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Organic &Biomolecular Chemistry

PERSPECTIVE

Cite this: Org. Biomol. Chem., 2013, 11,1582

Received 6th December 2012,Accepted 9th January 2013

DOI: 10.1039/c3ob27366k

www.rsc.org/obc

Recent applications of arene diazonium salts in organicsynthesis

Fanyang Mo,a,b Guangbin Dong,*b Yan Zhanga and Jianbo Wang*a

Arene diazonium salts are common, easily prepared and highly useful intermediates in organic synthesis

due to their rich reactivity and diverse transformations. In this review, recent advances involving arene

diazonium salts as starting materials or active intermediates for various synthetically useful applications

are summarized.

1 Introduction

Diazonium compounds, taught in almost every sophomoreorganic chemistry course, represent a large group of organiccompounds with the general formula R–NuN+X−, in which Rcan be alkyl or aryl and X is an organic or inorganic anionsuch as a halogen. Diazonium salts, especially those where Ris an aryl group, are important intermediates and have foundwide applications in organic synthesis. Since their first discov-ery in 1858,1 several prominent named reactions associated

with arene diazonium salts have evolved throughout the devel-opment of more than one century (Scheme 1).

In 1884, Sandmeyer disclosed that by treatment withcopper(I) chloride, benzenediazonium salt was converted intochlorobenzene.2 He also showed that bromobenzene could beformed when using copper(I) bromide, and benzonitrile wasobtained when copper(I) cyanide was used. 12 years later,Pschorr reported a method for the preparation of biaryltri-cyclics by intramolecular substitution of one arene with an arylradical, which is generated in situ from an aryl diazonium saltby copper catalysis.3 In 1924, Gomberg and Bachmann devel-oped an intermolecular version of Pschorr’s radical biaryl syn-thesis, which is now known as the Gomberg–Bachmannreaction.4 Only three years later, an important breakthroughwas achieved by Balz and Schiemann, who reported thermaldecomposition of aromatic diazonium tetrafluoroborates.

Fanyang Mo

Fanyang Mo was born in Liaon-ing Province of China in 1982.He received his B.Sc. and M.Sc.degrees from Beijing Institute ofTechnology (P. R. of China) in2004 and 2006 under the super-vision of Professor Zhiming Zhou.He then obtained his Ph.D. fromPeking University under thesupervision of Prof. Jianbo Wangin 2010. He is currently a post-doctoral fellow in Prof. Guang-bin Dong’s group at theUniversity of Texas at Austin.

Guangbin Dong

Guangbin Dong received his B.S.degree from Peking Universityand completed his Ph.D. degreein chemistry from Stanford Uni-versity with Professor BarryM. Trost, where he was a LarryYung Stanford Graduate fellow.In 2009, he began to researchwith Prof. Robert H. Grubbs atthe California Institute of Tech-nology, as a Camille and HenryDreyfus Environmental Chem-istry Fellow. In 2011, he joinedthe department of chemistry and

biochemistry at the University of Texas at Austin as an assistantprofessor and a CPRIT Scholar. His research interests include thedevelopment of powerful chemical tools for addressing questionsof biological importance.

aCollege of Chemistry and Molecular Engineering, Peking University, Beijing,

P. R. China. E-mail: [email protected] of Chemistry and Biochemistry, University of Texas at Austin, Austin,

Texas 78712, USA. E-mail: [email protected]

1582 | Org. Biomol. Chem., 2013, 11, 1582–1593 This journal is © The Royal Society of Chemistry 2013

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The reaction leads to the formation of aromatic fluorides,which cannot be accessed by the Sandmeyer reaction.5 In1939, Meerwein and co-workers reported an extensive study onthe reaction of aromatic diazonium salts with α,β-unsaturatedcarbonyl compounds. The reaction was later known as Meer-wein arylation, in which the aryl group adds across the doublebond.6

In 1977, Doyle and co-workers reported a different methodfor the generation of diazonium salts in which an aqueousacidic solution was no longer necessary.7 This developmentexpands the synthetic scope of diazonium salts in organic syn-thesis. Besides the above-mentioned classical reactions, diazo-nium salts also served as arylhalide surrogates, which havebeen utilized in Pd-catalyzed cross-coupling reactions forcarbon–carbon bond and carbon–heteroatom bond formation.These coupling reactions have been well established over thepast 40 years since the pioneering work of Kikukawa andMatsuda in 1977,8 and are comprehensively documented in aseries of excellent reviews.9 In addition, diazonium salts arealso highly useful in the dye and pigment industry for thepreparation of azo-compounds.10 Regardless of the longhistory, the arene diazonium compounds still attract attentionand new developments have been emerging constantly. In thisshort review article, we will focus on the most recentdevelopments.

2 Carbon–carbon bond formation

Arene diazonium salts have been utilized as reactive arylhalidesurrogates in Pd-catalyzed cross-coupling reactions for C–Cbond formation.9 The intrinsic electrophilicity of diazoniumsalts comes from N2 being a superb leaving group, whichallows the use of mild reaction conditions, and sometimeswithout an additional ligand and/or base. The first utilizationof the aryldiazonium salts as electrophiles in Pd-catalyzedSuzuki–Miyaura cross-couplings was achieved independentlyby Genêt11 and Sengupta.12 A recent example was shown byGras and co-workers who reported an application of diazo-nium salts 1 in a base-free cross-coupling reaction with self-activated dioxazaborocanes 2 under mild and user-friendlyconditions (Scheme 2).13

In their study, Pd(OAc)2 only showed moderate efficiencyand gave homo-coupling of dioxazaborocanes 2 as the majorproduct, whereas Pd/C was proved to be a highly selective cata-lyst towards cross-coupling products.

Although arene diazonium tetrafluoroborates have beenwell established as coupling partners in Pd-catalyzed reactions,the major drawback is that they are usually not commerciallyavailable and in many cases have to be newly prepared beforeuse. In this context, one-pot diazotization/cross-coupling isobviously more attractive. Such a one-pot approach has been

Yan Zhang

Yan Zhang obtained her B.S. in1997, and her Ph.D. in 2002from Lanzhou University (underthe supervision of Prof. ZiyiZhang). She continued herresearch as a postdoctoralassociate in Hong Kong,Germany, and the United States.She began her academic careerat Peking University in 2008 inProf. Jianbo Wang’s group. Herresearch focuses on the appli-cation of transition metal com-plexes of N-heterocyclic carbenes

and the synthesis of small molecules with important biologicalactivities.

Jianbo Wang

Jianbo Wang received his B.S.degree from Nanjing Universityof Science and Technology in1983, and his Ph.D. fromHokkaido University (under thesupervision of Prof. H.Suginome) in 1990. He was apostdoctoral associate at theUniversity of Geneva from 1990to 1993 (with Prof. C. W.Jefford), and at the University ofWisconsin-Madison from 1993 to1995 (with Prof. H. E. Zimmer-man and F. A. Fahien). He began

his academic career at Peking University in 1995. His researchinterests include catalytic metal carbene transformations.

Scheme 1 Brief history of diazonium salts.

Organic & Biomolecular Chemistry Perspective

This journal is © The Royal Society of Chemistry 2013 Org. Biomol. Chem., 2013, 11, 1582–1593 | 1583

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exploited previously by several groups.14 A recent example hasbeen shown by Wang and co-workers who have demonstrateda convenient Pd-catalyzed base-free Suzuki–Miyaura cross-coupling for the synthesis of biaryls using arylamines 4 as thestarting materials (Scheme 3).15 The mechanism of this diazo-tization-coupling is proposed to be the standard oxidativeaddition–reductive elimination mechanism of Pd-catalyzedcross-coupling.16

The Heck–Matsuda reaction is the diazonium salt versionof the Heck–Mizoroki reaction, where aryl-halides or -sulfo-nates serve as electrophiles. Although the seminal work wasachieved by Kikukawa and Matsuda in 1977,8 the Heck–Matsuda reaction had been overlooked until the late nineties.In recent years, the group of Felpin has made significant con-tributions toward the development of the Heck–Matsuda reac-tion.9,17 In 2010, they demonstrated a highly efficient Heck–Matsuda coupling of aryldiazonium salts with 2-arylacrylatesleading to cis-stilbene with good to excellent E stereoselectivity(Scheme 4).17e

It has been shown that 2-arylacrylates 6 with one substitu-ent on the aromatic ring at C2 coupling with diazonium salts 1under palladium catalysis gives exclusively cis-stilbenes. Inter-estingly, the high stereoselectivity observed does not seem tobe related to any stereoelectronic effect on either the acrylateor the diazonium salt. They also found that the catalystloading can be lowered to 0.005% in the coupling reaction ofdiazonium salts with methyl acrylate.

In subsequent work, the same group reported a Heck–Matsuda reaction using a substoichiometric amount of diazo-nium salt through a double catalytic cycle.17g A variety of acidswere examined in order to evaluate the influence of the acidityand the nature of the counter-ion (eqn (1)). The results showedthat although the tetrafluoroborate anion has been widelyused in the literature, it was not the most effective counter-ionregardless of the source of the acid (i.e., HBF4 and BF3·Et2O).Finally, MeSO3H was selected as the acid of choice based onthe cost and recoverability.

ð1Þ

This reaction is not sensitive to steric effects as some ortho-substituted diazonium salts gave even higher yields as com-pared with their para-substituted counterparts. Moreover, thiscoupling has been utilized to complete the synthesis of quino-lone 11 by their Heck-reduction–cyclization strategy(Scheme 5).

In 2012, König and co-workers developed an efficientvisible-light mediated arylation of alkenes, alkynes and enoneswith diazonium salts by photoredox catalysis (Scheme 6).18

The reaction scope comprises a range of different substitutedaryl diazonium salts and tolerates a variety of functionalgroups including aryl halides. Mechanistically, a radical

Scheme 2 Pd-catalyzed cross-coupling of diazonium salts anddioxazaborocanes.

Scheme 3 Pd-catalyzed one-pot diazotization/cross-coupling.

Scheme 4 Synthesis of stilbenes by the Heck–Matsuda reaction.

Perspective Organic & Biomolecular Chemistry


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pathway including one-electron oxidation and reduction stepsis likely for this photoredox arylation.

In early 2013, Gholinejad reported Heck–Matsuda andSuzuki–Miyaura coupling reactions of aryl diazonium saltscatalyzed by palladium nanoparticles supported on agarose.19

By using this new catalyst, reactions could be carried out inaqueous solution at lower temperature. Moreover, thisimmobilized catalyst could be recycled and reused severaltimes.

Although many Pd-catalyzed cross-coupling reactions, suchas the Heck, Suzuki–Miyaura and Stille reactions, have beendeveloped utilizing aryldiazonium salts as aryl halide surro-gates, the Sonogashira cross-coupling remained a challengeuntil the first two successful examples presented by the Sarkargroup and the Cacchi group in 2010, respectively (Schemes 7and 8).20,21 In Sarkar’s work, AuCl and PdCl2 were combinedas a catalyst and the reaction could even start with anilinederivatives by employing an in situ diazonium formation step.In Cacchi’s work, initial attempts with various Pd catalysts, sol-vents and bases did not produce the desired product. Theproblem was circumvented by a sequential iododediazonia-tion/cross-coupling strategy.

Carbopalladation of alkynes results in the formation ofalkenyl palladium species. Phenol diazonium salts 12 havebeen recently explored by Schmidt and co-workers in Pd-

catalyzed [2 + 2 + 1] cyclization, leading to the formation ofspirocyclic ketones 14 (Scheme 9).22

Besides traditional cross-coupling reactions, arene diazo-nium salts can also serve as an aryl radical source in tran-sition-metal-catalyzed C–H functionalization and metal-freeC–C bond forming reactions. For a recent example, theSanford group described a room-temperature ligand-directedC–H arylation reaction using aryldiazonium salts (Scheme 10).23

The linchpin for the success of this methodology is the combi-nation of visible-light photoredox catalysis and Pd-catalyzedC–H functionalization. This room-temperature C–H arylationreaction is effective for the substrates containing a wide rangeof directing groups, including 2-arylpyridines, amides,

Scheme 5 Heck–Matsuda reaction and synthesis of quinolone 11.

Scheme 6 Photocatalytic arylation of alkenes, alkynes and enones with diazo-nium salts.

Scheme 7 Pd–Au-catalyzed Sonogashira cross-coupling of arenediazoniumsalts.

Scheme 8 Sonogashira cross-coupling of aryldiazonium salts.

Scheme 9 Pd-catalyzed reaction of phenol diazonium salts with alkynes 13.



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pyrazoles, pyrimidines, oxime ethers, and free oximes. Theauthors have proposed a mechanism which involves a ruthe-nium catalyst cycle. The diazonium salt is decomposed to anaryl radical by Ru(bpy)3

2+*,24 which is formed by photoexcita-tion of Ru(bpy)3

2+. This aryl radical then participates in the Pd-catalyzed C–H functionalization cycle to oxidize Pd(II) to Pd(III),which is further oxidized by Ru(III) to form a Pd(IV) species andregenerate the photocatalyst. Finally, C–C bond-forming reduc-tive elimination releases the arylated product and regeneratesthe Pd(II) catalyst.

In 2012, König and co-workers reported a metal-free,visible-light-mediated direct C–H arylation of heteroareneswith aryldiazonium salts (Scheme 11).25

The reaction does not require transition-metal catalysts orbases and proceeds smoothly at room temperature. In contrastto Sanford’s ruthenium catalyst, this protocol uses eosin Y 17as the photoredox catalyst, and is presumed to proceedthrough a radical mechanism.26 The radical mechanism issupported by the fact that 2,2,6,6-tetramethylpiperidinoxyl(TEMPO) effectively inhibits the reaction, and trapped inter-mediates can be detected.

As shown in Scheme 11, the proposed mechanism startswith the formation of an aryl radical A by single-electron trans-fer (SET) from the excited state of eosin Y to the aryldiazoniumsalt. Addition of the aryl radical to heteroarene gives radicalintermediate B, which is further transformed to the carbo-cation intermediate C by two possible pathways: (a) oxidationof the radical intermediate B by the eosin Y radical cation togive C or (b) oxidation of B by aryldiazonium salt in a radicalchain transfer mechanism. Finally, deprotonation of inter-mediate C regenerates the aromatic ring and gives the finalcoupling product.

In the same year, the König group developed an eosin Ycatalyzed visible light photocatalytic reaction of o-methylthio-arenediazonium salts with alkynes. The reaction affords sub-stituted benzothiophenes through a similar radical annulationprocess.27 This method provides mild and efficient access tobenzothiophenes. This method was employed to preparethe key intermediate 21 for the synthesis of raloxifene28

(Scheme 12).In 2012, Studer and co-workers reported a transition-metal-

free oxyarylation of alkenes 23 with aryldiazonium salts andTEMPONa (Scheme 13).29 The mechanism involves aryl radicaladdition to alkenes with subsequent TEMPO trapping toafford the corresponding oxyarylation products 24. TEMPONais used as a reducing reagent to convert an aryldiazonium saltto the corresponding aryl radical through single-electron trans-fer. The product TEMPO-based alkoxyamines can be easilyconverted to more common and useful compounds by furtherchemical manipulation.

Aryldiazonium salts are typically considered as electrophilesto participate in various Pd-catalyzed cross-coupling reactions.However, much less attention has been paid to the syntheticutility of the homocoupling of aryldiazonium salts for synthe-sizing symmetrical biaryls.30 In 2012, Song and co-workersreported a simple and efficient FeCl2-promoted homocouplingof aryldiazonium tetrafluoroborates to afford symmetricalbiaryls 25 with broad substrate scope and high yields(Scheme 14).31 The authors suggest that the mechanisminvolves reductive homocoupling of the aryldiazonium saltwith the oxidation of the Fe2+ ion.

Aryldiazonium salts are not only excellent precursorsfor the generation of radical intermediates, but also aryl

Scheme 11 Eosin Y-catalyzed visible-light-mediated direct C–H arylation ofheteroarenes with aryldiazonium salts.

Scheme 10 Pd/Ru-catalyzed C–H arylation with diazonium salts.



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cations.32 One of the latest examples is shown in Scheme 15,which was reported by Ren and co-workers in 2012.33 It hasbeen shown that the reaction of aryltriazenes 26 with BF3·OEt2leads to the formation of polycyclic aromatic hydrocarbons 27through a Friedel–Crafts intramolecular arylation. Aryltria-zenes, which can be readily prepared from the corresponding

arylamine in high yields and are easy to handle, are widelyused as equivalents of aryldiazonium salts. In the presence ofLewis or Brønsted acid, aryltriazenes are activated and the cor-responding aryldiazonium salt is generated.

Zhou and co-workers reported a metal-free, visible light-induced [4 + 2] benzannulation of biaryldiazonium salts 28and alkynes with eosin Y as the photoredox catalyst.34 A varietyof 9-substituted or 9,10-disubstituted phenanthrenes 29 wereobtained via a cascade radical addition and cyclizationsequence. In general, electron deficient alkynes give higheryields as compared with electron rich ones (Scheme 16).

3 Carbon–boron bond formation

Since the discovery of diazonium salts by Griess in 1858,carbon–halogen, carbon–carbon, carbon–nitrogen, carbon–oxygen, carbon–sulfur bond formation have been achieved byutilizing intrinsic reactivity of diazonium salts. Althoughnumerous efforts have been made in Pd-catalyzed cross-coup-ling reactions using aryldiazonium salts since the pioneeringwork of Kikukawa and Matsuda reported in 1977, in terms ofmechanism, diazonium salts in these reactions only serve as“super” electrophiles and surrogates of aryl halides. Thus, it isnot unexpected that aryldiazonium salts can also be used in

Scheme 12 Benzothiophene synthesis from arene diazonium salts.

Scheme 13 Transition-metal-free oxyarylation of alkenes with aryldiazoniumsalts and TEMPONa.

Scheme 14 Homocoupling of aryldiazonium tetrafluoroborates.

Scheme 15 Synthesis of polycyclic aromatic hydrocarbons through Friedel–Crafts intramolecular arylation.



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the Pd-catalyzed Miyaura borylation reaction, which is animportant approach towards aromatic boronates.35

A recent example of transition-metal-catalyzed borylationof diazonium salts was shown by Yu and co-workers. Theyreported Cu(I)-catalyzed cross-coupling reactions of aryl diazo-nium salts with B2pin2 [bis(pinacolato)diboron] 30 in MeCN–H2O at room temperature, providing the corresponding aryl-boronates 31 in good to high yields (Scheme 17).36 They foundthat CuBr is superior to other inorganic salts and water in anorganic solvent is helpful to the reaction. Most of the sub-strates presented are those with electron withdrawing groupson the aromatic ring. However, the reaction tolerates halo andacidic substituents.

Recently, transition-metal-free methods for C–B bond for-mation reactions involving aryldiazonium salts as key inter-mediates have been reported.37–40 In 2010, Mo and Wangreported novel metal-free C–B bond formation by directly

converting arylamines into pinacolboronates 31 at room temp-erature. The starting material arylamine is first converted intothe corresponding diazonium ion by reaction with tert-butylni-trite 32, and then the diazonium ion reacts with the diboronreagent B2pin2 30 to deliver the final product (Scheme 18).37

The reaction occurs smoothly with meta- and para-substi-tuted arylamines, while the reactions with ortho-substitutedarylamines give diminished yields. In general, substrates withelectron-withdrawing groups at the para- and meta-positionsexhibit good reactivity. It is noteworthy that substrates bearinghalo substituents can also be employed in this reaction, pro-viding the possibility of multiple transition-metal-catalyzedcross-coupling. Since arylamines are inexpensive and ubiqui-tous starting materials, this borylation method is expected tofind wide applications in organic synthesis.

More recently, the substrate scope of the reaction wasfurther expanded, especially to heterocyclic amine derivatives,for which the corresponding boronate products are highlyimportant in both academic research and the pharmaceuticalindustry (Scheme 19).38 It was found that electron-deficientheterocyclic amines exhibit high reactivity with nearly com-plete conversion of B2pin2. However, electron-rich heterocyclicamines are prone to be oxidized in the presence of t-BuONO,resulting in diminished yields of the borylation products.

Based on the experimental observations, a possible reactionpathway involving radical species is proposed for this boryla-tion reaction as shown in Scheme 20. First, the tert-butoxideanion interacts with B2pin2 to form a tetra-coordinated boroncomplex A. Single electron transfer (SET) between the atecomplex A and the aryldiazonium ion then affords an arylradical D through N2 extrusion from radical B. Finally, reactionof aryl radical D with intermediate C gives the borylationproduct.

Scheme 16 Eosin Y-catalyzed visible light-induced [4 + 2] benzannulation ofbiaryldiazonium salts and alkynes.

Scheme 17 Cu(I)-catalyzed borylation of aryldiazonium salts.

Scheme 18 Direct conversion of arylamines to pinacol boronates.

Scheme 19 Heterocyclic boronates from heterocyclic amines.



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Subsequently, Yan and co-workers reported photoredox-mediated reduction of aryldiazonium salts, providing free arylradicals that are borylated in the presence of B2pin2

(Scheme 21).39 Both electron poor and electron rich aromaticsare tolerated and provided the corresponding borylated com-pounds in moderate to good yields.

In the proposed mechanism, an aryl radical is formed bysingle electron transfer (SET) from the excited state of eosin Yto aryldiazonium salt (Scheme 22). The aryl radical then reactswith complex A, which is generated in situ from the B2pin2

coordination tetrafluoroborate anion, affording borylationproduct 31 and the radical anion intermediate B. Oxidation ofB to C by the eosin Y radical cation completes the catalyticcycle. This transformation provides supportive evidence for theinvolvement of aryl radical species in the borylation withB2pin2 as shown in Scheme 20.

More recently, Yamane and Zhu described a related arylboro-nate synthesis via direct borylation of aryltriazene mediated byBF3·OEt2 (Scheme 23).40 The aryltriazenes, which are

considered as protected diazonium salts, can be easily pre-pared from the corresponding arylamines in high yields.41 Thereaction proceeds smoothly for a variety of aryltriazenes 35and provides moderate to high yields of arylboronates.

For the reaction mechanism, it is proposed that the for-mation of triazene–BF3 complex A is followed by the gener-ation of aryldiazonium salt B (Scheme 24). Then the fluorideanion transfers from the trifluoroborate anion onto B2pin2 togenerate C. Finally, nucleophilic substitution affords the bory-lation product and releases N2 and F–Bpin. Although this bory-lation is closely related to those shown in Scheme 18, amechanism involving radical species has been ruled out basedon the trapping experiment.

4 Carbon–sulfur bond formation

Direct chlorosulfonylation of diazonium salts to build carbon–sulfur bonds was first reported by Meerwein and co-workers in1957.42 In their original paper, the diazonium salt, formedfrom aniline using aqueous NaNO2 in a mixture of concen-trated aqueous HCl and acetic acid, is added to a saturatedsolution of SO2 in acetic acid in the presence of a catalyticamount of CuCl2. The reaction affords the corresponding arylsulfonyl chloride (eqn (2)).

ð2Þ

Scheme 20 Proposed mechanism for borylation of arylamine.

Scheme 21 Metal-free, visible-light-induced borylation of aryldiazonium salts.

Scheme 22 Proposed reaction mechanism.

Scheme 23 Borylation of aryltriazene mediated by BF3·OEt2.

Scheme 24 Proposed mechanistic for borylation of aryltriazene.



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Recently, Batanero and co-workers developed an electrolyticmethod towards diaryldisulfides synthesis by cathodicreduction of aryldiazonium tetrafluoroborates in CS2–EtOHand Bu4NClO4 (Scheme 25).43 In their previous paper,44 theyhad already demonstrated that the aryldiazonium salts areeasily reduced to the corresponding aryl radicals under electro-lysis, which can further react with solvents such as acetonitrile,DMF, or 1,2-dichloroethane to produce the dimethylaminocar-bonyl, cyanomethyl, or 1,2-dichloroethyl radicals, respectively.As shown in Scheme 26, the aryl radical, once generated in alow concentration, can react with CS2 forming intermediate A,which further decomposes to give carbon monosulfide andaryl sulfur radical B. Dimerization of intermediate B releasesthe final diaryldisulfide product.

In 2011, Ranu and co-workers reported a transition-metal-free procedure for the synthesis of S-aryl dithiocarbamates 37using water as a solvent at room temperature.45 The reaction isa one-pot multi-component condensation of aryldiazonium-tetrafluoroborate, carbon disulfide and an amine withoutmetal catalysts (Scheme 27).

Mechanistically, it was found that CS2 underwent a very fastreaction with piperidine in water at 0–5 °C to form piperidine-1-dithiocarbamic acid 38, which could be isolated and fullycharacterized. Compound 38 could react with aryldiazoniumtetrafluoroborate to give the corresponding dithiocarbamateproduct (Scheme 28).

Moreover, the same group has succeeded in using aryldia-zonium fluoroborates and diaryldichalcogenides 39 to accessunsymmetrical diarylchalcogenides 40 under microwave con-ditions (Scheme 29).46

It is known that diaryldichalcogenide could be reduced byZn dust via homo-cleavage to form Zn(Xaryl)2 species.47 ThisZn(Xaryl)2 species then reacts with aryldiazonium tetrafluoro-borate to provide the final product with extrusion of N2.

5 Miscellaneous reactions

The stability of aryldiazonium salts depends on the aromaticsubstituents and the nature of their counter-anion. The diazo-nium salts of tetrafluoroborates, tosylates and disulfonimidesrepresent the most stable ones. Recently Kachanov and co-workers have reported a modified method that introduces1,1,2,3,3-pentacyanopropenide as the anion of aryldiazoniumsalts.48 A number of aryldiazonium salts possessing the1,1,2,3,3-pentacyanopropenide anion have been prepared bymeans of the exchange reaction between aryldiazonium chlor-ides and pyridinium 1,1,2,3,3-pentacyanopropenide49 in water(Scheme 30).

Although aryldiazonium salts have been widely investigatedas sources of aryl radicals in the Sandmeyer, Meerwein,Gomberg–Bachmann, and Pschorr reactions, their use andapplication as nitrogen-centered radical surrogates has onlybeen marginally explored so far.50 In 2010, Heinrich and co-workers reported an iron(II)-mediated three-component reac-tion of hydroperoxides, olefins and aryldiazonium salts to giveazo compounds as products.51 The reaction starts with a

Scheme 26 Proposed reaction mechanism for 36.

Scheme 25 Formation of diaryldisulfides by electrolyses of aryldiazonium saltsin CS2.

Scheme 27 Transition-metal-free reaction of aryldiazonium salts with dithio-carbamate anions.

Scheme 28 Proposed reaction mechanism for the formation ofdithiocarbamates.

Scheme 29 Zn-mediated synthesis of diarylchalcogenides 40.



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fragmentation liberating acetic acid from hydroperoxide com-pound 41 to give radical A, which is trapped by olefin to giveanother radical B. The last step involves nucleophilic attack ofB by a diazonium ion via a reductive process to furnish thefinal product (Scheme 31).

Cyclization of the diazonium ion to form a heterocycle isanother common application of diazonium salts chemistry.In 2010, Flynn and co-workers reported a modified Richtercyclization52 by using 2-alkynylaryltriazene 43 as masked di-azonium salts, affording chemoselective access to 4-bromocin-noline 44, cinnolinones 45, ring-fused cinnolines 46 andindazoles 47 (Scheme 32).53

Deamination of aromatic amines is one of the importanttransformations in organic chemistry. Very recently, Müllerand co-workers reported an efficient and mild deaminationprocedure for 1-aminoanthraquinones 48 by using a zinc–ethanol system (Scheme 33).54

Recently, FLP (frustrated Lewis pair) has attracted atten-tion.55 The chemistry of diazonium salts has also been com-bined with the reaction of FLP. In 2012, Stephan and co-workers described a new and facile approach for the prep-aration of electrophilic vinyl boranes 52 starting from diazo-nium salts and alkynylborate salts 50 (Scheme 34).56a

Alkynylborate salts 50 are easily prepared from the reaction ofFLP tBu3P–B(C6F5)3 with a terminal alkyne by the samegroup.56b,c This methodology to electrophilic vinyl boranes canbe conveniently expanded to various alkynylborates and diazo-nium salts. The authors proposed a mechanism in which theinteraction of the electron-deficient cation derived from thediazonium salt and the alkyne fragment of the alkynylborategenerates a transient carbocation adjacent to the borate centre.This promotes the migration of the –C6F5 group from–B(C6F5)3 to the carbon cation to afford the vinyl borane.

6 Conclusions

In this review we have shown a number of synthetic appli-cations of aryldiazonium salts developed in recent years. Thereactive manner of the aryldiazonium salts in all these

Scheme 30 Preparation of aryldiazonium 1,1,2,3,3-pentacyanopropenide.

Scheme 31 Synthesis of the azo compound via iron(II)-mediated olefin func-tionalization with aryldiazonium salts.

Scheme 32 Cyclization of 2-alkynylaryl triazenes 43.

Scheme 33 Reductive deamination protocol using a Zn–EtOH system.

Scheme 34 Formation of electrophilic vinyl boranes.



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reactions can be summarized in three categories, namely arylradical precursors, aryl cation precursors, and “super” electro-philes in transition-metal-catalyzed cross-coupling reactions.Although the history of aryldiazonium salts can be dated backto the nonage of organic chemistry, from the selectedexamples shown in this review, it can be expected that aryldia-zonium salts, which are readily derived from inexpensive andubiquitous aromatic anilines, will continue to attract the atten-tion of synthetic chemists as valuable reactants in the comingyears.

Acknowledgements

Financial support from the 973 Program (No. 2012CB821600)and the National Natural Science Foundation of China is grate-fully acknowledged.

Notes and references

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