Photosensitized O2 enables intermolecular alkene cyclopropanation by active methylene compounds

Organophotocatalyst Enabled Deoxycyclopropanation of Alcohols

Cyclopropane fragments, which widely exist in marketed drugs and natural products, can confer special pharmacological properties to small-molecule drugs. Therefore, developing methods to construct cyclopropanes is of great significance. Nevertheless, the introduction of cyclopropane primarily relies on already-formed cyclopropyl groups, which significantly restricts the diversity of cyclopropane skeletons. Late-stage direct cyclopropanation is still a challenging task. Herein, a photo-induced intermolecular deoxycyclopropanation reaction that employs alcohols as substrates, and 1 mol.% of 2,3,5,6-tetrakis(carbazol-9-yl)-1,4-dicyanobenzene (4CzTPN) as organophotocatalyst is reported. This method proceeds with high transformation efficiency (up to 98% yield) and exhibits broad functional group tolerance, such as primary, secondary, and tertiary alcohols as well as various activated β-halogenated alkenes. This process is mild, easy to operate, and has low equipment requirements. The power of this technology is demonstrated by the late-stage functionalization of five marketed drugs and five natural products.

Strain-Release-Driven Electrochemical Skeletal Rearrangement of Non-Biased Alkyl Cyclopropanes/Butanes

Capitalizing the inherent strain energy within molecules, strain-release-driven reactions have been widely employed in organic synthesis. Small cycloalkanes like cyclopropanes and cyclobutanes, with their moderate ring strain, typically require dense functionalization to induce bias or distal activation of (hetero) aromatic rings via single-electron oxidation for relieving the tension. In this study, we present a pioneering direct activation of alkyl cyclopropanes/butanes through electrochemical oxidation. This approach not only showcases the potential for ring-opening of cyclopropane/butane under electrochemical conditions but also streamlines the synthesis of diverse oxazolines and oxazines. The applicability of our method is exemplified by its broad substrate scopes. Notably, the products derived from cyclobutanes undergo a formal ring contraction to cyclopropanes, introducing an intriguing aspect to our discoveries. These discoveries mark a significant advancement in strain-release-driven skeletal rearrangement reactions of moderately strained rings, offering sustainable and efficient synthetic pathways for future endeavours.

Anti-selective Cyclopropanation of Nonconjugated Alkenes with Diverse Pronucleophiles via Directed Nucleopalladation

A facile approach to obtaining densely functionalized cyclopropanes is described. The reaction proceeds under mild conditions via the directed nucleopalladation of nonconjugated alkenes with readily available pronucleophiles and gives excellent yields and good anti-selectivity using I2 and TBHP as oxidants. Pronucleophiles bearing a diverse collection of electron-withdrawing groups, including -CN, -CO2R, -COR, -SO2Ph, -CONHR, and -NO2, are well tolerated. Internal alkenes, which are generally challenging substrates in other cyclopropanation methods, provide excellent yields and good diastereoselectivity in this methodology, allowing for controlled access to cyclopropanes substituted at all three C atoms. DFT calculations and mechanistic experiments reveal that the major mechanistic pathway involves the initial α-iodination of the nucleophile, followed by anti-carbopalladation and intramolecular C(sp3)-I oxidative addition. Strain-release-promoted C(sp3)-C(sp3) reductive elimination then furnishes the cyclopropanated product.

The efficient synthesis of three-membered rings via photo- and electrochemical strategies

Three-membered rings, such as epoxides, aziridines, oxaziridines, cyclopropenes, vinyloxaziridines, and azirines, are recognized as crucial pharmacophores and building blocks in organic chemistry and drug discovery. Despite the significant advances in the synthesis of these rings through photo/electrochemical methods over the past decade, there has currently been no focused discussion and updated overviews on this topic. Therefore, we presented this review article on the efficient synthesis of three-membered rings using photo- and electrochemical strategies, covering the literature since 2015. In this study, a conceptual overview and detailed discussions were provided to illustrate the advancement of this field. Moreover, a brief discussion outlines the current challenges and opportunities in synthesizing the three-membered rings using these strategies.

Redox Approaches to Carbene Generation in Catalytic Cyclopropanation Reactions

Transition metal-catalyzed carbene transfer reactions have a century-old history in organic chemistry and are a primary method for the synthesis of cyclopropanes. Much of the work in this field has focused on the use of diazo compounds and related precursors, which can transfer a carbene fragment to a catalyst with concomitant loss of a stable byproduct. Despite the utility of this approach, there are persistent limitations in the scope of viable carbenes, most notably those lacking stabilizing substituents. By coupling carbene transfer chemistry with two-electron redox cycles, it is possible to expand the available starting materials that can be used as carbene precursors. In this Minireview, we discuss emerging catalytic reductive cyclopropanation reactions using either gem-dihaloalkanes or carbonyl compounds. This strategy is inspired by classic stoichiometric transformations, such as the Simmons-Smith cyclopropanation and the Clemmensen reduction, but instead entails the formation of a catalytically generated transition metal carbene or carbenoid. We also present recent efforts to generate carbenes directly from methylene (CR2H2) groups via a formal 1,1-dehydrogenation. These reactions are currently restricted to substrates containing electron-withdrawing substituents, which serve to facilitate deprotonation and subsequent oxidation of the anion.

Distinct Photochemistry of Odd-Carbon PAHs from the Even-Carbon Ones During the Photoaging and Analysis of Soot

Polycyclic aromatic hydrocarbons (PAHs) are the primary organic carbons in soot. In addition to PAHs with even carbon numbers (PAHeven), substantial odd-carbon PAHs (PAHodd) have been widely observed in soot and ambient particles. Analyzing and understanding the photoaging of these compounds are essential for assessing their environmental effects. Here, using laser desorption ionization mass spectrometry (LDI-MS), we reveal the substantially different photoreactivity of PAHodd from PAHeven in the aging process and their MS detection through their distinct behaviors in the presence and absence of elemental carbon (EC) in soot. During direct photooxidation of organic carbon (OC) alone, the PAHeven are oxidized more rapidly than the PAHodd. However, the degradation of PAHodd becomes preponderant over PAHeven in the presence of EC during photoaging of the whole soot. All of these observations are proposed to originate from the more rapid hydrogen abstraction reaction from PAHodd in the EC-photosensitized reaction, owing to its unique structure of a single sp3-hybridized carbon site. Our findings reveal the photoreactivity and reaction mechanism of PAHodd for the first time, providing a comprehensive understanding of the oxidation of PAHs at a molecular level during soot aging and highlight the enhanced effect of EC on PAHodd ionization in LDI-MS analysis.

Light Induced Cobalt(III) Carbene Radical Formation from Dimethyl Malonate As Carbene Precursor

Radical-type carbene transfer catalysis is an efficient method for the direct functionalization of C-H and C=C bonds. However, carbene radical complexes are currently formed via high-energy carbene precursors, such as diazo compounds or iodonium ylides. Many of these carbene precursors require additional synthetic steps, have an explosive nature, or generate halogenated waste. Consequently, the utilization of carbene radical catalysis is limited by specific carbene precursors that access the carbene radical intermediate. In this study, we generate a cobalt(III) carbene radical complex from dimethyl malonate, which is commercially available and bench-stable. EPR and NMR spectroscopy were used to identify the intermediates and showed that the cobalt(III) carbene radical complex is formed upon light irradiation. In the presence of styrene, carbene transfer occurred, forming cyclopropane as the product. With this photochemical method, we demonstrate that dimethyl malonate can be used as an alternative carbene precursor in the formation of a cobalt(III) carbene radical complex.

Transient-State Self-Bipolarized Organic Frameworks of Single Aromatic Units for Natural Sunlight-Driven Photosynthesis of H2O2

Constructing π-conjugated polymer structures through covalent bonds dominates the design of organic framework photocatalysts, which significantly depends on the selection of multiple donor-acceptor building blocks to narrow the optical gap and increase the lifetimes of charge carriers. In this work, self-bipolarized organic frameworks of single aromatic units are demonstrated as novel broad-spectrum-responsive photocatalysts for H2O2 production. The preparation of such photocatalysts is only to fix the aromatic units (such as 1,3,5-triphenylbenzene) with alkane linkers in 3D space. Self-bipolarized aromatic units can drive the H2O2 production from H2O and O2 under natural sunlight, wide pH ranges (3.0-10.0) and natural water sources. Moreover, it can be extended to catalyze the oxidative coupling of amines. Experimental and theoretical investigation demonstrate that such a strategy obeys the mechanism of through-space π-conjugation, where the closely face-to-face overlapped aromatic rings permit the electron and energy transfer through the large-area delocalization of the electron cloud under visible light irradiation. This work introduces a novel design concept for the development of organic photocatalysts, which will break the restriction of conventional through-band π-conjugation structure and will open a new way in the synthesis of organic photocatalysts.

Nitrogen-doped Carbon Dots as Efficient Photocatalysts for High Selectivity of Dehalogenative Oxyalkylation of Styrene

Carbon dots (CDs) are a type of carbon-based luminescent material with a zero-dimensional structure and a size of less than 10 nm, which are composed of sp2 /sp3 hybrid carbon nuclei and surface functional groups. Because CDs has strong photoluminescence and good light absorption in the ultraviolet and near visible regions, it is an excellent candidate for photocatalytic applications. However, the use of nonmetallic doped CDs as photosensitizers for direct photocatalytic organic reactions has been limited to several scattered reports. Herein, we present nitrogen-doped carbon dots (N-CDs) that has a capability for not only produce reactive oxygen species (ROS), including superoxide anion radical (O2 ⋅- ) and singlet oxygen (1 O2 ), but also provide an unprecedented high activity of dehalogenative oxyalkylation of styrene with a yield of 93 %. This work develops a novel opportunity to utilize cost-effective and easily accessible CDs for the advancement of photocatalysis.

Optoelectronic materials as emerging photocatalysts: opportunities in sustainable organic synthesis

To overcome the energy and environmental crisis, the development of efficient, sustainable photocatalysts to convert inexhaustible solar energy into chemical energy is of great significance. Due to their unique optoelectronic properties, organic electronic materials have been translated into the photocatalytic field. These emerging photocatalysts are attractive because of their metal-free nature, chemical stability, and structural diversity. However, as many small molecules fail to absorb visible light solely, incorporating them into crosslinked frameworks is found to be an effective strategy to extend the conjugation and enhance visible-light absorption. In addition, the photophysical properties of these heterogeneous materials can be adjusted through structural modification and linkage engineering. Finally, these insoluble photocatalysts exhibit good recyclability and reusability. As a representative illustration, this feature article describes recent examples of the use of two types of organic electronic materials including phenothiazine and truxene in heterogeneous photocatalytic organic transformations. The synthesis and key photophysical properties of both organic electronic material-based photocatalysts are discussed combined with specific synthetic applications. We anticipate this feature article will stimulate the implementation of more diverse organic electronic materials in the field of photocatalysis, which may lead to unprecedented synthetic applications.