A General Copper Catalyst for Photoredox Transformations of Organic Halides

Article abstract of DOI:10.1021/acs.orglett.7b01518

A broadly applicable copper catalyst for photoredox transformations of organic halides is reported. Upon visible light irradiation in the presence of catalytic amounts of [(DPEphos)(bcp)Cu]PF₆ and an amine, a range of unactivated aryl and alkyl halides were shown to be smoothly activated through a rare Cu(I)/Cu(I)?/Cu(0) catalytic cycle. This complex efficiently catalyzes a series of radical processes, including reductions, cyclizations, and direct arylation of arenes.

Full text of DOI:10.1021/acs.orglett.7b01518

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Letter
pubs.acs.org/OrgLett
A General Copper Catalyst for Photoredox Transformations of
Organic Halides
Bastien Michelet,† Christopher Deldaele,† Sofia Kajouj,‡ Cecile Moucheron,‡ and Gwilherm Evano*,†
́
‡
†Laboratoire de Chimie Organique and Laboratoire de Chimie Organique et Photochimie, Service de Chimie et PhysicoChimie
́
Organiques, Universite libre de Bruxelles (ULB), Avenue F. D. Roosevelt 50, CP160/06, 1050 Brussels, Belgium
S
* Supporting Information
ABSTRACT: A broadly applicable copper catalyst for photoredox trans-
formations of organic halides is reported. Upon visible light irradiation in the
presence of catalytic amounts of [(DPEphos)(bcp)Cu]PF6 and an amine, a
range of unactivated aryl and alkyl halides were shown to be smoothly activated
through a rare Cu(I)/Cu(I)*/Cu(0) catalytic cycle. This complex efficiently
catalyzes a series of radical processes, including reductions, cyclizations, and
direct arylation of arenes.
he development of photoredox catalysis has recently
enabled the design of remarkably powerful synthetic
complexes,14 combining both one-electron-deficient phenan-
T
throline and one-electron-rich chelating diphosphine10a,15 or
bis(isonitrile)8c (Table 1, entries 6−12). Among all heteroleptic
complexes evaluated, [(DPEphos)(bcp)Cu]PF6, whose effi-
ciency as a photosensitizer for the photocatalytic reduction of
water15b and carbon dioxide15c had been previously demon-
strated, proved to be superior. Further optimization of the
tools, notably based on the generation of radical species under
mild, safe, and environmentally friendly conditions.1 Following
pioneering studies reported at the end of 20th century,
spectacular applications of photoredox catalysis were developed
in the past decade. This field has been largely dominated by
ruthenium and iridium complexes associated with π-deficient
ligands whose main features encompass strong absorption in the
visible region, high oxidation and reduction potentials, and long-
lived excited states (typically in the μs range).1
sacrificial reductant revealed the higher efficiency of Hunig’s
̈
base, which enabled a clean reduction of 1a in 93% yield with a
decreased catalyst loading of 5 mol % (Table 1, entry 14).
Control experiments demonstrated the strict requirement of
both catalyst and light for the reaction to occur (Table 1, entries
15 and 16), the heteroleptic complex being also responsible for
the catalysis and not the corresponding homoleptic complexes
that might result from a poorly controlled speciation in solution
(entry 12 vs entries 4 and 5). Finally, note that the reaction can
also be performed by using simple blue LEDs to promote the
reaction (Table 1, entry 17) with only a slight erosion of the yield,
and that oxygen or TEMPO was highly detrimental (50 and 0%
yields, respectively).
The main alternatives mostly rely on the use of organic dyes2
or photoactivatable gold complexes3 that were introduced more
recently. In sharp contrast, much less attention has been devoted
to copper complexes, despite their strong potential as cheaper
catalysts and for the activation of a broader range of substrates.
Despite the early work of Kutal,4 Mitani,5 and Sauvage6 and the
recent remarkable reports by Peters and Fu7 and Reiser,8 who
demonstrated their unique potential, copper complexes have
been scarcely used in photoredox transformations,9,10 which can
mostly be attributed to their short-lived excited states. Based on
our recent interest in copper-catalyzed radical processes,11 we
report here a broadly applicable copper catalyst for photoredox
transformations of organic halides.
We initiated our studies by identifying a copper complex that
could activate the carbon−halogen bond in a representative
unactivated organic halide possessing a highly negative reduction
potential and whose activation is therefore known to be
challenging.12,13 The model reduction of aryl iodide 1a was
evaluated in the presence of excess triethylamine as the sacrificial
reductant in acetonitrile at rt for 16 h under irradiation at 420 nm
and in the presence of various photoactivatable copper
complexes: results from these studies are collected in Table 1
(entries 1−12). After the little success met with a series of
homoleptic complexes of phenanthroline and diphosphine
derivatives (Table 1, entries 1−5), we next moved to heteroleptic
With these optimized conditions in hand, and to gain insights
into the substrates that could be activated under these
conditions, we next moved to the study of the scope and
limitation of the light-mediated, [(DPEphos)(bcp)Cu]PF6-
catalyzed reduction of a series of aryl halides possessing
representative substituents and substitution patterns (Figure
1).12 Reduction of various aryl iodides 1 was first evaluated and
found to proceed smoothly in all cases, providing the
corresponding hydrodeiodinated arenes 2a-h in good yields
regardless of the substitution pattern and electronic properties of
the starting aryl iodide. Notably, the reaction was found to be
compatible with a range of functional groups, including a
boronate 2e, and could also be extended to heteroaryl iodides 2h.
Received: May 23, 2017
© XXXX American Chemical Society
A
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Letter
Table 1. Optimization of the Copper-Catalyzed Photoredox
Reduction of Aryl Halides
a
yield
entry
catalyst CuL1L2X
R3N
Et3N
irradiation
(%)
1
2
3
4
5
6
7
8
[(phen)2Cu]Cl (10 mol %)
[(dmp)2Cu]Cl (10 mol %)
[(dap)2Cu]Cl (10 mol %)
420 nm
420 nm
420 nm
420 nm
420 nm
420 nm
420 nm
420 nm
−
−
−
−
−
−
−
58
Et3N
Et3N
[(DPEphos)2Cu]PF6 (10 mol %) Et3N
[(bcp)2Cu]PF6 (10 mol %) Et3N
[(binc)(dmp)Cu]PF6 (10 mol %) Et3N
[(binc)(dap)Cu]PF6 (10 mol %)
Et3N
Et3N
[(Xantphos)(dmp)Cu]PF6
(10 mol %)
9
[(Xantphos)(bcp)Cu]PF6
(10 mol %)
Et3N
Et3N
Et3N
Et3N
420 nm
420 nm
420 nm
420 nm
65
−
Figure 2. Copper-catalyzed photoredox cyclization of aryl iodides
10 [(DPEphos)(dap)Cu]PF6
(10 mol %)
(airradiation with blue LEDs; b 1H NMR yield).
11 [(DPEphos)(dmp)Cu]PF6
(10 mol %)
60
70
88
93
To further test the efficiency and usefulness of our procedure
for the activation of aryl halides, we further studied its use for the
formation of C−C bonds in intramolecular processes. The
reductive radical cyclization of N-allyl-o-iodoanilines (Figure 2, 3,
Y = NR′′) afforded the corresponding polysubstituted indolines
4a-h3,12a,b in fair to good yields if 10 mol % of [(DPEphos)-
(bcp)Cu]PF6 was used in combination with irradiation in a
photoreactor at 420 nm (50 W/m2), which proved to be slightly
superior to the use of blue LEDs. A range of substituents were
well-tolerated on the nitrogen atom, including alkyl and common
protecting groups (4a-c), on the aromatic ring (4d-g), and on the
alkene (4h), although the reaction was less efficient in the last
case. This cyclization could be extended to the preparation of a
series of dihydrobenzofurans 4i-l3,12a,c,d and indanes 4m-p
starting from the corresponding allylaryl ethers and homoallylar-
enes, respectively, with similar efficiency. Various substituents
were found to be compatible with the reaction conditions in all
cases, and the yields of the cyclized products compared well with
the ones obtained through other procedures, clearly demonstrat-
ing the broad applicability of [(DPEphos)(bcp)Cu]PF6 as a
catalyst for photoredox transformations of aryl halides.
12 [(DPEphos)(bcp)Cu]PF6
(10 mol %)
13 [(DPEphos)(bcp)Cu]PF6
(10 mol %)
iPr2NEt 420 nm
iPr2NEt 420 nm
14 [(DPEphos)(bcp)Cu]PF6
(5 mol %)
15 no catalyst
iPr2NEt 420 nm
iPr2NEt no light
−
−
16 [(DPEphos)(bcp)Cu]PF6
(5 mol %)
17 [(DPEphos)(bcp)Cu]PF6
(5 mol %)
iPr2NEt blue
85
LEDs
a
NMR yield with p-anisaldehyde as internal standard.
Having confirmed the applicability of [(DPEphos)(bcp)Cu]-
PF6 in photoredox processes with aryl halides, we finally
evaluated the reductive cleavage of alkyl halides. Alkyl iodides
and bromides, even embedded in a complex structure such as 5,
were smoothly reduced under our standard conditions, yielding
the corresponding derivatives 6 and 8 in 78 and 74% yields,
respectively (Scheme 1). The copper catalyst can also initiate
radical cyclizations from suitably functionalized alkyl halides such
as 9 and 11, yielding the corresponding carbo- and heterocycles
Figure 1. Scope of the copper-catalyzed photoredox reduction of aryl
halides. Yields determined by 1H NMR analysis of crude reaction
mixtures using p-anisaldehyde as internal standard (aon a 1.0 mmol
scale; busing 10 mol % of catalyst).
10 and 12 with reasonable efficiency if the amount of Hunig’s
̈
base was decreased to minimize the competing reduction.
Next, we focused on an especially challenging transformation,
the direct arylation of C(sp2)−H bonds in arenes. Whereas
various photoredox processes have been recently reported for
this reaction, which affords a remarkably direct entry to
biaryls,13,17 they mostly rely on the use of diazonium salts,18a−d
diaryliodonium triflates,18e or aryl sulfonyl chlorides.18f Although
more challenging due to their potential competitive reduction,
We next evaluated the reduction of aryl bromides, which is
known to be much more challenging due to the slow C−Br bond
cleavage kinetics.12a,16 The reaction was efficient with electron-
poor substrates (1d,i,j, X = Br), and the presence of electron-
donating substituents strongly hampered the reaction (1f,k, X =
Br). Aryl chlorides, even activated, were found to be poor
reaction partners.
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The scope of this copper-catalyzed photoredox direct arylation
was next explored starting with the arylation of N-methylpyrrole
with various representative aryl iodides 1. As demonstrated in
Figure 3, the reaction was found to be rather general, the
corresponding arylated pyrroles 15 being obtained in fair to good
yields. Interestingly, aryl iodides substituted with a bromide or a
boronate could be smoothly and selectively used as arylating
reagents, therefore providing a useful handle for subsequent
functionalization and diversification of corresponding products
15e and 15f. Heteroaryl iodides performed equally well, as
demonstrated with their use in arylation of N-methylpyrrole to
corresponding 2-pyridyl and 2-thiophenyl derivatives 15h and
15i. Other pyrroles also reacted smoothly, including Boc-
protected ones, and direct arylation could be extended to
electron-rich benzene derivatives 16. Finally, electron-poor aryl
bromides could be used for the direct arylation, as demonstrated
with the synthesis of 15p and 15q.
Scheme 1. Copper-Catalyzed Photoredox Transformations of
Alkyl Halides
Photophysical studies of [(DPEphos)(bcp)Cu]PF6, which
was shown to be perfectly stable in the dark and afford only
minor amounts of catalytically inactive homoleptic complexes
upon irradiation, revealed a luminescence lifetime of 819 ns,
which is in the high range for a heteroleptic copper complex, and
an oxidation potential in the excited state of −1.02 V vs SCE.20
This value is in accordance with recently reported ones15c and
clearly demonstrates the inability of the excited copper catalyst to
reduce an organic halide by a Cu(I)/Cu(I)*/Cu(II) oxidative
quenching cycle (Figure 4, bottom). A rare Cu(I)/Cu(I)*/
aryl halides are considerably more practical and attractive readily
available arylation reagents.12c,19 To check whether our copper-
based catalytic system could efficiently promote this reaction, we
selected pyrroles as substrates as they are known to have high
reaction rates in the addition of radical species.12c Initial studies
revealed that the competitive reduction of the aryl halide by
Hunig’s base was a serious side reaction which could be
̈
suppressed using dicyclohexylisobutylamine.19d Biphenyl iodide
1a was found to smoothly react with an excess of N-
methylpyrrole, with the excess of the arene being required in
most processes reported to date, in the presence of 10 mol % of
[(DPEphos)(bcp)Cu]PF6, 0.5 equiv of Cy2NiBu, and 2 equiv of
potassium carbonate in acetonitrile at rt for 3 days to afford the
corresponding C2-arylated pyrrole 15a in 66% yield (Figure 3).
Figure 4. Mechanistic proposal, stability, and photophysical properties
of [(DPEphos)(bcp)Cu]PF6.
Cu(0) catalytic cycle21 (Figure 4, top) might then account for the
ability of [(DPEphos)(bcp)Cu]PF6 to catalyze photoredox
transformations of organic halides. The reaction would be
initiated by reduction of [(DPEphos)(bcp)Cu]PF6* with
iPr2NEt, a transformation which is supported by the reduction
potential in the excited state for this complex of +0.63 V vs SCE20
and by the reported oxidation potential of iPr2NEt (+0.68 V vs
SCE),22a generating a transient copper(0) complex
[(DPEphos)(bcp)Cu] whose oxidation potential was measured
to be −1.64 V vs SCE.20 Whereas this value might seem rather
high for this complex to be able to reduce unactivated aryl iodides
(reduction potentials of −1.2 to −0.33 V vs SCE for alkyl
iodides22b and −1.91 V vs SCE for iodobenzene7), similar results
have been reported for iridium complexes such as fac-Ir(ppy)3,
whose excited state (with an oxidation potential of −1.73 V vs
SCE) was shown to be quenched by compounds with reduction
potentials lower than −2.00 V vs SCE.12a,22c Moreover,
formation of copper nanoparticles/mirrors in the presence of
unreactive organic halides and strong quenching of [(DPEphos)-
Figure 3. Copper-catalyzed photoredox direct arylation of (hetero)-
arenes with aryl halides (aduring 5 days).
C
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(bcp)Cu]PF6* by iPr2NEt (kq = 3.4 × 107 M−1 s−1)20 strongly
support this elementary step. Limited quenching of
[(DPEphos)(bcp)Cu]PF6* by iodobenzene (kq = 6.9 × 106
M−1 s−1)20 also supports this mechanism. Reduction of the
organic halide R−X by single electron transfer from
[(DPEphos)(bcp)Cu] regenerates the copper(I) catalyst and
initiates generation of a radical R•, and upon its further
cyclization in the presence of a suitably placed functional
group or its addition to a radical acceptor such as an arene,
yielding another radical species R′•. This radical is finally reduced
by the amine radical cation to afford the final product. In the case
of the direct arylation of C(sp2)−H bonds in arenes, a reductant
is not formally needed. The final rearomatization can indeed be
triggered by reduction of the resulting radical species by either
the amine radical cation or [(DPEphos)(bcp)Cu]PF6*. In the
last case, a substoichiometric amount of the amine is sufficient for
the reaction to occur, which accounts for the successful use of
only 0.5 equiv of Cy2NiBu, which, combined with its low
solubility,19d minimizes the competitive reduction of the starting
halide.
In conclusion, we have reported a general and broadly
applicable copper catalyst enabling photoredox transformations
of organic halides. Both aryl and alkyl halides were found to be
readily transformed to the corresponding radicals upon reaction
with catalytic amounts of [(DPEphos)(bcp)Cu]PF6 in the
presence of an amine. A rare Cu(I)/Cu(I)*/Cu(0) catalytic
cycle was shown to be involved with this photocatalyst, whose
excited state was also shown to be long-lived. Besides providing
an excellent alternative to iridium and ruthenium complexes, this
opens new perspectives in the use of copper complexes for
photoredox transformations.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.7b01518.
■
S
Photophysical studies of [(DPEphos)(bcp)Cu]PF6, ex-
perimental procedures, characterization, and copies of
NMR spectra (PDF)
(16) Costentin, C.; Robert, M.; Savea
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AUTHOR INFORMATION
2958. (b) Xue, D.; Jia, Z.-H.; Zhao, C.-J.; Zhang, Y.-Y.; Wang, C.; Xiao, J.
Chem. - Eur. J. 2014, 20, 2960. (c) Zoller, J.; Fabry, D. C.; Rueping, M.
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Maestri, G.; Malacria, M. Org. Chem. Front. 2015, 2, 464. (e) Liu, Y.-X.;
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Tetrahedron 2016, 72, 2521.
■
Corresponding Author
*E-mail: gevano@ulb.ac.be.
ORCID
Gwilherm Evano: 0000-0002-2939-4766
Notes
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L.; Ghosh, I.; Esteban, F.; Konig, B. ACS Catal. 2016, 6, 6780.
̈
The authors declare no competing financial interest.
(c) Ghosh, I.; Konig, B. Angew. Chem., Int. Ed. 2016, 55, 7676. (d) Arora,
̈
A.; Weaver, J. D. Org. Lett. 2016, 18, 3996.
ACKNOWLEDGMENTS
Our work was supported by the Universite
■
(20) See Supporting Information for details.
́
libre de Bruxelles
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(ULB), the Fed
2014-2019), and the COST action CM1202. C.D. and S.K.
acknowledge the Fonds pour la formation a la Recherche dans
́
er
́
ation Wallonie-Bruxelles (ARC Consolidator
̀
l’Industrie et dans l’Agriculture (F.R.I.A.) for graduate fellow-
ships.
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