The Photocatalyzed Meerwein Arylation: Classic Reaction of Aryl Diazonium Salts in a New Light

The use of diazonium salts for aryl radical generation and C? H arylation processes has been known since 1896 when Pschorr first used the reaction for intramolecular cyclizations. Meerwein developed it further in the early 1900s into a general arylation method. However, this reaction could not compete with the transition‐metal‐mediated formation of C(sp²)? C(sp²) bonds. The replacement of the copper catalyst with iron and titanium compounds improved the situation, but the use of photocatalysis to induce the one‐electron reduction and activation of the diazonium salts is even more advantageous. The first photocatalyzed Pschorr cyclization was published in 1984, and just last year a series of papers described applications of photocatalytic Meerwein arylations leading to aryl–alkene coupling products. In this Minireview we summarize the origins of this reaction and its scope and applications.     

可见光光催化:缺电子杂环、苯环与芳基偶氮盐的直接芳基化偶联反应

发展了可见光照射下Ru(bpy)₃(PF₆)₂催化缺电子杂环C-H活化与芳基偶氮盐的交叉偶联反应.在室温、可见光照射下,各种缺电子杂环与芳基偶氮盐发生高效的偶联反应,同时在相似的条件下,普通苯环也能有效地与芳基偶氮盐偶联形成新C-C键化合物.     

Dihalogenated Methylperoxy Radicals: Spectroscopic Characterization and Photodecomposition by Release of HO.

Two atmospherically relevant dihalogenated methylperoxy radicals CHX₂OO^. (X=F and Cl) have been generated through O₂-oxidation of the corresponding alkyl radicals CHX₂^. in the gas phase. The IR spectroscopic characterization of both radicals in cryogenic Ar- and N₂-matrices (15 K) is supported by ¹⁸O-labeling and ab initio calculations at the UCCSD(T)/aug-cc-pVTZ level. Upon 266 nm laser irradiation, both radicals decompose mainly by releasing hydroxyl radicals (→HO^.+X₂CO) via the intermediacy of intriguing α-hydroperoxyalkyl radicals (^.CX₂OOH), implying that the photooxidation of dihalogenated hydrocarbons might serve as important sources of HO^. radicals in the atmosphere.     

Mechanistic Insight into the Catalytic Oxidation of Cyclohexane over Carbon Nanotubes: Kinetic and In Situ Spectroscopic Evidence

As some of the most interesting metal‐free catalysts, carbon nanotubes (CNTs) and other carbon‐based nanomaterials show great promise for some important chemical reactions, such as the selective oxidation of cyclohexane (C₆H₁₂). Due to the lack of fundamental understanding of carbon catalysis in liquid‐phase reactions, we have sought to unravel the role of CNTs in the catalytic oxidation of C₆H₁₂ through a combination of kinetic analysis, in situ spectroscopy, and density functional theory. The catalytic effect of CNTs originates from a weak interaction between radicals and their graphene skeletons, which confines the radicals around their surfaces. This, in turn, enhances the electron‐transfer catalysis of peroxides to yield the corresponding alcohol and ketone.     

Mechanistic Insight into Binding of Huperzine A with Human Serum Albumin: Computational and Spectroscopic Approaches

Human serum albumin (HSA) is the most abundant protein in plasma synthesized by the liver and the main modulator of fluid distribution between body compartments. It has an amazing capacity to bind with multiple ligands, offering a store and transporter for various endogenous and exogenous compounds. Huperzine A (HpzA) is a natural sesquiterpene alkaloid found in Huperzia serrata and used in various neurological conditions, including Alzheimer’s disease (AD). This study elucidated the binding of HpzA with HSA using advanced computational approaches such as molecular docking and molecular dynamic (MD) simulation followed by fluorescence-based binding assays. The molecular docking result showed plausible interaction between HpzA and HSA. The MD simulation and principal component analysis (PCA) results supported the stable interactions of the protein–ligand complex. The fluorescence assay further validated the in silico study, revealing significant binding affinity between HpzA and HSA. This study advocated that HpzA acts as a latent HSA binding partner, which may be investigated further in AD therapy in experimental settings.     

飞秒时间分辨的光电子谱对苯S2态的超快动力学研究

结合飞秒时间分辨的质谱技术与时间分辨的光电子影像技术对苯S₂激发态的超快动力学进行了研究.苯分子吸收两个400 nm的光子被激发到S₂态,之后再用一个267 nm的光子对其进行探测.获得的母体离子产率随泵浦探测时间延迟的变化曲线包含了两个不同的时间寿命组分.第一个时间寿命组分(90 ± 1) fs被归纳为S₂态到S₁/S₀态的内转换过程;第二个时间寿命组分(5.0 ± 0.2) ps被归纳为S₁态的衰减过程.实验中观察到的第二个寿命组分小于早前的研究结果,这表明了在S₁态的衰减过程中还可能存在其他的过程.从时间分辨的光电子影像提取得到的光电子能谱中发现了一个新的失活过程,该过程被归结为激发态S₁的振动态与“热”三重态T₃之间的系间交叉过程.     

Modulating the Electronic Structures of Single-walled Carbon Nanotubes by Filling with Electron Acceptors and the Effect on Their Reactivity with Aryl Diazonium Salts

The electronic structures of single-walled carbon nanotubes （SWCNTs） were modulated by filling with tetracyanoquinodimethane （TCNQ）, a strong electron acceptor. The structures of TCNQ-filled SWCNTs were checked by X-ray diffraction analysis, high-resolution transmission electron microscopy and Raman spectroscopy. Optical absorption spectroscopy demonstrated an enhanced reactivity between aryl diazonium and semiconducting SWCNTs.     

A Picosecond Time-Resolved Spectroscopic Investigation of the Effect of pH on Morin Fluorescence

In this work, we investigated for the first time morin in MeOH at different pH values by picosecond time-resolved fluorescence. We identified the two species responsible for the fluorescence at low and high pH. The solvated morin-solvent hydrogen-bonded complex has been experimentally observed for the first time. We give also the typical fluorescence spectra as well as the fluorescence lifetimes of the probable emitting species. In this work we put forward new insights concerning the contribution of free morin to the fluorescence. We hope that these new data improve the accuracy of the interpretation of the cation:morin complexes titration using fluorescence signal.     

Light‐Induced Mechanistic Divergence in Gold(I) Catalysis: Revisiting the Reactivity of Diazonium Salts

In a systematic study of the Au‐catalyzed reaction of o‐alkynylphenols with aryldiazonium salts, we find that essentially the same reaction conditions lead to a change in mechanism when a light source is applied. If the reaction is carried out at room temperature using a Au^I catalyst, the diazonium salt undergoes electrophilic deauration of a vinyl Au^I intermediate and provides access to substituted azobenzofurans. If the reaction mixture is irradiated with blue LED light, C?C bond formation due to N₂‐extrusion from the diazonium salt is realized selectively, using the same starting materials without the need for an additional photo(redox) catalyst under aerobic conditions. We report a series of experiments demonstrating that the same vinyl Au^I intermediate is capable of producing the observed products under photolytic and thermal conditions. The finding that a vinyl Au^I complex can directly, without the need for an additional photo(redox) catalyst, result in C?C bond formation under photolytic conditions is contrary to the proposed mechanistic pathways suggested in the literature till date and highlights that the role of oxidation state changes in photoredox catalysis involving Au is thus far only poorly understood and may hold surprises for the future. Computational results indicate that photochemical activation can occur directly from a donor–acceptor complex formed between the vinyl Au^I intermediate and the diazonium salt.     

Exploring the Reactivity of Multicomponent Photocatalysts: Insight into the Complex Valence Band of BiOBr

The band structure of multicomponent semiconductor photocatalysts, as well as their reactivity distinction under different wavelengths of light, is still unclear. BiOBr, which is a typical multicomponent semiconductor, may have two possible valence‐band structures, that is, two discrete valence bands constructed respectively from O 2p and Br 4p orbitals, or one valence band derived from the hybridization of these orbitals. In this work, aqueous photocatalytic hydroxylation is applied as the probe reaction to investigate the nature and reactions of photogenerated holes in BiOBr. Three organic compounds (microcystin‐LR, aniline, and benzoic acid) with different oxidation potentials were selected as substrates. Isotope labeling (H₂¹⁸O as the solvent) was used to determine the source of the O atom in the hydroxyl group of the products, which distinguishes the contribution of different hydroxylation pathways. Furthermore, a spin‐trapping ESR method was used to quantify the reactive oxygen species (^.OH and ^.OOH) formed in the reaction system. The different isotope abundances of the hydroxyl O atom of the products formed, as well as the reverse trend of the ^.OH/^.OOH ratio with the oxidative resistance of the substrate under UV and visible irradiation, reveal that BiOBr has two separate valence bands, which have different oxidation ability and respond to UV and visible light, respectively. This study shows that the band structure of semiconductor photocatalysts can be reliably analyzed with an isotope labeling method.     

Catalyst‐Free Photoredox Addition–Cyclisations: Exploitation of Natural Synergy between Aryl Acetic Acids and Maleimide

Suitably functionalised carboxylic acids undergo a previously unknown photoredox reaction when irradiated with UVA in the presence of maleimide. Maleimide was found to synergistically act as a radical generating photoxidant and as a radical acceptor, negating the need for an extrinsic photoredox catalyst. Modest to excellent yields of the product chromenopyrroledione, thiochromenopyrroledione and pyrroloquinolinedione derivatives were obtained in thirteen preparative photolyses. In situ NMR spectroscopy was used to study each reaction. Reactant decay and product build‐up were monitored, enabling reaction profiles to be plotted. A plausible mechanism, whereby photo‐excited maleimide acts as an oxidant to generate a radical ion pair, has been postulated and is supported by UV/Vis. spectroscopy and DFT computations. The radical‐cation reactive intermediates were also characterised in solution by EPR spectroscopy.     

几种光催化反应体系中羟基自由基表观生成率的实验研究

采用邻二氮啡-Fe(Ⅱ)(橙红色)分光光度法间接测定了Fenton、UV/Fenton、UV/草酸铁/H₂O₂、UV/TiO₂4种体系中在不同条件下·OH的表观生成率,初步探讨了体系中各影响因子的作用机制.结果分析表明:对于Fenton体系、UV/Fenton、UV/草酸铁/H₂O₂而言,其最佳反应条件为pH=4.0、m(H₂O₂)/m(Fe)=1/25、铁离子的质量浓度以25 mg/L左右为宜;TiO₂光催化体系催化产生的·OH尽管受环境pH的影响相对较小,但在同等条件下·OH的表观生成率最低;4种体系中以UV/草酸铁/H₂O₂体系中·OH表观生成率最高,而且以线性的规律递增.     

Towards Conformation-Sensitive Inhibition of Gyrase: Implications of Mechanistic Insight for the Identification and Improvement of Inhibitors

Gyrase is a bacterial type IIA topoisomerase that catalyzes negative supercoiling of DNA. The enzyme is essential in bacteria and is a validated drug target in the treatment of bacterial infections. Inhibition of gyrase activity is achieved by competitive inhibitors that interfere with ATP- or DNA-binding, or by gyrase poisons that stabilize cleavage complexes of gyrase covalently bound to the DNA, leading to double-strand breaks and cell death. Many of the current inhibitors suffer from severe side effects, while others rapidly lose their antibiotic activity due to resistance mutations, generating an unmet medical need for novel, improved gyrase inhibitors. DNA supercoiling by gyrase is associated with a series of nucleotide- and DNA-induced conformational changes, yet the full potential of interfering with these conformational changes as a strategy to identify novel, improved gyrase inhibitors has not been explored so far. This review highlights recent insights into the mechanism of DNA supercoiling by gyrase and illustrates the implications for the identification and development of conformation-sensitive and allosteric inhibitors.     

Ultrafast Spectroscopy of Fe(II) Complexes Designed for Solar-Energy Conversion: Current Status and Open Questions

One major challenge of future sustainable photochemistry is to replace precious and rare transition metals in applications such as energy conversion or electroluminescence by earth-abundant, cheap, and recyclable materials. This involves using coordination complexes of first row transition metals such as Cu, Cr, or Mn. In the case of iron, which is attractive due to its natural abundance, fundamental limitations imposed by the small ligand field splitting energy have recently been overcome. In this review article, we briefly summarize the present knowledge and understanding of the structure-property relationships of Fe(II) and Fe(III) complexes with excited state lifetimes in the nanosecond range. However, our main focus is to examine to which extent the ultrafast spectroscopy methods used so far provided insight into the excited state structure and the photo-induced dynamics of these complexes. Driven by the main question of how to spectroscopically, i. e. in energy and concentration, differentiate the population of ligand- vs. metal-centered states, the hitherto less exploited ultrafast vibrational spectroscopy is suggested to provide valuable complementary insights.     

Study of Excited States and Electron Transfer of Semiconductor-Metal-Complex Hybrid Photocatalysts for CO2 Reduction by Using Picosecond Time-Resolved Spectroscopies

A semiconductor-metal-complex hybrid photocatalyst was previously reported for CO₂ reduction; this photocatalyst is composed of nitrogen-doped Ta₂O₅ as a semiconductor photosensitizer and a Ru complex as a CO₂ reduction catalyst, operating under visible light (>400 nm), with high selectivity for HCOOH formation of more than 75 %. The electron transfer from a photoactive semiconductor to the metal-complex catalyst is a key process for photocatalytic CO₂ reduction with hybrid photocatalysts. Herein, the excited-state dynamics of several hybrid photocatalysts are described by using time-resolved emission and infrared absorption spectroscopies to understand the mechanism of electron transfer from a semiconductor to the metal-complex catalyst. The results show that electron transfer from the semiconductor to the metal-complex catalyst does not occur directly upon photoexcitation, but that the photoexcited electron transfers to a new excited state. On the basis of the present results and previous reports, it is suggested that the excited state is a charge-transfer state located between shallow defects of the semiconductor and the metal-complex catalyst.     

Planar Array Transient Infrared Spectroscopy: A New Tool for the Time-Resolved Analysis of Polymers

Summary: Planar array infrared spectroscopy (PA-IR) was used for the first time to record transient infrared (TIRS) spectra. In proof-of-concept experiments, it was demonstrated that PA-TIRS can record high quality spectra of common polymeric samples. It was shown the this new technique allows acquiring spectra in as little as 17 ms, opening the door to high speed, real-time monitoring applications.     

Matrix Isolation and Spectroscopic Characterization of the Phenylperoxy Radical and Its Rearranged Products

The phenylperoxy radical 1 has been synthesized by the reaction of the phenyl radical 2 with ³O₂. Radical 1 could be either generated in the gas phase and subsequently trapped in solid argon at 10 K, or directly synthesized in argon matrices. By reacting 2 as well as its perdeuterated isotopomer [D₅]‐ 2 with ¹⁶O₂ and with ¹⁸O₂, respectively, the four isotopomers [H₅]‐¹⁶O₂‐ 1 , [D₅]‐¹⁶O₂‐ 1 , [H₅]‐¹⁸O₂‐ 1 , and [D₅]‐¹⁸O₂‐ 1 were matrix‐isolated and characterized by IR spectroscopy. The experimental IR spectra are in excellent agreement with results from DFT calculations. Irradiation of 1 with visible light produces the 2‐oxepinoxy radical 5 in a clean reaction. Subsequent irradiation results in ring‐opening and formation of several conformers of ketoketene 6 . The radicals 1 , 5 , and 6 play an important role in the combustion of aromatic hydrocarbons and could now be isolated and spectroscopically characterized for the first time.