an unambiguous radical-radical dimerization 29 (1.5 : 1 dr) (Figure ) differed significantly from the iron-catalyzed hydrobenzylation (4 : 1 dr), suggesting that the C-C bonds form by different mechanisms. 9,30 Furthermore, direct radical-radical coupling required excess primary benzyl partner to achieve appreciable heterocoupling, which occurred in low yield and low selectivity. Preference for a heterodimer using iron catalysis likely reflects the different stabilities of 1°, 2° and 3° C-Fe complexes 10 : equilibria would disfavor tert-alkyl iron porphyrin complexes, which homolyze above 0 °C. Taken together, polar and alkene coordination pathways, as well as C• C• bond formation, appear unlikely mechanisms, whereas MHAT / SH2 governed by the single Fe(T4CPP)Cl catalyst fits the data well and enables high preferences for heteroselectivity.