Thesis advisor: X. Peter Zhang / Asymmetric C–H functionalization represents one of the central topics in modern organic chemistry, which allows for the direct installation of functional groups onto ubiquitous C–H bonds in organic molecules. Among numerous elegant strategies, transition metal-catalyzed C–H alkylation with diazo compounds represents one of the most powerful methods for C–C bond formation. Different from Fischer metallocarbene-based C–H insertion reactions, cobalt(II)-based metalloradical catalysis (MRC) is recently proven to be capable of activating acceptor/acceptor diazo compounds for radical C–H alkylation reactions via H-atom abstraction. In this dissertation, we have developed several systems by utilizing less-explored aryl and alkyl diazomethanes as new radical precursors for highly enantioselective radical C–H alkylation reactions, which permit the efficient synthesis of different optically active heterocyclic compounds. First, we have demonstrated the feasibility of using aryl aldehyde-derived sulfonylhydrazones as new radical precursors for enantioselective radical C–H alkylation to synthesis enantioenriched 2,3-dihydrobenzofuran derivatives. Notably, a general and mild way for in situ generation of diazo compounds have been identified by using 2,4,6-triisopropyl sulfonyl hydrazone as diazo precursor, which allow us to regulate the reaction temperature to achieve the high enantioselectivity for the desired radical reactions. Second, the utility of Co(II)-based MRC has been further highlighted by enantioselective indoline synthesis. Through the design and synthesis of new catalysts, the system is shown to have a broad spectrum of substrate scope, forming various 2-substituted indolines with up to 98% yield and 96% ee. A series of mechanistic studies further support the underlying stepwise radical alkylation pathway. Finally, we further expand the applicability of MRC to even more challenging diazo compounds, aliphatic diazomethanes. Starting from alkyl aldehyde-derived sulfonylhydrazones as diazo precursors, the Co(II)-based radical alkylation reactions allow for the enantioselective synthesis for common 2-substituted tetrahydrofuran structures with high yields and excellent enantioselectivities. / Thesis (PhD) — Boston College, 2019. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.

Thesis advisor: X. Peter Zhang / Among recent advances in devising different strategies for stereoselective homolytic reactions, metalloradical catalysis (MRC) has emerged as a conceptually new approach for controlling stereoselectivity of radical reactions. As stable metalloradicals, cobalt(II) complexes of D₂-symmetric chiral amidoporphyrins [Co(D₂-Por)] have proven to be effective catalysts for homolytically activating a series of diazo compounds to generate α-Co(III)-alkyl radicals for various C-centered radical transformations with well-confined reactivity and selectivity. Nevertheless, the applications of donor-, donor/donor- and alkyl diazo compounds have been largely underdeveloped. This dissertation mainly focuses on how the chemistry of these types of diazo compounds was initiated by using commonly available aldehyde-derived sulfonylhydrazones as diazo surrogates. In the context of Co(II)-MRC, in situ-generated diazo compounds can be effectively activated for various asymmetric radical transformations, including intermolecular radical cyclopropanation of alkenes and intramolecular radical alkylation of C–H bonds. First, as a proof of concept, we have demonstrated the feasibility of using aryl aldehyde-derived sulfonylhydrazones as new radical precursors for diastereo- and enantioselective radical cyclopropanation of alkenes, and proven that the diazo in situ-generation protocol is well compatible with the catalytic radical process. Second, we have expanded the application of Co(II)-based MRC to a new territory by employing aliphatic diazo compounds for asymmetric cyclopropanation. The system is highlighted by the excellent enantioselectivity together with remarkable cis-selectivity. Finally, with the utilization of linear aliphatic aldehyde sulfonylhydrazones as diazo precursors, we have presented a new radical cyclization mode, involving hydrogen atom abstraction and radical substitution, for enantioselective synthesis of common five-membered rings via radical C–H alkylation. The system would offer a new retrosynthetic paradigm for construction of ring structures, where C–C bond can be disconnected as common C=O and C–H units of linear aldehydes. / Thesis (PhD) — Boston College, 2018. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.