Phenoxyimine Iron Aryl and Alkoxide Complexes as Reactive Intermediates in Iron-Catalyzed C(sp)-C(sp) Suzuki-Miyaura Cross-Coupling.

The mechanism of phenoxyimine iron-catalyzed C(sp)-C(sp) Suzuki-Miyaura cross-coupling (SMC) mediated by alkoxide bases has been investigated through a combination of catalytic and stoichiometric experiments, focusing on the synthesis of catalytically relevant iron intermediates. Phenoxyimine (FI) iron bromide, alkoxide, and aryl complexes bearing pyridine and 4-dimethylaminopyridine (DMAP) ligands were synthesized and characterized by a combination of Fe Mössbauer spectroscopy, H NMR spectroscopy, magnetometry, and X-ray diffraction. Resting-state analysis supports turnover-limiting transmetalation from neutral aryl boron nucleophiles to high-spin, tetrahedral iron(II) alkoxide complexes to yield the corresponding iron(II) aryl derivatives. Based on the experimental data, these high-spin, tetrahedral iron complexes likely both capture the electrophile-derived C(sp) radical and initiate the radical chain responsible for C(sp)-C(sp) bond formation. High-spin iron(II) aryl complexes with both electron-donating and -withdrawing substituents were isolated, contrasting the scope limitations of the catalytic cross-coupling, where only aryl boronate nucleophiles with electron-deficient groups afforded a high yield of cross-coupled product. The iron alkoxide complexes converted to catalytically inactive bis(chelate) iron complexes and iron alkoxide aggregates over time through a pathway involving dissociation of the pyridine ligand. Thus, the success of the catalytic cross-coupling reaction is dependent on the relative rates of two competing processes: (i) the rate of transmetalation of the aryl organoboron nucleophile versus (ii) the rate of deactivation of the iron alkoxide resting state through conversion to the bis(chelate) iron complex. Addition of DMAP to both stoichiometric and catalytic reactions suppressed catalyst deactivation and improved cross-coupling performance.