The Study of Iron Complexes and Iodine Oxides for C-H Bond Activation and Functionalization

The production of alkyl arenes from benzene and olefins comprises a major sector of the petrochemical industry. These compounds are typically synthesized by Friedel-Crafts or zeolite catalysis. As a result of the acid-based mechanism, these reactions result in polyalkylation, which requires an energy-intensive trans-alkylation step to obtain the desired monoalkylated product, do not provide a way to make anti-Markovnikov addition products, and offer poor control of regioselectivity on substituted arenes. We have been studying an alternative mechanism that involves transition metal-mediated olefin insertion and aromatic C–H activation that may improve upon the deficiencies mentioned for acid-mediated benzene alkylation. Transition metal complexes that catalyze olefin hydroarylation by metal-mediated olefin insertion and C–H activation are based on expensive noble metals (e.g., Ru, Ir, Pt). Our group has previously studied olefin hydroarylation using TpRu(L)(NCMe)Ph complexes (Tp = hydridotris(pyrazolyl)borate, L = neutral, two-electron donor). This Dissertation is focused on extending the catalytic activity observed for TpRu(L)(NCMe)Ph complexes to ruthenium’s first row, Earth abundant counterpart, iron. However, examples of Fe complexes that can activate aromatic C–H bonds are rare.
The complex Cp*Fe(CO)(NCMe)Ph (Cp* = pentamethylcyclopentadienyl) was synthesized and characterized. It was found that this complex was able to activate the C–H bonds of benzene at 50 °C. Additionally, Cp*Fe(CO)(NCMe)Ph regioselectively activates the 2-position of furan, thiophene, and thiazole at, or below, room temperature. Cp*Fe(CO)(NCMe)Ph selectively activates the aromatic C–H bond of 2-methylfuran over the methyl C–H bond, which provides evidence against an H atom abstraction mechanism. A combined experimental and computational mechanistic study was undertaken for the C–H activation reaction of Cp*Fe(CO)(NCMe)Ph and furan. From this study, the mechanism of furan C–H activation involves reversible NCMe dissociation from Cp*Fe(CO)(NCMe)Ph, reversible coordination of furan followed by rate-determining C–H activation by a σ-bond metathesis transition state, and subsequent NCMe coordination.
Applying Cp*Fe(CO)(NCMe)Ph to catalytic ethylene hydrophenylation resulted in the production of 1.2 turnovers of styrene and 0.6 turnovers of ethylbenzene. Studies indicate that β-hydride elimination from Cp*Fe(CO)(CH2CH2Ph) to give an inactive Fe–hydride complex is likely competitive with benzene C–H activation. Attempts to catalyze ethylene hydroarylation using furan or thiophene were unsuccessful, which is attributed to prohibitively slow ethylene insertion into the Fe–aryl bond. Rather than catalyzing alkyne hydrophenylation, the reaction of Cp*Fe(CO)(NCMe)Ph and internal alkynes results in the formation of novel hydroxyindenyl and vinylidene ligands from intramolecular reactivity following alkyne insertion into the Fe–Ph bond of Cp*Fe(CO)(NCMe)Ph.
Under photolytic conditions, Cp*Fe[P(OCH2)3CEt]2Ph activates the C–H bond at the 2-position of furan and thiophene and the 5-position of 2-methylfuran. While no catalysis was achieved with this complex under thermal or photolytic conditions, it was discovered that the reaction of Cp*Fe[P(OCH2)3CEt]2(2-furyl) with excess 2-butyne affords a new ferrocenyl-type complex that forms via ring opening of the furyl ring. The synthesis of Fe complexes outside the Cp*Fe motif have also been investigated, including Fe complexes based on phosphine-tethered cyclopentadienyl ligands and 2,6-bis(dihydrocarbylphosphinomethyl)pyridine ligands.
Additionally, the partial oxidation of light alkanes using periodate and chloride salts in trifluoroacetic acid has been studied. It was discovered that KIO4 and KCl mediate the partial oxidation of methane to methyl trifluoroacetate and methyl chloride in 42% yield using low pressures of methane (860 kPa) at 200 °C in one hour. KIO4 and KCl also functionalize ethane and propane in >20% yields. These results are relevant to the development of new technologies for the conversion of natural gas into liquid fuels.