Fluorinated Carbocations: Design, Synthesis, and Applications in Low-energy Photoredox Catalysis

Abstract

Visible-light photoredox catalysis has reshaped modern organic synthesis by enabling single electron pathways under mild conditions; however, many widely used systems still depend on high-energy blue light with limited photostability and narrow redox windows, constraints that can curtail functional-group tolerance, scalability, and compatibility with sensitive substrates. This dissertation addresses these limitations by introducing difluorinated carbocations as a tunable platform for low-energy photoredox catalysis. We have developed modular synthetic routes to fluorinated triaryl carbenium frameworks, including acridinium, helicenium, and triangulenium scaffolds that exhibit bathochromic shift in absorption and emission, useful excited-state lifetimes, and widened excited-state redox windows. Comprehensive characterization of the difluorinated azadioxotriangulenium 2FADOTA⁺, including steady-state absorption/emission, quantum yield, fluorescence lifetimes, cyclic voltammetry, and excited-state thermodynamics (Rehm-Weller analysis), establishes it as a strong orange-light photooxidant for organic transformations. The utility of 2FADOTA⁺ is demonstrated in two synthetically distinct and practically relevant transformations. First, we use 2FADOTA⁺ as a photocatalyst for the direct conversion of substituted furans to the corresponding pyrroles using readily available primary amines under orange-light irradiation (595 nm). Mechanistic evidence is consistent with initial photooxidation of the furan to its radical cation, nucleophilic capture by the amine, and subsequent Paal-Knorr condensation to deliver pyrroles under mild conditions. Second, we have developed an orange-light mediated direct arene C-H amination strategy that constructs aryl C-N bonds via arene radical-cation chemistry, enabling late-stage diversification of complex scaffolds under low energy conditions using 2FADOTA⁺. Across representative scopes, 2FADOTA⁺ mediates efficient conversions with predictable site-selectivity patterns characteristic of arene radical-cation intermediates, and the platform accommodates diverse amine partners and electron-rich arene/heteroarenes without requiring precious metals. Together, these studies establish fluorinated carbocations, exemplified by 2FADOTA⁺, as a general, designable platform for low-energy photoredox catalysis that decouples strong excited-state oxidizing ability from the need for high-energy excitation. Finally, we present preliminary results on red-light mediated C-H functionalization of furans using organic helicenium (nPr-DMQA⁺) as the photocatalyst.