated imine intermediate involving the sulfonyl aldehyde product imine, temporarily removing the catalytic transient directing group from the cycle. This helps to explain why using the aldehyde substrate in excess gave an increased yield, which could help alleviate the inhibition effects by further favouring formation of the starting material imine hence shifting the equilibrium to favour the productive pathway.