Unfortunately, Fe(TPP)Cl (entry 1) only delivered a 14% yield of heterodimer 1a along with 6% of a styrene homodimer 1c (12% of its theoretical maximum: a 1:1 ratio of homo-to -heterodimer). After a lengthy exploratory campaign (see SI), effective conditions were developed that 1) employed the novel iron catalyst [Fe(T4CPP)Cl] (see Fig ), 2) did not require oxygen and 3) delivered 87% of 1a (5:1 ratio with 1b), effectively outcompeting the C• dimerization pathway, despite use of a 1:1 substrate ratio. The T4CPP ligand proved unique: replacement with a sulfonic acid could not recapitulate the effect of the carboxylic acid; Fe(acac)3 alone was completely ineffective (entries 4 and 5), consistent with its inability to turnover by BnBr. Addition of base (NaHCO3, entry 4 and 5) proved essential, possibly a result of carbonate-silane coordination to increase Si-H hydricity, 20 quench of strong acid, 7 iron ligand exchange or ligand deprotonation. Notably, the reaction could tolerate air, but O2 was not necessary for catalyst turnover. This stands in contrast to prior work in which O2, 21 TBHP 22 or N-fluoro-collidinium additives reoxidized catalysts. 8 NaBH4 could replace phenylsilane (entry 7) for large scale applications. The redox neutrality of this new single-catalyst system allows facile scaling without the constraints of mass-transfer control between the liquid phase and gas headspace. In contrast, higher material throughput of an O2-dependent dual-iron reaction 7 required scaling-out to multiple runs at 0.1 mmols. Entry 2 scaled to 1 mmol with no change in yield (see SI). The cross-coupling proved general to promote selective heterodimerization of different benzyl radicals across a wide variety of commercially available materials. To the best of our knowledge, only polymerization reactions have merged styrenes and benzyl bromides, two widespread building blocks. In these polymerization examples, benzyl radicals add to a styrene terminus (β-position), whereas in Table , the benzyl bromide connects to the internal carbon (α-position) exclusively. A variety of substituents are well tolerated in the reaction, even electron withdrawing groups that can accelerate β-addition. Cyclic styrenes performed particularly well, despite increased steric hindrance; even tetrasubstituted alkenes are productive. Hetereocycles were less effective when directly conjugated to the alkene (see SI for a list of unsuccessful partners), but a tetrahydropyridine (precursor to 2n) was among the best performing substrates. Ortho-substitution did not have a major impact on yield, even from 2,6-dichloro-α-methylstyrene, where di-planarity of the arene-alkene π-system is obstructed severely, suggesting that conjugation is not required for efficient MHAT. Similarly, Z-and Eβ-styrenes reacted with comparable efficiency, even though the cis-alkene is rotated out of conjugation (see also Table ).