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

Heme-Containing Oxygenases

Molecularly imprinted polymers and their use in biomimetic sensors.

The ability of platinum and gold catalysts to effect powerful atom-economic transformations has led to a marked increase in their utilization. The quite remarkable correlation of their catalytic behavior with the available structural data, coordination chemistry, and organometallic reactivity patterns, including relativistic effects, allows the underlying principles of catalytic carbophilic activation by π acids to be formulated. The spectrum of reactivity extends beyond their utility as catalytic and benign alternatives to conventional stoichiometric π acids. The resulting reactivity profile allows this entire field of catalysis to be rationalized, and brings together the apparently disparate electrophilic metal carbene and nonclassical carbocation explanations. The advances in coupling, cycloisomerization, and structural reorganization—from the design of new transformations to the improvement to known reactions—are highlighted in this Review. The application of platinum- and gold-catalyzed transformations in natural product synthesis is also discussed.

Catalytic Carbophilic Activation: Catalysis by Platinum and Gold π Acids

In the last few years, visible-light initiated organic transformations have attracted increasing attention. The development of visible-light-promoted photocatalytic reactions, which enable rapid and efficient synthesis of fine chemicals, is highly desirable from the viewpoint of cost, safety, availability, and environmental friendliness. In this Minireview, recent advances made in this fast developing area of research are discussed.

Visible-light photoredox catalysis.

Ab initio SCF/MP2 potentials are calculated on benzene–methane and benzene–benzene complexes. Although no energy stabilization appears at the Hartree–Fock level, a small but non-negligible stabilization in energy is observed at the MP2 level in both complexes, indicating the importance of the dispersion energy. Besides the dispersion energy, the electrostatic interaction plays some role in determining the relative stabilities in several cases. The most stable structure of the benzene–methane complex adopts a C3v symmetry with methane lying on the benzene C6 axis and one hydrogen atom pointing towards benzene. The binding energy of this structure is –1.95 kcal mol–1 from MP2/MIDI-4** calculations and –1.09 kcal mol–1 from MP2/6-31G** calculations, where a p-polarization function is added only on the H atoms of methane. The benzene–methane complex is much less stable than the benzene–benzene complex.

Structures and binding energies of benzene-methane and benzene-benzene complexes : an ab initio SCF/MP2 study

Electrochemical reduction of CO2catalysed by [RuL1(L2)(CO)2]2+[L1,L2=(bipy)2, (bipy)(dmbipy), (dmbipy)2, or (phen)2], [Ru(phen)2(CO)Cl]+(phen = 1,10-phenanthroline), and [RuL(CO)2Cl2][L = 2,2′-bipyridine (bipy) or 4,4′-dimethyl-2,2′-bipyridine (dmbipy)] were carried out by controlled-potential electrolysis at –1.30 V vs. saturated calomel electrode in acetonitrile–water (4 : 1, v/v), MeOH, or MeCN–MeOH (4 : 1, v/v). In acetonitrile–water (4 : 1, v/v) no difference in activities between the catalysts was observed, however in MeOH the amounts of carbon monoxide produced became larger than those of HCO2– upon introduction of the dmbipy ligand. This is attributed to the equilibrium constants among the reaction intermediates [RuL1(L2)(CO)2]2+, [RuL1(L2)(CO)-{C(O)OH}]+, and [RuL1(L2)(CO)(C02–)]+ which become smaller on substitution of bipy by dmbipy, because of the donor property of the CH3 group.

Ligand effects of ruthenium 2,2′-bipyridine and 1,10-phenanthroline complexes on the electrochemical reduction of CO2

Highly Effective Binding of Phosphomonoester with Neutral Cyclic Peptides which Include a Non-natural Amino Acid

The asymmetric synthesis of (R)-(+)-1,1′-bi-2-naphthol [or (R)-(+)-1,1 ′-bi-3-methoxy-2-naphthol] from 2-naphthol (or 3-methoxy-2-naphthol) is performed photocatalytically by using the chiral ruthenium complex, M(C3)-Δ-[Ru(menbpy)3]2+[M(C3)= counterclockwise molecular helicity along the C3 axis, menbpy = 4,4′-di(1R,2S,5R)-(–)-menthoxycarbonyl-2,2′-bipyridine] as a photosensitizer and[Co(acac)3](acac–= acetylacetonato) as an oxidant.

A novel photocatalytic asymmetric synthesis of (R)-(+)-1,1′-bi-2-naphthol derivatives by oxidative coupling of 3-substituted-2-naphthol with Δ-[Ru(menbpy)3]2+[menbpy = 4,4′-di(1R,2S,5R)-(–)-menthoxycarbonyl-2,2′-bipyridine], which posseses molecular helicity

Comparison of electronic structure, stereochemistry, and coordinate bonds between Ni(0)-SO2 complexes and nonmetal SO2 complexes and nonmetal SO2 complexes. An MO study

Katsutoshi Ohkubo

Papers

Structures and binding energies of benzene-methane and benzene-benzene complexes : an ab initio SCF/MP2 study

Ligand effects of ruthenium 2,2′-bipyridine and 1,10-phenanthroline complexes on the electrochemical reduction of CO2

Highly Effective Binding of Phosphomonoester with Neutral Cyclic Peptides which Include a Non-natural Amino Acid

A novel photocatalytic asymmetric synthesis of (R)-(+)-1,1′-bi-2-naphthol derivatives by oxidative coupling of 3-substituted-2-naphthol with Δ-[Ru(menbpy)3]2+[menbpy = 4,4′-di(1R,2S,5R)-(–)-menthoxycarbonyl-2,2′-bipyridine], which posseses molecular helicity

Comparison of electronic structure, stereochemistry, and coordinate bonds between Ni(0)-SO2 complexes and nonmetal SO2 complexes and nonmetal SO2 complexes. An MO study