Die photochemische Synthese von Feinchemikalien mit Sonnenlicht

Photoredox Organic Synthesis Employing Heterogeneous Photocatalysts with Emphasis on Halide Perovskite

Lately, heterogeneous semiconductor materials have been explored as an emerging type of efficient photocatalyst for photoredox organic synthesis. Among these semiconductors, lead halide perovskite materials demonstrate unique properties towards excellent charge separation and charge transfer, extremely long charge carrier migration, high efficiency in visible light absorption, and long excited states lifetimes, etc., as proved in ground-breaking solar cell applications, garnering necessary merits for an efficient catalytic system for photoredox organic reactions. Here, the latest progress in heterogeneous semiconductor materials towards this endeavor is examined, with particular emphasis on lead halide perovskite nanocrystals (NCs) in photocatalytic organic synthesis.

Effective Utilization of NIR Wavelengths for Photo-Controlled Polymerization: Penetration Through Thick Barriers and Parallel Solar Syntheses

This contribution details an efficient and controlled photopolymerization regulated by far-red (λ=680 nm) and NIR (λ=780 and 850 nm) light in the presence of aluminium phthalocyanine and aluminium naphthalocyanine. Initiating radicals are generated by photosensitization of peroxides affording an effective strategy that provides controlled polymerization of a variety of monomers with excellent living characteristics. Critically, long wavelength irradiation provides penetration through thick barriers, affording unprecedented rates of controlled polymerization that can open new and exciting applications. Furthermore, a more optimized approach to performing solar syntheses is presented. By combining the narrow Q-bands of these photocatalysts with others possessing complementary absorptions, layered, independent polymerizations and organic transformations may be performed in parallel under a single broadband emission source, such as sunlight.

Energy-Efficient Solar Photochemistry with Luminescent Solar Concentrator Based Photomicroreactors

The sun is the most sustainable light source available on our planet, therefore the direct use of sunlight for photochemistry is extremely appealing. Demonstrated here, for the first time, is that a diverse set of photon-driven transformations can be efficiently powered by solar irradiation with the use of solvent-resistant and cheap luminescent solar concentrator based photomicroreactors. Blue, green, and red reactors can accommodate both homogeneous and multiphase reaction conditions, including photochemical oxidations, photocatalytic trifluoromethylation chemistry, and metallaphotoredox transformations, thus spanning applications over the entire visible-light spectrum. To further illustrate the efficacy of these novel solar reactors, medicinally relevant molecules, such as ascaridole and an intermediate of artemisinin, were prepared as well.

A highly efficient supramolecular photoswitch for singlet oxygen generation in water

A series of water-soluble supramolecular assemblies were constructed from dithienylethene-modified permethyl-β-cyclodextrins and porphyrin derivatives, accompanied by a high FRET efficiency, and could be applied in the control of singlet oxygen generation in a 1% ethanol aqueous solution upon irradiation of different wavelength light. These findings will provide a feasible and convenient way to construct a potential photodynamic therapy material.

A novel porphyrinic photosensitizer based on the molecular complex of meso-tetraphenylporphyrin with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone: higher photocatalytic activity, photooxidative stability and solubility in non-chlorinated solvents

The 1 : 2 molecular complex ofmeso-tetraphenylporphyrin with 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) has been used as a promising photosensitizer for the aerobic oxidation of olefins in different chlorinated and non-chlorinated solvents.

Sunlight-Driven Decarboxylative Alkynylation of α-Keto Acids with Bromoacetylenes by Hypervalent Iodine Reagent Catalysis: A Facile Approach to Ynones

A novel and practical decarboxylative alkynylation of α-keto acids with bromoacetylenes is catalyzed by hypervalent iodine(III) reagents when irradiation by sunlight at room temperature. The product ynones are generated in good yields. Experiments show that results obtained with blue light (λ=450-455 nm) are comparable to those obtained when using sunlight. Mechanistic studies demonstrate that the sunlight-driven decarboxylation undergoes a radical process.

Semiconductor photocatalysis--mechanistic and synthetic aspects

Preceding work on photoelectrochemistry at semiconductor single-crystal electrodes has formed the basis for the tremendous growth in the three last decades in the field of photocatalysis at semiconductor powders. The reason for this is the unique ability of inorganic semiconductor surfaces to photocatalyze concerted reduction and oxidation reactions of a large variety of electron-donor and -acceptor substrates. Whereas great attention was paid to water splitting and the exhaustive aerobic degradation of pollutants, only a small amount of research also explored synthetic aspects. After introducing the basic mechanistic principles, standard experiments for the preparation and characterization of visible light active photocatalysts as well as the investigation of reaction mechanisms are discussed. Novel atom-economic C-C and C-N coupling reactions illustrate the relevance of semiconductor photocatalysis for organic synthesis, and demonstrate that the multidisciplinary field combines classical photochemistry with electrochemistry, solid-state chemistry, and heterogeneous catalysis.

Visible light photocatalysis of [2+2] styrene cycloadditions by energy transfer

Hip to be square: Styrenes participate in [2+2] cycloadditions upon irradiation with visible light in the presence of an iridium(III) polypyridyl complex. In contrast to previous reports of visible light photoredox catalysis, the mechanism of this process involves photosensitization by energy transfer and not electron transfer.

Homogeneous photocatalytic reactions with organometallic and coordination compounds--perspectives for sustainable chemistry

Since the time of Giacomo Ciamician at the beginning of the 20th century, photochemical transformations have been recognized as contributing to sustainable chemistry. Electronic excitation significantly changes the reactivity of chemical compounds. Thus, the application of activation reagents is frequently avoided and transformations can be performed under mild conditions. Catalysis plays a central role in sustainable chemistry. Stoichiometric amounts of activation reagents are often avoided. This fact and the milder catalytic reaction conditions diminish the formation of byproducts. In the case of homogeneous catalysis, organometallic compounds are often applied. The combination of both techniques develops synergistic effects in the sense of "Green Chemistry". Herein, metal carbonyl-mediated reactions are reported. These transformations are of considerable interest for the synthesis of complex polyfunctionalized compounds. Copper(I)-catalyzed [2+2] photocycloaddition gives access to a large variety of cyclobutane derivatives. Currently, a large number of publications deal with photochemical electron-transfer-induced reactions with organometallic and coordination compounds, particularly with ruthenium complexes. Several photochemically induced oxidations can easily be performed with air or molecular oxygen when they are catalyzed with organometallic complexes. Photochemical reaction conditions also play a certain role in C-H activation with organometallic catalysts, for instance, with alkanes, although such transformations are conveniently performed with a variety of other photochemical reactions.

Efficient oxidations and photooxidations with molecular oxygen using metal phthalocyanines as catalysts and photocatalysts

Metal phthalocyanines can be very efficient as catalysts and photocatalysts in oxidation reactions using molecular oxygen as oxidant. Different types of soluble low molecular weight or oligomeric and insoluble heterogeneous catalysts and photocatalysts were developed. The heterogeneous metal phthalocyanines exist either impregnated on SiO 2, Al 2 O 3, charcoal and TiO 2 or covalently and coordinatively bound on SiO 2 and organic polymers or ionically bound on an organic ion exchanger. The catalytic oxidations of toxic sulfide and thiol derivative are studied. In addition, toxic phenols were employed as substrates for the photooxidation. Heterogeneous catalysts can exhibit higher activities then low molecular weight phthalocyanines. These systems exhibit a good stability for re-use. Photooxidations are more efficient than oxidations. A Si(IV) phthalocyanine derivative on a polymer ion exchanger is most active and stable. Also some examples for photooxidations in the direction of photochemical synthesis are given.

Photochemical key steps in the synthesis of surfactants from furfural-derived intermediates

Furfural is oxidized to 2[5H]-furanone by using hydrogen peroxide or to 5-hydroxy-2[5H]-furanone by using photo-oxygenation. An amine function is introduced by photochemically induced radical addition of tertiairy amines, some of which carry an n-alkyl side chain as hydrophobic moiety. These amines are produced from fatty aldehydes and cyclic secondary amines. The resulting adducts are transformed into amphoteric surfactants possessing an ammonium and a carboxylate function. Amphoteric (pK(N) and isoelectric point) and surfactant properties such as the critical micelle concentration and the adsorption efficiency are determined.

Porphyrin-Functionalized Dendrimers: Synthesis and Application as Recyclable Photocatalysts in a Nanofiltration Membrane Reactor

The convergent synthesis of a series of porphyrin-functionalized pyrimidine dendrimers has been accomplished by a procedure involving the nucleophilic aromatic substitution (NAS) as a key reaction step. The resulting dendritic porphyrin catalysts show high activity in the light-induced generation of singlet oxygen ((1)O2) from ground-state oxygen. These materials are synthetically useful photosensitizers for the oxidation of various olefinic compounds to the corresponding allylic hydroperoxides. Catalytic activities and regio- and stereoselectivities of the dendritic photosensitizers are comparable to those observed for mononuclear porphyrin catalysts. Recycling of the dendrimer-enlarged homogeneous photocatalysts was possible by solvent-resistant nanofiltration (SRNF) by using an oxidatively stable membrane consisting of a polysiloxane polymer and ultrastable Y zeolite as inorganic filler. Moreover, this membrane technology provides a safe way to isolate the hydroperoxide products under very mild conditions. The membrane showed high retention for the macromolecular catalysts, even in chlorinated solvents, but some oxidative degradation of the porphyrin units of the dendrimer was observed over multiple catalytic runs.