Photoinduced Iron-Catalyzed ipso-Nitration of Aryl Halides via Single-Electron Transfer

Abstract

A photoinduced iron-catalyzed ipso-nitration of aryl halides with KNO 2 has been developed, in which aryl iodides, bromides, and some of aryl chlorides are feasible. The mechanism investigations show that the in situ formed iron complex by FeSO 4 , KNO 2 , and 1,10-phenanthroline acts as the light-harvesting photocatalyst with a longer lifetime of the excited state, and the reaction undergoes a photoinduced single-electron transfer (SET) process. This work represents an example for the photoinduced iron-catalyzed Ullmann-type couplings.

Green-Light-Driven Fe(III)(btz)3 Photocatalysis in the Radical Cationic [4+2] Cycloaddition Reaction

“…These complexes, including the recently disclosed Fe II (phen) nitrate, 5 were oxidatively quenched to afford the required singleelectron reduction of organohalide substrates and oxygen (Figure 1A). As a method for destabilizing MC states and including strong ligand fields, the electron-rich ligands, such as strongly σ-donating N-heterocyclic carbene (NHC) and also strongly π-accepting mesoionic carbene (MIC) ligands, were introduced by Warnmark in the noble low-spin d 5 Fe III complex [Fe(btz) 3 ] 3+ and [Fe(phtmeimb) 2 ] + {btz = 3,3′dimethyl-1,1′-bis(p-tolyl)-4,4′-bis(1,2,3-triazol-5-ylidene), and phtmeimb = phenyl[tris(3-methylimidazol-1-ylidene)]-borate}. 6 These Fe III complexes suppress deactivation processes sufficiently to realize excited-state lifetimes (100 ps and 1.96 ns, respectively) and room-temperature photoluminescence from the doublet ligand-to-metal charge-transfer ( 2 LMCT) state, which is rarely expected for low-spin d 5 metal complexes.…”

“…Two pioneer reactions of enantioselective alkylation by Cozzi and carbazole synthesis by Collins involved visible-light irradiation of Fe II (bpy) 3 and Fe II (phen) 3 catalysts. These complexes, including the recently disclosed Fe II (phen) nitrate, were oxidatively quenched to afford the required single-electron reduction of organohalide substrates and oxygen (Figure A). As a method for destabilizing MC states and including strong ligand fields, the electron-rich ligands, such as strongly σ-donating N-heterocyclic carbene (NHC) and also strongly π-accepting mesoionic carbene (MIC) ligands, were introduced by Wärnmark in the noble low-spin d 5 Fe III complex [Fe­(btz) 3 ] 3+ and [Fe­(phtmeimb) 2 ] + {btz = 3,3′-dimethyl-1,1′-bis­( p -tolyl)-4,4′-bis­(1,2,3-triazol-5-ylidene), and phtmeimb = phenyl­[tris­(3-methylimidazol-1-ylidene)]­borate} .…”

“…19 The UV−vis absorption spectrum obtained for a solution of Fe III (btz) 3 (1.0 mM) and 1a (0.10 M) exhibited a strong absorption band with peak wavelengths of 528 and 558 nm, which gradually disappeared after photoirradiation due to the formation of the Fe II (btz) 3 species (Figure 2a), showing broad intense bands at 446 and 688 nm. The X-band EPR spectrum of Fe III (btz) 3 (1 mM) and 1a (0.10 M) revealed the presence of an octahedral low-spin d 5 Fe III species with g values of 1.94 and 2.32. Photoirradiation reduced the EPR signature due to oneelectron reduction of Fe III (btz) 3 into a diamagnetic low-spin d 6…”

Two-in-one metallaphotoredox cross-couplings enabled by a photoactive ligand

“…The photocatalysis of Fe complexes remained rare in literature due in part to their fleeting excited state. 92,93 Pleasingly, the combination of our PPQN 2,4-di-OMe and simple ferric salt could promote the decarboxylative fluorination of the estrone-derived carboxylic acid (60), 94,95 which exemplified a convenient route to prepare the valuable monofluoromethoxylated product (61, Figure 5; Equation 1). 96,97 Analogous to the Ni metallaphotocatalysis, the Co 2+ /PPQN 2,4-di-OMe could also drive the reductive allylation of the aldehyde with the allyl ester in the presence of tertiary amine (58, Figure 5; Equation 2), providing more flexibility for the retrosynthetic planning of allylic alcohol preparation.…”

From N–H Nitration to Controllable Aromatic Mononitration and Dinitration─The Discovery of a Versatile and Powerful N-Nitropyrazole Nitrating Reagent

“… 3 The most influential and widely used nitration strategy in both laboratory and industry is electrophilic aromatic substitution (S E Ar), with the “mixed acids” protocol as the representative one, which requires the use of concentrated nitric acid and another assisting reagent (e.g., H 2 SO 4 , Tf 2 O or Ac 2 O) to generate the reactive nitronium ion ([NO 2 + ]). 3a Other strategies include transition metal-catalyzed cross coupling 4 or directed C–H activation 5 with nitrites or nitrates, ipso -nitration of prefunctionalized arenes 6 (e.g., aryl boronic acids, aryl carboxylic acids, and aryl metal species), oxidation of aryl azides 7 or anilines, 8 as well as enzymatic, 9 electrochemical, 10 and photochemical nitrations. 11 Despite their effectiveness, these methods suffer from certain limitations, including (a) poor functional group tolerance due to harsh reaction conditions (e.g., strong acids, strong oxidants or high temperature), (b) the requirement of prefunctionalization or directing group, and (c) adverse environmental impact.…”