Unveiling Extreme Photoreduction Potentials of Donor–Acceptor Cyanoarenes to Access Aryl Radicals from Aryl Chlorides

Since the seminal work of Zhang in 2016, donor-acceptor cyanoarene-based fluorophores, such as 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN), have been widely applied in photoredox catalysis, and used as excellent metal-free alternatives to noble metal Ir- and Ru-based photocatalysts. However, all the reported photoredox reactions involving this chromophore family are based on harnessing the energy from a single visible light photon, with a limited range of redox potentials from -1.92 V to +1.79 V. Here, we document the unprecedented discovery that this family of fluorophores can undergo consecutive photoinduced electron transfer (ConPET) to achieve very high reduction potentials. One of the newly synthesized catalysts, 2,4,5-tri(9H-carbazol-9-yl)-6-(ethyl(phenyl)amino)isophthalonitrile (3CzEPAIPN), possesses a long-lived (12.95 ns) excited radical anion form, 3CzEPAIPN•−*, which can be used to activate reductively recalcitrant aryl chlorides (Ered ≈ -1.9 to -2.9 V) under mild conditions. The resultant aryl radicals can be engaged in synthetically valuable aromatic C-B, C-P, and C-C bond formation to furnish arylboronates, arylphosphonium salts, arylphosphonates, and spirocyclic cyclohexadienes, respectively.

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Unveiling Extreme Photoreduction Potentials of Donor-Acceptor Cyanoarenes to Access Aryl Radicals from Aryl Chlorides

10.26434/chemrxiv.14747850 ◽

2021 ◽

Author(s):

Jinhui Xu ◽

Jilei Cao ◽

Xiangyang Wu ◽

Han Wang ◽

Xiaona Yang ◽

...

Keyword(s):

Limited Range ◽

Redox Potentials ◽

Aryl Chlorides ◽

Aryl Radicals ◽

Reduction Potentials ◽

Anion Form ◽

Donor Acceptor ◽

High Reduction ◽

Very High ◽

Excited Radical

Since the seminal work of Zhang in 2016, donor-acceptor cyanoarene-based fluorophores, such as 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN), have been widely applied in photoredox catalysis, and used as excellent metal-free alternatives to noble metal Ir- and Ru-based photocatalysts. However, all the reported photoredox reactions involving this chromophore family are based on harnessing the energy from a single visible light photon, with a limited range of redox potentials from -1.92 V to +1.79 V. Here, we document the unprecedented discovery that this family of fluorophores can undergo consecutive photoinduced electron transfer (ConPET) to achieve very high reduction potentials. One of the newly synthesized catalysts, 2,4,5-tri(9H-carbazol-9-yl)-6-(ethyl(phenyl)amino)isophthalonitrile (3CzEPAIPN), possesses a long-lived (12.95 ns) excited radical anion form, 3CzEPAIPN•−*, which can be used to activate reductively recalcitrant aryl chlorides (Ered ≈ -1.9 to -2.9 V) under mild conditions. The resultant aryl radicals can be engaged in synthetically valuable aromatic C-B, C-P, and C-C bond formation to furnish arylboronates, arylphosphonium salts, arylphosphonates, and spirocyclic cyclohexadienes, respectively.

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Halogen-bonding-promoted Photo-induced C–X Borylation of Aryl Halide using Phenol Derivatives

10.26434/chemrxiv-2022-glvkg ◽

2022 ◽

Author(s):

Kazuki Matsuo ◽

Eiji Yamaguchi ◽

Akichika Itoh

Keyword(s):

Functional Group ◽

Aryl Halide ◽

Halogen Bonding ◽

Mechanistic Studies ◽

Boronate Esters ◽

Aryl Radicals ◽

Phenol Derivatives ◽

Mild Conditions ◽

Donor Acceptor ◽

Photo Induced Electron Transfer

This study investigates the photo-induced C–X borylation reaction of aryl halides by forming a halogen-bonding complex. The method employs 2-naphthol as a halogen-bonding acceptor and proceeds under mild conditions without a photoredox catalyst under 420 nm blue light irradiation. The method is highly chemoselective, broadly functional group tolerant, and provides concise access to corresponding boronate esters. Mechanistic studies reveal that forming the halogen-bonding complex between aryl halide and naphthol acts as an electron donor-acceptor complex to furnish aryl radicals through photo-induced electron transfer.

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A study of the mechanism of the Sandmeyer reaction

Proceedings of the Royal Society of London Series A - Mathematical and Physical Sciences ◽

10.1098/rspa.1957.0150 ◽

1957 ◽

Vol 242 (1228) ◽

pp. 16-27 ◽

Cited By ~ 14

Keyword(s):

Electron Transfer ◽

High Yield ◽

Similar Process ◽

Electron Transfer Mechanism ◽

Sandmeyer Reaction ◽

Aryl Chlorides ◽

Aryl Radicals ◽

Vinyl Polymerization ◽

Initial Stage ◽

The One

The one-electron-transfer mechanism for the Sandmeyer reaction, proposed by W. A. Waters in 1942 has now been confirmed by diagnostic experiments. Its initial stage, Ar ·N + 2 + Cu + → Ar · + N 2 + Cu 2+ , has been shown to produce free aryl radicals capable of initiating vinyl polymerization; this observation is also true for related reactions of diazonium salts for which analogous one-electron-transfer mechanisms have been suggested. The cupric ions thus produced are oxidizers that, like ferric ions, can effect chain-ending of vinyl polymerization. They bring about the final step in the Sandmeyer reaction by a similar process that apparently involves chlorine atom transfer; Ar · + Cl—Cu—Cl → Ar —Cl + Cu—Cl. Product studies show that cupric ions are needed to enable the Sandmeyer reaction to give aryl chlorides in high yield; in their absence other reactions of aryl radicals can occur extensively. Molecular weight measurements with acrylonitrile polymers indicate that in dilute solution up to 35% of the initial aryldiazonium cations can produce free aryl radicals by interaction with cuprous ions in chloride solutions, but that in more concentrated solutions the secondary reaction with the cupric chloride generated occurs so extensively that the full cycle of the Sandmeyer reaction becomes predominantly an intramolecular process.

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