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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�
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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
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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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