For example, β-pinene as substrate leads to a 3:2 mixture of unrearranged and rearranged (U:R) hydrobenylated products (Figure ), indicative of radical ring opening. The corresponding anionic or organometallic pathways to ring opening are unlikely: protic solvent would immediately quench a tert-alkyl carbanion and steric repulsion from gem-dimethyls would preclude iron porphyrin approach syn to the cleaving cyclobutane bond. Furthermore, benzyl dimerization occurs in high yield when alkene is excluded (Figure ), consistent with Fe(II) reduction of the benzyl bromide and ruling out XAT as a C-Br homolysis pathway. The intermediacy of benzyl bromide-derived radicals are also signaled by clear benzaldehyde byproduct peaks in the 1 H NMR. We considered that the styrene-derived radical may be generated reversibly to suppress styrene homodimers, but according to deuterium labeling MHAT occurs irreversibly (Figure ) suggesting that SH2 outcompetes radical-radical dimerization. Similarly, we thought selective heterodimerization might occur by C• C• collision if the concentration of benzyl bromide-derived radical were suppressed by iron and a hindered styrene-derived radical were incapable of dimerization. However, the diastereoselectivity of