Continuous flow synthesis of azobenzenes via Baeyer–Mills reaction

Discovery of Novel Nicotinamide Derivatives by a Two-Step Strategy of Azo-Incorporating and Bioisosteric Replacement

Azobenzene moieties can serve as active fragments in antimicrobials and exert trans/cis conversions of molecules. Herein, a series of novel nicotinamide derivatives (NTMs) were developed by employing a two-step strategy, including azo-incorporating and bioisosteric replacement. Azo-incorporation can conveniently provide compounds that can be easily optically interconverted between trans/cis isomers, enhancing the structural diversity of azo compounds. It is noteworthy that the replacement of the azo bond with a 1,2,4-oxadiazole motif through further bioisosteric replacement led to the discovery of a novel compound, NTM18, which made a breakthrough in preventing rice sheath blight disease. A control effect value of 94.44% against Rhizoctonia solani could be observed on NTM18, while only 11.11% was determined for boscalid at 200 mg·L-1. Further mechanism validations were conducted, and the molecular docking analysis demonstrated that compound NTM18 might have a tight binding with SDH via an extra π-π interaction between the oxadiazole ring and residue of D_Y586. This work sets up a typical case for the united applications of azo-incorporating and bioisosteric replacement in fungicide design, posing an innovative approach in structural diversity-based development of pesticides.

In-situ Oxidation and Coupling of Anilines towards Unsymmetric Azobenzenes Using Flow Chemistry

Molecular switches, especially azobenzenes, are used in numerous applications, such as molecular solar thermal storage (MOST) systems and photopharmacology. The Baeyer-Mills reaction of anilines and nitrosobenzenes has been established as an efficient synthetic method for non-symmetric azobenzenes. However, nitrosobenzenes are not stable, depending on their substitution pattern and pose a health risk. An in-situ oxidation of anilines with Oxone® was optimized under continuous flow conditions avoiding isolation and contact. The in-situ generated nitrosobenzene derivatives were subjected to a telescoped Baeyer-Mills reaction in flow. That way azobenzenes with a broad substituent spectrum were made accessible.

Electrocatalytic Energy Release of Norbornadiene-Based Molecular Solar Thermal Systems: Tuning the Electrochemical Stability by Molecular Design

Molecular solar thermal (MOST) systems, such as the norbornadiene/quadricyclane (NBD/QC) couple, combine solar energy conversion, storage, and release in a simple one-photon one-molecule process. Triggering the energy release electrochemically enables high control of the process, high selectivity, and reversibility. In this work, the influence of the molecular design of the MOST couple on the electrochemically triggered back-conversion reaction was addressed for the first time. The MOST systems phenyl-ethyl ester-NBD/QC (NBD1/QC1) and p-methoxyphenyl-ethyl ester-NBD/QC (NBD2/QC2) were investigated by in-situ photoelectrochemical infrared spectroscopy, voltammetry, and density functional theory modelling. For QC1, partial decomposition (40 %) was observed upon back-conversion and along with a voltammetric peak at 0.6 V_fc , which was assigned primarily to decomposition. The back-conversion of QC2, however, occurred without detectable side products, and the corresponding peak at 0.45 V_fc was weaker by a factor of 10. It was concluded that the electrochemical stability of a NBD/QC couple is easy tunable by simple structural changes. Furthermore, the charge input and, therefore, the current for the electrochemically triggered energy release is very low, which ensures a high overall efficiency of the MOST system.

Synthesis of Non-Symmetric Azoarenes by Palladium-Catalyzed Cross-Coupling of Silicon-Masked Diazenyl Anions and (Hetero)Aryl Halides

The photoswitchable motif of azobenzenes is of great importance across the life and materials sciences. This maintains a constant demand for their efficient synthesis, especially that of non-symmetric derivatives. We disclose here a general strategy for their synthesis through an unprecedented C(sp2 )-N(sp2 ) cross-coupling where functionalized aryl-substituted diazenes masked with a silyl group are employed as diazenyl pronucleophiles. These equivalents of fragile diazenyl anions couple with a diverse set of (hetero)aryl bromides under palladium catalysis with no loss of dinitrogen. The competing denitrogenative biaryl formation is fully suppressed. The reaction requires only a minimal excess, that is 1.2 equivalents, of the diazenyl component. By this, a broad range of azoarenes decorated with two electron-rich/deficient aryl groups can be accessed in a predictable way with superb functional-group tolerance.