Decoupling Plasmonic Hot Carrier from Thermal Catalysis via Electrode Engineering.

Increased attention has been directed toward generating nonequilibrium hot carriers resulting from the decay of collective electronic oscillations on metal known as surface plasmons. Despite numerous experimental endeavors, demonstrating hot carrier-mediated photocatalysis without a heating contribution has proven challenging, particularly for single electron transfer reactions where the thermal contribution is generally detrimental. An innovative engineering solution is proposed to enable single electron transfer reactions with plasmonics. It consists of a photoelectrode designed as an energy filter and photocatalysis performed with light function modulation instead of continuously. The photoelectrode, consisting of FTO/TiO2 amorphous (10 nm)/Au nanoparticles, with TiO2 acting as a step-shape energy filter to enhance hot electron extraction and charge-separated state lifetime. The extracted hot electrons were directed toward the counter electrode, while the hot holes performed a single electron transfer oxidation reaction. Light modulation prevented local heat accumulation, effectively decoupling hot carrier catalysis from the thermal contribution.

Characterization and evaluation of photolabile (µ-peroxo)(µ-hydroxo)bis[bis(bipyridyl)cobalt caged oxygen compounds to facilitate time-resolved crystallographic studies of cytochrome c oxidase.

Photolabile (µ-peroxo)(µ-hydroxo)bis[bis(bipyridyl)-cobalt-based caged oxygen compounds have been synthesized and characterized by optical absorbance spectroscopy, X-ray crystallography. and the quantum yield and redox stability were investigated. Furthermore, conditions were established where redox incompatibilities encountered between caged oxygen compounds and oxygen-dependant cytochrome c oxidase (CcO) could be circumvented. Herein, we demonstrate that millimolar concentrations of molecular oxygen can be released from a caged oxygen compound with spatio-temporal control upon laser excitation, triggering enzymatic turnover in cytochrome c oxidase. Spectroscopic evidence confirms the attainment of a homogeneous reaction initiation at concentrations and conditions relevant for further crystallography studies. This was demonstrated by the oxidizing microcrystals of reduced CcO by liberation of millimolar concentrations of molecular oxygen from a caged oxygen compound. We believe this will expand the scope of available techniques for the detailed investigation of oxygen-dependant enzymes with its native substrate and facilitate further time-resolved X-ray based studies such as wide/small angle X-ray scattering and serial femtosecond crystallography.

Spatiotemporal Release of Singlet Oxygen in Low Molecular Weight Organo-Gels Upon Thermal or Photochemical External Stimuli.

The first example of a material capable of spatiotemporal catch and release of singlet oxygen (1O2) in gel phase is presented. Several low molecular weight organogelators based around an oxotriphenylhexanoate (OTHO) core are developed and optimized with regard to; their gelation properties, and ability of releasing 1O2 upon thermal and/or photochemical external stimuli, in both gel phase and solution. Remarkably, reversible phase transitioning between the gel and solution phase are also demonstrated. Taken together two complementary modes of releasing 1O2, one thermally controlled over time, and one rapid release by means of photochemical stimuli is disclosed. These findings represent the first phase reversible system where function and aggregation properties can be controlled independently, and thus pave the way for novel applications in material sciences as well as in life sciences.

Hypervalent Iodine Compounds in Carbohydrate Chemistry: Glycosylation, Functionalization and Oxidation.

This mini review article provides an overview on the use of hypervalent iodine compounds (HICs) in carbohydrate synthesis, focusing on their chemistry and recent applications. HICs are similar to transition metals in their reactivity but have the added benefit of being environmentally benign, and are therefore commonly used as selective oxidants and eco-friendly reagents in organic synthesis. Herein, we summarize various synthetic uses of hypervalent iodine reagents in reactions such as glycosylation, oxidations, functionalization, and C-C bond-forming reactions. The goal of this review is to illustrate the advantages and versatility of using HICs as an environmentally sustainable alternative to heavy metals in carbohydrate chemistry.

Utilizing a needle as a source of iron in synergistic dual photoredox catalytic generation of alkoxy radicals.

A visible-light mediated alkoxy radical generation is described, which allows for a structurally divergent oxidative C(sp3)-H bond functionalization. This protocol employs a photoredox/iron dual catalysis allowing for an unprecedented chemoselective single-step transformation of alcohol derivatives giving access to two valuable sets of products, tetrahydrofurans and γ-bromoketones, under one set of conditions. Addition of iron, by slow corrosion of a needle, provides superior reaction efficiency as compared to various iron(III) complexes.

Visible-Light Mediated Oxidative Fragmentation of Ethers and Acetals by Means of Fe(III) Catalysis.

A new method employing iron(III) acetylacetonate along with visible light is described to effect oxidative ring opening of cyclic ethers and acetals with unparalleled efficiency. The method allows for a photocatalytic radical chemistry approach to functionalize relatively inert cyclic ethers into useful synthetic intermediates. The methodology sheds further light on the use of underexplored iron complexes in visible-light photochemical contexts and illustrates that simple Fe(III) complexes can initiate redox processes from 4LMCT excited states.

Highly efficient direct visible-light-mediated oxidative esterification of aldehydes.

A very efficient one-pot procedure for the direct oxidation of aldehydes to esters mediated by visible-light is presented. Utilizing a combination of indium triflate and N-bromosuccinimide, NBS, with either ortho-esters or alcohols provided rapid access to a variety of esters. Certain substrates convert fully within a few seconds, other more challenging tert-butyl esters were formed in good yield after no more than 4 h.

Sustainable Ir-Photoredox Catalysis by Means of Heterogenization.

A heterogenized iridium catalyst was employed to perform photoredox catalysis for a collection of mechanistically orthogonal reactions using very low quantities of iridium (0.01-0.1 mol %). The heterogenized construct consists of an organometallic iridium coordination complex bonded to an aluminum metal oxide solid-state support via an anchoring group. The solid-state support allows for easy recovery and reusability of the catalyst. Evaluation of the catalytic activity was performed with five different reactions, showing broad applicability and demonstrating the general potential for a heterogenized strategy. Moreover, the heterogenized catalyst was shown to be reusable up to five times and also mediated the reactions with much higher efficiency than the original processes by employing the corresponding homogeneous catalyst. As a result of the low catalyst loadings employed, the feasibility of reusage, and faster reaction times, this catalyst offers a more sustainable option when precious metal catalysts are used in organic synthesis. Finally, the catalyst was successfully applied to a gram-scale reaction, showing it is susceptible to scalability.

Alcohol Etherification via Alkoxy Radicals Generated by Visible-Light Photoredox Catalysis.

A mechanistically divergent method is described that, employing a commercially available hypervalent iodine(III) reagent, generates alkoxy radicals from 1°, 2°, and 3° alcohols and allows their use in the functionalization of C(sp3)-H and C(sp2)-H bonds. This visible-light photoredox catalysis produces alkyl ethers via 1,5/6-hydrogen atom transfer or aryl ethers via 1,5-addition. This mild methodology provides a practical strategy for the synthesis of acetals, orthoesters, tetrahydrofurans, and chromanes.

Nickel-Catalyzed Selective Reduction of Carboxylic Acids to Aldehydes.

The direct reduction of carboxylic acids to aldehydes is a fundamental transformation in organic synthesis. The combination of an air-stable Ni precatalyst, dimethyl dicarbonate as an activator, and silane reductant effects this reduction for a wide variety of substrates, including pharmaceutically relevant structures, in good yields and with no overreduction to alcohols. Moreover, this methodology is scalable, allows access to deuterated aldehydes, and is also compatible with one-pot utilization of the aldehyde products.

Synthesis and biological evaluation of truncated derivatives of abyssomicin C as antibacterial agents.

The synthesis and antibacterial activity of two new highly truncated derivatives of the natural product abyssomicin C are reported. This work outlines the limits of structural truncation of the natural product and consequently provides insights for further structure-activity relationship studies towards novel antibiotics targeting 4-amino-4-deoxychorismate (ADC) synthase. Specifically, it is demonstrated that the synthetically challenging bicyclic motif is essential for activity towards methicillin-resistant Staphylococcus aureus (MRSA).

Redox-Neutral Dual Functionalization of Electron-Deficient Alkenes.

Visible-light photoredox catalysis has been utilized in a new multicomponent reaction forming ß-functionalized δ-diketones under mild conditions in an operationally convenient manner. Single-electron reduction of in situ generated carboxylic acid derivatives forms acyl radicals that react further via 1,2-acylalkylation of olefins in an intermolecular, three-components cascade reaction, giving valuable synthetic entities from readily available starting materials. A diverse set of substrates has been used, demonstrating robust methodology with broad substrate scope.

Visible-Light-Mediated Photocatalytic Difunctionalization of Olefins by Radical Acylarylation and Tandem Acylation/Semipinacol Rearrangement.

A novel method for the mild photoredox-mediated tandem radical acylarylation and tandem acylation/semipinacol rearrangement has been developed. The synthesis of highly functionalized ketones bearing all-carbon α- or ß-quaternary centers has been achieved using easily available symmetric aromatic carboxylic anhydrides as the acyl radical source. The method allows for a straightforward introduction of the keto functionality and concomitant construction of molecular complexity in a single operation.

Acyl Radicals from Aromatic Carboxylic Acids by Means of Visible-Light Photoredox Catalysis.

Simple and abundant carboxylic acids have been used as acyl radical precursor by means of visible-light photoredox catalysis. By the transient generation of a reactive anhydride intermediate, this redox-neutral approach offers a mild and rapid entry to high-value heterocyclic compounds without the need of UV irradiation, high temperature, high CO pressure, tin reagents, or peroxides.

On the radical nature of iron-catalyzed cross-coupling reactions.

The radical nature of iron-catalyzed cross-coupling between Grignard reagents and alkyl halides has been studied by using a combination of competitive kinetic experiments and DFT calculations. In contrast to the corresponding coupling with aryl halides, which commences through a classical two-electron oxidative addition/reductive elimination sequence, the presented data suggest that alkyl halides react through an atom-transfer-initiated radical pathway. Furthermore, a general iodine-based quenching methodology was developed to enable the determination of highly accurate concentrations of Grignard reagents, a capability that facilitates and increases the information output of kinetic investigations based on these substrates.

Photocatalytic decarboxylative reduction of carboxylic acids and its application in asymmetric synthesis.

The decarboxylative reduction of naturally abundant carboxylic acids such as α-amino acids and α-hydroxy acids has been achieved via visible-light photoredox catalysis. By using an organocatalytic photoredox system, this method offers a mild and rapid entry to a variety of high-value compounds including medicinally relevant scaffolds. Regioselective decarboxylation is achieved when differently substituted dicarboxylic acids are employed. The application of this method to the synthesis of enantioenriched 1-aryl-2,2,2-trifluoroethyl chiral amines starting from natural α-amino acids further testifies to the utility of the developed photocatalytic decarboxylative reduction protocol.

Highly accurate excited-state structure of [Os(bpy)2dcbpy](2+) determined by X-ray transient absorption spectroscopy.

Determining the electronic and geometric structures of photoexcited transient species with high accuracy is crucial for understanding their fundamental photochemistry and controlling their photoreactivity. We have applied X-ray transient absorption spectroscopy to measure the XANES and EXAFS spectra of a dilute (submillimolar) solution of the osmium(II) polypyridyl complex [Os(bpy)2dcbpy](PF6)2 (dcbpy = 4,4'-dicarboxy-2,2'-bipyridine) (OsL2L') in methanol at the Os LIII edge. We have obtained spectra of superb quality for both the ground state and the photoinduced (3)MLCT excited state that have allowed us not only to extract detailed information about the Os 5d orbitals but also to resolve very small differences of 0.010 ± 0.008 Å in the average Os-N bond lengths of the ground and excited states. Theoretical calculations using a recently developed DFT-based approach support the measured electronic structures and further identify the nature of the molecular orbitals that contribute to the main absorption bands in the XANES spectra.

Photoredox Activation and Anion Binding Catalysis in the Dual Catalytic Enantioselective Synthesis of ß-Amino Esters.

The enantioselective oxidative C-H functionalization of tetrahydroisoquinoline derivatives is achieved through the merger of photoredox and asymmetric anion-binding catalysis. This combination of two distinct catalysis concepts introduces a potentially general approach to asymmetric transformations in oxidative photocatalysis.

Synthesis of an orthogonal topological analogue of helicene.

The synthesis of an orthogonal topological pentamer analogue of helicene is presented. This analogue forms a tubular structure with its aromatic systems directed parallel to the axis of propagation, which creates a cavity with the potential to function as a host molecule. The synthetic strategy reported, based on a series of repeating Friedländer condensations that utilize pyrido[3,2-d]pyrimidine moieties as protected amino aldehydes, allows for the facile access of higher generations of helical, tubular structures. As a result of the synthetic strategy, only a helical isomer of the pentamer is possible. The structure and absolute configuration of the pentamer were elucidated from a combination of NMR spectroscopic data, optical properties, X-ray structures, and by comparison of an experimental electronic circular dichroism spectrum to a calculated spectrum.

Tröger's base twisted amides: endo functionalization and synthesis of an inverted crown ether.

Taking advantage of the unconventional reactivity of twisted mono- and bis-amides of Tröger's base (TB), rac-6 and rac-7, respectively, the first synthesis of a 6-endo-monosubstituted TB analogue, rac-9, and the first rational synthesis of a 6,12-endo,endo-disubstituted TB analogue, rac-11, have been achieved. The bis-TB crown ether, meso-13, was prepared starting from rac-7. Meso-13 constitutes a rare example of a crown ether with an inverted methylene bridge-to-bridge bis-TB conformation both in solution and in the solid state, resulting in a reluctance to act as a receptor for cations.