C-H on the Oxo Ferryl Wheel: Comparison of Pyridine and Imidazole-Substituted Ligands for C-H Activation and Functionalization
The selective and efficient transformation of hydrocarbon feedstocks is of high value for industry and research. While Shilov-type organometallic methods have facilitated this goal, systems designed after nature’s use of cheap and abundant iron-based enzymes are desired for wider-scale applications. This work establishes hydrocarbon oxidation efficiency of synthetic pyridine-based ligands (BPMEN, BPMPN) compared to commercially available TPA with in situ generated catalysts. Literature studies of traditionally synthesized BPMEN systems and initial in situ studies offered evidence for enhanced reactivity (TON) as compared to TPA. Expansion to a propyl backbone to produce BPMPN tested the increased chelate ring size’s impact on reactivity as compared to BPMEN. Optimized catalysis yielded higher TON efficiency with BPMEN as well as greater selectivity compared to TPA across all substrates and conditions. Utilization of the BPMPN architecture not only offered lower TON as compared to the BPMEN precursor, but resulted in excessively low product yields below that of TPA, often near or below the limit of quantification. We next turned to addressing the lack of imidazole-substituted models through ligands BItCHEN and nBuBIMEN. Translating the previously established BItCHEN structure to our simple and modular synthetic approach ultimately proved unsuccessful. Therefore, we focused on nBuBIMEN, established by our collaborator. Preliminary investigations indicated that while this ligand was less effective for oxidation, epoxidation capability was near or above that of BPMEN. Interestingly, nBuBIMEN also showed evidence of a more stable oxo-species intermediate, leading to computational assessment of oxo stability across these scaffolds.
in situ, non-heme oxo, catalysis, biomimetic, C-H functionalization, C-H activation, Inorganic Chemistry, Organic Chemistry
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