[Cu(dap)2Cl] as an efficient visible-light-driven photoredox catalyst in carbon-carbon bond-forming reactions

Article abstract of DOI:10.1002/chem.201200967

Copper sees the light of day: [Cu(dap)₂Cl] proved to be an excellent photoredox catalyst for atom-transfer radical addition reactions, as well as for allylation reactions (see scheme), providing an attractive alternative to commonly used iridium- and ruthenium-based catalysts. Copyright

Full text of DOI:10.1002/chem.201200967

DOI: 10.1002/chem.201200967
[CuACHTUNGTRENNUNG(dap)2Cl] As an Efficient Visible-Light-Driven Photoredox Catalyst in
Carbon–Carbon Bond-Forming Reactions
Michael Pirtsch,[a] Suva Paria,[a] Taisuke Matsuno,[a, b] Hiroyuki Isobe,[b] and
Oliver Reiser*[a]
Visible-light driven photoredox catalysis is rapidly devel-
oping into a versatile tool for organic synthesis that is espe-
cially attractive as a sustainable technology.[1] Excellent re-
sults have recently been reported by using organic dyes or
redox cofactors, such as flavine,[2] however, the most widely
utilized visible-light photoredox catalysts are metal com-
plexes based on ruthenium or iridium.[3,4] Although advanta-
geous with respect to stability and activity, the rarity of
these metals is disadvantageous for possible large-scale ap-
plications. Therefore, we have focused our attention on
copper, which has been successful in a number of photoreac-
tions, generally carried out under UV-light irradiation.[5]
We were especially intrigued by the report of Sauvage
et al. that described the reductive dimerization of para-nitro-
benzylbromide under irradiation (l=350 nm) in the pres-
edge, unprecedented under photoredox visible-light-induced
catalysis.
The synthesis of ligand 3 has previously been reported by
addition of 4-lithioanisole to phenanthroline, followed by
rearomatization with manganese dioxide.[7] As an operation-
ally convenient alternative and following literature prece-
dents of related derivatives,[8] we synthesized 4 by Suzuki
coupling of 2,9-dichloro-1,10-phenanthroline (1) with 4-me-
thoxyboronic acid, opening up a facile route to derivatives
of the dap ligand (Scheme 1). Complexation of 3 with CuCl
ence of triethylamine as an electron donor and [CuACTHUNRGTNEUNG(dap)2Cl]
(4; dap=2,9-bis(para-anisyl)-1,10-phenanthroline (3)).[6]
Moreover, in the presence of oxygen, the oxidation of para-
nitrobenzylbromide to the corresponding aldehyde was ob-
served. [*Cu
(À1.43 V) than [*Ru
dine) or [*Ir(dF(CF3)ppy
(CF3)ppy=2-(2,4-difluorophenyl)-5-trifluoromethylpyridine,
ACHTUNGTRENNUNG a slightly stronger reductant
(dap)2]+ is
AHCTUNGTRENNUNG
A
R
ACHTUNGTRENNUNG
dtbbpy=4,4-di-tert-butyl-2,2-dipyridyl) with a lifetime of
270 ns, making it an attractive, but to the best of our knowl-
edge largely unexplored, alternative to commonly employed
ruthenium or iridium complexes. We report, herein, that
[CuACHTUNGTRENNUNG(dap)2Cl] is indeed an excellent photoredox catalyst
upon excitation at l=530 nm for atom-transfer radical addi-
tions (ATRAs) to olefins, as well as for allylations of a-halo-
ketones, the latter process being, to the best of our knowl-
Scheme 1. Synthesis of [Cu
ACHTUNGRTENN(UNG dap)2Cl] (4): i) 1 (1.0 g, 4.0 mmol), 2 (1.6 g,
10.5 mmol), [Pd(PPh3)4] (15 mol%), toluene/aqueous Na2CO3, 958C,
AHCTUNGTRENNUNG
[a] M. Pirtsch, S. Paria, T. Matsuno, Prof. Dr. O. Reiser
Institut fꢀr Organische Chemie
18 h; ii) CuCl (1 mmol), 3 (2 mmol), CHCl3, 15 min, 558C.
Universitꢁt Regensburg
Universitꢁtsstr. 31, 93053 Regensburg (Germany)
Fax : (+49)941-9434121
E-mail: Oliver.Reiser@chemie.uni-regensburg.de
gives rise to complex 4 in quantitative yield, which can be
isolated and stored for extended periods of time without
special precautions.
[b] T. Matsuno, Prof. Dr. H. Isobe
Department of Chemistry
Tohoku University
[CuACTHNUTRGNE(UGN dap)2Cl] shows a strong absorption band between 400
and 600 nm, making it suitable for irradiation with green
LEDs at 530 nm. Under these reaction conditions, we found
that ATRAs of organic halides to alkenes[9–12] readily occur
at room temperature in good yields with catalyst loadings of
0.3–1.0 mol% (Table 1). The reactions can also be run in a
Aoba-ku, Sendai 980-8578 (Japan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201200967. It contains full ex-
perimental details and spectroscopic characterization of all starting
materials and products.
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Chem. Eur. J. 2012, 18, 7336 – 7340

COMMUNICATION
Table 1. Visible-light-induced [CuACTHNUTRGNEUNG
(dap)2Cl]-catalyzed ATRA reactions.[a]
(Table 1, entries 1, 2, and 8–16) with the exception of em-
ploying a-methylstyrene (Table 1, entry 7), for which only
the product resulting from HBr elimination from the initial
ATRA product could be obtained. The reactions also
showed remarkable stereoselectivity when mixtures of dia-
stereomeric ATRA products could have been obtained
(Table 1, entries 8 and 11).
Entry
Alkene
Halide
Product
Yield
[%]
1
88
92
8
4
0
2[b]
3[c]
4[d]
5[e]
6[f]
The products obtained are consistent with a mechanism
CBr4
(Scheme 2) in which the excited [*Cu
fers an electron to the organohalide, generating a radical
(dap)2]+-catalyst trans-
ACHTUNGTRENNUNG
42
7
CBr4
CBr4
38
8
88[g]
9
CBr4
CBr4
55
78
10
11
12
CBr4
CBr4
82[g]
76
Scheme 2. Proposed mechanism of the [Cu
action.
ACHTUNGRTEN(NUNG dap)2Cl]-catalyzed ATRA re-
13[h,i,j]
14[h,i,j]
67
75
that adds under electronic and steric control to the alkene.
The resulting radical combines with the halide with concur-
rent electron transfer back to CuII, thus, regenerating the
catalyst. The alternative mechanism that has been suggested
for the CuCl-catalyzed ATRA reaction under UV irradia-
tion,[9] invoking the oxidative addition of the halide to CuI,
seems to be less likely with complex 4. As can be concluded
from the oxidation potentials[13] of the various species in-
volved, the overall process is thermodynamically favored for
bromides in each step. The oxidation of nonafluorobutylio-
dide seems to approach the limit of copper catalyst 4, but
this halide could nevertheless be employed in the ATRA re-
action with good results (Table 1, entry 16). Neither inacti-
15[h,i,j]
98
63
16
n-C4F9I
[a] Alkene (1 equiv), halide (1 equiv), [CuACTHNUTRGENUN(G dap)2Cl] (0.3 mol%) in di-
chloromethane (1.5 mL), irradiation at 530 nm (green LED) for 20 h
(Ts=tosyl; Boc=tert-butoxycarbonyl). [b] Irradiation by sunlight; [c] No
[Cu
dark. [f] No CuCl, dap (0.6 mol%). [g] A single diastereomer. [h] Reac-
tion time: 24 h. [i] [Cu(dap)2Cl] (1 mol%), halide (2 equiv). [j] LiBr
ACHTUNGTRENNUNG(dap)2Cl]. [d] CuCl, no dap ligand. [e] Reaction carried out in the
ACHTUNGTRENUNG
(2 equiv) was added, solvent DMF/water 1:4.
vation of [CuACHTNUTRGNEUNG(dap)2Cl], even in the presence of a large
normal flask in sunlight with equally good results (Table 1,
entry 2), whereas no conversion is observed when the reac-
tions are run in the dark (Table 1, entry 5). The copper-cata-
lyzed ATRA reaction between alkenes and CBr4 at room
temperature, employing remarkably low concentrations
excess (up to 3000 fold) of CBr4, nor formation of hexabro-
moethane was observed, making a mechanism involving
direct metalation of the ATRA reagents unlikely.
The products of the ATRA reaction with bromo malonate
have already proven to be valuable synthons, for example,
they can be converted into cyclopropanes by 1,3-elimina-
tion.[10] Furthermore, the products obtained from addition of
CBr4 appear to be valuable intermediates. We found that
compounds 5 can be readily transformed with nucleophiles,
such as water, alcohols or amines, into the corresponding
substitution products 6 with concurrent elimination of HBr
(Scheme 3). Further oxidation of 6b gives rise to b,b’-dibro-
mo-a,b-unsaturated ketone 8, representing a class of com-
pounds that have shown to be versatile precursors for the
synthesis of nitrogen heterocycles, such as pyrazoles.[14] Fur-
thermore, these compounds should have considerable poten-
(down to 0.002 mol%) of [CuII
ACTHNUTRGNE(UNG tpma)X][X] (tpma=tris(2-
pyridylmethyl)amine, X=Cl, Br), has been achieved previ-
ously, however, 5 mol% of the radical initiator 2,2’-azobis(4-
methoxy-2,4-dimethyl valeronitrile) (V-70) must be em-
ployed as an additive.[11]
Further control experiments showed that the reactions do
not proceed to a significant extent in the absence of [Cu-
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
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O. Reiser et al.
Table 2. Visible-light-induced [Cu
ocarbonyl compounds.[a]
ACHTUNGRTENN(UNG dap)2Cl]-catalyzed allylation of a-hal-
Entry Halide
Time
[h]
Product
Yield
[%]
1[b]
2[c]
3
24
3
380
30
0[d]
4
5
85
5
6
4.5
5
74
79
Scheme 3. Synthetic utility of the CBr4 adducts (DBU=1,8-diazabicyclo-
[5.4.0]undec-7-ene; Nu=nucleophile; Bn=benzyl; py=pyridine).
7
8
4
6
74
68
ACHTUNGTRENNUNG
tial in metal-catalyzed cross-coupling reactions, as well as
allyl acetate 7, being available by acylation of 6b in high
yields.
As a second process, we investigated catalysis by [Cu-
ACHTUNGTRENNUNG(dap)2Cl] for the visible-light-driven allylation of a-chloro-
and a-bromoketones. Allylations of organohalides by allyl-
tributyltin have proven to be a powerful synthetic method
that can be carried out by initiation with azobisisobutyroni-
trile (AIBN) at 808C or by Et3B at ambient temperature,[15]
as well as under photochemical conditions in the absence of
catalysts.[16] However, to the best of our knowledge, the utili-
ty of this transformation under visible-light-induced photo-
redox catalysis has not yet been demonstrated.[17]
9
10
11
7
89
78
80
6
5.5
12
13
12
15
72
75
Control experiments established the necessity for a photo-
redox catalyst for the allylation by using visible-light LED
irradiation. Applying LED irradiation alone at 455 nm (blue
LED) at room temperature to a mixture of a-bromoaceto-
phenone and allyltributyltin resulted in a sluggish reaction,
giving the allylation product in only 30% yield after 24 h.
No conversion was observed when irradiation was carried
out at 530 nm (green LED).
14
15
X=Br
X=Cl
7
6
70
63
In contrast, in the presence of [CuACTHNUTRGENUN(G dap)2Cl] (1 mol%),
the allylation of a-bromoacetophenone is achieved in 80%
yield within 3 h (Table 2). Likewise, a variety of a-bromo-
and a-chloroketones, as well as a,a-dibromo- or a,a-dichlor-
oketones, are singly or doubly allylated, the last two being
useful precursors for ring-closing metathesis reactions. In all
cases, the reactions proceeded in good yields, without the
need for an excess of the allylation reagent, which is in con-
trast to thermal and photochemical protocols reported in
the literature.[15,16] It is noteworthy that the chlorinated pre-
cursors, despite the more stable carbon–chlorine bond (rela-
tive to a carbon–bromine bond), often gave somewhat
better yields than the corresponding brominated starting
materials.
16
17
4.5
8
70
80
[a] Organohalide (0.25 mmol), allyltributyltin (0.25 mmol per halide
atom), [Cu(dap)2Cl] (1 mol%) in acetonitrile (1 mL), irradiation at
530 nm (green LED). [b] Irradiation at 455 nm (blue LED) in the ab-
sence of [Cu(dap)2Cl]. [c] Irradiation at 530 nm (green LED) in the ab-
sence of [Cu(dap)2Cl]. [d] No reaction.
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
coupling with tetrabromomethane, giving rise to 4,4,4-tribro-
mobut-1-ene in 60% yield (Scheme 4).
The ecologically more benign allyltrimethylsilane can also
be used in visible-light-induced [Cu
G
It again appears reasonable to assume that [CuACHTUNGTRENNUNG(dap)2Cl]
lations, however, our preliminary investigations into the uti-
lization of this reagent were, so far, only successful in the
acts as an electron shuttle between the organic halide and
the allylmetal reagent (Scheme 5) as opposed to a mecha-
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Chem. Eur. J. 2012, 18, 7336 – 7340

A Visible-Light-Driven Photoredox Catalyst
COMMUNICATION
flask was purged with nitrogen and acetonitrile (1.0 mL) was added. The
resulting mixture was degassed for 5 min by nitrogen sparging and placed
at a distance of ꢀ0.5–1.0 cm from a 530 nm green LED lamp. After com-
pletion of the reaction (as judged by TLC analysis), the mixture was con-
centrated in vacuo. The residue was purified by chromatography on silica
gel.
Scheme 4. Visible-light-induced [CuACHTNUTRGENNG(U dap)2Cl]-catalyzed allylation of tet-
rabromomethane with allyltrimethylsilane.
Acknowledgements
This work was supported by the DFG (GRK 1626 Photocatalysis).
Keywords: allylation
· atom-transfer radical addition ·
copper · photochemistry · redox chemistry
´
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Scheme 5. Proposed mechanism of the visible-light-induced [Cu
catalyzed allylation reaction.
ACHTUNGRTENN(UNG dap)2Cl]-
nism of direct electron transfer[18] between allylstannane and
the haloketones. This process should again be thermody-
namically favored in each reaction step as judged by the oxi-
dation potentials of the individual species.
In conclusion, we have demonstrated that [CuACTHNUGTRENUNG(dap)2Cl] is
an effective visible-light photocatalyst for ATRA and allyla-
tion reactions, providing an more economically viable alter-
native to the widely used ruthenium and iridium catalysts.
Experimental Section
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Typical procedures for the photoredox-cataylzed ATRA reaction:
Carbontetrabromide or nonafluoroiodobutane as the ATRA reagents: A
solution of the alkene (1.5 mmol), the ATRA reagent (1.5 mmol), and
[CuACHTUNGTRENNUNG(dap)2Cl] (0.3 mol%) in CH2Cl2 (1.5 mL) was degassed by using one
freeze/pump/thaw cycle, placed under a nitrogen atmosphere, and then ir-
radiated with a 530 nm green LED lamp placed at a distance of ꢀ0.5–
1.0 cm for 20 h. The reaction mixture was concentrated in vacuo and the
residue was purified on silica gel.
[8] Compare with: J. Frey, T. Kraus, V. Heitz, J.-P. Sauvage, Chem. Eur.
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Diethylbromomalonate or a-bromoacetophenone as the ATRA reagents:
A solution of the alkene (1.0 mmol), the ATRA reagent (2.0 mmol),
LiBr (2.0 mmol), and [CuACTHNUTRGNEUNG(dap)2Cl] (1 mol%) in a DMF/water mixture
(0.2:0.8 mL) was degassed by using one freeze/pump/thaw cycle, placed
under a nitrogen atmosphere, and then irradiated with a 530 nm green
LED lamp placed at a distance of ꢀ0.5–1.0 cm for 24 h. The reaction
mixture was concentrated in vacuo and the residue was purified on silica
gel.
[9] For ATRA by using CuCl/UV irradiation, see: M. Mitani, I. Kato,
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ACHTNUTRGNE(NUG tpma)Br][Br] (tpma=tris(2-pyridylmethyl)a-
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thoxy-2,4-dimethyl valeronitrile) (V-70), see: T. Pintauer, W.-T. Eck-
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oven-dried vial (10 mL) equipped with a plastic septum and magnetic
stirring bar was charged with [Cu
(0.25 mmol, 1.0 equiv), and allyltri-n-butyltin (0.25 mmol, 1.0 equiv). The
(dap)2Cl] (1 mol%), the halide
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Chem. Eur. J. 2012, 18, 7336 – 7340
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ACHUTGTNRENNUG CAHTUNGTRENNUGN
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C
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Received: March 21, 2012
Published online: May 13, 2012
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Chem. Eur. J. 2012, 18, 7336 – 7340