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

Conversion constants for redox potentials measured versus different reference electrodes in acetonitrile solutions at 25°C

Nucleophilic substitution reactions by electron transfer

Publisher Summary Single electron transfer to or from molecules is often accompanied by other reactions involving the formation or the cleavage of bonds. The high energy intermediates thus formed can undergo further electron transfer with the same electron source or sink that initiated the reaction. Among the reactions accompanying electron transfer, bond breaking is a common mode by which a free radical and a diamagnetic leaving group are produced by single electron transfer to a diamagnetic molecule. Such reactions bridge single electron-transfer chemistry and radical chemistry. This chapter explores how the dynamics of concerted electron-transfer/bond-breaking reactions can be modeled and the contribution of bond breaking to the activation barrier. It discusses thermal heterogeneous and homogeneous reactions and molecular parameters that govern the concerted/stepwise dichotomy. The dynamics of the cleavage and coupling reactions and the electron transfer/bond breaking process are examined. Most investigations of dissociative electron transfer concerns thermal reactions and photoinduced-dissociative electron transfer. Quantum yield expressions for the concerted and stepwise cases are established and experimental examples are discussed.

Electron transfer, bond breaking, and bond formation

We report a new method for the formal [3+2] reaction of aryl cyclopropyl ketones with olefins to generate highly substituted cyclopentane ring systems. The key initiation step in this process is the one-electron reduction of the ketone to the corresponding radical anion, which is accomplished using a photocatalytic system comprising Ru(bpy)(3)(2+), La(OTf)(3), and TMEDA.

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[3+2] cycloadditions of aryl cyclopropyl ketones by visible light photocatalysis.

Aryl cyclopropyl ketones have frequently been utilized as diagnostic probes for single electron transfer (SET) for a variety of organic transformations. The implicit assumption in these studies is that the formation of rearranged product(s) signals the intermediacy of a ketyl anion. Through a detailed examination of the mechanism, kinetics, and products arising from the decay of several arylcyclopropylketyl anions (generated electrochemically), we have demonstrated that the assumptions made in the use of these substrates as SET probes are tenuous

Radical ion probes. 2. Evidence for the reversible ring opening of arylcyclopropylketyl anions. Implications for mechanistic studies

Radical ion probes. I. Cyclopropyl-carbinyl rearrangements of aryl cyclopropyl ketyl anions

Aryl cyclopropyl ketones have frequently been used as probes for single-electron transfer in organic reactions. The implicit assumption in these studies is that upon one-electron reduction, the corresponding arylcyclopropylketyl anions will undergo ring opening in analogy to the cyclopropylcarbinyl free radical. Earlier work in our laboratory has shown that ring opening of arylcyclopropylketyl anions is kinetically and thermodynamically unfavorable when the only substituents on the cyclopropane ring are alkyl or H. This sluggishness is attributable to a loss of resonance energy upon ring opening which is not fully compensated by the reliefofcyclopropane ring strain. Consequently, aryl cyclopropyl ketones are unsuitable probes for single-electron transfer. In this paper, we examine the effect of radical-stabilizing substituents (phenyl and vinyl) on the ring opening of arylcyclopropylketyl anions.

Radical Ion Probes. 3. The Importance of Resonance vs. Strain Energy in the Design of SET Probes Based upon the Cyclopropylcarbinyl-to-Homoallyl Radical Rearrangement

Ray E. Drumright

Papers

Radical ion probes. 2. Evidence for the reversible ring opening of arylcyclopropylketyl anions. Implications for mechanistic studies

Radical ion probes. I. Cyclopropyl-carbinyl rearrangements of aryl cyclopropyl ketyl anions

Radical Ion Probes. 3. The Importance of Resonance vs. Strain Energy in the Design of SET Probes Based upon the Cyclopropylcarbinyl-to-Homoallyl Radical Rearrangement