Photochemical and Electrochemical Applications of Proton-Coupled Electron Transfer in Organic Synthesis

Abstract

We present here a review of the photochemical and electrochemical applications of multi-site proton-coupled electron transfer (MS-PCET) in organic synthesis. MS-PCETs are redox mechanisms in which both an electron and a proton are exchanged together, often in a concerted elementary step. As such, MS-PCET can function as a non-classical mechanism for homolytic bond activation, providing opportunities to generate synthetically useful free radical intermediates directly from a wide variety of common organic functional groups. We present an introduction to MS-PCET and a practitioner's guide to reaction design, with an emphasis on the unique energetic and selectivity features that are characteristic of this reaction class. We then present chapters on oxidative N-H, O-H, S-H, and C-H bond homolysis methods, for the generation of the corresponding neutral radical species. Then, chapters for reductive PCET activations involving carbonyl, imine, other X═Y π-systems, and heteroarenes, where neutral ketyl, α-amino, and heteroarene-derived radicals can be generated. Finally, we present chapters on the applications of MS-PCET in asymmetric catalysis and in materials and device applications. Within each chapter, we subdivide by the functional group undergoing homolysis, and thereafter by the type of transformation being promoted. Methods published prior to the end of December 2020 are presented.

Figure 1

Limitations for (A) oxidative and (B) reductive HAT in synthetic chemistry. (C) BDFEs of common organic functional groups which have been difficult to activate via HAT. (D) MS-PCET approach to homolytic bond activation and formation.

Figure 2

(A) Thermodynamic scheme describing the BDFE of an E–H bond. (B) Thermodynamic scheme describing the effective BDFE of a pair of MS-PCET reagents using the relevant pK_a and E°. (C) Example of how to determine the thermodynamic driving force for a representative MS-PCET reaction.

Figure 3

Representative oxidative (top) and reductive (bottom) MS-PCET reagents pairs and the corresponding effective BDFEs in kcal mol⁻¹ using pK_a and E_1/2 values vs Fc⁺/Fc in MeCN. *Denotes photoexcited state complex as the oxidant / reductant.

Figure 4

(A) Square scheme representation of PCET mechanisms available for the cleavage of an E–H bond. The stepwise transfer of electrons and protons are shown on the edges of the square. The concerted transfer of the electron and proton via concerted PCET is shown along the diagonal. (B) Reaction coordinate of stepwise vs concerted PCET mechanisms. Stepwise PCET generates high-energy charged intermediates which are avoided in a concerted PCET pathway.

Figure 5

Pre-equilibrium hydrogen bonding can provide kinetic selectivity for homolysis of strong O–H bonds in the presence of much weaker C–H bonds.

Figure 6

Top: Reaction scheme for MS-PCET reduction of an aryl ketone to the corresponding ketyl radical. Bottom: Reaction coordinate diagram highlighting the kinetic and thermodynamic importance of pre-equilibrium hydrogen bonding in MS-PCET.

Figure 7

General schemes for substrate activation through (A) a photocatalytic oxidative quenching mechanism and (B) a photocatalytic reductive quenching mechanism, for overall redox-neutral transformations.

Chart 1. Structures of Photocatalysts Discussed in This Reviewa

Figure 8

General schemes for substrate activation through direct and mediated electrolysis.

Scheme 1. Photocatalytic Alkene Carboamidation of N-Aryl Amides through Concerted PCET (Knowles, 2015)

Scheme 2. Photocatalytic Alkene Hydroamidation of N-Aryl Amides through Concerted PCET (Knowles, 2015)

Scheme 3. Observed Rate Law for Ir(III) Luminescence Quenching in a Model Hydroamidation System (Knowles, 2019)

Scheme 4. Investigation into and Improvement of the Quantum Efficiency of Photocatalytic Intramolecular Olefin Hydroamidation (Nocera, 2018)

Scheme 5. Olefin Amido-alkynylation, Amido-alkenylation, and Amido-allylation of N-Aryl Amides with Sulfone Reagents (Rueping, 2018)

Scheme 6. Merger of Homolytic N–H Bond Activation through PCET and Ni-Catalysis Enables the Amido-arylation of Alkenes (Molander, 2019)

Scheme 7. Catalytic Olefin Amido-acylation of N-Aryl Amides and Acyl Electrophiles through Dual PCET/Ni Catalysis (Molander, 2020)

Scheme 8. Catalytic Intramolecular Alkene Hydroamidations of N-Alkyl Amides (Knowles, 2019)

*25 mol% n-Bu₄P⁺(t-BuO)₂P(O)O^–, 80 mol% TRIP-SH. **Without TRIP-SH, 30 mol% TRIP₂S₂.

Scheme 9. (A) Photocatalytic Synthesis of 4-Substituted 3,4-Dihydroisoquinolinones (Chen, 2017), (B) Photocatalytic Synthesis of 3-Substituted 3,4-Dihydroisoquinolinones (Huang, 2019), and (C) Proposed Reaction Mechanisms

*With 2.0 equiv of TEMPO.

Scheme 10. Photocatalytic Intramolecular Alkene Hydrosulfonamidation (Knowles, 2018)

Scheme 11. Design Plan for the Radical Cascade Approach to a Collection of 33 Indole Alkaloid Natural Products (Qin, 2017)

Scheme 12. Path A Type Assembly for the Synthesis of Eburnamine–Vincamine Family Alkaloids (Qin, 2017)

Scheme 13. Stereochemical Model for Radical Cascade Cyclization (Qin, 2017)

Scheme 14. Path C-Type Assembly for the Synthesis of Yohimbine Family Alkaloids (Qin, 2017)

Scheme 15. Path A-Type Assembly for the Synthesis of Eburnane Natural Products (Qin, 2018)

Scheme 16. Electrochemical Intramolecular Cyclization of Sulfonamides and Electron-Rich Olefins (Moeller, 2008)

Scheme 17. Intramolecular Competition Experiments to Study Sulfonamidyl Radical, and Olefin Radical Cation Pathways for Cyclization (Moeller, 2010)

Scheme 18. Electrochemical Intramolecular Cyclization of Carboxamides and Electron-Rich Olefins (Moeller, 2014)

Scheme 19. Electrochemical Intramolecular Oxyamidation of Olefins with N-Aryl Carbamates and Amides (Xu, 2014)

Scheme 20. Electrochemical Intramolecular Oxyamidation of Alkynes with N-Aryl Carbamates and Ureas (Xu, 2020)

*With 2.0 equiv of NaOAc instead of NaO₂CCF₃. **Without NaO₂CCF₃.

Scheme 21. Olefin Amido-Functionalization in an Electrochemical Flow Microreactor (Wirth, 2017)

Scheme 22. Electrochemical Synthesis of Cyclic Ureas (Ahmed, 2018)

Scheme 23. Electrocatalytic Intramolecular Olefin Hydroamidation (Xu, 2016)

Scheme 24. Electrocatalytic Intramolecular Carbamate and Urea Dicyclization (Xu, 2018)

Scheme 25. Electrothermal Intramolecular Aza-Wacker-Type Amidation of Olefins with N-PMP Carbamates and Amides (Xu, 2017)

*Reaction at 130 °C.

Scheme 26. Electrocatalytic Aza-Wacker-Type Amidation of Olefins with N-Aryl Carbamates and Amides (Hu, 2019)

*MeOH/PhCl (1:1) as solvent, 0.1 M n-Bu₄N⁺TsO^– as electrolyte, 4.0 equiv of NaOAc as base, 65 °C. **0.1 M n-Bu₄N⁺TsO^– as electrolyte.

Scheme 27. Electrocatalytic Intramolecular Indole Construction (Xu and Lu, 2016)

*From terminal alkyne (R₄ = H). **From TMS-alkyne (R₄ = SiMe₃).

Scheme 28. Electrocatalytic Intramolecular Indoline Construction (Xu, 2018)

*With 1.0 equiv of K₂CO₃ instead of 0.5 equiv of NaOAc.

Scheme 29. Electrocatalytic Intramolecular Polycyclic N-Heteroaromatic Synthesis (Xu, 2017)

Scheme 30. Electrocatalytic Synthesis of Imidazo-Fused N-Heteroaromatic Compounds (Xu and Lu, 2018)

*With 5 mol% 1, 1.0 equiv of NaHCO₃, in THF/MeOH (3:1), reflux. **With 0.5 equiv of NaHCO₃.

Scheme 31. Electrochemical Synthesis of Benzimidazolones and Benzoxazolones through a Cascade Cyclization (Xu, 2019)

*Without TFA.

Scheme 32. Electrochemical Synthesis of Isoxazolidine- and Oxazinane-Fused Isoquinolin-1(2H)-ones (Wen and Li, 2020)

*Constant current = 0.5 mA.

Scheme 33. Photocatalytic Intramolecular Arene C(sp²)–H Amidation with N-Aryl Amides (Hong, 2018)

Scheme 34. Catalytic Intramolecular Arene C(sp²)–H Amidation with N-Aryl Amides Employing an Organic Photocatalyst (Itoh, 2020)

Scheme 35. Synthesis of N-Aryl Phenanthridinones (A) and Carbazoles (B) via Electrochemical Intramolecular Arene C(sp²)–H Amidation (Waldvogel, 2018 and 2020)

*With 15% H₂O as cosolvent.

Scheme 36. Electrochemical Synthesis of 10H-Benzo[4,5]imidazo[1,2-a]indoles through Sulfonamidyl Radical Generation and Cyclization (Lei, 2019)

Scheme 37. Electrochemical Dehydrogenative Synthesis of Benzimidazolones from Trisubstituted N,N′-Diaryl Ureas (Li, 2020)

Scheme 38. Remote C-Centered Radical Generation via N–H Bond Homolysis and Subsequent Aryl Group Migration (Nevado, 2017)

*Using 3 equiv of Na₂CO₃ as base. **Using 2 equiv of Na₂CO₃ as base.

Scheme 39. Electrochemical Synthesis of Medium-Sized Benzo-Fused β-Keto-lactams via C–C Bond Cleavage (Ackermann and Ruan, 2020)

*50 °C.

Scheme 40. Electrocatalytic Sulfonamide Heteroarylation via Heteroaryl Group Migration (Guo, 2020)

Scheme 41. Photocatalytic Sulfonamide Heteroarylation via Heteroaryl Group Migration (Guo, 2020)

Scheme 42. Electrocatalytic Synthesis of N-Alkyl Triazolopyridines via C-to-N Alkyl Migration (Zhang, 2020)

Scheme 43. Intermolecular Hydrosulfonamidation of Unactivated Olefins Proceeding through Concerted PCET (Knowles, 2018)

*With 2 equiv of olefin. **From silyl enol ether.

Scheme 44. Intermolecular Hydroamidation between 4-Methoxybenzamide and 2,2,4-Trimethyl-1-pentene (Knowles, 2019)

Scheme 45. Electrocatalytic Synthesis of Indolines via Intermolecular [3+2] Annulation (Zhang and Lei, 2020)

*With 1.0 equiv of Na₂CO₃. **Constant current = 8 mA.

Scheme 46. Oxidative Benzylic C(sp³)–H Bond Amidation (Pandey, 2015)

Scheme 47. Electrochemical Intermolecular C(sp³)–H Amidation of Xanthenes (Zeng, 2018)

Scheme 48. (A) Photocatalytic C2-Selective C(sp²)–H Sulfonamidation of Indoles, Pyrroles, and Benzofurans (Yu, 2016) and (B) Photocatalytic C3-Selective C–H Sulfonamidation of Imidazo[1,2-a]pyridines (Sun, 2017)

*With 4.0 equiv of sulfonamide.

Scheme 49. Electrochemical C(sp²)–H Sulfonamidation of Heteroarenes (Zhang and Ackermann, 2020)

Scheme 50. Electrochemical Oxidative [4+2] Annulation of Indole-1H-carboxamides and N-Alkylindoles (Lei, 2020)

Scheme 51. Intermolecular Oxidative Heteroarene C(sp²)–H Imidation via Phthalimidyl Radicals (Itoh, 2017)

Scheme 52. Photocatalytic Arene and Heteroarene C(sp²)–H Sulfonimidation (Itami, 2017)

Scheme 53. Direct (Hetero)arene C(sp²)–H Sulfonimidation Mediated by Visible-Light Irradiation of DDQ (Itami, 2017)

Scheme 54. Electrochemical C(sp²)–H Sulfonimidation of (Hetero)arenes (Lei, 2019)

Scheme 55. Electrochemical C(sp²)–H Sulfonimidation of Olefins (Lei, 2019)

Scheme 56. Catalytic Remote Alkylation of C(sp³)–H Bonds in N-Acylamines (Knowles, 2016)

Scheme 57. (A) Catalytic Remote Alkylation of C(sp³)–H Bonds in N-Trifluoroacetamides (Rovis, 2016) and (B) γ-Selective C(sp³)–H Alkylation of Imides (Rovis, 2017)

*DMF solvent.

Scheme 58. γ-Selective C(sp³)–H Alkylation of Trifluoroacetamides with Alkyl Bromides (Rovis, 2019)

Scheme 59. γ-Selective C(sp³)–H Allylation of Trifluoroacetamides with Allyl Chlorides (Tambar, 2019)

Scheme 60. Remote Alkylation of C(sp³)–H Bonds of Sulfamate Esters through N–H Bond Homolysis and 1,6-HAT (Duan, Roizen, and Shu, 2019)

Scheme 61. Electrochemical Remote C(sp³)–H Sulfonamidation through an ET/PT Manifold (Muñiz, 2018)

*Constant current = 1 mA.

Scheme 62. Electrochemical Remote C(sp³)–H Sulfonamidation through a Concerted PCET Manifold (Lei, 2018)

Scheme 63. Electrochemical Remote C(sp³)–H Sulfonamidation through a PT/ET Manifold (Rueping, 2019)

Scheme 64. Electrochemical Synthesis of Pyrazolidin-3,5-diones through Intramolecular N–N Bond Formation (Waldvogel, 2016)

Scheme 65. Investigation of the Mechanism of Intramolecular N–N Bond Formation in Dianilide Substrates (Waldvogel, 2017)

Redox potentials vs Fc⁺/Fc in EtOH.

Scheme 66. Electrochemical Synthesis of Phthalic Hydrazides through N–N Bond Formation (Waldvogel, 2018)

Scheme 67. Electrochemical Intermolecular Homo-Coupling of Anilides via N–N Bond Formation (Waldvogel, 2020)

Scheme 68. Electrochemical Synthesis of Benzoxazoles (Waldvogel, 2017)

Scheme 69. Electrochemical Anodic Cross-Coupling of Anilides (Waldvogel, 2017)

*In HFIP solvent.

Scheme 70. 1,2-Allylsulfonylation of ortho-Vinyl Sulfonanilides (Xue, Chen, and Xiao, 2019)

*In MeCN solvent.

Scheme 71. An Early Example of Electrochemical Benzothiazole Formation through C–S Bond Formation (Tabaković, 1979)

*Using C anode.

Scheme 72. Aerobic Photocatalytic Synthesis of Benzothiazoles from Thiobenzanilides (Li, 2012)

Scheme 73. Dehydrogenative Synthesis of Benzothiazoles from Thiobenzanilides via N–H Bond Homolysis (Lei, 2015)

*Using 10% n-Bu₄N⁺OH^– in place of 1 equiv of Na⁺H₂NCH₂CO₂^–.

Scheme 74. Synthesis of 2-Aminobenzothiazoles from Aryl Isothiocyanates and Secondary Alkylamines (Lei, 2017)

Scheme 75. Catalyst- and Electrolyte-Free Flow Electrochemical Synthesis of Benzothiazoles and Thiazolopyridines (Wirth and Xu, 2018)

*With 3% v/v H₂O added as cosolvent.

Scheme 76. Photocatalytic Synthesis of Benzothiazoles (Gustafson, 2019)

Scheme 77. Electrosynthesis of Thiazolidin-2-imines from N-Allylthioureas (Ahmed and Wirth, 2019)

Scheme 78. Electrochemical Synthesis of 3,5-Diimido-1,2-dithiolanes (Waldvogel, 2018)

*With MeCN as solvent.

Scheme 79. Electrochemical Synthesis of 3-Substituted 5-Amino-1,2,4-thiadiazoles (Liu, Yang, and Zhou, 2020)

Scheme 80. Electrochemical C(sp²)–H Amination of Heteroarenes with Alkylamines (Ackermann, 2018)

*80 °C.

Scheme 81. Photocatalytic C(sp²)–H Amination of Quinoxalinones with Alkylamines (Wei and Zhou, 2018)

Scheme 82. Photocatalytic C(sp²)–H Amination of Phenols with Diarylamines (Xia, 2017)

Scheme 83. Electrochemical C(sp²)–H Amination of Phenols with Phenothiazines (Lei, 2018)

Scheme 84. Electrochemical C2-Selective Bioconjugation of Tyrosine Residues with Phenothiazines in (A) Isolated Amino Acids, (B) Tyrosine-Containing Dipeptides, and (C) Complex Polypeptides (Lei, Weng, and Chiang, 2019)

Scheme 85. Electrochemical C(sp²)–H Amination of Anilines and Electron-Rich (Hetero)arenes with Diarylamines (Lei, 2019)

*MeCN/AcOH (9:1) as solvent.

Scheme 86. Electrochemical C(sp²)–H Amination of Anilines with Diarylamines (Li and Song, 2019)

Scheme 87. Electrochemical Synthesis of Pyrido[1,2-a]benzimidazoles (Chen, 2020)

*With 10 mol% HSCH₂CO₂Me.

Scheme 88. Photocatalytic Aerobic C(sp³)–H Amination of THF and 2-MeTHF with Azoles (Lei, 2017)

*2-MeTHF as solvent.

Scheme 89. Electrochemical Oxidative C(sp³)–H Amination with Azoles (Lei, 2017)

Scheme 90. Intermolecular Electrochemical C–H Amination of Xanthenes with Azole Nucleophiles (Yang, Song, and Li, 2019)

Scheme 91. Photocatalytic C(sp²)–H Azolation of (A) 8-Aminoquinoline Amides and (B) Imidazo[1,2-a]pyridines (Adimurthy, 2017)

Scheme 92. Electrochemical C2 C(sp²)–H Azolation of Phenols and Anilides (Feng and Chen, 2019)

Scheme 93. Dehydrogenative C–N Cross-Coupling of Imidazo[1,2-a]pyridines or Benzo[d]imidazo[2,1-b]thiazoles with Azoles (Lei, 2019)

*With 3 equiv of azole.

Scheme 94. Electrochemical Synthesis of Tetrasubstituted Hydrazines (Xu, 2019)

*With 1 equiv of KOAc instead of pyridine, at reflux.

Scheme 95. Electrochemical N-Nitration of Azole Heterocycles and N-Nitrosation of Secondary Alkylamines (Lu, 2020)

*60 mol% n-Bu₄N⁺ClO₄^– as electrolyte.

Scheme 96. Synthesis of (Aza)benzimidazoles via Amidinyl Radical Generation and Cyclization (Xu, 2017)

*5:1 THF:MeOH.

Scheme 97. Dual Photoredox/Co(III)-Catalyzed Dehydrogenative Annulation of Aryl NH-Ketimines and Arylacetylenes (Li, 2018)

*Minor regioisomer.

Scheme 98. Photocatalytic N-Arylation of NH-Sulfoximines with Electron-Rich Arenes (König, 2018)

Scheme 99. Photocatalytic Syntheses of β-Keto Sulfones from Alkynes and Sulfonylhydrazides (Cai, 2016)

Scheme 100. Decarboxylative Sulfonylation of Cinnamic Acids with Arylsulfonyl Hydrazides for the Synthesis of Vinylsulfones (Cai, 2016)

Scheme 101. Electrochemical Decarboxylative Sulfonylation of Cinnamic Acids (Huang, 2017)

Scheme 102. Electrochemical Alkoxysulfonylation of Styrenes with Arylsulfonyl Hydrazides and Alcohols (Lei, 2018)

Scheme 103. Photocatalytic Oxysulfonylation of Styrenes (Zhu, 2019)

Scheme 104. Photocatalytic Decarboxylative Oxidative Sulfonylation of Atropic Acids with Sulfonyl Hydrazide Reagents (He and Guan, 2020)

Scheme 105. Electrochemical C–H Sulfonylation of Electron-Rich (Hetero)arenes with Arylsulfonyl Hydrazides (Lei and Huang, 2018)

*3.0 equiv of ArSO₂NHNH₂.

Scheme 106. Electrochemical C–H Sulfonylation of 2-Aryl-2H-indazoles with Arylsulfonyl Hydrazides (De Sarkar, 2020)

Scheme 107. Deoxygenative C2-Arylation of Quinoline N-Oxides with Arylsulfonyl Hydrazide Reagents (Lei, 2019)

Scheme 108. Oxidative Coupling of Arylsulfonyl Hydrazides and Tertiary Amines Proceeding with C–N Bond Cleavage (Sheykhan and Abbasnia, 2017)

Scheme 109. Electrochemical Synthesis of β-Aminovinyl Sulfones (Kim and Lee, 2019)

Scheme 110. (A) Electrolytic Co-generation of Iminium Ion and Sulfonyl Radical Reactive Intermediates, (B) Mechanism of Formation of Sulfonamide Products from These Reactive Intermediates (Sheykhan and Abbasnia, 2017), and (C) Mechanism of Formation of β-Aminovinyl Sulfone Products from These Reactive Intermediates (Kim and Lee, 2019)

Scheme 111. Electrochemical Synthesis of Unsymmetrical Thiosulfonates in the Cross-Coupling of Arylsulfonyl Hydrazide Reagents and Thiols (Tang, Pan, and Chen, 2018)

Scheme 112. Photocatalytic Oxidative Cleavage of N–N Bonds in Aromatic Hydrazines and Hydrazides (Zheng, 2011)

Scheme 113. Photocatalytic Synthesis of Pyrazoles from Hydrazines and Electron-Deficient Olefins (Zhu, 2016)

Scheme 114. Tandem Photocatalytic Aerobic Glaser Coupling/Hydrazine Annulation Cascade for the Synthesis of Pyrazoles (Zhu, 2019)

Scheme 115. Photocatalytic Oxidative Cleavage of C=C Bonds for the Synthesis of Hydrazones from Styrenes (Zhu, 2017)

Scheme 116. Photocatalytic Oxyarylation of Styrenes with Hydrazines (Zhu, 2017)

Scheme 117. Photocatalytic Synthesis of Sulfides and Selenides from Aryl Hydrazines and Thiols/Selenols (Hajra, 2018)

Scheme 118. Dual Ru(II)/Co(III) Catalytic System Enabling the Acceptorless Dehydrogenation of N,N′-Diaryl Hydrazines (Balaraman, 2018)

Scheme 119. Oxidative Arylation and Alkylation of Quinoxalin-2(1H)-ones with Hydrazine Reagents, Catalyzed by a 2D-COF Catalyst (Yang, 2020)

Scheme 120. Photocatalytic Intramolecular Hydrosulfonamidation of Allylic Sulfonylhydrazones (Xiao and Chen, 2014)

*With 2 equiv of TEMPO.

Scheme 121. Photocatalytic Intramolecular Olefin Sulfonamidation with Allylic Migration (Xiao and Chen, 2016)

Scheme 122. Photocatalytic Intramolecular Hydroxysulfonamidation of Allylic Sulfonylhydrazones (Xiao and Chen, 2016)

Scheme 123. Photocatalytic Synthesis of Benzosultams from Sulfonylhydrazones (Xiao and Chen, 2016)

Scheme 124. Photocatalytic Carbosulfonamidation of Allylic Sulfonylhydrazones with (A) Nozaki Allylsulfone Reagents and (B) Morita–Baylis–Hillman Allylic Acetate Reagents (Xiao and Chen, 2017)

Scheme 125. Synthesis of 3,5-cis-Isoxazolidines through N-Centered Radical Generation and Intramolecular Hydro(sulfon)amidation (A, Chen and Xiao, 2018; B, Nagasawa, 2019)

*With 2 equiv of TEMPO, 60% RSM. **With 10 equiv of KOtBu.

Scheme 126. Phthalazine Synthesis through Intramolecular Alkyne Sulfonamination and Smiles Rearrangement (Brachet and Belmont, 2016)

Scheme 127. Phthalazine Synthesis from Phosphoramidate-Appended Hydrazones (Brachet and Belmont, 2020)

Scheme 128. Electrochemical Synthesis of 1,2,4-Triazole-Fused Heterocycles (Zhang, 2018)

Scheme 129. Electrochemical Synthesis of 1,2,3-Triazoles (Xu, 2019)

Scheme 130. Photocatalytic Aerobic Oxidation of Dihydropyrimidines (Wu and Liu, 2014)

Scheme 131. Modular Assembly of Saturated Nitrogen Heterocycles through the PCET Activation of 4-Alkyldihydropyridine Precursors (Romanov-Michailidis, 2019)

Potentials measured in V vs SCE in MeCN.

Scheme 132. N-Acyl Sulfinamides as Alkyl Radical Precursors via N–H Bond Homolysis and C–S Bond Fragmentation (Qin, 2018)

Scheme 133. N-Acyl Sulfinamides as Alkyl Radical Precursors for the Reductive Alkylation of Benzothiophenes (Qin, 2018)

Scheme 134. Alkyl Carbazates as Precursors to Alkyl Radicals via Electrochemical Activation (Wang, 2020)

*MeCN solvent, n-Bu₄N⁺ClO₄^– electrolyte, constant current = 3 mA.

Scheme 135. Allylic and Benzylic C–H Bond Arylation Mediated by a Diarylsulfonamide HAT Catalyst (Kanai and Oisaki, 2018)

*Mixture of uncharacterized isomers.

Scheme 136. A Zwitterionic Amidate Catalyst for Photoinduced HAT Reactions (Ooi, 2020)

*With 10 equiv of C–H nucleophile.

Scheme 137. Photocatalytic C(sp³)–H Alkylation of Primary Amines (Cresswell, 2020)

*Using 1 mol% [Ir-6]PF₆ photocatalyst.

Scheme 138. Photocatalytic Generation of an N-Centered Radical for Catalytic Alkene 1,2-Carbosulfonylation (Xiao and Chen, 2019)

Scheme 139. Photocatalytic Generation of an N-Centered Radical for Catalytic [3+2] Cycloaddition (Xiao and Chen, 2020)

Scheme 140. Distinct Reactivity of an N–H Phenothiazine Photocatalyst Attributed to N–H MS-PCET (Larionov, 2020)

Potentials measured in V vs SCE in MeCN.

Scheme 141. Borylation of Energy-Demanding Arenes through MS-PCET-Induced Aryl Radical Generation (Larionov, 2020)

*With 1 equiv of H₂O; ketone protected as acetal, which was removed in situ with conversion to BF₃K salt.

Scheme 142. Proposed Mechanism of Aryl Phosphate Borylation via MS-PCET As Mediated by a Phenothiazine-Carbonate Hydrogen-Bonded Complex (Larionov, 2020)

Scheme 143. Photocatalytic Ring-Opening Reactions of Unstrained Cyclic Alcohols via Alkoxy Radical-Mediated C–C Bond β-Scission (Knowles, 2016)

*2 mol% [Ir-9]PF₆, 1.0 equiv of collidine, 4.0 equiv of SelectFluor, 1:1 CD₃CN:D₂O, blue LEDs, rt.

Scheme 144. Photocatalytic C–C Bond Cleavage via Alkoxy Radical β-Scission in Aliphatic Alcohols (Knowles, 2019)

*With n-Bu₄P⁺(PhO)₂P(O)O^–. **With n-Bu₄P⁺(MeO)₂P(O)O^– in PhCF₃.

Scheme 145. Photocatalytic (A) (n+2) and (B) (n+1) Ring Expansion of Cyclic Allylic Alcohols via C–C Bond β-Scission (Knowles, 2019)

*With n-Bu₄P⁺(PhO)₂P(O)O^–.

Scheme 146. Ring-Opening Functionalization of Cycloalkanols via C–C Bond Fragmentation (Zhu, 2018)

Scheme 147. Photocatalytic Ring-Opening (A) Allylation and (B) Formylation of Arylcycloalkanols via C–C Bond β-Scission (Xia, 2018)

Scheme 148. Dual Photoredox/Ni-Catalyzed Ring-Opening Arylation Enabled by PCET-Promoted C–C Bond Cleavage (Rueping, 2020)

Scheme 149. Synthesis of SCF₃-Containing Ketones via Alkoxy Radical Generation and C–C Bond β-Scission (Rueping, 2020)

Scheme 150. Photocatalyzed Oxidative Ring-Opening and β-Fluorination of Cyclopropanols (Lectka, 2015)

Scheme 151. Synthesis of Unsymmetric 1,8-Diketones Enabled by C–C Bond Fragmentation (Zhu, 2019)

*With 2-(benzothiazoyl)carbinol coupling partner. **With formyl alcohol coupling partner.

Scheme 152. Synthesis of 1,5-Diarylpyrazoles via Photocatalytic Cycloaddition of Arenediazonium Salts with Arylcyclopropanols (von Wangelin, 2020)

Scheme 153. Synthesis of 1-Tetralones via Regioselective Electrochemical Ring Expansion of 1-Arylcyclobutanols (Parsons, 2020)

*With 10 mol% Mn(OTf)₂.

Scheme 154. Photocatalytic Heteroarylation of Remote C(sp³)–H Bonds via Alkoxy Radical 1,5-HAT and Heteroaryl Migration (Zhu, 2018)

Scheme 155. Photocatalytic Cyanation of Remote C(sp³)–H Bonds via Alkoxy Radical 1,5-HAT and Nitrile Migration (Zhu, 2019)

Scheme 156. Catalytic Hydroetherification and Carboetherification of Unactivated Alkenes Enabled by PCET Activation of Alcohols (Knowles, 2020)

*With 2-methyl-2-oxazoline base catalyst and 4-(trifluoromethyl)thiophenol HAT co-catalyst.

Scheme 157. Photocatalytic α-Oxyamination of 1,3-Dicarbonyl Compounds Proceeding through Proposed TEMPO Radical Trapping (Tan, 2010)

Scheme 158. Photocatalytic α-Oxyamination of 1,3-Dicarbonyl Compounds Proceeding through Proposed Catalytic Disproportionation of TEMPO (Koike, Yasu, and Akita, 2012)

Scheme 159. Heterogeneous Photocatalysis of the α-Oxyamination of 1,3-Dicarbonyls (Huang and Wu, 2019)

Scheme 160. Photocatalytic C–C Bond Formation from 1,3-Dicarbonyl Compounds (Ollivier, 2014)

Scheme 161. Three-Component Coupling of 1,3-Dicarbonyl Compounds, Electron-Rich Olefins, and Heteroarenes (Xia, 2020)

Scheme 162. Photocatalytic Alkylation of Malonate Diesters with Styrenes (Kobayashi and Yamashita, 2020)

Scheme 163. Electrochemical C(sp²)–H Annulation of 2-Fluoro-1,3-amido Esters for the Synthesis of Oxindoles (Xu, 2017)

Scheme 164. Electrochemical C(sp²)–H Annulation for the Synthesis of Oxindoles, Quinoxalinones, and a Bicyclo-Fused Pyrrole (Xu, 2018)

Scheme 165. Electrochemical [4+1] and [4+2] Annulation of N-Allyl Amides with 1,3-Dicarbonyl Compounds to Access (A) Pyrrolidine and (B) Tetrahydropyridine Derivatives (Xu, 2018)

Scheme 166. Proposed Mechanism of Electrochemical [4+1] and [4+2] Annulation (Xu, 2018)

Scheme 167. Electrochemical [3+2] Annulation of 1,3-Dicarbonyl Compounds and Alkenes to Yield 2,3-Dihydrofurans (Lei, 2019)

*Using 2.0 equiv of NaOt-Bu instead of NaOAc as base. **Constant current of 5 mA. ***Using 1.0 equiv of K₂CO₃ instead of NaOAc as base.

Scheme 168. Electrochemical Synthesis of Dihydropyrano[4,3-b]indoles and 2,3-Dihydrofurans (Park, 2020)

*Applied potential = 3.5 V. **2.0–3.0 equiv of active methylene compound used.

Scheme 169. Dual Photoredox/Co(III) Catalysis for the Synthesis of 10-Phenanthenols via C(sp³)–H Alkylation (Wu, 2020)

Scheme 170. Electrochemical Synthesis of 10-Phenanthrenols via C(sp²)–H Alkylation (Mei, 2020)

Scheme 171. Photocatalytic Generation of Both Neutral Ketyl Radicals and C2-C-Centered Radicals from Aryl β-Keto Esters (Wu, 2020)

Scheme 172. Systematic Study of the Nature of the Mechanism of Phenol Oxidation under Neutral and Basic Conditions (Vermillion Jr. and Pearl, 1964)

Scheme 173. An Early Example of the Homo-Coupling of a Phenol via Electrochemical Phenoxyl Radical Generation

Regioselectivity of bond formation is dictated by steric control in the nature of the C1-substituent (Bobbitt, 1969 and 1970).

Scheme 174. Boron-Doped Diamond Electrodes Offered a Highly Chemoselective Ortho,ortho-Homo-Coupling of 2,4-Dimethylphenol (Waldvogel, 2006)

Scheme 175. Symmetric Phenol Homo-Coupling Mediated by Boron-Doped Diamond Electrode and Hexafluoroisopropanol (Waldvogel, 2009)

Scheme 176. Electrosynthesis of Biphenols Using Graphite Electrodes and Fluorinated Media (Waldvogel, 2011)

Scheme 177. (A) Electrochemical Anodic C(sp²)–H/C(sp²)–H Homo-Coupling of Phenols Bearing Electron-Withdrawing Groups and (B) Cross-Coupling in the Presence of Naphthalene (Waldvogel, 2020)

Scheme 178. Selective Anodic 2,2′-Cross-Coupling of Phenols (Waldvogel, 2014)

*In neat HFIP.

Scheme 179. Mechanistic Proposal for the Oxidative Coupling of 4-Alkylphenols for the Synthesis of Pummerer Ketones

Scheme 180. An Early Example of Electrochemical Anodic Ortho,para-Homo-Coupling of 4-Alkylphenols for the Synthesis of Pummerer Ketones (Miller, 1978)

Scheme 181. Unexpected Formation of a Trimeric Spirocyclic Lactone Product during the Electrolysis of 2,4-Dimethylphenol (Waldvogel, 2006)

Scheme 182. Improved Preparation of a Tetrameric Adduct via Anodic Oxidation of 2,4-Dimethylphenol (Waldvogel, 2008)

Scheme 183. Diversity-Oriented Synthesis of a Series of Complex Polycyclic Structures from Tetrameric Adduct 182.1 Resulting from Anodic Oxidation of 2,4-Dimethylphenol (Waldvogel, 2006–2016)

Scheme 184. Electrochemical Anodic Oxidative Coupling of Guaiacol Derivatives (Waldvogel, 2011)

Scheme 185. Electrochemical Anodic C(sp²)–H/C(sp²)–H Cross-Coupling of Phenol and Naphthol Derivatives, and Comparison to a Fe(III)/DTBP-Mediated Oxidative Coupling Reaction (Waldvogel, 2017)

*At 30 °C. **HFIP + 18 vol% MeOH solvent.

Scheme 186. Selective Cross- and Homo-4,4′-Coupling of 2,5- and 2,6-Disubstituted Phenols (Waldvogel, 2019)

*6 vol% aqueous co-solvent.

Scheme 187. Electrochemical C(sp²)–H/C(sp²)–H Cross-Coupling of Phenols with Electron-Rich Arenes (Waldvogel, 2010)

Scheme 188. A Study on the Effect of MeOH on Electrochemical C(sp²)–H/C(sp²)–H Cross-Coupling of Phenols with Electron-Rich Arenes (Waldvogel, 2012)

Scheme 189. Synthesis of Mono-TIPS-Protected 2,2′-Biphenols via Electrochemical Anodic Phenol–Arene Cross-Coupling (Waldvogel, 2016)

*With Ni cathode.

Scheme 190. Electrochemical Oxidative [3+2] Annulation of Phenols and N-Acetyl Indoles (Lei, 2017)

Scheme 191. Dehydrogenative Cross-Coupling of Phenols and Thiophenes or Benzothiophenes (Waldvogel, 2017 and 2018)

*With 1.0 equiv of phenol and 3.0 equiv of thiophene; **With 1.0 equiv of phenol and 1.5 equiv of benzothiophene; ***With 3.0 equiv of phenol and 1.0 equiv of thiophene.

Scheme 192. Convergent Synthesis of C3-(2-Hydroxyphenyl)benzofurans from Both C2- and C3-Substituted Benzofurans, with Furan Metathesis (Waldvogel, 2018)

Scheme 193. Furan metathesis in the oxidative coupling of phenols and benzofurans (Waldvogel, 2018)

Scheme 194. Synthesis of N,N-Diarylamides through Anodic Coupling of Phenols and Benzoxazoles (Waldvogel, 2019)

Scheme 195. Electrochemical Synthesis of Diisoeugenol (195.2) (Einaga and Waldvogel, 2018)

Scheme 196. Synthesis of Benzodihydrofurans via Electrochemical, Intermolecular [3+2] Annulation between Phenols and Electron-Deficient Olefins (Zhang and Wang, 2020)

Scheme 197. Electrochemical Sulfonylation of Phenols Using Sulfinates (Waldvogel, 2019)

Scheme 198. Photocatalytic Oxidative Phenol–Arene Cross-Coupling (König, 2017)

E_p^ox/V vs SCE in MeCN.

Scheme 199. Photocatalytic Homo- and Cross-Coupling of Phenols (Kozlowski, 2020)

*With 2.0 mol% [Ru-2](PF₆)₂ photocatalyst.

Scheme 200. Photochemical Heck-Type Arylation of Vinylphenols via an Excited-State Phenolate Intermediate (Xia, 2020)

Scheme 201. Photocatalytic Oxidative De-aromatization of Orcinaldehyde Derivatives (Narayan, 2020)

IBX conditions: 1.1 equiv of IBX, 0.1 equiv of n-Bu₄N⁺I^–, 5.0 equiv of TFA, 1,2-DCE, rt.

Scheme 202. Electrochemical Synthesis of Nitriles via Dehydration of Oximes (Waldvogel, 2015)

Scheme 203. Electrochemical Dehydration, and 1,3-Dipolar Cycloaddition of 2,6-Dichlorobenzaldoxime in a Flow Electrochemical Cell (Waldvogel, 2017)

Scheme 204. Photocatalytic 1,2-Dioxygenation of Alkenes: The Cyclization of Oximes (Chen, 2016)

Scheme 205. Electrochemical Synthesis of N-Heterocycles via Oxime Cyclization (Xu, 2018)

Scheme 206. Electrochemical [4+2] Annulation and Dioxygenation of Olefins with Hydroxamic Acids (Han, 2020)

Scheme 207. Photocatalytic Trifluoromethyl Thiolation of C(sp³)–H Bonds (Glorius, 2016)

Scheme 208. Photocatalytic Alkynylation of Formyl C(sp²)–H Bonds (Glorius, 2017)

Scheme 209. Photocatalytic Aliphatic C(sp³)–H Functionalization via Phosphoryl Radical-Mediated HAT (Alexanian, 2018)

Scheme 210. Photocatalytic Aliphatic C(sp³)–H Cyanation via Phosphoryl Radical-Mediated HAT (Kanai, 2018)

*With 3.0 equiv of TsCN.

Scheme 211. Visible-Light Photocatalytic Thiol–Ene Reaction (Yoon, 2013)

Scheme 212. Dual Photocatalytic/Aniline-Catalyzed Thiol–Ene Reaction (Yoon, 2014)

*Reaction conducted in water.

Scheme 213. Photocatalytic Titanium Dioxide-Promoted Thiol–Ene (Greaney, 2015)

*With 10 mol% TiO₂. **In the absence of TiO₂. ***With 100 mol% TiO₂.

Scheme 214. Organophotocatalytic Thiol–Yne Reaction (Ananikov, 2016)

Scheme 215. Light-Mediated Thiol–Ene Reaction through Organic Photoredox Catalysis (Wang, 2017)

Scheme 216. Photocatalytic Thiol–Yne Reactions (Wang, 2019)

*With 1 mol% [Acr-1]BF₄, 1.0 equiv of thiol, 1.7 equiv of alkyne.

Scheme 217. Light-Mediated Thiol–Ene Reaction through Concerted PCET (Dilman, 2019)

Scheme 218. Thiol–Yne Annulation Cascade for the Synthesis of Sulfenylated Dihydrochromenones (Volla, 2019)

Scheme 219. Preparation of β-Sulfenylated Alcohols through Photoredox Catalysis (Du, 2020)

Scheme 220. Photocatalytic Tandem α-Thiolation/Annulation of Carbonyls with 2-Mercaptobenzimidazoles (Huang, 2019)

Scheme 221. Photocatalytic 1,2-Thioamination of Alkynes for the Synthesis of 3-Sulfenylindoles (Kshirsagar, 2018)

Scheme 222. Electrochemical Oxysulfenylation and Aminosulfenylation of Alkenes (Lei, 2018)

*With 10.0 equiv of amine.

Scheme 223. Electrochemical Hydroxy- and Alkoxysulfenylation of Alkenes (Mei, Du, and Han, 2018)

*With 3.0 equiv of thiophenol, 1.0 equiv of p-TsOH·H₂O, in MeCN.

Scheme 224. Dual Photoredox/Ni Catalysis for the Cross-Coupling of (Hetero)aryl Iodides and Thiols (Oderinde and Johannes, 2016)

Scheme 225. Photocatalytic Cross-Coupling of Aryl Halides and Thiols (Fu, 2017)

Scheme 226. Photocatalytic Cross-Coupling of Heteroaryl Chlorides and Thiols (Glorius, 2020)

*Indicates minor site of thiolation. **With 1.0 equiv of thiol, 1.5 equiv of aryl halide, 1.2 equiv of K₂HPO₄. ***With 3.0 equiv of thiol.

Scheme 227. Electrocatalytic Cross-Coupling of (Hetero)aryl Iodides and Thiols (Wang, 2019)

Scheme 228. Electrochemical Cross-Coupling of (Hetero)aryl Halides and Thiols (Mei, 2019)

Scheme 229. Electrochemical C(sp²)–H Thiolation of Indoles and Electron-Rich Arenes (Lei, 2017)

Constant current = 6 mA.

Scheme 230. Photocatalytic C(sp²)–H Sulfenylation of Imidazopyridines (Barman, 2018)

Scheme 231. Electrochemical C3 C(sp²)–H Sulfenylation of Imidazopyridines (Lei and Tang, 2019)

*Constant current = 6 mA, 55 °C.

Scheme 232. Electrochemical C(sp²)–H Sulfenylation of Quinoxalin-2-ones (Li, 2020)

Scheme 233. Photocatalytic aerobic C(sp²)–H sulfenylation of indolizines (Lenardão and Silviera, 2020)

Scheme 234. Electrochemical C(sp²)–H Thiolation of Arenes (Wu, 2020)

*With n-Bu₄N⁺BF₄^– electrolyte in HFIP solvent. **Indicates minor site of arylation.

Scheme 235. Electrochemical [4+2] De-aromative Annulation of Indoles (Lei, 2020)

Scheme 236. Electrochemical C(sp²)–H Sulfenylation of β-Aminoacrylates (Lei, 2019)

Scheme 237. Electrochemical C(sp²)–H Sulfenylation of N,N-Dimethylenaminones (Lei, Lu, and Gao, 2020)

Scheme 238. Benzothiazole Synthesis through Photocatalytic Aerobic Oxidative Thiyl Radical Generation (Natarajan, 2018)

Scheme 239. Photocatalytic Aerobic Synthesis of Thiocarbamates through the Reaction of Thiols and Isonitriles (Wei, 2018)

Scheme 240. Photocatalytic Synthesis of Thioethers from Thiols and Hydrazones (Singh, 2019)

Scheme 241. Electrochemical Synthesis of Sulfur-Containing Enaminonitriles through the Coupling of Thiols and Acetonitrile. (Huang, 2019; Lei, 2019)

*60 mol% KI, **from dimethyl diselenide, 60 mol% KI

Scheme 242. Electrochemical Oxidative gem-Difunctionalization of Isocyanides (Lei, 2020)

Scheme 243. Photocatalytic Conversion of Thiols into Disulfides and H₂ Using CdSe Quantum Dots (Wu, 2014)

Scheme 244. Photocatalytic Synthesis of Symmetric and Unsymmetric Disulfides (Dethe, 2018)

*≤6% symmetrical disulfide observed. **≤15% symmetrical disulfide observed.

Scheme 245. Electrochemical Synthesis of Unsymmetrical Aryl-alkyl Disulfides through S–S Bond Formation (Lei, 2018)

*MeCN/CH₂Cl₂ (3:1) solvent. Constant current = 16 mA.

Scheme 246. Sulfonylation of thiols enabled by visible-light generation of thiyl radical (Volla, 2019)

Scheme 247. Electrochemical Synthesis of 2-Benzothiazolyl Sulfenamides (Torii, 1978)

*With stainless steel electrodes.

Scheme 248. Electrochemical Dehydrogenative Coupling of S–H/N–H Bonds (Li and Yuan, 2019)

Scheme 249. Electrochemical Dehydrogenative Cross-Coupling of Diaryl Phosphine Oxides and Thiols (Zheng, 2019)

*DMF used as solvent.

Scheme 250. Photocatalytic Cross-Coupling between Phosphines and Thiophenols (Wu and Xia, 2020)

*3.0 equiv of thiol, 2.0 equiv of benzaldehyde.

Scheme 251. Tandem Photocatalytic Thiol–Ene/S-Oxidation for the Synthesis of Sulfoxides (Alemán and Fraile, 2017; Wang and Wei, 2017)

Scheme 252. Tandem Photocatalytic Thiol–Ene/S-Oxidation for the Synthesis of Sulfoxides (Singh, 2018)

Scheme 253. Photoredox-Mediated Synthesis of Functionalized Sulfoxides from Terminal Alkynes (Shah, 2020)

*H₂O solvent.

Scheme 254. Electrochemical Synthesis of Sulfinic Acid Esters through the Oxidative Coupling of Thiols and Alcohols (Zhong, 2019)

*MeCN as solvent.

Scheme 255. Electrochemical Synthesis of Sulfinate Esters via Ni(II)-Catalyzed Oxidative Esterification of Thiols with Alcohols (Kaboudin, 2019)

Scheme 256. Light-Induced Oxidative Cross-Coupling of Thiophenols and Alcohols (Lei, 2017)

Scheme 257. Electrochemical Synthesis of Sulfonamides from Thiols and Amines (Noël, 2020)

*Batch reactor, 1 equiv of thiol, 1.5 equiv of amine, 1 equiv of Me₄N⁺BF₄^–. **1 equiv of pyridine added.

Scheme 258. Electrochemical Synthesis of Sulfonyl Fluorides through the Oxidative Coupling of Thiols and Potassium Fluorides (Noël, 2019)

Scheme 259. Photocatalytic Aerobic Oxidative Acylation of Amines with Thiocarboxylate Salts (Tan, 2016)

Scheme 260. Photocatalytic Acylation of Amines with Thiocarboxylic Acids Using a Heterogeneous Catalyst (Biswas, 2018)

Scheme 261. Photocatalytic Acylation of Amines with Thiocarboxylic Acids Using an Organic Photocatalyst (Song, 2020)

Scheme 262. Organophotocatalytic Amination of Heteroaromatic Thiols (Wacharasindhu, 2017)

*DMSO solvent.

Scheme 263. Photocatalytic C(sp³)–H Arylation of Benzylic Alcohols and Ethers (MacMillan, 2014)

*DMA/DMSO (1:1) solvent.

Scheme 264. Photocatalytic C(sp³)–H Aminoalkylation of Benzylic Ethers (MacMillan, 2014)

*10 equiv of silyl ether.

Scheme 265. Dual Photoredox/Thiol-Catalyzed Allylic C(sp³)–H Arylation (MacMillan, 2015)

*Indicates minor regioisomer. **25% K₂CO₃ base. ***From 2-cyclohexene-1-ol. ****25 mol% A.

Scheme 266. C(sp²)–H Alkylation of N-Heteroaromatics with Alcohols via Dual Photoredox/Thiol Catalysis (MacMillan, 2015)

*From 1,3-butanediol.

Scheme 267. C(sp³)–H Arylation of N-Protected Tetrahydroisoquinoline and Tetrahydro-β-carbolines (Wang, 2016)

Scheme 268. Dual Photoredox/Thiyl Catalysis for the Synthesis of Tertiary Alcohol via Allylic and Benzylic C(sp³)–H Cleavage (Liu, 2019)

*Indicates minor regioisomer.

Scheme 269. Allylic C(sp³)–H Alkylation with N-Aryl Aldimines and Ketimines (Huang and Rueping, 2020)

*Indicates minor regioisomer.

Scheme 270. Triple Photoredox/Thiyl/Pd(II) Catalytic Dehydrogenation of Tetrahydronaphthalenes (Kanai, 2017)

Scheme 271. Photocatalytic Aldehyde Allylation via Ternary Catalysis (Kanai and Mitsunuma, 2020)

Scheme 272. Dual Photoredox/Thiyl Catalytic Hydroxyalkylation of Heteroarenes (Kanai, 2020)

Scheme 273. Photocatalytic Carboxylation of Benzylic C(sp³)–H Bonds (König, 2019)

*3DPA2FBN photocatalyst. **3DPAFIPN photocatalyst.

Scheme 274. Photocatalytic Addition of Benzylic Carbanions to Ketones and Aldehydes (König, 2019)

*5 mol% 3DPA2FBN, 20 mol% 274.A, 50 mol% K₂CO₃, 3 equiv of aldehyde in MeCN.

Scheme 275. Photocatalytic Corey–Seebach Reaction (König, 2020)

*3 equiv of dithiane, 1 equiv of aldehyde.

Scheme 276. Photoredox/Thiobenzoic Acid Dual Catalysis for the α-Arylation of Benzylamines (Hamashima, 2018)

Scheme 277. Photoredox/Thiobenzoic Acid Co-catalyzed Reductive Homo-Coupling of Benzylic Amines and Alcohols (Hamashima, 2020)

Scheme 278. Dual Photoredox/Thiol Co-catalytic Hydrosilylation of Olefins (Wu, 2017)

Scheme 279. Joint Photoredox/Thiol-Catalyzed Inverse Hydroboration of Imines (Xie and Zhu, 2018)

Scheme 280. Dehydrodimerization of Cyclic Ethers and Olefins Catalyzed by ZnS (Kisch, 1985–1999)

Figure 9

Dependence of apparent quantum yield (Φ_app) on (A) substrate oxidation potential (E_ox) or (B) calculated C–H bond dissociation energies implies a PCET mechanism for H-atom abstraction. Reproduced with permission from ref (750). Copyright 1999 John Wiley and Sons.

Scheme 281. Sulfoxidation of Adamantane via a Titania–SO₂ Charge-Transfer Complex (Kisch, 2008 and 2012)

Scheme 282. Dimerization of Oxindoles with an Acridinium Betaine Catalyst (Ooi, 2017)

Scheme 283. Demonstration of Benzylic C(sp³)–H Bond Homolysis via Concerted MS-PCET (Mayer, 2018)

Scheme 284. Second-Order PCET Rate Constants for the Reaction of Fluorenyl Benzoates with Various pK_a’s and Oxidants of Various Potentials (Mayer, 2019)

Figure 10

Rate–driving force relationships in the concerted PCET activation of fluorenyl benzoates. (A) Plot of PCET rate constant versus driving force for PCET across a series of fluorenyl benzoates and oxidants. From top line to bottom line: R = NH₂, OMe, H, CF₃. The slope of each plot is the α value for ET for that substrate. (B) Plot of PCET rate constant versus driving force for PCET across a series of fluorenyl benzoates with a single oxidant (FeCp₂⁺). The slope of the plot is the α value for PT. (C) Double More O’Ferrall–Jencks plot for the PCET activation of a fluorenyl benzoate. The horizontal planes represent progress in the PT coordinate and electronic reorganization coordinate, while the jump between the two planes represents the instantaneous ET that occurs between the fluorenyl benzoate and the oxidant. Reproduced with permission from ref (763). Copyright 2019 ACS Publishing.

Figure 11

Rate constants of the reaction of the parent fluorenyl benzoate. Left: open blue circles, experimental data; black circles, predicted rate constant of the global reaction. Right: open red circles, predicted concerted PCET rate constant; open green circles, predicted stepwise PT-ET rate constant; black circles, predicted rate constant of the global reactions. Reproduced with permission from ref (765). Copyright 2020 RCS Publishing.

Scheme 285. Photocatalytic Deuteration and Oxidative Lactonization of Benzylic C(sp³)–H Bonds via Concerted PCET (Mayer, 2020)

Scheme 286. PCET Enables Catalytic C(sp³)–H Alkylation (Alexanian and Knowles, 2019)

Scheme 287. Alkylation of Indoles, Benzofurans, Benzothiophenes, and Enamines Enabled by Excitation of an EDA Complex (Glorius, 2019)

Scheme 288. Photocatalytic Carbonyl Reduction with an NADH Model (Pac, 1983 and 1987)

Scheme 289. Photoreduction of Carbonyl Compounds Using a Semiconducting Organic Polymer Photocatalyst (Pac, 1990)

Scheme 290. Epoxide and Aziridine Reduction and Reductive Allylation through Aryl Ketone PCET Activation (Ollivier and Fensterbank, 2011)

Scheme 291. Dual Photoredox/Brønsted Acid-Catalyzed Reductive Cyclization of Enones (Yoon, 2011)

Scheme 292. Photocatalytic Intramolecular Cyclization of Ketones with Olefins Enabled by Reductive PCET (Knowles, 2013)

Scheme 293. Photocatalytic Intramolecular Ketyl-Olefin Cyclization, Catalyzed by Cationic Heteroleptic Cu(I)(diamine)(bisphosphine) Complexes (Collins, 2018)

Scheme 294. Intramolecular ketyl-olefin coupling for the synthesis of chromanols via reductive PCET (Rueping, 2016)

Scheme 295. Umpolung Allylation of Carbonyls and Imines via Reductive PCET (Chen, 2016)

Scheme 296. Three-Component Coupling of Aryl Aldehydes, Anilines, and Allyl Sulfones for the Synthesis of Homoallylic N-Arylamines (Dixon, 2016)

Scheme 297. Three-Component Coupling of Aldehydes, Anilines, and a Dehydroalanine Derivative for the Synthesis of anti-1,3-Diamines (Dixon, 2018)

Scheme 298. Synthesis of Tetrahydroisoquinolines in a Photocatalytic Reverse Povarov Reaction (Dixon, 2018)

Scheme 299. Biomimetic Strategy for the α-C(sp³)–H Allylation of α,α-Disubstituted Primary Amines (Dixon, 2019)

Scheme 300. β-Selective Reductive Coupling of Aldehydes or Imines with Alkenylpyridines (Ngai, 2017)

Scheme 301. Photocatalyst-Free Intermolecular Reductive Coupling of 2,2,2-Tri- and 2,2-Difluoroacetophenones with 2-Vinylpyridine (Liu, 2018)

Scheme 302. Reductive Coupling of Aliphatic Carbonyl Compounds and Styrenes (Jamison, 2019)

*With 4.0 equiv of acetaldehyde.

Scheme 303. Olefin Hydroaminoalkyation Used to Access a Common Intermediate Toward the Total Synthesis of Alkaloids (−)-FR901483 (303.1) and (+)-TAN1251C (303.2) (Gaunt, 2020)

Scheme 304. Proposed Hydroaminoalkylation Mechanism through Stepwise PT/ET (Gaunt, 2020)

Scheme 305. Photocatalytic Intermolecular Imine–Olefin Reductive Coupling (Rueping, 2020)

Scheme 306. Photocatalytic Synthesis of Indoles and Isoquinolines through Reductive PCET-Mediated Neutral Ketyl Radical Generation and Subsequent Smiles Rearrangement (Ye, 2020)

Scheme 307. Mechanism of Photocatalytic Indole Formation via Neutral Ketyl Radical Initiated Smiles Rearrangement (Ye, 2020)

Scheme 308. Photocatalytic Reductive Pinacol-Type Coupling of Aldehydes, Ketones, and Imines (Rueping, 2015)

Scheme 309. Reductive Coupling of Aldehydes, Ketones, and Imines via Reductive PCET Using Perylene as a Photocatalyst (Sudo, 2016 and 2017)

Scheme 310. Visible-Light Photocatalytic Pinacol Coupling of α-Ketoesters (Wang and Yao, 2017)

Scheme 311. Pinacol Coupling of Aldehydes, Ketones, and Imines Using a Coumarin Photocatalyst (Cozzi, 2018)

Scheme 312. Photocatalytic Synthesis of Primary Amines from NH₃ and Aldehyde or Ketone Starting Materials (Gilmore, 2018)

Scheme 313. A Bifunctional Cu(I) Complex Mediates Photocatalytic Reductive PCET for Pinacol Coupling (Collins, 2019)

Scheme 314. Reductive Pinacol Coupling of Aryl Aldehydes Using a ZnIn₂S₄ Semiconductor as Photocatalyst (Sun, 2020)

Scheme 315. Electrochemical Reductive PCET of Acetophenone with a Cobaltocenium-Anilinium PCET Mediator (Peters, 2020)

Scheme 316. Photocatalytic Reductive Arylation of Aldehydes, Ketones, and Imines (Xia, 2017)

Scheme 317. Heteroarene Alkylation with Ketones and Aldehydes via Reductive PCET and Spin-Centered Shift (Wang, 2019)

Scheme 318. Synthesis of 1,3-Diazepanes from N-Aryl Imines and 4-Substituted Quinolines via Reductive PCET (Dixon and Duarte, 2020)

Scheme 319. Reductive Vinylogous Arylation of Arylidene Malonates with Cyanoheteroarenes (Scheidt, 2019)

Scheme 320. Photocatalytic β-Mannich Reaction of Cyclic Ketones with Preformed Imines (MacMillan, 2015)

*40 mol% Morpholine used in place of azepane. **20 mol% pyrrolidine used in place of azepane.

Scheme 321. Redox-Neutral Cyclization of Amino-ketones for the Synthesis of 3-Hydroxy Azetidines, Pyrrolidines, and Piperidines (Zhu, 2016)

Scheme 322. Photocatalytic Intermolecular Cross-Coupling Reaction between N-Arylamines and Aldehydes, Ketones, and Imines for the Synthesis of 1,2-Amino Alcohols and Vicinal Diamines (Wang, 2018)

Scheme 323. Synthesis of Vicinal Diamines via Photocatalytic Reductive Coupling of N-Aryl Aldimines and N,N-Dicyclohexlmethylamine (Cho, 2020)

Scheme 324. Photocatalytic Radical Alkylation of Aldimines/Ketimines with 4-Alkyl-1,4-dihydropyridines (Yu, 2017)

Scheme 325. Photoinduced Acylation of Imines with α-Ketoacids (Yu, 2019)

Scheme 326. Photocatalytic Synthesis of Masked Fluoroalkyl Amino Aldehydes from 1,3-Dioxoline and Fluoroalkyl Aldimines/Ketimines (Lu and Gong, 2019)

Scheme 327. Photocatalytic Decarboxylative Synthesis of Vicinal Amino Alcohols (Zeng and Zhong, 2020)

Scheme 328. Intermolecular Reductive Coupling of Acetylenic Ketones and Potassium Benzyltrifluoroborate (Sun, 2020)

Scheme 329. Photocatalytic Transfer Hydrogenation of Diarylimines (Polyzos, 2018)

Scheme 330. A Tandem Photoredox Mechanism Operates upon Irradiation of [Ir-3]PF₆ in the Presence of Et₃N Reductant, Permitting the In Situ Generation of the Potent Photoreductant [Ir(ppy)₂(H₃dtbbpy)]PF₆ for the Activation of Energy-Demanding Substrates (Connell, Polyzos, and Francis, 2019)

Scheme 331. Reductive Amination Enabled by Photoredox Catalysis and Polarity-Matched Hydrogen Atom Transfer (Guo and Wenger, 2018)

Scheme 332. Photocatalytic Reduction of Imines Using Semiconducting Quantum Dots (Pu and Shen, 2018)

Scheme 333. Electrochemical, Pd-Catalyzed Reduction of Benzaldehyde through Concerted PCET (Lercher, 2020)

Scheme 334. Photocatalytic Oxyalkylation of Styrenes with NHPI Esters (Glorius, 2017)

Scheme 335. Photocatalytic 1,2-Alkylation/Lactonization of Carboxylic Acid Tethered Styrenes (Pan, 2017)

Scheme 336. Decarboxylative Alkylation of Acetophenone-Derived Silyl-Enol Ethers for the Synthesis of Alkylated Ketones (Song, 2018)

Scheme 337. Cascade Cyclization for the Synthesis of Benzazepines Initiated via Reductive Radical Generation from NHPI Esters (Xiao, 2018)

Scheme 338. Synthesis of Complex Dihydroquinolinone Derivatives via the [2+2+1] Polycyclization of 1,7-Enynes (Paixão, 2019)

Scheme 339. [2+2+1] Polycyclization of 1,6-Enynes via Decarboxylative Radical Generation (Xu and Hu, 2019)

Scheme 340. Synthesis of Benzo[b]phosphole Oxides from Alkynylphosphine Oxides and NHPI Esters (Zhou and Dong, 2019)

Scheme 341. Intramolecular Alkylation of Arenes via Reductive Decarboxylative Radical Generation from NHPI Esters (Sherwood and Xiao, 2019)

Scheme 342. Photocatalytic C3-Alkylation of Coumarins with NHPI Ester Reagents (Jin and Sun, 2019)

Scheme 343. Photocatalytic C3-Alkylation of Quinoxalin-2(1H)-ones with NHPI Ester Reagents (Jin and Sun, 2019; Li, 2019)

Scheme 344. Decarboxylative Benzylic C(sp³)–H Alkylation of Tetrahydroisoquinolines with NHPI Esters (Ren and Cong, 2018)

Scheme 345. Photocatalytic Decarboxylative N-Alkylation of α-Diazoacetates with Alkyl NHPI Esters (Yu, 2019)

Scheme 346. Dual Photoredox/Cu-Catalyzed Alkyl Radical Sulfonylation (Li and Liu, 2020)

*TFA used in place of (n-BuO)₂PO₂H.

Scheme 347. Photocatalytic Late-Stage C(sp²)–H Methylation, Ethylation, and Cyclopropanation of Pharmaceuticals Mediated by PCET Activation of Peroxyacetates (DiRocco, 2014)

*Indicated minor site of alkylation.

Scheme 348. Photocatalytic, Late-Stage Hydroxymethylation of Heteroarene Pharmaceuticals (DiRocco, 2016)

Scheme 349. Heteroarene Minisci Alkylation through Dual Reductive PCET and HAT (Wang, 2018)

Scheme 350. Photocatalytic Minisci C(sp²)–H Alkylation Reaction of N-Heteroarenes with Alkyl Iodides, Proceeding through Halogen Atom Transfer (Wang, 2019)

Scheme 351. Addition of Olefins and Enol Ethers to 1,2-Diazenes through Semiconductor Photoredox Catalysis (Kisch, 1992 and 1995)

Scheme 352. Addition of Olefins and Enol Ethers to Imines through Semiconductor Photoredox Catalysis (Kisch, 1996, 1997, and 2002)

Scheme 353. Photocatalytic Traceless Olefin Hydroalkylation with α-Diazoacetates (Doyle, 2020)

Scheme 354. Photocatalytic Reduction of Nitroquinolines to the Corresponding Aminoquinolines (Helaja, 2019)

Scheme 355. Photocatalyst-Free Direct Deoxygenation of Pyridine N-Oxides and Nitroarenes with Hantzsch Ester (von Wangelin and Konev, 2020)

*With 2.4 equiv of Hantzsch ester. **With 2.2 equiv of Hantzsch ester.

Scheme 356. Photoinduced 4-Pyridination of C(sp³)–H Bonds (Inoue, 2013)

Scheme 357. Electrochemical Synthesis of Hindered Primary Amines through Iminium Heteroarylation with Cyanoheteroarene Reagents (Rovis and Lehnherr, 2020)

Scheme 358. Synthesis of α,α-Disubstituted Primary Amines through the Cross-Coupling of Imines or Oximes with Cyanoheteroarenes (Rovis, Lehnherr, and DiRocco, 2020)

Scheme 359. Photocatalytic Hydro-heteroarylation of Alkenes with 2-Bromoazoles (Weaver, 2015)

*Indicates minor site of heteroarylation

Scheme 360. Photocatalytic Meerwein-Type Hydro-heteroarylation of Electron-Deficient Olefins (Jui, 2017)

Scheme 361. Photocatalytic Synthesis of Heteroaryl Amino Acids (Jui, 2017)

Scheme 362. Photocatalytic Anti-Markovnikov Hydro-heteroarylation of Unbiased Olefins and Arenes (Jui, 2017)

*Indicates minor site of arylation.

Scheme 363. Photocatalytic Hydro-heteroarylation of Functionalized Olefins through Reductive PCET (Jui, 2018)

Scheme 364. Photoreductive Deprotection of Benzyloxypyridines for the Synthesis of Pyridones (Helaja, 2017)

Scheme 365. Photocatalytic Asymmetric Aza-pinacol Cyclization Enabled by Concerted PCET (Knowles, 2013)

Scheme 366. Enantioselective Synthesis of 1,2-Amino Alcohols Enabled by Visible-Light-Activated Iridium Catalysis (Meggers, 2016)

Scheme 367. Photocatalytic Enantioselective Conjugate Amination Enabled by Concerted PCET (Gong and Meggers, 2017)

Scheme 368. Enantioselective Remote Alkylation of Benzamides with α,β-Unsaturated 2-Acylimidazoles (Gong and Meggers, 2017)

Scheme 369. Enantioselective Pyrroloindoline Synthesis via Photocatalytic Indole PCET (Knowles, 2018)

Scheme 370. Enantioselective Synthesis of 1,2-Amino Alcohols through Catalytic Radical Coupling of Activated Ketones and N-Arylglycines (Jiang, 2018)

Scheme 371. Enantioselective Synthesis of γ-Functionalized Alkylpyridines via Addition of Prochiral Radicals to Vinylpyridines (Jiang, 2019)

*Reaction performed in toluene.

Scheme 372. Enantioselective Reduction of Azaarene Ketones by Cooperative Photoredox and Hydrogen-Bonding Catalysis (Jiang, 2019)

*Using modified chiral phosphoric acid. **Reaction run at rt.

Scheme 373. Enantioselective Synthesis of β-Benzylated Aldehydes via Photocatalytic C(sp³)–H Functionalization of Toluene and Derivatives (Melchiorre, 2018)

Scheme 374. Photocatalytic Enantioselective Hydrosulfonamidation of Alkenes (Knowles, 2020)

*Reaction at 0 °C with TRIP₂S₂.

Scheme 375. Enantioselective Synthesis of Pyrrolidine Derivatives through a Concurrent Photoredox and Enzymatic Catalysis (Ward and Wenger, 2018)

Scheme 376. Enantioselective Reductive Deoxygenation of α-Acetoxy Ketones (Hyster, 2018)

*With glucose-d₁

Scheme 377. Non-natural Reactivity of Ene-Reductases Induced by Synergistic Photoenzymatic Catalysis (Hyster, 2019)

Scheme 378. Photochemical Generation of Metal Nanoparticles (Scaiano, 2006)

Figure 12

Ru-catalyzed photo-cross-linking reaction of tyrosine-containing peptide hydrogels. (A) Structure of FmocFFGGGY and schematic illustration of hydrogelation and Ru-catalyzed photo-cross-linking of hydrogel. (B) Mechanism of photomediated cross-linking reaction of tyrosine to form the cross-linked dityrosine adduct. (C) UV–vis spectra experiments showing evidence of the formation of peptide dimer through a dityrosine bond. (D, E) TEM images of uncross-linked and cross-linked hydrogels, suggesting increased entanglement of the peptide fibrils upon photo-cross-linking. Reproduced with permission from ref (1068). Copyright 2013 ACS Publications.

Scheme 379. Two-Step, One-Pot Depolymerization of Native Lignin via Electrocatalysis and Photoredox Catalysis (Stephenson, 2017)

Scheme 380. Photocatalytic Depolymerization of Native Lignin through O–H Bond PCET (Knowles, 2020)

Scheme 381. Proton-Coupled Redox Switching in Rosarins and Octaphyrins (Sessler, 2013 and 2018)

Figure 13

Design of bistable electrochromic devices based on concerted intramolecular PCET. (A) General structure of polymeric electrochromic materials used in the study and proposed PCET mechanism responsible for color switching between a transparent state and magenta (R¹ = R³ = NEt₂, R² = H). (B) Schematic of an electrochromic device. (C) Multicolor ESL prototypes (the colored outlines represent the rim of ESLs). (D) A single-color ESL prototype (dashed blue line represents the ongoing process for the ESL display, and the solid blue line represents the completed process). Reproduced with permission from ref (1081). Copyright 2019 Springer Nature.

Figure 14

Development of a host–guest collaborative MOF as a photochromic material. (A) The host–guest collaborative framework structure of F14.1. (B) Proposed mechanism for the photoinduced formation of H₂ bpe^2+• radical, which is responsible for photochromic behavior of N. (C) Generation of different patterns on the same film of F14.1. Reproduced with permission from ref (1082). Copyright 2019. The Royal Society of Chemistry.

Figure 15

Use of redox molecules for the performance improvement of the carbon-based SCs. (A) Specific capacitances with current density in 0.1 M H₂SO₄ solution of four carbon-based SCs. (B) Specific capacitances with current density of four carbon-based SCs in HQ/H₂SO₄ solution (prepared by dissolving 0.38 M HQ in 0.1 M H₂SO₄). (C) Redox reaction of HQ/BQ system. Reproduced with permission from ref (1085). Copyright 2011 John Wiley and Sons.

Figure 16

Study of physisorbed HQ on activated charcoal material as a supercapacitor. (A) Cyclic voltammograms at scan rates of 5 (green trace), 10 (blue), 20 (red), and 50 (black) mV/s. (B) Proposed overall mechanism of HQ redox chemistry on electrode surface when HQ is physisorbed on AC. (C) Total capacitance, double layer capacitance, and pseudocapacitances calculated from cyclic voltammograms at 5 mV/s scan rate for HQ-AC in different pH electrolyte solutions. Reproduced with permission from ref (1087). Copyright 2015 ACS Publications.

Figure 17

Photoelectric conversion cell set up and reactions (Xie and Bakker 2014).

Figure 18

Electrochemical cell assembly and half-cell reactions during charging. Constant current = 0.1 mA; electrolyte = 0.1 M NaCl; OCP = 0.62 V; solvent = 1:1 v/v H₂O/CH₃CN (Haga, 2017).