Polysubstituted pyrimidines also participated in the N → N exchange as summarized in Figure . Importantly, 2-substituted pyrimidines 28 and 35 were found to undergo N labeling in serviceable yields (see the Supplementary Materials for details). For 2-substituted pyrimidines, there is a large steric barrier associated with the triflylation step, which correspondingly renders the triflyl motif more susceptible to attack by a nucleophile in the resulting pyrimidinium species. These factors likely explain the lower labeling yields observed for 2-substituted substrates. C5-Functionalized pyrimidines 30-32 also participated in the reaction, with higher chemical and isotopic yields observed for electron-rich pyrimidines. Finally, several elaborated pyrimidines underwent labeling in variable degrees of success (Figure ). Practically, the yields of several "complex" pyrimidines were limited by poor solubility in the reaction medium (see the Supplementary Materials for details). Additionally, the inherently low nucleophilicity of the pyrimidine heterocycle often led to competing triflylation at other, more reactive sites on these molecules, preventing the desired reaction or inducing side reactivity. Relative to the C5-aryl substituted pyrimidines, on average, the 4-arylpyrimidine substrates feature larger HOMO coefficients on the heterocyclic nitrogen atoms and higher-lying HOMOs, indicative of enhanced nucleophilicity. In fact, the HOMOs of the 5-arylpyrimidine substrate are minimally localized on the heterocyclic nitrogen atoms (Figure ). The N-triflyl derivatives of the 5-arylpyrimidines also feature greater LUMO character on the heterocyclic carbon atoms and are less sterically hindered at these sites, indicating a higher likelihood of deleterious bis-nucleophile addition (Figure ). These data are corroborated by the Fukui nucleophilicity and electrophilicity indices for the nitrogen and carbon atoms of the neutral and triflylated species, respectively (Figure ).