Uses of K2S2O8 in Metal-Catalyzed and Metal-Free Oxidative


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Uses of K2S2O8 in Metal-Catalyzed and Metal-Free Oxidative Transformations Sudip Mandal,† Tishyasoumya Bera,† Gurudutt Dubey,§ Jaideep Saha,*,† and Joydev K. Laha*,§ †

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Centre of Biomedical Research, Division of Molecular Synthesis and Drug Discovery, SGPGIMS Campus, Raebareli Road, Lucknow 226014, India § Department of Pharmaceutical Technology (Process Chemistry), National Institute of Pharmaceutical Education and Research, S. A. S. Nagar, Punjab 160062, India ABSTRACT: Carbon−carbon/carbon−heteroatom bond formation via oxidative transformations is a heavily explored topic at the frontier of chemistry. Potassium persulfate (K2S2O8) has emerged as a cost-effective, suitable inorganic oxidant for a wide array of oxidative transformations, ranging from laboratory experiments to industrial processes. The current review provides a comprehensive coverage of oxidative transformations aided with K2S2O8 in the presence or absence of a transition-metal catalyst, critical assessment of the results, and underlying mechanisms. Organic chemists may find this review to be a useful guide for the expedient synthesis of new chemical entities, to formulate mechanistic manifolds involving the sulfate radical anion, or to design novel oxidative transformations. A detailed understanding of the unsolved mechanisms could further enrich the field. KEYWORDS: potassium persulfate, C−H functionalizations, oxidative transformations, sulfate radical anion, radical reactions

1. INTRODUCTION Carbon−carbon (C−C)/carbon−heteroatom (C−X) bond formation between two C−H/C−H bonds or C−H/X−H bonds via oxidative transformation has become the central focus in C−C/C−X bond forming reactions obviating the use of prefunctionalized substrates and alleviating the generation of salt waste, thereby rendering superior sustainability and environmental compatibility.1 The dependence on these direct oxidative C−H functionalizations that feature higher atom economy and sustainability has witnessed a steady escalation of these reactions over the past two decades, which signifies the importance of and emergent interest in this synthetic technology. Despite an intrinsic challenge that lies in achieving high regio-selectivity, successful realizations of multiple regioselective C−H functionalizations have successfully been achieved.2 Transition-metalcatalyzed oxidative transformations represent one of the state-ofthe-art features at the frontiers of chemistry. An oxidant is invariably used in these transformations for regeneration of the catalyst. In many of these metal-catalyzed transformations, it has often been found that the judicious choice of the terminal oxidant plays a crucial role for the desired catalytic transformation. Many oxidants that have found legible use in these reactions include molecular oxygen, DDQ, p-benzoquinone, TBHP, PhI(OAc)2, I2, metal oxidants, oxone, persulfates, etc. Among these varieties of oxidants, including both inorganic and organic, potassium persulfate (K2S2O8) has emerged as a suitable inorganic oxidant for a wide array of oxidative transformations and has found broad © 2018 American Chemical Society

applications ranging from laboratory experiments to industrial processes. K2S2O8, as an inexpensive, readily available oxidant, has shown its unique utility since the discovery of Minisci reaction.3 The peroxydisulfate ion (S2O82−) is the most powerful oxidant among other peroxygen families of compounds, such as H2O2, KHSO4, etc. The standard redox potential is estimated to be 2.01 V in aqueous solution.3 Thermal, photolysis, radiolysis, or redox decomposition of S2O82− provides the sulfate radical anion (SO4• −) under mild conditions. Moreover, transition-metal ions are also able to activate K2S2O8 to form SO4• −. The SO4• − is regarded as a very strong one-electron oxidant with a redox potential of 2.5−3.1 V.4 It has a longer lifetime (∼4 s) than hydroxyl radicals, primarily because of their preference for electron transfer reactions. An electron transfer process could be exergonic or endergonic. The endergonic process could be slow, because of large activation energy. K2S2O8 can oxidize various metal salts and complexes, anions, nucleophilic radicals, and neutral organic compounds.3 A plethora of literature has appeared in recent years, leveraging the potential use of this oxidant in various metalcatalyzed and metal-free organic transformations. Not only are its substantial uses in palladium catalysis achieved after Minisci’s work, but different unconventional oxidative transformations Received: February 22, 2018 Revised: April 12, 2018 Published: April 13, 2018 5085

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ACS Catalysis

important to mention that, although the oxidizing ability of SO4• − is comparable to that of hydroxyl radical, the former is very selective and, thus, can afford specific oxidative transformations.7 A large volume of the silver-catalyzed reactions proceed through decarboxylative generation of a radical species. The Ag(I)/K2S2O8 combination is suitably poised for silvercatalyzed reactions, as SO4• − species selectively oxidizes Ag(I) to Ag(II) without directly affecting the substrates. The Ag(II) species, in turn, oxidizes the substrate, for example, oxidation of carboxylic acid or carboxylate results in alkyl or acyl radicals after decarboxylation. This synchronized process occurs most efficiently with the Ag(I)-peroxydisulfate combination, and, therefore, it finds frequent use in many decarboxylative and related oxidative transformations. In other cases, especially in C−N and C−S bond-forming reactions, stoichiometric Ag salts have been used and the counteranion of the salts are incorporated into the final products. In these reactions, the counteranion of the Ag salt is transformed to a radical, which subsequently participates in different bond-forming processes. Among the common peroxydisulfates (Na2S2O8, (NH4)2S2O8, and K2S2O8), K2S2O8 has been mostly used in organic transformations. It can be found that, in many different reactions, K2S2O8 resulted in better conversion, in comparison to its congeners. One possible reason could be its better solubility in organic media. In some cases, when Na2S2O8 or (NH4)2S2O8 gave comparable yields to K2S2O8 in the optimization, the author preferred K2S2O8 for the subsequent investigations. Reactions with Ag(I)/K2S2O8 mostly proceed via a radical mechanism. This section of the review will provide coverage on different silver-catalyzed and Ag-mediated oxidative transformations that are carried out in the presence of K2S2O8. Another important note is that, in many such transformations, multiple bond formation occurred in tandem fashion. We have categorized those reactions based on the nature of the first bond formed in the sequence. 2.1.1. C−C Bond Formation. Silver-catalyzed decarboxylative methods are among the most popular means to effect direct C−H functionalizations. Because of the easy availability of various carboxylic acids, such methods are gaining increasing attention in organic synthesis. As such, alkyl and aryl carboxylic acids serve as the source for the corresponding alkyl and aryl groups in silver-catalyzed transformation, whereas α-oxocarboxylic acids can be deployed as the source for acyl groups. Several decarboxylative coupling reactions for C−C bond formation were explored with the combination of Ag(I) and K2S2O8. In 2014, Muthusubramanian and co-workers developed a silvercatalyzed decarboxylative method for the acylation of pyridine N-oxides with α-oxocarboxylic acids (see Scheme 1).8 A radical mechanism was proposed wherein the acyl radical generated in the reaction added to pyridine N-oxides and subsequent hydrogen abstraction from the intermediate by SO4• − delivered the final product. Syntheses of coumarin derivatives have attracted considerable attention as they exhibit diverse biological activities. In 2015, Wang and co-workers developed a radical cyclization method to access various acylated coumarins via a silver-mediated cascade reaction where aryl alkynoates, α-keto acids, and K2S2O8 were used (see Scheme 2).9 Different N-alkyl-N-aryl cinnamamides or acrylamides were strategically used in analogous cascade radical cyclization processes with acyl or alkyl carboxylic acids (see Scheme 3). For example, Mai and co-workers developed one such tandem radical cyclization method with N-arylcinnamides and alkylcarboxylic

also have been realized solely with this oxidant and without adding any metal catalyst. It is noteworthy that other variants, such as Na2S2O8 and (NH4)2S2O8, are far less commonly used, compared to K2S2O8. This can be attributed to the better solubility of potassium salt in organic solvent rendering efficient transformation. The range of transformations that are achieved with K2S2O8 in metal-catalyzed and metal-free processes covers the C−C to C−X (X = N, O, S, P, B, Si, F, Br, I) bond forming reaction across a wide array of substrates. K2S2O8 has also proven to be very effective in degrading organic pollutants, particularly for the class of aromatic pollutants.5 Although selective organic transformations involving electron transfer to SO4• − has been covered in some early reviews,3,6 no comprehensive review has appeared on the oxidative transformations using persulfate and, more precisely, K2S2O8, with updates of recent advancements in the area. The current review is specifically focused on the applications of K2S2O8 in oxidative organic transformations. Any reported methods that use Na2S2O8 or (NH4)2S2O8 are beyond the scope of this review. Because of the large volume of the literature available with this oxidant, only important examples showcasing varieties of bond formations are presented herein. This review is organized based on uses of K2S2O8 in metalcatalyzed and metal-free processes, and those are categorized further by the types of the bonds being formed. Selected examples of substrates and important mechanisms are discussed for different methods highlighting their scopes and limitations. We hope that this review should serve as a useful reference to the chemists who are involved, or intend to be involved, in developing methodologies using K2S2O8 in metal-catalyzed and metal-free oxidative transformations and, therefore, should facilitate significant research progress in this area.

2. METAL-CATALYZED OXIDATIVE TRANSFORMATIONS WITH K2S2O8 2.1. Silver-Catalyzed Transformations. The initial belief with silver to have low catalytic efficacy has been overridden in recent years by the fast-paced development of Ag(I)-catalyzed transformations in organic synthesis. Subsequent to Minisci’s pioneering contributions,3 a wide array of oxidative C−H bond functionalization to form different C−C (such as arylation, alkenylation, alkylations via decarboxylative reactions or related processes), C−N (azidation, nitration etc.) and C−P bonds (synthesis of phosphonates) are achieved using silver catalysts in combination with K2S2O8. In some cases, especially in C−N and C−S bond-forming reactions, stoichiometric Ag salts have been used and the counteranions of the salts are incorporated into the final products. For the reactions that are discussed in this section, different oxidants such as ceric ammonium nitrate (CAN), H2O2, TBHP, DTBP, m-CPBA, oxone, molecular oxygen, PhI(OAc)2, BQ , and DDQ were screened during optimization, along with peroxydisulfates. The majority of the oxidants either failed to provide any desired product or resulted in poor conversion and therefore proved significantly inferior to peroxydisulfates. The probable reason is linked to the efficacy of peroxydisulfates in oxidizing Ag(I) to the metastable Ag(II) species (1.98 V) during the course of the reaction, which is the key step in most Ag(I)catalyzed or Ag(I)-mediated oxidative processes. Other oxidants have lower redox potentials (for example, hydrogen peroxides, 1.78 V; O2, 1.23 V; IO3−, 1.13 V; TEMPO, 1.48 V; KHSO5, 1.85 V), compared to the peroxydisulfate ion (S2O82− (2.01 V)) or sulfate radical anion (SO4• − (2.5−3.1 V)). While the oxidation of Ag(I) to Ag(II) could proceed with both S2O82− and SO4• −, the latter species is likely to be involved at an elevated temperature. It is 5086

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ACS Catalysis Scheme 1. Silver-Catalyzed Decarboxylative Acylation of Pyridine-N-Oxides with α-Oxocarboxylic Acidsa

a

Reprinted with permission from ref 8. Copyright 2014, Royal Society of Chemistry, London.

Scheme 2. Ag-Mediated Radical Cyclization of Alkynoates with α-Keto Acidsa

a

Reprinted with permission from ref 9. Copyright 2015, American Chemical Society, Washington, DC.

Scheme 3. Ag-Catalyzed Tandem Cyclization of N-Arylcinnamides/Acrylamides with Alkyl or Acyl Radicala

a Reprinted with permission from refs 10 (Copyright 2014, American Chemical Society, Washington, DC), 11 (Copyright 2016, American Chemical Society, Washington, DC), and 12a (Copyright 2013, Wiley−VCH, Weinheim, Germany).

alkyl carboxylic acid to α-ketoacids, which delivered the corresponding acylated products (Scheme 3b).11 For these reactions, radical cyclization occurred in 6-endo-trig fashion. A different course of cyclization was achieved when acrylamides were used under similar reaction conditions. For example, Guo and co-workers

acids using Ag(I)/K2S2O8 (see Scheme 3a).10 This reaction afforded substituted dihydroquinolin-2(1H)-ones. Using a similar reaction condition, they introduced a CF3 group on the dihydroquinolin-2(1H)-one moiety with CF3SO2Na. They subsequently extended the scope of this transformation by changing 5087

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The foregoing discussion on the silver-catalyzed oxidative radical addition−cyclization reactions (Schemes 3 and 4) followed the general mechanism outlined in Scheme 5. In all of these cases, the first step was the generation of radical species (alkyl or acyl) by catalytic Ag(I) and stoichiometric K2S2O8. The Ag(I) was oxidized to Ag(II), which, in turn, oxidized carboxylic acid into a carboxylate radical and subsequent decarboxylation afforded the radical species A. Next, it was added on the alkene or alkyne unit of the conjugated systems (B) and formed radical C or D. Regioselectivity of this addition step was dependent on the stability of the radical species formed. Intramolecular cyclization on the aromatic ring thus produced another aryl radical intermediate (E/F) with a five- or six-membered fused ring. In the next step, aromatization could occur in two different processes. One path could involve Ag(II)-mediated formation of the carbocation, followed by deprotonation (path A). The other one could proceed through direct hydrogen abstraction by SO4• − (path B).9−12 For 1,7-enynes, the first radical cyclization occurred on the alkyne to give a vinyl radical species, which subsequently cyclilized through a hydrogen abstraction−radical addition−oxidation sequence.13,14 In 2015, Zhang and Lei developed a silver-catalyzed method to prepare oxazoles from α-oxocarboxylates and isocyanides via a decarboxylative addition−cyclization cascade in the presence of K2S2O8 (Scheme 6).15 This reaction was proposed to proceed

reported the access to oxindoles, when substituted acrylamides were reacted with α-keto acids in the presence of Ag(I)/K2S2O8 (see Scheme 3c).12a This reaction proceeded through a 5-exo-trig cyclization. In these cases, either a five- or six-membered ring was formed, which was mainly governed by the regioselectivity of the initial radical addition step. It is important to note that, not only did the alkyl radicals from carboxylic acids participate in such addition−cyclization reactions, but 1,3-dicarbonyls also were employed in the same fashion. For the latter case, the key radical species was generated through methylenic C(sp3)−H bond cleavage in the presence of Ag(I)/K2S2O8.12b Use of different conjugated 1,7-enyne-based N-(arylsulfonyl)acrylamides for cascade radical cyclization was demonstrated, which afforded various spirocyclic systems with the reaction of alkylcarboxylic acids.13 A series of bond-forming processes involving an alkyne moiety occurred in the reaction that afforded the final spirocyclic compounds. Li and co-workers utilized a nonconjugated 1,7-enyne system and reaction with secondary alkyl carboxylic acids provided a different class of spirocyclic scaffold (see Scheme 4).14 These examples demonstrated the Scheme 4. Ag-Catalyzed C(sp3)-H Bond Cycloalkylation with Neighboring Carboxylic Acid Groupsa

Scheme 6. Ag-Catalyzed Oxidative Cyclization of α-Oxocarboxylate and Isocyanidea

a

Reprinted with permission from ref 14. Copyright 2017, American Chemical Society, Washington, DC.

a

utility of the Ag(I)/K2S2O8 system in generating complex molecular scaffolds through sequential radical addition−cyclization steps.

with the intermediacy of an acyl cation, which is rarely postulated in the literature.

Reprinted with permission from ref 15. Copyright 2015, Royal Society of Chemistry, London.

Scheme 5. Proposed Mechanism for Ag-Catalyzed Radical Addition−Cyclization Reactions with Alkynoates and Acrylamides

5088

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ACS Catalysis In 2012, Li and co-workers reported an efficient silvercatalyzed decarboxylative alkynylation method of different aliphatic carboxylic acids using phenylethynylbenziodoxolone as an electrophilic alkynylating agent (see Scheme 7).16a The overall

Scheme 8. Proposed Mechanism for Decarboxylative Alkynylationa

Scheme 7. Ag-Catalyzed Decarboxylative Alkynylation of Aliphatic Carboxylic Acids, as Reported by Li et al.a

a Reprinted with permission from ref 16a. Copyright 2012, American Chemical Society, Washington, DC.

Chen and co-workers demonstrated the use of silver-catalyzed oxidative decarboxylation strategy in developing a direct C-2 alkylation method for benzothiazoles, thiazoles, and benzoxazoles (see Scheme 9).18 K2S2O8 provided the best result (96%), compared to Na2S2O8 (51%), while other oxidants such as H2O2, TBHP, and DTBP gave no reaction. One important feature is that, unlike Minisci reaction, this transformation occurred under acid-free conditions at room temperature. Under such conditions, generation of the SO4• − ion was unlikely and it was proposed that the alkyl radical itself abstracted the C2 hydrogen of the heterocycles. Thus, union of the C2-centered radical with the alkyl radical delivered the product. This proposition was further supported by the excess requirement of carboxylic acid in the reaction. Alternatively, alkyl radical addition at C2, followed by oxidation, could also be feasible. In 2015, Mai et al. reported a similar decarboxylative alkylation of pyrimidines using different aliphatic carboxylic acids and Ag(I)/K2S2O8.19 This method delivered different 4-substituted pyrimidine derivatives in aqueous media. A radical addition− oxidation sequence was proposed in the report. Song and co-workers reported an efficient silver-catalyzed method for decarboxylative cross coupling of aliphatic carboxylic acids with sulfonyl oxime ethers to provide corresponding alkyl oxime ether derivatives (see Scheme 10).20 The overall transformation was a C(sp3)− C(sp2) coupling, which was carried out with 20 mol % AgNO3 and 1.5 equiv of K2S2O8 in a mixture of CH3CN/H2O (1:1) at 50 °C. Trial reactions carried out with other oxidants such as PIDA, TBHP, CuCl2, FeCl2, or MnBr3 resulted in no conversion. In the catalytic cycle, Ag(I)/K2S2O8 was mainly involved in generating the alkyl radical. In this reaction, addition of alkyl radical to sulfonyl oxime, followed by elimination of the sulfonyl radical, delivered the product. Recently, Li and co-workers developed a convenient approach for the synthesis of 6-alkyl phenanthridines and 1-alkylisoquinolines from 2-isocyanobiphenyls and vinyl isonitriles, respectively,

a

Reprinted with permission from ref 16a. Copyright 2012, American Chemical Society, Washington, DC.

transformation resulted in a C(sp3)−C(sp) coupling, which is particularly challenging to achieve with tertiary alkyl groups. Reactions involving arylethynylbenziodoxolones required the use of 10 mol % AgNO3 and 1 equiv of K2S2O8 for the alkynylation whereas higher loadings of catalyst and oxidant were necessary for triisopropylsilyl (TIPS)-substituted ethynylbenziodoxolones. This method could be applied to a wide array of alkyl carboxylic acids, even when functionalized with different groups such as amide, carbonyl, halide, ether, etc., and offers a means for chemoselective alkynylation between an alkyl and aromatic carboxylic acids. In the proposed mechanism (Scheme 8), alkyl radical A was generated by Ag(I)/K2S2O8, which was added on the electrophilic alkyne reagent to form B. The latter species underwent β-elimination and provided the desired product. Controlled experiments showed that Ag(I) and K2S2O8 were only involved in generating the alkyl radical.16a It is important to note that the group also reported the use of Ag(I)/tBuOCl or Ag(I)/Selectfluor for decarboxylative generation of alkyl radical from carboxylic acids. In those cases, however, the alkyl radical combined with the Cl or F atoms present in the oxidants and afforded the corresponding alkyl halides.16b,c Therefore, choice of Ag(I)/ K2S2O8 combination was the most appropriate for the present transformation. The Hashmi group reported a similar silver-catalyzed alkynylation strategy in the presence of K2S2O8.17 In this case, they used α,α-difluoroarylacetic acid and synthesized different fluorinated alkynes. 5089

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ACS Catalysis Scheme 9. Zhao’s Ag-Catalyzed Decarboxylative C2 Alkylation of Benzothiazolesa

a

Reprinted with permission from ref 18. Copyright 2014, Royal Society of Chemistry, London.

More recently, the group disclosed a decarboxylative trifluoromethylation strategy of aliphatic carboxylic acids employing (bpy)Cu(CF3)3 as the CF3 group transfer reagent in the presence of Ag(I)/ K2S2O8 (Scheme 12).22b In a previous

Scheme 10. Ag-Catalyzed Conversion of Aliphatic Carboxylic Acids to Oxime Ethersa

Scheme 12. Ag-Catalyzed Decarboxylative Trifluoromethylation of Aliphatic Carboxylic Acids, as Reported by Li et al.a

a

Reprinted with permission from ref 22b. Copyright 2017, American Chemical Society, Washington, DC.

report (see section 2.3), they demonstrated the use of this reagent in a reductive trifluoromethylation process. By tuning the reaction condition and adding ZnMe2, they were able to use it in conjunction with the decarboxylative generation of alkyl radicals in the presence of Ag(I)/K2S2O8, thus affording a broad range of trifluoromethylated compounds including complex molecules. They proposed that ZnMe2 converted (bpy)Cu(CF3)3 to the active anion species Cu(CF3)2−, which was subsequently oxidized to Cu(CF3)2 and transferred the CF3 group to alkyl radical to give the desired product. In the examples shown above, alkyl and acyl carboxylic acids were used in various decarboxylative coupling processes. In contrast, aryl carboxylic acids have found limited applications in these reactions. This is primarily due to the slow release of CO2 from the aroyloxy radical, which often leads to inefficient generation of aryl radical species. However, some studies have circumvented the challenges associated with the inefficient generation of aryl radicals and uses thereof. For example, Greaney and co-workers reported a facile decarboxylative generation of aryl radical with AgOAc/K2S2O8 and demonstrated its intramolecular cyclization to access different fluorenone derivatives (Scheme 13).23a It is important to note that other oxidants and metal catalysts proved inferior for this particular reaction. Benzophenone was the side product in this reaction, which was formed through the quenching of aryl radicals in an acetonitrile solvent. Since the reaction was only efficient in acetonitrile, deuterated acetonitrile was chosen in

a

Reprinted with permission from ref 20. Copyright 2016, American Chemical Society, Washington, DC.

via a silver-catalyzed radical addition−cyclization method with K2S2O8.21 Various alkyl groups originated from alkyl carboxylic acids were introduced at C-6 on phenanthridine scaffolds and at C-1 of isoquinolines. In 2016, Li and co-workers disclosed a decarboxylative allylation employing aliphatic carboxylic acids in the presence of Ag(I)/K2S2O8 (see Scheme 11).22a Scheme 11. Ag-Catalyzed Decarboxylative Allylation of Aliphatic Carboxylic Acids, as Reported by Li et al.a

a Reprinted with permission from ref 22a. Copyright 2016, American Chemical Society, Washington, DC.

This method devoids any prefunctionalization of the carboxylic acids, and excellent tolerance of the functional groups could allow it to be used in the late-stage allylation of complex molecules. 5090

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using Minisci’s condition, they developed a novel and operationally simple method for arylation of a wide range of electrondeficient heterocycles at ambient temperature (see Scheme 14).24 Heterocycles used in the study included pyridine, pyrimidine, pyridazine, pyrazine, quinoline, isoquinoline, and phthalazine. They also showed direct arylation of the natural product quinine using the method. In 2013, Patel and Flowers reported a thorough mechanistic study on the transformation reported by the Baran group, which revelead interesting insights (see Scheme 15).25 They proposed a pre-equilibrium step for the formation of a complex between Ag(I) and pyridine nitrogen atom. This complex was oxidized by S2O82− to form a Ag(II)-pyridine complex. This silver(II) species, in turn, oxidized the aryl boronic acid to form the aryl radical. They also mentioned that coordination of either pyridine or water to boronic acid (forming a quaternized boron) might facilitate the radical generation step. Subsequently, aryl radical addition on the protonated heterocycle and oxidation provided the final product.25 Similar to the use of arylboronic acid as precursor to aryl radical, aromatic triazenes were used by Wang and co-workers to serve the same purpose (see Scheme 16).26 Importantly, the author screened various oxidant and catalyst combinations, which all failed for the reaction, except Ag(I)/K2S2O8, which suggests the unique role played by these reagents. Reductive dissociation of aryl triazenes is known to generate aryl radicals in the presence of acid and catalytic metal salts.27a,b Based on these precedents, the author proposed that SO42− ions present in the reaction medium served as the one-electron reductant, whereas the Ag(II) species was involved in other oxidative steps of the transformation. Such a proposal on the Ag(I)/K2S2O8 system playing the roles of both oxidant and reductant was quite unique. In contrast, an oxidative decomposition pathway also could be possible27c and could necessitate additional mechanistic investigations. Other than decarboxylative processes, the generation of alkyl radicals from other precursors and their subsequent uses in

Scheme 13. Ag-Catalyzed Decarboxylative Radical Cyclization Toward Fluorenones, as Reported by Greaney et al.a

a

Reprinted with permission from ref 23a. Copyright 2012, American Chemical Society, Washington, DC.

order to suppress the side reaction. However, such a requirement would limit its practical applications. Capitalizing on the observation that acetonitrile quenched the aryl radical through hydrogen transfer, the group separately disclosed a protodecarboxylation method of benzoic acids in MeCN using Ag(I)/K2S2O8.23b In 2010, Baran and co-workers demonstrated the use of arylboronic acids as surrogate for aryl carboxylic acids for facile generation of aryl radical species, and, subsequently,

Scheme 14. Ag-Catalyzed Arylation of Electron Deficient Heterocycles with Arylboronic Acids, as Reported by Baran et al.a

a

Reprinted with permission from ref 24. Copyright 2010, American Chemical Society, Washington, DC. 5091

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Scheme 15. Proposed Mechanism for Arylation of Electron-Deficient Heterocycles with Ag(I)/K2S2O8, as Reported by Patel and Flowersa

a

Reprinted with permission from ref25. Copyright 2013 American Chemical Society, Washington, DC.

Scheme 16. Wang’s C−H arylation of electron-deficient heteroarenes with triazenes, as Reported by Wang et al.a

a

Reprinted with permission from ref 26. Copyright 2014, Royal Society of Chemistry, London.

Scheme 17. Ag-Catalyzed C−H Functionalization of Quinones with Cyclopropanolsa

a

Reprinted with permission from ref 28. Copyright 2013, American Chemical Society, Washington, DC.

addition−oxidation sequence (see Scheme 17).28 Although the transformation could be achieved with other one-electron oxidants, such as Mn(OAc)3, CAN, or FeSO4, in combination with K 2 S 2 O 8 , the author opted to use a Ag(I)/K 2 S 2 O 8

Minisci-like reactions were also explored with Ag(I)/K2S2O8. IIangovan et al. reported a strategy to generate β-keto radical from cyclopropanols using Ag(I)/K2S2O8 and demonstrated its utility in the functionalization of quinones via radical 5092

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Scheme 18. Silver-Catalyzed C-2 Functionalization of Heteroarenes with Tertiary Cycloalkanols, as Reported by Li et al.a

a

Reprinted with permission from ref 29b. Copyright 2017, Royal Society of Chemistry, London.

Scheme 19. Ag-Catalysed Amidation of Benzoylformic Acids with Tertiary Amines, as Reported by Wang et al.a

a

Reprinted with permission from ref 32. Copyright 2013, Royal Society of Chemistry, London.

Thus, a Ag(I)/Ag(0) catalytic cycle was predicted to be involved where Ag(I) could promote the oxidative cleavage of tertiary amine via an imine intermediate. Other metal catalysts, such as Cu(I), Cu(II), Fe(II), or Fe(III), proved inferior to Ag(I). However, it was difficult to assess why Ag(I)/K2S2O8 was bestsuited for this conversion. In 2014, Wu et al. reported a strategy for two different C−N bond formations in quinoxalines (Scheme 20).33 One C−N bond formation was via direct nitration of tertiary benzylic C−H bond and the other one involved the transformation of C2 methyl into a nitrile. This method required stoichiometric AgNO2 and K2S2O8. Nitration and cyanation were proposed to involve through the formation of NO2 and NO radicals, respectively, in the presence of Ag(I)/K2S2O8. In 2015, Li and co-workers reported an elegant one-step method to synthesize various alkyl azides through silvercatalyzed decarboxylative radical azidation process.34a Here, they used alkyl carboxylic acids and sulfonyl azides with catalytic AgNO3 (20 mol %) and K2S2O8 (Scheme 21). Pyridine-3sulfonyl azide and TsN3 were used as azide sources. The method provided a broad substrate scope for carboxylic acids. Mechanistically, the alkyl radical generated via decarboxylation with Ag(I)/K2S2O8 made a nucleophilic attack to sulfonyl azide and delivered the product. Song and co-workers reported a similar decarboxylative azidation of carboxylic acids, using AgF as a catalyst and PhSO2N3 as an azide source.34b 2.1.3. C−S Bond Formation. Different C−S bond forming reactions were also reported in the literature with stoichiometric silver salts and K2S2O8. Majority of these included trifluoromethylthiolation and sulfonylation reactions. Synthetic methods for trifluoromethylthiolation reaction have received significant attention, because of the use of −SCF3-containing compounds in agrochemistry and pharmaceutical industry. In these reactions, a base or additive is often added that improves the solubility and stability of AgSCF3 and also reduces the redox potential of higher-valency silver species. In some examples shown here, C−S bond was formed concurrently with a C−C or other type of bonds. In 2014, Shen and co-workers reported a silver-catalyzed decarboxylative trifluoromethylthiolation reaction to form the C(sp3)−SCF3 bonds. They used different

combination, because of its shorter reaction time and better selectivity for the monosubstituted product. Bao and Zhu demonstrated a silver-catalyzed ring-expansion method of cyclobutanol in an intramolecular setting toward the preparation of 1-tetralones.29a More recently, a similar work was reported from Li and co-workers, where different β- and γ-keto radical species from tertiary cycloalkanols were generated and reacted with heteroarenes to provide C2-functionalized products (Scheme 18).29b All these transformations shared a common mechanism that involved the addition of a keto-radical species to the aromatic systems, followed by oxidation/rearomatization. Apart from acylation, alkylation, and arylation reactions, fluoroalkylations with CF3SO2Na were reported with Ag(I)/ K2S2O8. Unlike decarboxylation, SO2 extrusion occurred in these cases to generate the radical species. In 2013, Maiti and co-workers developed an oxidative trifluoromethylation strategy of alkenes with CF3SO2Na, catalytic Ag(I), and stoichiometric K2S2O8 to prepare various α-trifluoromethyl substituted ketones.30 In this case, Ag(I) alone could oxidize the CF3SO2Na salts to CF3 radical with extrusion of SO2, whereas K2S2O8 maintained the Ag(I)/Ag(0) catalytic cycle. The source of oxygen atom incorporated in the product was the air which was confirmed by using 18O2 and XPS studies supported the Ag(I)/Ag(0) catalytic cycle. This mechanistic scenario was certainly different from that observed with alkyl carboxylic acids. Employing conjugated N-arylsulfonylated amides, Hu developed a silver-catalyzed cascade fluoroalkylation−aryl migration−SO2 extrusion cascade employing fluorinated sulfinate salts (RfSO2Na).31 2.1.2. C−N Bond Formation. Although K2S2O8 has found multitude applications as oxidant in silver-catalyzed C−C bondforming reactions, its uses in C−N bond-forming reactions are comparatively few. Available reports include amidation, azidation, and nitration reactions. Wang and co-workers reported a silver-catalyzed amidation of benzoylformic acids with tertiary amines, using K2S2O8 (see Scheme 19).32 The reaction proceeded through the cleavage of a C−N bond in tertiary amine and subsequent new C−N bond formation with carboxylic acid leading to α-ketoamides. Unlike other reports wherein generation of acyl radical was postulated via decarboxylation, in this case, however, no decarboxylation was seemingly involved. 5093

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ACS Catalysis Scheme 20. Ag-Mediated Functionalizations of Quinoxalines to Related Nitro and Nitrile Derivativesa

a

Reprinted with permission from ref 33. Copyright 2014, Royal Society of Chemistry, London.

Scheme 21. Ag-Catalyzed Conversion of Aliphatic Carboxylic Acids to Azides via Decarboxylationa

Scheme 22. Silver-Catalyzed Decarboxylative Trifluoromethylthiolation of Aliphatic Carboxylic Acids, as Reported by Shena

a

Reprinted with permission from ref 35. Copyright 2014, Wiley−VCH, Weinheim, Germany.

Scheme 23. Proposed Mechanism for Trifluoromethylthiolation of Carboxylic Acids with Ag(I)/ K2S2O8a

a Reprinted with permission from ref 34a. Copyright 2015, American Chemical Society, Washington, DC.

aliphatic carboxylic acids and an electrophilic trifluoromethylthiolating agent (T), instead of AgSCF3 (see Scheme 22).35 An aqueous emulsion of 20 mol % sodium dodecyl sulfate (SDS) was used as an additive, which minimized the overoxidation of the product. A Ag(II) carboxylate species (A) was first generated by K2S2O8 in water and penetrated into the droplets (Scheme 23). Decarboxylation occurred in the droplet and generated the alkyl radical B, which, in turn, reacted with the reagent T providing the desired product along with a tertiary alkoxy radical.35 The later species was finally decomposed into 1-(2-iodophenyl)-ethanone. Coumarin-3-carboxylic acids and their thio and nitrogen analogues were converted to the corresponding trifluoromethylthiolated

a

Reprinted with permission from ref 35. Copyright 2014, Wiley−VCH, Weinheim, Germany.

products with AgSCF3 and K2S2O8 (Scheme 24).36 Although (NH4)S2O8 gave a slight better yield than K2S2O8 during reaction optimization, the latter was used in the subsequent substrate screening. Some trifluoromethylthiolation reactions were based on the addition of F3CS• radical to unsaturated systems. For example, Cao reported the hydrotrifluoromethylthiolation of terminal 5094

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generated the F3CS• radical, which added onto alkyne to give vinyl radical A. A spiro-radical intermediate B was formed next via the addition of this radical species to the quaternary carbon of aromatic ring. The ring opening of the spiro intermediate was accompanied by a 1,4-aryl migration, affording allyl alkoxy radical species C. Finally, hydrogen atom abstraction by sulfate radical anion (SO4− •) furnished the product (see Scheme 27).

Scheme 24. Ag-Mediated Conversion of Coumarin-3Carboxylic Acids into Related Trifluoromethylthiolated Compoundsa

Scheme 27. Proposed Mechanism for 1,4-Aryl Migration− Trifluoromethylthiolation Cascadea

a

Reprinted with permission from ref 36. Copyright 2017, American Chemical Society, Washington, DC.

alkynes that gave either anti-Markovnikov or Markovnikovselective adducts, depending on the employed reaction condition (see Scheme 25).37 Anti-Markovnikov selective product was Scheme 25. Ag-Mediated Selective Hydrotrifluoromethylthiolation of Terminal Alkynesa

a

Reprinted with permission from ref 38. Copyright 2017, Wiley−VCH, Weinheim, Germany.

Liu recently reported a AgSCF3-mediated trifluoromethylthiolation of α,α-diaryl allylic alcohols that proceeded through radical neophyl rearrangement in the presence of K2S2O8 (see Scheme 28).39 This protocol rendered various α-aryl-βtrifluoromethylthiolated carbonyl compounds. Scheme 28. Trifluoromethylthiolation of α,α-Diaryl Allylic Alcohols with Ag(I)/K2S2O8a

a

Reprinted with permission from ref 37. Copyright 2016, American Chemical Society, Washington, DC.

achieved by employing 1.5 equiv AgSCF3 and 3 equiv K2S2O8, along with 50 mol % HMPA and 10 mol % 1,10-phenanthroline as an additive in DMF. Changing the solvent to dichloroethane and using 10 mol % CuCl, the reaction delivered the Markovnikov-selective product. This reaction was quite specific with the use of K2S2O8 as an oxidant. However, (NH4)S2O8 afforded lower yields, compared with the former. Very recently, a cascade process involving alkyne trifluoromethylthiolation and 1,4-arylmigration from oxygen to carbon was reported by Liu and co-workers. They used various arylpropynyl ethers in the presence of 1.5 equiv AgSCF3, 3 equiv K2S2O8 and 2 equiv K3PO4 as a base (see Scheme 26).38

a

Reprinted with permission from ref 39. Copyright 2017, Royal Society of Chemistry, London.

It has been discussed in the previous section (section 2.1.1) that acrylamides or related structures were suitable for developing radical addition−cyclization cascades. In 2014, Wang and co-workers demonstrated a trifluoromethylthiolation−cyclization process with acrylamide derivatives, using AgSCF3, K2S2O8, and HMPA, that afforded substituted oxindoles.40 K2S2O8 proved to be the best among a series of oxidants, such as PhI(OAc)2, TBHP, NFSI, Ce(SO4)2·4H2O, and (NH4)2S2O8. This could be correlated to the better ability of K2S2O8 in efficiently oxidizing AgSCF3 to the radical species. Similarly, Liu and co-workers used substituted propiolamides to obtain trifluoromethylthiolated azaspirocycles (see Scheme 29a).41a Both K2S2O8 (3.0 equiv) and TBHP (5.0 equiv) were required in this transformation. The addition of SCF3 radical on the conjugated alkyne facilitated the subsequent spirocyclization. Finally, the addition of tert-butylperoxy radical at the paraposition of the phenyl group afforded the quinol structure. Nevado and co-workers reported a cascade radical addition− cyclization process of N-(arylsulfonyl)acrylamides with AgSCF3/ K2S2O8 to access highly functionalized heterocyclic scaffolds.41b Liang and co-workers demonstrated a trifluoromethylthiolation−cyclization cascade on 1,6-enyne systems affording tricyclic scaffolds (see Scheme 29b).42

Scheme 26. Radical 1,4-Aryl Migration− Trifluoromethylthiolation Cascade Reaction from Aryl Propynyl Ethersa

a

Reprinted with permission from ref 38. Copyright 2017, Wiley−VCH, Weinheim, Germany.

The reaction delivered tetrasubstituted α,β-unsaturated aldehydes in good yields. In the mechanistic course, K2S2O8 5095

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Scheme 29. Ag-Mediated Alkyne Trifluoromethylthiolation and Cascade Cyclizations toward SCF3-Incorporated Compoundsa

a

Reprinted with permission from ref 41a. Copyright 2016, American Chemical Society, Washington, DC. Reprinted with permission from ref 42. Copyright 2015, American Chemical Society, Washington, DC.

Strategic use of oxidative ring-opening of cycloalkanols and subsequent trifluoromethylthiolation was reported by Qi and Zhang, using AgSCF3 and K2S2O8 (Scheme 30).43 Other oxidants such

quaternary center. In the same year, this group further reported the synthesis of sulfonylated benzofurans capitalizing on similar silver-catalyzed oxidative cyclization strategy (see Scheme 32).45b

Scheme 30. Ag-Mediated Process for Trifluoromethylthiolation of Cycloalkanolsa

Scheme 32. Ag-Catalyzed Access to Sulfonylated Benzofurans from 1,6-Enynea

a

Reprinted with permission from ref 42. Copyright 2015, American Chemical Society, Washington, DC.

a Reprinted with permission from ref 45b. Copyright 2017, American Chemical Society, Washington, DC.

as TBHP, PhI(OAc)2, BI−OH, and Mn(OAc)3 failed to provide any desired product. Although these oxidants could produce a keto radical from cycloalkanols, K2S2O8 could generate both the radical species (alkyl and SCF3) efficiently, thereby facilitating the reaction. Trifluoromethylthiolation of N-benzyl indoles was achieved with AgSCF3 and a combination of KI/K2S2O8/I2, as disclosed by Li and Wang.44 The reaction proceeded via an electrophilic substitution reaction with the in-situ-generated I-SCF3 species under the reaction condition. Similar to oxidative generation of SCF3 radical from silver salts, sulfonyl radical was generated from sodium arylsulfinates with the aid of oxidants such as Mn(III), Cu(II), or Ag(I)/K2S2O8. For those transformations performed in the presence of Ag(I)/K2S2O8 depicted herein, it is however not clear whether the sulfonyl radical was generated precisely by Ag(II) or K2S2O8. In 2017, Wu and Jiang reported a silvercatalyzed synthesis of sulfonylated lactones using 1,6-enynes with sodium sulfinate in the presence of K2S2O8 (see Scheme 31).45a

Very recently, Lu and Shen developed an electrophilic monofluoromethylthiolating reagent, S-(fluoromethyl)benzenesulfonothioate and showed that this reagent could react smoothly with unactivated alkenes in the presence of catalytic AgNO3 (10 mol %) and K2S2O8 (1 equiv) to deliver sulfonylated monofluoromethylthioethers (see Scheme 33).46 Scheme 33. Monofluoromethylthiolation of Aryl Boronic Acids with S-(fluoromethyl)benzenesulfonothioatea

a

Reprinted with permission from ref 46. Copyright 2017, Wiley−VCH, Weinheim, Germany.

A silver-catalyzed one-pot route to β-ketosulfones via aerobic oxysulfonylation of alkenes with thiophenols was reported by Yadav and co-workers, in the presence of K2S2O8 (see Scheme 34).47

Scheme 31. Ag-Catalyzed Synthesis of Sulfonylated Lactones via Cascade Cyclization of 1,6-Enynesa

Scheme 34. Ag-Catalyzed Oxysulfonylation of Alkenes with Thiophenolsa a

Reprinted with permission from ref 45a. Copyright 2017, American Chemical Society, Washington, DC.

This method gave access to a wide ranges of five-membered sulfonylated lactones with exocyclic double bond and a

a

Reprinted with permission from ref 47. Copyright 2014, Royal Society of Chemistry, London.

5096

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ACS Catalysis Controlled experiments revealed that Ag(I) was responsible for generating the thiyl radical from thiophenol and oxygen incorporation into the carbonyl functional group occurred from the air present in the reaction. Analogous to β-ketosulfide oxidation to corresponding sulfone, Hajra et al. also used a Ag(I)/K2S2O8 combination to transform benzo[b][1,4]thiazines to benzo[b][1,4]thiazines 1,1-dioxides.48 Wang et al. reported a silver-mediated C−S cross coupling reaction using aliphatic carboxylic acids and diphenyl disulfide (see Scheme 35).49

Scheme 37. Proposed Mechanism for Ag-Catalyzed CarboxylRadical-Assisted 1,5-Aryl Migrationa

Scheme 35. Ag-Mediated Decarboxylative C−S CrossCoupling between Aliphatic Carboxylic Acids and Diaryl Disulfidesa

a

Reprinted with permission from ref 49. Copyright 2014, American Chemical Society, Washington, DC.

Scheme 38. General Mechanism for the Generation of Phosphite Radical with Ag(I)/K2S2O8

Jana and co-workers developed a silver-catalyzed carboxyl-radicalassisted Smiles rearrangement of 2-aryloxy or 2-(arylthio)benzoic acid that afforded aryl-2-hydroxybenzoate or aryl-2-marcaptobenzoate, respectively, via a 1,5-aryl migration (see Scheme 36).50 The carboxyl radical (A) was formed in the presence of Ag(I)/ K2S2O8, which then made an intramolecular ipso attack to the arylether (or arylthioether) moiety and facilitated the 1,5-aryl migration via the formation of a spiro radical intermediate B (see Scheme 37). Ring opening (via C) and hydrogen abstraction rendered the product. Slow decarboxylation rate of aryl carboxylic acid allowed significant accumulation of carboxyl radical species, which ultimately facilitated this reaction path. 2.1.4. C−P Bond Formation. Oxidative phosphonylation of aromatics compounds was thoroughly studied in the early 1980s, using anodic oxidation as well as chemical oxidation with Ag(I)/ S2O82−.51 Since Na2S2O8 was used in the study, a higher proportionation of H2O (water/acetonitrile, 5:1) was required. The Ag(I)/S2O82− combination was found to be indispensible for the reaction with diethylphosphite, whereas Fe(II), Cu(I), or combinations such as Ag(I)/Cu(II) or Fe(II)/Cu(II) showed poor efficiency. Dialkyl H-phosphonate (RO)2P(OH) (A) exists in equilibrium with dialkyl phosphite (RO)2P(O)H (A′). In the mechanism, Ag(II) was proposed to oxidize the substrate into a radical cation first, and subsequent deprotonation rendered the P-centered radical (see Scheme 38). The hydrogen abstraction pathway from dialkyl H-phosphonate (A) was proposed to be less likely. Reaction of phosphite radical with different substrates and subsequent oxidation could deliver various products. Since the appearance of these earlier reports on oxidative phosphonylations,

a resurgence of interest to develop this chemistry is being observed in recent years with different strategic uses. In 2012, Huang reported silver(I)-catalyzed coupling of differnet heteroarenes and phosphite in the presence of K2S2O8.52 Zhu and Cheng reported Ag(I)/K2S2O8-mediated ortho-phosphonation of electron-deficient arenes such as N,N-dialkylbenzamides, N,N-dialkylbenzenesulfonamides, and nitrobenzenes.53a In 2015, Huang further reported a C(sp2)−P coupling method to access various para-acylphenylphosphonates (see Scheme 39).53b They employed different aromatic carbonyl compounds and dialkyl H-phosphonates in the reaction. In this case, phosphite radical was added at the para-position of the aromatic system, which was mostly driven by captodative and steric effects. Sun et al. disclosed a C5−H phosphonation of 8-aminoquinoline amides with Ag(I)/K2S2O8.53c This was also an example of para-selective phosphonation, with respect to the

Scheme 36. Ag-Catalyzed 1,5-Aryl Migration through Smiles Rearrangementa

a

Reprinted with permission from ref 50. Copyright 2016, Royal Society of Chemistry, London. 5097

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ACS Catalysis Scheme 39. Dehydrogenative Cross-Coupling between Aromatic Aldehydes and Ketones with Dialkyl H-Phosphonatesa

Scheme 41. Proposed Mechanism for Ag-Catalyzed Phosphorylation of Styrenesa

a Reprinted with permission from ref 53b. Copyright 2015, Royal Society of Chemistry, London.

amide group. In all of these aromatic phosphonation reactions, the Ag(I)/K2S2O8 combination worked most efficiently in the reaction optimization step. Tan and co-workers reported a silver-catalyzed stereoselective synthesis of vinylphosphonates employing styrenes and dialkyl H-phosphonates in the presence of K2S2O8 (see Scheme 40).54 In this case, a slightly different mechanism on the generation of phosphate radical was proposed and a Ag(I)/Ag(0) catalytic cycle was shown instead of Ag(II)/Ag(I). This method required 0.4 equiv of TEMPO as an additive to achieve the desired (E)-vinylphosphonates. Reaction between Ph2P(O)H and silver salt provided Ph2P(O)Ag, which was subsequently converted to phosphate radical and released Ag(0) (see Scheme 41). The addition of the phosphate radical to styrene delivered the desired product and K2S2O8 oxidized Ag(0) to Ag(I) to restore the catalytic cycle. Apart from the above methods that afforded different C(sp2)−P bonds, phosphonation reactions resulting in C(sp3)−P bond formation were also reported in the literature. Chen et al. demonstrated a silver−copper catalyzed one-pot strategy to access β-ketophosphonates via direct oxyphosphorylation of alkynes with H-phosphonates in the presence of oxygen (Scheme 42).55 In this case also, K2S2O8 as oxidant proved superior to other oxidants such as TBHP, H2O2, DTBP, and mCPBA. This reaction was proposed to proceed with a phosphite radical, which generated a vinyl radical A after addition to the alkyne. A ligated Cu(III)-oxygen complex next combined with this vinyl radical to afford complex B, and subsequently provided hydroperoxide C. Following a O−O bond cleavage, the resulting alkoxyl radical reacted with diethyl H-phosphonate to regenerate phosphonate

a

Reprinted with permission from ref 54. Copyright 2015, Royal Society of Chemistry, London.

Scheme 42. Ag−Cu-Catalyzed Oxyphosphorylation of Alkynes with H-Phosphonatesa

a

Reprinted with permission from ref 55. Copyright 2015, Royal Society of Chemistry, London.

radical and then, itself, was reduced to β-ketophosphonate product (Scheme 43). Another silver-catalyzed reaction between 1,3-dicarbonyl compounds and H-phosphonates was reported by Dong and co-workers (see Scheme 44).56 In this case, reaction proceeded with the phosphite radical addition to the enol form of dicarbonyls. Li and co-workers disclosed a straightforward approach to synthesize different phosphonate chroman-4-ones with Ag(I)/ K2S2O8 (see Scheme 45).57a After several controlled experiments, the author proposed the formation of acyl radical first through hydrogen abstraction from aldehyde, which upon cyclization on the alkene formed a primary radical and subsequently trapped by phosphonyl radical generated under the reaction condition. Many reports have been deposited in the literature, which discuss radical−radical cross couplings reactions yielding good to high yields of the products.57b−f These reports

Scheme 40. Ag-Catalyzed Phosphorylation of Styrenes toward Vinylphosphonate Synthesisa

a

Reprinted with permission from ref 54. Copyright 2015, Royal Society of Chemistry, London. 5098

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ACS Catalysis Scheme 43. Proposed Mechanism for Ag−Cu-Catalyzed Synthesis of β-Ketophosphonatesa

Scheme 45. Cascade Radical Cyclization toward the Synthesis of Chroman-4-ones with Ag(I)/K2S2O8a

a

Reprinted with permission from ref 57a. Copyright 2016, Royal Society of Chemistry, London.

etc. Since many of these oxidants are expensive and toxic, there has been a growing interest to find a less-expensive and environmentally benign alternative. In this context, molecular oxygen perhaps would be the best choice. However, this oxidant is most suited for the reactions that proceed via the Pd(II)/Pd(0) catalytic cycle. Peroxydisulfates, on the other hand, could also meet these criteria and both of the palladium catalytic cycles, Pd(II)/Pd(0) and Pd(IV)/Pd(II) have been postulated to exist with this oxidant. This oxidant exhibited unique efficacy in various palladium-catalyzed transformations and often are found to be superior to other oxidants. However, its precise role in the mechanism remained illusive in many reported transformations and therefore demands more-detailed investigations. For example, it is not clear whether S2O82−, SO4• −, or both are responsible for the oxidation of palladium. Some general considerations are are given as follows: (a) For decarboxylative acylation reactions, where K2S2O8 was used as the sole oxidant, controlled experiments supported a Pd(IV)/Pd(II) catalytic cycle. The reaction course could differ in cases when K2S2O8 was used as a co-oxidant with Ag(I). (b) Some acylation reactions with K2S2O8 proceeded through the formation of radical species and involved either Pd(III) or Pd(IV) intermediates.60 (c) Carbonylation reactions mostly proceeded with Pd(II)/ Pd(0) catalytic cycle. (d) Most arylation reactions proceeded with a Pd(IV)/Pd(II) catalytic cycle, while alkenylation processes involved Pd(II)/Pd(0). (e) The C−O, C−N, and C−P bond-forming reactions with Pd(II)/K2S2O8 involved higher-valency palladium species. This section of the review is not organized chronogically but presented on the basis of the nature of transformation and bonds being formed, viz directing-group-assisted decarboxylative acylation, arylation, alkenylation, carbonylation reactions, and other cross-coupling reactions. 2.2.1. C−C Bond Formation. In 2010, Ge and co-workers demonstrated the use of K2S2O8 as co-oxidant with Ag(I) salt in a directing-group-assisted palladium-catalyzed ortho-acylation of 2-phenylpyridines with different α-oxocarboxylic acids (see Scheme 46).61

a

Reprinted with permission from ref 55. Copyright 2015, Royal Society of Chemistry, London.

Scheme 44. Ag-Catalyzed Oxidative C(sp3)-P Bond Formation between 1,3-Dicarbonyls and H-Phosphonatesa

a

Reprinted with permission from ref 56. Copyright 2017, Wiley−VCH, Weinheim, Germany.

could collectively support the coupling of two radicals proposed in the mechanism. 2.2. Palladium-Catalyzed Transformations. Since the development of the Wacker process, the use of various oxidants for the maintenance of Pd(0)−Pd(II) catalytic cycle has become a practice.58 It is apparent from the literature that the efficacy of a reaction could be seriously affected, depending on the compatibility of a particular Pd(II) catalyst to the oxidant used in the mixture.1b Following the use of CuCl2/O2, benzoquinone, or tert-butyl hydroperoxide as an oxidant in palladium-catalyzed transformations, the use of K2S2O8 in palladium-catalyzed reactions could be traced back to the early 1980s by Fujiwara and co-workers.59 A large number of reports are now available that preferentially used K2S2O8 as the oxidant. These reactions include various C−C, C−O, C−N, C−S, and C−P bondforming reactions. In the majority of reactions discussed herein, product formation occurred through reductive elimination path, which was associated with a significant redox change at the Pd center. In these examples, reductive elimination of palladium from a PdII(Ar)(Ar′) or PdII(Ar)(Nu) afforded Pd0. On the other hand, in cases where the Pd-center was not sufficiently oxidizable, reductive elimination either became slower or thermodynamically unfavorable. Such reactions required oxidation of the Pd(II) center to a higher oxidation state, i.e., Pd(III) or Pd(IV), to facilitate the reductive elimination step. Selected oxidants could efficiently promote such process that included PhI(OAc)2, diaryliodonium salts, electrophilic fluorine reagents, transition-metal-based oxidants,

Scheme 46. Pd-Catalyzed Decarboxylative Acylation of 2-Arylpyridines with α-Oxocarboxylic Acids, as Reported by Ge et al.a

a

Reprinted with permission from ref 61. Copyright 2010, American Chemical Society, Washington, DC.

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earlier report on acylation of azobenzenes using aldehydes as the acylating agents.64c The azo group participated as the directing group in this reaction. Analogous ortho-acylations were reported with N-nitrosoanilines65a and aryl(β-carbolin-1-yl) methanones.65b For all these transformations, the mechanism involved a Pd(IV)/Pd(II) catalytic cycle. First, a nitrogen-coordinated five- or six-membered palladacycle with Pd(II) was formed through ortho C−H cleavage, which then underwent ligand exchange with α-keto acid. Next, decarboxylation and oxidation by K2S2O8 converted this to a Pd(IV) species. Finally, reductive elimination afforded the acylated product. Similar to the above C(sp2)−H functionalization/ acylation of aryl systems, palladium-catalyzed alkene C(sp2)−H acylation was reported by Duan and co-workers (Scheme 49).66 In this report, unactivated C−H bond of enamides were acylated with 10 mol % Pd(OAc)2, 1 equiv K2S2O8, and 2 equiv of Ag2O. In this reaction, K2S2O8 was superior to other persulfates and oxone used during reaction optimization. A six-membered palladacycle formation with amide oxygen was proposed, and the reaction proceeded with a Pd(II)/Pd(0) cycle. Importantly, attempted decarboxylative acylation on tetralone-derived enamide was unsuccessful. This indicated that the presence of an oxygen atom in the cyclic enamide was crucial for efficiently promoting the decarboxylative acylation step.66 A palladium-catalyzed ortho-acylation method for 2-(tertbutyl(phenyl)-phosphoryl)biphenyls was developed by Yang and co-workers (see Scheme 50).67 The R2(O)P group acted as a directing group and participated in forming a seven-membered palladacycle. Based on the precedents, the reaction was likely to proceed via a Pd(IV)/Pd(II) catalytic cycle and K2S2O8 resulted in better conversion, compared to (NH4)2S2O8, oxone, and Ag(I).

In 2013, Zhu and co-workers reported a Pd(II)-catalyzed method for C2-acylation of 1-(pyrimidin-2-yl)-1H-indole with arylglyoxylic acids in the presence of Ag2O and K2S2O8 (see Scheme 47).62 In this case, K2S2O8 also was used as the Scheme 47. Zhu’s Pd-Catalyzed Decarboxylative C-2-Acylation of Indoles, as Reported by Zhu et al.a

a

Reprinted with permission from ref 62. Copyright 2013, Royal Society of Chemistry, London.

co-oxidant. The pyrimidine moiety assisted as a directing group in this reaction, which could be removed afterward using NaOEt. Zhang developed a Pd-catalyzed decarboxylative acylation of 2-aryloxypyridines with α-oxo carboxylic acids, using a combination of Ag2O and K2S2O8.63 In this report, mechanistic investigations revealed that Ag(I) was crucial as an oxidant and K2S2O8 had some secondary roles in the transformation, which were not clear. For all these reports, further investigations are required to elucidate the role of K2S2O8 as a co-oxidant. Ortho-acylation of symmetrical and unsymmetrical azobenzenes with α-oxocarboxylic acids in the presence of Pd(II) catalyst and K2S2O8 as an oxidant was reported by Wang (see Scheme 48).64a,b K2S2O8 provided the best yield, compared to other oxidants such as Na2S2O8, (NH4)2S2O8, oxone, Cu(OAc)2, BQ , TBHP, and PhI(OAc)2. This method complements an

Scheme 48. Pd-Catalyzed Synthesis of ortho-Acylated Azobenzenes with α-Oxocarboxylic Acidsa

a

Reprinted with permission from refs 64a (Copyright 2013 American Chemical Society, Washington, DC) and 64b (Copyright 2013, Wiley−VCH, Weinheim, Germany). 5100

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ACS Catalysis Scheme 49. Pd-Catalyzed Decarboxylative Acylation of Cyclic Enamidesa

a

Reprinted with permission from ref 66. Copyright 2012, American Chemical Society, Washington, DC.

Scheme 50. Pd-Catalyzed C(sp2)−H Acylation of 2-Phosphorylbiphenyls with α-Oxocarboxylic Acids, as Reported by Yang et al.a

a

Reprinted with permission from ref 67. Copyright 2014, Royal Society of Chemistry, London.

of pyridine N-oxides with benzyl alcohol or benzyl chloride as acylating agents with palladium(II)/K2S2O8. Acylating agents used herein were oxidized into an acyl radical species by sulfate radical ion and the reaction involved either Pd(III) or Pd(IV) intermediates.68b A direct ortho C−H acylation of aromatic ketones using α-oxocarboxylic acids was described by Yu and co-workers (Scheme 52).69 This method obviated any prederivatization of

In 2016, Cui and co-workers disclosed an efficient C8 selective acylation method of quinoline N-oxide via a palladium(II)catalyzed process using α-oxocarboxylic acids in the presence of K2S2O8 (see Scheme 51).68a A five-membered palladacycle Scheme 51. Pd-Catalyzed C8-Selective Acylation of Quinoline N-Oxidesa

Scheme 52. Direct Pd-Catalyzed Decarboxylative Acylation of Aromatic Ketonesa a Reprinted with permission from ref 68a. Copyright 2016, American Chemical Society, Washington, DC.

intermediate was proposed to form via coordination of palladium to oxygen of N-oxide and subsequent electrophilic attack at C8. Ligand exchange with α-oxocarboxylic acid, decarboxylation, and reductive elimination from Pd(II) center delivered the final product and released Pd(0), which was oxidized by K2S2O8. While other oxidants like TBHP, DTBP, DDQ, BQ, O2, Ag(I) salts failed to give any significant conversion, only K2S2O8 was found to deliver the product. Any reasonable explanation for such distinct reactivity of K2S2O8 was not provided. Unlike a Pd(II)/ Pd(IV) catalytic cycle, a Pd(II)/Pd(0) catalytic cycle was proposed in the mechanism. Kianmehr et al. performed ortho-acylation

a

Reprinted with permission from ref 69. Copyright 2017, American Chemical Society, Washington, DC.

ketone to a N-directing group. Based on controlled experiments, a radical mechanism was proposed for this transformation. Ge and co-workers reported cross-coupling reactions of potassium aryltrifluoroborates with α-oxocarboxylic acid or oxamic acid or potassium oxalate monoester in the presence of Pd(II)/K2S2O8 system (see Scheme 53).70a−c These methods allowed smooth access to different aryl ketones, aryl amides, and 5101

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ACS Catalysis Scheme 53. Ge’s Decarboxylative Cross-Coupling of Potassium Aryltrifluoroborates and α-oxocarboxylic Acids with Pd(II)/K2S2O8, as Reported by Ge and Co-workersa

a

Reprinted with permission from refs 70a (Copyright 2011, American Chemical Society, Washington, DC), 70b (Copyright 2011, Royal Society of Chemistry, London), and 70c (Copyright 2016, Royal Society of Chemistry, London).

esters. Although (NH4)2S2O8 was also found to be efficient during optimization, the authors however used K2S2O8 in the subsequent studies for all the cases. These reactions proceeded thorugh the formation of an acyl radical, as confirmed by experiment with TEMPO. Since the reaction was performed at room temperature, S2O82− was more likely to be the actual oxidant. Palladium-catalyzed carboxylation of arenes with CO through direct aryl C−H functionalization with K2S2O8 as an oxidant was demonstrated by Fujiwara in 1995.71 Wang and co-workers reported a decarboxylative ortho-ethoxycarbonylation method through directing-group-assisted aryl ortho C−H functionalization using potassium oxalate monoester in the presence of

palladium(II) catalyst and K2S2O8 (see Scheme 54).72 Different substituted aryl O-methyl ketoximes and 2-arylpyridines were used in this study. The reaction was optimized with 10 mol % Pd(OAc)2, 2 equiv of K2S2O8, 2 equiv of Ag2CO3 with 0.5 equiv of D-CSA as additive. Na2S2O8 and (NH4)2S2O8, both afforded lower conversion, which could be linked to their poor solubility in DCE. A radical pathway was speculated as TEMPO inhibited the reaction. This radical process was proposed to involve either a Pd(III) or Pd(IV) intermediate and reductive elimination furnished the desired product (see Scheme 55). In the same vein, Wu and Luo reported an ortho sp2 C−H carboxylation of acetanilides with Pd(II)/K2S2O8 to synthesize anthranilic acid derivatives (Scheme 56).73 The reaction used

Scheme 54. Pd-Catalyzed Decarboxylative orthoEthoxycarbonylation Reactiona

Scheme 56. Pd-catalyzed Decarboxylative Cross Coupling of Acetanilides and N,N-Dimethyloxamic Acidsa

a

Reprinted with permission from ref 73. Copyright 2016, Royal Society of Chemistry, London.

N,N-dimethyloxamic acid, which selectively delivered the carboxylated product out of other possibilities. Thus, formation of a HOOC• radical species could be speculated to form in the presence of K2S2O8, which afforded the product and the residual

a

Reprinted with permission from ref 72. Copyright 2015, American Chemical Society, Washington, DC.

Scheme 55. Proposed Mechanism for Pd-Catalyzed ortho-Ethoxycarbonylationa

a

Reprinted with permission from ref 72. Copyright 2015, American Chemical Society, Washington, DC. 5102

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ACS Catalysis portion of N,N-dimethyloxamic acid was converted to DMF, as was observed by GC-MS. Some carbonylation reactions were reported with the palladium(II)/K2S2O8 combination. In 2014, Zhu reported a pyridocarbonylation of N-aryl-2-aminopyridines using CO gas (see Scheme 57).74a This reaction closely resembled the

developed by Lei and co-workers, using K2S2O8 as a terminal oxidant.75 Although this reaction could be performed with other oxidants such as MnO2 or BQ, the author used K2S2O8 in the subsequent screening to prepare various xanthones. All these carbonylation reactions with Pd(II)/K2S2O8 proceeded with the Pd(II)/Pd(0) catalytic cycle. Nozaki and co-workers reported an unexpected finding on simultaneous hydroxylation−carboxylation of biphenyls with formic acid in the presence of Pd(OCOCF3)2 and K2S2O8, affording 4′-hydroxy-4-biphenylcarboxylic acid (see Scheme 58).76 Two distinct C−H activation occurred on the same molecule under this reaction condition. This protocol, besides providing sequential hydroxylation−carboxylation of biphenyls, also proved to be efficient for the carbonylation of aromatic C−H bonds. Electron-rich arenes showed higher ortho−para selectivity, which indicated that the reaction most likely proceeded via electrophilic substitution. Apart from palladium-catalyzed acylations and carboxylations, arylation reactions were also reported with K2S2O8. In 2006, Lu reported intermolecular cross-coupling of simple arenes with Pd(II)/K2S2O8.77 Later, they disclosed homocoupling reactions of p-xylenes to afford biaryls or diarylmethanes. These reactions involved a Pd(II)/Pd(0) catalytic cycle. Some arylation reactions were reported where strong coordination from a directing group facilitated the regioselectivity. In 2012, palladium-catalyzed guanidine-directed arylation was first reported by Yu and co-workers (see Scheme 59).78 Use of BQ, Ag2CO3, Ag2O, Cu(OAc)2, or air gave futile results, compared to K2S2O8. In the mechanism, directing-group-assisted palladacycle formation was proposed with Pd(II), which was oxidized to a hexa-coordinated Pd(IV) by S2O82− and subsequently arene insertion occurred via ligand displacement. Finally, reductive elimination delivered the desired product. Similar ortho-arylation of tertiary benzamides was reported by Guan and co-workers.79 Notably, this method showed high paraselectivity for the arenes that were incorporated on benzamides. In situ formation of cationic Pd(OTf)2 was proposed for this transformation. The role of K2S2O8 in these reactions was to generate the Pd(IV) species to facilitate the reductive elimination step to product. In 2013, Kuang developed a room-temperature protocol for C5 alkenylation of pyrazoles in the presence of Pd catalyst and K2S2O8.80a The group further reported a palladium-catalyzed orthoalkenylation strategy of 2-benzyl-1,2,3-triazoles (see Scheme 60).80b

Scheme 57. Pyridocarbonylation of N-Aryl-2-aminopyridines with Palladium Catalysta

a Reprinted with permission from ref 74a. Copyright 2014, American Chemical Society, Washington, DC.

directing-group-assisted ortho-acylation of arenes. In this case, the pyridyl moiety acted as both a directing group and an internal nucleophile. Other persulfates and Cu(II) salts proved inferior to K2S2O8 for this reaction. Importantly, under such oxidative condition, the formation of urea byproduct involving two aniline molecules and CO gas could be possible, which, however, practically did not form in the presence of K2S2O8. This represented the selectivity and suitability of K2S2O8 over other oxidants. Using the same substrate, Wu and co-workers reported pyridocarbonylation using DMF as the carbonyl source in the presence of palladium(II)/K2S2O8.74b Similar to pyridocarbonylation, a palladium-catalyzed double C−H activation/carbonylation of diarylethers with CO gas was

Scheme 58. Nozaki’s Pd(II)-Catalyzed Sequential Hydroxylation−Carboxylation of Biphenyls, as Reported by Nozaki and Co-workersa

a

Reprinted with permission from ref 76. Copyright 2004, American Chemical Society, Washington, DC.

Scheme 59. Pd-Catalyzed C−H Arylation of Arenes Using Guanidine as a Directing Groupa

a

Reprinted with permission from ref 78. Copyright 2012, American Chemical Society, Washington, DC. 5103

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ACS Catalysis Scheme 60. Pd-Catalyzed ortho-Functionalization of 2-Benzyl-1,2,3-triazoles with Electron-Deficient Alkenesa

a

Reprinted with permission from ref 80b. Copyright 2015, Royal Society of Chemistry, London.

Scheme 61. Pd-Catalyzed Dehydrogenative Homocoupling/ortho-Hydroxylation of N-Phenylpyrazolesa

a

Reprinted with permission from ref 82. Copyright 2015, American Chemical Society, Washington, DC.

Interestingly, 2-benzyl pyrazole proved to be an incompetent substrate for this method. O’Shea reported an oxidative alkenylation method of arenes with Pd(II)/K2S2O8.81 Trans1,2-disubstituted alkenes were coupled to arenes in this reaction affording various trisubstituted alkenes. These alkenylation reactions afforded products through β-hydride elimination step and mostly proceeded through Pd(II)/Pd(0) catalytic cycle. Other oxidants, e.g., Ag(I), O2, BQ, MnO2 showed moderate efficiency, whereas K2S2O8 turned out to be the most efficient for desired product formation. Batra and co-workers employed N-arylpyrazoles for palladium-catalyzed oxidative dehydrogenative homocoupling and ortho-hydroxylation in the presence of K2S2O8 (see Scheme 61).82 Variation of substituents on N-aryl group or on the pyrazole ring and changes in substrate concentration afforded variying yields in the formation of two products. For this reaction, acyloxylation with trifluoroacetic acid, followed by hydrolysis, rendered the hydroxylated compound. Rao and co-workers reported a convenient access to 2,2′difunctional biaryl systems (homo and heterocoupled products) via Pd(II)-catalyzed dehydrogenative cross-coupling reaction

with the substrates having weak coordination sites, such as ester, carbamate, ketone, etc. (see Scheme 62).83 A combination of Scheme 62. Pd-Catalyzed Regioselective Oxidative Coupling for 2,2′-Difunctional Biaryl Synthesisa

a

Reprinted with permission from ref 83. Copyright 2015, American Chemical Society, Washington, DC.

oxidants, i.e., K2S2O8 (0.5 equiv) and NaIO4 (0.6 equiv), were found to be most efficient whereas the use of oxone, PhI(OAc)2, etc. was futile. The requirements for this combined oxidant in the mechanism of this transformation were not clear. 5104

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ACS Catalysis Scheme 63. Pd-Catalyzed meta-Arylation of O-β-Naphthyl Carbamatesa

a

Reprinted with permission from ref 84. Copyright 2015, Royal Society of Chemistry, London.

a simple Pd(II)/TFA/oxidant system, where either molecular oxygen or K2S2O8 could be used as an oxidant.85 The crucial factor that controlled the product disctribution of homo and hetero biaryl was the concentration of arene in the reaction and loading of TFA. K2S2O8 served as the better oxidant than oxygen for the reaction of trifluoromethyl benzene or nitrobenzene with ethyl benzoate. This reaction afforded 2,3′-biaryls with exclusive ortho-selectivity, with respect to the benzoate group, and metaselectivity, with respect to the nitro or trifluoromethyl group (Scheme 65).

Since individual use of NaIO4 in the reaction was not as effective as it was with K2S2O8, the latter most likely served as the terminal oxidant for the Pd(II)/Pd(IV) catalytic cycle. Zhang and co-workers first reported a palladium(II)-catalyzed directed meta-arylation of O-β-naphthyl carbamate with aryl boronic acids at room temperature (see Scheme 63).84 In order to favor the arylation to occur at meta over ortho-position, reaction was performed under acidic conditions using TFA:AcOH (2:1, v/v). Importantly, the carbamate moiety was only effective for this reaction, while the hydroxyl, ether, and acetate groups did not provide any product. This reaction, unlike other arylation processes with K2S2O8, involved a Pd(II)/Pd(0) catalytic cycle as the final product formation was speculated to occur through β-hydride elimination (see Scheme 64). However,

Scheme 65. Pd(II)-Catalyzed Cross-Coupling of Ethyl Benzoate with Electron-Deficient Arenesa

Scheme 64. Proposed Mechanism for meta-Selective Arylationa

a

Reprinted with permission from ref 85. Copyright 2014, American Chemical Society, Washington, DC.

A palladium-catalyzed cross-dehydrogenative coupling (CDC) method between 1,3,5-trialkoxybenzenes and simple arenes was developed by Greaney and co-workers (Scheme 66).86 It is Scheme 66. Pd-Catalyzed CDC Reaction of 1,3,5-Trialkoxybenzenes with Different Arenesa

a

Reprinted with permission from ref 86. Copyright 2014, Royal Society of Chemistry, London.

important to note that electron-rich aromatics used in the reaction did not undergo any oxidative side reactions in the presence of K2S2O8; rather, the desired SEAr path was only followed in the presence of Pd(II). Overall, this method offered a means to prepare multi-ortho-substituted biaryls. Similar to the synthesis of biaryls via C−H activation, analogous methods for heterobiaryls have appeared in the literature with Pd(II)/K2S2O8. Wang reported C−H arylation of azoles with different secondary aryl amides or their surrogates e.g., ArCONHOR, ArCONHNH2 as arylating agents (see Scheme 67).87 This reaction proceeded through a deaminative−decarbonylation sequence. K2S2O8 worked most efficiently among (NH4)2S2O8, Ag(I), DDQ , organic peroxides, PhI(OAc)2, and Cu(OAc)2. Based on the ratio of CO/CO2 released in the reaction as monitored by infrared (IR)

a

Reprinted with permission from ref 84. Copyright 2015, Royal Society of Chemistry, London.

the mechanism did not support directing-group-assisted metaarylation unequivocally. The development of a catalytic method to synthesize unsymmetrical biaryls via C−H activation has been receiving continuous attention in organic synthesis. In particular, arenes armed with a directing group are suitably poised for such transformations. However, the use of simple or deactivated arenes offer more challenges, in terms of reactivity and controlling the selectivity. In 2012, Lu and co-workers reported 5105

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ACS Catalysis Scheme 67. Pd-Catalyzed Arylation of Azoles via Deamidation of Arylamidesa

a

Reprinted with permission from ref 87. Copyright 2012, Royal Society of Chemistry, London.

futile. In the same year, another CDC reaction was disclosed by the same group for the construction of C−C bond between C(sp2)−H and C(sp3)−H of pyridine N-oxides and toluene, respectively (see Scheme 70).89b When 2-ethylpyridine N-oxide

spectroscopy, path B in the mechanism was claimed to be more likely over path A (see Scheme 68). Scheme 68. Catalytic Cycle for the Deamidative Arylation of Azolesa

Scheme 70. Pd-Catalyzed Regioselective Benzylation and Annulation of Pyridine N-Oxidesa

a Reprinted with permission from ref 89b. Copyright 2015, American Chemical Society, Washington, DC.

was used in this study, cyclic azafluorene N-oxides were obtained through the activation of four C−H bonds. This reaction was performed with 3 mol % Pd(OAc)2 and 0.75 equiv of K2S2O8. Na2S2O8 or (NH4)2S2O8 could also be used in this reaction albeit in lower yields. Formation of benzylic radical species was proposed herein, which possibly involved either Pd(III) or Pd(IV) species in the catalytic cycle (see Scheme 71). Liu et al. disclosed a palladium-catalyzed modular synthesis of dihydro-isoquinolines via a sequential sp2 C−H and sp3 C−H bond activation/annulations process (see Scheme 72).90 Different benzamides and tertiary alcohols were used to achieve the transformation. A radical pathway was proposed for the reaction. Wang and co-workers first introduced palladium-catalyzed heteroannulation strategies of [60]fullerene with different annulation partners via C−H activation.91 During the optimization of these reactions, K2S2O8 was found to be the most efficient among

a

Reprinted with permission from ref 87. Copyright 2012, Royal Society of Chemistry, London.

Wang and co-workers developed a method for heterobiaryl synthesis by direct C2 arylation of indoles with aryl boronic acids (Scheme 69).88 In this method, they used a recyclable Fe3O4 magnetic nanoparticles (MNP) immobilized Pd(II) catalyst and K2S2O8 as the terminal oxidant. The reaction required 2 mol % loading of the catalyst and stoichiometric K2S2O8. A regioselective CDC reaction involving C5−H bond of uracil and C2−H of benzofuran was disclosed by Kianmehr et al.89a In a previous report,42 the C5−H of uracil was selectively functionalized with palladium catalyst in the presence of a Ag(I) oxidant. The current report demonstrated the possibility for using K2S2O8 as an inexpensive alternative, while using BQ, O2, or Cu(II) was

Scheme 69. Use of MNP Immobilized Pd-Catalyst for Direct C-2 Arylation of Indoles with Arylboronic Acidsa

a

Reprinted with permission from ref 88. Copyright 2014, Royal Society of Chemistry, London. 5106

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ACS Catalysis Scheme 71. Catalytic Cycle for Pd-Catalyzed Benzylation/Annulation of Pyridine N-Oxidea

a

Reprinted with permission from ref 89b. Copyright 2015, American Chemical Society, Washington, DC.

Scheme 72. Pd-Catalyzed Synthesis of Dihydro-isoquinolines with Sequential C(sp2)−H and C(sp3)−H Bond Activationa

a

Reprinted with permission from ref 90. Copyright 2015, Royal Society of Chemistry, London.

Scheme 73. Wang’s Pd-catalyzed annulation of [60]fullerene with anilides, as Reported by Zhu and Wanga

a

Reprinted with permission from ref 91a. Copyright 2009, American Chemical Society, Washington, DC.

acetanilides (see Scheme 74). A five-membered palladacycle intermediate (A) was formed first via amide-directed Pd(II) insertion into the ortho C−H bond. Next, the C60 inserted onto arylpalladium bond (intermediate B) and reductive elimination from this complex completed the annulation with C−N bond

other oxidants, such as Cu(OAc)2, BQ, oxone, AgOAc, NFSI, PhI(OTf)2, etc. In 2009, they first reported the reaction of C60 with anilides to form [60]fulleroindolines (see Scheme 73) with 10 mol % Pd(OAc)2 and 5 equiv of K2S2O8.91a The proposed mechanism was analogues to other reported ortho C−H functionalization of 5107

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ACS Catalysis

compounds containing tetrahydroisoquinoline,91b sultone ring,91c tetrahydrobenzooxepine,91d isochroman,91d and benzofuran.91e In these transformations, N-benzylsulfonamides, arylsulfonic acids, phenylethyl alcohols, benzyl alcohols, and phenols were used as reaction partner with [60]fullerene. Synthesis of highly substituted naphthalenes by cycloaromatization of alkynes with arenes through C−H activation has remained underdeveloped. Wu et al. developed an efficient strategy to synthesize 5,6,7,8-tetraaryl-N-acetyl-1-aminonaphthalenes with palladium(II) catalyst and K2S2O8 starting from N-acetyl anilines and substituted alkynes (see Scheme 75).92 This reaction efficiently proceeded with electron-deficient diaryl acetylenes and electron-rich acetanilides. In 2015, Laha and co-workers reported a palladium-catalyzed tandem one-pot approach to access carbazoles (monosubstituted, disubstituted, and trisubstituted), as well as α-carbolines from readily available indoles or 7-azaindoles and alkenes with K2S2O8 (see Scheme 76).93 This reaction required the use of 50 mol % AgOAc as a co-oxidant for better results. This palladiumcatalyzed tandem reaction followed the sequence regioselective C3 alkenylation−C2 alkenylation−thermal electrocyclization. 2.2.2. C−O Bond Formation. Different C−O bond forming reactions were developed with a palladium catalyst and K2S2O8 that proceeded with or without the assistance of a directing group. This part of the review lists such C−O bond-forming

Scheme 74. Proposed Mechanism for Heteroannulation of [60] Fullerenea

a Reprinted with permission from ref 91a. Copyright 2009, American Chemical Society, Washington, DC.

formation. The Pd(0) was oxidized back to Pd(II) species by K2S2O8 and completed the catalytic cycle. In the subsequent studies, they expanded the scope of [60]fullerene functionalization and synthesized different C60-fused

Scheme 75. Pd-Catalyzed Synthesis of Substituted Naphthalenes from Amides and Alkynesa

a

Reprinted with permission from ref 92. Copyright 2010, Royal Society of Chemistry, London.

Scheme 76. Pd-Catalyzed Tandem Approach to Transform Indoles to Functionalized Carbazolesa

a

Reprinted with permission from ref 93. Copyright 2015, American Chemical Society, Washington, DC. 5108

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ACS Catalysis Scheme 77. Wang’s Pd-Catalyzed ortho-Acetoxylation and Alkoxylation of Anilidesa

a

Reprinted with permission from refs 94a (Copyright 2008, American Chemical Society, Washington, DC), 94b (Copyright 2010, American Chemical Society, Washington, DC), and 94c (Copyright 2012, American Chemical Society, Washington, DC).

Scheme 78. Ru(II) or Pd(II)-Catalyzed Regioselective Hydroxylation of Benzanilidesa

a

Reprinted with permission from ref 97. Copyright 2016, Royal Society of Chemistry, London.

reactions including acetoxylation, aroyloxylation, alkoxylation, and hydroxylation. Wang et al. reported ortho-acetoxylation/ alkoxylation of anilides and benzamides via oxidative C(sp2)−H functionalization (Scheme 77).94a−c The choice of K2S2O8 as an oxidant was crucial for these transformations, as PhI(OAc)2, oxone, mCPBA, TBHP, or O2 afforded poor conversion. The final step of C−O bond formation resulted from the reductive elimation step in the catalytic cycle and this favorably occurred from a higher-valency palladium(IV) species. K2S2O8 most likely played a crucial role in maintaining the Pd(II)−Pd(IV) catalytic cycle and afforded better conversion, compared to other oxidants. On the basis of Wang’s alkoxylation method,94b Sunoj and co-workers disclosed density functional theory (DFT) calculations on this transformation.95 This study revealed interesting mechanistic facts on Wang’s transformation, such as involvement of concerted metalation deprotonation (CMD) pathway. In addition to that, other interesting details on methanol acting as solvent as well as the alkoxylating agent and an acetate-assisted C−H bond activation were also conveyed.95 However, their calculations were based on a Pd(II)/Pd(0) catalytic cycle, which was in stark contrast to the proposed mechanism of the earlier reports on such transformation. In 2012, Rao and Dong independently reported sp2 C−H hydroxylation of aromatic ketones using a Pd(II) catalyst, where the ketone moiety served as the directing group.96 Several oxidants, such as NFSI, selectfluor, PhI(OAc)2, PhI(CF3CO2)2, or K2S2O8 could be used in this transformation, as indicated in the optimization step of the reaction. However, the authors used K2S2O8 for the subsequent development and substrate scope investigation. It was confirmed that TFA/TFAA played important roles in these C−H activation reactions and also served as the source of hydroxyl group. Thus, ring hydroxylation occurred through trifluoroacetoxylation, followed by its hydrolysis. In 2016, Zhang and co-workers disclosed a complementary strategy for regioselective sp2 C−H hydroxylation (via trifluoroacetoxylation) using either 5% [Ru(pcymene)Cl2]2 or 10% Pd(OAc)2 catalyst (see Scheme 78), where K2S2O8 served as an oxidant in both cases.97 In this study, the ortho-hydroxylation of acylated anilines with Pd(II) was analogous to the previous report of Wang’s discussed earlier.94a

On the other hand, when Ru(II) was used, the hydroxyl group was incorporated at the ortho position to the carbonyl group. Two different C−O bond formations by two catalyst systems could be explained on the basis of two different ring-metalation processes (see Scheme 79).97 Scheme 79. Different Metallocycles for Regioselective Hydroxylationa

a

Reprinted with permission from ref 97. Copyright 2016, Royal Society of Chemistry, London.

Triazole-assisted ortho-alkoxylation of 2-aryl-1,2,3-triazoles was reported by Kuang and co-workers with palladium(II)/ K2S2O8 (see Scheme 80).98 Also, in these cases, K2S2O8 worked more suitably among other oxidants, as was observed in previous examples.94 A similar acetoxylation on 1,4-disubstituted 1,2,3-triazole was recently reported by Jiang and co-workers.99 Employing sulfoximines as reusuable directing groups, Sahoo and co-workers developed ortho-selective acetoxylation of arenes with Pd(OAc)2/K2S2O8.100a In a later study, they introduced a new directing group, S-methyl-S-2-pyridylsulfoximine, for the ortho-acetoxylation of arylacetic acids.100b Azoxybenzene substrates were also used for similar C−O bond-forming reactions. Cui and co-workers reported acyloxyation with carboxylic acids in the presence of catalytic Pd(TFA)2 and stoichiometric K2S2O8 (see Scheme 81).101 Importantly, these substrates could not be efficiently alkoxylated with K2S2O8. An exo-oxime benzylether-directed aroyloxylation with benzoic acid on the aromaric ring was reported by Shao et al. in the presence of Pd(II)/K2S2O8.102 While oxone, PhI(OAc)2, or Na2S2O8 showed comparable efficiency to K2S2O8 in the optimization step, TBHP or Ag(I) showed no reaction. Batra and 5109

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ACS Catalysis Scheme 80. Pd-Catalyzed Synthesis of ortho-Alkoxylated 2-Aryl-1,2,3-triazolesa

a

Reprinted with permission from refs 98a (Copyright 2014, American Chemical Society, Washington, DC) and 98b (2014, Wiley−VCH, Weinheim, Germany) .

Scheme 81. Pd-Catalyzed Direct ortho-C−O Bond Construction of Azoxybenzenesa

a

Reprinted with permission from ref 101. Copyright 2015, Royal Society of Chemistry, London.

co-workers reported functionalization of various (β-carbolin-1yl)methanones via palladium-catalyzed regioselective alkoxylation in the presence of K2S2O8 (see Scheme 82).103 Oxygenation of simple benzene is highly challenging in the absence of any directing group. The use of Pd(OAc)2 in such reactions has been proven to be futile in many cases, because of slow reaction rate, lower turnover number (TON), and modest selectivity for the formation of PhOAc over undesired biphenyls. In 2013, Sanford and co-workers developed an efficient Pd(OAc)2-catalyzed ligand-directed C−H oxygenation of benzene with K2S2O8 (Scheme 83).104 For this transformation, the use of a cationic pyridine ligand was crucial, which, in addition to generating an active Pd-catalyst, also acted

Scheme 82. Regioselective Alkoxylation of Aryl(β-carbolin1-yl)methanones with Pd(II)/K2S2O8a

a Reprinted with permission from ref 103. Copyright 2015, Royal Society of Chemistry, London.

as a phase-transfer catalyst (renders better solubility of S2O82−) and thus resulted in the improved yield of the desired product. 5110

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ACS Catalysis Scheme 83. Pd-Catalyzed sp2 C−H Oxygenation of Benzenea

a

Reprinted with permission from ref 104. Copyright 2013, American Chemical Society, Washington, DC.

Scheme 84. Pd-Catalyzed sp3 C−H Activation/β-Acyloxylation of Amidesa

a

Reprinted with permission from ref 105a. Copyright 2014, American Chemical Society, Washington, DC.

Scheme 85. Proposed Mechanism for the β-Acyloxylation of Amide with Pd Catalysta

So far, the foregoing discussion includes C−O bond-forming reactions via C(sp2)−H bond activation. Apart from this, C(sp3)−H activation/oxygenations were also reported with K2S2O8. In 2014, Lu and co-workers developed a palladium-catalyzed β-acyloxylation of simple aliphatic amides (see Scheme 84).105a K2S2O8 was found to be compatible with the Pd(OAc)2/CF3COOH combination used in the reaction and played crucial roles in the formation of a higher-valency palladium intermediate (from A to B), leading to the desired product (Scheme 85). The use of other oxidants such as PhI(OAc)2, O2, or oxone was futile. Very recently, the group disclosed a similar β-mesylation strategy with Pd(II)/K2S2O8 and demonstrated the utility of the method in the synthesis of β-fluoroamides and β-lactams.105b 2.2.3. C−N Bond Formation. Compared to substantial uses of Pd(II)/K2S2O8 in oxidative C−C and C−O bond forming reactions, only selected C−N bond forming reactions such as amidation and nitration were reported with this combination of reagents. For oxidative amidation reactions, the choice of oxidant often becomes a crucial factor, as the presence of free N−H groups could lead to undesired reactions. In 2006, Yu and co-workers reported the palladium-catalyzed intermolecular ortho-amidation of C(sp2)−H/C(sp3)−H bonds of 2-phenylpyridines and O-methyl oximes with different acetamides (see Scheme 86).106 In the presence of K2S2O8, nitrene formation from the acetamide was speculated and some evidence gathered through the controlled experiments supported this notion. Therefore, the nitrene could insert into the Pd−C bond of the palladacycle to afford the final product. Zhang and Zhao reported a palladium-catalyzed intramolecular oxidative amination to the tethered arene with

a

Reprinted with permission from ref 105a. Copyright 2014, American Chemical Society, Washington, DC.

(N,N-dimethyl)oxamoyl amide (see Scheme 87).107 This cyclization could be achieved with K2S2O8, TBPB, DTBP, and H2O2, whereas the use of NFSI resulted in the ring cleavage to afford O-aminobenzaldehydes. The author found that a combination of two oxidants, viz., 4 equiv H2O2 and 2 equiv K2S2O8 was most effective for this transformation. However, the mechanism and precise roles of each oxidant was unclear. Liu and Wang developed a palladium-catalyzed decaboxylative amidation of α-oxocarboxylic acids in the presence of K2S2O8 (see Scheme 88).108 Pyridine containing secondary amines were used in the reaction. K2S2O8 generated the acyl radical 5111

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ACS Catalysis Scheme 86. Pd-Catalyzed Intermolecular Amidation of sp2 and sp3 C−H Bondsa

a

Reprinted with permission from ref 106. Copyright 2006, American Chemical Society, Washington, DC.

moiety exhibited the best directing group ability and the adjacent aromatic ring underwent smooth nitration. Oxime ether derivatives were also nitrated at the ortho-position, whereas pyridine, quinoline, or pyrazole motif showed moderate efficiency in delivering the desired product. Using similar conditions, Kapur and co-workers reported another method for regioselective ortho-nitration of anilines wherein a pyrimidine moiety served as a removable directing group (see Scheme 90).110 In 2014, Fan et al. reported a one-pot synthesis of benzo[c]pyrazolo[1,2-a]-cinnolin-1-ones via palladium-catalyzed dual C−H activation strategy (see Scheme 91).111 In this report, authors first optimized the individual C−H activation steps and then combined them together to design the one-pot transformation. In both steps, the pyrazol-5(4H)-one moiety served as the directing group. For the intramolecular C−N bondforming step, 10 mol % Pd(OAc)2 and 1.5 equiv of K2S2O8 were used. Another such sequential C−C/C−N bond-forming process to synthesize various tricyclic phenanthridinones employing benzamides and arynes was reported by Jeganmohan (see Scheme 92).112 In this reaction, benzamides and O-(trimethylsilyl)phenyltriflate as aryne precursors were combined with Pd(II)/K2S2O8 and CsF. Other oxidants such as Ag(I), PhI(OAc)2, or (NH4)2S2O8 were incompetent for this transformation. The reaction proceeded through the insertion of aryne into the Pd−C bond of the initial palladacycle formed from benzamide and then a reductive elimination was followed to give the desired C−N bond. However, Pd(0) is known to react with

Scheme 87. Pd-Catalyzed Transformation of Masked Benzyl Alcohols to o-Aminobenzaldehydesa

a

Reprinted with permission from ref 107. Copyright 2016, Royal Society of Chemistry, London.

in the reaction, and it maintained the Pd(II)/Pd(IV) catalytic cycle. Site selectivity during normal aromatic nitration is mostly dictated by the functional groups present on the aryl ring. However, such bias is alleviated in metal-catalyzed direct C−H activation/nitration methods. Some palladium-catalyzed nitrations were reported by employing AgNO2 or NaNO2 as the nitrating agent. In all of these cases, K2S2O8 was preferentially used as the oxidant, although oxone or CAN could be an alternative. Other oxidants such as BQ, PhI(OAc)2, Cu(OAc)2, and O2 were not effective in most of these cases. These reactions proceeded through the generation of a NO2 radical from the corresponding salts with K2S2O8 and involved Pd(III) or Pd(IV) species in the reaction. In 2010, Liu et al. first disclosed a Pd(OAc)2-catalyzed directing-group-assisted ortho-nitration of aryl C−H bond with AgNO2 and K2S2O8 (see Scheme 89).109a Later, they reported a more elaborate study on this method.109b,c The quinoxaline Scheme 88. Pd-Catalyzed Decarboxylative Amidation to Imidesa

a

Reprinted with permission from ref 108. Copyright 2016, Royal Society of Chemistry, London. 5112

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ACS Catalysis Scheme 89. Pd-Catalyzed Regiospecific Synthesis of Nitroarenesa

a

Reprinted with permission from ref 109a. Copyright 2010, Wiley−VCH, Weinheim, Germany.

varyings yields were obtained employing different oxidants such as K2S2O8, DBTP, TBHP, PhI(OAc)2, DDQ , Ag2O, BQ , and O2. Reaction with K2S2O8 afforded the best yield. The proposed mechanism involved the formation of enamide involving two substrates, followed by intramolecular cyclization through C−O, which was facilitated by the C−H activation. The exact role of Cu(II) and oxidant was not clear in the mechanism (see Scheme 94). A three-component cascade reaction employing amines, alkyne esters, and alkenes in the presence of Pd(II)/K2S2O8 was developed by Zhang et al. to access various 2,3,4-trisubstituted pyrroles (see Scheme 95).114 The overall transformation resulted in the formation of C(sp2)−C(sp2) and C(sp2)−N bonds. 2.2.4. C−S/C-P Bond Formation. Only a handful of examples are available on palladium-catalyzed C−S and C−P bondforming reactions that used K2S2O8 as an oxidant. Cui and co-workers disclosed a palladium-catalyzed ortho-sulfonylation of azobenzenes using arylsulfonyl chlorides and K 2 S 2 O 8 (see Scheme 96).115 A p-tolylsulfonyl radical was generated by K2S2O8, which subsequently entered into the Pd(II)/Pd(IV) catalytic cycle and furnished the desired product. Some phosphonation reactions involving the cleavage of C(sp2)−H and P(O)−H bonds were reported with Pd(II) catalysts and oxidants. In 2012, Li first reported a palladium-catalyzed direct phosphonation of azoles with dialkyl phosphates (see Scheme 97).116 The choice of K2S2O8 as an oxidant was important, because O2, BQ, and CuCl2 proved inferior for this transformation. The requirement of such a strong oxidant also indicated the presence of a Pd(IV)/Pd(II) catalytic cycle. Generally, nucleophilic phosphite coordinates to Pd(II) first and the complex is subsequently oxidized by K2S2O8 into a Pd(IV) intermediate. Finally, reductive elimination furnishes the product. It is important to note that mechanistic course for the Pd-catalyzed phosphonations is different from reactions performed with the Ag(I)/K2S2O8 combination or with K2S2O8 alone. In 2013, Huang and Wu reported a C3 selective dehydrogenative phosphonation of various coumarin derivatives with Pd(II)/ K2S2O8 (Scheme 98).117

Scheme 90. Pd-catalyzed Regioselective C−H Nitration of Anilinesa

a

Reprinted with permission from ref 110. Copyright 2016, American Chemical Society, Washington, DC.

Scheme 91. Pd-Catalyzed Synthesis of Benzo[c]pyrazolo[1,2a]-cinnolin-1-ones from 5-Pyrazolonesa

a

Reprinted with permission from ref 111. Copyright 2014, Royal Society of Chemistry, London.

aryne, which could lead to undesired reaction pathways. Therefore, an important role of the oxidant could be speculated to avoid such side reactions and thus the actual catalytic cycle might involve higher-valency palladium species (with Pd(II)/ Pd(IV) catalytic cycle). Using simple amides and ketones, Jiang developed a palladiumcatalyzed strategy to synthesize various oxazoles from readily available starting materials through sequential C−N/C−O bond formation (see Scheme 93).113 With the use of CuBr2 as additive, 5113

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ACS Catalysis Scheme 92. Pd-Catalyzed Benzamide Cyclization with Arynesa

a

Reprinted with permission from ref 112. Copyright 2014, Royal Society of Chemistry, London.

Scheme 93. Pd-Catalyzed Access to Oxazoles via Sequential C−N/C−O Bond Formationa

Scheme 95. Pd-Catalyzed Three-Component Reaction to 2,3,4-Trisubstituted Pyrrolesa

a

Reprinted with permission from ref 113. Copyright 2014, American Chemical Society, Washington, DC.

a Reprinted with permission from ref 114. Copyright 2016, American Chemical Society, Washington, DC.

In 2015, Wang reported a palladium-catalyzed method to introduce phosphonates at the C5 of imidazo[2,1-b]thiazoles (see Scheme 99).118

2.2.5. Miscellaneous. In 2011, Yu and co-workers developed a palladium-catalyzed ortho C−H borylation of N-arylbenzamides with B2pin2 (see Scheme 100).119 In this study, the −CONHAr

Scheme 94. Proposed Mechanism for Pd-Catalyzed Synthesis of Oxazole Derivativesa

a

Reprinted with permission from ref 113. Copyright 2014, American Chemical Society, Washington, DC. 5114

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ACS Catalysis Scheme 96. Pd-Catalyzed Sulfonylation Reaction of Azobenzenesa

a

Reprinted with permission from ref 115. Copyright 2015, American Chemical Society, Washington, DC.

Scheme 97. Phosphonation of Azoles with Pd(II)/K2S2O8a

a

Reprinted with permission from ref 116. Copyright 2012, Royal Society of Chemistry, London.

Scheme 100. Oxidative ortho C−H Borylation of Arenes with Pd(II)/K2S2O8a

Scheme 98. Regioselective Phosphonation of Coumarins with Pd(II)/K2S2O8a

a Reprinted with permission from ref 117. Copyright 2013, American Chemical Society, Washington, DC.

Scheme 99. Regioselective Phosphonation of Imidazo[2,1-b] Thiazoles with Pd(II)/K2S2O8a

a Reprinted with permission from ref 119. Copyright 2012, American Chemical Society, Washington, DC.

5 mol % [Pd(NCCH3)4](BF4)2 catalyst with 2 equiv of K2S2O8. 2.3. Cu-Catalyzed Reactions. A copper-catalyzed crossdehydrogenative coupling (CDC) reaction between thiazoles and cyclic ethers was developed by Jiang and co-workers providing access to C2-functionalized thiazoles and benzothiazoles (see Scheme 101).121 This transformation could be performed with TBHP, oxone, or K2S2O8. A very similar transformation was reported using visible light and K2S2O8, which, however, differed from the present reaction, in terms of mechanism (see Scheme 173 in section 3.5, presented later in this work). In this copper-catalyzed process, upon the addition of ether radical to organocopper species A, the formation of intermediate B instead of another possibility (B′) was proposed in the reaction (see Scheme 102). This was observed through the

a Reprinted with permission from ref 118. Copyright 2015, Elsevier, Amsterdam.

group [where Ar = (4-CF3)C6F4] on the substrate served as the auxiliary to promote effective ortho C−H activation in the presence of 10 mol % Pd(OAc)2, an electron-deficient dba ligand, and K2S2O8. A Pd(II)/Pd(IV) catalytic cycle is most likely to be operative in the presence of K2S2O8. Similar to the above reaction, Kanai and Kuninobu disclosed a palladium-catalyzed method for the sp2 C−H fluorosilylation (C−Si bond formation) of 2-phenylpyridines with amino(1,3,2dioxaborolan-2-yl)diphenylsilane.120 The reaction required 5115

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ACS Catalysis

Scheme 105. Cu-Mediated Trifluoromethylation of Different Alkyl Halidesa

Scheme 101. CDC Reaction of (Benzo)thiazoles and Cyclic Ethers with Cu(II)/K2S2O8a

a

Reprinted with permission from ref 121. Copyright 2013 American Chemical Society, Washington, DC.

Scheme 102. Key Intermediates for the Cu-Catalyzed CDC Reactiona

a Reprinted with permission from ref 121. Copyright 2013, American Chemical Society, Washington, DC.

a Reprinted with permission from ref 125. Copyright 2017, American Chemical Society, Washington, DC.

evidence collected via controlled experiments and DFT calculations. K2S2O8 was involved in generating the ether radical via α-sp3 C−H abstraction with SO4− •, as well as in oxidizing Cu(I) to Cu(II). Another cross-dehydrogenative coupling between N-arylglycine derivatives and olefins was developed by Liu et al. with Cu(I)/ K2S2O8 for the preparation of quinoline-2-carboxylates.122 Wang and co-workers developed a Cu(I)-catalyzed method to prepare cyano-containing oxindoles employing N-arylacrylamides and AIBN (see Scheme 103).123 While K2S2O8, (NH4)2S2O8,

from alkyl bromide or iodide. Screening of different organic and inorganic peroxides revealed that both BPO and K2S2O8 are viable as oxidants for this transformation. However, the author used K2S2O8 for subsequent development of the method. Some mechanistic insights were obtained through controlled experiments, which suggested the involvement of a Cu(II)−CF3 complex as a trifluoromethylating agent and Et3SiH/K2S2O8 as a radical initiator (Scheme 106). Although understanding the full Scheme 106. Proposed Mechanism for Cu-Mediated Trifluoromethylation via a Radical Processa

Scheme 103. Cu-Catalyzed Access to Cyano-Containing Oxindoles through Radical Cascade Reactiona

a Reprinted with permission from ref 123. Copyright 2014, Royal Society of Chemistry, London. a Reprinted with permission from ref 125. Copyright 2017, American Chemical Society, Washington, DC.

TBHP, DTBP, and PhI(OAc)2 exhibited comparable efficiencies, O2 and Na2S2O8 gave slighly lower yields. Ritter and co-workers developed a novel method for perfluoroalkylation where gaseous CF3I and CF3CF2I were transformed to liquids because of their 1:1 tetramethylguanidine (TMG) adduct for easy handling. These new reagents could perform efficient perfluoroalkylation of various arenes with the use of 2 equiv Cu(OAc)2·H2O and K2S2O8 in AcOH (Scheme 104).124

mechanism would be more intriguing, a unique reactivity exhibited by K2S2O8 in this transformation could stimulate its further strategic uses. In 2014, Lei and co-workers developed a copper-catalyzed methylenation of aryl ketones and 1-aryl-1-pyridinemethanes with DMF as the one carbon source and K2S2O8 as an oxidant.126 Overall, this reaction transformed a ketone to α,β-unsaturated ketone. More recently, Zhou and Chen disclosed a Cu(II)catalyzed three-component cross-dehydrogenative coupling/ cycloaddition reaction affording various 4-acyl-1,2,3-triazoles (see Scheme 107).127 The reaction was realized with the use of aryl ketones, organic azides, and DMF as one-carbon (C1) donor in the presence of catalytic Cu(NO3)2. The methylenation occurred in this reaction is similar to an earlier report.126 One-pot oxidative Mannich reaction, followed by a 1,3-dipolar cycloaddition, was the key feature of this transformation. The first step of this reaction proceeded most efficiently with K2S2O8, compared to other organic peroxides. More recently, Li and co-workers reported a Cu(II)-catalyzed C−N bond formation via N−H/C−H cross-dehydrogenative

Scheme 104. Cu-Mediated Perfluoroalkylation Reactions with a New Liquid Perfluoroalkylating Reagenta

a

Reprinted with permission from ref 124. Copyright 2015, Wiley−VCH, Weinheim, Germany.

Very recently, Li developed a copper-mediated, operationally simple and practical method for the trifluoromethylation of alkyl radicals (see Scheme 105).125 In this method, BPyCu(CF3)3 (where Bpy = 2,2′-bipyridine) was used as the CF3 source and a combination of Et3SiH/K2S2O8 was used to generate the radical 5116

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ACS Catalysis Scheme 107. Cu-Catalyzed CDC Reaction to 4-Acyl-1,2,3-triazolesa

a

Reprinted with permission from ref 127. Copyright 2017, American Chemical Society, Washington, DC.

Scheme 108. Cu-Catalyzed N-Amidoalkylation of NH-1,2,3-triazolesa

a

Reprinted with permission from ref 128. Copyright 2017, American Chemical Society, Washington, DC.

Scheme 109. Cu-Catalyzed Cascade Reaction toward Isoxazolinesa

a

Reprinted with permission from ref 129a. Copyright 2015, Royal Society of Chemistry, London.

coupling between NH-1,2,3-triazoles and N,N-dialkylamides (see Scheme 108).128 This reaction proceeded through the oxidative formation of an iminium ion from DMF, followed by the addition of 1,2,3-triazole. An interesting cascade reaction was developed by Yang where two different sp3 C−H bonds in the side chain of ethylazaarenes were transformed to a ketone and isoxazoline moiety, respectively, in the presence of 10 mol % CuBr and stoichiometric K2S2O8 and KNO3 (Scheme 109).129a Four new bond formations (CO, CN, C−C, C−O) could be taken into consideration in this transformation, involving ethylazaarenes and different alkene reaction partners. The overall transformation appeared quite complex and a possible mechanism is outlined in Scheme 110. The conversion of ethylazaarene A into ketone E occurred through stepwise oxidation of the benzylic C−H bonds with Cu(I)/K2S2O8. In this sequence, addition of NO3− to benzylic carbocation was proposed, which ultimately delivered

the oxygen atom to form the carbonyl group in E. Next, addition of NO2 radical to the double bond of a copper enolate (F) and subsequent oxidation of G could provide a nitrile oxide intermediate (H). Finally 1,3-dipolar cycloaddition with olefin could deliver the desired product.129a This group and others reported few oxidative phosphorylation reactions with Cu(II)/K2S2O8 system on different substrates using HP(O)R2.129b,130 Since both Cu(II) and S2O82− (or SO4− •) could function as the oxidant, it was unclear which species was involved in different oxidation steps of the transformation. Mechanistically, these reactions differed from those phosphorylation reactions that were performed with the Ag(I)/ K2S2O8 combination or K2S2O8 alone. Based on the literature precedents, it could be speculated that K2S2O8 was mostly involved in the generation of nucleophilic phosphinoyl radical and also maintained the metal-catalytic cycle, whereas substrate oxidation might occur with Cu(II) species. In 2015, Yang reported 5117

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ACS Catalysis Scheme 110. Proposed Mechanism for Isoxazoline Synthesis via Cu-Catalyzed Cascade Processa

a

Reprinted with permission from ref 129a. Copyright 2015, Royal Society of Chemistry, London.

Scheme 111. Different Cu(II)-Catalyzed Oxidative Phosphorylations: (a) Cu(II)-Catalyzed Oxidative Radical Cyclization of Various N-protected-2-allyl anilines with HP(O)Ph2 in the Presence of K2S2O8, (b) Oxidative sp2 C−H Phosphonation by Cu(II)/K2S2O8 to Provide Various α-Iminophosphine Oxides from Aldehyde Hydrazones and Diphenylphosphine Oxide, and (c) Cascade Reaction Involving Phosphorylation−Cyclization with Cu(II)/K2S2O8 System To Provide the Access to Different 2-Phosphorylated Pyrrolo[1,2-a]indolesa

a

Reprinted with permission from refs 129b (Copyright 2015, Royal Society of Chemistry, London), 130a (Copyright 2016, American Chemical Society, Washington, DC) and 130b (Copyright 2017, American Chemical Society, Washington, DC).

a Cu(II)-catalyzed oxidative radical cyclization of various N-protected-2-allyl anilines with HP(O)Ph2 in the presence of K2S2O8 (see Scheme 111a).129b This reaction delivered various phosphorated indolines. In 2016, Zhu developed an oxidative sp2 C−H phosphonation by Cu(II)/K2S2O8 to provide various α-iminophosphine oxides from aldehyde hydrazones and diphenylphosphine oxide (see Scheme 111b).130a In both cases, the generation of phosphinoyl radical and its nucleophilic addition on the substrates were proposed in the mechanism. The same group reported a cascade reaction involving phosphorylation−cyclization with Cu(II)/K2S2O8 system to provide the access to different 2-phosphorylated pyrrolo[1,2a]indoles (Scheme 111c).130b

An analogous sulfur-centered radical addition−cyclization cascade on the same scaffold was reported by Cheng.131 In 2016, Kianmehr et al. reported acetylamination of 2-arylpyridines with potassium cyanate in the presence of stoichiometric Cu(II)/K2S2O8 combination (Scheme 112).132 First, a pyridinecoordinated Cu(II) species A was proposed to form. Next, single electron transfer (SET) from the attached aryl ring to Cu(II) generated a radical cation intermediate (B), which was subsequently trapped by a proximate isocyanate ligand at the ortho-position of the aryl group in the presence of K2S2O8. The resulting isocyanate intermediate (C) was finally transformed to the product by the addition of acetic acid, followed by decarboxylation (see Scheme 113). 5118

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ACS Catalysis

Scheme 115. Trifluoromethylthiolation of Terminal Alkenes with Cu(II)/K2S2O8a

Scheme 112. Cu-Mediated Acetylamination of Arenes with Cyanate Saltsa

a

Reprinted with permission from ref 135b. Copyright 2014, Royal Society of Chemistry, London.

a Reprinted with permission from ref 132. Copyright 2016, American Chemical Society, Washington, DC.

Formation of regiospecific and stereospecific C(sp2)−S and C(sp2)−Se bonds through a three-component copper(I)catalyzed reaction was reported by Li and co-workers (see Scheme 116).137 This reaction employed alkynes, arylsulfonohydrazides, and diphenyl diselenide to furnish (E)-β-selenovinyl sulfones. Du Bois and co-workers disclosed a facile method for converting sp3 C−H bond into C−O bond via intramolecular addition of carboxylic acid in the presence of a Cu(II) catalyst and K2S2O8 (see Scheme 117).138 Different lactones were synthesized by this method. K2S2O8 furnished the best yield among Na2S2O8, (NH4)2S2O8, oxone, or CAN. Based on the controlled experiments, it was proposed that the sulfate radical ion (SO4− •) abstracted the C−H hydrogen and generated a benzylic radical first, which was subsequently oxidized into a carbocation by cupric ion. Nucleophilic addition by the tethered COOH group provided the desired lactones. It is important to note that this mechanism differed from the earlier reports on analogous reactions, which invoked carboxylate radical as the reactive intermediate. The author conducted a thorough massspectrometric analysis, which revealed the presence of a benzyl sulfate species (formed by coupling of benzylic radical to SO4− •). This observation firmly supported the postulated mechanism.138b Recently, a cascade reaction was developed by Liu with the Cu(II)/K2S2O8 system employing different alkynols and 2-azidobenzaldehydes that furnished 6H-isochromeno[4,3-c]quinolines (see Scheme 118).139 This reaction resulted in the formation of C−C, C−N, and C−O bonds in a single operation. 2.4. Fe-Catalyzed or Fe-Mediated Reactions. In 2000, Wille disclosed an interesting SO4− •-mediated transannular reaction that employed the Fenton redox system, i.e., a combination

Narula reported a Cu(I)-catalyzed ortho-azidation of 2-arylpyridines and other related systems with benzotriazole sulphonyl azide (BtSO2N3) in the presence of K2S2O8.133 The reaction proceeded with similar mechanistic path as presented for isocyanate addition in the earlier example. A decarboxylative C−N coupling between α-keto acids and sulfoximines with Cu(I)/K2S2O8 was disclosed by Yotphan (see Scheme 114).134 Formation of an acyl radical in the presence of Cu(II)-K2S2O8 and its reaction with either a Cu(II)-coordinated imine (A) or nitrogen-centered imine radical (B) were proposed for the formation of the product. In 2010, Qing developed a Cu(II)-mediated ortho-methylthiolation of 2-arylpyridines with DMSO in the presence of K2S2O8.135a In 2014, the same group developed a Cu(II)-mediated trifluoromethylthiolation of unactivated terminal alkenes with AgSCF3 (see Scheme 115).135b Using this method, different terminal alkenes were converted to trifluoromethylthiolated allylic compounds. The formation of SCF3 radicals was proposed to form with Cu(II)/K2S2O8, which, after sequential addition− oxidation steps, provided the desired product. A copper-catalyzed deoxygenative transformation between quinoline N-oxide and sodium sulfinate in the presence of K2S2O8 was reported by Han and Pan.136 This method afforded various C2-sulfonylated quinolines with 20 mol % CuBr2 and 50 mol % K2S2O8. The reaction proceeded through Minisci-like radical reactions with a sulfinate radical and the deoxygenation of quinoline N-oxide occurred in the final aromatization step via the elimination of water.

Scheme 113. Proposed Mechanism for Acetylamination of Arenes with Cu(II)/K2S2O8a

a

Reprinted with permission from ref 132. Copyright 2016, American Chemical Society, Washington, DC.

Scheme 114. Cu-Catalyzed Access to N-Aroylsulfoximines via Coupling of α-Keto Acids and Sulfoximinesa

a

Reprinted with permission from ref 134. Copyright 2017, Royal Society of Chemistry, London. 5119

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ACS Catalysis Scheme 116. Selenosulfonation of Alkynes with Cu(I)/K2S2O8a

a

Reprinted with permission from ref 137. Copyright 2017, American Chemical Society, Washington, DC.

Scheme 117. Cu-Catalyzed Oxidative Cyclization of Carboxylic Acids to Lactonea

Scheme 119. Sulfate-Radical-Mediated Transannular Reaction with Fenton Redox System, as Reported by Willea

a

Reprinted with permission from ref 140. Copyright 2000, American Chemical Society, Washington, DC.

Scheme 120. Proposed Mechanism for Transannular Reactiona

a Reprinted with permission from ref 140. Copyright 2000, American Chemical Society, Washington, DC.

Scheme 121. Fe-Catalyzed CDC Reaction toward the Synthesis of Vinylaromaticsa

a

Reprinted with permission from ref 138. Copyright 2016, American Chemical Society, Washington, DC.

of S2O82− and an Fe2+ source.140 In the reaction, Fe(EDTA)(SO4)2·4H2O and K2S2O8 were used with cycloalkynes, which afforded a mixture of α,β-epoxy ketones in almost-equal ratio (see Scheme 119). The intriguing mechanistic feature of this transformation was that the sulfate radical ion was directly added to the alkyne and generated a vinyl radical (see Scheme 120). This, in turn, was added on the carbonyl group to form an allyloxy radical and subsequent cyclization to olefin provided the intermediate oxiranyl radical. Finally, expulsion of SO3− • furnished the product. This report is unique, in terms of the addition of the sulfate radical ion to alkyne and such direct addition is rarely postulated in the literature. In 2012, Lou et al. developed a FeCl3-catalyzed benzylic methylenation of 2-methylazaarenes using N,N-dimethylacetamide (DMA) and K2S2O8 (see Scheme 121).141 This transformation was similar to that reported by the copper-catalyzed

a

Reprinted with permission from ref 141. Copyright 2012, Royal Society of Chemistry, London.

process, that is discussed in the earlier section of this review.124 A Fe(III)−Fe(II) catalytic cycle was proposed for this transformation. A similar strategy on the α-methylenation of ketones was reported by Li, using FeCl3·6H2O catalyst and K2S2O8 with DMA.142 This protocol afforded various α,β-unsaturated ketones. A silver-catalyzed arylation method for electron-deficient arenes with arylboronic acid in the presence of K2S2O8 was reported by Baran.24 Yu and co-workers demonstrated that the same reaction could be achieved with the use of stoichiometric

Scheme 118. Cu-Catalyzed Cascade Annulation to 6H-Isochromeno[4,3-c]quinolinea

a

Reprinted with permission from ref 139. Copyright 2017, American Chemical Society, Washington, DC. 5120

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ACS Catalysis FeS and K2S2O8.143 Arylations with p-quinones were also described in this report. The mechanism proposed herein for the aryl radical generation from arylboronic with Fe(II)/K2S2O8 was different from that reported with the Ag(I)/K2S2O8 system.25 Soon after this report, Singh and Vishwakarma disclosed a similar protocol using catalytic Fe(acac)2 and TBAB as a phase-transfer catalyst with K2S2O8 (see Scheme 122).144

In the same year, Maiti reported a Fe(NO3)3-catalyzed C−H arylation of heterocycles with arylboronic acids.145a Subsequently, they disclosed FeCl3-promoted decarboxylative trifluoromethylation of α,β-unsaturated carboxylic acids using NaSO2CF3 and K2S2O8.145b In 2015, Guo developed a Fe(II)-facilitated decarboxylative cross-coupling reaction employing two different types of carboxylic acids with K2S2O8 (see Scheme 123).146 Various α-oxocarboxylic acids and acrylic acids were used in this reaction that afforded α,β-unsaturated carbonyl compounds. 2.5. Other Metal-Catalyzed Reactions. In 2011, Lang et al. disclosed an efficient rhodium-catalyzed carbonylation at the C3 position of indoles with CO and different linear and cyclic alcohols (see Scheme 124).147 The reaction used 2 mol % of [Rh(COD)Cl]2 and 2 equiv of K2S2O8 at 110 °C. In the reaction, an active Rh(III)-catalyst was proposed to form in the presence of K2S2O8. A Rh(II)-catalyzed decarboxylative coupling method between acrylic acids and unsaturated oxime esters in the presence of

Scheme 122. Cross-Coupling Reactions of Electron-Deficient Heterocycles and Organoboron Species with Fe(II)/K2S2O8a

a Reprinted with permission from ref 144. Copyright 2013, American Chemical Society, Washington, DC.

Scheme 123. Decarboxylative Cross-Coupling between α-Oxocarboxylic Acids and Acrylic Acids with Fe(II)/K2S2O8a

a

Reprinted with permission from ref 146. Copyright 2015, American Chemical Society, Washington, DC.

Scheme 124. Preparation of Indole-3-carboxylates via Rh-Catalyzed Indole Carbonylationa

a

Reprinted with permission from ref 147. Copyright 2011, Royal Society of Chemistry, London. 5121

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ACS Catalysis AgOTs−K2S2O8 was developed by Rovis and co-workers for the synthesis of different substituted pyridines (see Scheme 125).148

In section 2.2.1 of this Review, Wang’s report on different palladiumcatalyzed oxidative functionalizations of [60]fullerenes has been discussed.91a−e Between those studies, the group also disclosed a Mn(OAc)3-mediated method to prepare [60]fullerenes-fused tetrahydronaphthalene and indane derivatives with 2-arylmalonates, 2-benzylmalonates, and 2-arylcyanoacetates in the presence of K2S2O8 (see Scheme 127).151 Unlike the palladium-catalyzed processes, the present reaction with Mn(III) proceeded via a radical mechanism. A regioselective C3 iodination of different azaarenes such as quinolones, uracil, and pyridine with NaI was reported by Lupton and co-workers.152 Two equivalents of Ce(NO3)3·6H2O and K2S2O8 were used in this reaction. Wang and Ji reported a cobalt-catalyzed oxidative insertion of isocyanide in amine-based bisnucleophiles (such as 2-aminophenol) to access various 2-amino-benzimidazoles/benzothiazoles/benzoxazoles.153 In 2016, Bazgir strategically employed the above method, using benzo[d]imidazole-aniline substrates and prepared various benzoimidazoquinazoline amines (see Scheme 128).154 Two different types of N−H bonds participated in this cyclization reaction in the presence of the Co(II)/K2S2O8 system. Fujiwara reported a VO(acac)2-catalyzed reaction between methane and carbon monoxide to give acetic acid.155a In this transformation, K2S2O8 worked as the best oxidant in TFA with a TON of 27.5. Bell and co-workers reported the calcium-saltpromoted synthesis of methanesulfonic acid via sulfonation of methane with SO2 in the presence of K2S2O8.155b Later, they reported the synthesis of mixed acid anhydrides with CH4 and CO2, using a combination of VO(acac)2/K2S2O8.155c

Scheme 125. Cross Coupling of Acrylic Acids and Unsaturated Oxime Esters with Rh(III)/K2S2O8a

a Reprinted with permission from ref 148. Copyright 2014, American Chemical Society, Washington, DC.

In this study, K2S2O8 worked as the co-oxidant and was involved in the generation of an active Rh(III) catalyst in the catalytic cycle. In 2015, Ackermann and co-workers developed a Ru(II)catalyzed ortho C−H acyloxylation of phenols bearing a removable auxiliary with different aromatic and aliphatic carboxylic acids (see Scheme 126).149 The use of 2.5 mol % of Scheme 126. Ru(II)-Catalyzed Phenol C−H Acyloxylationa

3. METAL-FREE OXIDATIVE TRANSFORMATIONS WITH K2S2O8 K2S2O8 is employed as the primary oxidant for the formation of a wide range of C−C/C−N/C−S/C−O bonds, giving access to different classes of cyclic and acyclic compounds. The oxidation step either directly involves S2O82− or sulfate radical anion (SO4− •), which, in turn, is generated by the decomposition of K2S2O8. 3.1. C−C Bond Formation. Among different types of C−C bond-forming reactions reported with K2S2O8 as sole oxidant, some important transformations include direct radical acylation,

a

Reprinted with permission from ref 149. Copyright 2015, Wiley−VCH, Weinheim, Germany.

[RuCl2(p-cymene)]2, 10 mol % of AgSbF6, and 1 equiv K2S2O8 were required for this transformation. Formation of a cationic Ru(II) catalyst was proposed under the reaction condition, which promoted the arene oxygenation. However, the precise role of the oxidant was not clear in the proposed mechanism. Similarly, Rao and co-workers reported a directing-groupassisted Ru(II)-catalyzed ortho-hydroxylation of ethyl benzoate, anilides, and aryl carbamates in the presence of K2S2O8.150a−c

Scheme 127. Mn(III)-Mediated Synthesis of [60]Fullerene-Fused Tetrahydronaphthalene and Indane Derivativesa

a

Reprinted with permission from ref 151. Copyright 2011, American Chemical Society, Washington, DC.

Scheme 128. Co-catalyzed Synthesis of Benzoimidazoquinazolines by Cascade Reaction with Isocyanidea

a

Reprinted with permission from ref 154. Copyright 2016, Royal Society of Chemistry, London. 5122

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ACS Catalysis Scheme 129. Jeganmohan’s Cross-Coupling of Two Different Phenols with K2S2O8a

a

Reprinted with permission from ref 156. Copyright 2015, American Chemical Society, Washington, DC.

Scheme 130. Proposed Mechanism for Synthesis of Unsymmetrical Biphenols with K2S2O8a

a

Reprinted with permission from ref 156. Copyright 2015 American Chemical Society, Washington, DC.

Scheme 131. Metal-Free Oxidative Spirocyclization of Hydroxymethylacrylamides with K2S2O8a

a

Reprinted with permission from ref 160. Copyright 2013, American Chemical Society, Washington, DC.

the reaction, which could stabilize this intermediate. Finally, nucleophilic attack by the second phenol partner to the radical cation furnished the final product (seeScheme 130). In 2015, silver-mediated sequential acylation and carbocyclization of alkynoates with α-ketoacid was reported for the synthesis of different 3-acyl-4-arylcoumarins, which has been discussed in section 2.1.1 of this Review. Almost at the same time, Mi et al. reported a transition-metal-free method for the synthesis of the same scaffold with TBAB/K2S2O8, wherein aldehydes served as the source for the acyl group.157 In many radical-mediated oxidative transformations, use of tetrabutylammonium halides were proven beneficial.158 In such cases, K2S2O8 is converted

alkylation, and arylation reactions (through cross-dehydrogenative coupling reactions and decarboxylative processes), cascade radical addition−cyclization processes, multifold bond-cleavage− bond-forming reactions and photoredox reactions. Jeganmohan reported an efficient synthesis of unsymmetrical biphenols via oxidative cross-coupling of two different phenols in the presence of K2S2O8 at ambient temperature (see Scheme 129).156 Oxidation potential of each participating phenol was vital in order to predict the coupling pattern. In the presence of CF3COOH, the phenol as reaction partner with lower oxidation potential was first oxidized by the sulfate radical anion (SO4− •) and afforded a radical cation intermediate. Catalytic Bu4N+·HSO3− was used in 5123

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ACS Catalysis to n-Bu4+SO4− •, which can render better solubility and appropriate oxidation potential for some substrates, thereby facilitating the reaction. In the screening, TBHP gave lower conversion, compared to K2S2O8; however, Na2S2O8 and (NH4)2S2O8 both gave comparable yields during optimization. Here, the aldehyde was first transformed to an acyl radical with n-Bu4+SO4− •, which subsequently underwent cascade addition−cyclization process as described earlier. Guo employed a TBAI/K2S2O8 combination for a similar cascade cyclization of N-methyl-N-arylacrylamides to oxindoles with aromatic aldehydes or benzenesulfonohydrazides.159 Guo and Duan reported the access of spirooxindoles from functionalized acrylamides using dicarbonyls as the reaction partner in the

Scheme 135. Intramolecular Oxidative Acylation of Benzaldehydes with K2S2O8a

Scheme 132. Metal-Free Synthesis of CF3-Containing Oxindoles with K2S2O8a

a Reprinted with permission from ref 161. Copyright 2014, American Chemical Society, Washington, DC.

a

Reprinted with permission from ref 158d. Copyright 2013 Royal Society of Chemistry, London.

Scheme 133. Amidoalkylation/Arylation of Benzothiazoles and Alkenes with N,N-Dialkylamides and K2S2O8a

presence of K2S2O8 (see Scheme 131).160 The use of other oxidants, such as oxone, BQ, or Mn(OAc)3 proved inefficient for this reaction. In 2014, Wang and co-workers successfully demonstrated the use of Langlois’s reagent with K2S2O8 and developed a metal-free access to CF3-incorporated oxindoles from N-arylacrylamides (see Scheme 132).161 This method is quite dissimilar from other reports wherein Langlois’s reagent is largely used in combination with a metal catalyst (Ag or Cu) and oxidant (such as K2S2O8, TBHP). In this particular case, stoichiometric K2S2O8 in a mixture of CH3CN/H2O (4:1) afforded a series of oxindoles bearing a quaternary center, whereas TBHP, DTBP, H2O2, or even Na2S2O8 or (NH4)2S2O8 gave significantly lower conversion.

a Reprinted with permission from ref 163. Copyright 2016, American Chemical Society, Washington, DC.

Scheme 134. Decarboxylative Alkynylation of α-Keto Acids and Oxamic Acids with K2S2O8a

a

Reprinted with permission from ref 165. Copyright 2015, American Chemical Society, Washington, DC. 5124

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ACS Catalysis

Analogously, Yadav reported α-amino radical formation from N-methylanilines, and its reaction to maleimides afforded various tetrahydroquinolines.164 In 2015, a K2S2O8-mediated decarboxylative alkynylation method of α-keto acids and oxamic acids using alkynyliodonium reagent was reported by Duan and co-workers (see Scheme 134).165 This method features the generation of acyl radical under metal-free conditions and its subsequent addition to the electrophile. It is important to note that the reaction was equally efficient with Na2S2O8 and (NH4)2S2O8 during optimization. However, the authors used K2S2O8 to investigate the substrate scope. A similar method on decarboxylative alkynylation with alkyl carboxylic acids was reported with Ag(I)/K2S2O8 and discussed in section 2.1.1.16 The C1 acylation of isoquinolines with α-keto acid was recently reported by Singh and co-workers with K2S2O8 in water.166 Glorius developed an intramolecular oxidative acylation with catalytic (10 mol %) tetraethylammonium bromide (TEAB) and 2 equiv of K2S2O8 by employing 2-aryl benzaldehydes as substrates (Scheme 135).158d Oxone, TBHP, Na2S2O8, or (NH4)2S2O8 resulted in poor conversion. Since the reaction was

Tian also reported a similar strategy using trifluoromethanesulfonyl hydrazides as trifluoromethylating agents in the presence of K2S2O8.162 Selective generation of amido alkyl radical species from N,N-dialkylamides with K2S2O8 and their uses in C2 amidoalkylation of benzothiols or oxindole was demonstrated by Huang and Zhu (see Scheme 133).163 This reaction was very specific to the use of K2S2O8, as PhI(OAc)2, TBHP, Na2S2O8, or (NH4)2S2O8 was proved ineffective. The outcome could be attributed to poor solubility of the other peroxydisulfate oxidants, compared to K2S2O8 in the reaction medium. Scheme 136. K2S2O8-Mediated Oxidative Condensation between Benzothiazoles and Aldehydesa

a

Reprinted with permission from ref 170. Copyright 2012, American Chemical Society, Washington, DC.

Scheme 139. Metal-Free Decarboxylative Cross-Coupling of Carboxylic Acids with Cyclic Ethersa

Scheme 137. Proposed Mechanism for the Oxidative Condensationa

a

Reprinted with permission from ref 173a. Copyright 2017, American Chemical Society, Washington, DC.

Scheme 140. Metal-Free α-Csp3−H Methylenation of Ketones with DMSOa

a Reprinted with permission from ref 170. Copyright 2012, American Chemical Society, Washington, DC.

a

Reprinted with permission from ref 173b. Copyright 2017, American Chemical Society, Washington, DC.

Scheme 138. K2S2O8-Mediated Oxidative Decarboxylation−Cyclization Cascade of 2-Isocyanobiphenylsa

a

Reprinted with permission from ref 171. Copyright 2015 American Chemical Society, Washington, DC. 5125

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ACS Catalysis Scheme 141. Proposed Mechanism for α-Csp3−-H Methylenation of Ketones with K2S2O8a

a

Reprinted with permission from ref 173b. Copyright 2017, American Chemical Society, Washington, DC.

carried out in organic solvent (DCE), it could be speculated that in situ formation of soluble peroxydisulfate, (n-Bu4)2S2O8 (which produces the n-Bu4+SO4− • ion) was crucial for this transformation, and cation exchange occurred most efficiently with K2S2O8. Importantly, Ag(I)/K2S2O8 system did not work for this reaction. It was proposed that the acyl radical was generated via cleavage of the aldehydic C−H bond, which added intramolecularly to the aryl group and afforded different fluorenone, xanthone, and anthrone derivatives. Recently, Laha et al. disclosed a complementary approach to access fluorenones where decarboxylative acylation strategy was used with an appropriately positioned α-oxocarboxylic acid.167 The key acyl radical generated in the reaction participated in analogues intramolecular cyclization, as reported by Glorius. This method only requires K2S2O8 for the generation of acyl radical and is applicable to the synthesis of different azafluorenones. Unlike Friedel−Crafts acylation, this protocol works for both electron-rich and electron-deficient arenes and tolerates acid-sensitive functional groups. Adib et al. disclosed an acylation process of 3-substituted coumarin with aromatic aldehydes to prepare various 4-aroylcoumarins in the presence of K2S2O8/Aliquat 336 (tricaprylmethylammonium chloride).168a They also reported the synthesis of 3-acylcoumarins from coumarin and aldehydes, using the same strategy.168b More recently, Khoobi and Jafarpour disclosed a metal-free decarboxylative process to access various 2-arylated flavones and 3-aryl coumarins from corresponding carboxylic acids and aryl boronic acids in the presence of K2S2O8.169 Tan and co-workers developed an oxidative C2 arylation method of benzothiazoles with K2S2O8.170 Interestingly, they employed aryl aldehydes and phenylglyoxylic acids as the arylating agent (see Scheme 136). In this case, the formation of any radical intermediate was ruled out by controlled experiments. Rather, oxidative ring opening of heterocycles to 2-amino thiophenol was proposed, which, upon condensation and cyclization with aldehydes, gave dihydrobenzothiazole and finally provided benzothiazoles via oxidation (see Scheme 137). It was likely that phenylglyoxylic acids also were converted to the aldehydes under the reaction condition. This was certainly an unexpected course of reaction mediated by K2S2O8. In 2015, Lu and co-workers disclosed another metal-free-radical decarboxylation−addition−cyclization cascade to access phenanthridines employing 2-isocyanobiphenyls and different alkyl/aryl carboxylic acids in the presence of K2S2O8 (see Scheme 138).171 The reaction first generated an alkyl radical and then an imidoyl radical through the addition of the former to the isocyanide moiety. Finally, the radical cyclilization on the aryl ring delivered the product. Some cross-dehydrogenative coupling (CDC) reactions were also reported under metal-free conditions employing K2S2O8 as the sole oxidant. One such CDC reaction was developed by

Scheme 142. Metal-Free Alkynylation and Ring Expansion of Alkenyl Cyclobutanolsa

a

Reprinted with permission from ref 177. Copyright 2016, American Chemical Society, Washington, DC.

Singh and co-workers between electron-deficient arenes and α-Csp3−H of ethers.172 Instead of the direct α-C−H abstraction by SO4− •, as shown in the report, a radical cation formation, followed by deprotonation is more likely for the generation of nucleophilic α-alkoxy radical. The latter species subsequently added on the protonated heterocycle to form another radical cation intermediate. A SET process involving either SO4− •or S2O82− delivered the final product. Different nitrogen-containing heteroarenes and naphthoquinones were similarly functionalized using this protocol. Recently, Guo developed a decarboxylative cross-coupling strategy between α,β-unsaturated carboxylic acids and cyclic ethers rendering various ketones in the presence of K2S2O8 (see Scheme 139).173a This reaction involved direct activation of α-sp3 C−H of different cyclic ethers, which efficiently proceeded with K2S2O8, in comparison to other oxidants screened [e.g., oxone, DTBP, Na2S2O8, (NH4)2S2O8]. The reaction required the presence of O2, which was incorporated in the final product as carbonyl oxygen. Importantly, only cyclic ethers worked successfully under the protocol, whereas related sulfur or nitrogen congeners failed to give any desired product. The group also developed a K2S 2O8 -mediated direct Csp3−H methylenation of aryl ketones employing DMSO (see Scheme 140).173b The latter served as the one-carbon source under the reaction conditions. This method describes an alternative means, as opposed to the use of DMA or DMF for similar methylenation.141 Importantly, K2S2O8 worked most efficiently among other oxidants such as KHSO5, H2O2, TBHP, and (NH4)2S2O8. Probable reasons could be the compatibility of this oxidant with the base used in this transformation and the ability to promote a Pummerer-like rearrangement with DMSO.173c In the depicted mechanism, intermediate A was 5126

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ACS Catalysis Scheme 143. Proposed Mechanism for Alkynylation/Ring-Expansion Rearrangementa

a

Reprinted with permission from ref 177. Copyright 2016, American Chemical Society, Washington, DC.

proposed to form from DMSO in the presence of K2S2O8, which combined with the enolate to give intermediate B. K2S2O8 further facilitated the demethylthiolation under oxidative conditions to afford the product (see Scheme 141). With a slight modification in the reaction conditions and using DABCO as a base, the group further reported a synthetic strategy to access different a-methylthiomethyl enones and 3-methylthiomethyl chroman-4-ones, where three methyl groups underwent Csp3−H bond coupling.174 Capitalizing on the K2S2O8mediated methylenation of ketones with DMSO, Tiwari and co-workers developed a method to synthesize various acylated quinolines. This reaction proceeded with the reaction of conjugated ketone and anthranils.175 Yadav and co-workers initially reported a silver-catalyzed aerobic oxidation method to transform alkene to β-ketosulfones with thiophenols (shown earlier in Scheme 34).47 Subsequently, they disclosed a metal-free K2S2O8-mediated method to achieve the same product, employing alkenes and sodium sulfinate salts.176 Chen and Yu strategically utilized the ring-expansion propensity of vinyl or alkynylcyclobutanes under oxidative conditions and coupled it with an electrophilic alkynylating agent, ethynylbenziodoxolones (EBX), to obtain various β-alkynylated cyclopentanones (see Scheme 142).177 This reaction could be performed with only K2S2O8 without any catalyst. Other oxidants and even (NH4)2S2O8 were found to be inferior in this reaction. In the mechanistic proposal, first, formation of an oxygen centered radical species was proposed to form via hydrogen abstraction with S2O82− or SO4− • (see Scheme 143). Radical migration/ring expansion subsequently followed, to generate another alkyl radical intermediate A (a vinyl radical is generated when alkynyl cyclobutane was used in the reaction). Addition of this radical to EBX, followed by β-elimination of the benziodoxolonyl radical, gave the desired product. Zhu reported an interesting K2S2O8-promoted transformation of benzyl-2(3-hydroxypropynyl)-benzoates to polysubstituted cyclobutanes.178 An allenylic ester was proposed to form from the starting material in the presence of K 2 S 2 O 8 , and intermolecular dimerization of the former species afforded the product. A TEMPO-catalyzed K2S2O8-promoted oxidative annulation of N-benzyl-N′-phenyl benzimidamides to quinazoline was reported by Long and co-workers (see Scheme 144).179 This protocol resulted in C(sp3)−C(sp2) and C(sp2)−N bond formations under metal-free conditions. The absolute requirement of TEMPO as a catalyst and the precise role of K2S2O8 in this transformation were unclear.

Scheme 144. Metal-Free Oxidative Annulations of Arylamidinesa

a Reprinted with permission from ref 179. Copyright 2014, American Chemical Society, Washington, DC.

3.2. C−N Bond Formation. Different nitration, azidation, and intramolecular C−N bond-forming reactions were reported under metal-free conditions with K2S2O8. Unlike the decarboxylative generation of alkyl radicals from carboxylic acids, or generating sulfur-centered radicals from the corresponding metal salts, silver or other metal catalysts are not essential for the generation of nitrogen dioxide or azide radicals (redox potentials of nitrogen dioxide and azide radicals are +1.04 V and +1.33 V, respectively)180 and K2S2O8 alone could serve the purpose. By employing arylacrylamides and unactivated alkenes, Yang developed a metal-free carbonitration method with K2S2O8 and NaNO2 for the synthesis of nitro-containing oxindoles and 3,4-dihydro-2(1H)-quinolinones (see Scheme 145).181 This Scheme 145. Metal-Free Carbonitration of Alkenes Using K2S2O8, as Reported by Yang and Co-workersa

a Reprinted with permission from ref 181. Copyright 2013, Royal Society of Chemistry, London.

transformation could also be performed with other oxidants, such as (NH4)2S2O8, Na2S2O8, or oxone with moderate efficacy. In these cases, nitrogen dioxide radical was first added to the olefin and the ensuing radical was cyclized on the aromatic ring. Similar to Yang’s nitration strategy,181 a carboazidation method was developed by Qiu et al., employing arylacrylamides and NaN3, which afforded N3-substituted oxindoles.182 Guo reported a transition-metal-free nitration protocol of alkenes using NaNO2 as the nitrating agent in the presence of K2S2O8 and TEMPO.183 This protocol afforded various (E)-nitroalkenes. Tang developed a metal-free method for selective functionalization of aliphatic C−H bonds into azides using methyl 5127

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ACS Catalysis Scheme 146. Metal-Free Transimination−Intramolecular Cyclization to Heterocyclesa

a Reprinted with permission from refs 185a (Copyright 2015, American Chemical Society, Washington, DC) and 185b (Copyright 2016, Royal Society of Chemistry, London)..

Scheme 147. Proposed Mechanism for Metal-Free Transimination-Intramolecular Cyclizationa

a

Reprinted with permission from refs 185b. Copyright 2016, Royal Society of Chemistry, London.

Scheme 148. Decarboxylative Amidation-Cyclization to N-Heterocyclesa

a

Reprinted with permission from ref 185c. Copyright 2016, Royal Society of Chemistry, London.

N-arylbenzyl amines185a or imines185b with ortho-substituted anilines (see Scheme 146). One key feature of this transformation was that N-arylbenzyl amines could serve as the imine precursors that formed in situ, in the presence of K2S2O8, and subsequently underwent transimination with the reaction partner (see Scheme 147). DDQ and oxone were moderately efficient for this transformation. It appeared that K2S2O8 could oxidize benzylamine to imine very selectively and efficiently, even in the presence of aniline. Other oxidants were apparently not compatible to give the desired product. K2S2O8 also affected the

2-(azidosulfonyl)benzoate as azide source and K2S2O8 as an oxidant.184 The union of alkyl and azide radicals was proposed as the possible route to the product formation where K2S2O8 mediated the generation of both the radical species. The reaction was most efficient for the formation of tertiary azides, whereas the secondary azides could be obtained only with moderate yields. The utility of this method was further demonstrated in achieving late-stage azidation of complex molecules. Laha and co-workers developed a K 2 S 2 O 8 -mediated method for the synthesis of different N-heterocycles using 5128

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ACS Catalysis final step in converting dihydroquinazolinones to quinazolinones (see Scheme 147). In 2016, the same group reported a K2S2O8-mediated decarboxylative acylation of amino group present in anthranilic acids, amino phenols, etc. with α-oxocarboxylic acids, followed by an intramolecular cyclization to access various N-heterocycles (see Scheme 148).185c The reaction featured an oxidative amidation reaction in the first step, followed by a condensation reaction that did not require any additional base. In the mechanism, an acyl radical could be generated from phenylglyoxylic acid in a predictable fashion, which, in turn, could react with an aryl amine to form the N-acylated product. However, the mechanism for the amidation step involving K2S2O8 requires further investigation. In the subsequent step, a base such as HSO4− or SO42−, which are present in the reaction medium,

could mediate the condensation reaction. The group demonstrated the utility of the method in the laboratory scale synthesis of a top-selling drug, sildenafil (Viagra). Luo et al. engaged the Knoevenagel product of β-ketoamides in an intramolecular Csp2−H amidation reaction in the presence of K2S2O8 (see Scheme 149).186a In the presence of an oxidant, formation of an amidyl radical was proposed, which, added on the aromatic ring and subsequent oxidation with SO4− •, delivered the product. More recently, they developed another K2S2O8-mediated dehydrogenative cascade aromatization−intramolecular Csp2−H amidation process by using 1,4-dihydropyridine substrates.186b Sawant and co-workers showed a K2S2O8-promoted transamidation with aromatic and aliphatic amines. Only primary amines delivered the desired products.187 Oxidative cyclization of enamines by K2S2O8 toward the synthesis of diversely functionalized pyrroles was reported by Guo and co-workers (see Scheme 150).188 In this transformation, two enamine molecules reacted with each other. One molecule formed a cationic amino radical via single electron transfer to K2S2O8, which was attacked by the second enamine molecule, and stepwise oxidation−cyclization led to the final product. 3.3. C−S/C−P/C−O/C−Se/C−Halogen Bond Formation. Some methods are reported on the oxidative C−S, C−P, C−O/C−Se, and C−halogen bond-forming reactions using K2S2O8 as the sole oxidant. In some cases, these bonds are formed in concert with a C−C bond. Prabhu and co-workers developed a synthetic method to access α-sulfanylated β-diketones employing benzazole and 1,3-diketone as starting materials (see Scheme 151).189a This reaction could also be performed with other oxidants (for example, (NH4)2S2O8, Na2S2O8, oxone, TBHP, DDQ ) that, however, would give moderate to low yields. A Bronsted acid, HClO4, was specifically required for this reaction, which drove the thione−thiol equilibrium toward thiol and the oxidation of the latter produced an electrophilic sulfur intermediate. Subsequent nucleophilic attack by 1,3-dikenone through its enol form rendered the final product (see Scheme 152).

Scheme 149. Intramolecular C(sp2)−H Amidation to N-Aryl 2-quinolinonesa

a

Reprinted with permission from ref 186a. Copyright 2016, Royal Society of Chemistry, London.

Scheme 150. Metal-Free Oxidative Enamine Cyclization to Polycarbonyl Pyrrolesa

a

Reprinted with permission from ref 188. Copyright 2016, American Chemical Society, Washington, DC.

Scheme 151. K2S2O8-Mediated Sulfanylation of β-Diketonesa

Scheme 153. Synthesis of 2-Sulfenylindenones via Meyer− Schuster Rearrangement and Radical Cyclizationa

a

a Reprinted with permission from ref 193. Copyright 2016, American Chemical Society, Washington, DC.

Reprinted with permission from ref 189a. Copyright 2015, American Chemical Society, Washington, DC.

Scheme 152. Proposed Mechanism for Sulfanylation of β-Diketonesa

a

Reprinted with permission from ref 189a. Copyright 2015, American Chemical Society, Washington, DC. 5129

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ACS Catalysis They further reported the synthesis of different α-sulfenyl monoketones under slightly modified conditions with benzooxazole-2-thione and different aromatic or aliphatic ketones with α-hydrogens.189b In 2015, Lee reported a different strategy to access α-thio-β-dicarbonyl compounds with catalytic I2 and a stoichiometric K2S2O8 combination employing dialkyl disulfide and β-diketone as starting materials.190 The use of H2O2, TBPB also gave good conversion for this transformation, albeit in lower yield (ca. 10%−15%), compared to K2S2O8. Zhu reported a TEAB-catalyzed oxidative coupling between alcohols/aldehydes and thiophenols in the presence of K2S2O8 to access various thioesters.191 Disulfides could also be used as the sulfur source for this transformation. An interesting feature of this S2O8 2−-mediated reaction is that the acyl radical formation (oxidation of aldehyde) occurred selectively over thiol oxidation. Such selectivity was obtained with K2S2O8 over other oxidants used in the transformation. In 2015, Yang and Wang developed a metal-free decarboxylative coupling between α-keto acid and thiols in the presence of K2S2O8, which also gave access to thioesters.192 Zhang and Zhang demonstrated a tandem Meyer−Schuster rearrangement of arylpropynols coupled to radical cyclization with disulfides in synthesizing different 2-sulfenylindenones (see Scheme 153).193 This method required 30 mol % of benzoyl peroxide (BPO), 2 equiv I2, and 2 equiv K2S2O8. In the plausible mechanism, the author showed the formation of thiyl radical in the presence of I2/BPO (see Scheme 154). Thiyl radical addition

Scheme 155. Metal-Free Synthesis of 1,2,3-Thiadiazole from Hydrazones and Sulfura

a Reprinted with permission from ref 194. Copyright 2016, American Chemical Society, Washington, DC.

Apart from thiol-based oxidative transformations, different thiocyanation reactions were performed with K2S2O8 as the primary oxidant. Wang and co-workers developed a method to introduce thiocyanate at the C-3 position of imidazopyridines employing KSCN and K2S2O8.195 The formation of either the · SCN radical or electrophilic thiocyanogen (SCN)2 as the active species was proposed for the reaction. However, this proposition requires further mechanistic scrutiny. In the same year, Guo reported a thiocyanooxygenation strategy to prepare different SCN-containing heterocycles in the presence of K2S2O8 under metal-free conditions (see Scheme 156).196 K2S2O8 gave the best Scheme 156. K2S2O8-Mediated Thiocyanooxygenation of Olefin-Containing Amidesa

Scheme 154. Proposed Mechanism for Tandem Meyer− Schuster Rearrangement and Radical Cyclizationa

a

Reprinted with permission from ref 196. Copyright 2015, American Chemical Society, Washington, DC.

result during the optimization step, although Na2S2O8 could be an alternative. Using isomeric substrates, this method allowed access to either six-membered benzooxazines or five-membered imino-isobenzofurans via C−O and C−S bond formation, respectively. In the mechanism, the addition of the SCN radical to the olefin moiety triggered the cyclization process. More recently, Bhat reported a K2S2O8-mediated paraselective thiocyanation of phenols and anilines.197 High regioselectivity was also observed for heterocycles such as indoles (C3-thiocyanation). Yu reported a direct approach to access 3-thiocyanato-4H-chromen-4-ones from various 2-hydroxyaryl enaminones via thiocyanation and C−O cyclization in the presence of K2S2O8 (see Scheme 157).198 This reaction

a Reprinted with permission from ref 193. Copyright 2016, American Chemical Society, Washington, DC.

to the rearranged allene moiety, cyclization of the ensuing radical onto the aromatic ring, and subsequent oxidation delivered the final product. Since this reaction involved three different oxidants, their actual and synergistic effect on the overall reaction would be difficult to assess from the limited controlled experiments performed in the report. Recently, conversion of N-tosylhydrazones into 1,2,3thiadiazole with sulfur was disclosed by Cheng and co-workers by using catalytic TBAI (20 mol %) and 2 equiv of K2S2O8 (see Scheme 155).194 This reaction might involve more soluble (Bu4N)2S2O8 species formed in situ in the reaction. The use of other oxidants, such as DDQ, H2O2, etc., gave inferior results. It was proposed that K2S2O8 would oxidize I− to I2, which led to α-iodation of acetophenone N-tosylhydrazones. This key intermediate could react with S8 and deliver the final product in a stepwise fashion.

Scheme 157. K2S2O8-Mediated Synthesis of 3-Thiocyanato4H-chromen-4-onesa

a Reprinted with permission from ref 198. Copyright 2016, Royal Society of Chemistry, London.

was proposed to proceed with the formation of nucleophilic SCN radical. 5130

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or transition-metal catalysts. Recently, the Sanford group demonstrated an operationally simple and economical method for this reaction, using K2S2O8 and H2SO4, which rendered various hydroxylated amino compounds (see Scheme 160).203

In 2014, Rao disclosed an unprecedented strategy to access symmetrical and unsymmetrical diarylsulfones from simple arenes via double C−S bond formation (see Scheme 158).199 Scheme 158. K2S2O8-Mediated Synthesis of Diarylsulfones from Simple Arenesa

Scheme 160. K2S2O8-Promoted C(sp3)−H Oxygenation of Protonated Aliphatic Aminesa

a Reprinted with permission from ref 199. Copyright 2014, Royal Society of Chemistry, London.

This transformation demonstrated the use of K2S2O8 as an efficient sulfonating agent. The use of (NH4)2S2O8 also showed similar efficiency during the optimization process. Although phosphorylation/phosphonation were frequently performed with the Ag(I)/K2S2O8 combination (see section 2.2.4), some reactions were reported using only K2S2O8 as oxidants. For example, Wang reported C−P bond formation between ketene dithioacetals and diphenylphosphine oxide in the presence of 3 equiv of K2S2O8.200a The use of other oxidants for the purpose appeared to be futile. This could be correlated to the higher redox potential of the sulfate radical anion, which efficiently generated the phosphorus centered radical to initiate the reaction, even in the absence of a silver catalyst. Cui and co-workers reported phosphonation of quinoxaline-2(1H)-ones employing similar reaction conditions.200b Guo and Cai reported C2 phosphonation of benzoxazoles and benzothiazoles under metal-free conditions with K2S2O8.200c Some C−O bondforming reactions were also realized with the use of K2S2O8 as an oxidant. In 2013, Gevorgyan disclosed a C−H oxygenation of arene through the addition of a remotely placed pendant carboxylic acid group in the presence of K2S2O8.201 In previous reports, this reaction was performed with toxic oxidants, such as Pb(IV), Cr(VI), Cu(II), etc. Replacing those with K2S2O8 imparted the development of a more sustainable chemistry for this transformation. The reaction proceeded through the formation of an O-centered carboxyl radical by K2S2O8, which added intramolecularly onto the arene ring and delivered the product after oxidation. Xu reported a KI-catalyzed oxidative C−O bond formation between acetone and aromatic carboxylic acids in the presence of K2S2O8 that afforded different esters (see Scheme 159).202

a Reprinted with permission from ref 203. Copyright 2017, American Chemical Society, Washington, DC.

A remote C−H functionalization was selectively achieved in this case, as the α-C−H bonds in the substrate were deactivated through protonation of nitrogen under strong acidic conditions. Tertiary and secondary C−H bond oxygenations were only demonstrated in this report. A K2S2O8-mediated selenoamination of alkenes was reported by Sun and co-workers, where diphenyl diselenide and different amines, including saccharin, dibenzenesulfonimide, benzotriazole, pyrazole, 1,2,4-triazole, 6-chloropurine, etc. were used (see Scheme 161).204 The reaction could involve either the selenyl radical or a three-membered seleniranium intermediate (ionic). However, both processes would require oxidation of Ph2Se2, and K2S2O8 turned out to be the most efficient among oxidants such as H2O2, DTBP, or TBHP. K2S2O8 has also found some important applications in oxidative halogenation reactions. Kitamura reported the synthesis of diaryliodonium triflates employing arenes and elemental iodine in the presence of K2S2O8 and trifluoroacetic acid.205 In 2013, Zhang developed an oxidative chlorination method of different aromatic compounds using saturated NaCl/NH4Cl aqueous solution with 2−4 equiv of K2S2O8.206 The use of NaCl solution mostly provided dichloro compounds, while the use of NH4Cl primarily gave monochloro derivatives. Anilines, halo aromatics, and anisoles were used as substrates in this transformation. The authors did not attempt any other oxidant for this reaction. It was likely that sulfate radical ion oxidized the Cl− ion (E° = 1.359 V) into an electrophilic chlorine species, which then promoted the eletrophilic aromatic substitution reaction. A transition-metal-free method for direct benzylic C−H fluorination was developed by Yi and co-workers with Selectfluor and K2S2O8 (see Scheme 162).207 Despite other existing methods for this transformation, the use of K2S2O8 as an inexpensive oxidant under metal-free conditions could make this method more practically viable. Formation of a benzylic radical with sulfate radical ion was most likely to occur, which then combined with a F atom from Selectfluor (see Scheme 163).

Scheme 159. KI−K2S2O8-Mediated α-Acyloxylation of Acetone with Carboxylic Acidsa

a Reprinted with permission from ref 202. Copyright 2016, Royal Society of Chemistry, London.

Apart from aromatic carboxylic acids, use of 4-phenylbutanoic acid, cinnamic acid, or 3-phenylpropiolic acid also gave the desired transformation. Although KI/K2S2O8-mediated reactions could proceed through either ionic or radical mechanisms, in this case, a radical mechanism was proposed, based on controlled experiments. Remote Csp3−H oxygenation of different aliphatic amines were previously reported with methyl(trifluoromethyl)dioxirane 5131

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ACS Catalysis Scheme 161. K2S2O8-Promoted Selenoamination of Alkenesa

a

Reprinted with permission from ref 204. Copyright 2016, Royal Society of Chemistry, London.

Minisci210a in the presence of Ag(I)/S2O82− under acidic conditions and with that notion, S2O82− appeared to be indispensable for this reaction as well. The key hydroxymethyl radical species was efficiently generated by K2S2O8,210b,c whereas other oxidants such as (PhCOO)2, TBHP, CAN, DDQ , BQ , PhI(OAc)2 afforded poor conversions. Some important control experiments were described in this report, which gave important insights into the mechanism. The key step was the addition of hydroxymethyl radical to heterocycles (the formation of A), which then underwent oxidation and isomerization (forming B and C, respectively) and subsequently transformed to the final product aided by molecular oxygen (see Scheme 165). Direct aerobic dehydrogenative coupling between two nucleophilic sp3 C−H and sp2 C−H center by K2S2O8 was developed by Jeganmohan, which led to the α-diarylation of benzyl ketones.211 In this reaction, two new C−C bonds were formed after an initial C−C bond cleavage in the starting material (see Scheme 166). Although (NH4)2S2O8 provided comparable yield to K2S2O8 in the reaction, DDQ, Ag2O, BQ, PhI(OAc)2, etc. were not effective during optimization. The reaction could be initiated through the formation of the α-keto radical by the sulfate radical ion. The α-keto radical upon subsequent reaction with dioxygen was converted to intermediate A (see Scheme 167). Following an ipso-substitution rearrangement in an acidic medium (Hock-type rearrangement), intermediate B was formed, which then suffered a nucleophilic attack by arenes to form intermediate C. Next, a carbocation could be generated from intermediate C, either via radical or ionic pathway, which, upon a nuclephilic attack by a second arene, delivered the final compound. An interesting transformation involving oxytrifluoromethylation of alkenes and aerobic oxygenation of Cvinyl-heteroatom bond was reported by Lei and co-workers, using CF3SO2Na as the source for the trifluoromethyl group (see Scheme 168).212 This reaction only required catalytic K2S2O8 (25 mol %) and did not proceed in the absence of either O2 or K2S2O8, which

Scheme 162. K2S2O8-Promoted Benzylic Monofluorination and Difluorinationa

a

Reprinted with permission from ref 207. Copyright 2015, Royal Society of Chemistry, London.

Scheme 163. Proposed Mechanism for Venzylic C−H Fluorinationa

a

Reprinted with permission from ref 207. Copyright 2015 Royal Society of Chemistry, London.

In 2013, Jiang et al. developed a tandem K2S2O8-mediated method for hydroxybromination−oxidation of styrenes with KBr in water. Here, K2S2O8 played a role in oxidizing Br − to Br2, which led to the final product in the presence of water.208 3.4. Multiple Bond-Cleavage and Bond-Forming Reactions. K2S2O8 has been employed to promote some reactions that involve multiple bond cleavage and formation in a single operation and lead to important structural motifs. These reaction courses are more complex, and their mechanisms are illusive, in most of the cases. Liu and co-workers reported one such type of reaction using quinoxalines (or benzothiazoles) and methanol in the presence of K2S2O8, which resulted in the formation of carbaldehyde dimethyl acetals (see Scheme 164).209 A very similar transformation was previously reported by 5132

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ACS Catalysis Scheme 164. K2S2O8-Mediated Multiple Bond Cleavage and Formation between MeOH and Quinoxalinesa

a

Reprinted with permission from ref 209. Copyright 2013, American Chemical Society, Washington, DC.

Scheme 165. Proposed Mechanism for Synthesis of Carbaldehyde Dimethyl Acetals through Bond Cleavage and Formationa

a

Reprinted with permission from ref 209. Copyright 2013, American Chemical Society, Washington, DC.

intermediate species as an oxidant. In the mechanism, CF3 radical addition to the olefin formed intermediate A, which then combined with oxygen and formed B (see Scheme 169). In situ formation of dialkyl peroxide from B could be the key for the propagation of the reaction without K2S2O8 (i.e., generation of CF3 radical through intermediate C). Finally, the elimination of bromide from intermediate D furnished the product. In 2014, Laha and co-workers reported a tandem oxidative conversion of 10,11-dihydro-5H-dibenzo[b,e][1,4] diazepines to phenazines with K2S2O8 (see Scheme 170).213 The overall process involved oxidative removal of benzylic methylene group via C−C and C−N bond cleavage and a simultaneous intramolecular aryl C−N bond formation. This reaction could be performed with DDQ as well. However, the use of KHSO5

Scheme 166. K2S2O8-Mediated Aerobic Dehydrogenative α-Diarylation of Benzyl Ketones with Arenesa

a Reprinted with permission from ref 211. Copyright 2014, American Chemical Society, Washington, DC.

indicated the complementary effect of both oxidants. Since regeneration of K2S2O8 was not possible, it could be speculated that K2S2O8 only generated the CF3 radical as an initiator from CF 3SO2 Na and the reaction propagated through other 5133

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ACS Catalysis Scheme 167. Proposed Mechanism for α-Diarylation of Benzyl Ketones with Arenesa

a

Reprinted with permission from ref 211. Copyright 2014, American Chemical Society, Washington, DC.

A metal-free access to aryl thioamides was developed by Jiang, using alkyl/aryl aldehydes, sodium sulfide, and N-substituted formamides.214 Bhat recently reported oxidative decarboxylation of arylacetic acids to aldehydes in the presence of K2S2O8.215 The reaction proceeded with the cleavage of a C−C via decarboxylation, and a new C−O bond was formed through the addition of water, followed by oxidation. 3.5. Photoredox Reactions. Recently, some photoredox or visible-light-mediated transformations have appeared in the literature where K2S2O8 was used as the terminal oxidant. Using an iridium photocatalyst and stoichiometric K2S2O8, a radical cascade reaction to form C−P and C−C bonds was realized with diphenyl phosphine oxide and biaryl isocyanides as substrates (see Scheme 172).216 There are precedents that K2S2O8 could efficiently promote P-centered radical formation from diphenyl

Scheme 168. Oxytrifluoromethylation of Alkenes Using CF3SO2Naa

a

Reprinted with permission from ref 212. Copyright 2014, Royal Society of Chemistry, London.

gave only a moderate conversion. In the reaction course, stepwise oxidation at the benzylic position generated a hemiaminal intermediate A, which led to C−N cleavage and ring opening. The ensuing aminyl radical B was subsequently added on the arene and sequential decarboxylation−oxidation furnished the ring-contracted product (see Scheme 171).

Scheme 169. Proposed Mechanism for Oxytrifluoromethylation of Alkenesa

a

Reprinted with permission from ref 212. Copyright 2014, Royal Society of Chemistry, London. 5134

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species to Ir(IV), which ultimately oxidized the advanced intermediate to product and completed the catalytic cycle. In the same year, Shah reported a visible-light-promoted formation of a Csp3−Csp2 bond at room temperature employing ethers and electron-deficient arenes (see Scheme 173).217 This reaction could be regarded as a milder alternative to a previously reported Minisci-type reaction in the presence of K2S2O8.172 Another interesting feature was that this protocol obviated the need of any acid, which was usually required to affect the protonation of heteroaromatics. In this case, an electron donor− acceptor (EDA) complex was proposed to form between arene and peroxysulfate anion (S2O8−2), which dissociated to the sulfate radical anion (SO4− •) in the presence of visible light. Other steps in the mechanism were very similar to earlier examples.172 Xia developed a visible-light-promoted intermolecular crossdehydrogenative coupling amination method between phenols and cyclic anilines in the presence of K2S2O8.218 Although several phenol substrates could be used for this reaction, the choice for aryl amine was restricted to phenothiazine (E° = 0.78 V), which could be oxidized to a N-radical form. Partner phenols used herein had similar oxidation potentials and, thus, afforded the

Scheme 170. Tandem Oxidative Conversion of 10,11Dihydro-5H-dibenzo[b,e][1,4]diazepines to Phenazinesa

a

Reprinted with permission from ref 213. Copyright 2014, American Chemical Society, Washington, DC.

phosphine oxides and that was also the crucial step for this transformation. K2S2O8 converted the excited-state Ir(III)*

Scheme 171. Proposed Mechanism for Oxidative Removal of Benzylic Methylene Group and New Bond Formations to Phenazinesa

a

Reprinted with permission from ref 213. Copyright 2014, American Chemical Society, Washington, DC.

Scheme 172. Cascade Reactions to Form C−C and C−P Bond Formation with Visible Lighta

a

Reprinted with permission from ref 216. Copyright 2016, American Chemical Society, Washington, DC. 5135

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made impressive progress in recent years. This review highlights those oxidative transformations that have been carried out either in the presence or absence of a transition-metal catalyst and using K2S2O8 as either terminal or primary oxidant. Many new catalytic systems involving a palladium catalyst and K2S2O8 have been realized since the development of Minisci reaction and selected examples have been included in this review. Overall, this review provides a comprehensive coverage of different classes of transformations with K2S2O8 and relevant mechanisms. For silver-catalyzed processes, the majority of reactions proceed through decarboxylative generation of radical species and their subsequent reactions deliver the desired products. In silvermediated processes, however, the radicals are generated by oxidation of the counteranions of the silver salts. For the palladium-catalyzed reactions with K2S2O8, the major proportions involve the directing-group-assisted arene C−H functionalizations. Notably, other types of Pd-catalyzed oxidative processes, viz, hydroamination, hydroalkoxylation, and carbocyclization, have not been significantly explored with this oxidant. The use of K2S2O8 as a compatible oxidant in Rh and Ru-catalyzed transformations have begun only in recent years. Apart from metal-catalyzed transformations, a wide range of transformation with different types of bond formation is achieved with K2S2O8 as the sole oxidant. Recently, their uses in visiblelight and photoredox-catalyzed transformations has been realized. Even with the widespread use of K2S2O8 in oxidative transformations, in many cases, its exact role in the mechanism could be a matter of debate and has not been thoroughly addressed with experiments. For example, when K2S2O8 is used as the terminal oxidant in the presence of a metal catalyst, regardless of whether S2O82− or SO4• −, or both, are responsible for the oxidation of the metal, is not very clear. Furthermore, in some metal-catalyzed transformations, oxidation of intermediates to product (e.g., aromatization of an aromatic radical

Scheme 173. C−H Functionalization of Ethers and ElectronDeficient Arenes with Visible Lighta

a

Reprinted with permission from ref 217. Copyright 2016, Royal Society of Chemistry, London.

desired products through radical−radical coupling. Such controlled reactivity of K2S2O8 for generating two radical species subsiding undesired homocoupling reactions was quite interesting and could open new avenues for aromatic C−N bond-forming reactions. Recently, Molander reported coupling between 2-trifluoroborato-4-chromanones and heteroarenes with an organic photoredox catalyst, mesitylacridinium perchlorate in the presence of K2S2O8 and a 25W CFL (see Scheme 174).219 This reaction featured the Minisci reaction with broader applicability when combined with a photoredox catalyst. Using a metal photoredox catalyst, Ravelli and Ryu disclosed a cross-dehydrogenative coupling of heteroarenes with hydrogen donors such as cyclic alkanes, cyclic ethers, formamides, aliphatic aldehydes, cyclic ketones, etc.220 In this reaction, tetrabutylammonium decatungstate was used as the metal photocatalyst with K2S2O8 as an oxidant. This protocol broadened the substrate scopes for Minisci-type reactions. Using Ru(bpy)3Cl2 as a photocatalyst and K2S2O8 as an oxidant, Cho developed an oxidative C−N bond cleavage protocol.221 Using this method, different primary secondary and tertiary amines were converted to the corresponding carbonyl compounds.

4. CONCLUSIONS Oxidative transformations leading to C−C or C−X bond formation belong to an important class of reactions that has

Scheme 174. Heteroarylation 2-Trifluoroboratochromanones with Photoredox Organocatalysisa

a

Reprinted with permission from ref 219. Copyright 2017, American Chemical Society, Washington, DC. 5136

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ACS Catalysis species) could proceed with either S2O82− or SO4• −, or by metal species in higher oxidation states. Similar questions could also arise for some metal-free transformations, where K2S2O8 is used as a primary oxidant, or in those cases where K2S2O8 is used in combination with another nonmetal catalytic oxidant. More mechanistic investigations, such as those exemplified using the mechanistic studies of Patel and Flowers25 would be needed to expand the scope and utility of many such transformations outlined in this Review. Nonetheless, based on the current standing on the use of this oxidant in various transformations, a resurgence of new methodologies are quite expected in near future and this can be accelerated further with a thorough understanding of mechanistic pathways.



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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] *E-mail: [email protected] ORCID

Joydev K. Laha: 0000-0003-0481-5891 Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS Financial support was provided to J.S. by SERB, New Delhi (No. ECR/2016/001474) and Centre of Biomedical Research, Lucknow. J.K.L. also thanks SERB, New Delhi for financial support.



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