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Despite comprising a variety of bioactive compounds that can be utilized as effective synthetic precursors, the construction of halogenated arenes often relies on hazardous reagents and conditions that pose regioselectivity issues in complex systems. Halodecarboxylation using vanadium-dependent haloperoxidases (VHPOs) has emerged as a sustainable alternative for the synthesis of halogenated arenes. In the Biegasiewicz group, we recently discovered that VHPOs can furnish 3-bromooxindoles from 3-carboxyindoles through a decarboxylation event, followed by oxidation. While this tandem process was exciting, the intermediates of this process, 3- bromoindoles are independently valuable reagents, which necessitated further investigation. Herein we examine the biocatalytic access to bromoindoles for which we addressed the major challenge of undesired oxidation event. The first preventative approach acylated the indole nitrogen, resulting in 1-acetylindole-3-CO2H. This could then be subjected to optimized enzymatic bromination conditions to produce 1-acetyl-3-bromoindole in 98% yield with CiVCPO. The second preventative approach was to modify the reaction conditions, furnishing 1-methyl-3-bromoindole in 73% yield from 1-methylindole-3- CO2H with AmVBPO.
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Transition metals have been extensively employed to address various challenges
related to catalytic organic transformations, small molecule activation, and energy storage
over the last few decades. Inspired by recent catalytic advances mediated by redox noninnocent
pyridine diimine (PDI) and α-diimine (DI) ligand supported transition metals,
our group has designed new PDI and DI ligands by modifying the imine substituents to
feature donor atoms. My doctoral research is focused on the development of PDI and DI
ligand supported low valent first row metal complexes (Mn, Fe, Co) and their application
in bond activation reactions and the hydrofunctionalization of unsaturated bonds.
First two chapters of this dissertation are centered on the synthesis and
application of redox non-innocent ligand supported low valent iron complexes. Notably,
reduction of a DI-based iron dibromide led to the formation of a low valent iron
dinitrogen compound. This compound was found to undergo a sequential C-H and C-P
bond activation processes upon heating to form a dimeric compound. The plausible
mechanism for dimer formation is also described here.
Inspired by the excellent carbonyl hydrosilylation activity of our previously
reported Mn catalyst, (Ph2PPrPDI)Mn, attempts were made to synthesize second generation
Mn catalyst, which is described in the third chapter. Reduction of (PyEtPDI)MnCl2
furnished a deprotonated backbone methyl group containing Mn compound
[(PyEtPDEA)Mn] whereas reduction of (Ph2PEtPDI)MnCl2 produced a dimeric compound,
[(Ph2PEtPDI)Mn]2. Both compounds were characterized by NMR spectroscopy and XRD
analysis. Hydrosilylation of aldehydes and ketones have been studied using
[(PyEtPDEA)Mn] as a pre-catalyst. Similarly, 14 different aldehydes and 6 different
ii
formates were successfully hydrosilylated using [(Ph2PEtPDI)Mn]2 as a pre-catalyst.
Encouraged by the limited number of cobalt catalysts for nitrile hydroboration, we
sought to develop a cobalt catalyst that is active for hydroboration under mild conditions,
which is discussed in the last chapter. Treatment of (PyEtPDI)CoCl2 with excess NaEt3BH
furnished a diamagnetic Co(I) complex [(PyEtPDIH)Co], which exhibits a reduced imine
functionality. Having this compound characterized, a broad substrate scope for both
nitriles and imines have been investigated. The operative mechanism for nitrile
dihydroboration has been investigated based on the outcomes of a series of stoichiometric
reactions using NMR spectroscopy.