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Contents lists available at ScienceDirect

Journal of Hazardous Materials

jou rn al hom epage: www.elsev ier .com/ locate / jhazmat

omplex of 2-(methylthio)aniline with palladium(II) as an efficientatalyst for Suzuki–Miyaura C C coupling in eco-friendly water

yandshwar K. Rao, Arun Kumar, Mrinal Bhunia, Mahabir P. Singh, Ajai Kumar Singh ∗

epartment of Chemistry, Indian Institute of Technology Delhi, New Delhi 110016, India

i g h l i g h t s

Synthesis of palladium complex of abidentate ligand of (N, S) type.Determination of single crystal struc-ture of the complex.Complex showed excellent activityfor Suzuki–Miyaura coupling reac-tions in water.TON values up to 93,000 wereachieved.

g r a p h i c a l a b s t r a c t

r t i c l e i n f o

rticle history:eceived 27 August 2013eceived in revised form4 November 2013ccepted 21 November 2013vailable online xxx

a b s t r a c t

2-(Methylthio)aniline (L1), a bidentate (S,N) ligand synthesized by the reaction of o-aminothiophenolwith methyl iodide, on reacting with Na2PdCl4 in acetone and water gives a complex [PdL1Cl2] (1). Singlecrystal X-ray diffraction studies have revealed that the geometry of palladium in 1 is nearly square-planar and the ligand L1 is bound to the palladium through S and N in a bidentate coordination modeforming a five membered chelate ring. This complex functions as a thermally and air stable catalyst ofhigh efficiency for Suzuki–Miyaura C C coupling reactions in water. It catalyzes C C coupling betweenvarious aryl bromides and phenylboronic acid under mild reaction conditions in water. TON value up to

eywords:alladium(II) complexuzuki–Miyaura coupling-(Methylthio)anilinerystal structureater

93,000 has been obtained.© 2013 Elsevier B.V. All rights reserved.

atalyst

. Introduction

Palladium catalyzed cross coupling reactions have been recog-ized as cutting age technologies [1–3] in the recent past as theyave been widely used in a variety of applications ranging from

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atural product synthesis, medicinal chemistry to material science1,4–8]. Suzuki–Miyaura C C cross-coupling has been consideredery important among them as reaction conditions required for it

∗ Corresponding author. Tel.: +91 11 26591379; fax: +91 11 26581102.E-mail addresses: aksingh@chemistry.iitd.ac.in, ajai57@hotmail.com

A.K. Singh).

304-3894/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.jhazmat.2013.11.045

are mild and arylboronic acids are easily available or synthesized[9,10]. Several polychlorinated biphenyls (PCBs) [11], hydroxylatedPCBs [12], dioxin-like monofluorinated PCBs [13], non-dioxin-likePCBs [14], and their derivatives [15,16] required for environmentalstudies [such as study of bacterial degradation of polychlorinatedbiphenyls (PCB’s) in environment] have also been synthesized bythis reaction. This reaction is generally carried out in organic sol-vents containing small amount of water resulting in environmentunfriendly waste. The use of eco-friendly pure water as a solvent

)aniline with palladium(II) as an efficient catalyst for Suzuki–Miyauraorg/10.1016/j.jhazmat.2013.11.045

appeals to chemists [17] and its use for the present coupling reac-tion important to generate compounds for environmental studies,can bring it to a level of low hazard. However, in general, water hasnot been considered an appropriate solvent for synthetic chemistry

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ARTICLEAZMAT-15570; No. of Pages 6

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ecause of several issues such as substrate molecules themselvesay react with water. Immiscibility of most of the organic reactantsith water makes the reaction mixture heterogeneous, anotherorrisome point. Even though reactants and/or reagents are notomogenized in aqueous medium, several reactions have been

ound accelerated [18–20] in water. The replacement of organic sol-ents with water in transition metal based homogeneous catalysisas received remarkable attention, as water is inexpensive, non-oxic, non-flammable, environmentally sustainable [21]. For theseeasons, intensive research efforts are being directed to performhe Suzuki–Miyaura coupling either via aqueous biphasic cataly-is or in neat water [22–26]. Initially, water-soluble phosphinesere used as ligands for the cross-coupling reactions in aqueousedia [27]. As the organophosphorous compounds are poisonous

nd also prone to oxidation due to its trivalent nature when useds ligands, our research group [28–38] as well as several otheresearch [39–49] workers have focused on the development of newhosphine-free ligands by incorporating the chalcogen (S, Se or Te)tom in ligand’s framework. Such ligands have been used as build-ng blocks to design moisture and air-insensitive Pd based catalysts.owever, reports on catalysis of this coupling reaction in aqueousedium using such catalysts [30,50] or other phosphine free cat-

lytic systems such as (C,N,C) pincer palladium complexes [51] areimited in number. Taking into account the potential of palladiumomplexes of organochalcogen ligands as air and moisture insensi-ive catalysts of high efficiency and limited reports on catalysis of

chemical processes in aqueous medium using such catalysts, inhis paper we are presenting the synthesis of a potentially bidentateS,N) ligand L1 and its palladium(II) complex 1 and application ofhis complex for catalyzing Suzuki–Miyaura C C Coupling reactionn H2O with high efficiency.

. Experimental

.1. Starting materials

2-Aminothiophenol, 4-chlorobenzaldehyde, 4-bromobenzald-hyde, 4-bromobenzonitrile, 1-bromo-4-nitrobenzene,-bromotoluene, 4-bromoanisole, bromobenzene, 4-bromo--chlorobenzene, 1-chloro-4-iodobenzene, 4-bromopyridine,-bromopyridine and disodium tetrachloropalladate was procuredrom Sigma–Aldrich (USA). Commercially available reagents weresed as received without further purification. The solvents viz.eOH and acetone were dried by the standard methods. All

eactions were carried out in glassware dried in an oven undermbient conditions except the synthesis of L1 which was carriedut under N2 atmosphere. The commercial nitrogen gas was usedfter passing it successively through traps containing solutionsf alkaline anthraquinone-sodium dithionite, alkaline pyrogallol

conc. H2SO4. Nitrogen atmosphere was created using Schlenkechnique. The progress of the reactions was monitored by NMRpectroscopy. Solvents were evaporated on a rotary evaporatornder reduced pressure. Yields refer to isolated yield of productfter column chromatography.

.2. Characterization techniques

The 1H, 13C{1H} and DEPT–135 NMR spectra were recorded on Bruker Spectrospin DPX-300 NMR spectrometer at 300.13 and5.47 respectively. Quaternary carbon signals in L1 as well as 1ere assigned with the help of DEPT-135 NMR spectral data. Single-

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rystal structure data were collected with a Bruker AXS SMARTpex CCD diffractometer using Mo K� (0.71073 A) radiation at98(2) K. The software SADABS was used for absorption correctionif needed) and SHELXTL for space group, structure determination,

PRESS Materials xxx (2013) xxx– xxx

and refinements. Hydrogen atoms were included in idealized pos-itions with isotropic thermal parameters set at 1.2 times that of thecarbon atom to which they were attached in all cases. IR spectra inthe range 4000–250 cm−1 were recorded on a Nicolet Protégé 460FT-IR spectrometer as KBr pellets.

2.3. Synthesis of ligand (L1)

2-Aminothiophenol (20 mmol; 2.50 g) was reacted with sodiummetal (0.75 g) in dry methanol (30 mL) for 1 h at room temperature.There after 4 mL of methyl iodide was added and the reaction mix-ture was refluxed for 2 h. After cooling, the mixture was pouredinto water (50 mL) and extracted with diethylether. The organiclayer was dried with sodium sulphate and purified by columnchromatography on silica gel. 1H NMR (300.13 MHz, CDCl3, 25 ◦C):ı = 2.38 (s, 3H, SMe), 4.26 (bs, 2H, NH2), 6.73–6.77 (m, 2H, H1 + H2),7.12 (dt, J = 7.5, 1.5 Hz, 1H, H3), 7.38 (dd, J = 7.8, 1.5 Hz, 1H, H4).13C{1H} NMR (75.47 MHz, CDCl3, 25 ◦C): ı = 17.6 (SCH3), 114.9 (C1),118.7 (C3), 120.2 (C5), 128.8 (C2), 133.3 (C4), 146.9 (C6). IR (KBr,cm−1): 3061, 2918, 1607, 1478, 1443, 1304, 749.

2.4. Synthesis of complex 1

A solution of Na2PdCl4 (0.147 g, 0.5 mmol) made in 5 mL of waterwas mixed with a solution of L1 (0.070 g, 0.5 mmol) in 10 mL ofacetone with vigorous stirring at room temperature for 2 h. Theyellow solution was extracted with CHCl3, dried with anhydrousNa2SO4 and evaporated to dryness under vacuum to obtain 1 asan orange colored powder. Single crystals of 1 were grown froma 1:1 mixture of acetonitrile and methanol. Yield: 0.2793 g (85%).1H NMR (300.13 MHz, DMSO-d6, 25 ◦C): ı = 2.83 (s, 3H, SMe), 7.33(d, J = 7.8 Hz, 1H, H1), 7.39–7.62 (m, 4H, H2 + H3 + NH2), 7.84 (d,J = 7.5 Hz, 1H, H4). 13C{1H} NMR (75.47 MHz, DMSO-d6, 25 ◦C):ı = 27.6 (SMe), 127.3 (C1), 128.6 (C2), 130.7 (C5), 131.2 (C4), 131.5(C3), 146.1 (C6). IR (KBr, cm−1): 3007, 2921, 1618, 1580, 1441, 1219,967, 756.

2.5. General procedure for Suzuki reaction of aryl halides withphenylboronic acid

An oven-dried flask was charged with aryl halide (1.0 mmol),phenylboronic acid (1.2 mmol), K2CO3 (2.0 mmol) and H2O(4.0 mL). A solution of catalyst 1 (amount as per mol% of Pd used inan experiment) in DMF (100 �L) was added via syringe. The flaskwas placed on an oil bath maintained at 90 ◦C under ambient condi-tions and the reaction mixture stirred until maximum conversion ofaryl halide to product occurred. The reaction mixture was cooledand poured into 50 mL of water. The resulting aqueous mixturewas extracted with diethylether (2× 10 mL). The organic layer wasdried with Na2SO4 and its solvent was removed and recovered withrotary evaporator. The resulting residue was purified by a columnchromatography on silica gel using chloroform and hexane mixture(5:95% to 15:85%) as eluent. The solvents were recovered for safedisposal.

2.6. Hg poisoning test

A mixture of complex 1 (0.5 mol% in 5 × 102 �L of DMF) andelemental Hg (Pd:Hg::1:1000) was taken in a reaction flask andstirred at room temperature for 10 min before the addition of cou-pling reactants. Thereafter mixture of 4-bromoanisole (2.0 mmol),

)aniline with palladium(II) as an efficient catalyst for Suzuki–Miyauraorg/10.1016/j.jhazmat.2013.11.045

phenylboronic acid (2.5 mmol) and water (4.0 mL) was added, andthe reaction under optimum conditions was carried out. The yieldof expected product (4-methoxybiphenyl) was only ∼8% after 6 h.The Hg was recovered and stored safely.

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SH2NPd

MeSNH2 Me

SHNH2

1

2 3

4

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Na2PdCl4

Acet one- Water

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L1

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3

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3

daLaT

FlPN8

Scheme 1. Synthe

. Results and discussion

.1. General

The syntheses of ligand L1 and its Pd(II) complex (1) are given incheme 1. The synthesis of complex involves the use of Na2PdCl4s precursor for complexation.

Ligand L1 has higher solubility in CHCl3, CH2Cl2, CH3CN, CH3OH,2H5OH and diethyl ether than complex 1. Ligand L1 and complex

both are sparingly soluble in hexane.

.2. NMR spectra

The 1H, 13C{1H} NMR spectra of L1 were found to be charac-eristic. In 1H NMR spectrum of complex 1, signals of SMe andH2 protons were found to be deshielded by ∼0.5 and 3.2 ppm

espectively in comparison to those of free L1. The magnitude ofeshielding was also noticed to be quite high in case of 13C{1H}MR data. The signals of SMe carbon and C5 (quaternary carbon) ofomplex 1 are deshielded by ∼10 ppm with respect to that of free1. However, other quaternary carbon i.e. C6 (H2N C) gets slightlyhielded (∼0.8 ppm) on complex formation.

.3. Molecular structure of complex 1

The structure of 1 has been elucidated by single crystal X-rayiffraction. It is shown in Fig. 1 with its selected bond lengths

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nd angles. The Pd has nearly square planar geometry. The ligand1 coordinates with Pd in a neutral bidentate mode through Snd N mode forming a five membered chelate ring around Pd.he Cl· · ·H(aromatic) and Cl· · ·H ( SCH3) secondary interactions

ig. 1. ORTEP diagram of 1 with 40% probability ellipsoids. Selected bondengths (Å): Pd(1) N(1) 2.034(3); Pd(1) S(1) 2.2478(12); Pd(1) Cl(2) 2.2887(13);d(1) Cl(1) 2.3242(12). Selected bond angles (◦): N(1) Pd(1) S(1) 86.86(10);(1) Pd(1) Cl(2) 174.52(11); S(1) Pd(1) Cl(2) 90.53(5); N(1) Pd(1) Cl(1)9.94(10); S(1) Pd(1) Cl(1) 175.63(5); Cl(2) Pd(1) Cl(1) 92.91(5).

ClCl

L1 and complex 1.

in the crystal of 1 result in the formation of a three dimen-sional extended structure as shown in Fig. 2. The Pd N bondlength 2.034(3) A in 1 is almost similar to the reported value of2.018(6) A reported for a palladium(II) complex of a tridentate sele-nated Schiff base ligand [29] and is longer than 1.986(6) A reported[34] for [PdCl{C6H5(C6H4 2 O−)C N (CH2)3SePh}]. Two types ofbond distances for Pd Cl [2.2887(13) and 2.3242(12) A] have beenobtained in 1. They are shorter or comparable to those reportedfor [PdCl{C6H5(C6H4 2 O−)C N (CH2)2SPh}] [34]. The bond dis-tances of Pd S 2.2478(12) A was almost similar to 2.2563(6) Areported for [PdCl{C6H5(C6H4 2 O−)C N (CH2)2SPh}] [34]. Thecrystal data, refinement parameter, bond lengths and bond anglesare given in Supplementary Data (Tables S1 and S2).

3.4. Catalysis of Suzuki–Miyaura C C coupling reaction bycomplex 1

Suzuki–Miyaura coupling (SMC) reaction which producesbiaryls has become more popular in comparison to various othermethods (e.g. Kharash coupling, Negishi coupling, Stille coupling,Himaya coupling, Liebeskind–Srogl coupling and Kumuda cou-pling) available for construction of C C bonds. The preference forSMC reaction over other Pd-catalysed cross-coupling reactions isnot incidental and has been attributed to several factors. This pro-cess involves boronic acids which are much less toxic (i.e. saferfor the environment) than organostannane and organozinc com-pounds. Use of water as a solvent makes this reaction furthereco-friendly. The complexes of organochalcogen ligands with pal-ladium are considered less hazardous substitutes of palladiumcomplexes of organophosphorous ligands (commonly used for thecatalysis of this reaction), as many organophosphorous compoundshave been listed in high toxicity category [52] by World HealthOrganization (WHO).

Other advantages of SMC over other coupling reactions are themild reaction conditions and the commercial availability of thediverse boronic acids [53]. Boronic esters and organotrifluorobo-rate salts may also be used [53] instead of boronic acids. Further,the handling and removal of boron-containing byproducts is easy,compared to other organometallic reagents, especially in scale-upsynthesis. The use of water as a solvent, further expands thescope of the coupling reaction with respect to substrates, boronicacid derivatives and related species. Further SMC is feasible foraryl-vinyl coupling and coupling of alkyl bromides [54] withvarious types of boronic acids. The pseudohalides such as triflates(OTf), may replace halides. In Heck reaction, in which aryl or vinylhalide and alkene are coupled to give a more substituted alkeneby Pd catalysts, the intermolecular reaction often proceeds wellwhen the alkene is electrophilic. With nucleophilic substituents,the reaction gives less satisfactory results. The Kumada couplingis very sensitive to air and the presence of radical inhibitors. Thus

)aniline with palladium(II) as an efficient catalyst for Suzuki–Miyauraorg/10.1016/j.jhazmat.2013.11.045

it is of limited use. In the Stille reaction, stannates are used assubstrates, and many of these are environmentally hazardous.There is no toxicity issue involved with organoborane reagents.These reagents are usually easy to prepare in the laboratory, and

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Table 1Suzuki coupling reactions catalyzed by 1.a

Entry no. Aryl halide/hetero-aryl halide Time (h) 1

Mol% Pd Yieldb TON

1 ClOHC 24 0.5 – –

2BrOHC

1 0.002 95 47,5003c,d 1 0.002 28 14,0004c,e 1 0.002 70 35,0005c,f 1 0.002 86 43,000

6c

BrH3C1 0.005 33 6600

7c 22 0.005 54 10,8008 1 0.01 85 8500

9c

BrMeO24 0.002 32 16,000

10c 22 0.005 50 10,00011c 22 0.008 51 637512 1 0.02 80 4000

13 ICl 1 0.001 93 93,000

14 BrCl 1 0.002 94 47,000

15c

NBr

16 0.002 18 900016 2 0.1 86 860

17c

N Br16 0.002 50 25,000

18 2 0.1 92 920

a Reaction conditions: 1.0 equiv. of aryl or heteroaryl halide, 1.3 equiv. of phenylboronic acid, and 2 equiv. of base (K2CO3), solvent: water and temperature of bath 90 ◦C.b Isolated yield.c NMR percentage yield.

tcGopaa

d Base used was 2 equiv. of NEt3.e Base used was 2 equiv. of NaOAc.f Base used was 2 equiv. of Na2CO3.

he workup procedures are simpler than the ones used in otherross-coupling methods. The cross-coupling reactions based onrignard reagents have several drawbacks. These include abilityf the Grignard reagents to attack the reactive functional groups

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resent in the starting materials whereas the SMC process tolerates variety of functional groups in the starting partners as discussedbove. Although tin compounds can be used to minimize the

Fig. 2. Molecular packing of 1 showing Cl· · ·H (aromatic) and Cl· · ·H (SMe) int

disadvantages posed by Grignard reagents, they offer a less attrac-tive choice due to toxicity and the difficulties associated with thepurification of certain tin compounds. The usage of other accessiblemethods is restrained, as the availability of the corresponding

)aniline with palladium(II) as an efficient catalyst for Suzuki–Miyauraorg/10.1016/j.jhazmat.2013.11.045

organometallic reagents is somewhat limited. In view of thesefactors, Suzuki coupling (Scheme 2) is an attractive choice for alarge-scale C C coupling.

eractions [Cl H bond lengths are in the range of 2.761(2)–2.949(1) A].

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fha2wWswiioFcu14El(tosdtp(oc9rdemhtuaiac

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Scheme 2. Suzuki–Miyaura coupling for C C bond formation.

The catalytic activity of the palladium complex 1 was testedor the Suzuki–Miyaura C C cross-coupling of aryl and heteroarylalides with phenylboronic acid. K2CO3 was found to be the mostppropriate base for the functioning of catalyst (Table 1: Entries–5). For alternatives viz. NEt3, NaOAc or Na2CO3 lesser conversionere obtained within same reaction time (Table 1: Entries 2–5).ater was used as solvent during catalysis of SMC of all the sub-

trates. All the catalytic reactions were carried out at 90 ◦C for a timehich varied generally with substrates. The coupling of 1-chloro-4-

odobenzene with phenylboronic acid resulted in 4-chlorobiphenyln 93% yield (Table 1: Entry 13) in 1 h at very lower catalyst loadingf 1 (0.001 mol%) and a very high TON value (93,000) was achieved.or the coupling of 1-bromo-4-chlorobenzene higher amount ofatalyst 1 is required (0.002 mol%) to achieve almost similar prod-ct yield but TON value (47,000) achieved is less (Table 1; Entry4). Coupling of deactivated aryl bromides (4-bromotoluene and-bromoanisole) was also carried out using this catalyst (Table 1:ntries 6–12). For the coupling of 4-bromoanisole higher catalystoading (0.02 mol%) is required than that used for 4-bromotoluene0.01 mol%) under similar reaction condition and for 1 h reactionime (Table 1: Entries 8 and 12). For catalyst 1 the dependencef catalytic activity on the electron withdrawing properties of theubstituent R at the aryl bromide was observed i.e. electron with-rawing groups on the aryl ring increased the reaction rate. Forhe deactivated 4-bromoanisole, the reaction gave the coupledroduct (MeO C6H4 Ph) with conversions in the range 32–80%Table 1: Entry 9–12). In order to explore the catalytic potentialf complex 1 for heteroaryl halides, its activity was tested in theoupling of 2- and 4-bromopyridine and good conversions (up to2%) were obtained (Table 1: Entries 15–18). All these catalyticeactions were performed under ambient conditions. Complex 1id not show any catalytic activity for aryl chlorides in the pres-nce of its 0.5 mol% (Table 1: Entry 1). Coupling of aryl chlorides isore difficult [30] than those of bromide and iodide analogs and

ence less reported. This is due to low reactivity of chlorides underhe conditions employed to couple bromides and iodides and issually attributed [30] to high strength of the C Cl bond [dissoci-tion energy (kcal mol−1) for Ph X: X Cl: 96; Br: 81; I: 65]. Theres a reluctance (due to the strength of the C Cl bond) in oxidativeddition of ArCl to Pd (0), considered as a critical step in palladium-atalyzed SMC.

It is very well known that palladium(II) complexes are pre-ursors serving as a reservoir of catalytically active Pd(0) whichight be in different phases like palladium nanoparticles or low-

oordinated palladium(0) species [30,55–58] and generally there isnvolvement of a Pd(0)–Pd(II) cycle in such reactions [57]. In viewf these reports, mercury poisoning test [55] was carried out toscertain the role of 1 in catalysis. In this test a significant suppres-ion in catalysis was observed in presence of excess of Hg as only8% conversion took place. This indicates that complex 1, a palla-ium(II) species, is a pre-catalyst which dispenses Pd(0) species aseal catalyst in the course of catalysis.

. Conclusions

A (S,N) bidentate ligand (L1) and its palladium (II) complex

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1) have been synthesized and characterized using multi-nucleiMR spectroscopy. Single crystal structure of complex 1 was deter-ined. The ligand L1 coordinated with palladium in bidentate (S,N)

oordination mode. The palladium complex 1 has been found to

[

PRESS Materials xxx (2013) xxx– xxx 5

show high efficiency for Suzuki–Miyaura C C coupling reactionsand that too in aqueous medium. TON values up to 93,000 wereachieved. Complex 1 functions in aerobic conditions, an additionaladvantage apart from high efficiency.

Acknowledgement

This work was supported by Council of Scientific and IndustrialResearch, India through the award of RA and SRF to Arun Kumarand G.K. Rao respectively. Funds were also made available by CSIRthrough project 01 (2421)10/EMR-II. UGC is also acknowledged forthe award of JRF to M. P. Singh.

Appendix A. Supplementary data

Supplementary data associated with this article can be found,in the online version, at http://dx.doi.org/10.1016/j.jhazmat.2013.11.045

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[2] N. Miyaura, in: N. Miyaura (Ed.), Cross-coupling Reactions, vol. 219, Springer,Berlin/Heidelberg, 2002, pp. 1–241.

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[7] S. Kotha, K. Lahiri, D. Kashinath, Recent applications of the Suzuki–Miyauracross-coupling reaction in organic synthesis, Tetrahedron 58 (2002)9633–9695.

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12] H.-J. Lehmler, L.W. Robertson, Synthesis of hydroxylated PCB metabolites withthe Suzuki-coupling, Chemosphere 45 (2001) 1119–1127.

13] R. Sott, C. Hawner, J.E. Johansen, Synthesis of dioxin-like monofluorinatedPCBs: for the use as internal standards for PCB analysis, Tetrahedron 64 (2008)4135–4142.

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