Hydrogen-Borrowing Reduction/Dehydrogenative Aromatization of Nitroarenes through Visible-Light-Induced Energy Transfer: An Entry to Pyrimidoindazoles and Carbazoles

Abstract

Herein, we describe a novel visible-light-induced protocol for hydrogen-borrowing reduction/dehydrogenation aromatization/cyclization of nitroarenes by energy transfer. The present protocol does not require additional oxidants, hydrogen acceptors, and hydrogen evolution metal catalysts. The mechanistic studies demonstrated that the hydrogen-borrowing reduction/ dehydrogenative aromatization process was initiated by the formation of active singlet species through efficient energy transfer of excited Ir[dF(CF 3 )ppy] 2 (dtbpy)PF 6 to nitroarenes.

Nitroarenes and nitroalkenes as potential amino sources for the synthesis of N-heterocycles

“…Recently, Huang et al illustrated a visible-light-induced approach for the hydrogen-borrowing cyclization of nitroarenes to afford pyrimidoindazoles (Scheme 23). 48 The optimal conditions involve the use of 63 (0.2 mmol), Ir[dF(CF 3 ) ppy] 2 (dtbpy)PF 6 (2 mol%), 1,4-dioxane (2.0 mL), at 60 °C under an Ar atmosphere and 35 W blue LED irradiation for 72 h. The system does not require additional oxidants, hydrogen acceptors, and hydrogen evolution metal catalysts. Both the elec- tron-donating and electron-withdrawing nitroarenes are effectively used in the reaction.…”

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

“…A mild and novel visible-light-induced hydrogen-borrowing reduction/dehydrogenation aromatization strategy for the intramolecular dehydrative cyclization of 2-substituted nitroarenes bearing cyclohexene and tetrahydropyrimidine has recently been developed and investigated. [73] The photochemical reaction is efficiently initiated through energy transfer of excited Ir[dF(CF 3 )ppy] 2 (dtbpy)PF 6 to nitroarenes providing valuable pyrimidoindazoles and carbazoles in good to excellent yields. However, for the preparation of carbazoles the present protocol does require the combinational use of PdCl 2 and Ph 3 P in catalytic amounts that benefits the desired transformation furnishing the expected carbazole derivatives in modest to good yields (Scheme 38).…”

Continuous Flow Synthesis of Nitrosoarenes via Photochemical Rearrangement of Aryl Imines

“…Due to the intrinsic value of nitrosoarenes as building blocks for organic synthesis, we developed a complementary approach exploiting o -nitrophenylimines that would undergo an internal redox process upon photoexcitation. Recent years have already showcased ever-growing interest in photochemistry as a green and effective synthetic methodology, whereby nitroarenes have been exploited as oxygen atom transfer reagents and toward the generation of medicinally relevant heterocycles such as indazolones, indoles, 2-indolinones, 2- H -indazoles, carbazoles, or pyrido­[1,2- b ]­indazoles . Furthermore, 2-nitrobenzaldehyde, which we utilize as the starting material to prepare the corresponding nitrophenylimines, is a recognized actinometric compound, and its photochemical reactivity has been examined in diverse studies .…”