Visible-light-mediated decarboxylation/oxidative amidation of α-keto acids with amines under mild reaction conditions using O2

Article abstract of DOI:10.1002/anie.201308614

Photochemistry has ushered in a new era in the development of chemistry, and photoredox catalysis has become a hot topic, especially over the last five years, with the combination of visible-light photoredox catalysis and radical reactions. A novel, simple, and efficient radical oxidative decarboxylative coupling with the assistant of the photocatalyst [Ru(phen)₃]Cl ₂ is described. Various functional groups are well-tolerated in this reaction and thus provides a new approach to developing advanced methods for aerobic oxidative decarboxylation. The preliminary mechanistic studies revealed that: 1)an SET process between [Ru(phen)₃]²⁺* and aniline play an important role; 2)O₂ activation might be the rate-determining step; and 3)the decarboxylation step is an irreversible and fast process. Bring to light: A new approach to oxidative decarboxylation under visible light using O₂ as the oxidant has been developed. A variety of functional groups were well-tolerated in this reaction and insights into the mechanism were investigated with the assistance of EPR spectroscopy, cyclic voltammetry, and theoretical studies.

Full text of DOI:10.1002/anie.201308614

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DOI: 10.1002/anie.201308614
Photochemistry
Visible-Light-Mediated Decarboxylation/Oxidative Amidation of a-
Keto Acids with Amines under Mild Reaction Conditions Using O2**
Jie Liu, Qiang Liu, Hong Yi, Chu Qin, Ruopeng Bai, Xiaotian Qi, Yu Lan,* and Aiwen Lei*
Abstract: Photochemistry has ushered in a new era in the
development of chemistry, and photoredox catalysis has
become a hot topic, especially over the last five years, with
the combination of visible-light photoredox catalysis and
radical reactions. A novel, simple, and efficient radical
oxidative decarboxylative coupling with the assistant of the
photocatalyst [Ru(phen)3]Cl2 is described. Various functional
groups are well-tolerated in this reaction and thus provides
a new approach to developing advanced methods for aerobic
oxidative decarboxylation. The preliminary mechanistic stud-
ies revealed that: 1) an SET process between [Ru(phen)3]2+*
and aniline play an important role; 2) O2 activation might be
the rate-determining step; and 3) the decarboxylation step is an
irreversible and fast process.
years,[4] decarboxylation of a-keto acids as acyl surrogates has
received less attention.[5] Gooßen et al. firstly demonstrated
a palladium-catalyzed decarboxylative acylation of aryl
bromides with a-keto carboxylate salts as acyl anion equiv-
alents to afford diaryl ketones at 1708C.[5b] In addition,
directing-group-assisted ortho-selective decarboxylative acy-
À
lation of aromatic C H bonds with a-keto acids has also been
achieved using persulfates as oxidants.[5e,f,h,i] In spite of the
significant progress offered by these reactions, there are still
certain limitations including elevated temperature, super-
fluous usage of strong oxidants, and high catalyst loading.
Thus, it is necessary to improve upon the harsh reaction
conditions and develop a powerful synthetic method for
oxidative decarboxylation of a-keto acids.
Recently, by taking advantage of visible light with the
assistance of photocatalysts, a variety of efficient organic
synthetic reactions have been realized under mild reaction
conditions through a single-electron-transfer process.[6] More-
over, one impressive feature of visible-light photocatalysis is
that O2 can act as a terminal oxidant for catalyst reoxidation
and afford an active superoxide radical anion.[7] Therefore, we
envision that it is possible to accomplish visible-light-induced
oxidative decarboxylation in the presence of O2 as the oxidant
(Scheme 1). The superoxide radical anion generated from the
D
ecarboxylation of pyruvate, the simplest a-keto acid,
popularly prevails in nearly all organisms.[1] At the first stage
of cellular respiration in aerobic organisms, pyruvate reacts
with coenzyme A (a thiol) to produce acetyl-CoA catalyzed
by the pyruvate dehydrogenase complex.[1,2] This mild and
efficient decarboxylative process is not only an important link
between the metabolic pathways of glycolysis and the citric
acid cycle, but also crucial for vital processes occurring in
living organisms.[3] Inspired by natureꢀs efficiency, it is
desirable to develop a mild and efficient method to construct
functional acyl compounds by decarboxylation of the a-keto
acids. Though transition-metal-catalyzed decarboxylative
coupling of carboxylic acids has become an important
À
À
protocol to form C C and C heteroatom bonds in recent
[*] J. Liu,[+] Dr. Q. Liu,[+] H. Yi, C. Qin, R. Bai, X. Qi, Prof. Y. Lan,
Prof. A. Lei
Scheme 1. Visible-light-mediated aerobic oxidative decarboxylation.
PC=photocatalyst.
College of Chemistry and Molecular Sciences
Wuhan University, 430072, Wuhan (P. R. China)
E-mail: aiwenlei@whu.edu.cn
photoredox process can promote the decarboxylation process
with subsequent nucleophilic attack to generate the final
product. In this work, we choose amines as the nucleophiles,
thus generating the corresponding amides. To the best of our
knowledge, this is the first photocatalyzed aerobic oxidative
decarboxylation of a-keto acids mediated by visible light, and
it proceeds under very mild reaction conditions like the
above-mentioned biological decarboxylation of pyruvate.
Initially, we started our evaluation of the reaction
parameters employing benzoylformic acid (1a) and 4-meth-
ylaniline (2a) as model substrates. The combination of
1 mol% of [Ru(phen)3]Cl2 and 1.5 equivalents of 4-methyl-
aniline, exposed to a 25 W household fluorescent lamp, in
DMSO under the atmosphere of O2 (balloon) for 36 hours
gave the best yield of up to 85%. Other data illustrating the
Prof. A. Lei
National Research Center for Carbohydrate Synthesis
Jiangxi Normal University
Nanchang 330022, Jiangxi, (P. R. China)
Prof. Y. Lan
School of Chemistry and Chemical Engineering
Chongqing University, 400030, Chongqing (P. R. China)
[+] These authors contributed equally to this work.
[**] This work was supported by the 973 Program (2012CB725302), the
National Natural Science Foundation of China (21025206,
21272180, and 21302148), the Program for Changjiang Scholars
and Innovative Research Team in University (IRT1030), and the
Research Fund for the Doctoral Program of Higher Education of
China (20120141130002).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201308614.
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Chemie
Table 1: Control experiments.[a]
addition, a-keto acids with a naphthyl group (3g) also
participated in this decarboxylative process with a high
reactivity. Heterocyclic a-keto acid was found to be favored
under this catalytic system to afford the corresponding
product (3h) in moderate yield. It is noteworthy that an
aliphatic a-keto acid was compatible with the reaction as well
and gave moderate yield of the desired product (3i).
Entry
Visible light
Photocatalyst
O2
Yield [%][b]
1
+
+
À
+
+
À
+
+
+
+
+
À
85
9
trace
n.d.
Furthermore, the reactivity of different amines was also
investigated (Table 3). Anilines bearing electron-neutral and
2[c]
3[d]
4[e]
Table 3: Scope of the substituted amines 2 for visible-light-mediated
decarboxylation with 1a.[a]
[a] Reaction conditions: 1a (0.25 mmol), 2a (0.375 mmol), photocata-
lyst (0.0025 mmol), DMSO (1 mL), irradiation with 25 W household light
bulb, 328C, 36 h. [b] Yields determined by GC. [c] Reaction was carried
out in the absence of catalyst. [d] Reaction was carried out in the dark.
[e] Reaction was carried out under N2. DMSO=dimethylsulfoxide;
phen=1,10-phenathorline; n.d.=not detected.
impact of different parameters on the efficiency of this
reaction is shown in the Supporting Information (Table S2).
Furthermore, a series of control experiments were examined.
The reaction yield was only 9% without the assistance of
a photocatalyst (Table 1, entry 2). Trace amounts of product
were obtained in the absence of either visible light or oxygen
(Table 1, entries 3 and 4), thus demonstrating that this
decarboxylative process is promoted by both photocatalyst
and oxygen, and mediated by visible light.
With the optimal reaction conditions in hand, we exam-
ined the scope of this reaction by testing the decarboxylative
coupling of a variety of substituted a-keto acids with 2a
(Table 2). Our catalytic system was successfully amenable to
a wide range of a-keto acids, and good to excellent yields were
achieved with substrates bearing both electron-deficient (3 f)
and electron-rich (3b and 3c) substituents. This transforma-
tion also showed satisfactory tolerance of halogen groups (3d
and 3e), which provide useful handles for further trans-
formations through traditional cross-coupling reactions. In
[a] Reaction conditions: 1a (0.5 mmol), 2 (0.75 mmol), photocatalyst
(0.005 mmol), DMSO (2 mL), irradiation with 25 W household light
bulb, 328C, 36 h. Yields of isolated products. [b] 1.0 mmol 2 was added.
[c] 2.5 mmol 2 was added. [d] 5 mmol 2 was added.
election-rich groups at the para and ortho positions under-
went this decarboxylation process smoothly to afford the
desired products (3j–o) in moderate to good yields. In
addition, this reaction was applicable to aliphatic amines
and gave good yields in the presence of 5–10 equivalents
amines (3p and 3q). Propargylamine was compatible in this
transformation, albeit in lower yield (3r).
Table 2: Scope of the substituted a-keto acids 1 for visible-light-
mediated decarboxylation with 2a.[a]
This method can be further applied to construct hetero-
cyclic compounds such as benzimidazole, benzoxazole, and
benzothiazole when the ortho positions of the anilines were
bore NH2, OH, and SH groups, respectively (Table 4). It is
known that these heterocyclic skeletons can be found in
numerous pharmaceutical agents with a diverse spectrum of
biological properties.[8] This method provides a novel and
straightforward procedure for the synthesis of these nitrogen
heterocycles under very mild reaction conditions.
We proposed two pathways for the mechanism (Scheme 2,
Path A and B): irradiation of I with visible light leads to the
excited compound II by metal-to-ligand charge transfer
(MLCT),[9] and is initially quenched by an amine to form III
by a reductive quenching mechanism (Path A).[10] Then O2
undergoes a single-electron-transfer (SET) process to render
the superoxide radical anion and regenerate I. The superoxide
radical anion then abstracts an electron from 1 to generate the
intermediates 6 and 7. Then 7 undergoes decarboxylation to
[a] Reaction conditions: 1 (0.5 mmol), 2a (0.75 mmol), photocatalyst
(0.005 mmol), DMSO (2 mL), irradiation with 25 W household light
bulb, 328C, 36 h. Yields of isolated products.
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Table 4: Scope of the ortho-substituted aniline 2 for visible-light-
mediated decarboxylation with 1.[a]
Figure 1. EPR spectra (X band, 9.4 GHz, 160 K).
[a] Reaction conditions: 1 (0.5 mmol), 2 (0.75 mmol), photocatalyst
(0.005 mmol), DMSO (2 mL), irradiation with 25 W household light
bulb, 328C, 36 h. Yields of isolated products.
[Ru(phen)3]+ and an organic radical. When 1a was added, this
organic radical may be consumed and only [Ru(phen)3]+
remained. Therefore, from the EPR experiments, Path A
(Scheme 2) is the more favored and serves as a feasible
reaction pathway for aryl amines.[15] In addition, [Ru-
(phen)3]+, the stable species detected by EPR, is the resting
state of photocatalyst in this transformation, thus indicating
that O2 activation by [Ru(phen)3]+ was possibly involved in
the rate-determining step.
Moreover, to investigate the decarboxylation of a-keto
acids, cyclic voltammetry (CV) experiments were carried out.
As shown in the Figure 2A, in the presence of PhCOCOOK
in DMSO, an obvious oxidative peak was detected. According
to the Figure 2C, this oxidative peak can be attributed to the
oxidation of PhCOCOOK. However, the corresponding
reductive peak was not observed (Figure 2A) even when
the scan rate was increased (Figure 2B). This irreversible
process may result from instability of the oxidation product of
PhCOCOOK. So from the results of the CV experiments, we
may confirm that PhCOCOOÀ can experience an oxidative
process to form PhCOCOOC, with subsequent fast and
irreversible decarboxylation with the generation of the
PhCOC intermediate.[12b] (for detailed CV experiments, see
Figure S7 in the Supporting Information).
Scheme 2. Proposed mechanism.
generate the acyl radical 8, which may subsequently react with
an amine to deliver the amide radical anion 9[11] in the
presence of 6, as the base, and then undergo SET to give the
desired product amide. Alternatively, in Path B, II can form
IV in the presence of O2 by an oxidative quenching
process[7b,12] and subsequently IV abstracts an electron from
1 to give 7 and regenerate I to finish the catalytic cycle. Then
intermediate 7 follows the same process as described for
Path A.
To confirm the possible pathway of this transformation,
electron paramagnetic resonance (EPR) was applied to study
the photocatalytic reaction (Figure 1).[7h,k,13] No EPR signal
was observed when 2a alone was tested under irradiation of
visible light (Figure 1A), similar to the mixture of 2a and
[Ru(phen)3]Cl2 in the absence of visible light (Figure 1B).
However, some strong signals were detected upon irradiation
of the mixture of 2a and [Ru(phen)3]Cl2 (Figure 1C). When
1a was added to the mixture of 2a and [Ru(phen)3]Cl2 under
irradiation by visible light (Figure 1D), the EPR spectrum
displayed resonance character similar to that of a [Ru(bpy)3]+
species.[14] On the basis of these results, we speculate that:
1) electron transfer may occur directly between the [Ru-
(phen)3]2+* and amines according to Figure 1C; 2) the EPR
signals in the Figure 1C may consist of two components:
To further confirm the existence of the radical intermedi-
ate 8,
a radical-trapping experiment was carried out
(Scheme 3). The addition of TEMPO profoundly suppressed
the reaction and afforded the addition product, 6a, of the
Scheme 3. Radical-trapping experiment. TEMPO=2,2,6,6-tetramethyl-
1-piperidinyloxy.
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Figure 3. Free-energy profile for the formation of F. The values are
calculated (M06-2X) free energies in DMSO solvent.
Figure 2. Cyclic voltammetry of PhCOCOOK in DMSO, NBu4BF4
(0.1m) under nitrogen. A) At a scan rate of v=0.5 VsÀ1 at a steady
glassy carbon disk electrode. B) At a scan rate of v=5.0 VsÀ1 at
a steady glassy carbon disk electrode. C) Different concentration of
PhCOCOOK at a scan rate of v=0.5 VsÀ1 at a rotating electrode.
TEMPO and acyl radical, thus suggesting that 8 was a possible
intermediate in this transformation.[16]
Furthermore, the density functional theory M06-2X
method with a standard 6-311 + G(d) basis set was employed
to investigate the reaction mechanism for the addition of B to
A and the formation of the complex F. As depicted in
Figure 3, A, B, and O2HÀ is set to zero in the free-energy
profile, and two competitive pathways for the formation of F
are computed. In Path-I, the nucleophilic addition of B to A
takes place via the transition-state C-TS with only a 9.1 kcal
molÀ1 barrier. The natural population analysis of D indicates
that D has a dipole character, in which positive and negative
charge is located on the nitrogen and oxygen atoms,
respectively (Figure 4).[17] Subsequently, D combines with
O2HÀ through intermolecular hydrogen bonds, thus forming
the intermediate E exothermically (4.0 kcalmolÀ1). Release
of H2O2 gives the amide radical anion F as the product and is
exothermic by 10.5 kcalmolÀ1, thus indicating that Path-I is
thermodynamically favorable. Meanwhile, the addition of an
amide anion to an acyl radical has also been considered in
Path-II. However, the calculated result shows that the amide
anion is unstable because the formation of the amide anions
G and H is endothermic by 27.1 kcalmolÀ1 and 18.9 kcal
molÀ1, respectively. Therefore, the overall barrier of amide
anion addition towards an acyl radical via the transition state
Figure 4. The electrostatic potential of D.
I-TS is as high as 37.3 kcalmolÀ1, which is 28.2 kcalmolÀ1
higher than that of Path-I. The theoretical study indicates that
F can be formed by the nucleophilic attack of B with the
radical A. The formation of F is exothermic by 14.5 kcal
molÀ1, with a barrier of only 9.1 kcalmolÀ1.
In conclusion, we have disclosed the first example of
visible-light-mediated decarboxylation/oxidative amidation
of a-keto acids with amines under mild reaction conditions
using O2 as the terminal oxidant. This transformation
provides
a new approach to developing an advanced
method of oxidative decarboxylation. The preliminary mech-
anism studies were investigated step by step and revealed
that: 1) an SET process between [Ru(phen)3]2+* and aniline
played an important role; 2) O2 activation might be the rate-
determining step; 3) decarboxylation was an irreversible and
fast process. The application of this powerful strategy to other
oxidative decarboxylation reactions with various nucleophiles
and further mechanistic investigations are underway in our
laboratory.
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[7] For selected works on O2 as the oxidant in photoredox catalysis,
see: a) A. G. Condie, J. C. Gonzꢅlez-Gꢆmez, C. R. J. Stephen-
son, J. Am. Chem. Soc. 2010, 132, 1464; b) Y. Cheng, J. Yang, Y.
Qu, P. Li, Org. Lett. 2011, 13, 98; c) D. P. Hari, B. Kçnig, Org.
Lett. 2011, 13, 3852; d) S. Lin, M. A. Ischay, C. G. Fry, T. P. Yoon,
J. Am. Chem. Soc. 2011, 133, 19350; e) Y. Pan, C. W. Kee, L.
Chen, C.-H. Tan, Green Chem. 2011, 13, 2682; f) Y. Pan, S. Wang,
C. W. Kee, E. Dubuisson, Y. Yang, K. P. Loh, C.-H. Tan, Green
Chem. 2011, 13, 3341; g) M. Rueping, C. Vila, R. M. Koenigs, K.
Poscharny, D. C. Fabry, Chem. Commun. 2011, 47, 2360; h) Y. Su,
L. Zhang, N. Jiao, Org. Lett. 2011, 13, 2168; i) S. Cai, X. Zhao, X.
Wang, Q. Liu, Z. Li, D. Z. Wang, Angew. Chem. 2012, 124, 8174;
Angew. Chem. Int. Ed. 2012, 51, 8050; j) W. Fu, W. Guo, G. Zou,
C. Xu, J. Fluorine Chem. 2012, 140, 88; k) Q. Liu, Y.-N. Li, H.-H.
Zhang, B. Chen, C.-H. Tung, L.-Z. Wu, Chem. Eur. J. 2012, 18,
620; l) S. Maity, N. Zheng, Angew. Chem. 2012, 124, 9700;
Angew. Chem. Int. Ed. 2012, 51, 9562; m) Z.-Q. Wang, M. Hu,
X.-C. Huang, L.-B. Gong, Y.-X. Xie, J.-H. Li, J. Org. Chem. 2012,
77, 8705; n) Y.-Q. Zou, J.-R. Chen, X.-P. Liu, L.-Q. Lu, R. L.
Davis, K. A. Jørgensen, W.-J. Xiao, Angew. Chem. 2012, 124,
808; Angew. Chem. Int. Ed. 2012, 51, 784; o) S. Zhu, A. Das, L.
Bui, H. Zhou, D. P. Curran, M. Rueping, J. Am. Chem. Soc. 2013,
135, 1823.
Experimental Section
General procedure for visible-light-mediated aerobic oxidative
decarboxylation of a-keto acids with amines: A dried Schlenk tube
equipped with a stir bar was loaded with a-keto acid (0.5 mmol),
amine (0.75 mmol), [Ru(phen)3]Cl2 (0.005 mmol, 1 mol%), and
DMSO (2 mL) under an atmosphere of O2. The tube was then stirred
under visible light (with a 25 W fluorescent household light bulb
(distance app. 10 cm) 328C) for 36 h. After completion of the
reaction, it was quenched by water and extracted with ethyl acetate
(3 ꢁ 10 mL). The organic layers were combined and the pure product
was obtained by flash column chromatography on silica gel.
Received: October 3, 2013
Published online: November 24, 2013
Keywords: amides · computational chemistry · photochemistry ·
.
reaction mechanism · synthetic methods
[1] P. Raven, G. Johnson, J. Losos, K. Mason, S. Singer, Biology, 8th
ed., McGraw Hill, New York, 2008.
[2] B. Alberts, A. Johnson, J. Lewis, M. Raff, K. Roberts, P. Walter,
Molecular Biology of the Cell, Garland Science, New York, 2001.
[3] E. Chabriꢂre, X. Vernꢂde, B. Guigliarelli, M.-H. Charon, E. C.
Hatchikian, J. C. Fontecilla-Camps, Science 2001, 294, 2559.
[4] For recent reviews on decarboxylation, see: a) O. Baudoin,
Angew. Chem. 2007, 119, 1395; Angew. Chem. Int. Ed. 2007, 46,
1373; b) L. J. Gooßen, N. Rodrꢃguez, K. Gooßen, Angew. Chem.
2008, 120, 3144; Angew. Chem. Int. Ed. 2008, 47, 3100; c) T.
Satoh, M. Miura, Synthesis 2010, 3395; d) N. Rodrꢃguez, L. J.
Gooßen, Chem. Soc. Rev. 2011, 40, 5030; e) R. Shang, L. Liu, Sci.
Chi. Chem. 2011, 54, 1670; f) J. D. Weaver, A. Recio, A. J.
Grenning, J. A. Tunge, Chem. Rev. 2011, 111, 1846; g) J.
Cornella, I. Larrosa, Synthesis 2012, 653; h) W. I. Dzik, P. P.
Lange, L. J. Gooßen, Chem. Sci. 2012, 3, 2671; i) Z. Li, Y. Y.
Jiang, A. A. Yeagley, J. P. Bour, L. Liu, J. J. Chruma, Y. Fu,
Chem. Eur. J. 2012, 18, 14527.
[5] a) F. Fontana, F. Minisci, M. C. Nogueira Barbosa, E. Vismara, J.
Org. Chem. 1991, 56, 2866; b) L. J. Gooßen, F. Rudolphi, C.
Oppel, N. Rodrꢃguez, Angew. Chem. 2008, 120, 3085; Angew.
Chem. Int. Ed. 2008, 47, 3043; c) L. J. Gooßen, B. Zimmermann,
T. Knauber, Angew. Chem. 2008, 120, 7211; Angew. Chem. Int.
Ed. 2008, 47, 7103; d) C.-C. Cho, J.-N. Liu, C.-H. Chien, J.-J. Shie,
Y.-C. Chen, J.-M. Fang, J. Org. Chem. 2009, 74, 1549; e) P. Fang,
M. Li, H. Ge, J. Am. Chem. Soc. 2010, 132, 11898; f) M. Li, H.
Ge, Org. Lett. 2010, 12, 3464; g) N. Rodrꢃguez, F. Manjolinho,
M. F. Grꢄnberg, L. J. Gooßen, Chem. Eur. J. 2011, 17, 13688;
h) H. Wang, L.-N. Guo, X.-H. Duan, Org. Lett. 2012, 14, 4358;
i) M. Kim, J. Park, S. Sharma, A. Kim, E. Park, J. H. Kwak, Y. H.
Jung, I. S. Kim, Chem. Commun. 2013, 49, 925.
[8] a) S.-T. Huang, I. J. Hsei, C. Chen, Bioorg. Med. Chem. 2006, 14,
6106; b) C. J. Lion, C. S. Matthews, G. Wells, T. D. Bradshaw,
M. F. G. Stevens, A. D. Westwell, Bioorg. Med. Chem. Lett. 2006,
16, 5005; c) N. Siddiqui, A. Rana, S. A. Khan, M. A. Bhat, S. E.
Haque, Bioorg. Med. Chem. Lett. 2007, 17, 4178.
[9] X. Wang, A. Del Guerzo, H. Tunuguntla, R. H. Schmehl, Res.
Chem. Intermed. 2007, 33, 63.
[10] Since the amine is in excess in the reaction, the amine radical
cation formed in initiation step may be quenched later or
experienced
a SET process with amide radical anion to
regenerate amine.
[11] a) Y. Uenoyama, T. Fukuyama, O. Nobuta, H. Matsubara, I.
Ryu, Angew. Chem. 2005, 117, 1099; Angew. Chem. Int. Ed. 2005,
44, 1075; b) Y. Uenoyama, T. Fukuyama, I. Ryu, Org. Lett. 2007,
9, 935; c) H. Zhang, R. Shi, A. Ding, L. Lu, B. Chen, A. Lei,
Angew. Chem. 2012, 124, 12710; Angew. Chem. Int. Ed. 2012, 51,
12542.
[12] a) P. K. Ghosh, B. S. Brunschwig, M. Chou, C. Creutz, N. Sutin, J.
Am. Chem. Soc. 1984, 106, 4772; b) D. Kotkar, V. Joshi, P. K.
Ghosh, J. Chem. Soc. Chem. Commun. 1987, 4; c) X. Zhang,
M. A. J. Rodgers, J. Phys. Chem. 1995, 99, 12797; d) A. Das, V.
Joshi, D. Kotkar, V. S. Pathak, V. Swayambunathan, P. V. Kamat,
P. K. Ghosh, J. Phys. Chem. A 2001, 105, 6945.
[13] D. Hollmann, F. Gꢇrtner, R. Ludwig, E. Barsch, H. Junge, M.
Blug, S. Hoch, M. Beller, A. Brꢄckner, Angew. Chem. 2011, 123,
10429; Angew. Chem. Int. Ed. 2011, 50, 10246.
[14] a) A. G. Motten, K. Hanck, M. K. DeArmound, Chem. Phys.
Lett. 1981, 79, 541; b) K. Matsuura, L. Kevan, Radiat. Phys.
Chem. 2001, 33, 399.
[15] Though we cannot exclude path B in this reaction, the EPR
experiments and fluorescence quenching results (see Figures S1
and S2) demonstrates path A is more reasonable for aryl amines.
The aliphatic amines may experience a different pathway as
discussed in the Supporting Information.
[16] a) A. G. Davies, R. Sutcliffe, J. Chem. Soc. Perkin Trans. 2 1980,
819; b) C. Chatgilialoglu, D. Crich, M. Komatsu, I. Ryu, Chem.
Rev. 1999, 99, 1991.
[6] For reviews on visible-light photoredox catalysis, see: a) K.
Zeitler, Angew. Chem. 2009, 121, 9969; Angew. Chem. Int. Ed.
2009, 48, 9785; b) T. P. Yoon, M. A. Ischay, J. Du, Nat. Chem.
2010, 2, 527; c) J. M. R. Narayanam, C. R. J. Stephenson, Chem.
´
Soc. Rev. 2011, 40, 102; d) F. Teply, Collect. Czech. Chem.
Commun. 2011, 76, 859; e) M. A. Ischay, T. P. Yoon, Eur. J. Org.
Chem. 2012, 3359; f) D. Ravelli, M. Fagnoni, ChemCatChem
2012, 4, 169; g) J. W. Tucker, C. R. J. Stephenson, J. Org. Chem.
2012, 77, 1617; h) J. Xuan, W.-J. Xiao, Angew. Chem. 2012, 124,
6934; Angew. Chem. Int. Ed. 2012, 51, 6828; i) C. K. Prier, D. A.
Rankic, D. W. C. MacMillan, Chem. Rev. 2013, 113, 5322; j) Y.
Xi, H. Yi, A. Lei, Org. Biomol. Chem. 2013, 11, 2387.
[17] For a colorful figure see Figure S9.
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