A fundamental aim in the field of catalysis is the development of new modes of small molecule activation. One approach toward the catalytic activation of organic molecules that has received much attention recently is visible light photoredox catalysis. In a general sense, this approach relies on the ability of metal complexes and organic dyes to engage in single-electron-transfer (SET) processes with organic substrates upon photoexcitation with visible light.

Many of the most commonly employed visible light photocatalysts are polypyridyl complexes of ruthenium and iridium, and are typified by the complex tris(2,2′-bipyridine) ruthenium(II), or Ru(bpy)32+ (Figure 1). These complexes absorb light in the visible region of the electromagnetic spectrum to give stable, long-lived photoexcited states.1,2 The lifetime of the excited species is sufficiently long (1100 ns for Ru(bpy)32+) that it may engage in bimolecular electron-transfer reactions in competition with deactivation pathways.3 Although these species are poor single-electron oxidants and reductants in the ground state, excitation of an electron affords excited states that are very potent single-electron-transfer reagents. Importantly, the conversion of these bench stable, benign catalysts to redox-active species upon irradiation with simple household lightbulbs represents a remarkably chemoselective trigger to induce unique and valuable catalytic processes.






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Figure 1


Ruthenium polypyridyl complexes: versatile visible light photocatalysts.

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Visible Light Photoredox Catalysis with Transition Metal Complexes: Applications in Organic Synthesis

2.3. [4 + 4] Cycloadditions 1058 2.4. Photocycloadditions of Aromatic Compounds 1058 2.4.1. Benzene Derivatives 1058 2.4.2. Condensed Aromatic Compounds 1060 3. Photochemical Rearrangements 1061 4. Cyclizations 1064 4.1. Pericyclizations 1064 4.2. Norrish−Yang Reaction 1066 5. Photochemical Extrusion of Small Molecules 1067 6. Photochemical Electron Transfer 1068 6.1. Photochemical Electron-Transfer Reactions with a Catalytic Sensitizer 1068

Photochemical Reactions as Key Steps in Organic Synthesis

With the exception of palladium-catalyzed cross-couplings, no other group of reactions has had such a profound impact on the formation of carbon-carbon bonds and the art of total synthesis in the last quarter of a century than the metathesis reactions of olefins, enynes, and alkynes. Herein, we highlight a number of selected examples of total syntheses in which such processes played a crucial role and which imparted to these endeavors certain elements of novelty, elegance, and efficiency. Judging from their short but impressive history, the influence of these reactions in chemical synthesis is destined to increase.

Metathesis Reactions in Total Synthesis

The use of photochemical transformations is a powerful strategy that allows for the formation of a high degree of molecular complexity from relatively simple building blocks in a single step. A central feature of all light-promoted transformations is the involvement of electronically excited states, generated upon absorption of photons. This produces transient reactive intermediates and significantly alters the reactivity of a chemical compound. The input of energy provided by light thus offers a means to produce strained and unique target compounds that cannot be assembled using thermal protocols. This review aims at highlighting photochemical transformations as a tool for rapidly accessing structurally and stereochemically diverse scaffolds. Synthetic designs based on photochemical transformations have the potential to afford complex polycyclic carbon skeletons with impressive efficiency, which are of high value in total synthesis.

Photochemical Approaches to Complex Chemotypes: Applications in Natural Product Synthesis

Photocatalysis for the formation of the C-C bond.

The first total synthesis of (±)-ingenol has been achieved. The key features of the synthesis include the use of a highly diastereoselective Michael reaction to fix the C-11 methyl stereochemistry and the incorporation of the dimethylcyclopropane via diastereoselective carbene addition to the Δ13,14 olefin. The intramolecular dioxenone photoaddition-fragmentation sequence leads to the establishment of the critical C-8/C-10 trans intrabridgehead stereochemistry, a central challenge in the synthesis of ingenanes. The completion of the synthesis proceeds using the C-6α hydroxymethyl group as the sole handle for oxidation of seven contiguous carbon centers.

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The first total synthesis of ingenol

Radical cyclization reactions of .alpha.-silyl amine .alpha.,.beta.-unsaturated ketone and ester systems promoted by single electron transfer photosensitization

Novel electron-transfer photocyclization reactions of .alpha.-silyl amine .alpha.,.beta.-unsaturated ketone and ester systems

Studies have been conducted to explore single transfer (SET) induced photoaddition reactions of acenaphtylenedione (ACND) with the n-electron donors Et 2 NCH 2 SiMe 3 , n-PrSCH 2 SiMe 3 , EtOCH 2 SiMe 3 , EtCO 2 CH 2 N(CH 2 Ph)Ch 2 SiMe 3 , and EtCO 2 CH 2 N(CH 2 Ph)CH 3

Photoaddition reactions of acenaphthylenedione with .alpha.-silyl n-electron donors via triplet single electron transfer desilylation and triplet hydrogen atom abstraction pathways

Yoon T. Jeon

Papers

The first total synthesis of ingenol

Radical cyclization reactions of .alpha.-silyl amine .alpha.,.beta.-unsaturated ketone and ester systems promoted by single electron transfer photosensitization

Novel electron-transfer photocyclization reactions of .alpha.-silyl amine .alpha.,.beta.-unsaturated ketone and ester systems

Photoaddition reactions of acenaphthylenedione with .alpha.-silyl n-electron donors via triplet single electron transfer desilylation and triplet hydrogen atom abstraction pathways

The First Total Synthesis of (.+‐.)‐Ingenol.