A mechanistic investigation of the photoinduced, copper-mediated cross-coupling of an aryl thiol with an aryl halide

Article abstract of DOI:10.1039/c5sc04709a

Photoinduced, copper-catalyzed cross-coupling can offer a complementary approach to thermal (non-photoinduced) methods for generating C-X (X = C, N, O, S, etc.) bonds. In this report, we describe the first detailed mechanistic investigation of one of the processes that we have developed, specifically, the (stoichiometric) coupling of a copper-thiolate with an aryl iodide. In particular, we focus on the chemistry of a discrete [Cu^I(SAr)₂]^- complex (Ar = 2,6-dimethylphenyl), applying a range of techniques, including ESI-MS, cyclic voltammetry, transient luminescence spectroscopy, optical spectroscopy, DFT calculations, Stern-Volmer analysis, EPR spectroscopy, actinometry, and reactivity studies. The available data are consistent with the viability of a pathway in which photoexcited [Cu^I(SAr)₂]^-? serves as an electron donor to an aryl iodide to afford an aryl radical, which then reacts in cage with the newly generated copper(ii)-thiolate to furnish the cross-coupling product in a non-chain process.

Full text of DOI:10.1039/c5sc04709a

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Page 1 of 17
Chemical Science
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DOI: 10.1039/C5SC04709A
AꢀMechanisticꢀInvestigationꢀofꢀtheꢀPhotoinduced,ꢀCopper-MediatedꢀCross-Couplingꢀ
ofꢀanꢀArylꢀThiolꢀwithꢀanꢀArylꢀHalideꢀ
ꢀ
MilesꢀW.ꢀJohnson,ꢀKareemꢀI.ꢀHannoun,ꢀYichenꢀTan,ꢀGregoryꢀC.ꢀFu,*ꢀandꢀJonasꢀC.ꢀPeters*ꢀ
ꢀ
ABSTRACTꢀꢀ
Photoinduced,ꢀcopper-catalyzedꢀcross-couplingꢀcanꢀofferꢀaꢀcomplementaryꢀapproachꢀtoꢀthermalꢀ(non-photoinduced)ꢀmethodsꢀforꢀ
generatingꢀC–Xꢀ(Xꢀ=ꢀC,ꢀN,ꢀO,ꢀS,ꢀetc.)ꢀbonds.ꢀꢀInꢀthisꢀreport,ꢀweꢀdescribeꢀtheꢀfirstꢀdetailedꢀmechanisticꢀinvestigationꢀofꢀoneꢀofꢀtheꢀprocessesꢀ
thatꢀweꢀhaveꢀdeveloped,ꢀspecifically,ꢀtheꢀ(stoichiometric)ꢀcouplingꢀofꢀaꢀcopper–thiolateꢀwithꢀanꢀarylꢀiodide.ꢀꢀInꢀparticular,ꢀweꢀfocusꢀonꢀtheꢀ
I
–
2
chemistryꢀofꢀaꢀdiscreteꢀ[Cu (SAr) ] ꢀcomplexꢀ(Arꢀ=ꢀ2,6-dimethylphenyl),ꢀapplyingꢀaꢀrangeꢀofꢀtechniques,ꢀincludingꢀESI–MS,ꢀcyclicꢀ
voltammetry,ꢀtransientꢀluminescenceꢀspectroscopy,ꢀopticalꢀspectroscopy,ꢀDFTꢀcalculations,ꢀStern–Volmerꢀanalysis,ꢀEPRꢀspectroscopy,ꢀ
I
–
actinometry,ꢀandꢀreactivityꢀstudies.ꢀꢀTheꢀavailableꢀdataꢀareꢀconsistentꢀwithꢀtheꢀviabilityꢀofꢀaꢀpathwayꢀinꢀwhichꢀphotoexcitedꢀ[Cu (SAr)
2
] *ꢀ
servesꢀasꢀanꢀelectronꢀdonorꢀtoꢀanꢀarylꢀiodideꢀtoꢀaffordꢀanꢀarylꢀradical,ꢀwhichꢀthenꢀreactsꢀinꢀcageꢀwithꢀtheꢀnewlyꢀgeneratedꢀcopper(II)–
thiolateꢀtoꢀfurnishꢀtheꢀcross-couplingꢀproductꢀinꢀaꢀnon-chainꢀprocess.ꢀ
ꢀ
Introductionꢀ
Theꢀutilityꢀofꢀcross-couplingꢀchemistryꢀhasꢀcontinuedꢀtoꢀexpandꢀatꢀaꢀrapidꢀrateꢀasꢀnovelꢀorꢀunderexploredꢀreactionꢀpathwaysꢀareꢀ
1
exploitedꢀtoꢀachieveꢀimportantꢀnewꢀfamiliesꢀofꢀbondꢀconstructions. ꢀꢀWeꢀhaveꢀrecentlyꢀreportedꢀthat,ꢀinꢀtheꢀpresenceꢀofꢀlightꢀandꢀaꢀsimpleꢀ
copperꢀcatalyst,ꢀcouplingꢀreactionsꢀofꢀaꢀvarietyꢀofꢀnucleophilesꢀ(nitrogen,ꢀsulfur,ꢀoxygen,ꢀandꢀcarbon)ꢀwithꢀarylꢀorꢀalkylꢀelectrophilesꢀcanꢀbeꢀ
2
,3,4
accomplishedꢀunderꢀmildꢀconditionsꢀ(–40ꢀtoꢀ30ꢀ°C;ꢀeqꢀ1).
ꢀ
ꢀ
cat. CuI
hν
R
X
H Nu
R Nu (1)
base
–
40 to 30 °C
aryl or alkyl carbazole, indole,
halide
imidazole, amide,
aryl thiol, phenol,
etc.
ꢀ
ꢀ
5
Weꢀhaveꢀsuggestedꢀtheꢀoutlineꢀofꢀaꢀpossibleꢀpathwayꢀforꢀtheseꢀprocessesꢀ(Figureꢀ1,ꢀillustratedꢀforꢀC–Sꢀcoupling), ꢀrecognizingꢀthatꢀtheꢀ
courseꢀofꢀtheꢀcross-couplingꢀisꢀlikelyꢀtoꢀvaryꢀwithꢀdifferentꢀcouplingꢀpartnersꢀandꢀreactionꢀconditions.ꢀꢀWeꢀhaveꢀbeenꢀinterestedꢀinꢀ
mechanisticꢀsimilaritiesꢀandꢀdichotomiesꢀwithꢀphotoredoxꢀcatalysis,ꢀaꢀmodeꢀofꢀreactivityꢀthatꢀhasꢀbeenꢀtheꢀfocusꢀofꢀgreatꢀinterestꢀinꢀrecentꢀ
6
years. ꢀꢀForꢀexample,ꢀweꢀhaveꢀhypothesizedꢀthat,ꢀdistinctꢀfromꢀaꢀclassicalꢀphotoredoxꢀcatalystꢀwhereinꢀaꢀparticularꢀmetalꢀcomplexꢀservesꢀ
exclusivelyꢀasꢀanꢀelectronꢀdonor/acceptor,ꢀinꢀourꢀprocessesꢀtheꢀcopperꢀcomplexꢀmayꢀplayꢀaꢀroleꢀbothꢀinꢀelectronꢀtransferꢀandꢀinꢀtheꢀkeyꢀ
2
a–d,3,7
bond-formingꢀstepꢀ(e.g.,ꢀC–SꢀbondꢀconstructionꢀinꢀFigureꢀ1).
ꢀꢀFurthermore,ꢀtheꢀmechanismꢀdepictedꢀinꢀFigureꢀ1ꢀisꢀnotꢀaꢀradical-chainꢀ
8
process;ꢀalthoughꢀnon-chainꢀpathwaysꢀhaveꢀfrequentlyꢀbeenꢀinvokedꢀinꢀearlierꢀstudiesꢀofꢀphotoredoxꢀcatalysis, ꢀYoonꢀhasꢀrecentlyꢀ
concludedꢀthat,ꢀforꢀthreeꢀrepresentativeꢀandꢀmechanisticallyꢀdistinctꢀtransformations,ꢀtheꢀphotoredoxꢀcatalystꢀservesꢀtoꢀinitiateꢀaꢀchainꢀ
9
reaction. ꢀꢀInꢀthisꢀreport,ꢀweꢀdescribeꢀourꢀfirstꢀstudyꢀfocusedꢀprimarilyꢀonꢀtheꢀmechanismꢀofꢀaꢀphotoinduced,ꢀcopper-mediatedꢀcross-
1
0
coupling,ꢀspecifically,ꢀanꢀinvestigationꢀofꢀtheꢀstoichiometricꢀcouplingꢀofꢀanꢀarylꢀiodideꢀwithꢀaꢀcopper–thiolateꢀ(eqꢀ2). ꢀ
ꢀ
1

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X–
I
[
L Cu (SAr)]
n
hν
A
ligand
exchange
–
SAr
excitation
I
I
[
L Cu X]
D
B
[L Cu (SAr)]*
n
n
electron
transfer
coupling
C
Ar1 SAr
Ar1
X
II
[
L Cu (SAr)]X
n
Ar1 •
ꢀ
Figureꢀ1.ꢀꢀOutlineꢀofꢀaꢀpossibleꢀcatalyticꢀcycleꢀforꢀphotoinduced,ꢀcopper-catalyzedꢀcross-coupling:ꢀCouplingꢀofꢀanꢀarylꢀradicalꢀwithꢀaꢀ
5,11
copper(II)–thiolateꢀasꢀaꢀkeyꢀstep.
ꢀ
ꢀ
hν (100-watt Hg lamp)
Ar1
I
I
–
Ar1 SAr (2)
[Cu (SAr) ]
2
CH CN, 0 °C
3
1
.0 equiv
ꢀ
ꢀ
ResultsꢀandꢀDiscussionꢀ
Background.ꢀꢀ
Theꢀphotoinducedꢀcouplingꢀofꢀarylꢀthiolsꢀwithꢀarylꢀhalidesꢀinꢀliquidꢀammonia,ꢀinꢀtheꢀabsenceꢀofꢀaꢀcatalyst,ꢀthroughꢀanꢀSRN1ꢀmechanismꢀ
1
2,13,14
isꢀwell-establishedꢀthroughꢀtheꢀworkꢀofꢀBunnett.
ꢀꢀInꢀourꢀinitialꢀreport,ꢀweꢀobservedꢀthatꢀaꢀmodelꢀphotoinducedꢀcross-couplingꢀ
2
b
proceedsꢀsignificantlyꢀmoreꢀrapidlyꢀinꢀtheꢀpresenceꢀofꢀaꢀcopperꢀcatalystꢀthanꢀinꢀitsꢀabsenceꢀ(eqꢀ3). ꢀ
ꢀ
Me
Me
1
0% CuI
hν (100-watt Hg lamp)
I
H
SPh
SPh (3)
1
.0 equiv NaOt-Bu
1
.0 equiv
CH CN, 0 °C, 5 h
Me
3
Me
8
5%
(10% without CuI)
ꢀ
ꢀ
Underꢀourꢀreportedꢀconditions,ꢀtheꢀreactionꢀmixtureꢀisꢀheterogeneous,ꢀwithꢀaꢀsubstantialꢀportionꢀofꢀtheꢀNaSPhꢀpresentꢀasꢀaꢀsolid.ꢀꢀWeꢀ
haveꢀdeterminedꢀthat,ꢀwhenꢀtheꢀsameꢀpartnersꢀareꢀcoupledꢀatꢀmuchꢀlowerꢀconcentrationꢀinꢀaꢀhomogeneousꢀsolution,ꢀtheꢀratesꢀofꢀproductꢀ
1
5
formationꢀcanꢀbeꢀsimilarꢀinꢀtheꢀpresenceꢀandꢀinꢀtheꢀabsenceꢀofꢀCuI. ꢀꢀThus,ꢀaꢀcopper-mediatedꢀpathwayꢀandꢀaꢀcopper-freeꢀpathwayꢀforꢀC–Sꢀ
bondꢀformationꢀareꢀpossible,ꢀandꢀwhichꢀoneꢀisꢀdominantꢀcanꢀdependꢀonꢀtheꢀrelativeꢀconcentrationꢀinꢀsolutionꢀofꢀsodiumꢀversusꢀcopperꢀ
thiolatesꢀ(theꢀlatterꢀareꢀgenerallyꢀmoreꢀsolubleꢀinꢀCH
pathway.ꢀ
3
CN).ꢀꢀInꢀtheꢀpresentꢀinvestigation,ꢀweꢀseekꢀtoꢀgainꢀinsightꢀintoꢀtheꢀcopper-mediatedꢀ
Inꢀtheꢀmechanisticꢀframeworkꢀthatꢀweꢀhaveꢀpreviouslyꢀdescribedꢀ(Figureꢀ1),ꢀirradiationꢀofꢀaꢀcopper(I)–thiolateꢀcomplexꢀ(A)ꢀleadsꢀtoꢀaꢀ
photoexcitedꢀstateꢀ(B).ꢀꢀElectronꢀtransferꢀfromꢀBꢀtoꢀtheꢀarylꢀhalideꢀfurnishesꢀaꢀcopper(II)–thiolateꢀcomplexꢀ(C)ꢀandꢀanꢀarylꢀradical.ꢀꢀRadicalꢀ
1
6
recombinationꢀthenꢀformsꢀtheꢀC–Sꢀbondꢀofꢀtheꢀthioether,ꢀeitherꢀdirectly ꢀorꢀthroughꢀaꢀcopper(III)ꢀintermediate,ꢀandꢀaꢀcopper(I)–halideꢀ
ꢀ
complexꢀ(D). ꢀDisplacementꢀofꢀtheꢀhalideꢀofꢀcomplexꢀDꢀbyꢀthiolateꢀthenꢀregeneratesꢀcopper(I)–thiolateꢀcomplexꢀA.ꢀ
2

Page 3 of 17
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Weꢀhaveꢀalsoꢀconsideredꢀaꢀvarietyꢀofꢀotherꢀmechanisms,ꢀincludingꢀtheꢀthreeꢀillustratedꢀinꢀFiguresꢀ2–4.ꢀꢀTheꢀpathwayꢀdepictedꢀinꢀFigureꢀ
ꢀproducesꢀanꢀarylꢀradicalꢀandꢀaꢀcopper(II)–thiolateꢀ(C)ꢀthroughꢀtheꢀsameꢀinitialꢀstepsꢀasꢀinꢀFigureꢀ1.ꢀꢀNext,ꢀtheꢀarylꢀradicalꢀreactsꢀwithꢀaꢀ
2
1
7
copper(I)–thiolateꢀ(A),ꢀratherꢀthanꢀaꢀcopper(II)–thiolateꢀ(C;ꢀFigureꢀ1),ꢀtoꢀformꢀtheꢀthioether,ꢀasꢀwellꢀasꢀcopper(0)ꢀ(E). ꢀꢀComproportionationꢀ
ofꢀcopper(0)ꢀwithꢀcopper(II)–thiolateꢀCꢀcouldꢀregenerateꢀcopper(I)–thiolateꢀA.ꢀ
ꢀ
X–
I
[
L Cu (SAr)]
n
hν
–
A
SAr
II
[
L Cu (SAr)]X
n
C
0
I
[
L Cu ]
E
B
[L Cu (SAr)]*
n
n
Ar1
X
Ar1 SAr
Ar1 •
I
II
[
L Cu (SAr)]
[L Cu (SAr)]X
n
n
A
C
ꢀ
5
Figureꢀ2.ꢀꢀAnꢀalternativeꢀmechanism:ꢀCouplingꢀofꢀanꢀarylꢀradicalꢀwithꢀaꢀcopper(I)–thiolateꢀasꢀaꢀkeyꢀstep. ꢀ
ꢀ
12
AnotherꢀmechanismꢀunderꢀconsiderationꢀlargelyꢀfollowsꢀtheꢀSRN1ꢀpathwayꢀforꢀcopper-freeꢀC–Sꢀcouplingꢀreactions, ꢀtheꢀdifferenceꢀ
beingꢀthatꢀaꢀphotoexcitedꢀcopper(I)–thiolateꢀ(B),ꢀratherꢀthanꢀaꢀphotoexcitedꢀcopper-freeꢀthiolate,ꢀservesꢀasꢀtheꢀinitiatingꢀelectronꢀdonorꢀtoꢀ
1
8
theꢀarylꢀhalide,ꢀtherebyꢀgeneratingꢀaꢀradicalꢀanionꢀ(F)ꢀthatꢀcanꢀparticipateꢀinꢀaꢀchainꢀreactionꢀtoꢀformꢀtheꢀthioetherꢀ(Figureꢀ3). ꢀ
ꢀ
hν
I
I
[
L Cu (SAr)]
[L Cu (SAr)]*
n
n
A
B
Ar1
X
– •
[
Ar1 X]
Ar1 SAr
F
Ar1
X
Ar1 SAr]
– •
Ar • X–
1
[
X–
–
SAr
ꢀ
5
Figureꢀ3.ꢀꢀAnꢀalternativeꢀmechanism:ꢀSRN1ꢀ(radicalꢀchainꢀprocess). ꢀ
ꢀ
Weꢀhaveꢀalsoꢀconsideredꢀmechanismsꢀthatꢀdoꢀnotꢀinvolveꢀanꢀorganicꢀradicalꢀasꢀanꢀintermediate.ꢀꢀForꢀexample,ꢀinꢀtheꢀpathwayꢀdepictedꢀ
inꢀFigureꢀ4,ꢀphotoexcitedꢀcomplexꢀBꢀreactsꢀwithꢀtheꢀarylꢀhalideꢀtoꢀcleaveꢀtheꢀC–Xꢀbondꢀinꢀaꢀconcertedꢀprocessꢀwithoutꢀtheꢀintermediacyꢀofꢀ
3

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1
9
1
anꢀarylꢀradical. ꢀReductiveꢀeliminationꢀofꢀtheꢀresultingꢀcopper(III)ꢀcomplexꢀ(G)ꢀleadsꢀtoꢀtheꢀthioetherꢀproductꢀ(Ar –SAr)ꢀandꢀcopper(I)–
halideꢀadductꢀD.ꢀꢀLigandꢀexchangeꢀthenꢀcompletesꢀtheꢀcatalyticꢀcycleꢀbyꢀregeneratingꢀcopper(I)–thiolateꢀA.ꢀ
ꢀ
X–
I
[
L Cu (SAr)]
n
hν
A
–
SAr
I
I
[
L Cu X]
D
B
[L Cu (SAr)]*
n
n
G
Ar1 SAr
Ar1
X
III
1
[
L Cu Ar X(SAr)]
n
ꢀ
5
,20
ꢀ
Figureꢀ4.ꢀꢀAnꢀalternativeꢀmechanism:ꢀConcertedꢀoxidativeꢀaddition.
ꢀ
2
b
Previouslyꢀreportedꢀmechanisticꢀobservations. ꢀ
Inꢀourꢀoriginalꢀreport,ꢀweꢀdescribedꢀcyclization/stereochemistryꢀdataꢀ(eqꢀ4)ꢀthatꢀareꢀmoreꢀreadilyꢀaccommodatedꢀbyꢀaꢀradical/electron-
transferꢀpathwayꢀ(Figuresꢀ1–3)ꢀthanꢀbyꢀaꢀconcertedꢀpathwayꢀ(Figureꢀ4)ꢀforꢀC–Xꢀbondꢀcleavage.ꢀꢀFurthermore,ꢀinꢀaꢀrelative-reactivityꢀstudyꢀ
2
1
(
eqꢀ5),ꢀweꢀdeterminedꢀthatꢀtheꢀarylꢀhalideꢀthatꢀisꢀmoreꢀeasilyꢀreducedꢀviaꢀelectronꢀtransferꢀ(4-chlorobenzonitrile;ꢀ–2.03ꢀVꢀvs.ꢀSCEꢀinꢀDMF )ꢀ
21
isꢀmoreꢀreactiveꢀthanꢀtheꢀoneꢀwithꢀtheꢀweakerꢀC–Xꢀbondꢀ(1-bromonaphthalene;ꢀ–2.17ꢀVꢀvs.ꢀSCEꢀinꢀDMF );ꢀthisꢀcontrastsꢀwithꢀthermalꢀ(non-
photoinduced)ꢀcopper-catalyzedꢀS-arylation,ꢀwhereinꢀessentiallyꢀexclusiveꢀcouplingꢀofꢀtheꢀarylꢀbromideꢀisꢀproposedꢀtoꢀresultꢀfromꢀ
19,22
concertedꢀoxidativeꢀaddition.
ꢀ
ꢀ
D
SPh
H
D
see eq 3
I
(4)
H
SPh
O
1
.0 equiv
O
1
:1 mixture
of diastereomers
D
via (?)
O
Cl
SPh
NC
5
0%
1
.0 equiv PhSH
see eq 3
5
.0 equiv
Br
NC
(5)
SPh
1
9%
5
.0 equiv
ꢀ
4

Page 5 of 17
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ꢀ
I
–
Ourꢀeffortsꢀtoꢀisolateꢀaꢀmononuclearꢀ[Cu (SPh)
2
] ꢀcomplexꢀ(e.g.,ꢀAꢀinꢀFiguresꢀ1–4),ꢀwhichꢀweꢀhadꢀdetectedꢀinꢀanꢀESI–MSꢀstudyꢀofꢀaꢀC–Sꢀ
I
couplingꢀreaction,ꢀledꢀinsteadꢀtoꢀaꢀcopper(I)–thiolateꢀcluster,ꢀ[Cu
5
(SPh)
7
][Na(12-crown-4)
2
]
2
.ꢀꢀThisꢀclusterꢀdid,ꢀhowever,ꢀserveꢀasꢀaꢀsuitableꢀ
stoichiometricꢀcouplingꢀpartnerꢀwithꢀanꢀarylꢀiodide,ꢀasꢀwellꢀasꢀanꢀeffectiveꢀ(pre)catalystꢀforꢀaꢀphotoinducedꢀC–Sꢀcross-coupling.ꢀ
Althoughꢀtheseꢀobservationsꢀareꢀconsistentꢀwithꢀourꢀinitialꢀworkingꢀhypothesisꢀforꢀtheꢀmechanismꢀofꢀphotoinduced,ꢀcopper-catalyzedꢀ
C–Sꢀcross-couplingsꢀ(Figureꢀ1),ꢀweꢀconcludedꢀthatꢀaꢀmoreꢀdetailedꢀinvestigationꢀwasꢀwarranted.ꢀ
ꢀ
Synthesisꢀandꢀcharacterizationꢀofꢀaꢀmonomericꢀcopper(I)–thiolateꢀmodelꢀcomplex.ꢀꢀꢀ
Aꢀcopper(I)–thiolateꢀ(A)ꢀisꢀtheꢀstartingꢀpointꢀinꢀeachꢀofꢀtheꢀpathwaysꢀillustratedꢀinꢀFiguresꢀ1–4.ꢀꢀForꢀeaseꢀofꢀanalysisꢀinꢀtheꢀpresentꢀ
investigation,ꢀweꢀsoughtꢀaꢀmodelꢀsystemꢀofꢀsimpleꢀspeciationꢀ(monomeric).ꢀꢀAsꢀdemonstratedꢀbyꢀTshuva,ꢀtheꢀuseꢀofꢀaꢀhinderedꢀarylthiolateꢀ
2
3
(
2,6-dimethylthiophenolate;ꢀSAr,ꢀArꢀ=ꢀ2,6-dimethylphenyl)ꢀcanꢀavoidꢀtheꢀformationꢀofꢀaꢀcluster; ꢀfurthermore,ꢀweꢀhadꢀreportedꢀinꢀourꢀ
2
b
initialꢀstudyꢀthatꢀthisꢀarylthiolateꢀservesꢀasꢀaꢀsuitableꢀcouplingꢀpartnerꢀinꢀphotoinducedꢀC–Sꢀcross-couplings. ꢀꢀReactionꢀofꢀmesitylcopper(I),ꢀ
,6-dimethylthiophenol,ꢀandꢀNaOt-BuꢀinꢀCH CN,ꢀfollowedꢀbyꢀtheꢀadditionꢀofꢀ12-crown-4,ꢀprovidedꢀtheꢀdesiredꢀsodiumꢀsalt,ꢀ
]Na”;ꢀeqꢀ6ꢀandꢀFigureꢀ5).ꢀ
2
3
I
I
[
Cu (SAr)
2
][Na(12-crown-4)
2
]ꢀ(1;ꢀ“[Cu (SAr)
2
ꢀ
1) 1.0 equiv NaOt-Bu
CH CN, 25 °C
3
I
MesCu
ArSH
[Cu (SAr) ][Na(12-crown-4) ] (6)
2 2
2
) 2.0 equiv 12-crown-4
Mes = mesityl
2
.0 equiv
I
"
[Cu (SAr) ]Na"
2
1
Ar = 2,6-dimethylphenyl
ꢀ
ꢀ
ꢀ
ꢀ
I
Figureꢀ5.ꢀꢀX-rayꢀcrystalꢀstructureꢀofꢀ[Cu (SAr)
2
][Na(12-crown-4)
2
]ꢀ(1).ꢀꢀEllipsoidsꢀareꢀshownꢀatꢀ50%ꢀprobability,ꢀandꢀhydrogenꢀatomsꢀhaveꢀ
beenꢀomittedꢀforꢀclarity.ꢀꢀSelectedꢀbondꢀlengthsꢀandꢀbondꢀangle:ꢀCu–Sꢀ=ꢀ2.1477(5)ꢀÅꢀandꢀ2.1499(5)ꢀÅ;ꢀS–Cu–S,ꢀ166.82(2)°.ꢀ
ꢀ
Ourꢀavailableꢀdataꢀareꢀconsistentꢀwithꢀtheꢀsuggestionꢀthatꢀthisꢀcopper(I)–thiolateꢀisꢀaꢀmonomerꢀinꢀsolution,ꢀasꢀinꢀtheꢀsolidꢀstate.ꢀꢀOnꢀ
theꢀbasisꢀofꢀdiffusion-orderedꢀNMRꢀspectroscopyꢀ(DOSY),ꢀweꢀestimateꢀtheꢀhydrodynamicꢀradiiꢀofꢀtheꢀanionꢀandꢀtheꢀcationꢀtoꢀbeꢀ4.2ꢀandꢀ4.4ꢀ
Å,ꢀrespectively,ꢀwhichꢀareꢀcomparableꢀtoꢀtheꢀcorrespondingꢀcomputedꢀradiiꢀofꢀ4.5ꢀandꢀ4.8ꢀÅ.ꢀꢀFurthermore,ꢀtheꢀmolarꢀconductivityꢀforꢀ
2
–1
2
–1
complexꢀ1ꢀinꢀacetonitrile,ꢀ128.5ꢀSꢀcm ꢀmol ,ꢀfallsꢀwithinꢀtheꢀrangeꢀ(120–160ꢀSꢀcm ꢀmol )ꢀforꢀotherꢀcoordinationꢀcompoundsꢀthatꢀareꢀ1:1ꢀ
2
4
electrolytes. ꢀ
NaSArꢀ(Arꢀ=ꢀ2,6-dimethylphenyl)ꢀisꢀsignificantlyꢀmoreꢀsolubleꢀinꢀCH
3
CNꢀthanꢀisꢀNaSPh;ꢀconsequently,ꢀforꢀtheꢀphotoinducedꢀcouplingꢀofꢀ
NaSArꢀwithꢀPh–Iꢀunderꢀourꢀstandardꢀconditions,ꢀtheꢀratesꢀofꢀreactionꢀinꢀtheꢀabsenceꢀandꢀinꢀtheꢀpresenceꢀofꢀCuIꢀareꢀsimilar,ꢀinꢀcontrastꢀtoꢀ
I
ourꢀobservationsꢀwithꢀNaSPhꢀ(eqꢀ3).ꢀꢀNevertheless,ꢀweꢀhaveꢀdeterminedꢀthatꢀ[Cu (SAr)
2
]Naꢀ(1)ꢀcouplesꢀatꢀ0ꢀ°CꢀwithꢀPh–Iꢀinꢀ56%ꢀyield,ꢀ
therebyꢀsubstantiatingꢀtheꢀviabilityꢀofꢀphotoinducedꢀcopper-mediatedꢀS–arylationꢀwithꢀthisꢀmodelꢀcomplexꢀ(eqꢀ7).ꢀ
ꢀ
5

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hν (100-watt Hg lamp)
I
[
Cu (SAr) ]Na Ph
I
Ph SAr (7)
6%
2
CH CN, 0 °C, 5 h
3
5
(
1)
1.0 equiv
Ar = 2,6-dimethylphenyl (no hν: 2%)
ꢀ
ꢀ
InꢀanꢀESI–MSꢀstudyꢀofꢀtheꢀcouplingꢀofꢀArSHꢀwithꢀPh–Iꢀunderꢀourꢀstandardꢀcopper-catalyzedꢀcross-couplingꢀconditionsꢀ(eqꢀ3),ꢀweꢀhaveꢀ
I
–
detectedꢀanꢀanionꢀwithꢀaꢀmolecularꢀweightꢀofꢀ337.2,ꢀwhichꢀcorrespondsꢀtoꢀthatꢀofꢀ[Cu (SAr)
2
] ;ꢀunderꢀtheseꢀconditions,ꢀweꢀdoꢀnotꢀobserveꢀ
I
2–
1
[
Cu (SAr)
3
] ,ꢀdespiteꢀtheꢀlargeꢀexcessꢀofꢀthiolateꢀrelativeꢀtoꢀcopper.ꢀꢀFurthermore,ꢀ HꢀNMRꢀandꢀopticalꢀabsorptionꢀspectraꢀofꢀcomplexꢀ1ꢀinꢀ
2
5
I
2–
theꢀpresenceꢀofꢀexcessꢀthiolate,ꢀasꢀwellꢀasꢀDFTꢀcalculations, ꢀindicateꢀthatꢀformationꢀofꢀ[Cu (SAr)
3
] ꢀisꢀunfavorable.ꢀꢀCollectively,ꢀourꢀdataꢀ
suggestꢀthatꢀcomplexꢀ1ꢀexistsꢀasꢀaꢀtwo-coordinateꢀmonomerꢀinꢀsolution,ꢀevenꢀinꢀtheꢀpresenceꢀofꢀexcessꢀthiolate.ꢀ
ꢀ
Electrochemistryꢀꢀ
I
Weꢀhaveꢀexaminedꢀthroughꢀelectrochemistryꢀtheꢀredoxꢀbehaviorꢀofꢀ[Cu (SAr)
2
]Naꢀ(1)ꢀandꢀofꢀNaSArꢀ(Figureꢀ6).ꢀꢀTheꢀcyclicꢀ
p
voltammogramꢀofꢀcomplexꢀ1ꢀshowsꢀanꢀirreversibleꢀoxidativeꢀfeatureꢀatꢀE ꢀ=ꢀ–0.18ꢀVꢀvs.ꢀSCEꢀthatꢀisꢀalsoꢀirreversibleꢀatꢀ–20ꢀ°Cꢀandꢀatꢀscanꢀ
ratesꢀupꢀtoꢀ1.5ꢀV/sꢀatꢀ25ꢀ°C.ꢀꢀFollowingꢀoxidationꢀofꢀ1,ꢀanꢀirreversibleꢀfeatureꢀisꢀobservedꢀatꢀ–1.85ꢀVꢀvs.ꢀSCE,ꢀwhichꢀcorrespondsꢀtoꢀtheꢀ
reductionꢀofꢀbis(2,6-dimethylphenyl)ꢀdisulfideꢀ(ArS–SAr),ꢀpresumablyꢀformedꢀfromꢀcomplexꢀ1ꢀuponꢀelectrochemicalꢀoxidationꢀ(oxidationꢀofꢀ
1
ꢀwithꢀ[FeCp
2
][PF
6
]ꢀalsoꢀleadsꢀtoꢀtheꢀformationꢀofꢀArS–SAr).ꢀ
ꢀ
ꢀ
I
Figureꢀ6.ꢀꢀCyclicꢀvoltammogramsꢀofꢀ[Cu (SAr)
2
]Naꢀ(1;ꢀtop)ꢀandꢀofꢀNaSArꢀ(bottom).ꢀꢀConditions:ꢀScanꢀrate:ꢀ100ꢀmV/s;ꢀsupportingꢀelectrolyte:ꢀ
0
4 6 5 4
.08ꢀMꢀ[(n-Bu) N][B(C F ) ];ꢀworkingꢀelectrode:ꢀglassyꢀcarbon;ꢀreferenceꢀelectrode:ꢀAg/AgNO3ꢀ(0.1ꢀmM)/acetonitrile;ꢀauxiliaryꢀelectrode:ꢀ
platinumꢀwire;ꢀtemperature:ꢀ25ꢀ°C.ꢀ
ꢀ
Toꢀgainꢀinsightꢀintoꢀwhetherꢀfreeꢀthiolate,ꢀgeneratedꢀeitherꢀbyꢀsimpleꢀligandꢀdissociationꢀorꢀthroughꢀirradiation,ꢀmightꢀplayꢀaꢀsignificantꢀ
4 6 5 4
roleꢀinꢀstoichiometricꢀreactionsꢀofꢀcomplexꢀ1,ꢀweꢀhaveꢀmonitoredꢀbyꢀcyclicꢀvoltammetryꢀaꢀsolutionꢀofꢀ1ꢀ(0.020ꢀM)ꢀandꢀ[(n-Bu) N][B(C F ) ]ꢀasꢀ
electrolyteꢀinꢀacetonitrile.ꢀꢀTheꢀcyclicꢀvoltammogramꢀisꢀunchangedꢀoverꢀ15ꢀminutesꢀofꢀirradiationꢀwithꢀaꢀHgꢀlamp,ꢀsuggestingꢀthatꢀirradiationꢀ
ofꢀcomplexꢀ1ꢀdoesꢀnotꢀleadꢀtoꢀtheꢀreleaseꢀofꢀaꢀdetectableꢀamountꢀofꢀfreeꢀthiolate.ꢀ
ꢀ
I
Photophysicalꢀstudyꢀofꢀ[Cu (SAr)
2
]Naꢀ(1)ꢀ
–
1
–1
Complexꢀ1ꢀabsorbsꢀstronglyꢀinꢀtheꢀultravioletꢀregionꢀ(topꢀofꢀFigureꢀ7),ꢀalthoughꢀonlyꢀweaklyꢀatꢀ365ꢀnmꢀ(e365ꢀ=ꢀ3ꢀM ꢀcm ),ꢀaꢀprominentꢀ
2
6
emissionꢀbandꢀforꢀtheꢀ100-wattꢀHgꢀlampꢀusedꢀinꢀourꢀphotoinducedꢀC–Sꢀcouplings. ꢀꢀTheꢀcomplexꢀluminescesꢀuponꢀexcitationꢀatꢀ355ꢀnmꢀ
withꢀaꢀlifetimeꢀofꢀ~7ꢀµsꢀinꢀacetonitrile,ꢀasꢀdeterminedꢀbyꢀtransientꢀluminescenceꢀspectroscopyꢀ(bottomꢀofꢀFigureꢀ7).ꢀꢀTheꢀlifetimeꢀofꢀtheꢀ
emissiveꢀstateꢀdoesꢀnotꢀchangeꢀasꢀaꢀfunctionꢀofꢀtheꢀobservationꢀwavelength,ꢀconsistentꢀwithꢀaꢀsingleꢀspeciesꢀbeingꢀtheꢀsourceꢀofꢀ
I
II
luminescence.ꢀꢀWhileꢀtheꢀlackꢀofꢀaꢀreversibleꢀCu /Cu ꢀredoxꢀcoupleꢀprecludesꢀaꢀtrueꢀevaluationꢀofꢀtheꢀexcited-stateꢀreductionꢀpotentialꢀforꢀ
p
complexꢀ1,ꢀweꢀestimateꢀthisꢀpotentialꢀtoꢀbeꢀ–2.5ꢀtoꢀ–2.7ꢀVꢀonꢀtheꢀbasisꢀofꢀtheꢀfirstꢀground-stateꢀoxidativeꢀfeatureꢀ(E ꢀ=ꢀ–0.18ꢀVꢀvs.ꢀSCE)ꢀ(seeꢀ
6

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0
0
27,28
Electrochemistry)ꢀandꢀanꢀapproximateꢀE ꢀofꢀ2.3–2.5ꢀeV.
ꢀꢀTheseꢀdataꢀsuggestꢀthatꢀtheꢀexcitedꢀstateꢀofꢀcomplexꢀ1ꢀisꢀsufficientlyꢀlong-livedꢀ
21
andꢀreducingꢀtoꢀengageꢀinꢀelectronꢀtransferꢀwithꢀelectrophilesꢀsuchꢀasꢀarylꢀiodidesꢀ(Ph–I:ꢀ–1.91ꢀVꢀvs.ꢀSCEꢀinꢀDMF ).ꢀ
ꢀ
!
ꢀ
I
4
–1
–1
Figureꢀ7.ꢀꢀTop:ꢀOpticalꢀspectrumꢀofꢀ[Cu (SAr)
2
]Naꢀ(1)ꢀ(lmaxꢀ=ꢀ258ꢀnm,ꢀeꢀ=ꢀ2.3ꢀ×ꢀ10 ꢀcm ꢀM );ꢀinset:ꢀexcitationꢀspectrumꢀatꢀ675ꢀnmꢀemissionꢀ
(
dashedꢀline)ꢀandꢀemissionꢀspectrumꢀatꢀ353ꢀnmꢀexcitationꢀ(solidꢀline);ꢀinꢀacetonitrileꢀatꢀ25ꢀ°C.ꢀꢀBottom:ꢀTime-resolvedꢀdecayꢀofꢀtheꢀ
luminescenceꢀintensityꢀofꢀ1*;ꢀinꢀacetonitrileꢀatꢀ25ꢀ°Cꢀ(25ꢀmM);ꢀXeꢀarcꢀlampꢀ(425ꢀW)ꢀatꢀ355ꢀnmꢀexcitation;ꢀobservationꢀwavelength:ꢀ675ꢀnm.ꢀ
ꢀ
Toꢀgainꢀinsightꢀintoꢀtheꢀpredictedꢀelectronicꢀstructureꢀofꢀtheꢀexcitedꢀstateꢀofꢀcomplexꢀ1,ꢀweꢀhaveꢀperformedꢀtime-dependentꢀDFTꢀ
2
9
calculations. ꢀꢀTheseꢀcalculationsꢀindicateꢀthatꢀtheꢀlowestꢀenergyꢀsingletꢀstateꢀ(λcalcꢀ=ꢀ325ꢀnm)ꢀconsistsꢀofꢀaꢀtransitionꢀfromꢀtheꢀHOMOꢀ(Cu–Sꢀ
antibonding)ꢀtoꢀtheꢀareneꢀπ*ꢀ(Figureꢀ8).ꢀꢀTheꢀpopulationꢀofꢀaꢀhigh-energyꢀareneꢀπ*ꢀorbitalꢀinꢀtheꢀexcitedꢀstateꢀisꢀconsistentꢀwithꢀ1ꢀbeingꢀaꢀ
potentꢀphotoreductant.ꢀ
ꢀ
ꢀ
I
Figureꢀ8.ꢀꢀDifferenceꢀdensityꢀplotꢀforꢀtheꢀlowestꢀenergyꢀabsorptionꢀbandꢀofꢀ[Cu (SAr)
2
]Naꢀ(1).ꢀꢀTheꢀdonorꢀorbitalꢀisꢀshownꢀinꢀred,ꢀandꢀtheꢀ
acceptorꢀorbitalꢀisꢀshownꢀinꢀblueꢀ(isovalueꢀ=ꢀ0.02).ꢀ
ꢀ
Stern–Volmerꢀkineticꢀanalysis.ꢀꢀꢀ
TheꢀmechanismsꢀoutlinedꢀinꢀFiguresꢀ1–3ꢀbeginꢀwithꢀelectronꢀtransferꢀfromꢀaꢀphotoexcitedꢀcopper(I)–thiolateꢀ(B)ꢀtoꢀtheꢀarylꢀhalide.ꢀꢀWeꢀ
I
haveꢀconductedꢀaꢀStern–Volmerꢀkineticꢀanalysisꢀofꢀthisꢀelementaryꢀstep,ꢀspecifically,ꢀtheꢀreactionꢀofꢀtheꢀexcitedꢀstateꢀofꢀ[Cu (SAr)
2
]Naꢀ(1)ꢀ
I
–
21
withꢀPh–Iꢀ(reductionꢀpotentials:ꢀ[Cu (SAr)
2
] *:ꢀ~–2.6ꢀV;ꢀPh–I:ꢀ–1.91ꢀVꢀvs.ꢀSCEꢀinꢀDMF ),ꢀandꢀweꢀhaveꢀdeterminedꢀthatꢀtheꢀrateꢀconstantꢀforꢀ
7

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5
–1 –1
quenchingꢀisꢀ8ꢀxꢀ10 ꢀM s .ꢀꢀAsꢀexpected,ꢀanꢀincreaseꢀinꢀtheꢀconcentrationꢀofꢀPh–Iꢀleadsꢀtoꢀaꢀdecreaseꢀinꢀtheꢀlifetimeꢀofꢀtheꢀexcitedꢀstateꢀ
(
Figureꢀ9).ꢀꢀTheꢀobservedꢀquenchingꢀresultsꢀfromꢀelectronꢀtransfer,ꢀnotꢀenergyꢀtransfer;ꢀtheꢀemissionꢀspectrumꢀofꢀcomplexꢀ1ꢀexhibitsꢀnoꢀ
3
0,31
overlapꢀwithꢀtheꢀabsorptionꢀspectrumꢀofꢀPh–I.
ꢀ
ꢀ
ꢀ
I
2
Figureꢀ9.ꢀꢀStern–Volmerꢀplotꢀforꢀtheꢀluminescenceꢀquenchingꢀofꢀ[Cu (SAr) ]Na*ꢀinꢀtheꢀpresenceꢀofꢀPh–I.ꢀ
ꢀ
Considerationꢀofꢀaꢀradicalꢀchainꢀmechanism:ꢀQuantumꢀyieldꢀandꢀchainꢀlength.ꢀꢀꢀ
Weꢀhaveꢀestablishedꢀthatꢀtheꢀquantumꢀyieldꢀ(Φ)ꢀforꢀtheꢀstoichiometricꢀcouplingꢀofꢀ[Cu (SAr)
I
2
]Naꢀ(1)ꢀwithꢀPh–Iꢀwhenꢀirradiatedꢀatꢀ365ꢀ
3
2
nmꢀisꢀ0.08(2), ꢀaꢀvalueꢀthatꢀcanꢀbeꢀaccommodatedꢀeitherꢀbyꢀaꢀnon-chainꢀmechanismꢀorꢀbyꢀaꢀchainꢀmechanismꢀwithꢀrapidꢀchainꢀ
termination.ꢀꢀByꢀdividingꢀtheꢀquantumꢀyieldꢀbyꢀtheꢀStern–Volmerꢀquenchingꢀfractionꢀ(Q),ꢀweꢀhaveꢀdeterminedꢀtheꢀchainꢀlengthꢀ(theꢀnumberꢀ
ofꢀmoleculesꢀofꢀproductꢀformedꢀperꢀphotoinducedꢀelectron-transferꢀevent)ꢀforꢀtheꢀC–Sꢀcouplingꢀofꢀcomplexꢀ1ꢀwithꢀPh–Iꢀtoꢀbeꢀ0.8ꢀ(eqꢀ8).ꢀꢀ
Thisꢀsuggestsꢀthatꢀthisꢀcross-couplingꢀproceedsꢀviaꢀaꢀnon-chainꢀpathway,ꢀasꢀaꢀchainꢀmechanismꢀwouldꢀbeꢀexpectedꢀtoꢀfurnishꢀmoreꢀthanꢀ
oneꢀmoleculeꢀofꢀproductꢀfromꢀeachꢀphotoinducedꢀelectronꢀtransfer.ꢀꢀInꢀcontrast,ꢀYoonꢀconcludedꢀonꢀtheꢀbasisꢀofꢀaꢀsimilarꢀanalysisꢀthatꢀ
9
threeꢀrepresentativeꢀreactionsꢀthatꢀinvolveꢀphotoredoxꢀcatalysisꢀproceedꢀthroughꢀaꢀchainꢀpathway. ꢀ
ꢀ
Φ = quantum yield = 0.08
(8)
Q = Stern–Volmer quenching fraction
kq[Ph–I ]
τ0 = excited-state lifetime of complex 1
=
=
τ0–1 + k [Ph–I ] k
= rate of Stern–Volmer quenching of Ph–I
q
q
5
–1 –1
(8 x 10 M s )(0.02 M)
= 0.10
5
–1 –1
(
7 x 10–6 s)–1 + (8 x 10 M s )(0.02 M)
Φ
Q
chain length =
= 0.8
ꢀ
ꢀ
ꢀ
Viabilityꢀofꢀcouplingꢀanꢀarylꢀradicalꢀwithꢀaꢀcopper–thiolate.ꢀꢀꢀ
Theꢀmechanisticꢀobservationsꢀdescribedꢀaboveꢀareꢀconsistentꢀwithꢀtheꢀsuggestionꢀthatꢀanꢀarylꢀradicalꢀisꢀgeneratedꢀunderꢀourꢀcross-
couplingꢀconditions.ꢀꢀThisꢀintermediateꢀcouldꢀsubsequentlyꢀformꢀaꢀC–SꢀbondꢀbyꢀreactingꢀwithꢀspeciesꢀsuchꢀasꢀaꢀCu(II)–thiolateꢀ(Figureꢀ1)ꢀorꢀaꢀ
Cu(I)–thiolateꢀ(Figureꢀ2).ꢀꢀWeꢀsoughtꢀinsightꢀintoꢀtheꢀviabilityꢀofꢀsuchꢀcouplingsꢀbyꢀexploringꢀreactionsꢀofꢀanꢀaryldiazoniumꢀsalt,ꢀwhichꢀcanꢀ
2d
readilyꢀbeꢀconvertedꢀintoꢀanꢀarylꢀradicalꢀviaꢀone-electronꢀreduction. ꢀ
I
33
Whenꢀ[Cu (SAr)
2
]Naꢀ(1;ꢀE
p
ꢀ=ꢀ–0.18ꢀVꢀvs.ꢀSCE)ꢀandꢀ4-methoxyphenyldiazoniumꢀtetrafluoroborateꢀ(E°ꢀ=ꢀ0.14ꢀVꢀvs.ꢀSCE )ꢀareꢀallowedꢀtoꢀ
1
reactꢀinꢀCD
3
CNꢀatꢀ–20ꢀ°Cꢀforꢀ30ꢀminutes,ꢀnoꢀcouplingꢀisꢀevidentꢀbyꢀ HꢀNMRꢀspectroscopyꢀ(Tableꢀ1,ꢀentryꢀ1).ꢀꢀHowever,ꢀuponꢀwarmingꢀtheꢀ
mixtureꢀtoꢀroomꢀtemperatureꢀforꢀ30ꢀminutes,ꢀC–Sꢀbondꢀformationꢀproceedsꢀinꢀ57%ꢀyieldꢀ(entryꢀ2).ꢀꢀOneꢀpossibleꢀpathwayꢀforꢀthisꢀ
8

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1
transformationꢀbeginsꢀwithꢀelectronꢀtransferꢀfromꢀcomplexꢀ1ꢀtoꢀtheꢀaryldiazoniumꢀsaltꢀtoꢀaffordꢀaꢀcopper(II)–thiolateꢀandꢀAr –N
2
,ꢀwhichꢀ
3
4
losesꢀN
ꢀ
2
ꢀtoꢀgenerateꢀanꢀarylꢀradicalꢀthatꢀcouplesꢀwithꢀtheꢀcopper(II)–thiolateꢀtoꢀformꢀtheꢀC–Sꢀbondꢀ(eqꢀ9). ꢀ
Tableꢀ1.ꢀꢀReactionsꢀofꢀaꢀcopper–thiolateꢀwithꢀanꢀaryldiazoniumꢀsalt.ꢀ
I
[Ar1 N ][BF ]
[Ar1 SAr]
[
Cu (SAr) ]Na
2
2
4
CH CN
3
(
1)
1.0 equiv
Ar = 2,6-dimethylphenyl
1
Ar = p-anisyl
entry
conditions
yield (%)
1
2
3
4
–20 °C, 30 min
r.t., 30 min
<2a
57
22
56
1.1 equiv FeCp* , –20° C, 30 min
2
r.t., 30 min, 1.1 equiv [FeCp* ][BF ]
2
4
a
Run in CD CN; unreacted diazonium salt: >90%.
3
ꢀ
ꢀ
ꢀ
I
Ar1 SAr
[
Cu (SAr) ]Na [Ar1 N ][BF ]
(9)
2
2
4
(
1)
Cu (SAr) Ar1 •
II
2
–
NaBF , N
4 2
ꢀ
Toꢀassessꢀtheꢀviabilityꢀofꢀtheꢀcouplingꢀofꢀanꢀarylꢀradicalꢀwithꢀaꢀcopper(I)–thiolate,ꢀweꢀsoughtꢀaꢀreductantꢀthatꢀwouldꢀreduceꢀtheꢀ
aryldiazoniumꢀsaltꢀandꢀtherebyꢀgenerateꢀanꢀarylꢀradicalꢀunderꢀconditionsꢀinꢀwhichꢀcopper(I)–thiolateꢀ1ꢀwouldꢀnotꢀ(CD CN,ꢀ–20ꢀ°C;ꢀTableꢀ1,ꢀ
;ꢀE°ꢀ=ꢀ–0.12ꢀVꢀvs.ꢀSCE ), ꢀtheꢀaryldiazoniumꢀsaltꢀisꢀ
3
3
5 36
entryꢀ1).ꢀꢀWeꢀdeterminedꢀthat,ꢀinꢀtheꢀpresenceꢀofꢀdecamethylferroceneꢀ(FeCp*
2
completelyꢀconsumedꢀwithinꢀ30ꢀminutesꢀatꢀ–20ꢀ°C,ꢀfurnishingꢀaꢀmixtureꢀofꢀcompoundsꢀthatꢀincludesꢀaꢀ22%ꢀyieldꢀofꢀtheꢀC–Sꢀcouplingꢀ
productꢀ(entryꢀ3).ꢀꢀTheꢀlowꢀyieldꢀofꢀtheꢀdiarylsulfideꢀindicatesꢀthatꢀunderꢀtheseꢀconditionsꢀanꢀarylꢀradicalꢀreactsꢀinefficiently,ꢀatꢀbest,ꢀwithꢀaꢀ
copper(I)–thiolateꢀtoꢀformꢀaꢀC–Sꢀbond;ꢀcontrolꢀexperimentsꢀsuggestꢀtheꢀalternativeꢀpossibilityꢀthatꢀatꢀleastꢀsomeꢀofꢀtheꢀcross-couplingꢀ
productꢀmayꢀbeꢀformedꢀfromꢀreactionꢀofꢀtheꢀarylꢀradicalꢀwithꢀaꢀsmallꢀamountꢀofꢀcopper(II)–thiolateꢀthatꢀisꢀgeneratedꢀthroughꢀaꢀredoxꢀ
I
III
II
II
37
equilibriumꢀbetweenꢀCu /Fe ꢀandꢀCu /Fe ꢀasꢀtheꢀferroceniumꢀionꢀisꢀformed. ꢀꢀWhenꢀtheꢀcouplingꢀillustratedꢀinꢀentryꢀ2ꢀisꢀconductedꢀinꢀtheꢀ
presenceꢀofꢀ[FeCp* ][BF ]ꢀ(entryꢀ4),ꢀtheꢀyieldꢀofꢀtheꢀdiarylsulfideꢀisꢀessentiallyꢀunchangedꢀ(56%;ꢀentryꢀ2ꢀversusꢀentryꢀ4).ꢀꢀThisꢀresultꢀindicatesꢀ
2
4
thatꢀtheꢀferroceniumꢀionꢀthatꢀisꢀproducedꢀinꢀentryꢀ3ꢀisꢀnotꢀresponsibleꢀforꢀtheꢀdiminishedꢀyieldꢀinꢀthatꢀreaction.ꢀ
ꢀ
Rateꢀofꢀcaptureꢀofꢀanꢀarylꢀradicalꢀbyꢀaꢀcopper–thiolate;ꢀin-cageꢀversusꢀout-of-cageꢀcoupling.ꢀꢀꢀ
Toꢀobtainꢀinsightꢀintoꢀtheꢀrateꢀofꢀcaptureꢀofꢀtheꢀarylꢀradicalꢀintermediate,ꢀweꢀhaveꢀdeterminedꢀtheꢀratioꢀofꢀuncyclized/cyclizedꢀ
3
8,39
productsꢀforꢀC–Sꢀcouplingsꢀofꢀseveralꢀarylꢀiodidesꢀthatꢀhaveꢀpreviouslyꢀbeenꢀemployedꢀinꢀradical-clockꢀstudiesꢀ(Tableꢀ2).
ꢀꢀOurꢀdataꢀ
indicateꢀthatꢀcaptureꢀofꢀtheꢀarylꢀradicalꢀbyꢀaꢀcopper–thiolateꢀoccursꢀcompetitivelyꢀwithꢀaꢀcyclizationꢀprocessꢀthatꢀhasꢀaꢀfirst-orderꢀrateꢀ
8
–1
constantꢀofꢀ~4ꢀ×ꢀ10 ꢀs ꢀinꢀbenzene.ꢀ
ꢀ
Tableꢀ2.ꢀꢀReactionꢀofꢀanꢀarylꢀradical:ꢀCyclizationꢀversusꢀcaptureꢀbyꢀaꢀcopper–thiolate.ꢀ
9

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hν (100-watt Hg lamp)
I
Ar1
1.0 equiv
[
Cu (SAr) ]Na
I
products
2
CH CN, 0 °C, 5 h
3
(
1)
Ar = 2,6-dimethylphenyl
productsa
Ar1
I
k (s )
–1 b
uncyclized
cyclized
SAr
I
SAr
O
9
(
.6 x 109
DMSO)
O
O
1
: >20
SAr
I
SAr
4
(
.2 x 108
benzene)
O
O
O
O
1
: 5
O
I
O
8
(
.0 x 105
water)
SAr
SAr
>
20 : 1
aRatio determined through GC analysis with dodecane as an internal standard
b
(
average of two experiments). Rate of cyclization of the aryl radical (20–25 °C).
ꢀ
ꢀ
I
Weꢀhaveꢀexaminedꢀtheꢀrelationshipꢀbetweenꢀtheꢀamountꢀofꢀ[Cu (SAr)
2
]Naꢀ(1)ꢀandꢀtheꢀratioꢀofꢀuncyclized/cyclizedꢀproducts,ꢀandꢀweꢀ
haveꢀdeterminedꢀthatꢀtheꢀproductꢀratioꢀremainsꢀessentiallyꢀconstantꢀasꢀweꢀalterꢀtheꢀquantityꢀofꢀcomplexꢀ1ꢀorꢀtheꢀoverallꢀconcentrationꢀ(eqꢀ
0).ꢀꢀTheseꢀobservationsꢀcanꢀbeꢀaccommodatedꢀbyꢀtheꢀmechanismꢀillustratedꢀinꢀFigureꢀ1,ꢀifꢀC–Sꢀbondꢀformationꢀoccursꢀbetweenꢀtheꢀarylꢀ
1
radicalꢀandꢀcopper(II)–thiolateꢀCꢀwithinꢀtheꢀsolventꢀcageꢀ(i.e.,ꢀaꢀsingleꢀcopperꢀcomplexꢀservesꢀfirstꢀasꢀtheꢀelectronꢀdonorꢀandꢀthenꢀasꢀtheꢀ
sourceꢀofꢀSAr).ꢀꢀInꢀcontrast,ꢀforꢀtheꢀmechanismꢀillustratedꢀinꢀFigureꢀ2,ꢀtheꢀcyclized/uncyclizedꢀproductꢀratioꢀshouldꢀdependꢀonꢀparametersꢀ
suchꢀasꢀstoichiometryꢀandꢀconcentration,ꢀsinceꢀC–Sꢀbondꢀformationꢀrequiresꢀtheꢀarylꢀradicalꢀtoꢀleaveꢀtheꢀsolventꢀcageꢀandꢀtoꢀencounterꢀaꢀ
4
0,41
copper(I)–thiolateꢀ(i.e.,ꢀoneꢀcopperꢀcomplexꢀservesꢀasꢀtheꢀelectronꢀdonorꢀandꢀaꢀdifferentꢀcopperꢀcomplexꢀprovidesꢀtheꢀSArꢀgroup).
ꢀ
ꢀ
SAr
I
[
Cu (SAr) ]Na
2
SAr
O
(
1; X equiv)
hν (100-watt Hg lamp)
(10)
I
CH3CN, 0 °C, 5 h
O
Ar = 2,6-dimethylphenyl
X
ratio of products
O
0
.010 M
1
1
1
2
2
.0
1 : 4.6
1 : 4.8
1 : 4.2
1 : 3.9
1 : 4.6
.0 (0.020 M)
.5
.0
.5
ꢀ
ꢀ
1
0

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Spectroscopicꢀevidenceꢀforꢀaꢀcopper(II)–thiolate.ꢀꢀꢀ
Asꢀnotedꢀabove,ꢀelectronꢀtransferꢀfromꢀanꢀexcited-stateꢀcopper(I)–thiolateꢀ(B)ꢀtoꢀanꢀarylꢀhalideꢀtoꢀgenerateꢀaꢀcopper(II)–thiolateꢀ(C)ꢀisꢀaꢀ
keyꢀstepꢀinꢀseveralꢀofꢀtheꢀmechanismsꢀunderꢀconsideration.ꢀꢀCopper(II)ꢀspeciesꢀareꢀSꢀ=ꢀ½ꢀandꢀthereforeꢀreadilyꢀdetectedꢀbyꢀEPRꢀ
4
2
I
spectroscopy,ꢀasꢀisꢀtheꢀcaseꢀforꢀcopper(II)–thiolateꢀcomplexes. ꢀꢀIndeed,ꢀphotolysisꢀofꢀaꢀsolutionꢀofꢀ[Cu (SAr)
2
]Naꢀ(1)ꢀinꢀtheꢀpresenceꢀofꢀ
excessꢀPh–IꢀandꢀNaSArꢀinꢀpropionitrile:butyronitrileꢀ(1:1)ꢀatꢀ–78ꢀ°Cꢀresultsꢀinꢀaꢀblueꢀsolution,ꢀtheꢀEPRꢀspectrumꢀofꢀwhichꢀisꢀconsistentꢀwithꢀ
4
3
theꢀpresenceꢀofꢀsomeꢀamountꢀofꢀaꢀcopper(II)–thiolateꢀradicalꢀ(Figureꢀ10). ꢀ
ꢀ
Ph–I (5 equiv)
NaSAr (5 equiv)
I
II
[
Cu (SAr) ]Na
Cu –thiolate
2
hν (100-watt Hg lamp)
propionitrile:butyronitrile (1:1)
(
1)
–78 °C
Ar = 2,6-dimethylphenyl
ꢀ
ꢀ
1 2
Figureꢀ10.ꢀꢀX-bandꢀEPRꢀspectrumꢀ(77ꢀK)ꢀofꢀaꢀcouplingꢀreactionꢀfollowingꢀirradiationꢀforꢀ5ꢀmin.ꢀꢀSimulationꢀparameters:ꢀg ꢀ=ꢀ2.022,ꢀg ꢀ=ꢀ2.032,ꢀ
g
1 2 3
3ꢀ=ꢀ2.104,ꢀA (Cu)ꢀ=ꢀ85ꢀMHz,ꢀA (Cu)ꢀ=ꢀ130ꢀMHz,ꢀandꢀA (Cu)ꢀ=ꢀ360ꢀMHz.ꢀ
ꢀ
Theꢀfour-lineꢀhyperfineꢀcouplingꢀisꢀconsistentꢀwithꢀanꢀIꢀ=ꢀ3/2ꢀparamagneticꢀcopperꢀcomplexꢀwithꢀaꢀsingleꢀmetalꢀcenter.ꢀꢀTheꢀspectrumꢀ
42
showsꢀmodestꢀgꢀanisotropyꢀcomparedꢀtoꢀotherꢀcopper(II)–thiolateꢀcomplexes, ꢀwhichꢀisꢀconsistentꢀwithꢀaꢀhighlyꢀcovalentꢀCu–Sꢀ
4
4
interaction. ꢀꢀThisꢀsuggestsꢀthatꢀsignificantꢀradicalꢀcharacterꢀresidesꢀinꢀsulfurꢀpꢀorbitals,ꢀandꢀDFTꢀcalculationsꢀsupportꢀthisꢀassessmentꢀ(seeꢀ
below).ꢀ
Opticalꢀspectroscopyꢀcanꢀserveꢀasꢀanꢀadditionalꢀtechniqueꢀforꢀcharacterizingꢀtheꢀputativeꢀcopper(II)–thiolate.ꢀꢀUponꢀirradiatingꢀ
complexꢀ1ꢀinꢀtheꢀpresenceꢀofꢀPh–IꢀandꢀNaSArꢀinꢀpropionitrileꢀatꢀ–78ꢀ°C,ꢀaꢀfeatureꢀatꢀ582ꢀnmꢀisꢀobservedꢀ(Figureꢀ11),ꢀwhichꢀisꢀconsistentꢀwithꢀ
4
5
theꢀblueꢀcolorꢀofꢀtheꢀreactionꢀmixture.ꢀꢀThisꢀfeatureꢀisꢀnearꢀtheꢀrangeꢀfoundꢀforꢀsulfur-richꢀcopper(II)ꢀproteinsꢀ(593ꢀtoꢀ610ꢀnm). ꢀ
ꢀ
ꢀ
Figureꢀ11.ꢀꢀOpticalꢀspectrumꢀofꢀaꢀcouplingꢀreactionꢀpriorꢀtoꢀphotolysisꢀ(red)ꢀandꢀafterꢀphotolysisꢀ(blue)ꢀinꢀpropionitrileꢀatꢀ–78ꢀ°C.ꢀ
1
1

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ꢀ
Whileꢀtheꢀaboveꢀdataꢀprovideꢀstrongꢀevidenceꢀforꢀtheꢀgenerationꢀofꢀaꢀcopper(II)–thiolateꢀradicalꢀuponꢀphotolysisꢀofꢀaꢀmixtureꢀofꢀ
complexꢀ1,ꢀPh–I,ꢀandꢀNaSAr,ꢀtheyꢀdoꢀnotꢀidentifyꢀtheꢀspecificꢀparamagneticꢀcopperꢀspecies,ꢀandꢀfurtherꢀcharacterizationꢀisꢀcomplicatedꢀbyꢀ
itsꢀinstabilityꢀevenꢀatꢀ–78ꢀ°C.ꢀꢀTheꢀpresenceꢀnotꢀonlyꢀofꢀPh–I,ꢀbutꢀalsoꢀofꢀNaSAr,ꢀisꢀrequiredꢀforꢀdetectionꢀofꢀthisꢀcopper(II)–thiolateꢀradicalꢀbyꢀ
II
–
] ,ꢀ
opticalꢀandꢀbyꢀEPRꢀspectroscopy.ꢀꢀInꢀviewꢀofꢀtheꢀneedꢀforꢀexogenousꢀthiolate,ꢀweꢀhypothesizeꢀthatꢀaꢀcopper(II)ꢀtris(thiolate),ꢀ[Cu (SAr)
3
I
–
II
46
mayꢀbeꢀformed,ꢀe.g.,ꢀviaꢀelectronꢀtransferꢀfromꢀ[Cu (SAr)
2
2 ꢀ
] *ꢀtoꢀtheꢀarylꢀhalideꢀtoꢀformꢀCu (SAr) , followedꢀbyꢀtrappingꢀbyꢀNaSArꢀ(eqꢀ11). ꢀꢀ
II
47,48
DFTꢀcalculationsꢀsuggestꢀthatꢀbindingꢀofꢀanꢀarylthiolateꢀtoꢀCu (SAr)
2
ꢀisꢀexergonicꢀbyꢀ~4ꢀkcal/mol.
ꢀ
ꢀ
hν
I
II
[
Cu (SAr) ]Na Ph I NaSAr
[Cu (SAr) ]Na (11)
2
3
hν
NaSAr
Ph I
Ph•, NaI
I
II
[
Cu (SAr) ]*Na
Cu (SAr)2
2
–
ꢀ
ꢀ
II
–
3
Alternatively,ꢀtheꢀcopper(II)ꢀtris(thiolate),ꢀ[Cu (SAr) ] ,ꢀcouldꢀbeꢀgeneratedꢀbyꢀelectronꢀtransferꢀtoꢀPh–IꢀfromꢀtheꢀexcitedꢀstateꢀofꢀNaSAr,ꢀ
I
–
2
followedꢀbyꢀcouplingꢀofꢀtheꢀthiylꢀradicalꢀwithꢀ[Cu (SAr) ] ꢀ(eqꢀ12).ꢀꢀOurꢀobservationsꢀtoꢀdateꢀdoꢀnotꢀallowꢀusꢀtoꢀdefinitivelyꢀdistinguishꢀ
II
–
betweenꢀtheseꢀtwoꢀpathwaysꢀforꢀtheꢀformationꢀofꢀputativeꢀ[Cu (SAr)
3
] .ꢀ
ꢀ
hν
I
II
[
Cu (SAr) ]Na Ph I NaSAr
[Cu (SAr) ]Na (12)
3
2
hν
I
[
Cu (SAr) ]Na
2
Ph I
Ph•, NaI
Na[SAr]*
• SAr
–
ꢀ
ꢀ
II
–
II
DFTꢀcalculationsꢀofꢀ[Cu (SAr)
3
] ꢀandꢀofꢀCu (SAr)2ꢀpredictꢀthatꢀsignificantꢀspinꢀdensityꢀwouldꢀresideꢀonꢀtheꢀthiolateꢀligandsꢀforꢀeitherꢀ
compound,ꢀwhichꢀsuggestsꢀthatꢀC–Sꢀbondꢀformationꢀcouldꢀoccurꢀthroughꢀdirectꢀreactionꢀofꢀtheꢀarylꢀradicalꢀwithꢀtheꢀcopper-boundꢀthiolateꢀ
II
–
–
–
II
–
–
(
[Cu (SAr)
3
] :ꢀCuꢀ0.23ꢀe ,ꢀ3Sꢀ0.57ꢀe ;ꢀCu (SAr)
2
:ꢀCuꢀ0.14ꢀe ,ꢀ2Sꢀ0.63ꢀe ;ꢀFigureꢀ12).ꢀ
ꢀ
ꢀ
1
2

Page 13 of 17
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DOI: 10.1039/C5SC04709A
II
–
II
3
Figureꢀ12.ꢀꢀSpinꢀdensityꢀplotsꢀ(0.002ꢀisocontours)ꢀofꢀ[Cu (SAr) ] ꢀ(left)ꢀandꢀCu (SAr)2ꢀ(right).ꢀ
ꢀ
Conclusionsꢀ
Inꢀthisꢀreport,ꢀweꢀdescribeꢀtheꢀfirstꢀdetailedꢀmechanisticꢀinvestigationꢀofꢀoneꢀofꢀtheꢀphotoinduced,ꢀcopper-mediatedꢀcross-couplingsꢀ
thatꢀweꢀhaveꢀdeveloped,ꢀspecifically,ꢀtheꢀcouplingꢀofꢀaꢀthiolꢀwithꢀanꢀarylꢀiodide.ꢀꢀDueꢀtoꢀtheꢀexistenceꢀofꢀaꢀparallel,ꢀcopper-freeꢀC–Sꢀbond-
formingꢀpathway,ꢀweꢀhaveꢀfocusedꢀourꢀattentionꢀonꢀunderstandingꢀtheꢀstoichiometricꢀchemistryꢀofꢀaꢀkeyꢀproposedꢀintermediate,ꢀ
I
–
[
(
Cu (SAr)
2
] ꢀ(Arꢀ=ꢀ2,6-dimethylphenyl);ꢀourꢀobservationsꢀtoꢀdateꢀareꢀconsistentꢀwithꢀtheꢀviabilityꢀofꢀtheꢀelementaryꢀstepsꢀoutlinedꢀinꢀFigureꢀ1ꢀ
Aꢀ→ꢀD).ꢀ
I
Weꢀhaveꢀestablishedꢀthatꢀ[Cu (SAr)
2
]Naꢀ(1)ꢀisꢀaꢀtwo-coordinateꢀmonomerꢀbothꢀinꢀtheꢀsolidꢀstateꢀandꢀinꢀsolution,ꢀandꢀweꢀhaveꢀdetectedꢀ
I
–
[
2
Cu (SAr) ] ꢀthroughꢀESI–MSꢀunderꢀcross-couplingꢀconditions.ꢀꢀComplexꢀ1ꢀundergoesꢀexcitationꢀuponꢀirradiationꢀatꢀ365ꢀnmꢀ(aꢀprominentꢀ
emissionꢀbandꢀofꢀaꢀHgꢀlamp),ꢀandꢀitꢀluminescesꢀwithꢀaꢀlifetimeꢀofꢀ~7ꢀms;ꢀweꢀestimateꢀitsꢀexcited-stateꢀreductionꢀpotentialꢀtoꢀbeꢀ~–2.6ꢀV.ꢀꢀ
ThroughꢀaꢀStern–Volmerꢀstudy,ꢀweꢀhaveꢀdeterminedꢀthatꢀtheꢀexcitedꢀstateꢀisꢀeffectivelyꢀquenchedꢀbyꢀPh–I,ꢀasꢀexpectedꢀonꢀtheꢀbasisꢀofꢀitsꢀ
reductionꢀpotential;ꢀcorrespondingly,ꢀcomplexꢀ1ꢀreactsꢀwithꢀPh–IꢀuponꢀirradiationꢀtoꢀaffordꢀtheꢀC–Sꢀcouplingꢀproduct.ꢀꢀByꢀemployingꢀ
actinometry,ꢀweꢀhaveꢀestablishedꢀthatꢀtheꢀchainꢀlengthꢀforꢀtheꢀcouplingꢀofꢀcomplexꢀ1ꢀwithꢀPh–Iꢀisꢀ0.8,ꢀindicatingꢀaꢀnon-chainꢀmechanism.ꢀꢀ
OurꢀEPRꢀandꢀopticalꢀspectroscopyꢀdataꢀsuggestꢀthatꢀaꢀcopper(II)–thiolateꢀisꢀformedꢀwhenꢀcomplexꢀ1ꢀisꢀirradiatedꢀinꢀtheꢀpresenceꢀofꢀPh–Iꢀandꢀ
NaSAr.ꢀꢀFurthermore,ꢀthroughꢀtheꢀuseꢀofꢀanꢀaryldiazoniumꢀsalt,ꢀweꢀhaveꢀindependentlyꢀgeneratedꢀanꢀarylꢀradicalꢀinꢀtheꢀpresenceꢀofꢀ
copper(I)–ꢀandꢀcopper(II)–thiolates,ꢀandꢀweꢀhaveꢀprovidedꢀevidenceꢀthatꢀC–Sꢀbondꢀformationꢀisꢀmoreꢀefficientꢀinꢀtheꢀcaseꢀofꢀaꢀcopper(II)–
thiolate.ꢀꢀFinally,ꢀwithꢀtheꢀaidꢀofꢀradicalꢀclocks,ꢀweꢀhaveꢀestablishedꢀthatꢀC–Sꢀbondꢀformationꢀlikelyꢀoccursꢀviaꢀanꢀin-cageꢀmechanismꢀinꢀ
I
II
whichꢀaꢀsingleꢀcopperꢀcomplexꢀservesꢀbothꢀasꢀanꢀelectronꢀdonorꢀ(Cu ꢀ→ꢀCu )ꢀandꢀaꢀsourceꢀofꢀSArꢀ(copper(II)–thiolate).ꢀꢀThus,ꢀtheꢀavailableꢀ
dataꢀsupportꢀtheꢀviabilityꢀofꢀtheꢀelementaryꢀstepsꢀforꢀphotoinducedꢀC–SꢀcouplingꢀthatꢀareꢀillustratedꢀinꢀFigureꢀ1ꢀ(Aꢀ→ꢀD),ꢀaꢀmechanismꢀthatꢀ
isꢀdistinctꢀfromꢀmostꢀapplicationsꢀofꢀphotoredoxꢀcatalystsꢀinꢀorganicꢀsynthesis;ꢀotherꢀC–Sꢀcouplingꢀpathways,ꢀforꢀexampleꢀinvolvingꢀinitialꢀ
photoreductionꢀofꢀtheꢀarylꢀhalideꢀbyꢀaꢀphotoexcitedꢀcopper-freeꢀthiolate,ꢀmayꢀalsoꢀbeꢀoperative.ꢀꢀOurꢀcurrentꢀeffortsꢀareꢀdirectedꢀatꢀ
evaluatingꢀtheꢀdegreeꢀtoꢀwhichꢀtheꢀmechanismꢀillustratedꢀinꢀFigureꢀ1,ꢀorꢀalternativeꢀmechanisms,ꢀappliesꢀtoꢀotherꢀphotoinduced,ꢀcopper-
catalyzedꢀcross-couplings.ꢀ
ꢀ
Acknowledgmentsꢀ
WeꢀthankꢀJunꢀMyunꢀAhn,ꢀDr.ꢀAngelꢀJ.ꢀDiꢀBilio,ꢀLawrenceꢀM.ꢀHenling,ꢀTanviꢀS.ꢀRatani,ꢀDr.ꢀDavidꢀVanderVelde,ꢀDr.ꢀJayꢀR.ꢀWinkler,ꢀandꢀ
membersꢀofꢀtheꢀlaboratoryꢀofꢀProf.ꢀHarryꢀB.ꢀGrayꢀforꢀassistance,ꢀandꢀweꢀthankꢀtheꢀGordonꢀandꢀBettyꢀMooreꢀFoundationꢀandꢀtheꢀNIHꢀ
(
NIGMS:ꢀR01ꢀGM109194)ꢀforꢀfunding.ꢀꢀPhotophysicalꢀstudiesꢀwereꢀconductedꢀatꢀtheꢀBeckmanꢀInstituteꢀLaserꢀResourceꢀCenter.ꢀ
ꢀ
NotesꢀandꢀReferencesꢀ
(
1)ꢀ Forꢀ recentꢀ monographs,ꢀ see:ꢀ (a)ꢀ T.ꢀ Colacot,ꢀ Newꢀ Trendsꢀ inꢀ Cross-Coupling,ꢀ Royalꢀ Societyꢀ ofꢀ
Chemistry,ꢀ Cambridge,ꢀ 2015;ꢀ (b)ꢀ A.ꢀ deꢀ Meijereꢀ andꢀ M.ꢀ Oestreich,ꢀ Metal-Catalyzedꢀ Cross-Couplingꢀ
Reactionsꢀ andꢀ More,ꢀ Wiley–VCH,ꢀ Weinheim,ꢀ 2014;ꢀ (c)ꢀ G.ꢀ Evanoꢀ andꢀ N.ꢀ Blanchard,ꢀ Copper-Mediatedꢀ
Cross-CouplingꢀReactions,ꢀJohnꢀWileyꢀ&ꢀSons,ꢀHoboken,ꢀNJ,ꢀ2014.ꢀ
(
2)ꢀArylations:ꢀ(a)ꢀcarbazolesꢀasꢀnucleophiles:ꢀS.ꢀE.ꢀCreutz,ꢀK.ꢀJ.ꢀLotito,ꢀG.ꢀC.ꢀFuꢀandꢀJ.ꢀC.ꢀPeters,ꢀScience,ꢀ
2012,ꢀ338,ꢀ647;ꢀ(b)ꢀarylꢀthiolsꢀasꢀnucleophiles:ꢀC.ꢀUyeda,ꢀY.ꢀTan,ꢀG.ꢀC.ꢀFuꢀandꢀJ.ꢀC.ꢀPeters,ꢀJ.ꢀAm.ꢀChem.ꢀ
Soc.,ꢀ 2013,ꢀ 135,ꢀ 9548;ꢀ (c)ꢀ heteroaromaticꢀ nitrogenꢀ nucleophiles:ꢀ D.ꢀ T.ꢀ Ziegler,ꢀ J.ꢀ Choi,ꢀ J.ꢀ M.ꢀ Muñoz-
Molina,ꢀA.ꢀC.ꢀBissember,ꢀJ.ꢀC.ꢀPetersꢀandꢀG.ꢀC.ꢀFu,ꢀJ.ꢀAm.ꢀChem.ꢀSoc.,ꢀ2013,ꢀ135,ꢀ13107;ꢀ(d)ꢀphenolsꢀasꢀ
nucleophiles:ꢀY.ꢀTan,ꢀJ.ꢀM.ꢀMuñoz-Molina,ꢀG.ꢀC.ꢀFuꢀandꢀJ.ꢀC.ꢀPeters,ꢀChem.ꢀSci.,ꢀ2014,ꢀ5,ꢀ2831;ꢀ(e)ꢀForꢀaꢀ
relatedꢀstudy,ꢀsee:ꢀW.-J.ꢀYoo,ꢀT.ꢀTsukamotoꢀandꢀS.ꢀKobayashi,ꢀOrg.ꢀLett.,ꢀ2015,ꢀ17,ꢀ3640ꢀ(carbazolesꢀasꢀ
nucleophiles).ꢀ
(
3)ꢀAlkylations:ꢀ(a)ꢀcarbazolesꢀasꢀnucleophiles:ꢀA.ꢀC.ꢀBissember,ꢀR.ꢀJ.ꢀLundgren,ꢀS.ꢀE.ꢀCreutz,ꢀJ.ꢀC.ꢀPetersꢀ
andꢀG.ꢀC.ꢀFu,ꢀAngew.ꢀChem.,ꢀInt.ꢀEd.,ꢀ2013,ꢀ52,ꢀ5129;ꢀ(b)ꢀamidesꢀasꢀnucleophiles:ꢀH.-Q.ꢀDo,ꢀS.ꢀBachman,ꢀ
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A.ꢀC.ꢀBissember,ꢀJ.ꢀC.ꢀPetersꢀandꢀG.ꢀC.ꢀFu,ꢀJ.ꢀAm.ꢀChem.ꢀSoc.,ꢀ2014,ꢀ136,ꢀ2162;ꢀ(c)ꢀcyanideꢀasꢀnucleophile:ꢀ
T.ꢀS.ꢀRatani,ꢀS.ꢀBachman,ꢀG.ꢀC.ꢀFuꢀandꢀJ.ꢀC.ꢀPeters,ꢀJ.ꢀAm.ꢀChem.ꢀSoc.,ꢀ2015,ꢀ137,ꢀ13902.ꢀ
(
4)ꢀForꢀrecentꢀoverviewsꢀofꢀphotoinduced,ꢀcopper-catalyzedꢀreactions,ꢀsee:ꢀ(a)ꢀM.ꢀMajekꢀandꢀA.ꢀJ.ꢀvonꢀ
Wangelin,ꢀAngew.ꢀChem.,ꢀInt.ꢀEd.,ꢀ2013,ꢀ52,ꢀ5919;ꢀ(b)ꢀS.ꢀPariaꢀandꢀO.ꢀReiser,ꢀChemCatChem,ꢀ2014,ꢀ6,ꢀ
477.ꢀ
5)ꢀForꢀtheꢀsakeꢀofꢀsimplicityꢀinꢀFiguresꢀ1–4,ꢀtheꢀcopperꢀcomplexesꢀareꢀshownꢀwithoutꢀcharges,ꢀandꢀallꢀ
2
(
processesꢀ areꢀ depictedꢀ asꢀ beingꢀ irreversible.ꢀ Halidesꢀ (X)ꢀ mayꢀ beꢀ boundꢀ asꢀ inner-sphereꢀ orꢀ asꢀ outer-
sphereꢀligands.ꢀ
(
6)ꢀForꢀrecentꢀreviews,ꢀsee:ꢀC.ꢀK.ꢀPrier,ꢀD.ꢀA.ꢀRankicꢀandꢀD.ꢀW.ꢀC.ꢀMacMillan,ꢀChem.ꢀRev.,ꢀ2013,ꢀ113,ꢀ
322;ꢀ(b)ꢀD.ꢀM.ꢀSchultzꢀandꢀT.ꢀP.ꢀYoon,ꢀScience,ꢀ2014,ꢀ343,ꢀ1239176.ꢀ
7)ꢀForꢀaꢀpreviousꢀexampleꢀofꢀaꢀtransition-metalꢀphotoredoxꢀcatalystꢀthatꢀalsoꢀparticipatesꢀinꢀbondꢀ
5
(
formation,ꢀsee:ꢀH.ꢀHuo,ꢀX.ꢀShen,ꢀC.ꢀWang,ꢀL.ꢀZhang,ꢀP.ꢀRöse,ꢀL.-A.ꢀChen,ꢀK.ꢀHarms,ꢀM.ꢀMarsch,ꢀG.ꢀHiltꢀandꢀ
E.ꢀMeggers,ꢀNature,ꢀ2014,ꢀ515,ꢀ100.ꢀ
(
(
(
8)ꢀForꢀexample,ꢀsee:ꢀD.ꢀA.ꢀNicewiczꢀandꢀD.ꢀW.ꢀC.ꢀMacMillan,ꢀScience,ꢀ2008,ꢀ322,ꢀ77.ꢀ
9)ꢀM.ꢀA.ꢀCismesiaꢀandꢀT.ꢀP.ꢀYoon,ꢀChem.ꢀSci.,ꢀ2015,ꢀ6,ꢀ5426.ꢀ
10)ꢀ Forꢀ aꢀ recentꢀ reviewꢀ withꢀ leadingꢀ referencesꢀ onꢀ metal-catalyzedꢀ cross-couplingsꢀ toꢀ formꢀ C–Sꢀ
bonds,ꢀsee:ꢀC.-F.ꢀLee,ꢀY.-C.ꢀLiuꢀandꢀS.ꢀS.ꢀBadsara,ꢀChem.–AsianꢀJ.,ꢀ2014,ꢀ9,ꢀ706.ꢀ
II
1
(
n
11)ꢀAlthoughꢀnotꢀexplicitlyꢀshown,ꢀcaptureꢀofꢀ[L Cu (SAr)]XꢀbyꢀAr ⋅ꢀcouldꢀgenerateꢀaꢀCu(III)ꢀ
III
1
1
n
intermediate,ꢀ[L Cu (Ar )(SAr)]X,ꢀwhichꢀcouldꢀthenꢀreductivelyꢀeliminateꢀtoꢀaffordꢀAr SArꢀandꢀcomplexꢀD.ꢀ
(
(
12)ꢀJ.ꢀF.ꢀBunnettꢀandꢀX.ꢀCreary,ꢀJ.ꢀOrg.ꢀChem.,ꢀ1974,ꢀ39,ꢀ3173.ꢀ
13)ꢀForꢀearlyꢀoverviewsꢀofꢀSRN1ꢀreactions,ꢀsee:ꢀ(a)ꢀJ.ꢀF.ꢀBunnett,ꢀAcc.ꢀChem.ꢀRes.,ꢀ1978,ꢀ11,ꢀ413;ꢀꢀ(b)ꢀJ.ꢀ
PinsonꢀandꢀJ.ꢀM.ꢀSaveant,ꢀinꢀElectroorganicꢀSynthesis,ꢀed.ꢀR.ꢀD.ꢀLittleꢀandꢀN.ꢀL.ꢀWeinbert,ꢀDekker,ꢀNewꢀ
York,ꢀ1991.ꢀ
(
14)ꢀForꢀreviewsꢀofꢀphotoinducedꢀSRN1ꢀreactions,ꢀsee:ꢀ(a)ꢀM.ꢀE.ꢀBuden,ꢀS.ꢀE.ꢀMartinꢀandꢀR.ꢀA.ꢀRossi,ꢀinꢀ
CRCꢀHandbookꢀofꢀOrganicꢀPhotochemistryꢀandꢀPhotobiology;ꢀed.ꢀA.ꢀGriesbeck,ꢀM.ꢀOelgemollerꢀandꢀF.ꢀ
Ghetti,ꢀCRCꢀPress,ꢀBocaꢀRaton,ꢀFlorida,ꢀ2012,ꢀppꢀ347–368;ꢀ(b)ꢀJ.ꢀI.ꢀBerdagi,ꢀV.ꢀA.ꢀVaillardꢀandꢀR.ꢀA.ꢀRossi,ꢀ
inꢀEncyclopediaꢀofꢀRadicalsꢀinꢀChemistry,ꢀBiologyꢀandꢀMaterials,ꢀed.ꢀC.ꢀChatgilialogluꢀandꢀA.ꢀStuder,ꢀJohnꢀ
Wileyꢀ&ꢀSons,ꢀChichester,ꢀU.K.,ꢀ2012,ꢀChapterꢀ14;ꢀ(c)ꢀA.ꢀB.ꢀPeñéñoryꢀandꢀJ.ꢀE.ꢀArgüello,ꢀinꢀHandbookꢀofꢀ
SyntheticꢀPhotochemistry,ꢀed.ꢀA.ꢀAlbiniꢀandꢀM.ꢀFagnoni,ꢀWiley–VCH,ꢀWeinheim,ꢀGermany,ꢀ2010,ꢀChapterꢀ
1
0;ꢀ (d)ꢀ W.ꢀ R.ꢀ Bowman,ꢀ inꢀ Photoinducedꢀ Electronꢀ Transfer,ꢀ ed.ꢀ M.ꢀ A.ꢀ Foxꢀ andꢀ M.ꢀ Chanon,ꢀ Elsevier,ꢀ
Amsterdam,ꢀ1990,ꢀppꢀ487–552.ꢀ
15)ꢀForꢀadditionalꢀinformation,ꢀseeꢀtheꢀSupportingꢀInformation.ꢀꢀForꢀotherꢀclassesꢀofꢀnucleophilesꢀthatꢀ
(
weꢀhaveꢀexamined,ꢀtheꢀrateꢀinꢀtheꢀpresenceꢀofꢀcopperꢀisꢀsignificantlyꢀgreaterꢀthanꢀinꢀtheꢀabsenceꢀofꢀ
copper.ꢀꢀForꢀexample,ꢀseeꢀreferencesꢀ2ꢀandꢀ3.ꢀ
(
16)ꢀ Forꢀ earlyꢀ examplesꢀ ofꢀ copper-mediatedꢀ C–Sꢀ couplingꢀ processesꢀ thatꢀ haveꢀ beenꢀ suggestedꢀ toꢀ
proceedꢀwithoutꢀtheꢀformationꢀofꢀaꢀCu(III)ꢀintermediate,ꢀsee:ꢀ(a)ꢀC.ꢀL.ꢀJenkinsꢀandꢀJ.ꢀK.ꢀKochi,ꢀJ.ꢀOrg.ꢀ
Chem.,ꢀ1971,ꢀ36,ꢀ3095;ꢀ(b)ꢀC.ꢀL.ꢀJenkinsꢀandꢀJ.ꢀK.ꢀKochi,ꢀJ.ꢀAm.ꢀChemꢀSoc.,ꢀ1972,ꢀ94,ꢀ856.ꢀ
(
17)ꢀCopper(0)ꢀ“complex”ꢀEꢀcouldꢀbeꢀcopperꢀmetalꢀorꢀaꢀformalꢀcopper(0)ꢀcomplexꢀthatꢀbearsꢀaꢀredox-
activeꢀligand.ꢀ
18)ꢀForꢀanꢀexampleꢀofꢀaꢀnon-chainꢀprocess,ꢀsee:ꢀM.ꢀAhbala,ꢀP.ꢀHapiot,ꢀA.ꢀHoumam,ꢀM.ꢀJouini,ꢀJ.ꢀPinsonꢀ
andꢀJ.-M.ꢀSaveant,ꢀJ.ꢀAm.ꢀChem.ꢀSoc.,ꢀ1995,ꢀ117,ꢀ11488.ꢀ
19)ꢀAꢀconcertedꢀpathwayꢀforꢀC–Xꢀbondꢀcleavageꢀisꢀgenerallyꢀinvokedꢀinꢀthermalꢀ(non-photoinduced)ꢀ
copper-catalyzedꢀ cross-couplings.ꢀ Forꢀ example,ꢀ see:ꢀ (a)ꢀ C.ꢀ Chen,ꢀ Z.ꢀ Wengꢀ andꢀ J.ꢀ F.ꢀ Hartwig,ꢀ
(
(
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Organometallics,ꢀ2012,ꢀ31,ꢀ8031;ꢀ(b)ꢀM.ꢀFont,ꢀT.ꢀParella,ꢀM.ꢀCostasꢀandꢀX.ꢀRibas,ꢀOrganometallics,ꢀ2012,ꢀ
1,ꢀ7976.ꢀ
20) Aꢀstep-wiseꢀETꢀprocessꢀwherebyꢀ[L
Gꢀisꢀalsoꢀplausible.ꢀꢀSeeꢀalsoꢀreferenceꢀ11.
21)ꢀR.ꢀJ.ꢀEnemaerke,ꢀT.ꢀB.ꢀChristensen,ꢀH.ꢀJensenꢀandꢀK.ꢀDaasbjerg,ꢀJ.ꢀChem.ꢀSoc.ꢀPerkinꢀTrans.,ꢀ2,ꢀ2001,ꢀ
620.ꢀ
22)ꢀ Forꢀ aꢀ mechanismꢀ whereinꢀ C–Xꢀ bondꢀ cleavageꢀ occursꢀ throughꢀ halogen-atomꢀ transfer,ꢀ 1-
bromonaphthaleneꢀwouldꢀgenerallyꢀbeꢀexpectedꢀtoꢀbeꢀmoreꢀreactiveꢀthanꢀ4-chlorobenzonitrile.ꢀ
23)ꢀS.ꢀZeeviꢀandꢀE.ꢀY.ꢀTshuva,ꢀEur.ꢀJ.ꢀInorg.ꢀChem.,ꢀ2007,ꢀ5369.ꢀTheseꢀworkersꢀhaveꢀcrystallographicallyꢀ
3
II
1
(
n
Cu (SAr)]XꢀisꢀgeneratedꢀandꢀthenꢀcapturedꢀbyꢀAr ⋅ꢀtoꢀgenerateꢀ
(
1
(
(
I
characterizedꢀ[Cu (SAr)
(
(
2
][NEt
24)ꢀW.ꢀJ.ꢀGeary,ꢀCoord.ꢀChem.ꢀRev.ꢀ1971,ꢀ7,ꢀ81.ꢀ
25)ꢀAꢀDFTꢀstudyꢀindicatesꢀthatꢀbindingꢀofꢀthiolateꢀtoꢀ[Cu (SAr)
4
].ꢀ
I
–
I
2–
2
3
] ꢀtoꢀproduceꢀ[Cu (SAr) ] ꢀisꢀunfavorableꢀ
byꢀ~20ꢀkcal/molꢀ(seeꢀtheꢀSupportingꢀInformation).ꢀ
26)ꢀLessꢀthanꢀ5%ꢀdecompositionꢀofꢀcomplexꢀ1ꢀwasꢀobservedꢀafterꢀirradiationꢀ(Hgꢀlamp)ꢀinꢀCH
Cꢀforꢀfiveꢀhours.ꢀ
27)ꢀTheꢀlowerꢀlimitꢀofꢀthisꢀrangeꢀisꢀbasedꢀonꢀtheꢀonsetꢀofꢀemission,ꢀandꢀtheꢀupperꢀlimitꢀofꢀthisꢀrangeꢀisꢀ
calculatedꢀfromꢀtheꢀmidpointꢀofꢀtheꢀmaximaꢀofꢀtheꢀemissionꢀandꢀexcitationꢀspectraꢀ(insetꢀofꢀFigureꢀ7).ꢀ
(
3
CNꢀatꢀ0ꢀ
°
(
(
(
28)ꢀD.ꢀRehmꢀandꢀA.ꢀWeller,ꢀIsr.ꢀJ.ꢀChem.,ꢀ1970,ꢀ8,ꢀ259.ꢀ
29)ꢀTime-dependentꢀDFTꢀcalculationsꢀwereꢀperformedꢀusingꢀtheꢀBP86ꢀfunctionalꢀwithinꢀtheꢀTamm–
Dancoffꢀ approximationꢀ employingꢀ theꢀ def2-TZVPꢀ basisꢀ set.ꢀ Theꢀ 50ꢀ lowest-lyingꢀ singletꢀ excitedꢀ statesꢀ
wereꢀcalculated,ꢀbasedꢀonꢀtheꢀDFT-optimizedꢀgeometryꢀofꢀcomplexꢀ1.ꢀ
(
30) J.ꢀFergusonꢀandꢀT.ꢀIredale,ꢀJ.ꢀChem.ꢀSoc.ꢀRes.,ꢀ1953,ꢀ2959.
31)ꢀTheꢀtripletꢀstateꢀofꢀPh–Iꢀisꢀ3.5-3.6ꢀeVꢀ(seeꢀreferenceꢀ30),ꢀwhereasꢀtheꢀtriplet-stateꢀenergyꢀofꢀ1*ꢀisꢀ
.8ꢀeVꢀbasedꢀonꢀanꢀemissionꢀmaximumꢀofꢀ675ꢀnm.ꢀꢀTheseꢀdataꢀexcludeꢀtheꢀpossibilityꢀofꢀDexterꢀenergyꢀ
transfer.ꢀ
(
1
(
32)ꢀForꢀanꢀexampleꢀofꢀanꢀearlierꢀdeterminationꢀofꢀtheꢀquantumꢀyieldꢀofꢀaꢀphotoinducedꢀcouplingꢀofꢀaꢀ
sulfurꢀnucleophileꢀwithꢀanꢀarylꢀhalide,ꢀsee:ꢀL.ꢀC.ꢀSchmidt,ꢀJ.ꢀE.ꢀArgüelloꢀandꢀA.ꢀB.ꢀPeñéñory,ꢀJ.ꢀOrg.ꢀChem.ꢀ
2007,ꢀ72,ꢀ2936.ꢀ
(
(
33)ꢀR.ꢀM.ꢀElofsonꢀandꢀF.ꢀF.ꢀGadallah,ꢀJ.ꢀOrg.ꢀChem.ꢀ1969,ꢀ34,ꢀ854.ꢀ
34)ꢀWeꢀhaveꢀinvestigatedꢀtheꢀpossibilityꢀthatꢀC–Sꢀbondꢀformationꢀisꢀoccurringꢀthroughꢀaꢀnucleophilicꢀ
N
aromaticꢀsubstitutionꢀreactionꢀ(S Ar)ꢀofꢀtheꢀcopper(I)–thiolateꢀwithꢀtheꢀaryldiazoniumꢀsalt.ꢀForꢀtheꢀcross-
couplingꢀofꢀcomplexꢀ1ꢀwithꢀanꢀaryldiazoniumꢀsaltꢀthatꢀbearsꢀaꢀpendantꢀolefinꢀthatꢀisꢀsuitablyꢀpositionedꢀ
forꢀ intramolecularꢀ additionꢀ byꢀ anꢀ arylꢀ radical,ꢀ weꢀ observeꢀ noneꢀ ofꢀ theꢀ “direct”ꢀ couplingꢀ product,ꢀ
indicatingꢀ thatꢀ substitutionꢀ viaꢀ anꢀ S
SupportingꢀInformation).ꢀ
N
Arꢀ pathwayꢀ isꢀ notꢀ favoredꢀ underꢀ theseꢀ conditionsꢀ (seeꢀ theꢀ
(
(
35)ꢀJ.ꢀL.ꢀRobbins,ꢀN.ꢀEdelstein,ꢀB.ꢀSpencerꢀandꢀJ.ꢀC.ꢀSmart,ꢀJ.ꢀAm.ꢀChem.ꢀSoc.,ꢀ1982,ꢀ104,ꢀ1882.ꢀ
36)ꢀWhenꢀtheꢀaryldiazoniumꢀsaltꢀisꢀmixedꢀwithꢀFeCp* ꢀinꢀCD CNꢀatꢀ–20ꢀ°Cꢀforꢀ30ꢀminutes,ꢀanisoleꢀisꢀ
2
3
producedꢀinꢀ~85%ꢀyield.ꢀ
(
(
37)ꢀForꢀdetails,ꢀseeꢀtheꢀSupportingꢀInformation.ꢀ
38)ꢀRadicalꢀcyclizationꢀrates:ꢀ(a)ꢀentriesꢀ1ꢀandꢀ2:ꢀA.ꢀAnnunziata,ꢀC.ꢀGalli,ꢀM.ꢀMarinelliꢀandꢀT.ꢀPau,ꢀEur.ꢀJ.ꢀ
Org.ꢀChem.,ꢀ2001,ꢀ1323;ꢀ(b)ꢀentryꢀ3:ꢀP.ꢀHanson,ꢀR.ꢀC.ꢀHammond,ꢀP.ꢀR.ꢀGoodacre,ꢀJ.ꢀPurcellꢀandꢀA.ꢀW.ꢀ
Timms,ꢀJ.ꢀChem.ꢀSoc.ꢀPerkinꢀTrans.ꢀ2,ꢀ1994,ꢀ691.ꢀ
(
39)ꢀ Weꢀ haveꢀ conductedꢀ controlꢀ experimentsꢀ thatꢀ establishꢀ thatꢀ theseꢀ arylꢀ iodidesꢀ areꢀ stableꢀ toꢀ
irradiationꢀunderꢀtheꢀindicatedꢀconditions.ꢀ
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DOI: 10.1039/C5SC04709A
(
40)ꢀWhenꢀtheꢀcross-couplingꢀillustratedꢀinꢀentryꢀ2ꢀofꢀTableꢀ1ꢀisꢀconductedꢀatꢀdifferentꢀconcentrationsꢀ
0.010ꢀMꢀversusꢀ0.0050ꢀM),ꢀtheꢀyieldꢀofꢀtheꢀcouplingꢀproductꢀdoesꢀnotꢀchange,ꢀwhichꢀisꢀalsoꢀconsistentꢀ
withꢀaꢀsingleꢀcopperꢀcomplexꢀservingꢀasꢀtheꢀelectronꢀdonorꢀandꢀtheꢀsourceꢀofꢀSAr.ꢀ
2
41)ꢀWhenꢀtheꢀcross-couplingꢀillustratedꢀinꢀeqꢀ10ꢀisꢀconductedꢀwithꢀNaSArꢀinꢀplaceꢀofꢀ[Cu (SAr) ]Na,ꢀaꢀ
(
I
(
differentꢀ ratioꢀ (1:18)ꢀ ofꢀ uncyclized:cyclizedꢀ productsꢀ isꢀ observed,ꢀ consistentꢀ withꢀ theꢀ suggestionꢀ thatꢀ
differentꢀthiolatesꢀ(copperꢀversusꢀsodium)ꢀareꢀcouplingꢀwithꢀtheꢀorganicꢀradicalꢀinꢀtheꢀtwoꢀprocesses.ꢀ
(
42)ꢀForꢀexample,ꢀsee:ꢀ(a)ꢀP.ꢀL.ꢀHollandꢀandꢀW.ꢀB.ꢀTolman,ꢀJ.ꢀAm.ꢀChem.ꢀSoc.,ꢀ1999,ꢀ121,ꢀ7270;ꢀꢀ(b)ꢀP.ꢀL.ꢀ
HollandꢀandꢀW.ꢀB.ꢀTolman,ꢀJ.ꢀAm.ꢀChem.ꢀSoc.,ꢀ2000,ꢀ122,ꢀ6331.ꢀ
43)ꢀAsꢀdeterminedꢀbyꢀspinꢀquantification,ꢀtheꢀcopper(II)ꢀspeciesꢀisꢀpresentꢀatꢀaꢀconcentrationꢀofꢀ0.2ꢀ
mM,ꢀwhichꢀcorrespondsꢀtoꢀ~5%ꢀofꢀtheꢀstartingꢀcopper(I)ꢀcomplex.ꢀ
(
(
(
(
44)ꢀP.ꢀChen,ꢀK.ꢀFujisawaꢀandꢀE.ꢀI.ꢀSolomon,ꢀJ.ꢀAm.ꢀChem.ꢀSoc.,ꢀ2000,ꢀ122,ꢀ10177.ꢀ
45)ꢀE.ꢀI.ꢀSolomon,ꢀM.ꢀJ.ꢀBaldwinꢀandꢀM.ꢀD.ꢀLowery,ꢀChem.ꢀRev.,ꢀ1992,ꢀ92,ꢀ521.ꢀ
II
46)ꢀIntermolecularꢀtrappingꢀofꢀCu (SAr)
2
ꢀbyꢀfreeꢀthiolateꢀrequiresꢀcageꢀescapeꢀbyꢀtheꢀarylꢀradicalꢀ
generatedꢀviaꢀelectronꢀtransferꢀtoꢀAr–I.ꢀWhileꢀourꢀradical-clockꢀstudiesꢀareꢀmostꢀconsistentꢀwithꢀin-cageꢀ
recombinationꢀbetweenꢀaꢀcopper(II)–thiolateꢀandꢀanꢀarylꢀradicalꢀatꢀ0ꢀ°C,ꢀbimolecularꢀreactionsꢀthatꢀ
followꢀEyringꢀbehaviorꢀcan,ꢀwhenꢀthereꢀisꢀnoꢀcoulombicꢀtermꢀandꢀtheꢀviscosityꢀchangeꢀisꢀnotꢀdramatic,ꢀ
beꢀexpectedꢀtoꢀexhibitꢀenhancedꢀcageꢀescapeꢀasꢀtheꢀtemperatureꢀisꢀloweredꢀinꢀfluidꢀsolution,ꢀaccordingꢀ
toꢀtheꢀEigenꢀequationꢀ(see:ꢀJ.ꢀOlmstedꢀandꢀT.ꢀJ.ꢀMeyer,ꢀJ.ꢀPhys.ꢀChem.,ꢀ1987,ꢀ91,ꢀ1649).ꢀInꢀgeneral,ꢀtheꢀ
temperatureꢀdependenceꢀofꢀtheꢀpartitioningꢀbetweenꢀin-cageꢀversusꢀout-of-cageꢀradicalꢀpathwaysꢀisꢀ
challengingꢀtoꢀpredictꢀ(seeꢀtheꢀSupportingꢀInformationꢀforꢀadditionalꢀinformation).ꢀ
II
(
47)ꢀ Accordingꢀ toꢀ aꢀ DFTꢀ calculation,ꢀ bindingꢀ ofꢀ iodideꢀ anionꢀ toꢀ Cu (SAr)
2
ꢀ isꢀ approximatelyꢀ
thermoneutralꢀ(seeꢀtheꢀSupportingꢀInformation).ꢀ
48)ꢀ Weꢀ haveꢀ examinedꢀ whetherꢀ evidenceꢀ forꢀ [Cu (SAr)
oxidationꢀofꢀ[Cu (SAr)
II
–
(
3
] ꢀ mightꢀ beꢀ obtained,ꢀ ifꢀ electrochemicalꢀ
2
]Naꢀ(1)ꢀisꢀconductedꢀinꢀtheꢀpresenceꢀofꢀNaSAr.ꢀHowever,ꢀtheꢀcyclicꢀvoltammogramꢀ
I
ofꢀ complexꢀ 1ꢀ isꢀ unchangedꢀ inꢀ theꢀ presenceꢀ ofꢀ 10ꢀ equivalentsꢀ ofꢀ NaSAr,ꢀ andꢀ itsꢀ oxidationꢀ remainsꢀ
irreversible.ꢀ Thisꢀ resultꢀ isꢀ consistentꢀ withꢀ ourꢀ expectations.ꢀ Asꢀ evidentꢀ inꢀ Figureꢀ 6,ꢀ theꢀ onsetꢀ forꢀ theꢀ
oxidationꢀfeatureꢀofꢀNaSArꢀisꢀcathodicallyꢀshiftedꢀrelativeꢀtoꢀthatꢀforꢀcomplexꢀ1.ꢀ
ꢀ
ꢀ
ꢀ
1
6

Page 17 of 17
Chemical Science
View Article Online
DOI: 10.1039/C5SC04709A
TOC graphic and synopsis:
In-cage
coupling
I
LnCu SAr
*+
Ar
Ph
ET
S
Ph
I
LnCuII SAr Ph
I
-
L Cu -I
n
photoreductant
A mechanistic study establishes the viability of a copper-mediated pathway for photoinduced C-S cross-
coupling involving single electron transfer and in-cage radical recombination.
ꢀ
2