The rate drops even further when performing the amination exclusively with deuterated substrate 1a-d2. Interestingly, the rate of the competition experiment using a 1:1 ratio of 1a and 1a-d2 is exactly the average of the rates from runs using 1a and 1a-d2 individually. Obviously, this dependence does not result from a regular KIE, as even in the competition experiment, a 1:1 ratio of the two isotopologue products is maintained throughout the whole reaction. Instead, a secondary KIE may rationalize the observed rate dependence. For example, stronger coordination of the deuterated pyrrolidine 1b-d2 compared to 1b decreases catalytic activity, in line with product inhibition as deduced from kinetic measurements. Slightly stronger coordination of 1b-d2 is also predicted by density functional theory (DFT) calculations on the B3LYP 49,50 /def2-TZVP level, 53,54 as the exchange of coordinated 1b with 1b-d2 at Fe(HMDS)2 is computed to be exergonic by 0.2 kcal mol -1 (Table -S6).