Solid State Hydroxylation of Unactivated C-H Bonds Using FeIIIbTAML Catalyst

Sajeevan, Amritha (2022) Solid State Hydroxylation of Unactivated C-H Bonds Using FeIIIbTAML Catalyst. Masters thesis, Indian Institute of Science Education and Research Kolkata.

Text (MS dissertation of Amritha Sajeevan (17MS207)) 17MS207_Thesis_file.pdf - Submitted Version Restricted to Repository staff only Download (1MB)

Abstract

The direct introduction of oxidized functionalities into isolated, unactivated, aliphatic C-H bonds is of immense interest in the context of organic synthesis. Nature has evolved a multitude of enzymes like oxidoreductases that can stitch on the functional groups to the rim of the hydrocarbon skeleton with excellent regio- and stereoselectivities. Enzymes like cytochrome P450 employ such a reaction through a high valent metal oxo species that can abstract H atom from alkane and the resulting radical immediately rebounds with the oxygen to result in a hydroxylated product. Several studies are reported on catalysts mimicking the structure and reactivity of these enzymes. But the intermediate in those systems are not stable and the mechanistic pathway is not studied for FeV(O) whereas FeIV(O) radical cation species have been reported. The first report of the mechanistic study of a FeV(O) species came in the late 2000s in a tetra amido macrocyclic ligand system. A biuret modified system was developed and it was found to catalyze the oxidation of a wide range of substrates. The C-H bond reactions in the solvent system have certain limitations like plausible solvent oxidation, solubility issues of hydrophobic substrates, use of harsh solvents, and waste solvents generated by these reactions. Hence, we proposed that it will be more sustainable to carry out the C-H bond oxidation in solventless conditions. This led us in exploring the field of mechanochemical activation. There have been many reports of mechanochemical C-H bond activation in the past two decades. These reactions showed better yield, and selectivity in less reaction time compared to the corresponding solvent-based reaction condition. Inspired by these, we studied the mechanochemical activation of C-H bonds using an (Et₄N₂)[(NO₂- bTAML)FeIIICl] system in presence of mCPBA. Since mCPBA was not a green oxidant, we explored sodium percarbonate. The H₂O₂ in sodium percarbonate was not activated by the catalyst because of the formation of FeIV(O) dimer. Hence, we switched to a derivative of the catalyst that has been synthesized in our lab for the C-H bond hydroxylation. We observed that the catalyst was able to hydroxylate many substrates in good yields in less reaction time under the atmospheric condition with a preferential selectivity for tertiary sites. This is the first-ever report of a mechanochemical C-H bond activation reaction through a high valent metal oxo intermediate.

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