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

The Huisgen 1,3-dipolar cycloaddition reaction of organic azides and alkynes has gained considerable attention in recent years due to the introduction in 2001 of Cu(1) catalysis by Tornoe and Meldal, leading to a major improvement in both rate and regioselectivity of the reaction, as realized independently by the Meldal and the Sharpless laboratories. The great success of the Cu(1) catalyzed reaction is rooted in the fact that it is a virtually quantitative, very robust, insensitive, general, and orthogonal ligation reaction, suitable for even biomolecular ligation and in vivo tagging or as a polymerization reaction for synthesis of long linear polymers. The triazole formed is essentially chemically inert to reactive conditions, e.g. oxidation, reduction, and hydrolysis, and has an intermediate polarity with a dipolar moment of ∼5 D. The basis for the unique properties and rate enhancement for triazole formation under Cu(1) catalysis should be found in the high ∆G of the reaction in combination with the low character of polarity of the dipole of the noncatalyzed thermal reaction, which leads to a considerable activation barrier. In order to understand the reaction in detail, it therefore seems important to spend a moment to consider the structural and mechanistic aspects of the catalysis. The reaction is quite insensitive to reaction conditions as long as Cu(1) is present and may be performed in an aqueous or organic environment both in solution and on solid support.

Cu-catalyzed azide-alkyne cycloaddition.

Covalent organic frameworks (COFs) represent an exciting new type of porous organic materials, which are ingeniously constructed with organic building units via strong covalent bonds. The well-defined crystalline porous structures together with tailored functionalities have offered the COF materials superior potential in diverse applications, such as gas storage, adsorption, optoelectricity, and catalysis. Since the seminal work of Yaghi and co-workers in 2005, the rapid development in this research area has attracted intensive interest from researchers with diverse expertise. This critical review describes the state-of-the-art development in the design, synthesis, characterisation, and application of the crystalline porous COF materials. Our own opinions on further development of the COF materials are also presented for discussion (155 references).

Covalent organic frameworks (COFs): from design to applications

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Metal–Organic Frameworks and Self-Assembled Supramolecular Coordination Complexes: Comparing and Contrasting the Design, Synthesis, and Functionality of Metal–Organic Materials

Covalent organic frameworks (COFs) are a class of crystalline porous polymers that allow the atomically precise integration of organic units to create predesigned skeletons and nanopores. They have recently emerged as a new molecular platform for designing promising organic materials for gas storage, catalysis, and optoelectronic applications. The reversibility of dynamic covalent reactions, diversity of building blocks, and geometry retention are three key factors involved in the reticular design and synthesis of COFs. This tutorial review describes the basic design concepts, the recent synthetic advancements and structural studies, and the frontiers of functional exploration.

Covalent organic frameworks

Described herein is a manganese-catalyzed dehydrogenative [4+2] annulation of NH imines and alkynes, a reaction providing highly atom-economical access to diverse isoquinolines. This transformation represents the first example of manganese-catalyzed CH activation of imines; the stoichiometric variant of the cyclomanganation was reported in 1971. The redox neutral reaction produces H2 as the major byproduct and eliminates the need for any oxidants, external ligands, or additives, thus standing out from known isoquinoline synthesis by transition-metal-catalyzed CH activation. Mechanistic studies revealed the five-membered manganacycle and manganese hydride species as key reaction intermediates in the catalytic cycle.

Manganese-Catalyzed Dehydrogenative [4+2] Annulation of NH Imines and Alkynes by CH/NH Activation

Synthesis, structure, and [60]fullerene complexation properties of azacalix[m]arene[n]pyridines.

Anion recognition has attracted much attention because of its importance in biological and environmental sciences. A large number of synthetic host molecules have been synthesized for the purpose of anion complexation. Noticeably, almost all synthetic anion receptors were designed to exploit hydrogen bonding, electrostatic interactions, hydrophobic effects, and coordination to metal ions. In recent years, however, there has been a growing interest in anion–p interactions, the interaction between an anion and an electron-deficient p-arene species. A number of theoretical studies, for example, have reported the noncovalent interactions of anions with aromatic compounds such as perfluoro-, nitro-, and cyano-substituted benzene, pyridine, pyrazine, and triazine derivatives. Experimental evidence to support the purely noncovalent anion–p interaction with charge-neutral arenes, however, is very rare. A recent study indicated indeed that the noncovalent anion–p interaction contributed less significantly than hydrogen bonding to the formation of the complex chloride–p-C6FnH6 n (n= 0– 5). Heteroatom-bridged heteroaromatic calixarenes are an emerging type of novel macrocyclic molecules. Because of their electronic nature, some heteroatoms, such as nitrogen, can adopt different configurations to form varying degrees of conjugation with adjacent heteroaromatic rings, resulting in macrocyclic heteroatom-linked heteroaromatic calixarenes with conformations and sizes different to those of conventional calixarenes. They have recently been utilized as versatile host molecules in supramolecular chemistry. As a typical example of heteroatom-bridged heteroaromatic calixarenes, tetraoxacalix[2]arene[2]triazine 1 (Scheme 1) has been reported to adopt a pre-organized 1,3-alternate conformation, yielding a cleft formed by two p-electron deficient triazine rings. We envisioned that this pelectron-deficient cavity would act as a receptor to interact with anions through p–anion interactions. Herein, we report halide recognition by tetraoxacalix[2]arene[2]triazine host molecules, and considerable substituent effects of the triazine on the halide–p interaction. Most astonishingly, X-ray crystallography revealed the concurrent formation of noncovalent p–halide and p–lone-pair electron interactions between water, chloride, or bromide, and the dichlorosubstituted tetraoxacalix[2]arene[2]triazine host. By means of spectrophotometric measurements, we examined the interaction of halides with dichloro-substituted tetraoxacalix[2]arene[2]triazine 1, a macrocycle readily accessible in large scale and good yield from the coupling of resorcinol and cyanuric acid chloride, under very mild conditions. As illustrated in Figure 1 (top), a new absorption band formed at 302 nm in the UV/Vis spectrum of 1 upon titration with tetrabutylammonium fluoride. Interaction of 1 with tetrabutylammonium fluoride also led to quenching of the host molecule emission signal at 304 nm with the concomitant emergence of a new fluorescence emission at 452 nm (Figure 1 (bottom)). These spectral changes were not caused by the cation or by moisture in the sample, as the control experiments using tetrabutylammonium perchlorate or water as a titrant showed no influence on either the absorption or the emission spectrum of 1. We then investigated the spectrophotometric titration of 1 with tetrabutylammonium chloride and tetrabutylammonium bromide. Under identical conditions, the outcomes of titration with chloride and bromide were very different to that for the titration with fluoride. For example, addition of chloride or bromide did not affect the UV/Vis spectrum of the host Scheme 1. Dichloro-substituted tetraoxacalix[2]arene[2]triazine 1 and its transformations into 2 and 3. a) H2, Pd/C, NaOAc/HOAc, 50 8C, 12 h, 82%. b) Me2NH·HCl, K2CO3, THF, 70 8C, 9 h, 65%.

Halide Recognition by Tetraoxacalix[2]arene[2]triazine Receptors: Concurrent Noncovalent Halide–π and Lone-pair–π Interactions in Host–Halide–Water Ternary Complexes

Qi-Yu Zheng

Papers

Manganese-Catalyzed Dehydrogenative [4+2] Annulation of NH Imines and Alkynes by CH/NH Activation

Synthesis, structure, and [60]fullerene complexation properties of azacalix[m]arene[n]pyridines.

Halide Recognition by Tetraoxacalix[2]arene[2]triazine Receptors: Concurrent Noncovalent Halide–π and Lone-pair–π Interactions in Host–Halide–Water Ternary Complexes

Visible light-induced 3-sulfenylation of N-methylindoles with arylsulfonyl chlorides.

Isoquinoline skeleton synthesis via chelation-assisted C−H activation