Photocatalytic Functionalization of Alkenes, C(sp³)–H Activation and Smiles Rearrangement

Pal, Koustav (2025) Photocatalytic Functionalization of Alkenes, C(sp³)–H Activation and Smiles Rearrangement. PhD thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

Photoredox catalysis has become a powerful approach in modern organic synthesis, enabling a wide range of bond-forming reactions under mild, sustainable conditions. Traditionally, this area has relied on metal-based photocatalysts, particularly ruthenium and iridium complexes, valued for their favorable redox properties and long-lived excited states. However, issues related to high cost, toxicity, limited availability, and metal contamination have prompted a shift toward metal-free photocatalysis. Organic dyes and small-molecule photocatalysts have recently emerged as effective and environmentally benign alternatives. These metal-free systems offer several advantages, including low toxicity, cost-efficiency, ease of structural modification, and compatibility with green chemistry principles. They are especially attractive for pharmaceutical and agrochemical applications, where metal residues are undesirable or strictly regulated. Recognizing the significance of this area, we began developing novel metal-free photocatalytic strategies to construct structurally complex and functionally diverse organic molecules. Through the rational design of photochemical processes, our work demonstrates efficient and sustainable methods for C–C, C–S, C-O, and C-N bond formation under visible light irradiation. Initially, a photocatalyzed difunctionalization protocol involves the in situ generation of difluoromethyl (CF₂) radicals using a metal- free organic photocatalyst. This approach enables the installation of fluorinated functionalities under mild, redox-neutral conditions. Next, extends this methodology to achieve remote C(sp³)–H bond functionalization, showcasing the selective activation of distal aliphatic positions via radical translocation pathways. Later, a distal Smiles rearrangement is explored as a key transformation for synthesizing distal oxindole-based heterocycles. This rearrangement proceeds through a photochemically generated aryl radical intermediate, enabling efficient C–C bond migration and heterocycle formation. Finally, we highlight two additional photochemical transformations: (i) a visible-light-induced amination reaction mediated via an Electron Donor–Acceptor (EDA) complex, and (ii) the synthesis of pyrazole derivatives under metal-free photocatalytic conditions. Both transformations proceed with broad substrate scope and functional group tolerance, underscoring the synthetic utility of metal-free photoredox catalysis.

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