Intermolecular competition experiments validate these computational predictions (Figure ). We reacted one equivalent each of 2e and [ 2 H]-2e until this mixture consumed one equivalent of NIS. The [ 2 H]-2e/2e ratio of 2.70 measured at the end of the reaction indicates that 2e is consumed preferentially and supports the selectivity-determining deprotonation in Figure . The same experiment using NBS did not show a significant bias between [ 2 H]-2e and 2e at the end of the reaction, in line with selectivity determined by C-Br bond formation.