Photoinduced, Copper-Catalyzed C-N and C-C Bond Formation and Photocatalytic Co-Mediated Nitrite Reduction to N₂O: Reactivity and Mechanism

Citation

Ratani, Tanvi Siraj (2018) Photoinduced, Copper-Catalyzed C-N and C-C Bond Formation and Photocatalytic Co-Mediated Nitrite Reduction to N₂O: Reactivity and Mechanism. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Z9ZP44BF. https://resolver.caltech.edu/CaltechTHESIS:10312017-153922571

Abstract

Photocatalytic reactions with first-row transition metals are presented as a method for sustainable chemistry with great potential for new forms of reactivity and mechanistic pathways. Chapters 2 and 3 of this thesis discuss mechanism and reactivity of photoinduced, copper-catalyzed bond constructions. The Peters and Fu groups have reported that a variety of couplings of nitrogen, sulfur, oxygen, and carbon nucleophiles with organic halides can be achieved under mild conditions (−40 to 30 °C) through the use of light and a copper catalyst. Insight into the various mechanisms by which these reactions proceed may enhance our understanding of chemical reactivity and facilitate the development of new methods. We apply an array of tools (EPR, NMR, transient absorption, and UV−vis spectroscopy; ESI−MS; X-ray crystallography; DFT calculations; reactivity, stereochemical, and product studies) to investigate the photoinduced, copper-catalyzed coupling of carbazole with alkyl bromides. Our observations are consistent with pathways wherein both an excited state of the copper(I) carbazolide complex ([Cu^I(carb)₂]⁻) and an excited state of the nucleophile (Li(carb)) can serve as photoreductants of the alkyl bromide. The catalytically dominant pathway proceeds from the excited state of Li(carb), generating a carbazyl radical and an alkyl radical. The cross-coupling of these radicals is catalyzed by copper via an out-of-cage mechanism in which [Cu^I(carb)₂]⁻ and [Cu^II(carb)₃]⁻ (carb = carbazolide), both of which have been identified under coupling conditions, are key intermediates, and [Cu^II(carb)₃]⁻ serves as the persistent radical that is responsible for predominant cross-coupling. This study underscores the versatility of copper(II) complexes in engaging with radical intermediates that are generated by disparate pathways, en route to targeted bond constructions.

In Chapter 3, we establish that photoinduced, copper-catalyzed alkylation can also be applied to C−C bond formation, specifically, that the cyanation of unactivated secondary alkyl chlorides can be achieved at room temperature to afford nitriles, an important class of target molecules. In the presence of an inexpensive copper catalyst (CuI; no ligand coadditive) and a readily available light source (UVC compact fluorescent light bulb), a wide array of alkyl halides undergo cyanation in good yield. Our initial mechanistic studies are consistent with the hypothesis that an excited state of [Cu(CN)₂]⁻ may play a role, via single electron transfer, in this process. This investigation provides a rare example of a transition metal-catalyzed cyanation of an alkyl halide, as well as the first illustrations of photoinduced, copper-catalyzed alkylation with either a carbon nucleophile or a secondary alkyl chloride.

Chapter 4 presents a mechanistic study of the photocatalytic reduction of nitrite to nitrous oxide with the use of an Ir photocatalyst ([Ir(ppy)₂(phen)][PF₆]) and a bimetallic CoMg co-catalyst with a diimine-dioxime ligand platform. Insights into the mechanism of this reaction may enhance our current understanding of N–N coupling processes relative to other pathways of reactivity for nitrosyl ligands, such as nitroxyl (HNO) dimerization. We propose a mechanism in which a coordinated and an uncoordinated •NO are coupled at a single Co center. One electron reduction of [(Cl)(NO)Co(^Medoen)Mg(Me₃TACN)(H₂O)][BPh₄] ({CoNO}⁸), a species we show to be catalytically relevant, forms a {CoNO}⁹ species that is characterized by UV-Vis, EPR, and FT-IR spectroscopy and whose electronic structure is supported by density functional theory (DFT). We formulate the {CoNO}⁹ as a 5-coordinate, S = 3/2 Co(II) antiferromagnetically coupled with an anionic S = 1 ³NO^– ligand. Experimental data suggest a mechanism in which this {CoNO}⁹ intermediate can release •NO, thereby reducing the Co(II) center to Co(I). This free •NO can react with another {CoNO}⁹ complex to generate a Co(NONO) intermediate which was observed by step-scan time-resolved IR spectroscopy and whose assignment was supported with DFT calculations. This Co(NONO) species, which can generate N₂O and H₂O, is formulated as a neutral hyponitrite intermediate with significant neutral radical character on both nitrosyl nitrogen atoms and a weak N–N bond.

Thesis Files