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Synthesis and Reactivity of5-Bromopenta-2,4-diynenitrile (BrC 5 N) an Access to

π-Conjugated ScaffoldsN. Kerisit, R. Ligny, E.S. Gauthier, J.-P. Guegan, Loic Toupet, J.-C.

Guillemin, Y. Trolez

To cite this version:N. Kerisit, R. Ligny, E.S. Gauthier, J.-P. Guegan, Loic Toupet, et al.. Synthesis and Reactivity of5-Bromopenta-2,4-diynenitrile (BrC 5 N) an Access to π-Conjugated Scaffolds. Helvetica ChimicaActa, Wiley, 2019, 102 (3), pp.e1800232. �10.1002/hlca.201800232�. �hal-02089223�

1

SynthesisandReactivityof5-Bromopenta-2,4-diynenitrile(BrC5N):anAccesstoπ-ConjugatedScaffolds

NicolasKerisit,[a]RomainLigny,[a]EtienneS.Gauthier,[a]Jean-PaulGuégan,[a]LoïcToupet,[b]Jean-ClaudeGuillemin,[a]YannTrolez*[a]

[a]Dr.N.Kerisit,Dr.R.Ligny,E.S.Gauthier,J.-P.Guégan,Dr.J.-C.Guillemin,Dr.Y.Trolez,UnivRennes, EcoleNationale SupérieuredeChimiedeRennes,CNRS, ISCR -UMR6226, F-35000Rennes,France.Tel:(+33)2-23-23-80-69;E-mail:yann.trolez@ensc-rennes.fr[b]Dr.L.Toupet,UnivRennes,CNRS,IPR-UMR6251,F-35000Rennes,France

ThisarticleisdedicatedtoFrançoisDiederichontheoccasionofhisretirement.

Abstract

Thesynthesisof5-bromopenta-2,4-diynenitrile(BrC5N)in3stepsfromcommercially

available compounds is reported. Reacting 5-bromopenta-2,4-diynenitrile with secondary

amines led to the formation of stable butadiynamines or enynenitriles, depending on the

nature of the amine reactant. The reaction of 5-bromopenta-2,4-diynenitrile with simple

terminal alkynes in the presence of secondary amines, copper and palladium catalysts,

provided a straightforward access to original polyfunctional carbon-rich scaffolds. In this

work, different alkynes and secondary amines were tested, which allowed for the

preparation of a family of substituted dienes. Given the high synthetic potential of 5-

bromopenta-2,4-diynenitrile,wealsopreparediodinatedcounterpartsofthiscompound,i.e.

5-iodopenta-2,4-diynenitrile and its lower homologue 3-iodopropiolonitrile. TheUV-visible

spectrumofsomerelevantcompoundswasalsorecorded.

Keywords

Alkynes,dienes,enynes,ynamines

Acknowledgements

WeacknowledgeDrEstelleMétayforfruitfuldiscussions.Thisstudyisapartoftheproject

ANR-13-BS05-0008 IMOLABS from the Agence Nationale pour la Recherche. N.K.

acknowledgestheFrenchMinistryofResearchandHigherEducationforallowinghimaPhD

fellowship.We also thank the PCMI program (INSU-CNRS) and the CNES (CentreNational

d'EtudesSpatiales)fortheirfinancialsupport.

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AuthorContributionStatementJ.-C.G.andY.T.designed theproject. J.-C.G.,Y.T.,N.K.,R. L.andE.S.G.discussed the

resultsandwrotethepaper.N.K.,R.L.andE.S.G.performedtheexperimentsandanalyzed

thedata. L.T.performedtheX-raycrystallographicanalyses. J.-P.G.performedadditional

characterizationbyNMRanalyses.

Tableofcontents

Hydrocarbonscaffolds:

The synthesis of 5-bromopenta-2,4-diynenitrile in 3 steps is reported. Its reaction with

secondaryaminesledtotheformationofstablebutadiynaminesorenynenitriles,depending

on thenatureof theamine. In Sonogashira conditionswith terminal alkynes, a secondary

amine, copper and palladium catalysts, this compound afforded original polyfunctional

carbon-rich scaffolds. The UV-visible spectrum of some relevant compounds was also

recorded.

Br CN

R[PdCl2(PPh3)2]

CuI

NHR'2

CN

N

R

R

R’R‘

N CNR

R

NHO

NHR2

N

CNNO

O

Br CN

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Thereactivityof1-halogenatedalkynesisofparticularinterestinthefieldoforganic

synthesis.[1,2,3]TheprobablymostpopularreactionusingsuchkindofspeciesistheCadiot-

Chodkiewicz coupling consisting in a copper-catalyzed heterocoupling of alkynes.[4] C–N

couplingsmayalsobeinitiatedinthepresenceofcoppertoformynamides.[5]Thisproperty

isessentiallydue to theabilityof thesecompounds toundergoanoxidativeadditionwith

metals[6 ,7 ] and further react. This reactivity prompted us to evaluate the reactivity of

bromopropynenitrile1(BrC3N)[8]withterminalalkynesinthepresenceofCuI,[PdCl2(PPh3)2]

and a secondary amine in the context of space science (tentative synthesis of

cyanobutadiynes[9,10]).Nevertheless, thetargetedunsymmetricaldiyneswerenotobtained

butenynenitrileswere isolated instead (scheme1).[11]Thispeculiar reactivity relieson the

presence of the secondary amine thatmost probably rapidly reactswith the electrophilic

alkynebeforetheoxidativeaddition,deviatingthustheusualsyntheticreactionpathway.In

asimilarfashion,wehaverecentlydemonstratedthatthe5-bromopenta-2,4-diynenitrile2

(BrC5N),whichbearsonemoreC≡C triplebond thanBrC3N, reacted in anoriginal fashion

with triisopropylsilylacetylene in the above-mentioned conditions.[12] As with BrC3N, the

unsymmetricalcouplingproductwasnotobtained.Instead,aconjugateddienynenitrilewas

isolated.

In the continuation of this work, we would now like to report on an alternative

synthesis of BrC5N and the further exploration of its reactivitywith different alkynes and

differentaminesinthesameconditions.Itsreactivitywithsecondaryamineshasalsobeen

investigated.Theopticalpropertiesofthemostrelevantproductsarealsoreportedinthis

paper.

Scheme 1. Reactivity of BrC3N 1 and BrC5N 2 with alkynes, [PdCl2(PPh3)2], CuI and asecondaryamine.

Br CNR H +

PdCl2(PPh3)2CuI

NHR'22

CN

N

R

R

Br CN+R H

PdCl2(PPh3)2CuI

NHR’2 N

CN

R

1

This work

R‘R’

R’R‘

Previous workA

ccep

ted

Man

uscr

ipt

Helvetica Chimica Acta

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ThesynthesisofBrC5N2haspreviouslybeendescribed[12]followingthebrominative

desilylationprocedureofKimandco-workers.[13]Thesynthesisof5-(triisopropylsilyl)penta-

2,4-diynenitrile3hasalreadybeenachievedfromthereactionoftriisopropylsilylbutadiyne4

(which was formed in two steps from commercially available compounds), n-butyllithium

and tosyl cyanide. Alternatively, we developed a more straightforward synthetic way to

compound 3. Triisopropylsilylacetylene was reacted with (E)-1,2-dichloroethylene in the

presenceofCuI, [PdCl2(PPh3)2] anddiisopropylamine to lead toenyne5 in54%yield, ina

reaction fashion inspired by a precedent publication of the Negishi group.[14] The latter

compound was subsequently reacted with n-butyllithium and tosyl cyanide to afford

compound3in38%yield.Onlytwostepsarethusrequiredandthesynthesiswasoperated

on the gram scale. This way also enabled the convenient synthesis of

triisopropylsilylbutadiyne 4 by simply replacing tosyl cyanide by a saturated ammonium

chloridesolution(scheme2).

Scheme 2. Synthesis of 5-triisopropylsilylpenta-2,4-diynenitrile 3 andtriisopropylsilylbutadiyne4.

Compound 3 was then reacted with N-bromosuccinimide (NBS) and AgF in

acetonitrileinthedark,affording2in99%yieldaftersublimationoftheresidue.Kimandco-

workers also described different conditions in order to carry out the desilylative

bromination.[13] When 3 was reacted with 4 equivalents of NBS, 1 equivalent of

tetrabutylammonium fluoride (TBAF) and a catalytic amount of AgNO3 in

dimethylformamide(DMF),2wasnotobtained.Instead,(E)-2,3,4,5,5-pentabromopenta-2,4-

dienenitrile6wasisolatedasthemajorproductofthereactionin14%yield.Theunexpected

formationof6arises,mostlikely,fromthepolybrominationof3,inasimilarmannerasthe

reportedbrominationof1,4-dibromobuta-1,3-diyne.[15]

By replacing NBS byN-iodosuccinimide (NIS) in the first conditions, the iodinated

analogue 5-iodopenta-2,4-diynenitrile 7 was conveniently prepared and obtained in 71%

TIPS H +Cl

Cl

[PdCl2(PPh3)2]CuI

iPr2NH54%

TIPSCl5

1) nBuLi2) TsCN38%

TIPS CN3

1) nBuLi

2) NH4Cl86% TIPS H

4

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yield as a pale yellow solid, after purification by column chromatography over neutral

alumina.The latter seems tobe thermallymore stable than2,whichmustbe storedat–

20°Ctobepreservedfromdegradation.Inthecaseof7,noapparentdecompositioncould

beobservedafterseveraldaysatroomtemperature.Giventheeasysynthesisofcompound

7, we also prepared the lower homologue, 3-iodopropiolonitrile 8, following the same

procedure.Thus,triisopropylsilylacetylenewasreactedwithn-butyllithiumandtosylcyanide

to afford 3-(triisopropylsilyl)propiolonitrile 9 (90% yield), which was then subjected to a

desilylativeiodinationinthepresenceofNISandAgF.3-Iodopropiolonitrile8wasobtained

in76%yield.Thesynthesisof8hasalreadybeenreportedbyKloster-Jensen,butonlyin60%

yieldandrequiredfirstthepreparationofcyanoacetyleneintwoextrasteps(scheme3).[16]

The structure of 6 could unambiguously be confirmed by single-crystal X-ray

diffraction.1Themost remarkable feature of thismolecule is the dihedral angle of 92(1)°

betweenthetwodoublebondsofthebutadienemoiety,whichisprobablyduetothesteric

hindranceinducedbybromineatoms.Asaconsequence,despitethepresenceofthenitrile

group, π-electrons aremostly localized as evidenced by the carbon–carbon double bonds

lengthswhichareclosetothelengthofthedoublebondofethylene(ca1.34Å):1.31(1)Å

for C1–C2 and 1.28(1) Å for C3–C4.[17] This observation means that the π-system is not

delocalized,despitethepresenceofthenitrilegroup.

Scheme3.Syntheticschemesof2,6–9andX-raystructureofcompound6.

1CCDC-995341(6)containsthesupplementarycrystallographicdataforthispaper.ThesedatacanbeobtainedfreeofchargefromTheCambridgeCrystallographicDataCentreviawww.ccdc.cam.ac.uk/data_request/cif.

TIPS H1) n-BuLi

2) TsCN90%

TIPS CN3 Br

Br Br

Br

Br

CN

Br CN

I CN

2

NBS, AgF

99%

NBS (4 equiv), TBAF, AgNO3

14%

NIS, AgF

71%

6

7

TIPS CNNIS, AgF

76%I CN

9 8

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The formation of ynamines by reacting 1-bromobutadiynes and secondary amines

hasrecentlybeendescribedbySzafertandco-workers.[18]Wealsoinvestigatedthereactivity

ofBrC5Nwithdiversesecondaryamines(3equivalents)inTHFatroomtemperature(Table

1). Even though this research group showed the sole formation of ynamines (even in the

presenceofanexcessofamine),theyhadenvisionedthepotentialformationofdiamination

products.Whenwe performed the reaction,we have also taken this into account.When

diisopropylamine was used, the corresponding ynamine 10a was obtained in 90% yield

(entry 1). Dibenzylamine and (−)-bis[(S)-1-phenylethyl]amine allowed for the formation of

thecorrespondingynamines10band10c in55and45%respectively(entries2and3).The

difference of yields obtained for 10b,c compared to 10a might be attributed to a less

importantsterichindranceoftheaminogroups,which,inthecaseof10a,contributesmore

importantly in stabilizing the ynamine and let it tolerate purification by chromatography.

When2wasreactedwithmorpholine,theexpectedynamine10dwasnotobtained.Instead,

the product of double addition 11d was isolated in 48% yield (entry 4). Morpholine is

probably less hindered than the other amines tested. Moreover, due to its rigid

conformation, it isalsomorenucleophilic.Thesetworeasonscouldexplaintheabsenceof

formationoftherelatedynamine.

X-rayqualitycrystalswereobtainedforcompound11d2thatconfirmedthestructure

determinedonthebasisof 1Hand13CNMRspectroscopy.Thecompoundcrystallizedasa

dimer in the unit cell (See Supporting Information, Figure SI-2), but only onemolecule is

represented in Table 1. The π-system of compound 11d is mostly planar, and the two

morpholine6-memberedringsadoptachairconformation.Delocalizationofelectronsfrom

thenitrogenatomofthemorpholinemoietiestowardthecyanogroupcanbeevidencedby

carbon–carbonbondlengths.ThedoublebondlengthC1–C2is1.372(2)Å,whichisslightly

longerthanausualC=Cdoublebond(ca1.34Å).SinglebondsC2–C3andC4–C5arerather

short,withbondlengthsof1.398(2)and1.375(3)Årespectively,whichcanbecomparedto

anaveragenon-conjugatedsinglebondlengthof1.54Å.TriplebondC3–C4islessaffected

withabondlengthof1.205(2)Å,whichisinagreementwiththelengthofausualC≡Ctriple

bond(about1.19Å).

2CCDC-994731 (11d) contains the supplementary crystallographic data for this paper. These data can beobtained free of charge from The Cambridge Crystallographic Data Centre viawww.ccdc.cam.ac.uk/data_request/cif.

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Table1.ReactivityofBrC5N(2)withsecondaryamines.a

Entry R2NH 10(yieldin%)b 11(yieldin%)b1

10a(90)

11b(0)

2

10b(55) 11b(0)

3

10c(57) 11c(0)

4

10d(0)

11d(48)

a)2(1equiv.),amine(3equiv.).b)Isolatedyields.

Since we recently showed that ynamides readily react with tetracyanoethylene

(TCNE)toyield1,1,4,4-tetracyanobutadienes(TCBDs),[19]compound10awasalsomixedwith

1 equivalent of TCNE. It resulted in the formation of many strongly colored products

(according toTLCanalysis) thatwewerenotable to isolate.Thesamekindof resultswas

observed when reacting compound 11d with TCNE. It confirms that N-alkylynamines,

contrarytoN-arylynamines,[20,21]arenotabletoaffordTCBDsbyreactionwithTCNEinthese

conditions.

Asexplainedintheintroduction,wehaverecentlydescribedthatBrC5Ndidnotreact

inCadiot-Chodkiewiczcouplingfashionwithtriisopropylsilylacetylene,palladiumandcopper

co-catalysts in the presenceof diisopropylamine. Instead, a dienewas obtained, involving

thereactionoftheamineandtwoequivalentsoftriisopropylsilylacetylene.[12]Therefore,we

wantedtoinvestigatethescopeofthismulti-componentreactionbyusingdifferentterminal

alkynesanddifferentsecondaryamines. Ineachcase,CuIand [PdCl2(PPh3)2]wereusedas

co-catalystsinTHFatroomtemperature(Table2).Whentriisopropylsilylacetylenewasused

alongwithdiisopropylamine,diene12awasobtainedin54%yield(entry1).Whenusing2-

Br CN2

+ NHR

RCNN

R

RTHF, rt+

10

CN

NN

RR

R

R11

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methylbut-3-yn-2-ol,amixtureoftheZandEisomersofcompound12bwasisolatedin36%

yield(entry2).TheZ-isomerremainedthemajorproductwithaZ/E-ratioof77/23.Thefact

thatE isomercouldbe formed in thiscasecanbeexplainedby theuseofa lesshindered

alkynethanthepreviouslydescribedtriisopropylsilylacetylenederivative12a.Therefore,the

cyanogroupisabletofindsomeroomtopointtowardstheotherdirection.Nevertheless,

thissterichindranceisprobablytheoriginoftheselectivityinfavoroftheZisomer.Similar

results were obtained with 2-ethynyl-1,3,5-triisopropylbenzene and ferrocenylacetylene

(entries3and4).They ledtothecorrespondingdienes12cand12d in68and65%yields

respectively(Z/E-ratio:86/14and74/26respectively).

Other secondary amines were also tested, namely dibenzylamine and (-)-bis[(S)-1-

phenylethyl]amine with triisopropylsilylacetylene (entries 5 and 6). Surprisingly, with

dibenzylamine, no diene could be obtained, and with (-)-bis[(S)-1-phenylethyl]amine, the

correspondingdiene12fwasobtainedinonly8%yield.Diisopropylamineis,byfar,thebest

aminethatwasused.Suchadifferenceishardtoexplain.Oneexplanationcouldbefoundin

the formationof theynaminethatweprovedtobethe firststepof themechanism.With

diisopropylamine,wecouldisolatetheynaminein90%yieldaspreviouslymentioned(table

1) whereas with (-)-bis[(S)-1-phenylethyl]amine and dibenzylamine, the yields are

significantly lower. Therefore, the less efficient formation of the ynamine could explain

finallythelowformationofthecorrespondingdiene.

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Table2.Synthesesofdienes12usingdifferentaminesanddifferentalkynes.a

Entry Alkyne Amine Yieldof12(Z/E-

ratio)

1

12a:54%(100/0)

2

12b:36%(77/23)

3

12c:68%(86/14)

4

12d:65%(74/26)

5

12e:0%

6

12f:8%(100/0)

a)2(1equiv.),alkyne(2equiv.),amine(5equiv.),[PdCl2(PPh3)2](10mol%),CuI(10mol%).

Thefirststepbeingtheformationofynamine,[12]thefactthatcopperandpalladium

co-catalystswerenecessarytothereactionletusthinkthatSonogashiracouplingsoccurred.

Moreover,we proved that the presence of halogenides (coming fromBrC5N for instance)

wasmandatoryforthesuccessofthereaction.Thus,wededucedthatthemechanismwasa

combination of two Sonogashira couplings and two halogenide additions[ 22 ] after the

ynamine was formed. In a previous paper, we showed that 1,6-addition on a similar

structure(i.e.Me–C≡C–C≡C–CN)waslargelyfavoredover1,4-additioninTHF.[23]Therefore,

R' H + Br CN

[PdCl2(PPh3)2]CuI

R2NH N

CN

R'

R’

R R

Z

N

NCR'

R’

R R

E

(Z)-12 (E)-12

+

2

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in this case,wealso suggest that1,6-additionofhalogenideon the intermediateynamine

happensbeforethe1,4-addition.Itcouldthusexplaintheselectivityobservedinfavorof(Z)-

12versus(E)-12,asindicatedinScheme4.

Scheme4.Proposedmechanismfortheformationofcompound12.

Theopticalpropertiesofcompounds10a,b,11dand12awereinvestigatedbymeans

ofUV/Visabsorptionspectroscopy.AllspectrawererecordedinCH2Cl2at293K.Absorption

spectraofynamines10aand10bareverysimilar(Figure1).Theirabsorptionspectraexhibit

fiveabsorptionmaximabetween250and350nmwithabsorptioncoefficientsrangingfrom

1.5to8.0x103M–1cm–1.Theirmostintensetransitionlieathigherenergy,withamaximum

between230and240nm,andreachmorethan6.0x104M–1cm–1.Noabsorptioncouldbe

observedinthevisibleregion.

The absorption spectrumof11d is significantly different from10a,b and shows an

intenseabsorptionbandat334nm(ε =2.3x104M–1cm–1)withashoulderatabout317nm

(Figure4).Oneless intensebandisvisibleat264nm(ε =6.8x103M–1cm–1). Importantly,

thiscompoundsignificantlyabsorbsinthevisibleregionupto500nm.

N

R‘

RR

R’

N CNR

RCNBrNH

R

R+

N

CN

R R

Br

NR R

R‘

N

CN

R R

+ Br-R‘

Br

Sonogashiracoupling

R2NH R2NH2+

Ynamine formation 10a2

+ Br-

R2NHR2NH2+

1,6-addition of HBr

R2NH

R2NH2+

Pd(II)Cu(I)

R2NH2+R2NH

1,4-addition of HBr

R2NH2+ R2NH

Sonogashiracoupling

(E)-12(minor)

R‘

N

CN

R R

R‘Br

+

CN

N

R‘

RR

R’

(Z)-12(major)

+

R‘

NCNC

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Figure3.Absorptionspectraofynamines10a(blueline),10b(redline)and11d(greenline)recordedinCH2Cl2.

The absorption spectrum of 12a shows two bands with absorption maxima being

reachedat266nm(ε =1.8x104M–1cm–1)and421(ε =1.3x104M–1cm–1)(Figure4,blue

line).Thelatterismostprobablyaconsequenceofachargetransferbetweentheaminoand

thecyanogroups. Inorder toevidence thispush-pull character,which is responsible fora

low-lying energy band in the visible region of the absorption spectrum, we added

trifluoroaceticacid (TFA) toa solutionof12a inCH2Cl2,protonating thus theaminogroup

andpreventing thedelocalizationof the lonepairof thenitrogenatom toward the cyano

group. The most significant consequence of this protonation in the spectrum was the

disappearanceof thebandof lowest energy, previously situated at 421nm (Figure 4, red

line).Noabsorptioninthevisiblerangecouldthusbeobservedanymore.

Addition of triethylamine (NEt3) to the latter solution regenerated the initial

absorptionspectrumof12a,theonlyexceptionbeingtheareahigherinenergythan300nm,

due to the absorption caused by the excess of NEt3 added (Figure 4, green line). These

observationsevidencethechargetransferoccurringoncompound12aandthereversibility

oftheprotonation-deprotonationsequence.Indeed,thisindicatesthatnodegradationwas

observeduponadditionofTFAintheexperimenttimescale,despitetheextendedπ-system

thatcouldpotentiallyhaveinducedunexpectedby-reactionsintra-orintermolecularly.

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Figure4.Absorptionspectraofcompound12a(blueline),compound12aandTFA(redline),diene12aplusTFAandNEt3(greenline),recordedinCH2Cl2.

Inconclusion,thereactivityof5-bromopenta-2,4-diynenitrile(BrC5N,2)wasexplored

indifferentconditionsandrevealed itsability to formπ-extendedsystems. Inparticular,a

seriesofdieneswassynthesizedinvariableyields,farfromtheCadiot-Chodkiewiczproducts

thatwere initially targeted. This unprecedented reactivity shows thismolecule has a very

high synthetic potential and paves the way to the synthesis of other original carbon-rich

scaffolds. In the continuation of this work, we are now working on the synthesis of its

ethynylogue 7-bromohepta-2,4,6-triynenitrile (BrC7N) that promises to exhibit an

exceptionalsyntheticpotentialifitreactsthesamewayasBrC5N.Itcouldalsobeusedasa

potentialprecursorofC7N−anionlikeBrC5NservedasaprecursorofC5N−anioninthegas

phaseforinterstellarsimulations.[24]

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[6] B. Pigulski, N. Gulia, S. Szafert, ‘Synthesis of Long, Palladium End-Capped Polyynes

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[7] S.Mader, L.Molinari,M. Rudolph, F. Rominger, A. S. K. Hashmi, ‘Dual Gold-Catalyzed

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andGeneralPropertiesofBromocyanoacetylene’,ActaChem.Scand.1963,17,1862–1865.

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