Graduation Year

2014

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Chemistry

Degree Granting Department

Chemistry

Major Professor

X. Peter Zhang, Ph.D.

Committee Member

Jon Antilla, Ph.D

Committee Member

Jianfeng Cai, Ph.D.

Committee Member

Edward Turos, Ph.D.

Keywords

azide, cobalt, cyclopropanation, diazoacetate, porphyrin

Abstract

Radical chemistry has attracted a large amount of research interest over the last few decades and radical reactions have recently been recognized as powerful tools for organic synthesis. The synthetic applications of radicals have been demonstrated in many fields, including in the synthesis of complex natural products. Radical reactions have a number of inherent synthetic advantages over their ionic counterparts. For example, they typically proceed at fast reaction rates under mild and neutral conditions in a broad spectrum of solvents and show significantly greater functional group tolerance. Furthermore, radical processes have the capability of performing in a cascade fashion, allowing for the rapid construction of complex molecular structures with multiple stereogenic centers. To further enhance the synthetic applications of radical reactions, current efforts are devoted toward the development of effective approaches for the regioselective control of their reactivity as well as stereoselectivity, especially enantioselectivity, a challenging issue that is intrinsically challenged by the "free" nature of radical chemistry.

This research has identified a fundamentally new approach to radical reactions based on the concept of metalloradical catalysis (MRC) for controlling the stereoselectivity of both C- and N-centered radical reactions. Cobalt(II) porphyrins [Co(Por)], are stable metalloradicals, and have been shown to enable the activation of diazo reagents and azides to cleanly generate C- and N-centered radicals, respectively, with N2 as the only byproduct in a controlled and catalytic manner. In addition to the radical nature of [Co(Por)], the low bond dissociation energy of Co-C/Co-N bonds plays a key role in the successful turnover of the Co(II)-based catalytic carbene and nitrene transfers. Through the support of porphyrin ligands with tunable electronic, steric, and chiral environments, this general concept of Co(II)-based metalloradical catalysis (Co-MRC) has been successfully applied to the development of various radical processes that enable stereoselective carbene and nitrene transfers.

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