We hypothesize that higher concentrations of iron may increase the rate of the racemic background iron-only reactivity excluding thiol, in analogy to Shenvi and Herzon's hydrogenation systems, 8e, f resulting in the observed degradation of er. Overall, the various spiking studies described above support the possibility of catalyst deactivation arising from both iron and silane, a result consistent with the kinetic results above. However, the practical utility is low; the yield increase from spiking is only moderate even when carried out for both iron catalyst and silane.