Aryl radical formation by copper(I) photocatalyzed reduction of diaryliodonium salts: NMR Evidence for a CuII/CuI mechanism

Article abstract of DOI:10.1002/chem.201301449

Photocatalyzed reduction of diaryliodonium salts was achieved by using [Cu(dpp)₂][PF₆] as a photoactive complex and DIPEA as a reductive quench. The application of a copper catalyst allows the generation of aryl radicals under mild conditions and maintains their reactivity for C-C bond formation processes.

Full text of DOI:10.1002/chem.201301449

COMMUNICATION
Photocatalyzed reduction of diarylio-
donium salts was achieved by using
Diaryliodonium Salts
[CuACHTUNGTRENNUNG(dpp)2]ACHTUNGTRENNUNG[PF6] as photoactive com-
A. Baralle, L. Fensterbank,*
plex and DIPEA as reductive quench.
The application of a copper catalyst
allows the generation of aryl radicals
under mild conditions and maintains
J.-P. Goddard,* C. Ollivier* &&&& — &&&&
Aryl Radical Formation by Copper(I)
Photocatalyzed Reduction of Diarylio-
donium Salts: NMR Evidence for a
CuII/CuI Mechanism
À
their reactivity for C C bond forma-
tion processes.
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DOI: 10.1002/chem.201301449
Aryl Radical Formation by Copper(I) Photocatalyzed Reduction of
Diaryliodonium Salts: NMR Evidence for a CuII/CuI Mechanism
Alexandre Baralle, Louis Fensterbank,* Jean-Philippe Goddard,* and Cyril Ollivier*[a]
Hypervalent iodine chemistry is currently a field of in-
tense development.[1] Thanks to the variety of possible de-
rivatives, iodonium salts have proven to be useful and versa-
tile reagents in organic synthesis.[2] Whereas most investiga-
tions focus on electrophilic arylation and alkenylation, ho-
molytic reduction of iodonium salts has rarely been exam-
complexes, with cheaper and more sustainable photocata-
lysts than ruthenium(II) and iridiumACTHNUTRGNEUNG(III) complexes but
with comparable reductive potentials.[17] In late 1970s,
McMillin et al. reported a new family of photoactive cop-
per(I)–bisphenanthroline complexes.[18] Owing to their pho-
tophysical properties and electrochemical properties, [Cu-
ined.
Such
transformations
have
typically
been
(dap)2]+ (dap=2,9-bis(para-anisyl)-1,10-phenanthroline; 1a)
ACHTUNGTRENNUNG
accomplished either by electrochemical or photochemical
methods or single electron transfer (SET) reagents.[3] In par-
ticular, the ability of diaryl iodonium salts to generate aryl
radicals under UV irradiation in the presence of a photoca-
talyst or by electrochemical reduction has been exploited as
a tool to initiate polymerization and for surface functionali-
zation. However, their use in mainstream organic synthetic
processes would require milder conditions. The development
of new photocatalytic systems may provide a solution to this
challenge. Recently, in the context of green chemistry, visi-
ble-light photoredox catalysis relying on the use of transition
metal complexes for the generation of radicals has witnessed
a rebirth.[4,5] For that purpose, a variety of ruthenium(II)[6]
and [Cu
1b; Figure 1) emerged as promising candidates for photo-
(dpp)2]+ (dpp=2,9-diphenyl-1,10-phenanthroline;
ACHTUNGTRENNUNG
Figure 1. Structures of [CuCAHTUGNTENRNU(GN dap)2]ACHTUNGTERNNUG[PF6] and [CuACHTUNEGTRNN(GUN dpp)2]ACTHNUGTREN[NUNG PF6].
AHCTUNGTRENNUNG
and iridium
N
A
ACHTUNGTRENNUNG
to catalyze single-electron transfer to unsaturated or cyclo-
propyl ketones,[8] halides,[9] N-alkoxyphthalimides,[10] keto
epoxydes/azeridines,[11] azides,[12] diazonium,[13] and sulfoni-
um reagents under visible-light.[14] Moreover, the previous
work of Lalevꢁe et al.[15] on cationic photopolymerization of
epoxides, initiated by the system TTMSS/Ph2I+/RuII or IrIII,
1
E
vs. SCE).[19] The first application of such complexes was pub-
lished by Sauvage et al. and concerns the reductive coupling
of para-nitrobenzylbromide catalyzed by [CuACHTUNGRTNENUG(dap)2]ACHTUNGTRENNUNG[BF4]
under visible light in the presence of triethylamine.[20] More
recently, Reiser et al. successfully applied the same photoca-
talyst to atom-transfer radical addition to olefins and allyla-
tion of a-haloketones.[9w] In both cases, a CuII/CuI redox sys-
tems may be involved through an oxidative quench of the
suggests that iodonium salts (E =
ACHTUNGTRENNUNG
2
SCE) should be appealing aryl radical precursors under pho-
toreductive conditions. Recently, Sanford et al. and Xue and
Xiao et al. took advantage of this property for the C H ary-
*[Cu
reducing copper(I) complex [CuACHTNUGTRENNUNG
(dap)2]+ complex. However, the reactivity of the least
À
lation of various arenes and heteroarenes with diaryliodoni-
(dpp)2]+ under visible-light
um salts.[16]
irradiation was never exploited in synthesis. In this context,
we explored the photocatalytic reduction of diaryliodonium
salts with [CuACHTNUGRTENNUG(dpp)2]ACHTUTGNREN[NGUN PF6], which shows a suitable reduction
The objective of this work is the application of iodonium
salts as alternative aryl radical sources to rare-metal-based
potential, to generate aryl radicals. The so-formed radicals
were engaged in allylation processes with allylsulfones as
radical acceptors.
In a typical experiment, diphenyliodonium hexafluoro-
phosphate (2a) was irradiated in acetonitrile (0.2m) in the
[a] A. Baralle, Prof. Dr. L. Fensterbank, Dr. J.-P. Goddard,
Dr. C. Ollivier
Institut Parisien de Chimie Molꢁculaire (UMR CNRS 7201)
UPMC Univ Paris 06, Sorbonne Universitꢁs
4 place Jussieu, C. 229, 75005 Paris (France)
Fax : (+33)1-44-27-73-60
E-mail: louis.fensterbank@upmc.fr
jean-philippe.goddard@upmc.fr
presence of 0.5 mol% of [Cu
ACHUTGTNERN(NGU dpp)2]ACHTUTGNREN[NUGN PF6], N,N-diisopropyl-
AHCTUNGTREGeNNNU thylamine (DIPEA, 2 equiv) and allylsulfone 3a (5 equiv).
After 30 min, a full conversion of 2a was observed and the
corresponding allylation product 4a was formed in 80%
yield (Table 1, entry 1). Considering that this copper com-
plex absorbs between 400 and 600 nm,[19] we selected green
cyril.ollivier@upmc.fr
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201301449.
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Photocatalyzed Reduction of Diaryliodonium Salts
COMMUNICATION
Table 1. Generation of phenyl radical by photocatalytic reduction of di-
phenyliodonium salts with copper(I) complexes.
Table 2. Allylation of phenyl radical under photocatalytic reduction con-
ditions.
Entry
Acceptor
Product, yield [%][a]
4a, 82
Entry
Catalyst
Conv. [%]
Yield [%][a]
1
2
3
4
5
1
2
3
4
5
6
7
8
9
[Cu
[Cu
[Cu
[Cu
N
G
100
100
100
100
100
100
100
0
80
E
80[b]
62[c]
82[d,e]
81[f]
80[f]
80
4b, 43[b]
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
4c, 64[b]
Ru
Ir
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
4d, 66[b,c]
[Cu
[Cu
[Cu
–
N
ACHTUNGTRENNUNG
N
0[g]
4e, 55[d,e]
T
0
0
20
0[h]
10
11
0[g]
[a] Isolated yield with full conversion. [b] Full conversion was obtained in
1 h. [c] NMR yield. [d] Full conversion was obtained in 4 h. [e] 1 mol%
of catalyst was used.
–
20
[a] NMR yield. [b] [(Ph)2I]ACHTUNRTGNEN[UG TfO] 2b was used instead of [(Ph)2I]ACHTUNTGNER[NUNG PF6] 2a.
[c] 2 equivalents of 3a were used. [d] Isolated yield. [e] In acetonitrile.
[f] Fluorescent bulb was used as visible light source. [g] Without DIPEA.
[h] Without light.
20% of 4a (Table 1, entry 11) as PET product (95% conver-
sion was observed in 22 h to yield 4a in 79%).
With the optimized conditions in hand, we extended the
scope of the allylation to a range of allyl sulfones (Table 2).
When the internal position of the olefin moiety was substi-
tuted by a phenyl or a tolyl group (3b and 3c), the yields
decreased to 43% and 64%, respectively (Table 2, entries 2
and 3). In these cases, longer times (1 hour) are required for
a full conversion of the iodonium salts. The same behavior
was observed with a methyl substituent (3d). A yield of
LED, emitting at 530 nm, far from the UV range, as a suita-
ble light source to limit potential UV side reactions such as
photoinduced electron transfer (PET) reactions from
DIPEA and degradation of iodoniums. The screening of the
experimental conditions was carried out in CD3CN to follow
the conversion and quantify the yield of the transformation
1
with precision by H NMR (butadiene sulfone was the inter-
1
nal standard). Changing the counter anion to triflate has no
influence on the reaction. Under the same reaction condi-
tions, diphenyliodonium triflate (2b) gave similar results
(Table 1, entry 2). These optimized conditions also allowed
the reduction of the amount of acceptor to two equivalents
(Table 1, entry 3). Full conversion of iodonium salt was ob-
served in 30 min but the yield decreased slightly to 62%.
Then, the reaction was scaled to one millimole in order to il-
lustrate the synthetic potential of this transformation
(Table 1, entry 4). A reproducible isolated yield of 82% was
obtained.
66% in 4d was determined by H NMR due to the volatility
of the resulting allylation product (Table 2, entry 4). A
chlorine atom at the internal position impacted the course
of the reaction and full conversion was reached in 4 h with
1 mol% of the copper catalyst (Table 1, entry 5). The corre-
sponding allylation product, 4e, was isolated in 55% yield.
In our conditions, allyl sulfone 3a proved to be the best ac-
ceptor and we decided to use it to test the scope of diaryl-
AHCTUNGTRENNUNG
generation of the corresponding substituted aryl radical was
also investigated (Table 3). A range of commercially availa-
ble diaryliodonium salts was selected. Alkyl substitution did
not modify the reaction time to full conversion. Indeed,
after 30 min of irradiation, iodonium salts 2c and 2e furnish-
ed allylated adducts 5 and 6 in 71% and 67% yield, respec-
tively (Table 3, entries 1 and 3). The influence of the coun-
ter-ion on the reduction of diaryliodonium salts was negligi-
ble as illustrated by the formation of 5 in 67% yield from
the triflate salt (Table 3, entry 2). When the aromatic ring
was para-substituted by a bromine atom, an extended reac-
tion time of 16 h was needed to obtain full conversion
(Table 3, entry 4). The corresponding allylated compound 7
was obtained in 63% yield. Interestingly, changing the bro-
mine to fluorine atom allowed to obtain 8 in 64% yield
within a shorter reaction time of 1.5 h (Table 3, entry 5).
Dissymmetric iodonium salts have been investigated to
test the influence of steric and electronic effects on the for-
Comparison with known photocatalysts showed that
standard [RuACHTUNGTRENNUNG(bpy)3]Cl2 and [IrACHTUNGTERN(NUGN ppy)3] catalyzed this transfor-
mation with fluorescent light irradiation (Table 1, entries 5
and 6). No significant improvement was observed, demon-
strating that [Cu
known photocatalysts. We also investigated the behavior of
[Cu
(dap)2]+ in our photocatalyzed reductive process
ACHTUNGTRENNUNG
(dpp)2]+ remains a valuable alternative to
ACHTUNGTRENNUNG
(Table 1, entry 7). This complex was also effective and 4a
was obtained in 80% yield. This comparison highlights the
potential of such complexes either in reductive or oxidative
transformations. When [CuACHTNUGRTNEUNG
(dpp)2]+ was used without
DIPEA, no 4a was observed (Table 1, entry 8). The control
experiments confirmed that the copper complex, DIPEA
and irradiation with visible-light are needed for good con-
versions (Table 1, entries 9 and 10). Without light, copper
salt and DIPEA proved to be inactive in this reduction
(Table 1, entry 9) and irradiation with DIPEA gave only
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L. Fensterbank, J.-P. Goddard, C. Ollivier and A. Baralle
Table 3. Scope of diaryliodonium salts as aryl radical sources.
Entry
1
Diaryliodonium salt
Product, yield [%][a]
5, 71[b]
2
3
4
5
5, 67[b]
6, 67[b]
7, 63[c]
8, 64[d]
[a] Isolated yield with full conversion. [b] Full conversion was obtained in
30 min. [c] Full conversion was obtained in 16 h. [d] Full conversion was
obtained in 1.5 h.
mation of aryl radicals (Scheme 1). Iodonium salts 2h–j
were applied to the allylation reaction with allylsulfone 3a
as the radical acceptor. Full conversions were obtained in
30 min and NMR analysis of the crude reaction mixture al-
lowed the quantification of both the aryl iodides and the al-
lylation products 4a, 4h–j. With iodonium salt 2h, no steric
Figure 2. 1H NMR investigations of iodonium photocatalytic reduction
(scale: 10–5.5 ppm, full scale spectra are in the Supporting Information).
tion to form iodobenzene and phenyl radical. The simultane-
ous oxidation of the copper(I) complex occurred and no
more characteristic signals of the initial complex were visi-
ble. Only broad peaks appeared that can be attributed to
the corresponding CuII species were observed and compared
with an authentic sample of [CuACHTUNRGTNENG(U dpp)2]ACHTUNTGRENN[GUN PF6]2 (Figure 2D).
Scheme 1. Behavior of dissymmetric iodonium salts. Conversions and
ratios were determined by 1H NMR spectroscopy (crude NMR spectra
are provided in the Supporting Information).
Two equivalents of DIPEA were then added and the irradia-
tion was maintained for 30 min (Figure 2C). The total con-
version of the iodonium salt was observed concomitantly
with an increase in the amount of iodobenzene and benzene.
discrimination was observed in the fragmentation step and
the aryl radical addition to acceptor 3a. In the same fashion,
no selectivity was obtained in the reduction of iodonium
salts bearing either an electron-poor (2i) or an electron-rich
(2j) aryl ring. The lack of selectivity in these cases deterred
us from exploring further the reduction of dissymmetric aryl
iodoniums.
Moreover, the signals of [CuACTHGNUTERNNUG
(dpp)2]+ were recovered with-
out significant modification. In order to support the reduc-
tion of CuII generated during the reduction of iodonium salt,
an authentic sample of [CuACTHNUTRGENNUG(dpp)2]ACHTUNTGREN[NUNG PF6]2 (Figure 2D) was
treated with DIPEA without irradiation. The initial dark
blue solution immediately turned to dark red suggesting the
1
reduction of CuII to CuI. The H NMR spectrum of the re-
To gain further insight into the catalytic cycle, we decided
to monitor the reduction of diphenyliodonium salt by
sulting solution confirmed the formation of [Cu
(Figure 2E).
ACHTUNGTRENNUNG
(dpp)2]+
1H NMR (Figure 2). [Cu
(dpp)2]
E
Based on these results, we can propose the following
mechanism (Scheme 2). Upon light irradiation, the excited
state of [Cu
um salt to generate [CuACTHNUGTRNENUG
(dpp)2]+ is oxidatively quenched by the iodoni-
ACHTUNGTRENNUNG
(dpp)2]2+ and the intermediate io-
danyl radical. Its fragmentation generates the aryl iodide
and aryl radical concomitantly. This suggestion is strongly
supported by the fact that aryl iodide by-products were iso-
lated from the reaction mixture in 53–100% yields. Con-
versely, the so-formed aryl radical reacts with the allyl sul-
LED (Figure 2B). The characteristic signals of [CuACTHNUTRGNEUNG
(dpp)2]+
disappeared and signals of PhI and benzene appeared,
which indicates the reduction of the iodonium salt into the
corresponding iodanyl radical and its subsequent fragmenta-
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Photocatalyzed Reduction of Diaryliodonium Salts
COMMUNICATION
residue was purified by flash chromatography on silica gel to afford the
corresponding product 5–8.
Acknowledgements
We thank UPMC, CNRS, IUF (L.F.), ANR (grant “Credox”) and Emer-
gence UPMC 2010. Technical assistance was generously offered by FR
2769.
Keywords: allylation · copper · iodonium · photocatalysis ·
radical reactions
Scheme 2. Proposed mechanism.
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2012, 66, 425.
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not an alternative source of aryl radical in such experimen-
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after reduction of [Cu
G
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AHCTUNGTRENNUNG
In summary, we report the photocatalytic reduction of di-
aryliodonium salts as an efficient way to generate the corre-
sponding aryl radicals. This mild process relies on a copper
complex as photocatalyst. The experimental conditions de-
veloped are compatible with the radical intermediates in-
voked. Carbon–carbon bond formation appears to be effi-
cient as illustrated by the formation of allylated products in
good yields which suggest the radical character of this trans-
formation. General studies extending to other radical pre-
cursors and the use of this copper complex as alternative
photocatalyst to polypyridine Ru/Ir complexes will be re-
ported in due course.
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Experimental Section
Photoreductive transformation of diphenyliodonium hexafluorophos-
phate 2a (General Procedure 1): A solution of diphenyliodonium hexa-
fluorophosphate (1 mmol, 428 mg, 1 equiv), [CuACTHNUGTRENNUG(dpp)2]ACHTUNTGREN[NGUN PF6] (0.005 mmol,
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4 mg, 0.5 mol%, unless stated otherwise), N,N-diisopropylethylamine
(2.0 mmol, 350 mL, 2 equiv) and radical acceptor (5 mmol, 5 equiv) in dis-
tilled CH3CN (5 mL) were introduced into a schlenk tube equipped with
a magnetic stirring bar. The mixture was degassed by freeze–pump–thaw
cycles (ꢂ2) then was irradiated with visible light (530 nm LED approxi-
mately 2 cm away from the glassware) for 30 min (unless stated other-
wise). The reaction mixture was then concentrated under reduced pres-
sure to give the crude product. The residue was purified by flash chroma-
tography on silica gel to afford the desired product 4a–4e.
[7] L. Flamigni, A. Barbieri, C. Sabatini, B. Ventura, F. Barigelletti,
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A
solution of iodonium salt (1 mmol, 1 equiv), [CuACHTUNGTERNNU(G dpp)2]ACHTUNGRTEN[NUNG PF6]
(0.005 mmol, 4 mg, 0.5 mol%, unless stated otherwise), N,N-diisopropyle-
thylamine (2.0 mmol, 350 mL, 2 equiv) and ethyl p-toluenesulfonylmetha-
crylate 3a (5 mmol, 1,340 g, 5 equiv) in distilled CH3CN (5 mL) were in-
troduced into a schlenk tube equipped with a magnetic stiring bar. The
mixture was degassed by freeze-pump-thaw cycles (ꢂ2) then irradiated
with visible light (530 nm LED approximately 2 cm away from the glass-
ware) during 30 min (unless stated otherwise). The reaction mixture was
then concentrated under reduced pressure to give the crude product. The
Chem. Eur. J. 2013, 00, 0 – 0
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(2 equiv) was irradiated with a green LED (530 nm) for 1 h, no reac-
tion was observed.
Received: April 16, 2013
Published online: && &&, 0000
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