The Photoinduced Electrocyclization Reaction of Triphenylamine (TPA) in Sustainable and Confined Micellar Solutions: A Steady‐State and Laser Flash Photolysis Approach

Syntheses of Carbazoles by Photosensitized Electrocyclization of Triarylamines. Effect of Inductive Electron-Withdrawing Groups on the Photocyclization

The acetone-sensitized [6π]-electrocyclization of unsubstituted, mono-, and disubstituted triarylamines (TAAs: substituents H, CH₃, CH₃O, Cl, CnF₂n+1; n=1, 3, 4, 6, 8, 10) under 310 nm light led to the formation of endo- and exo-carbazoles in high yields; particularly, TAAs substituted with inductive electron-withdrawing groups (Cl, CnF2n+1) exhibit remarkable endo-selectivity towards cyclization. For TAAs substituted with both inductive electron- withdrawing and donating groups, the photocyclization towards endo carbazoles is dictated by the presence of the electron-withdrawing groups. Chemical yields (%), quantum yields of conversion (φR), apparent cyclization rate constants (kcyc), and Hammett plots correlations are illustrated for all substituted TAAs in acetone. Photophysical studies reveal that TAAs substituted with resonance electron-withdrawing groups (OAc, NO2, CHO) do not undergo electrocyclization in acetone at 310 nm due to formation of charge-transfer states that vastly deactivate the triplet reactive manifold, paralleling the behavior observed in MeCN as solvent. Absorption and emission spectra, Stokes shifts, and singlet excited state energies are illustrated for substituted TAAs and carbazoles. In-silico studies support the high stereoselectivity observed for the preferred endo- photosensitized [6π]-electrocyclization of Cl- and perfluoroalkyl-substituted TAAs.

Effect of Confined and Micellar Media on the Photo-Fries Reaction of 4-Phenoxyphenol Esters: A Valuable Key Step Toward the Preparation of Aryloxyethyl Selenocyanates

The irradiation of a series of 4-phenoxyphenol esters in a sustainable micellar environment has been studied from both preparative and mechanistic viewpoints, and the results were compared with those obtained in cyclohexane solutions. These esters underwent the photo-Fries rearrangement reaction, and the microheterogeneous media induced a noticeable selectivity in favor of the ortho-regioisomer formation with yields up to 96% yield. UV-visible and 1D and 2D NMR (DCS, NOESY, and DOSY) spectroscopies have been employed to determine the binding constant (Kb) and the location of the esters within the hydrophobic core of the spherical micelles. Furthermore, the diffusion coefficient (D) and hydrodynamic radius (rs) were also measured. Application of the photo-Fries reaction of esters in microheterogeneous media as a key step in a multistep sequence has been carried out, leading to the preparation of (4-phenoxy)-(2-n-pentylcarbonyl)-phenoxyethyl selenocyanate (10), an interesting target molecule showing potential biological activity against Trypanosoma cruzi.

On the application of nanosecond laser flash photolysis to the investigation of the photoreactivity of di and triarylamines

This review aims to provide the readership with an overview of the applications of the time-resolved spectroscopic techniques (often combined with steady-state measurements) in the elucidation of the mechanism of the photochemical behaviour of diaryl- and triarylamines. Such derivatives undergo cyclization to the carbazoles or ionization to the corresponding amine radical cations depending on both the nature of the starting substrates and the reaction conditions. In this context, time-resolved spectroscopy techniques allowed for the detection and the full characterization of all the excited states and the transients involved in these processes. The main characteristics of such species, including lifetime, time-resolved absorption spectra, quantum yield values and rate constants of formation and decay in both homogeneous and micro-heterogeneous environment (e.g. aqueous surfactant solutions) are thus summarized in the presented paper.

Preparation of Carbazoles Involving 6π-Electrocyclization, Photoredox-, Electrochemical-, and Thermal Cyclization Reactions: Mechanistic Insights

Carbazole is a heterocyclic motif that can be found in a diverse array of natural and unnatural products displaying a wide range of biological and physiological properties. Furthermore, this heterocycle is part of electronic materials like photoconducting polymers and organic optoelectronic materials owing to its excellent photophysical characteristics. Consequently, the development of synthetic strategies for carbazole scaffolds holds potential significance in biological and material fields. In this regard, a variety of preparation methods has been developed to exploit their efficient and distinct formation of new C-C and C-heteroatom bonds under mild conditions and enabling broad substrate diversity and functional group tolerance. Therefore, this review focuses on the synthesis of a set of carbazole derivatives describing a variety of methodologies that involve direct irradiation, photosensitization, photoredox, electrochemical and thermal cyclization reactions.

Triarylamines as catalytic donors in light-mediated electron donor–acceptor complexes

EDA complexes with catalytic triarylamines allow C–H perfluoroalkylation of arenes and heteroarenes under visible light irradiation in pH- and redox-neutral conditions. A detailed photophysical characterization of the EDA complex is provided.

Photoinduced [6π]-Electrocyclic Reaction of Mono-, Di-, and Trisubstituted Triphenylamines in Acetonitrile. A Steady-State Investigation

Direct irradiation of mono-, di-, and trisubstituted triphenylamine derivatives in acetonitrile as solvent with light of 254 nm has been systematically investigated, revealing that the exo/endo carbazole derivatives were formed as the main photoproducts from modest to good yields for triphenylamines substituted with electron-donor and neutral substituents. The kinetic profiles of the photoreaction were also recorded, and the consumption rate constants (k) were measured. These kinetic parameters show dependence on the nature of the substituents, and linear Hammett correlations were carried out to showcase the substituent effect. On the other hand, the spectroscopic behavior of the electron-rich substituted triphenylamines has been analyzed, suggesting that the fluorescence emission spectra display a mirror image of the lower energy absorption bands, while for those amines bearing electron-acceptor groups the formation of charge-transfer complexes and their fluorescence emissions constitute the main deactivation pathway of the photoreaction.

Product selectivity in the photoreaction of aryl sulfonates and mesylate of estrone derivatives in sustainable and micellar media: a steady-state investigation

The photochemical reaction of sulfonate steroids in a sustainable environment was carried out successfully under steady-state conditions. Significant selectivity in photoproduct formation was also observed.