Photocatalytic and light-mediated C–C bond forming reactions

Abstract

The entire work revolved around C–C bond formation of the main focuses of the present thesis is to carry out functionalization of various unsaturated C–C bonds under main group Lewis acids catalysis. Keywords: Sulfons; alkyl radical, aryl radical, photoexcitation, 1,4-dihydropyridine, visible light, photoredox catalysis, carbon centered radicals. radical cross coupling, transition-metal-free, heterogeneous photocatalyst. Chapter 1: Introduction The Introduction section of this research thesis explores a number of fascinating aspects of free radicals in the context of organic synthesis in an effort to give a brief overview of their essential characteristics. These characteristics include importance of light, a variety of reactivity patterns, Ccentered radical stability, and ways to radical generation that include photoredox chemistry, excited state chemistry, classical methods, and EDA charge transfer mechanism. These techniques are applied in the development of innovative synthetic methodologies, which result in the production of C–C bonds in various ways. Chapter 2: Redox-active Alkylsulfones as a Precursor for Alkyl Radicals under Photoredox Catalysis In this chapter, a method for generating alkyl radicals using visible-light photoredox catalysis is described. This strategy efficiently enables a diverse collection of 1o, 2o, and 3o alkyl radicals generated via the single-electron transfer to sulfones under mild reaction conditions. These alkyl radicals are produced via the reductive desulfonylation of readily synthesized stable alkylsulfones, can be engaged in C–C bond formation in the presence of a suitable acceptor following the Giese reaction. This technique is also applicable to achieve allylation in the presence of allyl sulfone as a trapping reagent, thereby constructing a new C–C linkage. A series of fluorescence quenching experiments were executed to shed light on the mechanism, demonstrating that SET to alkylsulfones precursor is the crucial process for generating alkyl radical species. The major success of this strategy relies on the generation of primary alkyl radicals from redox-active primary alkylsulfones under mild reaction conditions utilizing the power of visible-light photoredox catalysis. Chapter 3: C(sp3)–C(sp3) Radical-Cross-Coupling Reaction via Photo-Excitation In this chapter, we described the photo-excitation of 4-alkyl-1,4-dihydropyridines (alkyl-DHPs) in presence of a base triggers the single-electron-transfer mediated desulfonative radical-cross-coupling (RCC) reaction without the need of any metal or photocatalyst. 4-alkyl substituted 1,4-DHPs as electron donor (reductant) and alkyl sulfones as electron acceptor (oxidant) are chosen strategically as the two best-matched modular radical precursors for the construction of C(sp3)–C(sp3) bonds. The UV LEDs (365 nm) have proven to be adequate to induce the SET phenomenon between two radical precursors in the excited state. Following this designed strategy, a diverse collection of 1o, 2o, and 3o persistent alkyl radicals from both the radical precursors have been engaged to forge C(sp3)–C(sp3) bonds. This blueprint features good functional group compatibility, broad scope, and detailed mechanistic investigation. Chapter 4: Ultrathin Bismuthene Nanosheets with Edge-Rich Coordinative Unsaturation Promote Photocatalytic Arylation of Heteroarenes In this chapter, we described a novel approach employing atomic-level thin semimetallic bismuthene nanosheets (2 nm) as the photocatalyst to facilitate a visible light mediated C(sp2)–C(sp2) cross coupling reaction. Under the irradiation of visible light (456 nm), aryl diazonium salts undergo a remarkable transformation, generating aryl radical species in the presence of a heterogeneous bismuthene-based photocatalyst. These radicals, in turn, engaged in dynamic interactions with heteroarenes, facilitating the facile formation of aryl-substituted heteroarenes. Our methodology showcased remarkable versatility, demonstrating mild reaction conditions and broad substrate tolerance. This method further extended beyond conventional C(sp2)–C(sp2) cross-coupling to borylation and halogenation reactions, unlocking versatile possibilities for molecular design and synthesis.