Application of microflow conditions to visible light photoredox catalysis

Applications of microflow conditions for visible light photoredox catalysis have successfully been developed. Operationally simple microreactor and FEP (fluorinated ethylene propylene copolymer) tube reactor systems enable significant improvement of several photoredox reactions using different photocatalysts such as [Ru(bpy)(3)](2+) and Eosin Y. Apart from rate acceleration, this approach facilitates previously challenging transformations of nonstabilized intermediates. Additionally, the productivity of the synergistic, catalytic enantioselective photoredox α-alkylation of aldehydes was demonstrated to be increased by 2 orders of magnitude.     

Oxidative photoredox catalysis: mild and selective deprotection of PMB ethers mediated by visible light

Herein we report an advancement in the application of visible light photoredox catalysts in the oxidation of electron-rich arenes resulting in the selective deprotection of para-methoxybenzyl (PMB) ethers. This method is highlighted by excellent functional group tolerance, protecting group orthogonality, mild reaction conditions and avoidance of stoichiometric redox byproducts.     

Dynamic,multimodal hydrogel actuators using porphyrin-based visible light photoredox catalysis in a thermoresponsive polymer network

Hydrogels that can respond to multiple external stimuli represent the next generation of advanced functional biomaterials. Here, a series of multimodal hydrogels were synthesized that can contract and expand reversibly over several cycles while changing their mechanical properties in response to blue and red light, as well as heat (∼50 °C). The light-responsive behavior was achieved through a photoredox-based mechanism consisting of photoinduced electron transfer from a zinc porphyrin photocatalyst in its excited state to oligoviologen-based macrocrosslinkers, both of which were integrated into the hydrogel polymer network during gel formation. Orthogonal thermoresponsive properties were also realized by introducing N-isopropyl acrylamide (NIPAM) monomer simultaneously with hydroxyethyl acrylate (HEA) in the pre-gel mixture to produce a statistical 60 : 40 HEA : NIPAM polymer network. The resultant hydrogel actuators – crosslinked with either a styrenated viologen dimer (2V4+-St) or hexamer (6V12+-St) – were exposed to red or blue light, or heat, for up to 5 h, and their rate of contraction, as well as the corresponding changes in their physical properties (i.e., stiffness, tensile strength, Young''s modulus, etc.), were measured. The combined application of blue light and heat to the 6V12+-St-based hydrogels was also demonstrated, resulting in hydrogels with more than two-fold faster contraction kinetics and dramatically enhanced mechanical robustness when fully contracted. We envision that the reported materials and the corresponding methods of remotely manipulating the dynamic hydrogels may serve as a useful blueprint for future adaptive materials used in biomedical applications.

Orthogonal modes of activation in thermoresponsive hydrogel actuators using porphyrin-based visible light photoredox catalysis, viologen-based crosslinkers, and poly(N-isopropylacrylamide).     

Visible light photoredox catalysis: aerobic oxidation of perimidines to perimidinones

Aerobic oxidation of a series of 2,3-dihydro-1H-perimidines to the corresponding 4- and 6-perimidinones via visible light photoredox catalysis using Ru(bpy)₃²⁺ as a catalyst was reported. The scope and limitation of this oxidation were investigated and a possible photochemical mechanism was proposed.     

Shining light on photoredox catalysis: theory and synthetic applications

Photoredox catalysis is emerging as a powerful tool in synthetic organic chemistry. The aim of this synopsis is to provide an overview of the photoelectronic properties of photoredox catalysts as they are applied to organic transformations. In addition, recent synthetic applications of photoredox catalysis are presented.     

Conjugated polymers as photoredox catalysts: a new catalytic system using visible light to promote aryl aldehyde pinacol couplings

The conjugated polymer poly-(p)-phenylene (PPP) was synthesized and used as a photoredox catalyst to promote pinacol coupling of aryl-aldehydes with visible light. The reaction required the use of a sacrificial electron donor (Et₃N), and was accelerated by the addition of Lewis and Brønsted acids. A distinct advantage of this photocatalytic system is the robust nature of the system, which is not overly sensitive to impurities, oxygen, or temperature, and proceeds cleanly with few side reactions. As a comparison with the PPP system, the reactivity of Ru(bpy)₃Cl₂, a popular photoredox catalyst was compared. The PPP system was superior to the Ru(bpy)₃Cl₂ for the pinacol couplings in both rate and yield.     

Perylene as a visible light photoredox catalyst for photoinduced electron transfer-reversible addition-fragmentation chain transfer (PET-RAFT) polymerization of MMA

Abstract

The organic photocatalyst, perylene, was used to mediate photoinduced electron transfer (PET) reversible addition-fragmentation chain transfer polymerization (RAFT) of methyl methhacrylate (MMA) under light irradiation in N,N-dimethylformamide (DMF) at 25°C with 4-cyanopentanoic acid dithiobenzoate (CPADB) as chain transfer agent (CTA). Kinetic studies confirmed that the polymerization obeyed the first order kinetic m'odel. The production of PMMAs with a good control of molecular weights (M_n,GPC) and narrow polymer molecular weight distribution (M_w/M_n) were obtained. It is found that well-controlled PET RAFT polymerization of MMA can be manipulated even with the amount of perylene decreasing to ppm level. No polymer was obtained in the absence of light irradiation, implying that the model of PET RAFT polymerization of MMA is an ideal light “on”-“off” switchable system. Furthermore, the speed of PET RAFT polymerization of MMA was also finely tunable by the external light irradiation intensity. The resultant PMMA macro-CTA was characterized by ¹H nuclear magnetic resonance spectrum (¹H NMR) and gel permeation chromatography (GPC). The accessibility of the high end group fidelity was further demonstrated by chain extension experiments.     

Pyrrolidine-containing polypyridines: new ligands for improved visible light absorption by ruthenium complexes

[reaction: see text] A range of new electron-releasing pyrrolidine-containing bipyridines and terpyridines has been prepared via selective metalation-cross-coupling sequences. The obtained ligands have been involved in microwave-assisted ruthenium complexation leading to homoleptic complexes in high yield. The electron-donor effect of the pyrrolidine nucleus led to a notable improvement of visible light absorption and strong changes in the electrochemical behavior, opening new opportunities for the design of photovoltaic devices.     

New trends in polyoxometalate photoredox chemistry: from photosensitisation to water oxidation catalysis

Molecular metal oxide clusters, so-called polyoxometalates (POM) have been extensively used as homogeneous photocatalysts in various photoredox reactions such as the oxidation of alkanes, alkenes and alcohols as well as the light-induced mineralisation of various organic and inorganic pollutants. The more general application of POMs as photoactive compounds, in particular in solar energy harnessing, has been hampered as the clusters typically absorb light in the UV-region only. Over the past decade, concepts have been put forward on how the reactivity of this class of compounds can be optimised to improve their overall photoactivity, and a particular focus has been on the design of photocatalytic processes which allow the conversion of solar light into useful chemical reactivity. This perspective gives a brief overview of general aspects of POM photochemistry and critically discusses the advantages and challenges of a range of POM-based systems for photooxidations and photoreductions with a focus on the development of sustainable solar light conversion systems.     

Visible‐light‐sensitive photoredox catalysis by iron complexes: Applications in cationic and radical polymerization reactions

The development of iron complexes for the photoredox catalysis is a huge challenge. Indeed, Iron complexes can be ideal candidates due to their potential visible light absorption and redox properties but also because they are less toxic, inexpensive and environmentally friendly compared to other catalysts. In the present paper, a series of novel iron complexes have been synthesized and utilized to initiate the free radical promoted cationic polymerization of epoxides or the free radical polymerization of acrylates through photoredox catalysis processes upon exposure to near UV (385 nm) or visible violet (405 nm) light emitting diodes (LEDs). When combined with an iodonium salt and N‐vinylcarbazole, the iron complex‐based photoinitiating systems are able to generate radicals, cations, and radical cations. The initiation efficiency is investigated through real‐time Fourier transform infrared spectroscopy and a satisfactory initiating ability is found. The mechanisms for the generation of the reactive initiating species through photoredox catalysis are studied by different methods (steady state photolysis, cyclic voltammetry and electron spin resonance spin trapping techniques) and discussed in detail. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 2247–2253     

Asymmetric halogeno-bridged complexes: new reagents in organometallic synthesis and catalysis

Several methods to synthesize bimetallic complexes in which two different metal fragments are connected by halide bridges are described. Using simple starting materials a large pool of structurally defined bimetallic complexes with unique chemical reactivities can be prepared in short time. Applications in organometallic synthesis and homogeneous catalysis are discussed.     

Chiral metal cluster and nanocluster complexes and their application in asymmetric catalysis

Asymmetric catalysis has always been the focus of attention for synthetic chemistry. In this review, we summarize recent advances in the synthesis and asymmetric catalysis application of chiral metal clusters, nanoclusters and nanoparticles.     

First metal complexes of 6,6'-dihydroxy-2,2'-bipyridine: from molecular wires to applications in carbonylation catalysis

The first square planar rhodium(I) complexes containing the 6,6'-dihydroxy-2,2'-bipyridine ligand have been prepared. The complexes form molecular wires in the solid state and are active catalysts for the carbonylation of methyl acetate.     

Configurational control in stereochemically pure ligands and metal complexes for asymmetric catalysis

Enantioselective synthesis relies on suitable chiral mediators, which, in many cases, owe their stereochemical information to chiral ligands coordinated to metals. Like nature, which uses (diastereomerically pure) enzymes with several stereogenic centers to catalyze biological processes, chemists, for their purposes, tend more and more to turn their attention towards ligands and metal complexes with more than one stereogenic center or element of chirality. Selected issues of the resulting diastereomeric interactions as well as the advantages that result from the use of such complexes in catalysis are presented and discussed here.     

Rare earth oxides in zirconium dioxide:How to turn a wide band gap metal oxide into a visible light active photocatalyst

In the present study, we investigated the effect of cerium and erbium doping of the zirconium dioxide matrix. We synthesized doped samples using hydrothermal process. The amounts of dopant used were 0.5%, 1% and 5% molar(rare earth oxide over zirconium dioxide) respectively. The samples have been studied via X-ray Diffraction measurements for the structural characterization. UV visible diffuse reflectance was used for the optical analysis, Brunauer-Emmett-Teller(BET) model for the measurement of the surface area. Finally the samples have been analysed via electron paramagnetic resonance(EPR) for the electronic characterization. Then we tested the new synthetized materials to determine their photocatalytic activity in the reaction of degradation of methylene blue performed under irradiation by diodes(LEDs) emitting exclusively visible light.     

Axial donating ligands: a new strategy for late transition metal olefin polymerization catalysis

An alpha-diimine ligand (1) containing an axial donating pyridine group is developed for late metal polymerization catalysis. Despite having no substitution on the bottom face of the ligand, the nickel and palladium complexes of 1 are highly active for ethylene polymerization, producing linear high molecular weight polymers. For example, 1-NiBr2 (3) forms PE with a Mn of up to 109 224 g/mol with 1.4 branches/1000 C's. Similarly, 1-PdMeCl (5) forms PE with a Mn of up to 880 379 g/mol with 5.1 branches/1000 C's. In sharp contrast, catalysts containing the control ligand (2) consisting of a noncoordinating phenyl group gave only low molecular weight branched oligomers. It is observed that AlMe2Cl plays a specific role in generating the active species for the pyridine-based complexes. Presumably, the pyridine group may interact with AlMe2Cl to form a bimetallic species which suppresses the beta-hydride elimination process, hence resulting in reduced chain transfer and more linear structure.     

Reductive coupling of carbodiimides with metallic lithium: a new route to metal oxalamidino complexes

Carbodiimides (RN)2C were reductively coupled with Li powder into [{Li(py)2}2(mu-C2N4R4)] (R = p-tolyl) (1) and [{Li(THF)}2(mu-C2N4R4)] (R = Cy) (2), and 2 reacted with UCl4 to give [Li(py)4]2[{UCl4(py)}2(mu-C2N4R4)] (R = Cy) (3); the crystal structures of 1 and 3 x 2py, the first oxalamidino compounds of an alkali metal and of a 5f-element, have been determined.