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Relevant academic research and scientific papers

Rh(iii)-Catalyzed regioselective mono- and di-iodination of azobenzenes using alkyl iodide

A new approach for highly regioselective iodination of azobenzenes with alkyl iodide as the iodinating reagent enabled by Rh-catalyzed oxidative C-H activation has been developed. By changing the oxidant, various mono- and di-iodinated azobenzenes were smoothly obtained in moderate to excellent yields, respectively. The preliminary mechanistic study reveals that the reaction process might undergo electrophilic substitution of the directed ortho metalated five-membered rhodacycle compound by an iodine cationic species generated in situ from alkyl iodide and oxidant.

Palladium-catalyzed regioselective halogenation of aromatic azo compounds

A highly regioselective halogenation reaction of symmetrical and unsymmetrical aromatic azo compounds has been developed at room temperature or at 50 °C. In the presence of 5 mol% palladium diacetate and 0.5 equiv. of p-toluenesulfonic acid, a range of symmetrical aromatic azo compounds smoothly undergo monobromination with N-bromosuccinimide to give the corresponding unsymmetrical aromatic azo compounds in good to excellent yields with >99:1 ortho-selectivity. This chemistry has been successfully extended to unsymmetrical aromatic azo compounds, whose electronricher aryl groups prefer to be monobrominated. Moreover, replacing N-bromosuccinimide with Niodosuccinimide in the reaction allows the synthesis of monoiodinated aromatic azo compounds with >99:1 regioselectivity.

Control of the equilibrium between 2-phosphinoazobenzenes and inner phosphonium salts by heat, solvent, acid, and photoirradiation

Several 2-phosphinoazobenzenes, which are in equilibrium with inner phosphonium salts, were synthesized. Effects of substituents, solvents, and acidic additives on their equilibria are described. Thermodynamic parameters of the equilibria in various solvents suggest that the acceptor character of the solvents is mainly responsible for the solvent effects. Addition of phenols changed the equilibria depending on their acidity. Substituents at the 4- and 4′-positions of the azobenzene also affected the equilibrium constants, which shifted the equilibrium toward the phosphonium salt in the order of their electron-withdrawing ability. Photoisomerization of the 2-phosphinoazobenzenes bearing electron-donating substituents at the 4- and 4′-positions, which shifted the equilibrium toward the 2-phosphinoazobenzene, proceeded successfully. While the phosphonium salt in equilibrium with the (E)-isomer of the 2-phosphinoazobenzene was protonated by perchlorophenol, the (Z)-isomer did not react with a proton source because it could not take on the form of an inner phosphonium salt. Thus, the properties and reactivity of the inner phosphonium salts in equilibrium with the phosphines can be successfully controlled by photoirradiation.