Towards elucidating photochemical reaction pathways in nickel catalyzed cross coupling and organocatalyzed Birch reduction
Date
2021
Authors
Kudisch, Max, author
Miyake, Garret, advisor
Finke, Richard, committee member
Chung, Jean, committee member
Reisfeld, Brad, committee member
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Abstract
Carbon-nitrogen (C─N) bond forming reactions to couple aryl halides with amines are essential for the discovery and production of medicinal compounds. The state-of-the-art method uses a precious metal palladium catalyst at high temperatures which poses sustainability concerns. Recently, a method was reported in which an iridium photocatalyst (PC) works in tandem with a nickel catalyst under blue light irradiation to achieve C─N bond formation at room temperature. Herein, it was discovered that the iridium PC could be omitted if 365 nm light is used, constituting a precious metal-free approach. This discovery suggests that a nickel-centered excited state can mediate C─N bond formation, raising the possibility of an energy transfer type pathway in dual catalytic systems. The nickel complexes formed were identified for the first time and mechanistic evidence was found that is consistent with energy transfer with both [Ru(bpy)3]2+ (bpy = 2,2'-bipyridine) and a phenoxazine PC. A series of [NiBr2(amine)n] complexes were isolated, characterized, and detected in C─N coupling reaction mixtures. A theoretical framework for predicting energy transfer rate constant ratios based on Förster theory and UV-visible spectroscopy was developed. The phenoxazine PC was both predicted and found to exhibit faster energy transfer and enhanced reaction performance when compared with [Ru(bpy)3]2+. In addition, a light-driven, organocatalyzed system for Birch reduction was developed. Historically, Birch reduction to reduce an arene to a 1,4-cyclohexadiene has been limited by the required use of alkali metals which are pyrophoric and can be explosive. Under violet light, a benzo[ghi]perylene imide PC was found to reduce challenging arenes such as benzene, constituting the first visible light driven approach capable of this reactivity. Mechanistic studies were performed that are consistent with a catalytic cycle involving addition of OH─ to the PC to form an adduct, [PC─OH]─. Photolysis of the adduct forms OH• and the PC radical anion which subsequently undergoes photoionization, ejecting a solvated electron that reduces the substrate.
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Subject
C-N coupling
energy transfer
photochemistry
catalysis
birch reduction
light driven