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Thesis defences

PhD Oral Exam - Mohammad Sharif Askari, Chemistry

Copper-Mediated Oxygenation and Amination of Phenols


Date & time
Thursday, July 30, 2015
2 p.m. – 5 p.m.
Cost

This event is free

Organization

School of Graduate Studies

Contact

Sharon Carey
514-848-2424 ext. 3802

Where

Richard J. Renaud Science Complex
7141 Sherbrooke W.
Room 265.29

Wheel chair accessible

Yes

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

Phenols are abundant feedstock chemicals and their functionalization constitutes an attractive method for the synthesis of functional compounds. In biological systems the copper containing enzyme tyrosinase catalyzes the ortho-oxygenation of tyrosine into L-dopaquinone in the first step of the melanogenesis process. The copper centres in the enzyme activate O2 hrough the formation of μ-η22-peroxodicopper(II) (SP) and subsequent electrophilic aromatic substitution on tyrosine provides L-dopaquinone. Despite many efforts for the past 50 years to develop a synthetic mimic for the aerobic oxygenation of phenols a truly catalytic system for the selective ortho-oxygenation of phenols was only recently reported. The copper(I) complex of N,N’-di-tert-butylethylenediamine (DBED) is used as the precatalyst that activates oxygen and catalyzes the ortho-oxygenation of phenols. Investigating the mechanism of this catalytic process is the first objective of this research whereby spectroscopic techniques are used to identify intermediates in this reaction and perform kinetic studies. The second objective of the research is the development of analogous phenol ortho-amination using nitrosoarenes as substitutes for oxygen.

The mechanistic studies were performed by spectroscopic characterization of intermediates and comparison with independently prepared complexes or literature spectra. In situ UV-vis spectroscopy of the reaction at 25 ºC demonstrated the formation of a copper(II)-semiquinone complex that persists during the oxygenation process. In order to identify the preceding intermediates, low temperature UV-vis spectroscopy was used and the intermediates were characterized spectroscopically through comparison with similarly reported species. Based on the observed intermediates a mechanism is proposed whereby the copper complex activates oxygen to forming SP; an oxygen atom is then transferred to a bound phenolate through electrophilic aromatic substitution. Kinetic studies using low-temperature stopped-flow techniques proved the binuclear mechanism of O2 activation and no dependence on phenol. The mechanistic investigation suggests that oxygen activation and phenol functionalization proceed through the same pathway as the one observed with tyrosinase thus proving the bio-mimetic nature of this catalytic reaction.

Nitrosoarenes (ArNO) are isoelectronic with singlet oxygen, therefore copper(I)/ArNO complexes are structural and electronic mimics of Cu/O2 complexes and are expected to react in a iv similar way with external substrates. The redox lability of nitrosoarenes was proved by preparing and characterizing a copper(II)-nitrosoarene radical-anion complex. Structural and computational investigation confirmed its similarity to end-on copper(II)-superoxo species. The two-electron reduction of nitrosoarene was performed using the copper(I) complex of diamine ligand and forms a side-on copper(II)-peroxo (SP) mimic. This complex undergoes stoichiometric reaction with phenolates to form aminophenols after reductive work-up, thus demonstrating similar reactivity to tyrosinase models.

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