Direct Observation of Triplet States in the Isomerization of Alkenylboronates by Energy Transfer Catalysis

Isomerization of E-Cinnamamides into Z-Cinnamamides Using a Recycling Photoreactor

The photocatalytic synthesis of thermodynamically less-stable Z-alkenes has received considerable research attention in recent years. In this study, a recycling photoreactor was applied to the photoisomerization of E-alkenes (cinnamamide and Weinreb amide derivatives) to produce Z-alkenes. The closed-loop recycling system comprises an immobilized photosensitizer to achieve rapid photoisomerization and a high-performance liquid chromatography instrument for separation of the Z/E diastereomers. After 4-10 cycles, the desired pure Z-alkenes were obtained efficiently. In the photoreactor system, a photosensitizer (thioxanthone) was covalently immobilized on silica gel via amide bonding, which led to an enhanced photocatalytic activity compared to the parent thioxanthone. This recycling photoreactor shows promise as an alternative system for the production of Z-alkenes.

PolyBorylated Alkenes as Energy-Transfer Reactive Groups: Access to Multi-Borylated Cyclobutanes Combined with Hydrogen Atom Transfer Event

While polyborylated alkenes are being recognized for their elevated status as highly valuable reagents in modern organic synthesis, allowing efficient access to a diverse array of transformations, including the formation of C-C and C-heteroatom bonds, their potential as energy-transfer reactive groups has remained unexplored. Yet, this potential holds the key to generating elusive polyborylated biradical species, which can be captured by olefins, thereby leading to the construction of new highly-borylated scaffolds. Herein, we report a designed energy-transfer strategy for photosensitized [2+2]-cycloadditions of poly-borylated alkenes with various olefins enabling the regioselective synthesis of diverse poly-borylated cyclobutane motifs, including the 1,1-di-, 1,1,2-tri-, and 1,1,2,2-tetra-borylated cyclobutanes. In fact, these compounds belong to a family that presently lacks efficient synthetic pathways. Interestingly, when α-methylstyrene was used, the reaction involves an interesting 1,5-hydrogen atom transfer (HAT). Mechanistic deuterium-labeling studies have provided insight into the outcome of 1,5-hydrogen atom transfer process. In addition, the polyborylated cyclobutanes are then demonstrated to be useful in selective oxidation processes resulting in the formation of cyclobutanones and γ-lactones.

Photoswitched Stereodivergent Synthesis of Allylic Sulfones

The present Letter demonstrates a photoswitched stereodivergent synthesis of allylic sulfones from sodium sulfinates, triphenylvinylphosphonium chloride, and (hetero)aromatic aldehydes in a single step. Mechanistically, cis-allylic sulfones, generated from the unstabilized ylide intermediates and aldehydes in situ, could be finally converted to trans-allylic sulfones via photochemical isomerization in the presence of a catalytic amount of bis(2-thienyl) ketone.

Mechanistic investigations of polyaza[7]helicene in photoredox and energy transfer catalysis

Organic photocatalysts frequently possess dual singlet and triplet photoreactivity and a thorough photochemical characterization is essential for efficient light-driven applications. In this article, the mode of action of a polyazahelicene catalyst (Aza-H) was investigated using laser flash photolysis (LFP). The study revealed that the chromophore can function as a singlet-state photoredox catalyst in the sulfonylation/arylation of styrenes and as a triplet sensitizer in energy transfer catalysis. The singlet lifetime is sufficiently long to exploit the exceptional excited state reduction potential for the activation of 4-cyanopyridine. Photoinduced electron transfer generating the radical cation was directly observed confirming the previously proposed mechanism of a three-component reaction. Several steps of the photoredox cycle were investigated separately, providing deep insights into the complex mechanism. The triplet-excited Aza-H, which was studied with quantitative LFP, is formed with a quantum yield of 0.34. The pronounced triplet formation was exploited for the isomerization reaction of (E)-stilbene to the Z-isomer and the cyclization of cinnamyl chloride. Catalyst degradation mainly occurs through the long-lived Aza-H triplet (28 µs), but the photostability is greatly increased when the triplet efficiently reacts in a catalytic cycle such that turnover numbers exceeding 4400 are achievable with this organocatalyst.

Regio- and Stereo-Selective Isomerization of Borylated 1,3-Dienes Enabled by Selective Energy Transfer Catalysis

Configurationally-defined dienes are pervasive across the bioactive natural product spectrum, where they typically manifest themselves as sorbic acid-based fragments. These C5 motifs reflect the biosynthesis algorithms that facilitate their construction. To complement established biosynthetic paradigms, a chemical platform to facilitate the construction of stereochemically defined, functionalizable dienes by light-enabled isomerization has been devised. Enabled by selective energy transfer catalysis, a variety of substituted β-boryl sorbic acid derivatives can be isomerized in a regio- and stereo-selective manner (up to 97 : 3). Directionality is guided by a stabilizing nO→pB interaction in the product: this constitutes a formal anti-hydroboration of the starting alkyne. This operationally simple reaction employs low catalyst loadings (1 mol %) and is complete in 1 h. X-ray analysis supports the hypothesis that the nO→pB interaction leads to chromophore bifurcation: this provides a structural foundation for selective energy transfer.

Synthesis of Borylated Carbocycles by [2 + 2]-Cycloadditions and Photo-Ene Reactions

Saturated bicyclic compounds make up a valuable class of building blocks in the development of agrochemicals and pharmaceuticals. Here, we present the synthesis of borylated bicyclo[2.1.1]hexanes via crossed [2 + 2]-cycloaddition. Due to the presence of the C-B bond, a variety of structures can be easily prepared that are not accessible by other methods. Moreover, a rare photo-ene reaction is also disclosed, allowing for the diastereoselective synthesis of trisubstituted borylated cyclopentanes.

Isonitriles as Alkyl Radical Precursors in Visible Light Mediated Hydro- and Deuterodeamination Reactions

Herein, we report the use of isonitriles as alkyl radical precursors in light-mediated hydro- and deuterodeamination reactions. The reaction is scalable, shows broad functional group compatibility and potential to be used in late-stage functionalization. Importantly, the method is general for Cα -primary, Cα -secondary and Cα -tertiary alkyl isonitriles. For most examples, high yields were obtained through direct visible-light irradiation of the isonitrile in the presence of a silyl radical precursor. Interestingly, in the presence of an organic photocatalyst (4CzIPN) a dramatic acceleration was observed. In-depth mechanistic studies using UV/Vis absorption, steady-state and time-resolved photoluminescence, and transient absorption spectroscopy suggest that the excited state of 4CzIPN can engage in a single-electron transfer with the isonitrile.

Dicationic Acridinium/Carbene Hybrids as Strongly Oxidizing Photocatalysts

A new design concept for organic, strongly oxidizing photocatalysts is described based upon dicationic acridinium/carbene hybrids. A highly modular synthesis of such hybrids is presented, and the dications are utilized as novel, tailor-made photoredox catalysts in the direct oxidative C-N coupling. Under optimized conditions, benzene and even electron-deficient arenes can be oxidized and coupled with a range of N-heterocycles in high to excellent yields with a single low-energy photon per catalytic turnover, while commonly used acridinium photocatalysts are not able to perform the challenging oxidation step. In contrast to traditional photocatalysts, the hybrid photocatalysts reported here feature a reversible two-electron redox system with regular or inverted redox potentials for the two-electron transfer. The different oxidation states could be isolated and structurally characterized supported by NMR, EPR, and X-ray analysis. Mechanistic experiments employing time-resolved emission and transient absorption spectroscopy unambiguously reveal the outstanding excited-state potential of our best-performing catalyst (+2.5 V vs SCE), and they provide evidence for mechanistic key steps and intermediates.

Coulomb interactions for mediator-enhanced sensitized triplet-triplet annihilation upconversion in solution

Sensitized triplet-triplet annihilation upconversion offers an attractive possibility to replace a high-energy photon by two photons with lower energy through the combination of a light-harvesting triplet sensitizer and an annihilator for the formation of a fluorescent singlet state. Typically, high annihilator concentrations are required to achieve an efficient initial energy transfer and as a direct consequence the most highly energetic emission is often not detectable due to intrinsic reabsorption by the annihilator itself. Herein, we demonstrate that the addition of a charge-adapted mediator drastically improves the energy transfer efficiency at low annihilator concentrations via an energy transfer cascade. Inspired by molecular dyads and recent developments in nanocrystal-sensitized upconversion, our system exploits a concept to minimize intrinsic filter effects, while boosting the upconversion quantum yield in solution. A sensitizer-annihilator combination consisting of a ruthenium-based complex and 9,10-diphenylanthracene (DPA) is explored as model system and a sulfonated pyrene serves as mediator. The impact of opposite charges between sensitizer and mediator - to induce coulombic attraction and subsequently result in accelerated energy transfer rate constants - is analyzed in detail by different spectroscopic methods. Ion pairing and the resulting static energy transfer in both directions is a minor process, resulting in an improved overall performance. Finally, the more intense upconverted emission in the presence of the mediator is used to drive two catalytic photoreactions in a two-chamber setup, illustrating the advantages of our approach, in particular for photoreactions requiring oxygen that would interfere with the upconversion system.

Divergent 1,2-carboallylation of terminal alkynes enabled by metallaphotoredox catalysis with switchable triplet energy transfer

We report a metallaphotoredox strategy for stereodivergent three-component carboallylation of terminal alkynes with allylic carbonates and alkyl trifluoroborates. This redox-neutral dual catalytic protocol utilizes commercially available organic photocatalyst 4CzIPN and nickel catalysts to trigger a radical addition/alkenyl-allyl coupling sequence, enabling straightforward access to functionalized 1,4-dienes in a highly chemo-, regio-selective, and stereodivergent fashion. This reaction features a broad substrate generality and a tunable triplet energy transfer control with pyrene as a simple triplet energy modulator, offering a facile synthesis of complex trans- and cis-selective skipped dienes with the same set of readily available substrates.