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


Apr 13, 2018 - Because of the large volume of the literature available with this oxidant, only important examples showcasing varieties of bond formati...
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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 ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00743 • Publication Date (Web): 13 Apr 2018 Downloaded from http://pubs.acs.org on April 13, 2018

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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 §*



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

Pharmaceutical Education and Research, S. A. S. Nagar, Punjab 160062, India

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Institute

of

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Abstract: Carbon-carbon/carbon-heteroatom bond formation via oxidative transformation presents one of the state-of-the-arts at the frontiers 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. The organic chemists could find this review as a useful guide for the expedient synthesis of new chemical entities, to formulate new mechanistic manifolds involving 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

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1. Introduction Carbon-carbon (C-C)/carbon-heteroatom (C-X) bond formation between two C-H/C-H bonds or CH/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-metal catalyzed oxidative transformations present one of the state-of-the-art 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 applications ranging from laboratory experiments to industrial processes. K2S2O8, as a cheap, 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 family of compounds like 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 sulphate radical anion (SO4

) under mild

conditions. Moreover, transition-metal ions are also able to activate K2S2O8 to form SO4 SO4

. The

is regarded as a very strong one-electron oxidant with a redox potential of 2.5-3.1 V. 4

It has a longer life-time (~4 s) than hydroxyl radicals, primarily due to 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 metal catalyzed and metal free organic transformations. Not only its substantial uses in palladium catalysis is achieved after Minisci’s work, but also different unconventional oxidative transformations have been realized solely with this oxidant and without adding any metal catalyst. It is noteworthy that, other variants like Na2S2O8, (NH4)2S2O8 are far less commonly used compared 3

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to K2S2O8. This can be attributed to better solubility of potassium salt in organic solvent rendering efficient transformation. Range of transformations that are achieved with K2S2O8 in metal-catalyzed and metal free processes covers C-C to C-X (X = N, O, S, P, B, Si, F, Br, I) bond forming reaction across 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 on 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 uses Na2S2O8 or (NH4)2S2O8 is beyond the scope of this review. Due to a 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 metal catalyzed 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 Ag-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 counter anion 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, DDQ were screened during optimization along with peroxydisulfates. 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 (1.98V) during the 4

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course of the reaction, which is the key step in most Ag(I) catalyzed or 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 peroxydisulfate ion, S2O82(2.01 V) or sulphate radical anion, SO4 2-

proceed with both, S2O8 and SO4

(2.5-3.1 V). While the oxidation of Ag(I) to Ag(II) could , the later species is likely to be involved at an elevated

temperature. It is important to mention that, although the oxidizing ability of SO4

is comparable

to that of hydroxyl radical, the former is much selective, and thus can afford specific oxidative transformations.7 A large volume of the Ag-catalyzed reactions proceed through decarboxylative generation of a radical species. The Ag(I)/K2S2O8 combination is suitably poised for Ag-catalyzed reactions, as SO4

species selectively oxidizes Ag(I) to Ag(II) without directly effecting 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 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 counter anion of the salts are incorporated into the final products. In these reactions, the counter anion of the Ag-salt is transformed into 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 medium. In some cases, when Na2S2O8 or (NH4)2S2O8 gave comparable yields to K2S2O8 in the optimization, the author preffered 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 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 first bond formed in the sequence. 2.2.1 C-C bond formation. Silver catalyzed decarboxylative methods are among the most popular means to effect direct C-H functionalizations. Due to 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 silver5

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catalyzed decarboxylative method for acylation of pyridine N-oxides with α-oxocarboxylic acids (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. R1

R1

O + R2

CO2H

N O

Ag2CO3,K2S2O8 DCM/H2O 500 C,12 h

R2 N O

O

Scheme 1. Ag-catalyzed decarboxylative acylation of pyridine-N-oxides with α-oxocarboxylic acids. Reprinted with Permission from Ref 8. Copyright 2014 Royal Society of Chemistry

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, αketo acids and K2S2O8 were used (Scheme 2).9

Scheme 2. Ag-mediated radical cyclization of alkynoates with α-keto acids. Reprinted with Permission from Ref 9. Copyright 2015 American Chemical Society

Different N-alkyl-N-aryl cinnamamides or acrylamides were strategically used in analogous cascade radical cyclization processes with acyl or alkyl carboxylic acids (Scheme 3).

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R2 R R1 N

O

R1

R3

AgNO3 (20 mol%) K2S2O8 (3 eq)

N

1000C, 12h CH3CN/H2O (1:1)

O

R2

CF3SO2Na

RCOOH AgNO3 (20 mol%) K2S2O8 (2 eq)

R2

R3

CF3 (a)

R1

0C,

100 12h CH3CN/H2O (1:1)

N

O

R3

-----------------------------------------------------------------------------------------------------------------------------------------------------------------------

R3

O

R1

O X

N

R3 +

R2

OH

R4 O

X=C,N

O

AgNO3 (20 mol%) K2S2O8 (4 eq) CH3CN/H2O (1:1) 100 0C, 12 h

R4

R1 X

N

(b)

O

R2

----------------------------------------------------------------------------------------------------------------------------------------------------------------------O O

O

R1

+

N R2

R3

OH

Ph O

AgNO3(10 mol%) K2S2O8(1 eq)

R1

Ph

R3

(c)

H2O, 50 0C N R2

O

Scheme 3. Ag-catalyzed tandem cyclization of N-arylcinnamides/ acrylamides with alkyl or acyl radical. Reprinted with permission from refs 10-11, and 12a. Copyright 2014 American Chemical Society; 2016 American Chemical Society; and 2013 Wiley-VCH

For example, Mai, Sun, Xiao and co-workers developed one such tandem radical cyclization method with N-arylcinnamides and alkylcarboxylic acids using Ag(I)/K2S2O8 (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 alkyl carboxylic acid to αketoacids, 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, Duan and coworkers reported the access to oxindoles, when substituted acrylamides were reacted with α-keto acids in presence of Ag(I)/K2S2O8 (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 regio-selectivity of the initial radical addition step. It is important to note that, not only the alkyl radicals from carboxylic acids were participated in such addition-cyclization reactions, 1,3-dicarbonyls were also employed in the same fashion. For the later case, the key radical species was generated through methylenic C(sp3)-H bond cleavage in 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 spiro-cyclic 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 spiro-cyclic compounds. Li and co-workers utilized a non7

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conjugated 1,7-enyne system and reaction with secondary alkyl carboxylic acids provided a different class of spirocyclic scaffold (Scheme 4).14 These examples demonstrated the utility of Ag(I)/K2S2O8 system in generating complex molecular scaffolds through sequential radical addition-cyclization steps.

Scheme 4. Ag-catalyzed C(sp3)-H bond cycloalkyaltion with neighbouring carboxylic acid group. Reprinted with Permission from Ref 14. Copyright 2017 American Chemical Society

The foregoing discussion on the silver catalyzed oxidative radical addition-cyclization reactions (Scheme 3-4) followed the general mechanism as outlined in Scheme 5. In all 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

hydrogen

abstraction-radical

sequence.13,14

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addition-oxidation

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Scheme 5. Proposed mechanism for Ag-catalyzed radical addition-cyclization reactions with alkynoates and acrylamides.

In 2015, Zhang and Lei developed a silver-catalyzed method to prepare oxazoles from αoxocarboxylates and isocyanides via decarboxylative addition-cyclization cascade in presence of K2S2O8 (Scheme 6).15 This reaction was proposed to proceed with the intermediacy of an acyl cation, which is rarely postulated in the literature.

Scheme 6. Ag-catalyzed oxidative cyclization of α-oxocarboxylate and isocyanide. Reprinted with Permission from Ref 15. Copyright 2015 Royal Society of Chemistry

In 2012, Li and co-workers reported an efficient silver-catalyzed decarboxylative alkynylation method of different aliphatic carboxylic acids using phenylethynylbenziodoxolone as electrophilic alkynylating agent (Scheme 7).16a The overall 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 loading 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 so, when functionalized with different groups such as amide, carbonyl, halide, ether etc. and offers a mean for chemoselective alkynylation between an alkyl and aromatic carboxylic acids. 9

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Scheme 7. Li’s Ag-catalyzed decarboxylative alkynylation of aliphatic carboxylic acids. Reprinted with Permission from Ref 16a. Copyright 2012 American Chemical Society

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 later 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, alkyl radical combined with the chlorine or fluorine atom 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.

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Scheme 8. Proposed mechanism for decarboxylative alkynylation. Reprinted with Permission from Ref 16a. Copyright 2012 American Chemical Society

Hashmi group reported a similar silver-catalyzed alkynylation strategy in presence of K2S2O8.17 In this case, they used α,α-difluoroarylacetic acid and synthesized different fluorinated alkynes. Chen, Zhao 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 (Scheme 9).18 K2S2O8 provided the best result (96%) compared to Na2S2O8 (51%) while other oxidants such as H2O2, TBHP, DTBP gave no reaction. One important feature is that, unlike Minisci reaction, this transformation occurred under acid free condition at room temperature. Under such condition, generation of 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 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.

Scheme 9. Zhao’s Ag-catalyzed decarboxylative C2-alkylation of benzothiazoles. Reprinted with Permission from Ref 18. Copyright 2014 Royal Society of Chemistry

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 medium. A radical addition-oxidation sequence was proposed in the report. Song, Jiao and co-workers reported an efficient Ag-catalyzed method for decarboxylative cross coupling of aliphatic carboxylic acids with sulfonyl oxime ethers to provide corresponding alkyl oxime ether derivatives (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 11

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mixture of CH3CN/H2O (1:1) at 50 oC. 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 sulfonyl radical delivered the product.

Scheme 10. Ag-catalyzed conversion of aliphatic carboxylic acids to oxime ethers. Reprinted with Permission from Ref 20. Copyright 2016 American Chemical Society

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, 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 presence of Ag(I)/K2S2O8 (Scheme 11).22a 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.

Scheme 11. Li’s Ag-catalyzed decarboxylative allylation of aliphatic carboxylic acids. Reprinted with Permission from Ref 22a. Copyright 2016 American Chemical Society

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 presence of Ag(I)/ K2S2O8 (Scheme 12).22b In a previous report (see in section 2.3), they demonstrated the use of this reagent in a reductive trifluoromethylation process. By tuning the reaction condition and adding 12

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ZnMe2, they were able to use it in conjunction with decarboxylative generation of alkyl radical in 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 into 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.

R COOH

AgNO3 (cat)/K2S2O8 (bpy)Cu(CF3)3, ZnMe2 CH3CN/H2O, 40 oC, 10 h

R-CF3

Scheme 12. Li’s Ag-catalyzed decarboxylative trifluoromethylation of aliphatic carboxylic acids. Reprinted with Permission from Ref 22b. Copyright 2017 American Chemical Society

In the examples shown above, alkyl and acyl carboxylic acids were used in various decarboxylative coupling processes. On the contrary, aryl carboxylic acids have found limited applications in these reactions. This is primarily due to slow release of CO2 from the aroyloxy radical, which often leads to inefficient generation of aryl radical species. Some studies however, 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 radical in acetonitrile solvent. Since the reaction was only efficient in acetonitrile, deuterated acetonitrile was chosen in order to suppress the side reaction. Such requirement however would limit its practical applications.

Scheme 13. Greaney’s Ag-catalyzed decarboxylative radical cyclization towards fluorenones. Reprinted with 13

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Permission from Ref 23a. Copyright 2012 American Chemical Society

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 using Minisci’s condition, they developed a novel and operationally simple method for arylation of a wide range of electron-deficient heterocycles at ambient temperature (Scheme 14).24 TFA (1 equiv) AgNO3 (0.2 equiv) K2S2O8 (3 equiv)

R1 (HO)2B N

+

R2

Me

Me

Me

N N

N 96% (1:2 C2:C4)

68% (2:1 C2:C4)

R2

N

DCM/H2O (1:1) rt, 3-12 h

N

N

R1

t Bu

N

58% (1:3 C3:C4)

R R=NO2, 45% R=CF3, 37%

N

N

N

OH MeO

Me

40%

OPh

N

N

R

N

N

Me

Me

Me

Isoquinoline, 33% R=NO2, 43%

50%

51%

Scheme 14. Baran’s Ag-catalyzed arylation of electron deficient heterocycles with arylboronic acids. Reprinted with Permission from Ref 24. Copyright 2010 American Chemical Society

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, Flowers and co-workers reported a thorough mechanistic study on the transformation reported by Baran group, which revelead interesting insights (Scheme 15).25 They proposed a pre-equlibrium 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

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Scheme 15. Flower’s proposed mechanism for arylation of electron deficient heterocycles withAg(I)/K2S2O8. Reprinted with Permission from Ref 25. Copyright 2013 American Chemical Society

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 (Scheme 16).26 Importantly, the author screened various oxidant and catalyst combinations, which all failed for the reaction except Ag(I)/K2S2O8 suggesting a unique role played by these reagents. Reductive dissociation of aryl triazenes is known, that generates aryl radical in presence of acid and catalytic metal salts.27a-b Based on these precedents, the author proposed that SO42- ion present in the reaction medium served as the one electron reductant, whereas Ag(II) species was involved in other oxidative steps of the transformation. Such proposal on the Ag(I)/K2S2O8 system playing the roles for both oxidant and reductant was quite unique. On the contrary, an oxidative decomposition pathway could be also possible27c and could necessitate additional mechanistic investigations.

Scheme 16. Wang’s C-H arylation of electron-deficient heteroarenes with triazenes. Reprinted with Permission from Ref 26. Copyright 2014 Royal Society of Chemistry

Other than decarboxylative processes, generation of alkyl radicals from other precursors and their subsequent uses in 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 addition-oxidation sequence 15

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(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 K2S2O8, the author opted to use Ag(I)/K2S2O8 combination for its shorter reaction time and better selectivity for the monosubstituted product.

Scheme 17. Ag-catalyzed C-H functionalization of quinones with cyclopropanols. Reprinted with Permission from Ref 28. Copyright 2013 American Chemical Society

Bao and Zhu demonstrated a Ag-catalyzed ring-expansion method of cyclobutanol in an intramolecular setting towards 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.

Scheme 18. Li’s Ag-catalyzed C-2 functionalization of heteroarenes with tertiary cycloalkanols. Reprinted with Permission from Ref 29b. Copyright 2017 Royal Society of Chemistry

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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

18

O2 and XPS studies

supported the Ag(I)/Ag(0) catalytic cycle. This mechanistic scenario was certainly different from the one 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 2.2.2 C-N bond formation. Although K2S2O8 has found multitude applications as oxidant in silver catalyzed C-C bond forming 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 (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. 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). It was however difficult to assess why Ag(I)/K2S2O8 suited best for this conversion. O R1

COOH + R2

R4 N R3

CCl4:DMF (4:1),120

R4 N

O

Ag2CO3 (20 mol%) K2S2O8 (2 equiv) oC

R1

R3

O

Scheme 19. Wang’s Ag-catalysed amidation of benzoylformic acids with tertiary amines. Reprinted with Permission from Ref 32. Copyright 2013 Royal Society of Chemistry

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 presence of Ag(I)/K2S2O8. 17

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Scheme 20. Ag-mediated functionalizations of quinoxalines to related nitro and nitrile derivatives. Reprinted with Permission from Ref 33. Copyright 2014 Royal Society of Chemistry

In 2015, Li and co-workers reported an elegant one step method to synthesize various alkyl azides through silver catalyzed decarboxylative radical azidation process.34a Here they used alkyl carboxylic acids and sulfonyl azides with catalytic AgNO3 (20 mol %) and K2S2O8 (Scheme 21). Pyridine-3-sulfonyl azide and TsN3 were used as azide sources. The method provided a broad substrate scope for carboxylic acids.

Scheme 21. Ag-catalyzed conversion of aliphatic carboxylic acids to azides via decarboxylation. Reprinted with Permission from Ref 34a. Copyright 2015 American Chemical Society

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Mechanistically, the alkyl radical generated via decarboxylation with Ag(I)/K2S2O8 made a nucleophilic attack to sulfonyl azide and delivered the product. Song, Jiao and co-workers reported a similar decarboxylative azidation of carboxylic acids using AgF as catalyst and PhSO2N3 as azide source.34b 2.2.3 C-S bond formation. Different C-S bond forming reactions were also reported in literature with stoichiometric silver salts and K2S2O8. Majority of these included trifluoromethylthiolation and sulfonylation reactions. Synthetic methods for trifluoromethylthiolation reaction have received significant attention due to 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 lowers the redox potential of higher valent 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 aliphatic carboxylic acids and an electrophilic trifluoromethylthiolating agent (T) instead of AgSCF3 (Scheme 22).35 An aqueous emulsion of 20 mol % sodium dodecyl sulphate (SDS) was used as additive which minimized the over oxidation 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. AgNO3 (30 mol%) nC12H25SO3Na (20 mol%)

Me R COOH +

O SCF3 Me Ar=2-iodophenyl (T) Ar

K2S2O8 (1 equiv) H2O

R SCF3

Scheme 22. Shen’s Ag-catalyzed decarboxylative trifluoromethylthiolation of aliphatic carboxylic acids. Reprinted with Permission from Ref 35. Copyright 2014 Wiley-VCH

Scheme 23. Proposed mechanism for trifluoromethylthiolation of carboxylic acids with Ag(I)/K2S2O8. Reprinted with Permission from Ref 35. Copyright 2014 Wiley-VCH

Coumarin-3-carboxylic acids and their thio and nitrogen analogues were converted into the corresponding trifluoromethylthiolated products with AgSCF3 and K2S2O8 (Scheme 24).36 Although 19

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(NH4)S2O8 gave a slight better yield than K2S2O8 during reaction optimization, the later was used in the subsequent substrate screening.

Scheme 24. Ag-mediated conversion of coumarin-3-carboxylic acids into related trifluoromethylthiolated compounds. Reprinted with Permission from Ref 36. Copyright 2017 American Chemical Society

Some trifluoromethylthiolation reactions were based on the addition of F3CS· radical to unsaturated systems. For example, Cao reported hydrotrifluoromethylthiolation of terminal alkynes that gave either anti-Markovnikov or Markovnikov-selective adduct depending upon the employed reaction condition (Scheme 25).37 Anti-Markovnikov selective product was achieved by employing 1.5 equiv. AgSCF3, 3 equiv. K2S2O8 along with 50 mol% HMPA and 10 mol% 1,10-phenanthroline as 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 oxidant. However, (NH4)S2O8 afforded lower yield compared with the former.

Scheme 25. Ag-mediated selective hydrotrifluromethylthiolation of terminal alkynes. Reprinted with Permission from Ref 37. Copyright 2016 American Chemical Society

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 presence of 1.5 equiv. AgSCF3, 3 equiv. K2S2O8 and 2 equiv. K3PO4 as base (Scheme 26).38 The reaction delivered tetrasubstituted α,β-unsaturated aldehydes in good yields. In the mechanistic course, K2S2O8 generated F3CS· radical, which added onto alkyne to give vinyl radical A. A spiroradical intermediate B was formed next via 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 20

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migration affording allyl alkoxy radical species C. Finally, hydrogen atom abstraction by sulfate radical anion (SO4−•) furnished the product (Scheme 27).

Scheme 26. A radical 1,4-aryl migration- trifluoromethylthiolation cascade reaction from aryl propynyl ethers. Reprinted with Permission from Ref 38. Copyright 2017 Wiley-VCH

Scheme 27. Proposed mechanism for 1,4-aryl migration- trifluoromethylthiolation cascade. Reprinted with Permission from Ref 38. Copyright 2017 Wiley-VCH

Liu recently reported a AgSCF3-mediated trifluoromethylthiolation of α,α-diaryl allylic alcohols that proceeded through radical neophyl rearrangement in presence of K2S2O8 (Scheme 28).39 This protocol rendered various α-aryl-β-trifluoromethylthiolated carbonyl compounds.

Scheme 28. Trifluoromethylthiolation of α,α-diaryl allylic alcohols with Ag(I)/ K2S2O8. Reprinted with Permission from Ref 39. Copyright 2017 Royal Society of Chemistry

It has been discussed in the previous section (2.1.1) that acrylamides or related structures were suitable for developing radical addition-cyclization cascades. In 2014, Wang and co-workers demonstrated 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 like PhI(OAc)2, TBHP, NFSI, Ce(SO4)2.4H2O and (NH4)2S2O8. This could be correlated to better ability of K2S2O8 in efficiently oxidizing AgSCF3 into the radical species.

Likewise,

Liu

and

co-workers

used

substituted

propiolamides

to

obtain

trifluoromethylthiolated azaspirocycles (Scheme 29a).41a Both K2S2O8 (3.0 equiv) and TBHP (5.0 21

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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 para-position 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 trifluoromethylthiolation-cyclization cascade on 1,6-enyne systems affording tricyclic scaffolds (Scheme 29b).42 K2S2O8 (3 equiv) TBHP (5 equiv) HMPA (0.5 equiv)

R2 + AgSCF3

R3 N R1

O

X

SCF3 (a)

O N R3 R 1

CH3CN, 80 oC, 12 h

AgSCF3 (1.5 equiv) K2S2O8 (3 equiv) HMPA (0.5 equiv) Ligand (10 mol%)

Y

R2

R

80 oC, 12 h

O

Y X

(b) SCF3

X=NTs, C(CO2Me)2 Y = CH2 X = O, Y = carbonyl L=

N N

N

Scheme 29. Ag-mediated alkyne trifluoromethylthiolation and cascade cyclizations towards SCF3incorporated compounds. Reprinted with Permission from Ref 41a and 42. Copyright 2016 American Chemical Society; 2015 American Chemical Society

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 as TBHP, PhI(OAc)2, BI-OH, 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.

Scheme 30. Ag-mediated process for Trifluoromethylthiolation of cycloalkanols. Reprinted with Permission from Ref 42. Copyright 2015 American Chemical Society

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 22

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sodium arylsulfinates with the aid of oxidants like 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 silver-catalyzed synthesis of sulfonylated lactones using 1,6-enynes with sodium sulfinate in presence of K2S2O8 (Scheme 31).45a This method gave access to a wide ranges of fivemembered sulfonylated lactones with exocyclic double bond and a quaternary center. In the same year, this group further reported the synthesis of sulfonylated benzofurans capitalizing on similar Ag-catalyzed oxidative cyclization strategy (Scheme 32).45b

Scheme 31. Ag-catalyzed synthesis of sulfonylated lactones via cascade cyclization of 1,6-enynes. Reprinted with Permission from Ref 45a. Copyright 2017 American Chemical Society

Scheme 32. Ag-catalyzed access to sulfonylated benzofurans from 1,6-enyne. Reprinted with Permission from Ref 45b. Copyright 2017 American Chemical Society

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 presence of catalytic AgNO3 (10 mol%) and K2S2O8 (1 equiv) to deliver sulfonylated monofluoromethylthioethers (Scheme 33).46

Scheme 33. Monofluoromethylthiolation of aryl boronic acids with S-(floromethyl)benzenesulfonothioate. Reprinted with Permission from Ref 46. Copyright 2017 Wiley-VCH

A silver-catalyzed one-pot route to β-ketosulfones via aerobic oxysulfonylation of alkenes with thiophenols was reported by Yadav and co-workers in presence of K2S2O8 (Scheme 34).47 Controlled experiments revealed that Ag(I) was responsible for generating the thiyl radical from 23

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thiophenol and oxygen incorporation into the carbonyl functional group occurred from the air present in the reaction. O

AgNO3 (20 mol%)

R1 R2

+ HS R3 + O2

K2S2O8 (3 equiv) DMF, rt

R1

O S 3 R O R2

Scheme 34. Ag-catalyzed oxysulfonylation of alkenes with thiophenols. Reprinted with Permission from Ref 47. Copyright 2014 Royal Society of Chemistry

Analogous to β-ketosulfide oxidation to corresponding sulfone, Hajra et al. also used Ag(I)/K2S2O8 combination to transform benzo[b][1,4]thiazines into 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 (Scheme 35).49

R COOH + Ar

S

S

AgNO3 (1 equiv) K2S2O8 (3 equiv)

Ar

R

CH3CN/H2O (1:1,2 ml) Ar, 60 oC, 24 h

S

Ar

Scheme 35. Ag-mediated decarboxylative C-S cross-coupling between aliphatic carboxylic acids and diaryl disulfides. Reprinted with Permission from Ref 49. Copyright 2014 American Chemical Society

Jana and co-workers developed a silver-catalyzed carboxyl radical assisted 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 (Scheme 36).50 The carboxyl radical (A) was formed in 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 formation of a spiro radical intermediate B (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.

Scheme 36. Ag-catalyzed 1,5-aryl migration through Smiles rearrangement. Reprinted with Permission from Ref 50. Copyright 2016 Royal Society of Chemistry

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Scheme 37. Proposed mechanism for Ag-catalyzed carboxyl radical assisted 1,5-aryl migration. Reprinted with Permission from Ref 50. Copyright 2016 Royal Society of Chemistry

2.2.4 C-P bond formation. Oxidative phosphonylation of aromatics compounds was thoroughly studied in early 80’s using anodic oxidation as well as by 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)/S2O82combination was found to be indespensible for the reaction with diethylphosphite, whereas Fe(II), Cu(I) or combination like Ag(I)/Cu(II) or Fe(II)/Cu(II) showed poor efficiency. Dialkyl Hphosphonate (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 (Scheme 38). The hydrogen abstraction pathway from dialkyl H-phosphonate (A) was proposed to be less likely. Reaction of phosphite radical with different substrtes 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.

Scheme 38. General mechanism for the generation of phosphite radical with Ag(I)/K2S2O8.

In 2012, Huang reported silver(I) catalyzed coupling of differnet heteroarenes and phosphite in 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 25

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para-acylphenylphosphonates (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 effect. 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 amide group. In all these aromatic phosphonation reactions, Ag(I)/K2S2O8 combination worked most efficiently in the reaction optimization step. O

R2

R1

+ H

O H P OR3 OR3

Ag2O (5 mol%) K2S2O8 (2 equiv)

O R1

CH3CN/H2O,100 0C

R2 OR3 3 OR P O

Scheme 39. Dehydrogenative cross-coupling between aromatic aldehydes and ketones with dialkyl Hphosphonates. Reprinted with Permission from Ref 53b. Copyright 2015 Royal Society of Chemistry

Tan and co-workers reported a silver catalyzed stereoselective synthesis of vinylphosphonates employing styrenes and dialkyl H-phosphonates in presence of K2S2O8 (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 additive to achieve the desired (E)-vinylphosphonates. Reaction between Ph2P(O)H and silver salt provided Ph2P(O)Ag which was subsequently converted into phosphate radical and released Ag(0) (Scheme 41). Addition of the phosphate radical to styrene delivered the desired product and K2S2O8 oxidized Ag(0) to Ag(I) to restore the catalytic cycle.

Scheme 40. Ag-catalyzed phosphorylation of styrenes towards vinylphosphonate synthesis. Reprinted with Permission from Ref 54. Copyright 2015 Royal Society of Chemistry

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Scheme 41. Proposed mechanism for Ag-catalyzed phosphorylation of styrenes. Reprinted with Permission from Ref 54. Copyright 2015 Royal Society of Chemistry

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 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 radical and itself was reduced to β-ketophosphonate product (Scheme 43).

R1

R2

O R3 PH + O + 2 R4

CuSO4.5H2O/AgNO3 K2S2O8 (4 equiv)

O R1

CH2Cl2:H2O (1:1), r.t, 2-3 h

R2

O 3 P R R4

Scheme 42. Ag-Cu-catalyzed oxyphosphorylation of alkynes with H-phosphonates. Reprinted with Permission from Ref 55. Copyright 2015 Royal Society of Chemistry

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Scheme 43. Proposed mechanism for Ag-Cu-catalyzed synthesis of β-ketophosphonates. Reprinted with Permission from Ref 55. Copyright 2015 Royal Society of Chemistry

Another silver-catalyzed reaction between 1,3-dicarbonyl compounds and H-phosphonates was reported by Dong, Ma and Peng (Scheme 44).56 In this case, reaction proceeded with the phosphite radical addition to the enol form of dicarbonyls.

Scheme 44. Ag-catalyzed oxidative C(sp3)-P bond formation between 1,3-dicarbonyls and H-phosphonates. Reprinted with Permission from Ref 56. Copyright 2017 Wiley-VCH

Li and co-workers disclosed a straightforward approach to synthesize different phosphonate chroman-4-ones with Ag(I)/K2S2O8 (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 yield of the products.57b-f These reports could collectively support the coupling of two radicals proposed in the mechanism.

Scheme 45. Cascade radical cyclization towards the synthesis of chroman-4-ones with Ag(I)/K2S2O8. 28

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Reprinted with Permission from Ref 57a. Copyright 2016 Royal Society of Chemistry

2.2 Palladium catalyzed transformations

Since the development of Wacker process, use of various oxidants for the maintenance of Pd(0)Pd(II) catalytic cycle had become a practice.58 It is apparent from the literature that the efficacy of a reaction could be seriously affected depending upon 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 oxidant in palladium catalyzed transformations, use of K2S2O8 in palladium catalyzed reaction could be traced back to early 1980’s 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 bond forming reactions. In the majority of reactions discussed herein, product formation occured through reductive elimination path, which was associated with a significant redox change at the Pd centre. In these examples, reductive elimination of palladium from a PdII(Ar)(Aŕ) 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) so as 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 etc. Since many of these oxidants are expensive and toxic, there has been a growing interest to find a cheaper 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 with Pd(II)/Pd(0) catalytic cycle. Peroxydisulfates, on the other hand, could also meet these criteria and both 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 found superior to other oxidants. It’s precise role in the mechanism however remained illusive in many reported transformations and therefore demands more detailed investigations. For example, it is not clear whether S2O82- or SO4

or both are responsible for the oxidation of palladium. Some general

considerations are the following; (a) for decarboxylative acylation reactions where K2S2O8 was used as the sole oxidant, controlled experiments supported for a Pd(IV)/Pd(II) catalytic cycle. The reaction course could differ in cases when K2S2O8 was used as 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 29

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cycle whereas, 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 valent 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 2phenylpyridines with different α-oxocarboxylic acids (Scheme 46).61

Scheme 46. Ge’s Pd-catalyzed decarboxylative acylation of 2-arylpyridines with α-oxocarboxylic acids. Reprinted with Permission from Ref 61. Copyright 2010 American Chemical Society

In 2013, Zhu and co-workers reported a Pd(II) catalyzed method for C2-acylation of 1-(pyrimidin2-yl)-1H-indole with arylglyoxylic acids in presence of Ag2O and K2S2O8 (Scheme 47).62 In this case also, K2S2O8 was used as the co-oxidant. The pyrimidine moiety assisted as directing group in this reaction, which could be removed afterwards 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 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 co-oxidant.

Scheme 47. Zhu’s Pd-catalyzed decarboxylative C-2-acylation of indoles. Reprinted with Permission from Ref 62. Copyright 2013 Royal Society of Chemistry

Ortho-acylation of symmetrical and unsymmetrical azobenzenes with α-oxocarboxylic acids in presence of Pd(II) catalyst and K2S2O8 as oxidant was reported by Wang (Scheme 48).64a-b K2S2O8 provided the best yield compared to other oxidants like Na2S2O8 , (NH4)2S2O8 , oxone, Cu(OAc)2, BQ, TBHP, PhI(OAc)2. This method complements an earlier report on acylation of azobenzenes using aldehydes as the acylating agents.64c Azo-group participated as the directing group in this 30

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reaction.

Scheme 48. Pd-catalyzed synthesis of ortho-acylated azobenzenes with α-oxocarboxylic acids. Reprinted with permission from refs 64a-b. Copyright 2013 American Chemical Society; and 2013 Wiley-VCH

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 into 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 coworkers (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, presence of an oxygen-atom in the cyclic enamide was crucial for efficiently promoting the decarboxylative acylation step.66

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Scheme 49. Pd-catalyzed decarboxylative acylation of cyclic enamides. Reprinted with Permission from Ref 66. Copyright 2012 American Chemical Society

A palladium-catalyzed ortho-acylation method for 2-(tert-butyl(phenyl)-phosphoryl)biphenyls was developed by Yang and co-workers (Scheme 50).67 The R2(O)P group acted as directing group and participateed 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).

Scheme 50. Yang’s Pd-catalyzed C(sp2)-H acylation of 2-phosphorylbiphenyls with α-oxocarboxylic acids. Reprinted with Permission from Ref 67. Copyright 2014 Royal Society of Chemistry

In 2016, Cui, Wu and co-workers disclosed an efficient C8 selective acylation method of quinoline N-oxide via palladium(II)-catalyzed process using α-oxocarboxylic acids in presence of K2S2O8 (Scheme 51).68a A five-membered palladacycle 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 32

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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 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) intermediate.68b

Scheme 51. Pd-catalyzed C8-selective acylation of quinoline N-oxides. Reprinted with Permission from Ref 68a. Copyright 2016 American Chemical Society

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 pre-derivatization of ketone to a Ndirecting group. Based on controlled experiments, a radical mechanism was proposed for this transformation.

Scheme 52. Direct Pd-catalyzed decarboxylative acylation of aromatic ketones. Reprinted with Permission from Ref 69. Copyright 2017 American Chemical Society

Ge and co-workers reported cross-coupling reactions of potassium aryltrifluoroborates with αoxocarboxylic acid or oxamic acid or potassium oxalate monoester in presence of Pd(II)/K2S2O8 system (Scheme 53).70a-c These methods allowed smooth access to different aryl ketones, aryl amides and 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.

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Scheme 53. Ge’s decarboxylative cross-coupling of potassium aryltrifluoroborates and α-oxocarboxylic acids with Pd(II)/K2S2O8. Reprinted with permission from refs 70a-c. Copyright 2011 American Chemical Society; 2011 Royal Society of Chemistry; and 2016 Royal Society of Chemistry

Palladium-catalyzed carboxylation of arenes with CO through direct aryl C-H functionalization with K2S2O8 as 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 presence of palladium(II) catalyst and K2S2O8 (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 (Scheme 55).

Scheme 54. Pd-catalyzed decarboxylative ortho-ethoxycarbonylation reaction. Reprinted with Permission from Ref 72. Copyright 2015 American Chemical Society

Scheme 55. Proposed mechanism for Pd-catalyzed ortho-ethoxycarbonylation. Reprinted with Permission from Ref 72. Copyright 2015 American Chemical Society

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 N,N34

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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 presence of K2S2O8 which afforded the product and the residul portion of N,N-dimethyloxamic acid was converted into DMF as was observed by GC-MS.

Scheme 56. Pd-catalyzed decarboxylative cross coupling of acetanilides and N,N-dimethyloxamic acids. Reprinted with Permission from Ref 73. Copyright 2016 Royal Society of Chemistry

Some carbonylation reactions were reported with palladium(II)/K2S2O8 combination. In 2014, Zhu reported a pyridocarbonylation of N-aryl-2-aminopyridines using CO gas (Scheme 57).74a This reaction closely resembled to directing group assisted ortho-acylation of arenes. In this case, the pyridyl moiety exhibited as both directing group and internal nucleophile. Other persulfates and Cu(II) salts proved inferior to K2S2O8 for this reaction. Importantly, under such oxidative condition, formation of urea byproduct involving two aniline molecules and CO gas could be possible, which however practically didn’t form in 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 presence of palladium(II)/K2S2O8.74b

Scheme 57. Pyridocarbonylation of N-aryl-2-aminopyridines with palladium catalyst. Reprinted with Permission from Ref 74a. Copyright 2014 American Chemical Society

Similar to pyridocarbonylation, a palladium-catalyzed double C-H activation/carbonylation of diarylethers with CO gas was developed by Lei and co-workers using K2S2O8 as terminal oxidant.75 35

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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 Pd(II)/Pd(0) catalytic cycle. Nozaki and co-workers reported an unexpected finding on simultaneous hydroxylationcarboxylation of biphenyls with formic acid in presence of Pd(OCOCF3)2 and K2S2O8 affording 4´hydroxy-4-biphenylcarboxylic acid (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.

Scheme 58. Nozaki’s Pd(II)-catalyzed sequential hydroxylation-carboxylation of biphenyls. Reprinted with Permission from Ref 76. Copyright 2004 American Chemical Society

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 (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.

Scheme 59. Pd-catalyzed C-H arylation of arenes using guanidine as directing group. Reprinted with Permission from Ref 78. Copyright 2012 American Chemical Society

Similar ortho-arylation of tertiary benzamides was reported by Guan and co-workers.79 Notably, this method showed high para-selectivity for the arenes that were incorporated on benzamides. In

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situ formation of cationic Pd(OTf)2 was proposed for this transformation. Role of K2S2O8 in these reaction 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 presence of Pd-catalyst and K2S2O8.80a The group further reported a palladium-catalyzed orthoalkenylation strategy of 2-benzyl-1,2,3-triazoles (Scheme 60).80b 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 Trans-1,2-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.

Scheme 60. Pd-catalyzed ortho-functionalization of 2-benzyl-1,2,3-triazoles with electron deficient alkenes. Reprinted with Permission from Ref 80b. Copyright 2015 Royal Society of Chemistry

Batra and co-workers employed N-arylpyrazoles for palladium-catalyzed oxidative dehydrogenative homocoupling and ortho-hydroxylation in presence of K2S2O8 (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.

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Scheme 61. Pd-catalyzed dehydrogenative homocoupling /ortho-hydroxylation of N-phenylpyrazoles. Reprinted with Permission from Ref 82. Copyright 2015 American Chemical Society

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 (Scheme 62).83 A combination of oxidants, i.e., K2S2O8 (0.5 equiv) and NaIO4 (0.6 equiv) were found to be most efficient whereas oxone, PhI(OAc)2 etc. were futile. Requirement for this combined oxidant in the mechanism of this transformation was not clear. Since individual use of NaIO4 in the reaction was not as effective as it was with K2S2O8, the later most likely served as the terminal oxidant for the Pd(II)/Pd(IV) catalytic cycle .

FG1 R1 H

FG2 + R2

Pd(OAc)2 NaIO4 (0.6 equiv) K2S2O8 (0.5 equiv) TfOH (2 equiv)

H

O

O N

R2

FG1

HFIP, rt - 70 oC

FG =

FG2

R1

O O

Scheme 62. Pd-catalyzed regioselective oxidative coupling for 2,2′-difunctional biaryl synthesis. Reprinted with Permission from Ref 83. Copyright 2015 American Chemical Society

Zhang, Wang and co-workers first reported a palladium(II) catalyzed directed meta-arylation of Oβ-naphthyl carbamate with aryl boronic acids at room temperature (Scheme 63).84 In order to favor the arylation to occur at meta over ortho-position, reaction was performed under acidic condition using TFA:AcOH (2:1, v/v). Importantly, carbamate moiety was only effective for this reaction, whereas hydroxyl, ether or 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 (Scheme 64). The mechanism, however, did not support directing group assisted meta-arylation unequivocally.

Scheme 63. Pd-catalyzed meta-arylation of O-β-naphthyl carbamates. Reprinted with Permission from Ref 84. Copyright 2015 Royal Society of Chemistry 38

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Scheme 64. Proposed mechanism for meta-selective arylation. Reprinted with Permission from Ref 84. Copyright 2015 Royal Society of Chemistry

Development of a catalytic method to synthesize unsymmetrical biaryls via C-H activation is receiving continuous attention in organic synthesis. In particular, arenes armed with a directing group are suitably poised for such transformations. However, use of simple or deactivated arenes offer more challenges in terms of reactivity and controlling the selectivity. In 2012, Lu and coworkers reported a simple Pd(II)/TFA/oxidant system where either molecular oxygen or K2S2O8 could be used as 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 benzoate group and meta-selectivity with respect to nitro or trifluoromethyl group (Scheme 65).

Scheme 65. Pd(II)-catalyzed cross-coupling of ethyl benzoate with electron-deficient arenes. Reprinted with Permission from Ref 85. Copyright 201 American Chemical Society

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A palladium catalyzed cross-dehydrogenative coupling (CDC) method between 1,3,5trialkoxybenzenes and simple arenes was developed by Greaney and co-workers (Scheme 66).86 It is important to note that, electron rich aromatics used in the reaction did not undergo any oxidative side reactions in presence of K2S2O8 rather, the desired SEAr path was only followed in presence of Pd(II). Overall, this method offered a mean to prepare multi-ortho-substituted biaryls.

Scheme 66. Pd-catalyzed CDC reaction of 1,3,5-trialkoxybenzenes with different arenes. Reprinted with Permission from Ref 86. Copyright 2014 Royal Society of Chemistry

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 (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 IR, path B in the mechanism was claimed to be more likely over path A (Scheme 68).

Scheme 67. Pd-catalyzed arylation of azoles via deamidation of arylamides. Reprinted with Permission from Ref 87. Copyright 2012 Royal Society of Chemistry

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O CO2

NOMe PdX

Ar

H 2O O C NOMe

H O N

Path A

Ar PdX

O

CO

Ar

Ar

NHOMe

O Ar

H

O

NOMe PdX A Path B

Ar

NOMe PdX

PdX2 X=OAc

PdX N Pd0

O B HPdX

K2S2O8

Ar N

O

Scheme 68. Catalytic cycle for the deamidative arylation of azoles. Reprinted with Permission from Ref 87. Copyright 2012 Royal Society of Chemistry

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.

Scheme 69. Use of MNP immobilized Pd-catalyst for direct C-2 arylation of indoles with arylboronic acids. Reprinted with Permission from Ref 88. Copyright 2014 Royal Society of Chemistry

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 presence of a Ag(I) oxidant. The current report demonstrated the possibility for using K2S2O8 as a cheap alternative, whereas BQ, O2 or Cu(II) were 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 (Scheme 70).89b When 2-ethylpyridine N-oxide was used in this study, cyclic azafluorene N-oxides were obtained through four C-H bonds activation. This reaction was performed with 3 mol % Pd(OAc)2 and 0.75 equiv. of K2S2O8. Na2S2O8 or (NH4)2S2O8 could also be

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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 (Scheme 71).

Scheme 70. Pd-catalyzed regioselective benzylation and annulation of pyridine N-oxides. Reprinted with Permission from Ref 89b. Copyright 2015 American Chemical Society

CH3 S2O8-2

2 SO4 -HSO4

Et CH2

R N O

X

N O Et

Pd X OAc

OAc Pd

N O

Oxidation

Oxidation OAc H Pd R

Cycle I N O

Pd OAc

Reductive elimination

Cycle II

Et

N O H

N

R

AcOH

Pd(OAc)2

N O

R N O

O H AcO Pd

Et

R = 6-Et

H

Scheme 71. Catalytic cycle for Pd-catalyzed benzylation/annulation of pyridine N-oxide. Reprinted with Permission from Ref 89b. Copyright 2015 American Chemical Society

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 (Scheme 72).90 Different benzamides and tertiary alcohols were used to achieve the transformation. A radical pathway was proposed for the reaction.

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H

O N H

OMe

OH + R H

O

Pd(OAc)2 K2S2O8, TFA

O N

OMe

N

+

cyclohexane, 100 °C

OMe R

R

O

O N

OMe

84%

O N

OMe

O N

OMe

OMe

8%

63%

O

N

O N

OMe

N

OMe

Et 29%

33%

0%

Scheme 72. Pd-catalyzed synthesis of dihydro-isoquinolines with sequential C(sp2)-H and C(sp3)-H bond activation. Reprinted with Permission from Ref 90. Copyright 2015 Royal Society of Chemistry

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 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 (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 acetanilides (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 formation. The Pd(0) was oxidized back to Pd(II) species by K2S2O8 and completed the catalytic cycle.

Scheme 73. Wang’s Pd-catalyzed annulation of [60]fullerene with anilides. Reprinted with Permission from Ref 91a. Copyright 2009 American Chemical Society

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Scheme 74. Proposed mechanism for heteroannulation of [60] fullerene. Reprinted with Permission from Ref 91a. Copyright 2009 American Chemical Society

In the subsequent studies, they expanded the scope of [60]fullerene functionalization and synthesized different C60-fused compounds containing tetrahydroisoquinoline,91b sultone ring,91c tetrahydrobenzooxepine,91d isochroman,91d and benzofuran.91e In these transformations, Nbenzylsulfonamides, 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 (Scheme 75).92 This reaction efficiently proceeded with electron deficient diaryl acetylenes and electron rich acetanilides.

Scheme 75. Pd-catalyzed synthesis of substituted naphthalenes from amides and alkynes. Reprinted with Permission from Ref 92. Copyright 2010 Royal Society of Chemistry

In 2015, Laha and co-workers reported a palladium catalyzed tandem one-pot approach to access carbazoles (mono-, di-, and trisubstituted) as well as α-carbolines from readily available indoles or 7-azaindoles and alkenes with K2S2O8 (Scheme 76).93 This reaction required the use of 50 mol % 44

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AgOAc as co-oxidant for better results. This palladium-catalyzed tandem reaction followed the sequence as regioselective C3 alkenylation-C2 alkenylation-thermal electrocyclization.

Scheme 76. A Pd-catallyzed tandem approach to transform indoles to functionalized carbazoles. Reprinted with Permission from Ref 93. Copyright 2015 American Chemical Society

2.2.2 C-O bond formation. Different C-O bond forming reactions were developed with 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 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 Choice of K2S2O8 as 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 valent 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. R2 N R1

OAc R2 N

R3 Pd(OAc) (5 mol%),K S O 2 2 2 8 O

AcOH/DCE (1:1),1000C,48 h

R1

O

O

O R1

R3

N H

O + R2 OH

Pd(OAc)2 (5 mol%),K2S2O8,4A0 MS Dioxane,550C

R1

N H OR2

O

Scheme 77. Wang’s Pd-catalyzed ortho-acetoxylation and alkoxylation of anilides. Reprinted with permission from refs 94a-c. Copyright 2008 American Chemical Society; 2010 American Chemical Society;and 2012 American Chemical Society

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On the basis of Wang’s alkoxylation method,94b Sunoj and co-workers disclosed a density functional theory (DFT) calculations on this transformation.95 This study revealed interesting mechanistic facts on Wang’s transformation, such as involvement of concerted metallation 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 Pd(II) catalyst, where the ketone moiety served as the directing group.96 Several oxidants, like 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. The authors, however, 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 trifluroacetoxylation followed by its hydrolysis. In 2016, Zhang, Rao and co-workers disclosed a complementary strategy for regioselective sp2 C-H hydroxylation (via trifluoroacetoxylation) using either 5% [Ru(p-cymene)Cl2]2 or 10 % Pd(OAc)2 catalyst (Scheme 78) where K2S2O8 served as oxidant in both the cases.97 In this study, the orthohydroxylation 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-metallation processes (Scheme 79).97

Scheme 78. Ru(II) or Pd(II)-catalyzed regioselective hydroxylation of benzanilides. Reprinted with Permission from Ref 97. Copyright 2016 Royal Society of Chemistry

Scheme 79. Different metallocycles for regioselective hydroxylation. Reprinted with Permission from Ref 97. Copyright 2016 Royal Society of Chemistry 46

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Triazole-assisted ortho-alkoxylation of 2-aryl-1,2,3-triazoles was reported by Kuang and coworkers with palladium(II)/K2S2O8 (Scheme 80).98 In these cases also, 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

Scheme 80. Pd-catalyzed synthesis of ortho-alkoxylated 2-aryl-1,2,3-triazoles. Reprinted with permission from refs 98a-b. Copyright 2014 American Chemical Society; and 2014 Wiley-VCH

Employing sulfoximines as reusuable directing group, Sahoo and co-workers developed orthoselective 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 ortho-acetoxylation of arylacetic acids.100b Azoxybenzene substrates were also used for similar C-O bond forming reaction. Cui, Wu and coworkers reported acyloxyation with carboxylic acids in presence of catalytic Pd(TFA)2 and stoichiometric K2S2O8 (Scheme 81).101 Importantly, these substrates could not be efficiently alkoxylated with K2S2O8.

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Scheme 81. Pd-catalyzed direct ortho-C-O bond construction of azoxybenzenes. Reprinted with Permission from Ref 101. Copyright 2015 Royal Society of Chemistry

An exo-oxime benzylether directed aroyloxylation with benzoic acid on the aromaric ring was reported by Shao et al. in 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 co-workers reported functionalization of various (β-carbolin-1-yl)methanones via palladium-catalyzed regioselective alkoxylation in presence of K2S2O8 (Scheme 82).103

Scheme 82. Regioselective alkoxylation of aryl(β-carbolin-1-yl)methanones with Pd(II)/ K2S2O8. Reprinted with Permission from Ref 103. Copyright 2015 Royal Society of Chemistry

Oxygenation of simple benzene is highly challenging in the absence of any directing group. The use of Pd(OAc)2 in such reaction had proven futile in many cases due to slow reaction rate, lower TON and modest selectivity for the PhOAc formation over undesired biphenyls. In 2013, Sanford and coworkers developed an efficient Pd(OAc)2 catalyzed ligand-directed C-H oxygenation of benzene with K2S2O8 (Scheme 83).104 For this transformation use of a cationic pyridine ligand was crucial, which in addition to generating an active Pd-catalyst also acted as a phase transfer catalyst (renders better solubility of S2O8-2 ) and thus resulted in the improved yield of the desired product.

Pd(OAc)2 (2 mol %) Ligand (2 mol %) K2S2O8 (1 equiv)

OAc N

BF4-

Ligand:

AcOH:Ac2O (9:1), 80 oC, 24 h

N

Scheme 83. Pd-catalyzed sp2 C-H oxygenation of benzene. Reprinted with Permission from Ref 104. Copyright 2013 American Chemical Society

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 (Scheme 48

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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 valent palladium intermediate (from A to B) leading to the desired product (Scheme 85). Other oxidants like PhI(OAc)2, O2 or oxone were 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

Scheme 84. Pd-catalyzed sp3 C-H activation /β-acyloxylation of amides. Reprinted with Permission from Ref 105a. Copyright 2014 American Chemical Society

Scheme 85. Proposed mechanism for amide’s β-acyloxylation with Pd-catalyst. Reprinted with Permission from Ref 105a. Copyright 2014 American Chemical Society

2.2.3 C-N bond formation. Compared to substantial uses of Pd(II)/K2S2O8 in oxidative C-C and CO 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, choice of oxidant often becomes a crucial factor, as the presence of free N-H group could lead to undesired reactions. In 2006, Yu, Che and co-workers reported a palladium catalyzed intermolecular orthoamidation of C(sp2)-H/C(sp3)-H bonds of 2-phenylpyridines and O-methyl oximes with different acetamides (Scheme 86).106 In presence of K2S2O8, nitrene formation from the acetamide was 49

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speculated and some evidences gathered thorugh the controlled experiments supported this notion. The nitrene therefore could insert into the Pd-C bond of the palladacycle to afford the final product.

Scheme 86. Pd-catalyzed intermolecular amidation of sp2 and sp3 C-H bonds. Reprinted with Permission from Ref 106. Copyright 2006 American Chemical Society

Zhang and Zhao reported a palladium catalyzed intramolecular oxidative amination to the tethered arene with (N,N-dimethyl)oxamoyl amide (Scheme 87).107 This cyclization could be achieved with K2S2O8, TBPB, DTBP, H2O2, whereas use of NFSI resulted in the ring cleavage to afford Oaminobenzaldehydes. The author found that a combination of two oxidants, viz. 4 equiv. H2O2 and 2 equiv. K2S2O8 was most effective for this transformation. The mechanism and precise roles of each oxidant however was unclear.

FG

O HN

Pd(OAc)2 (5 mol%), H2O2 (4 equiv) K2S2O8 (2 equiv) FG DCB (0.5 equiv)

O

O

O N

NMe2 HFIP, 100 oC, 4-16 h

O

O NMe2

Scheme 87. Pd-catalyzed transformation of masked benzyl alcohols to o-aminobenzaldehydes. Reprinted with Permission from Ref 107. Copyright 2016 Royal Society of Chemistry

Liu and Wang developed a palladium catalyzed decaboxylative amidation of α-oxocarboxylic acids in presence K2S2O8 (Scheme 88).108 Pyridine containing secondary amines were used in the reaction. K2S2O8 generated the acyl radical in the reaction as well as maintained the Pd(II)/Pd(IV) catalytic cycle.

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Scheme 88. Pd-catalyzed decarboxylative amidation to imides. Reprinted with Permission from Ref 108. Copyright 2016 Royal Society of Chemistry

Site selectivity during normal aromatic nitration is mostly dictated by the functional groups present on the aryl ring. Such bias is however 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 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, 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 (Scheme 89).109a Later they reported a more elaborate study on this method.109b-c Quinoxaline moiety exhibited the best directing group ability and the adjacent aromatic ring underwent smooth nitration. Oxime ether derivatives were also nitrated at the orthoposition, whereas pyridine, quinoline or pyrazole motif showed moderate efficiency in delivering the desired product.

Scheme 89. Pd-catalyzed regiospecific synthesis of nitroarenes. Reprinted with Permission from Ref 109a. Copyright 2010 Wiley-VCH

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Using similar conditions, Kapur and co-workers reported another method for regioselective orthonitration of anilines wherein a pyrimidine moiety served as a removable directing group (Scheme 90).110

Scheme 90. Pd-catalyzed regioselective C-H nitration of anilines. Reprinted with Permission from Ref 110. Copyright 2016 American Chemical Society

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 (Scheme 91).111 In this report, authors first optimized individual C-H activation step and then combined them together to design the one-pot transformation. In both the steps, pyrazol-5(4H)-one moiety served as the directing group. For the intramolecular C-N bond forming step, 10 mol% Pd(OAc)2 and 1.5 equiv. of K2S2O8 were used. R3

One Pot 1) Pd (OAc)2 (10 mol%) AgOAc (1.5 equiv) TFA (0.2 M), 1200C

I R1

N N

R1

+ R2

O

N N

R3

2) Pd(OAc)2 (10 mol%) K2S2O8 (105 equiv) K2CO3 (2 equiv)

R2 O

R3

R1 step-1

N

step-2

N R2

C-C

O

C-N

Scheme 91. Pd-catalyzed synthesis of benzo[c]pyrazolo[1,2-a]-cinnolin-1-ones from 5-pyrazolones. Reprinted with Permission from Ref 111. Copyright 2014 Royal Society of Chemistry

Another such sequential C-C/C-N bond forming process to synthesize various tricyclic phenanthridinones employing benzamides and arynes was reported by Jeganmohan (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 aryne that could lead to undesired reaction pathways. Therefore, an important role of the oxidant could be speculated to

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avoid such side reaction and thus the actual catalytic cycle might involve higher valent palladium species (with Pd(II)/Pd(IV) catalytic cycle).

Scheme 92. Pd-catalyzed benzamide cyclization with arynes. Reprinted with Permission from Ref 112. Copyright 2014 Royal Society of Chemistry

Using simple amides and ketones, Jiang developed a palladium catalyzed strategy to synthesize various oxazoles from readily available starting materials through sequential C-N/C-O bond formation (Scheme 93).113 With the use of CuBr2 as additive, varyings yields were obtained employing different oxidants such as K2S2O8 , DBTP, TBHP, PhI(OAc)2, DDQ, Ag2O, BQ, 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 (Scheme 94). R3 O

PdCl2 (10 mol%) CuBr (20 mol%)

O R1

NH2

+

R2

R3

O R1

N

R2

K2S2O8 (1.2 equiv) NaHCO3 (1.5 equiv) DCE,120 0C

Scheme 93. Pd-catalyzed access to oxazoles via sequential C-N/C-O bond formation. Reprinted with Permission from Ref 113. Copyright 2014 American Chemical Society

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Scheme 94. Proposed mechanism for Pd-catalyzed synthesis of oxazole derivatives. Reprinted with Permission from Ref 113. Copyright 2014 American Chemical Society

A three-component cascade reaction employing amines, alkyne esters and alkenes in presence of Pd(II)/K2S2O8 was developed by Zhang et al. to access various 2,3,4-trisubstituted pyrroles (Scheme 95).114 The overall transformation resulted in the formation of C(sp2)-C(sp2) and C(sp2)-N

bonds.

Scheme 95. Pd-catalyzed three component reaction to 2,3,4-trisubstituted pyrroles. Reprinted with Permission from Ref 114. Copyright 2016 American Chemical Society

2.2.4 C-S/C-P bond formation. Only handful examples are available on palladium-catalyzed C-S and C-P bond forming reactions that used K2S2O8 as oxidant. Cui, Wu and co-workers disclosed a palladium catalyzed ortho-sulfonylation of azobenzenes using arylsulfonyl chlorides and K2S2O8 (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. O N N R1

R2 + Cl

O

Pd(OAc)2 (5 mol%) K2S2O8 (1.1 equiv)

S

0

R3

DCE,120 C 136 h,air

N

R2

N R1

S O O

R3

Scheme 96. Pd-catalyzed sulfonylation reaction of azobenzenes. Reprinted with Permission from Ref 115. Copyright 2015 American Chemical Society

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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 (Scheme 97).116 Choice of K2S2O8 as oxidant was important as O2, BQ or CuCl2 proved inferior for this transforamtion. Requirement of such a strong oxidant also indicated the presence of a Pd(IV)/Pd(II) catalytic cycle. In general, 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 Ag(I)/K2S2O8 combination or K2S2O8 alone.

Scheme 97. Phosphonation of azoles with Pd(II)/ K2S2O8. Reprinted with Permission from Ref 116. Copyright 2012 Royal Society of Chemistry

In 2013, Huang and Wu reported a C3 selective dehydrogenative phosphonation of various coumarin derivatives with Pd(II)/K2S2O8 (Scheme 98).117 R1 +

R2 O

O HP(OR3)2

O

PdCl2 (10 mol%),bipy

R1

O 3 P OR OR3

O

O

R2

K2S2O8,CH3CN 100 0C, air

Scheme 98. Regioselective phosphonation of coumarins with Pd(II)/ K2S2O8. Reprinted with Permission from Ref 117. Copyright 2013 American Chemical Society

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

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Scheme 99. Regioselective phosphonation of imidazo[2,1-b] thiazoles with Pd(II)/ K2S2O8. Reprinted with Permission from Ref 118. Copyright 2015 Elsevier

2.2.5 Miscellaneous. In 2011, Yu and co-workers developed a palladium catalyzed ortho C-H borylation of N-arylbenzamides with B2pin2 (Scheme 100).119 In this study, the -CONHAr [Ar = (4CF3)C6F4] group on the substrate served as the auxiliary to promote effective ortho C-H activation in 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 presence of K2S2O8. Pd(OAc)2 /L K2S2O8, TsONa

O R1

NHAr

+

B2pin2

O R1

Ar = (4-CF3)C6F4

NH2Ar Bpin

CH3CN 80 0C, 24 h O

L=

F 3C

CF3

Scheme 100. Oxidative ortho C-H borylation of arenes with Pd(II)/ K2S2O8. Reprinted with Permission from Ref 119. Copyright 2012 American Chemical Society

Simiar 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 5 mol % [Pd(NCCH3)4](BF4)2 catalyst with 2 equiv. of K2S2O8. 2.3 Cu-catalyzed reactions A copper-catalyzed cross-dehydrogenative coupling (CDC) reaction between thiazoles and cyclic ethers was developed by Jiang, Chen and co-workers providing access to C2 functionalized thiazoles and benzothiazoles (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 section 3.5, Scheme 173). In this copper-catalyzed process, upon addition of ether radical to organocopper species A, formation of intermediate B instead of another possibility, B′ was proposed in the reaction. (Scheme 102). It was through the evidences 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). 56

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Scheme 101. CDC reaction of (benzo)thiazoles and cyclic ethers with Cu(II)/K2S2O8. Reprinted with Permission from Ref 121. Copyright 2013 American Chemical Society

N

N CuOTf

S

OR

S

A

B

N

CuOTf

O

CuOTf S

O

B’

Scheme 102. Key intermediates for the Cu-catalyzed CDC reaction. Reprinted with Permission from Ref 121. Copyright 2013 American Chemical Society

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 (Scheme 103).123 While K2S2O8, (NH4)2S2O8 , TBHP, DTBP and PhI(OAc)2 exhibited comparable efficiencies, O2 or Na2S2O8 gave slighly lower yields.

Scheme 103. Cu-catalyzed access to cyano-containing oxindoles through radical cascade reaction. Reprinted with Permission from Ref 123. Copyright 2014 Royal Society of Chemistry

Ritter and co-workers developed a novel method for perfluoroalkylation where gaseous CF3I and CF3CF2I were transformed into liquid as 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

Scheme 104. Cu-mediated perfluoroalkylation reactions with a new liquid perfluoroalkylating reagent. Reprinted with Permission from Ref 124. Copyright 2015 Wiley-VCH

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Very recently, Li developed a copper-mediated operationally simple and practical method for trifluoromethylation of alkyl radicals (Scheme 105).125 In this method, BPyCu(CF3)3 (Bpy =2,2′bipyridine) was used as the CF3 source and a combination of Et3SiH/K2S2O8 was used to generate the radical from alkyl bromide or iodide. Screening of differnet organic and inorganic peroxides revealed that both BPO and K2S2O8 are viable as oxidant for this transformation. The author however used K2S2O8 for subsequent development of the method. Some mechanistic insights were obtained though controlled experiments, which suggested the involvement of a Cu(II)-CF3 complex as trifluoromethylating agent and Et3SiH/K2S2O8 as radical initiator (Scheme 106). Although understanding the full mechanism would be more intriguing, a unique reactivity exhibited by K2S2O8 in this transformation could stimulate its further strategic uses.

Scheme 105. Cu-mediated trifluoromethylation of different alkyl halides. Reprinted with Permission from Ref 125. Copyright 2017 American Chemical Society

Scheme 106. Proposed mechanism for Cu-mediated trifluoromethylation via radical process. Reprinted with Permission from Ref 125. Copyright 2017 American Chemical Society

In 2014, Lei and co-workers developed a copper-catalyzed methylenation of aryl ketones and 1aryl-1-pyridinemethanes with DMF as the one carbon source and K2S2O8 as oxidant.126 Overall, this reaction transformed a ketone into 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 (Scheme 107).127 The reaction was realized with the use of 58

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aryl ketones, organic azides and DMF as one-carbon (C1) donor in 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. O

O

O CH3 +

N3 +

N

Cu(NO3)2 / K2S2O8 H

100 oC, 3 h

N N N

Scheme 107. Cu-catalyzed CDC reaction to 4-acyl-1,2,3-triazoles. Reprinted with Permission from Ref 127. Copyright 2017 American Chemical Society

More recently, Li, Chen and co-workers reported a Cu(II)-catalyzed C-N bond formation via NH/C-H cross-dehydrogenative coupling between NH-1,2,3-triazoles and N,N-dialkylamides (Scheme 108).128 This reaction proceeded through the oxidative formation of an iminium ion from DMF followed by addition of 1,2,3-triazole.

Scheme 108. Cu-catalyzed N-amidoalkylation of NH-1,2,3-triazoles. Reprinted with Permission from Ref 128. Copyright 2017 American Chemical Society

An interesting cascade reaction was developed by Yang where two different sp3 C-H bonds in the side chain of ethylazaarenes were transformed into a ketone and isoxazoline moiety respectively in presence of 10 mol % CuBr and stoichiometric K2S2O8 and KNO3 (Scheme 109).129a Four new bond 59

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formation (C=O, C=N, C-C, C-O) could be accounted for this transformation involving ethylazaarenes an different alkene reaction partners. The overall transformation appreared 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

Scheme 109. Cu-catalyzed cascade reaction towards isoxazolines. Reprinted with Permission from Ref 129a. Copyright 2015 Royal Society of Chemistry

Scheme 110. Proposed mechanism for isoxazoline synthesis via Cu-catalyzed cascade process. Reprinted with Permission from Ref 129a. Copyright 2015 Royal Society of Chemistry

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 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 60

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oxidation might occur with Cu(II) species. In 2015, Yang reported a Cu(II)-catalyzed oxidative radical cyclization of various N-protected-2-allyl anilines with HP(O)Ph2 in presence of K2S2O8 (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 (Scheme 111b).130a In both the cases, generation of phosphinoyl radical and its nucleophilic addition on the substrates were proposed in the mechanism.

+

O R1 H P R2

+

O R2 H P R3

NHR4 R3

N R1

N(alkyl)2 H

N

K2S2O8 (3 equiv) CH3CN

R3

K2S2O8 (2 equiv) CH3CN

R1

+

O H P

R4

R4

(a)

N(alkyl)2

N

Cu(OAc)2 (2.5 mol %)

O P R2 R3

Cu(SO 4)2 (20 mol %) K2S2O8 (2 equiv) CH3CN

(b)

R2

R1

R2

R1

R4 O R1 P N R2

Cu(ClO4)2. 8H2O (20 mol %)

N

R4 (c) P O R4

R3

Scheme 111. Different Cu(II)-catalyzed oxidative phosphorylations. Reprinted with permission from refs 129b and 130a-b. Copyright 2015 Royal Society of Chemistry; 2016 American Chemical Society; and 2017 American Chemical Society

The same group reported a cascade reaction involving phosphorylation-cyclization with

Cu(II)/K2S2O8 system to provide the access to different 2-phosphorylated pyrrolo[1,2-a]indoles (Scheme 111c).130b

An analogues 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 presence of stoichiometric Cu(II)/K2S2O8 combination (Scheme 112).132 First, a pyridine coordinated 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 presence of K2S2O8. The resulting isocyanate intermediate (C) was finally transformed into the product by the addition of acetic acid followed by decarboxylation (Scheme 113).

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Scheme 112. Cu-mediated acetylamination of arenes with cyanate salts. Reprinted with Permission from Ref 132. Copyright 2016 American Chemical Society

Scheme 113. Proposed mechanism for acetylamination of arenes with Cu(II)/K2S2O8. Reprinted with Permission from Ref 132. Copyright 2016 American Chemical Society

Narula reported a Cu(I)-catalyzed ortho-azidation of 2-arylpyridines and other related systems with benzotriazole sulphonyl azide (BtSO2N3) in 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 (Scheme 114).134 Formation of an acyl radical in 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. O R

OH + HN S O R2 R3

1

O

CuBr K2S2O8

O 1

°

CH3CN, 75 C

R

O S 3 N 2R R

CuII N O S 3 R2 R A

or

N O S 3 R2 R B

Scheme 114. Cu-catalyzed access to N-aroylsulfoximines via coupling of α -keto acids and sulfoximines. Reprinted with Permission from Ref 134. Copyright 2017 Royal Society of Chemistry

In 2010, Qing developed a Cu(II)-mediated ortho-methylthiolation of 2-aryl pyridines with DMSO in presence of

K2S2O8.135a In 2014, the same group developed a Cu(II)-mediated

trifluoromethylthiolation of unactivated terminal alkenes with AgSCF3 (Scheme 115).135b Using this method, different terminal alkenes were converted into trifluoromethylthiolated allylic compounds. Formation of SCF3 radical was proposed to form with Cu(II)/K2S2O8, which after sequential addition-oxidation steps, provided the desired product. R

+

AgSCF3

Cu(OAc)2 / K2S2O8 K3PO4, DMSO, 60 oC, 6 h

R

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SCF3

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Scheme 115. Trifluoromethylthiolation of terminal alkenes with Cu(II)/ K2S2O8. Reprinted with Permission from Ref 135b. Copyright 2014 Royal Society of Chemistry

A copper-catalyzed deoxygenative transformation between quinoline N-oxide and sodium sulfinate in presence of K2S2O8 was reported by Han and Pan.136 This method afforded various C2sulfonylated quinolines with 20 mol % CuBr2 and 50 mol % K2S2O8. The reaction proceeded through Minisci-like radical reaction with a sulfinate radical and deoxygenation of quinoline Noxide occurred in the final aromatization step via elimination of water. Formation of regio and stereospecific C(sp2)-S and C(sp2)-Se bonds through a three component copper(I)-catalyzed reaction was reported by Li, Zhang and co-workers (Scheme 116).137 This reaction employed alkynes, arylsulfonohydrazides and diphenyl diselenide to furnish (E)-βselenovinyl sulfones.

Scheme 116. Selenosulfonation of alkynes with Cu(I)/ K2S2O8. Reprinted with Permission from Ref 137. Copyright 2017 American Chemical Society

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 presence of Cu(II)-catalyst and K2S2O8 (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 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 mass-spectrometric 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

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Scheme 117. Cu-catalyzed oxidative cyclization of carboxylic acids to lactone. Reprinted with Permission from Ref 138. Copyright 2016 American Chemical Society

Recently, a cascade reaction was developed by Liu with Cu(II)/K2S2O8 system employing different alkynols and 2-azidobenzaldehydes that furnished 6H-isochromeno[4,3-c]quinolines (Scheme 118).139 This reaction resulted in the formation of C-C, C-N and C-O bonds in a single operation.

Scheme 118. Cu-catalyzed cascade annulation to 6H-Isochromeno[4,3-c]quinoline. Reprinted with Permission from Ref 139. Copyright 2017 American Chemical Society

2.4 Fe-catalyzed or mediated reactions In 2000, Wille disclosed an interesting SO4−• mediated transannular reaction employing the Fenton redox

system,

i.e.,

a

combination

of S2O82- and

Fe2+

source.140

In

the reaction,

Fe(EDTA)(SO4)2.4H2O and K2S2O8 were used with cycloalkynes which afforded a mixture of α,βepoxy ketones in almost equal ratio (Scheme 119). The intriguing mechanistic feature of this transformation was that the sulfate radical ion directly added to the alkyne and generated a vinyl radical (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

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SO3−• furnished the product. This report is unique in terms of addition of the sulfate radical ion to alkyne and such direct addition is rarely postulated in the literature.

Scheme 119. Wille’s sulfate radical mediated transannular reaction with Fenton redox system. Reprinted with Permission from Ref 140. Copyright 2000 American Chemical Society

Scheme 120. Proposed mechanism for transannular reaction. Reprinted with Permission from Ref 140. Copyright 2000 American Chemical Society

In 2012, Lou et al. developed a FeCl3 catalyzed benzylic methylenation of 2-methylazaarenes using N,N-dimethylacetamide (DMA) and K2S2O8 (Scheme 121).141 This transformation was similar to the one reported copper-catalyzed process, that is discussed in the earlier section of this review.124 A Fe(III)-Fe(II) catalytic cycle was proposed for this transformation.

Scheme 121. Fe-catalyzed CDC reaction towards the synthesis of vinylaromatics. Reprinted with Permission from Ref 141. Copyright 2012 Royal Society of Chemistry

A similar strategy on 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 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 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 the one reported with Ag(I)/K2S2O8 system.25 Soon after this

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report, Singh and Vishwakarma disclosed a similar protocol using catalytic Fe(acac)2 and TBAB as a phase transfer catalyst with K2S2O8 (Scheme 122).144

Scheme 122. Cross-coupling reactions of electron-deficient heterocycles and organoboron species with Fe(II)/ K2S2O8. Reprinted with Permission from Ref 144. Copyright 2013 American Chemical Society

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 (Scheme 123).146 Various α-oxocarboxylic acids and acrylic acids were used in this reaction that afforded α,β-unsaturated carbonyl compounds.

Scheme 123. Decarboxylative cross-coupling between α-oxocarboxylic acids and acrylic acids with Fe(II)/ K2S2O8. Reprinted with Permission from Ref 146. Copyright 2015 American Chemical Society

2.5 Other metal-catalyzed reactions In 2011, Lang et al. disclosed an efficient rhodium-catalyzed carbonylation at C3 position of indoles with CO and different linear and cyclic alcohols (Scheme 124).147 The reaction used 2 mol % of [Rh(COD)Cl]2 and 2 equiv. of K2S2O8 at 110 oC. In the reaction, an active Rh(III)-catalyst was proposed to form in presence of K2S2O8. 66

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Scheme 124. Preparation of indole-3-carboxylates via Rh-catalyzed indole carbonylaion. Reprinted with Permission from Ref 147. Copyright 2011 Royal Society of Chemistry

A Rh(II)-catalyzed decarboxylative coupling method between acrylic acids and unsaturated oxime esters in presence of AgOTs-K2S2O8 was developed by Rovis and co-workers for the synthesis of different substituted pyridines (Scheme 125).148 In this study, K2S2O8 worked as the co-oxidant and involved in the generation of an active Rh(III) catalyst in the catalytic cycle.

Scheme 125. Cross coupling of acrylic acids and unsaturated oxime esters with Rh(III)/ K2S2O8 . Reprinted with Permission from Ref 148. Copyright 2014 American Chemical Society

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 (Scheme 126).149 Use of 2.5 mol % of [RuCl2(p-cymene)]2, 10 mol % of AgSbF6 and one 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.

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Scheme 126. Ru(II)-catalyzed phenol C-H acyloxylation. Reprinted with Permission from Ref 149. Copyright 2015 Wiley-VCH

Similarly, Rao and co-workers reported a directing group assisted Ru(II)-catalyzed orthohydroxylation of ethyl benzoate, anilides and aryl carbamates in presence of K2S2O8.150a-c In the 2.2.1 section of the review, Wang’s report on different palladium-catalyzed oxidative functionalizations of [60]fullerenes have been discussed.91a-e In 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 presence K2S2O8 (Scheme 127).151 Unlike the palladium-catalyzed processes, the present reaction with Mn(III) proceeded via a radical mechanism.

R2 n CO2Et

+ R1

R2 CO2Et

Mn(OAc) 3.2H2O AlCl3, K2S2O8

n

ODCB-CH3CN/heat

R1

n=0,1

Scheme 127. Mn(III)-mediated synthesis of [60]fullerene-fused tetrahydronaphthalene and indane derivativs. Reprinted with Permission from Ref 151. Copyright 2011 American Chemical Society

A regioselective C3 iodination of different azaarenes such as quinolones, uracil and pyridine with NaI was reported by Lupton, Maiti and co-workers.152 Two equivalent 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 (Scheme 128).154 Two different types of N-H bonds participated in this cyclization reaction in presence of Co(II)/K2S2O8 system.

Scheme 128. Co-catalyzed synthesis of benzoimidazoquinazolines by cascade reaction with isocyanide. Reprinted with Permission from Ref 154. Copyright 2016 Royal Society of Chemistry 68

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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 TON of 27.5. Bell and co-workers reported calcium-salt promoted synthesis of methanesulfonic acid via sulfonation of methane with SO2 in 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

3. Metal Free Oxidative Transformations with K2S2O8 K2S2O8 is employed as 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, alkylation and arylation reactions (through cross-dehydrogenative coupling reactions and decarboxylative processes), cascade radical addition-cyclization processes, multifold bond cleavage-bond forming reactions and photo-redox reactions. Jeganmohan reported an efficient synthesis of unsymmetrical biphenols via oxidative cross-coupling of two different phenols in presence of K2S2O8 at ambient temperature (Scheme 129).156 Oxidation potential of each participating phenol was vital in order to predict the coupling pattern. In presence of CF3COOH, the phenol as reaction partner with lower oxidation potential was first oxidized by sulphate radical anion (SO4−•) and afforded a radical cation intermediate. Catalytic Bu4N+ · HSO3− was used in the reaction, which could stabilize this intermediate. Finally, nucleophilic attack by the second phenol partner to the radical cation furnished the final product (Scheme 130). OH OH

R1 OH

R1

+

R2

K2S2O8 (2 equiv) CF3COOH Bu4NHSO3 (10 mol%)

OH R2

rt,under air

Scheme 129. Jeganmohan’s cross-coupling of two different phenols with K2S2O8. Reprinted with Permission from Ref 156. Copyright 2015 American Chemical Society

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Scheme 130. Proposed mechanism for synthesis of unsymmetrical biphenols with K2S2O8. Reprinted with Permission from Ref 156. Copyright 2015 American Chemical Society

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 same scaffold with TBAB/K2S2O8, wherein aldehydes served as the source for acyl group.157 In many radical mediated oxidative transformations, use of tetrabutylammonium halides were proven beneficial.158 In such cases, K2S2O8 is converted to nBu4+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, (NH4)2S2O8 both gave comparable yields during optimization. Here, aldehyde was first transformed into an acyl radical with n-Bu4+SO4−•, which subsequently underwent cascade addition-cyclization process as described earlier. Guo employed TBAI/K2S2O8 combination for a similar cascade cyclization of N-methyl-Narylacrylamides 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 presence of K2S2O8 (Scheme 131).160 Use of other oxidants like oxone, BQ or Mn(OAc)3 proved inefficient for this reaction. In 2014, Wang and co-workers sucessfully demonstrated the use of Langlois’s reagent with K2S2O8 and developed a metal-free access to CF3incorporated oxindoles from N-arylacrylamides (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.

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Scheme 131. Metal-free oxidative spirocyclization of hydroxymethylacrylamides with K2S2O8. Reprinted with Permission from Ref 160. Copyright 2013 American Chemical Society

Scheme 132. Metal free synthesis of CF3-containing oxindoles with K2S2O8. Reprinted with Permission from Ref 161. Copyright 2014 American Chemical Society

Tian

also

reported

a

similar

strategy

using

trifluoromethanesulfonyl

hydrazides

as

trifluoromethylating agent in 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 (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. Poor solubility of the other peroxydisulfate oxidants compared to K2S2O8 in the reaction medium could be attributed for the outcome.

Scheme 133. Amidoalkylation/arylation of benzothiazoles and alkenes with N,N-dialkylamides and K2S2O8. 71

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Reprinted with Permission from Ref 163. Copyright 2016 American Chemical Society

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 (Scheme 134).165 This method features the generation of acyl radical under metal free condition 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 in investigating 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

Scheme 134. Decarboxylative alkynylation of α-keto acids and oxamic acids with K2S2O8 . Reprinted with Permission from Ref 165. Copyright 2015 American Chemical Society

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 carried out in organic solvent (DCE), it could be

speculated that in situ formation of soluble peroxydisulfate, (n-Bu4)2S2O8 (which produces nBu4+SO4−• ion) was crucial for this transformation and cation exchange occurred most efficiently with K2S2O8. Importantly, Ag(I)/K2S2O8 system didn’t work for this reaction. It was proposed that, the acyl radical was generated via cleavage of the aldehydic C-H bond, which added 72

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intramolecularly to the aryl group and afforded different fluorenone, xanthone and anthrone derivatives.

Scheme 135. Intramolecular oxidative acylation of benzaldehydes with K2S2O8 . Reprinted with Permission from Ref 158d. Copyright 2013 Royal Society of Chemistry

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 presence of K2S2O8/aliquat 336 (tricaprylmethylammonium chloride).168a They also reported the synthesis of 3-acylcoumarins from coumarin and aldehydes using 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 (Scheme 136). In this case, 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 (Scheme 137). It was likely that phenylglyoxylic acids also 73

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were converted to the aldehydes under the reaction condition. This was certainly an unexpected course of reaction mediated by K2S2O8.

Scheme 136. K2S2O8-mediated oxidative condensation between benzothiazoles and aldehydes. Reprinted with Permission from Ref 170. Copyright 2012 American Chemical Society

Scheme 137. Proposed mechanism for the oxidative condensation. Reprinted with Permission from Ref 170. Copyright 2012 American Chemical Society

In 2015, Lu and co-workers disclosed another metal free radical decarboxylation-additioncyclization cascade to access phenanthridines employing 2-isocyanobiphenyls and different alkyl/aryl carboxylic acids in presence of K2S2O8 (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.

Scheme 138. K2S2O8-mediated oxidative decarboxylation-cyclization cascade of 2-isocyanobiphenyls. Reprinted with Permission from Ref 171. Copyright 2015 American Chemical Society 74

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Some cross-dehydrogenative coupling (CDC) reactions were also reported under metal-free condition employing K2S2O8 as sole oxidant. One such CDC reaction was developed by Singh and co-workers between electron deficient arenes and α-Csp3-H of ethers.172 Instead of the direct alpha 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 later 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 hetero-arenes 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 presence of K2S2O8 (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.

Scheme 139. Metal-free decarboxylative cross-coupling of carboxylic acids with cyclic ethers. Reprinted with Permission from Ref 173a. Copyright 2017 American Chemical Society

The group also developed a K2S2O8 mediated direct Csp3-H methylenation of aryl ketones employing DMSO (Scheme 140).173b The later served as the one carbon source under the reaction condition. This method describes an alternative mean 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, (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 proposed to form from DMSO in presence of K2S2O8, which combined with the enolate to give intermediate B. K2S2O8 further facilitated the demethylthiolation under oxidative condition to afford the product (Scheme 141).

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Scheme 140. Metal-free α-Csp3-H methylenation of ketones with DMSO. Reprinted with Permission from Ref 173b. Copyright 2017 American Chemical Society

Scheme 141. Proposed mechanism for α-Csp3-H methylenation of ketones with K2S2O8 . Reprinted with Permission from Ref 173b. Copyright 2017 American Chemical Society

With a slight modification in the reaction condition and using DABCO as base, the group further reported a synthetic strategy to access different ܽ-methylthiomethyl enones and 3-methylthiomethyl chroman-4-ones, where three methyl groups underwent Csp3-H bond coupling.174 Capitalizing on the K2S2O8 mediated 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 into β-ketosulfones with thiophenols (shown 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 condition and coupled it with electrophilic alkynylating agent, ethynylbenziodoxolones (EBX) to obtain various β-alkynylated cyclopentanones (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−• (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 benziodoxolonyl radical gave the desired product.

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Scheme 142. Metal-free alkynylation and ring-expansion of alkenyl cyclobutanols. Reprinted with Permission from Ref 177. Copyright 2016 American Chemical Society

Scheme 143. Proposed mechanism for alkynylation/ring-expansion rearrangement. Reprinted with Permission from Ref 177. Copyright 2016 American Chemical Society

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 presence of K2S2O8 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 (Scheme 144).179 This protocol resulted in C(sp3)-C(sp2) and C(sp2)-N bond formations under metal free condition. The absolute requirement of TEMPO as catalyst and the precise role of K2S2O8 in this transformation were unclear.

Scheme 144. Metal-free oxidative annulations of arylamidines. Reprinted with Permission from Ref 179. Copyright 2014 American Chemical Society

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3.2 C-N bond formation Different nitration, azidation and intramolecular C-N bond forming reactions were reported under metal free condition 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. 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-dihydro2(1H)-quinolinones (Scheme 145).181 This 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 cyclized on the aromatic ring. H R3 N R1

R2 K2S2O8 (2 equiv)

O

+ NaNO2 R2 n=0,1

CH3CN,120 oC

NO2 n=0,1

R3 N

O

R1

Scheme 145. Yang’s metal-free carbonitration of alkenes using K2S2O8. Reprinted with Permission from Ref 181. Copyright 2013 Royal Society of Chemistry

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 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 2-(azidosulfonyl)benzoate as azide source and K2S2O8 as oxidant.184 Union of alkyl and azide radical 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 K2S2O8 mediated method for the synthesis of different Nheterocycles using N-arylbenzyl amines185a or imines185b with ortho-substituted anilines (Scheme 146). One key feature of this transformation was that, N-arylbenzyl amines could serve as the imine 78

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precursors that formed in situ in presence of K2S2O8 and subsequently underwent transimination with the reaction partner (Scheme 147). DDQ, 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 final step in converting dihydroquinazolinones to quinazolinones (Scheme 147).

Scheme 146. Metal-free transimination-intramolecular cyclization to heterocycles. Reprinted with permission from refs 185a-b. Copyright 2015 American Chemical Society; and 2016 Royal Society of Chemistry

Scheme 147. Proposed mechanism for Metal-free transimination-intramolecular cyclization. Reprinted with permission from refs 185b. Copyright 2016 Royal Society of Chemistry

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 (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. The mechanism for the amidation step involving K2S2O8 however requires further investigation. In the subsequent step, a base such as HSO4- or SO42- that 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 (viagraTM).

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Scheme 148. Decarboxylative amidation-cyclization to N-heterocycles. Reprinted with Permission from Ref 185c. Copyright 2016 Royal Society of Chemistry

Luo et al. engaged the Knoevenagel product of β-ketoamides in an intramolecular Csp2-H amidation reaction in presence of K2S2O8 (Scheme 149).186a In 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 1,4dihydropyridine substrates.186b

Scheme 149. Intramolecular C(sp2)-H amidation to N-aryl 2-quinolinones. Reprinted with Permission from Ref 186a. Copyright 2016 Royal Society of Chemistry

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 towards the synthesis of diversely functionalized pyrroles was reported by Guo, Guan and co-workers (Scheme 150).188 In this transformation, two enamine molecules reacted with each other. One molecule formed cationic amino radical by single electron transfer to K2S2O8, which was attacked by the second enamine molecule and stepwise oxidation-cyclization led to final product.

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Scheme 150. Metal-free oxidative enamine cyclization to polycarbonyl pyrroles. Reprinted with Permission from Ref 188. Copyright 2016 American Chemical Society

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-sulfenylated βdiketones employing benzazole and 1,3-diketone starting materials (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 to thiol and the oxidation of the later produced an electrophilic sulfur intermediate. Subsequent nucleophilic attack by 1,3-dikenone through its enol form rendered the final product (Scheme 152).

Scheme 151. K2S2O8 mediated Sulfenylation of β-diketones. Reprinted with Permission from Ref 189a. Copyright 2015 American Chemical Society

Scheme 152. Proposed mechanism for Sulfenylation of β-diketones. Reprinted with Permission from Ref 189a. Copyright 2015 American Chemical Society

They further reported the synthesis of different α-sulfenyl mono ketones under slightly modified conditions with benzooxazole-2-thione and different aromatic or aliphatic ketones with αhydrogens.189b In 2015, Lee reported a different strategy to access α-thio-β-dicarbonyl compounds with catalytic I2 and stoichiometric K2S2O8 combination employing dialkyl disulfide and β-diketone starting materials.190 Use of H2O2, TBPB also gave good conversion for this transformation, albeit in lower yield (ca. 10-15 %) compared to K2S2O8. 81

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Zhu reported a TEAB catalyzed oxidative coupling between alcohols/aldehydes and thiophenols in presence of K2S2O8 to access various thioesters.191 Disulfides could also be used as the sulphur 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 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 disulphides in synthesizing different 2-sulfenylindenones (Scheme 153).193 This method required 30 mol% of benzoyl peroxide (BPO), 2 equiv. of I2 and 2 equiv. of K2S2O8. In the plausible mechanism, the author showed the formation of thiyl radical in presence of I2/BPO (Scheme 154). Thiyl radical addition 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.

Scheme 153. Synthesis of 2-sulfenylindenones via Meyer-Schuster rearrangement and radical cyclization. Reprinted with Permission from Ref 193. Copyright 2016 American Chemical Society

Scheme 154. Proposed mechanism for tandem Meyer-Schuster rearrangement and radical cyclization. Reprinted with Permission from Ref 193. Copyright 2016 American Chemical Society 82

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Recently, conversion of N-tosylhydrazones into 1,2,3-thiadiazole with sulphur was disclosed by Cheng and co-workers by using catalytic TBAI (20 mol %) and 2 equiv. of K2S2O8 (Scheme 155).194 This reaction might involve more soluble (Bu4N)2S2O8 species formed in situ in the reaction. Use of other oxidants like 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 stepwise fashion.

Scheme 155. Metal-free synthesis of 1,2,3-thiadiazole from hydrazones and sulfur. Reprinted with Permission from Ref 194. Copyright 2016 American Chemical Society

Apart from thiol based oxidative transformations, different thiocyanation reactions were performed with K2S2O8 as primary oxidant. Wang and co-workers developed a method to introduce thiocyanate at the C-3 position of imidazopyridines employing KSCN and K2S2O8.195 Formation of either ·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 presence of K2S2O8 under metal free condition (Scheme 156).196 K2S2O8 gave the best result during optimization step although Na2S2O8 could be an alternative. Using isomeric substrates, this method allowed the access to either six-membered benzooxazines or five-membered imino-isobenzofurans via C-O and C-S bond formation respectively. In the mechanism, addition of SCN radical to olefin moiety triggered the cyclization process.

Scheme 156. K2S2O8 mediated thiocyanooxygenation of olefin containing amides. Reprinted with Permission from Ref 196. Copyright 2015 American Chemical Society

More recently, Bhat reported a K2S2O8-mediated para-selective thiocyanation of phenols and anilines.197 High regioselectivity was also observed for heterocycles such as indoles (C383

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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 presence of K2S2O8 (Scheme 157).198 This reaction was proposed to proceed with the formation of nucleophilic SCN radical.

Scheme 157. K2S2O8 mediated synthesis of 3-thiocyanato-4H-chromen-4-ones. Reprinted with Permission from Ref 198. Copyright 2016 Royal Society of Chemistry

In 2014, Rao disclosed an unprecedented strategy to access symmetrical and unsymmetrical diarylsulfones from simple arenes via double C-S bond formation (Scheme 158).199 This transformation demonstrated the use of K2S2O8 as efficient sulfonating agent. The use of (NH4)2S2O8 also showed similar efficiency during the optimization process.

Scheme 158. K2S2O8 mediated synthesis of diarylsulfones from simple arenes. Reprinted with Permission from Ref 199. Copyright 2014 Royal Society of Chemistry

Although

phosphorylation/phosphonation

were

frequently

performed

with

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 presence of 3 equiv. of K2S2O8.200a Use of other oxidants for the purpose appeared to be futile. This could be correlated to higher redox potential of sulfate radical anion which efficiently generated the phosphorous 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 condition.200b Guo and Cai reported C2 phosphonation of benzoxazoles and benzothiazoles under metal free condition with K2S2O8.200c Some C-O bond forming reactions were also realized with the use of K2S2O8 as oxidant. In 2013, Gevorgyan disclosed a C-H oxygenation of arene though the addition of a remotely placed pendant carboxylic acid group in 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

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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 presence of K2S2O8 that afforded different esters (Scheme 159).202 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 mechanism, in this case, a radical mechanism was proposed based on controlled experiments.

Scheme 159. KI- K2S2O8 mediated α-acyloxylation of acetone with carboxylic acids. Reprinted with Permission from Ref 202. Copyright 2016 Royal Society of Chemistry

Remote Csp3-H oxygenation of different aliphatic amines were previously reported with methyl(trifluoromethyl)dioxirane or transition metal

catalysts.

Recently,

Sanford

group

demonstrated an operationally simple and economical method for this reaction using K2S2O8 and H2SO4, which rendered various hydroxylated amino compounds (Scheme 160).203 A remote C-H functionalization was selectively achieved in this case, as the ߙ-C-H bonds in the substrate were deactivated through protonation of nitrogen under strong acidic condition. Tertiary and secondary C-H bond oxygenations were only demonstrated in this report.

Scheme 160. K2S2O8 promoted C(sp3)-H oxygenation of protonated aliphatic amines. Reprinted with Permission from Ref 203. Copyright 2017 American Chemical Society

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A K2S2O8 mediated selenoamination of alkenes was reported by Sun, Liu and co-workers where diphenyl diselenide and different amines including saccharin, dibenzenesulfonimide, benzotriazole, pyrazole, 1,2,4-triazole, 6-chloropurine etc. were used (Scheme 161).204 The reaction could involve either selenyl radical or a three-membered seleniranium intermediate (ionic). Both the process however would require oxidation of Ph2Se2 and K2S2O8 turned out to be the most efficient among oxidants like H2O2, DTBP or TBHP.

Scheme 161. K2S2O8 promoted selenoamination of alkenes. Reprinted with Permission from Ref 204. Copyright 2016 Royal Society of Chemistry

K2S2O8 has also found some important applications in oxidative halogenation reactions. Kitamura reported the synthesis of diaryliodonium triflates employing arenes and elemental iodine in 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 equivalent of K2S2O8.206 Use of NaCl solution mostly provided dichloro compounds whereas NH4Cl primarily gave mono-chloro derivatives. Anilines, halo aromatics and anisoles were used as substrates in this transformation. The authors didn’t attempt any other oxidant for this reaction. It was likely that sulfate radical ion oxidized the chloride ion (Eo = 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 coworkers with Selectfluor™ and K2S2O8 (Scheme 162).207 Despite of other existing methods for this transformation, use of K2S2O8 as cheap oxidant under metal free condition could make this method 86

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more practically viable. Formation of a benzylic radical with sulfate radical ion was most likely to occur which then combined with a fluorine atom from Selectfluor™ (Scheme 63).

Scheme 162. K2S2O8 promoted benzylic mono- and difluorination. Reprinted with Permission from Ref 207. Copyright 2015 Royal Society of Chemistry

Scheme 163. Proposed mechanism for benzylic C-H fluorination. Reprinted with Permission from Ref 207. Copyright 2015 Royal Society of Chemistry

In 2013, Jiang et al. developed a tandem K2S2O8-mediated method for hydroxybrominationoxidation of styrenes with KBr in water. Here K2S2O8 played the role in oxidizing Br which led to final product in presence of water.



to Br2,

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 presence of K2S2O8 which resulted in the formation of carbaldehyde dimethyl acetals (Scheme 164).209 A very similar transformation was previously reported by Minisci210a in presence of Ag(I)/S2O8 2- under acidic condition and with that notion, S2O8 2- 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 (formation of A), which then underwent oxidation, isomerization (forming B and C respectively) and subsequently transformed into the final product aided by molecular oxygen (Scheme 165).

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Scheme 164. K2S2O8-mediated multiple bond cleavage and formation between MeOH and quinoxalines. Reprinted with Permission from Ref 209. Copyright 2013 American Chemical Society

Scheme 165. Proposed mechanism for synthesis of carbaldehyde dimethyl acetals through bond cleavage and formation. Reprinted with Permission from Ref 209. Copyright 2013 American Chemical Society

Direct aerobic dehydrogenative coupling between two nucleophilic sp3 C-H and sp2 C-H center by K2S2O8 was developed by Jeganmohan that led to 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 (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 α-keto radical by sulfate radical ion. The α-keto radical upon subsequent reaction with dioxygen was converted to intermediate A (Scheme 167). Following an ipsosubstitution rearrangement in acidic medium (Hock-type rearrangement), intermediate B was formed, which then suffered a nucleophilic attack by arenes to form intermediate C. Next, a 88

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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.

Scheme 166. K2S2O8-mediated aerobic dehydrogenative α-diarylation of benzyl ketones with arenes. Reprinted with Permission from Ref 211. Copyright 2014 American Chemical Society

Scheme 167. Proposed mechanism for α-diarylation of benzyl ketones with arenes. Reprinted with Permission from Ref 211. Copyright 2014 American Chemical Society

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 trifluoromethyl group (Scheme 168).212 This reaction only required catalytic (25 mol %) K2S2O8 and did not proceed in absence of either O2 or K2S2O8, which indicated the complementary effect of both the oxidants. Since regeneration of K2S2O8 was not possible, it could be speculated that K2S2O8 only generated the CF3 radical as initiator from CF3SO2Na and the reaction propagated through other intermediate species as oxidant. In the mechanism, CF3 radical addition to olefin formed intermediate A, which then combined to oxygen and formed B (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, elimination of bromide from intermediate D furnished the product. O X R1

R2

+

CF3SO2Na

+

O2

K2S2O8 (25 mol %) 45 oC, DMSO

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R2 CF3

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Scheme 168. Oxytrifluoromethylation of alkenes using CF3SO2Na. Reprinted with Permission from Ref 212. Copyright 2014 Royal Society of Chemistry

Scheme 169. Proposed mechanism for oxytrifluoromethylation of alkenes. Reprinted with Permission from Ref 212. Copyright 2014 Royal Society of Chemistry

In 2014, Laha and co-workers reported a tandem oxidative conversion of 10,11-dihydro-5Hdibenzo[b.e][1,4] diazepines to phenazines with K2S2O8 (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 gave only a moderate conversion. In the reaction course, stepwise oxidation at the benzylic position generated a hemiaminal intermediate A, which led to CN cleavage and ring opening. The ensuing aminyl radical B was subsequently added on the arene and sequential decarboxylation-oxidation furnished the ring-contracted product (Scheme 171).

Scheme 170. Tandem oxidative conversion of 10,11-dihydro-5H-dibenzo[b,e][1,4]diazepines to phenazines. Reprinted with Permission from Ref 213. Copyright 2014 American Chemical Society

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Scheme 171. Proposed mechanism for oxidative removal of benzylic methylene group and new bond formations to phenazines. Reprinted with Permission from Ref 213. Copyright 2014 American Chemical Society

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 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 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 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 (Scheme 172).216 There are precedents that K2S2O8 could efficiently promote P-centered radical formation from diphenyl phosphine oxides and that was also the crucial step for this transformation. K2S2O8 converted the excited state Ir(III)* species to Ir(IV), which ultimately oxidized the advanced intermediate to product and completed the catalytic cycle.

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Scheme 172. Cascade reactions to form C-C and C-P bond formation with visible-light. Reprinted with Permission from Ref 216. Copyright 2016 American Chemical Society

In the same year, Shah reported a visible light promoted formation of Csp3-Csp2 bond at room temperature employing ethers and electron deficient arenes (Scheme 173).217 This reaction could be regarded as a milder alternative to a previously reported Minisci-type reaction in presence of K2S2O8.172Another 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 donoracceptor (EDA) complex was proposed to form between arene and peroxysulfate anion (S2O8-2), which dissociated into sulfate radical anion (SO4−•) in presence of visible light. Other steps in the mechanism were very similar to earlier examples.172

Scheme 173. C-H functionalization of ethers and electron-deficient arenes with visible light. Reprinted with Permission from Ref 217. Copyright 2016 Royal Society of Chemistry

Xia developed a visible-light promoted intermolecular cross-dehydrogenative coupling amination method between phenols and cyclic anilines in presence of K2S2O8.218 Although several phenol substrates could be used for this reaction, choice for aryl amine was restricted to phenothiazine (Eo = 0.78 V), which could be oxidized to a N-radical form. Partner phenols used herein had similar oxidation potential and thus afforded the 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 presence of K2S2O8 and a 25W CFL (Scheme 174).219 This reaction featured the Minisci reaction with broader applicability when combined with a photoredox catalyst.

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Scheme 174. Heteroarylation 2-trifluoroboratochromanones with photoredox organocatalysis. Reprinted with Permission from Ref 219. Copyright 2017 American Chemical Society

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, tetrabutylammoniumdecatungstate was used as the metal photocatalyst with K2S2O8 as oxidant. This protocol broadened the substrate scopes for Minisci type reactions. Using Ru(bpy)3Cl2 as photocatalyst and K2S2O8 as 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 reaction that has made an 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, 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 counter anions of the silver salts. For the palladium-catalyzed reactions with K2S2O8, the major proportions involve the 93

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directing group assisted arene C-H functionalizations. Notably, other types of Pd-catalyzed oxidative processes viz. hydroamination, hydroalkoxylation, carbocyclization have not been significantly explored with this oxidant. Uses of K2S2O8 as a compatible oxidant in Rh and Rucatalyzed 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 sole oxidant. Recently, their uses in visible-light and photoredox-catalyzed transformations has been realized. Even with the widespread use of K2S2O8 in oxidative transformations, in many cases, it’s exact role in the mechanism could be a matter of debate and not been thoroughly addressed with experiments. For example, when K2S2O8 is used as terminal oxidant in the presence of a metal-catalyst, 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 species) could proceed with either S2O82- or SO4•¯ , or by the metal species in higher oxidation state. Similar question could also arise for some metal free transformations, where K2S2O8 is used as primary oxidant or in those cases, where K2S2O8 is used in combination with another non-metal catalytic oxidant. More mechanistic investigations, like the one examplified with Flower II’s mechanistic studies 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.

AUTHOR INFORMATION Corresponding Author * Email: [email protected] * Email: [email protected] Notes

The authors declare no competing financial interest. ACKNOWLEDGEMENTS

Finalcial support was provided to J.S by SERB, New Delhi (ECR/2016/001474) and Centre of Biomedical Research, Lucknow. J.K.L thanks SERB, New Delhi for financial support. 94

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Radical

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