Diaryliodonium salts - Chemistry & Biology Interface

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Chemistry & Biology Interface, 2016, 6, 5, 270-281

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ISSN: 2249 –4820

CHEMISTRY & BIOLOGY INTERFACE An official Journal of ISCB, Journal homepage; www.cbijournal.com

Diaryliodonium salts: Emerging reagents for arylations and heterocycles synthesis Dalip Kumar*, V. Arun, Meenakshi Pilania, Manish K Mehra and Santosh B Khandagale Department of Chemistry, Birla Institute of Technology and Science, Pilani-333031, Rajasthan, India E-mail: [email protected] Received 3 October 2016; Accepted 22 October 2016

Abstract: Since the last one decade, a substantial research has been developed in the synthetic applications of diaryliodonium salts. They provide mild and general protocols for the construction of C-C and C-heteroatom bonds which are indispensable in many useful medicinal, agrochemicals and industrial molecules. This review is focused on the recent advances in synthetic utilities of diaryliodonium salts leading to the formation of heterocyclic frameworks, arylation using C-H functionalization strategies and their related mechanistic details. Keywords: Diaryliodonium salts, heterocycles, metal-catalysts, C-H functionalization.

1.1 Introduction

safety profile, air and moisture stability.[4]

Diaryliodonium salts are well renowned for their powerful electrophilic arylating behaviour. They have been used as aryl coupling partners for diverse range of alkenes, alkynes and heterocycles under metal and metal-free reaction conditions.[1-3] In current scenario diaryliodonium salts are frequently used as arylating agents besides their interesting applications in the construction of valuable heterocycles with five- and six-membered ring systems. The salient features of diaryliodonium salts are easy to synthesize, non-toxic, benign

1.1.1 Structure and Geometry

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Diaryliodonium salts 1 (Ar2IX) are a class of iodine (III) reagents with two aryl moieties and an anionic part (X). with ten electrons. The geometry of Ar2IX is pseudo trigonal bipyramidal with the weakly bonded anionic part at apical position, two aryl groups at apical and equatorial and two lone pairs occupied at equatorial position. X-ray studies showed diaryliodonium salts are T-shaped geometry (figure 1) with the bond angle of Ar-I-Ar is 90°. The I-X bond

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in 2 is longer than the average covalent bond diaryliodonium triflate salts. Next, Widdowson length. The high reactivity of iodonium salts and co-workers prepared Koser's work, 1980 1 is illustrated by the leaving group ability of Beringer's work, 1953 IO I(OH)OTs TMS iodobenzene (106 times > triflate) released from AcOH, Ac O MeCN + + Ar Ar Ar Ar reflux the corresponding diaryliodonium salt. Given H SO the superior solubility and non-nucleophilic I B(OH) I X m-CPBA I + Ar character, diaryliodonium salts 1 with triflate Ar + Ar m-CPBA Ar Ar Ar TfOH BF .Et O 0 °C - rt = X HSO , OTs, OTf Olofsson's work, 2007 and tetrafluoroborate anions are most frequently Olofsson's work, 2008 BF I(OAc) I(OH)OTs B(OH) employed in synthetic transformations.[5] 2

1

2

4

3

2

1

2

1

1

1

4

4

2

2

I

OTf Ar1

Ar 1a

I Ar

OTs Ar1

I Ar

1b

Ar1 1c

X

BF4 Ar 2

δ

Ar

+

Ar1

TfOH

TfOH -30 °C to RT Widdowson's work, 2000

I

Ar1

X = -OTf, -OTs, -BF4

DCM,1h

Ar

+

Ar1

Kitamura's work, 2006

Figure 2 Methods to prepare diaryliodonium salts

Figure 1 Structures and geometry of commonly diaryliodonium salts by employing IBD and used diaryliodonium salts arylboronic acids in triflic acid.[8] Recently, 1.1.2 Preparation of diaryliodonium salts Olofsson’s group developed an operationally simple, one-pot general protocol to achieve In general, preparation of diaryliodonium salts diaryliodonium triflates in good to excellent involves two steps, first oxidation of an aryl yields.[9] This convenient method uses iodide (I) to iodine (III) species and next ligand the reaction of aryl iodides and arenes in exchange with arenes or organometallic reagents. the presence of m-chloroperbenzoic acid Diaryliodonium salts 1 were first prepared by (m-CPBA) and triflic acid. Use of mild oxidant Meyer and Hartmann in 1894.[4a] Nevertheless, m-CPBA is advantageous due to its less the synthetic procedure was time-consuming solubility in organic solvents which facilitates and low-yielding. In 20th century this reagent its removal from the reaction mixture. The was rediscovered with the myriad of synthetic protocol is applicable to a range of iodoarenes transformations. In 1950, Beringer reported the and arenes. In an alternative route, the same synthesis of symmetrical and unsymmetrical research group[10] prepared various symmetrical diaryliodonium salts using hypervalent iodine and unsymmetrical diaryliodonium salts from (III) compounds namely, iodosylarenes (ArI=O), iodoarenes and arylboronic acids as outlined in (diacetoxyiodo)benzene (IBD), iodoxyarenes Figure 2. (ArIO2) and electron-rich arenes in the presence of sulphuric acid.[6] Later, in 1980 Koser and 1.1.3 Plausible catalytic cycle co-workers prepared diaryliodonium tosylates by employing hydroxy(tosyloxy)iodobenzene The reactivity of diaryliodonium salts and arylsilanes.[7] Afterwards, Kitamura et al. could be enhanced in the presence of disclosed an improved procedure to access copper and palladium catalysts. Research diaryliodonium salts by involving the reaction groups of Gaunt and Sanford disclosed the of (diacetoxyiodo)arenes and electron-rich metal-catalyzed (Cu and Pd) arylation of indoles arenes in triflic acid. Notably, triflic acid was using diaryliodonium salts, and investigated found to be an effective alternative to sulfuric the mechanistic pathways. In 2008, Gaunt et acid, acetic acid and p-toluenesulfonic acid al. proposed an elegant hypothesis that Cu(I) in the terms of reactivity and isolation of reduce the iodonium salt with the release of Chemistry & Biology Interface

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highly electrophilic arylcopper(III) species and of quinolines, phenanthridines, oxindoles, iodoarenes.[11] This in situ generated reactive arylcoumarins, acridines, and acridones. species could rapidly undergo functionalization with the nucleophile under fairly mild reaction 1.2.1 Quinolines conditions (Figure 3a). Chen’s group[13] developed a general and elegant pathway for the synthesis of quinolines 6 using multicomponent approach by employing diaryliodonium salts 3, nitriles 4 and alkynes 5. The reaction proceeded via [2+2+2] regioselective cascade annulation strategy to generate quinoline derivatives in good to excellent yields. Iodonium salts 3 bearing nonFigure 3 Plausible catalytic pathways involving coordinating anion PF6 gave better yields than Cu and Pd the salts with -OTf, Br- and -OTs counter ions. In 2006, Sanford and co-workers described a Use of electron deficient nitriles like ethyl Pd(II)-catalyzed C-H arylation of indoles and cyanoformate and diethyl cyanophosphate failed proposed a persuasive PdII/IV catalytic pathway to deliver the annulated products. Identified for the formation of 2-arylindoles (Figure 3b). reaction conditions covers variety of internal [12] alkynes, asymmetric alkynes, 1,3-hexadiyenes and diaryliodonium salts to prepare various 1.2 Heterocycles synthesis quinolines (Scheme 1.1). Ar2IOTf

-ArI

III Ar-Cu-OTf X

Cu(I)X

Nu-H

Nu-Ar

Nu-H

(III) Ar-Cu-OTf X

(III) Ar-Cu-Nu X

TfOH

-AcOH

(II)

Pd(II)(OAc)2

Ar

highly electrophilic

Figure 3a

Nu-Pd-OAc

Ar2IOTf

Nu-Ar

OTf Nu-Pd-OAc Ar (IV)

-ArI

Figure 3b

Heterocyclic compounds are extensively distributed in many natural products, life saving drugs and organic materials. Owing to the high significance and applications in drug discovery research construction of heterocycles is a longstanding interest. Assembly of heterocyclic compounds particularly, five- and six-membered ring systems and arylated heterocycles with interesting biological properties and for being important building blocks which are frequently encountered in natural products and various therapeutic agents. Though there are plethora of protocols to construct useful five and sixmembered heterocycles but still straightforward and eco-friendly methods to access these heterocycles are highly desirable. In view of significant advantages, diaryliodonium salts have been widely utilized in the direct arylation and preparation of bio-active heterocycles. In this review article, we have summarized the recent applications of diaryliodonium salts in the arylation of heterocycles and construction Chemistry & Biology Interface

I+

PF6

R2 R + R1CN +

R 3 N

R3 5

4 Ph

N

CH3 Ph CH3 6a, 66%

Cu(OTf)2 (10 mol %)

N

DCE, 120 °C

S C 2H5

6b, 82%

R1

R 6

R3

R2

35 examples 41-95% CH3

N

Ph

C 2H 5 H 3C

6c, 43%

N

Ph Et

Ph 6d, 74%

Scheme 1.1 Synthesis of quinolines and representative examples 1.2.2 Phenanthiridines Li et al.[14] reported a copper-catalyzed cascade coupling strategy for the synthesis of phenanthiridine derivatives 9 using diaryliodonium salts as an aryl source. The reaction was smoothly proceeded in the presence of Cu(OTf)2 and dichloroethane at 150 °C to produce 9 in good yields. Mechanistically, in situ generated copper(I) species from Cu(OTf)2 underwent oxidative insertion with diaryliodonium salt to furnish a highly

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electrophilic Ar-Cu(III) species. Consecutive nucleophilic addition of nitriles and annulation led to the corresponding phenanthiridines in moderate to excellent yields (Scheme 1.2).

14 and 2,6-ditertbutylpyidine (DTBP) as a base. Later, two different research groups [17-18] have independently reported the synthesis of oxindoles 12 under base-free conditions in the presence of a copper catalyst and diaryliodonium salts 14 (Scheme .131). R1

R2

R2

R N

O

+ Ar2IOTf

A, B ,C

R1 Ar

R 12

N

O

Conditions A:CuCl(15 mol %), DCM, DTBP, 60 °C, 24h B: Cu(OTf)2(10 mol %),DCE, 130 °C, 16h C: CuI(2.5 mol %), N2, DCE, 100 °C, 24-48h

14

13

Scheme 1.31 Synthesis of oxindoles 1.2.4 Carbocyclization

Scheme 1.2 Synthesis of phenanthiridine derivatives 1.2.3 Oxindoles Zhou co-workers described the coppercatalyzed arylation/vinylation of electrondeficient alkenes 10 to furnish highly substituted oxindoles 12 using diaryliodonium salts 11 as coupling partners.[15] After exploring various parameters, cuprous chloride in the presence of sodium bicarbonate found to be the best catalytic conditions. Formation of C-C bond proceeded with electrophilic addition of Cu(III)-aryl intermediate, followed by aromatization and reductive elimination to produce oxindoles 12 in good yields. The obtained oxindoles 12 were utilized to prepare complex bio-active natural product (±)-esermethole (Scheme 1.3). R N

O

+

OTf I+

11

10

N

26 examples 34-96%

MeO

Ph

Ph N

O

12a, 88%

N

O

N

12

12b, 45%

O 12c, 74%

R2

15 R1

16 18 examples 52-76%

CuTC( 5 mol%) DCM, 70 °C

17

Ar2IOTf

H

H

16a, 76%

3

CuCl( 5 mol%), DTBP R2 DCM, 70 °C

14

Br

4

16c, 52%

H

N

5

H

H 16b, 73%

18 10 examples 51-78%

16d, 70%

N

O

16e, 52%

MeO

18a, 51%

N

(±)-Esermethole

Ph N

R

R1

Ph

O

OMe MeO

R`

MeO

Ar

CuCl (15 mol %) R R1 NaHCO3, DCE, 80 °C

Interestingly, Gaunt and co-workers[19] described the efficient transformation of easily available alkenes and alkynes into substituted tetralins and cyclopentenes in good yields. Air-stable, copper(I) thiophene-2-carboxylate (CuTC), copper(I) chloride and sterically-hindered 2,6-ditertbutylpyidine (DTBP) are the suitable catalysts and base for the carbocyclization of 15 and 17. Detailed mechanistic investigations revealed that concerted 1,2-hydride and 1,5-hydride shifts led to the desired products 16 and 18 in good yields (Scheme 1.4).

MeO

18b, 70%

i−Bu

MeO

18c, 65%

Ph 18d, 78%

Scheme 1.4 Synthesis of substituted tetralins and cyclopentenes

O 12d, 60%

1.2.5 Benzocoumarins

Scheme 1.3 Synthesis of oxindoles

An unprecedented, Pd-catalyzed two consecutive C-C bonds forming strategy was employed Likewise, Tang et al.[16] explored the synthesis between diaryliodonium salts 14 and coumarins of oxindoles 12 using diaryliodonium salts Chemistry & Biology Interface

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19 to construct π-expanded 4,5-dibenzo- 23 (Scheme 1.6). coumarins 20. Formation of dibenzocoumarins 20 was smoothly proceeded without any external oxidant, ligand or directing groups. Interestingly, coumarins with hydroxyl group produced 20 without any O-arylation. However, sterically-hindered bis(2,4,6,-trimethylphenyl) iodonium triflate failed to afford the desired fused product 20. The proposed diarylation proceeded via Pd(II/IV) catalytic cycles with the synergistic activations of C-I and vicinal C-H bonds in diaryliodonium salts (Scheme Scheme 1.6 Synthesis and selected examples of 1.5).[20] 4-arylcoumarins Ar F

Ar

O

Ar1

Ar2I OTf 14

OEt

Ar1

Pd(OAc)2, DMF 80 °C, 3h

OH

21

Ph

HO

O O Cl 22a, 77%

F

O O 5-lipoxygenase inhibitor 23

Ph

Ph

Cl

N N N

F3C

O O 22 27examples 60-95%

N

O O 22b, 87%

Ph

O O 22c, 85%

CF3

O O 22d, 70%

CO2Et

F

NO2

O O 22e, 70%

Ar2I OTf 14

R 19

O

O

Pd(OAc)2, DMF 100 °C, 24h

R

20 O

O O 20a, 50%

Ph

O O 20b, 57%

O O OCH3 20c, 91%

O O 20e, 70%

O O 20f, 88%

F

O O 20d, 68% Me

F3C

O O 20g, 88%

O O 20h, 90%

Scheme 1.5 Synthesis of 4,5-dibenzocoumarins and their representative examples

O

22h, 86%

Recently, Pang et al.[22] reported a domino arylation and Friedel-Crafts acylation between o-acylanilines 24 and diaryliodonium salts 14 to achieve valuable acridine derivatives 25. Interestingly, this reaction proceeded effectively either under copper-catalyzed or metal-free conditions at an elevated temperature (130-135 °C). The reaction of o-aminobenzoates 26 with diaryliodonium salts 14 generated acridone framework 27 in high yields (Scheme 1.7). O O

1.2.6 4-Arylcoumarins

O

OMe 26 NH2

R Ar

R

Yang et al.[21] extended the synthetic utility of diaryliodonium salts in the construction of 4-arylcoumarins 23 via Pd-catalyzed arylations and cyclizations of o-hydroxylcinnamates 21. In the presence of CuI and Cu(OTf)2, O-phenylated cinnamates were obtained rather than expected arylcoumarins 22. By changing the catalyst from copper to palladium the reaction progressed in anticipated manner to afford 4-arylcoumarins 23 in better yields. This ligand and base-free approach is well-suited for diverse diaryliodonium salts and o-hydroxyl cinnamates and provided an alternative convenient route for 5-lipoxygenase inhibitor Chemistry & Biology Interface

O

O

22g, 71%

1.2.7 Acridines and Acridones

O

30 examples 50-90 %

HO

O

O O 22f, 60%

I Ar

condition 1 : CuI, 65 °C condition 2: 135 °C

N 27 H 7 examples 78-96%

OTf

NH2

Ar 14

Cu(OTf)2 (20%) DCE, 130°C, 12h

O

O

Me

24

R1

Ar

R1

N

25 28 examples 72-94%

O

Br

O F

N H 25b, 83%

N H 25a, 78% Ph

CH3

CO2Me

N H 25c, 96%

CH3

Ph

N

N 27c, 76%

O N 27a, 89%

O

27b, 84%

Cl N H 25d, 95% CH3

N 27d, 84%

Scheme 1.7 Some selected examples of prepared acridines and acridones 1.2.8 Quinazolinimines Recently, a new tandem approach was developed by Chen and co-workers[23] for the synthesis of quinazolinimines 29 and acridines 30 from easily accessible o-cyanoanilines 28

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Chemistry & Biology Interface, 2016, 6, 5, 270-281 OTf and diaryliodonium salts 14. With the more I PEG-400, MW + RSO Ar Ar RSO Na Ar favourable cyano group in cyanoanilines 50 °C, 10 min 32 31 14 17 examples for arylation , the protocol is well-suited for R= Phenyl, tolyl 81-96% methyl 1-amino-2-cynaocyclopentene and cyclohexene O O O O S S S S S to afford pyrimidine 29d analogues. Detailed Me O O O O H C CH CH optimization reaction conditions and 32a, 96% 32c, 82% 32d, 83% 32b, 91% mechanistic experiments disclosed that half an Scheme 1.9 Synthesis of diaryl sulfones and equivalent of 14 afforded quinazolinones 29, their representative examples whereas, its excess quantity (2.0 equiv) led to acridines 30 in good yields (Scheme 1.8). 1.2.10 Diaryloxazines 2

2

3

3

OTf

I Ar

NH

Ar

N

R

Ar CN

(0.5 equiv) Cu(OTf)2 20% DCE, 110 °C

N H 2N

I

Ar

14

R NH2 28

OTf

14

Ar

Cu(OTf)2 10% DCE, 65 °C

R

Ar N 30

29

11 examples 62-92%

30 examples 51-95%

NH

Cl

N

H 2N

H 2N 29b, 77%

29a, 79%

NH

NH

N

N

N

N

N 30a, 88%

OMe

NH

N

HN

A chiral copper(II)bisoxazoline-catalyzed reaction of readily available allylic amides 33 and diaryliodonium salts 3 led to enantioselective synthesis of 1,3-oxazines 34 and β,β’-diaryl enamides 35. Sterically hindered base 2,6-ditertbutylpyidine (DTBP) was used in order to avoid the decomposition of chiral catalyst by the release of HPF6. The electronic nature of diaryliodonium salts was found to be crucial for the enantioselective synthesis of 34 and 35. (Scheme 1.10).[25]

Ar

HN

( 2.0 equiv)

N

N H 2N 29c, 59%

HN 29d, 71%

CH3 F

CO2Me HN

HN

HN

CH3 N

CO2Me H 3C

30b, 69%

N

N

30c, 77%

30d, 69%

Scheme 1.8 Synthesis of quinazolinimines and acridines

I Ar O 34 R Ar = electron rich 14 examples 26-74%

PF6

PF6

CuTC (10 mol%) DCM , DTBP

R

34b, 80%

O

O

N O

O

34c, 64%

34d, 84%

N H

N H CO2Et

35b, 52%

OMe

O Cl Cl

Cl 35a, 53%

Br

O N H

OMe

OMe

O

O N H

35 Ar = electron poor 15 examples 46-83%

N O

N 34a, 61% SO2Ph

Ar

R

33

N O

N H

CuTC (10 mol%) DCM, DTBP

OMe

N

O

Ar

N H

OMe

Kumar et al.[24] disclosed a microwave-assisted, metal-free protocol for the synthesis of diaryl sulfones 32 using easily available diaryliodonium salts 14 and arenesulfinates 31. This high yielding protocol proceeded in benign and biodegradable solvent polyethyleneglycol-400 (PEG-400). Iodonium salts bearing different substitutents (Cl, Br, Me, COOH, OMe) were smoothly coupled to afford the corresponding sulfones in good to excellent yields. In the presence of cuprous iodide, unsymmetrical iodonium salts selectively transferred the electron-rich aryl moiety to produce appropriate sulfones in good yields. Under the optimized conditions, alkyl sulfone 32d was also achieved in high yield (Scheme 1.9).

I 3

O

Ar

3

N

1.2.9 Diaryl sulfones

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3

35c, 60%

CF3

35d, 58%

Scheme 1.10 Synthesis of various 1,3-oxazines and β,β’-diaryl enamides 1.2.11 β-arylation of ketones Huang and co-workers illustrated the application of diaryliodonium salts 14 in the selective β-arylation of ketones 36 using Pd-catalyst.[26] This method enabled to arylate various cyclic and linear ketones in excellent yields under oxidant free conditions. Under the reaction conditions, potassium trifluoroacetate (KTFA)

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and trifluoroacetic acid (TFA) behaved as buffer pair to maintain the required acidity of reaction medium. From the detailed mechanistic investigation and control experiments, involvement of Pd nanoparticles was suggested in the present catalytic system (Scheme 1.11).

were activated by the concerted metallation– deprotonation process and oxidation of Ir(III) to Ir(IV) (Scheme 1.12).[27] 1.2.13 2-Arylazoles

Kumar and co-workers[28] developed a general O O and high-yielding protocol for the C-H OTf Pd(OAc) (10 mol%) I Ph L (10 mol%) R + R S NHTs arylation of various azaheterocycles such as KTFA (2.0 equiv) L= 14 water, dioxane, TFA R 37 36 12h 80 °C, (1:20:2), oxadiazoles, thiadiazoles, benzoxazoles and TsHN S 28 examples Ph` 38-70% benzothiazoles using diaryliodonium salts 14 at O O O room temperature. The C-H arylation required Ph O Ph F Ph C simple catalytic system (CuBr/t-BuOLi) with N EtO C Me reduced reaction time (15 min) to produce 37d, 50% 37c, 61% 37a, 70% 37b, 66% Scheme 1.11 Synthesis of β-arylated ketones arylated azoles 42-43 in fairly good yields. The reactivity order of different heterocycles 40-41 and their representative examples was rationalized by the variable acidity of C2-H (oxadiazoles = benzoxazoles (pKa = 24.8) > 1.2.12 Arylheterocyles thiadiazoles >benzothiazoles (pKa = 27.3). Under the optimized conditions, C2-H arylation of benzothiazole could not be achieved. Modified reaction conditions involving the use of MW successfully afforded 2-arylbenzothiazoles in good yields. The synthetic utility of developed protocol was extended to prepare a Tafamidis analogue 43b which is a well known drug for neurodegenerative diseases (Scheme 1.13). 2

+

3

2

DG

I+

H

OTf

Ar

14

38

Me

DG

N

N

39b, 87%

39a, 86%

O

Me

39

62 examples 45-99%

Me

N-OMe

Me 2N

O

39d, 99%

O

Me

Me

N

Me

N

39c, 52%

O

Me

NH

Ar

[(Cp∗IrCl2) 2] 20 mol%

AgNTf 2 15 mol% PivOH, cyclohexane 100 °C

O

O

OH

OH

O

NiPr 2

Me 39e , 80%

39f , 92%

Me 39g, 42%

Me 39h, 52%

Me N

39i , 92% R

X

R Ar

42

X = O, S 23 examples 60-89%

Scheme 1.12 Synthesis of arylheterocycles

N

First example of Ir(III)-catalyzed β-arylation of aliphatic sp3 C-H bonds present in oximes and nitrogen-containing heterocycles like pyrazines, pyrazoles, quinolines, isoxazole, pyridine derivatives, substituted enamides and carboxylic acids, has been discovered. This protocol displayed high functional groups tolerance and also proved an effective synthetic tool for late-stage regioselective C-H arylation of complex triterpenoid molecules, for example, Lanosterol and Oleanolic acid. Outcome of the detailed mechanistic studies and density functional theory calculations suggested that sp3 C-H bond present in the substrates 38 Chemistry & Biology Interface

N

N

N X 40

H

CuBr (20-30 mol%) t-BuOLi, rt

OMe 42a, 60%

N

MeO

MeO

O 43a, 72%

N

R1

Ar1

41

N

N

O 43b, 69%

N

R1

Y 43

Ar

Y = O, S 22 examples 69-89%

N

N

S

42b, 74% N

Me

H

Y

CuBr/I (20-30 mol%) t-BuOLi, rt/MW

O

O MeO

14

N

N

OTf

I Ar

N S

Cl

42c, 84%

42d, 72%

Me

Cl N Cl

S 43c, 85%

F

N

S

S 43d, 78%

Scheme 1.13 Synthesis of various 2-arylazoles 1.2.14 Arylation of boron dipyrromethenes Jiao et al.[29] developed a metal-free, α-selective C-H arylation of boron dipyrromethenes 44 by employing mild coupling partners, diaryliodonium salts 14. The developed protocol furnished an array of mono 45 and di-arylated 46 boron dipyrromethenes in moderate to good

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yields. No product was formed upon addition of radical inhibitor (BHT) 2,6-di-tert-butyl-4methylphenol suggested the involvement of an aryl radical. The arylated compounds 45 and 46 displayed promising photophysical properties (Scheme1.14).

OTf

I

R + ArCN + Ar1CHO

R

Ar

N N B F F Ar 46 6 examples 23-45%

Cl

NaOH, DCE

Ar2IOTf (1.5 equiv) NaOH, DCE

N N B F F

R1 = 1,3,5-trimethylphenyl R1

R1

N N B F F

N N B F F

N N B F F

MeO

45b, 39%

OMe

t-Bu

45c, 43%

R1

R1

R1

N N B F F

N N B F F

N N B F F

46a, 27%

S

46b, 37%

Cl

Me

49 Ar 1 35 examples 50-94%

N

N

Ph O

N O

Me

N

Ph Me

O

Me

Ph O Br

O

N N B F F Ar 45 11 examples 17-63%

44

Cl

DCE, 120 °C, 24h

Ar O

R1

R1

45a, 45%

N

R1

Ar2IOTf (3.0 equiv)

R

OMe

Me R1

48

47

14

N

CuBr (10 mol%)

F

CHO 49a, 71%

O

49d, 76%

49c, 56%

49b, 64%

Scheme 1.15 Synthesis of benzoxazines and their representative examples 1.2.16 3-Iodochromenes

t-Bu

46c, 17% NO2

Scheme 1.14 Synthesis of α-arylated boron dipyrromethenes 1.2.15 Benzoxazines Very recently, an interesting Cu-catalyzed onepot [2+2+2] cascade annulation of nitriles 47, aldehydes 48 and diaryliodonium salts 14 has been developed by Jinhu et al. to construct benzoxazines 49 frameworks.[30] Iodonium salts 14 bearing functional groups such as aldehyde, halogens, alkyl and naphthyl were tolerated to afford 49 under the optimized reaction conditions. When two aldehyde groups were present in p-phthalaldehyde, then selectively one of –CHO underwent cyclization to give corresponding product (Scheme 1.15).

Togo and his co-workers [31] developed an efficient metal-free synthesis of 3-iodochromenes 51 from easily accessible diaryliodonium salts and aryl/alkly-propyn-1-ols 50. This elegant transformation proceeded via onepot O-arylation of aryl/alkly-propyn-1-ols in the presence of base t-BuONa and subsequent iodocyclization using N-iodosuccinimide and Lewis acid BF3Et2O. Iodochromenes 51 could be converted into beneficial molecules using metal-catalyzed C-C bond forming strategies (Scheme 1.16). I

a) t-BuONa, MgSO4 OH DCE, benzene 0 to 60 °C, 3 h

OTf R+

R 14

Ar

b) NIS, BF3.E t2O DCM, 0°C, 1 h

50

O

O

Me 51a, 62%

N

I Cl

I

51 Ar 35 examples 50-94% O

O I

I O

O R

O I

I

SO2Ph

51b, 54%

51c, 77%

51d, 70%

51e, 72%

Scheme 1.16 Synthesis of iodochromenes 1.2.17 2/4-Aryloxyquinolines Kumar et al.[32] disclosed a metal- and ligand-free approach for the synthesis of aryloxyquinolines 53 and 55 by the direct O-arylation of readily accessible quinolones Chemistry & Biology Interface

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52/54 using diaryliodonium salts 14 as aryl source. The reaction conditions were compatible with 2-/4-quinolones 52/54 and diaryliodonium salts 14 bearing sterically congested and sensitive functional substituents such as mesityl and halides. Moreover, use of microwave energy, mild reaction conditions, good product yields (55-80%) and short reaction time (5 Scheme 1.18 Synthesis of thiochromeno[2,3-b] min) are significant features of the protocol. indoles Prepared 4-aryloxyquinolines 55 were utilized in the construction of biologically important 1.2.19 α-Aryl-β-substituted cyclic ketones benzofuro[3,2-c]quinolines (Scheme 1.17). Pan et al.[34] reported an efficient method for the construction of α-aryl-β-substituted cyclic ketones 58 using diaryliodonium salts 14. The current transformation involved Cupromoted one-pot Michael addition followed by stereoselective arylation of cyclohexene-1one and cyclohepten-1-one using alkyl, aryl and sterically challenging lithium reagents as Michael donors and different diaryliodonium salts 14 as aryl partner. Proposed mechanistic pathway involves reaction of Cu(I) species Scheme 1.17 Synthesis of 2-and and diaryliodonium salts to generate an highly electrophilic Cu(III)-aryl species, which 4-aryloxyquinolines believed to react with enolate to produce the 1.2.18 Thiochromeno[2,3-b]indoles desired substituted cyclic ketones 59 in excellent yields (Scheme 1.19). An efficient Cu(OTf)2-promoted synthesis O O of thiochromeno[2,3-b]indoles 57 was Ar a) R CuLi.LiCN, Et O, -78 °C developed from alkynylaryl isothiocyanates R b) Ar IOTf 14, Cu(OAc) (20 mol%) 56 and diaryliodonium salts 14. The reaction DMF, -20 °C 59 58 33 examples involves successive S-arylation and cyclization 32-72% to furnish fused heterocycles 57. Under the CF Cl O O O O optimized conditions both electron-rich and N electron-poor iodonium salts were smoothly coupled with isothiocyanates 56 to afford the 59a, 41% 59b, 38% 59c, 38% 59d, 32% corresponding fused heterocycles 57.in 3570% yields. Addition of a radical inhibitor Scheme 1.19 Synthesis of α-aryl-β-substituted led to 57 in excepted yield which suggested cyclic ketones the involvement of carbocation mechanism 1.2.20 Oxazolines (Scheme 1.18).[33] R1

R

+

NCS

R1

OTf

I

Ar1

Ar

Cu(OTf)2, K2CO3

R

DCE, 50 °C

Ar

N

57

14

56

24 examples 35-70%

S

Ph

Ph

Me

N

S

N

57a, 70%

Me

N

S

S 57c, 35% Me

57b, 51%

F

N

S

57d, 60%

O

Ar

Me

N O 53 7 examples 65-80%

S

N 52 H

Ar

K2CO3, toluene 100 °C, 5 min, MW

Me

R

O

Ar

I OTf Ar

N 54 H

K2CO3, DMF 100 °C, 5 min, MW

14

N O 53a, 65%

R N 55 10 examples 60-80%

O

Ar

N Benzofuroquinolines

Me

Me

OMe

O

Me

Br

Me

N O 53b, 67% Me

N

O

53c, 73%

Cl

O

Me

O

O

Br

N 55a, 58%

N 55b, 70%

N

Ph

55c, 76%

2

2

2

2

3

Adam et al. demonstrated a novel coppercatalyzed ring closure-carboarylation strategy for the construction of oxazoline derivatives 61 Chemistry & Biology Interface

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using easily accessible alkylpropargylamides 60 and diaryliodonium salts 11. Iodonium salts 11 having ortho and para substitutents delivered 61 in relatively better yields. This one-pot method provides access to novel oxazolines heterocyclic core with highly substituted exo double bond (Scheme 1.20).[35] Ar O N H

R

I

EtOAc, 50 °C 6-12h

Ar 60

N

N 61a, 86%

Br N

61b, 56%

N 61c, 61%

O

63b, 98 %

O

63d, 65 % O

O

N

N

N

t-Bu

Ph

Br

t-Bu

N

63c, 99 %

O

Br

t-Bu

t-Bu

Ph

63f, 54 %

63g, 74 %

63h, 24 %

N

O

61d, 43%

Scheme 1.20 Synthesis of oxazolines and their representative examples 1.2.21 Pyridones

O

O

Me

N

1.2.22 2-(Phenylthio)pyrimidine

Br

O

63a, 98 %

Pirfenidone (antifibrotic drug)

Scheme 1.21 N-arylation of 2-pyridones using diaryliodonium salts

O

S

MeO2C

N 63i, 99%

Me

O t-Bu

N

63e, 95 %

R

Me

O

O

N

61 23 examples 40-86%

11

R

Ar

63 27 examples 23-99%

t-Bu

N

O

toluene, Et3N, r.t.

N

62

O

Br

CuCl (10 mol%)

NH

R

Br

CuCl (2.5 mol%)

Ar1

+

+

14

Ar1

OTf

Ar

Ar

O

O

O

OTf

I+

In 2013, Karade et al.[37] reported the synthesis of biologically important scaffold 2-(phenylthio)pyrimidine (65) via C-S coupling of 4-aryl-3,4-dihydropyrimidine-2(1H)thione (64) using diaryliodonium salts 14 in the presence of catalytic amount of CuO nanoparticles. This protocol showed good compatibility towards various functional groups and furnished 65 in good to excellent yields. In the case of unsymmetrical iodonium salts, mixture of S-arylated products were obtained. The CuO nanoparticles was recycled and reused for three times without any loss of catalytic activity (Scheme 1.22).

Recently, Kim and co-workers reported the copper-catalyzed N-arylation of 2-pyridones 62 using solid aryl coupling partner diaryliodonium salts 14 at room temperature. Various symmetrical and unsymmetrical diaryliodonium salts have been employed with 2-pyridones, to furnish the desired products in excellent yields. Pyridones with bulkier C6-phenyl, for example, 2-hydroxy-6-phenylpyridine,upon treatment with iodonium salt produced O-arylate 63g as the major product.. Further investigation of counter ion effect in diaryliodonium salts revealed that OTf, BF4 and PF6 ions yielded the desired products in 63 good yields in shorter time, while OTs, Cl, and Br showed the negative impact on the reaction and delivered the desired products low yields and longer reaction times. Next, the Scheme 1.22 Synthesis of 4-aryl-3,4developed protocol was effectively utilized to dihydropyrimidine-2(1H)-thiones prepare an antifibrotic drug, Pirfenidone (63i) which is used in the treatment of idiopathic 1.2.23 N-(2-Hydroxyaryl)benzotriazoles pulmonary fibrosis (Scheme 1.21).[36] Very recently, Dong-Liang Mo and his research group developed a metal-free approach for the synthesis of N-(2-hydroxyaryl)benzotriazoles 68 using N-hydroxybenzotriazoles 66 and O

R1

R2

NH

N H 64

Me

I

OTf

R

R

S

nano CuO (10 mol%) K2CO3( 2.5 equiv.) DMF, reflux, 12 h

14

OMe

Me

N

S

65a, 84%

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N 65

N

N

R

S

16 examples 63-88% Cl

O

N

EtO

N

EtO

Me

Me

O

O

N

R1

NO2

O

EtO

O

R2

S

65b, 76%

Me

N

65c, 64%

N

EtO

S

Me

N

S

Cl

65d, 70%

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diaryliodonium salts 14.[38] Initially, reaction of N-hydroxybenzotriazoles 66 with 14 at room temperature delivered the O-arylated product 67, which underwent [3,3] sigmatropic rearrangement at 60 °C to provide N-(2hydroxyaryl)benzotriazoles 68 in good yields. A variety of diaryliodonium salts and N-hydroxybenzotriazoles were examined to show the generality of developed protocol. Authors also showed practical utility of the protocol by the synthesis of novel P, N-type ligands 69 in two steps (Scheme 1.23). N

R

N

N OH

Ar2I OTf 14

N

t-BuOK R MeCN, r.t.

N O

3, 3

N

68

N N

Me

67b, 89 % Me N

N

N

OH

OH

N

N OH

68b, 79%

N N

1.

4.

N N O

5.

67d, 86 %

N

References

3.

CO2Me

67c, 90 %

We are grateful to CSIR, New Delhi for financial support

2.

Ph2P

69, 40% P, N-ligand

Me

Acknowledgments

O

N

N N O

N N

N

Ar

6.

N OH

7.

MeO 68a, 69%

N

OH

N

N N O

67a, 99 %

N

N

16 examples 43-89%

N

N N O

N

Ar

23 examples 40-99% N

N

MeCN, 60 °C

67

66

R

the design of novel transformations in the fields of metal-free and transition-metal-catalyzed reactions in the future.

68c, 82%

68d, 68%

Scheme 1.23 Synthesis of N‑(2-hydroxyaryl) benzotriazoles and representative examples Conclusions

9. 10.

In summary, we have described the recent achievements of diaryliodonium salts in the constructions of various C-C, C-N, C-S and C-O bonds enabling to access valuable heterocycles under mild reaction conditions. The impressive features of diaryliodonium salts are enhanced electrophilicity, stable solid compounds, no special precaution to handle, easy to prepare and recyclability of released iodoarenes during the reaction. Hopefully, these unique advantageous properties are highly favourable for synthetic community. In recent years, a range of new substrates have been explored for the arylation and manifold novel transformations for the assembly of heterocyclic frameworks. Undoubtedly, the distinctive properties of diaryliodonium salts will open a new avenue for Chemistry & Biology Interface

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11. 12. 13. 14. 15. 16. 17. 18. 19.

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