A Bioinspired Catalytic Aerobic Oxidative C-H Functionalization of Primary Aliphatic Amines: Synthesis of 1,2-Disubstituted Benzimidazoles

Article abstract of DOI:10.1002/chem.201502487

Aerobic oxidative C-H functionalization of primary aliphatic amines has been accomplished with a biomimetic cooperative catalytic system to furnish 1,2-disubstituted benzimidazoles that play an important role as drug discovery targets. This one-pot atom-economical multistep process, which proceeds under mild conditions, with ambient air and equimolar amounts of each coupling partner, constitutes a convenient environmentally friendly strategy to functionalize non-activated aliphatic amines that remain challenging substrates for non-enzymatic catalytic aerobic systems.

Full text of DOI:10.1002/chem.201502487

DOI: 10.1002/chem.201502487
Communication
&
Bioinspired Catalysis |Hot Paper|
A Bioinspired Catalytic Aerobic Oxidative CÀH Functionalization
of Primary Aliphatic Amines: Synthesis of 1,2-Disubstituted
Benzimidazoles
Khac Minh Huy Nguyen and Martine Largeron*[a]
doline or tetrahydroquinoline,[6] two reaction types that natural
Abstract: Aerobic oxidative CÀH functionalization of pri-
CuAOs are not able to accomplish.
mary aliphatic amines has been accomplished with a bio-
mimetic cooperative catalytic system to furnish 1,2-disub-
stituted benzimidazoles that play an important role as
drug discovery targets. This one-pot atom-economical
multistep process, which proceeds under mild conditions,
with ambient air and equimolar amounts of each coupling
partner, constitutes a convenient environmentally friendly
strategy to functionalize non-activated aliphatic amines
that remain challenging substrates for non-enzymatic cat-
alytic aerobic systems.
In contrast, the catalytic oxidation of non-activated primary
alkylamines, which are natural substrates for CuAOs, has re-
ceived little attention, probably because the generated alkyli-
mines very easily isomerize into the unstable enamine tauto-
mers. This is also the reason why alkylimines are difficult to
prepare from the condensation of aliphatic aldehydes with
amines. As a result, the catalytic oxidation of non-activated pri-
mary amines to imines remains a challenging task.
Recently, we reported a CuAO-like homogeneous co-catalytic
system for the atom-economical oxidation of primary amines
to imines under ambient air.[7] The catalytic process combines
two redox couples in a way reminiscent of other biomimetic
catalytic systems:[1b] the o-iminoquinone organocatalyst 1ox,
Naturally occurring metalloenzymes constitute a rich source of
inspiration for the design of synthetic catalysts because of
their ability to perform controlled aerobic oxidations under
very mild conditions.[1] Among metalloenzymes, copper amine
oxidases (CuAOs) promote selective aerobic oxidation of pri-
mary amines through the cooperation of a quinone-based co-
factor (topaquinone) and copper.[2] Recently, there has been
a boost in the development of various catalytic methods for
the aerobic oxidation of amines to imines,[3] owing to the im-
portance of imines as pivotal intermediates in the synthesis of
fine chemicals and pharmaceuticals. As a consequence, the use
of CuAO-like catalytic systems has emerged in synthetic strat-
egies,[4] especially because some biomimetic catalysts may
offer advantages over CuAOs for expanding the scope of possi-
ble substrates. For example, a simple bioinspired ortho-qui-
none catalyst was reported for the oxidation of a-branched
primary benzylic amines[5] and cooperative catalytic systems
have been identified as effective mediators for the oxidation of
secondary amines including nitrogen heterocycles such as in-
generated in situ from the corresponding o-aminophenol 1red
,
is the substrate-selective catalyst, whereas the copper(II) salt
serves as an electron transfer mediator. Interestingly, low load-
ings of biocompatible CuII and organocatalyst 1ox are sufficient
to activate the a-CÀH bond of primary aliphatic amines, which
are converted to cross-coupled imines through a transamina-
tion process that leads to the homocoupled imine intermedi-
ate, followed by dynamic transimination.[7b] The mild reaction
conditions are highly favorable from a synthetic viewpoint, in
particular for trapping the unstable alkylimines in situ for fur-
ther reactions. We therefore envisioned the use of our bioin-
spired co-catalytic system in the one-pot synthesis of 1,2-dis-
ubstituted benzimidazoles. These compounds are important
targets in drug discovery, as shown by the profusion in the
market of pharmaceutical products such as telmisartan and
candesartan as antihypertensives, or astemizole, clemizole, and
bilastine as antihistaminic agents.[8]
Over the past few years, novel methodologies involving
transition-metal-catalyzed CÀH functionalization reactions[9]
have been directed toward the regiocontrolled synthesis of
1,2-disubstituted benzimidazoles.[10] Meanwhile, aerobic cata-
lytic oxidative cross-coupling reactions employing either alco-
hols or amines as substrates have also been reported for the
synthesis of benzazoles.[11] However, several of these methods
suffer drawbacks such as elevated temperatures (90–1608C)
and/or oxygen pressure. In particular, non-activated primary al-
cohols or amines are generally ineffective starting materials for
this transformation. In addition, metal-free oxidative cross-cou-
pling reactions of primary alcohols or amines with o-amino ani-
lines have been described, but the scope is limited to benzylic-
type substrates.[12]
[a] K. M. H. Nguyen, Dr. M. Largeron
UMR 8638 CNRS-UniversitØ Paris Descartes (Paris 5), Sorbonne Paris CitØ
FacultØ de Pharmacie de Paris, 4 avenue de l’Observatoire
75270 Paris cedex 06 (France)
E-mail: martine.largeron@parisdescartes.fr
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201502487.
ꢀ 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
This is an open access article under the terms of Creative Commons Attri-
bution NonCommercial-NoDerivs License, which permits use and distribu-
tion in any medium, provided the original work is properly cited, the use is
non-commercial and no modifications or adaptations are made.
Chem. Eur. J. 2015, 21, 12606 – 12610
12606 ꢀ 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
the sole catalyst. These results
confirmed
the
cooperative
action of organocatalyst 1ox as
the substrate-selective catalyst
and copper salt as the electron
transfer mediator to facilitate the
aerobic oxidation of amines to
imines.[7]
Scheme 1. Cross-coupling of primary amines to give 1,2-disubstituted benzimidazoles by using a bioinspired co-
catalytic system.
With the optimized reaction
conditions in hand (Table 1,
Herein, we describe a bioinspired catalytic oxidative cou-
pling of a diverse range of activated and non-activated primary
amines with o-amino anilines under ambient air that leads to
1,2-disubstituted benzimidazoles through multistep oxidation
and nucleophilic addition reactions (Scheme 1).
entry 5), we examined the reactions of a series of primary
amine substrates. Variously substituted benzylamines afforded
the corresponding 1,2-benzimidazoles in 74–82% yield regard-
less of the electronic character of the substituents (Table 2, en-
tries 1–8). In contrast, the rate of the reaction was affected by
steric effects[11b,14] as shown by benzylic amine 2i which gave
benzimidazole 4i in 75% yield after 48 h (Table 2, entry 9). 1-
Naphthylmethylamine 2j could also be used as the amine sub-
strate leading to 1,2-benzimidazole 4j in 71% yield (Table 2,
entry 10). Likewise, heterocyclic compounds such as 2-thiophe-
nemethylamine 2k and furfurylamine 2l could be converted
into the corresponding 1,2-benzimidazoles 4k and 4l in good
yields with a prolonged reaction time (Table 2, entries 11 and
12).
As a starting point, we examined the aerobic oxidative
cross-coupling of benzylamine 2a with o-aminoaniline 3a
under our previously optimized ambient conditions for the
preparation of cross-coupled imines,[7b] but no significant
amounts of 1,2-benzimidazole 4a were isolated after 24 h
(Table 1, entry 1). Significant improvements were achieved by
heating the coupling-reaction mixture at 458C and by using
CuBr2 as the copper catalyst,[11c] which afforded 1,2-benzimid-
azole 4a in 82% yield of the isolated product (Table 1, entry 5).
When the reaction was conducted at higher temperature
(608C), the yield of 4a decreased to 75% due to the slow de-
composition of the o-iminoquinone organocatalyst 1ox to mela-
nin-like polymers (Table 1, entry 6).[13] As expected,[7b] further
variation of the solvent led only to a decrease in the yield
(Table 1, entries 7–9). Control studies revealed that the synthe-
sis of 4a could also be performed in the absence of copper
catalyst, albeit with a markedly reduced yield (30%), owing to
the slow spontaneous oxidation of 1red to organocatalyst 1ox,
whereas no reaction occurred at 458C when CuBr2 was used as
Our optimized conditions were further applied to the oxida-
tion of non-activated primary aliphatic amines (Table 2, en-
tries 13–19). Then, the aerobic oxidative cross-coupling of ami-
nomethylcyclohexane 2m with o-aminoaniline 3a afforded
1,2-benzimidazole 4m in only 36% yield after 24 h. The lower
yield could be due to the instability of the generated alkyl-
imine intermediate which readily isomerizes into the enamine
tautomer.[7b,11b,14] However, the yield of 1,2-benzimidazole 4m
could be improved to 58% at a slightly elevated reaction tem-
perature of 608C (Table 2, entry 13). Other aliphatic amines
were also oxidized to the corresponding benzimidazoles in
moderate to good yields (Table 2, entries 14–19). As previously
reported for catalyzed aerobic oxidation of non-activated alco-
hols,[15] we observed a decreased reactivity for b-branched al-
kylamines, but some activity could be regained by increasing
the reaction time (Table 2, entry 19).
Table 1. Optimization of the CuII/1ox-catalyzed aerobic oxidative cross-
coupling of benzylamine 2a with o-aminoaniline 3a.[a]
In a final series of experiments, we carried out reactions
using two commercially available o-aminoanilines as in situ
imine traps. N-methyl-1,2-phenylenediamine (Table 3, entries 1
and 3) gave similar results to its N-phenyl congener (Table 2,
entries 1 and 14), whereas N-(4-chlorophenyl)-1,2-phenylenedi-
amine delivered the desired products with acceptable yields
only after 48 h (Table 3, entries 2 and 4).
Entry
Copper catalyst
Solvent
Temperature [8C]
Yield [%][b]
1
2
3
4
5
6
7
8
9
Cu(OAc)2
Cu(OAc)2
CuCN
CuBr2
CuBr2
CuBr2
CuBr2
CuBr2
CuBr2
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
EtOH
25
40
40
40
45
60
45
45
45
5
48
40
70
82
75
68
45
10
A proposed reaction pathway is shown in Scheme 2. The ini-
tial step is the formation of the cross-coupled imine through
a transamination process that leads to the homocoupled imine
intermediate followed by dynamic transimination.[7b] The intra-
molecular addition of the amine group affords the cyclic inter-
mediate A, which is further oxidized to 1,2-disubstituted benzi-
midazole 4.
We have validated our cascade reaction sequence with two
control experiments (Scheme 3). Firstly, N-benzylidenebenzyla-
mine was reacted with two equivalents of o-amino aniline 3a
MeCN
THF
[a] The reactions were carried out using equimolar amounts of benzyla-
mine 2a and o-aminoaniline 3a on a 1.25 mmol scale, in the presence of
4 mol% of 1red and 0.4 mol% of copper salt, in 25 mL of solvent, under
ambient air at the indicated temperature for 24 h. After 6 h, an additional
aliquot of 1red (2 mol%) was added; [b] yield of isolated product.
Chem. Eur. J. 2015, 21, 12606 – 12610
www.chemeurj.org
12607 ꢀ 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
Table 2. CuBr2/1ox-catalyzed aerobic oxidative cross-coupling of a range
of primary activated and non-activated amines 2 with o-aminoaniline
3a.[a]
Table 2. (Continued)
Entry
Amine Substrate 2
Benzimidazole Product 4
Yield
[%][c]
Entry
Amine Substrate 2
Benzimidazole Product 4
Yield
[%][c]
16[b] 2p
4p
61
1
2
3
4
2a
2b
2c
2d
4a
82
80
78
80
17[b] 2q
18[b] 2r
19[b] 2s
4q
4r
4s
58
4b
4c
4d
53
51[d]
[a] The reactions were carried out using equimolar amounts of primary
amines 2 and o-aminoaniline 3a on a 1.25 mmol scale, in the presence of
4 mol% of 1red and 0.4 mol% of CuBr2, in 25 mL of MeOH, under ambient
air for 24 h. After 6 h, an additional aliquot of 1red (2 mol%) was added;
[b] T=608C; [c] yield of isolated product; [d] yield after 48 h; [e] as vola-
tile alkylamine 2n was lost at 608C, an additional 0.5 equivalent of alkyla-
mine was added after 6 h.
5
2e
4e
74[d]
6
7
8
2 f
2g
2h
4 f
4g
4h
75
80
74
Table 3. CuBr2/1ox-catalyzed aerobic oxidative cross-coupling of benzyla-
mine 2a or aminomethylcyclopropane 2n with o-aminoanilines 3.[a]
9
2i
4i
75[d]
Entry R2
R1
Benzimidazole Product 4 Yield [%][c]
10
11
12
2j
2k
2l
4j
4k
4l
71
1
2
3[b]
4[b]
phenyl
phenyl
cyclopropyl methyl
methyl
p-chlorophenyl
4t
4u
4v
4w
81
84[d]
73[e]
72[d,e]
81[d]
70[d]
58
cyclopropyl p-chlorophenyl
[a] The reactions were carried out using equimolar amounts of primary
amine 2a (or 2n) and o-aminoaniline 3 on a 1.25 mmol scale, in the pres-
ence of 4 mol% of 1red and 0.4 mol% of CuBr2, in 25 mL of MeOH, under
ambient air for 24 h. After 6 h, an additional aliquot of 1red (2 mol%) was
added; [b] T=608C; [c] yield of isolated product; [d] yield after 48 h;
[e] as volatile alkylamine 2n was lost at 608C, an additional 0.5 equivalent
of alkylamine was added after 6 h (entries 3 and 4) and after 24 h
(entry 4).
13[b] 2m
14[b] 2n
15[b] 2o
4m
4n
4o
68[e]
55
under the standard conditions (Table 1, entry 5). Then, 1,2-dis-
ubstituted benzimidazole 4a was isolated in 82% yield, in
agreement with N-benzylidenebenzylamine as the first imine
intermediate. Secondly, when the same reaction was realized
in the absence of CuBr2 catalyst, only 60% yield of benzimid-
Chem. Eur. J. 2015, 21, 12606 – 12610
www.chemeurj.org
12608 ꢀ 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
The above procedure is generally representative for all of the prod-
ucts shown in Tables 2 and 3. Any deviations from this protocol are
specified in the footnotes of the tables.
Acknowledgements
We would like to thank Dr M.-B. Fleury, Emeritus Professor at
the Paris Descartes University, for fruitful discussions. We also
thank ADRAPHARM (Association pour le DØveloppement de la
Recherche AppliquØe à la Pharmacologie) for financial support.
Keywords: aerobic oxidation · amines · benzimidazoles · CÀH
functionalization · homogeneous catalysis
Scheme 2. Proposed mechanism for the CuBr2/1ox-mediated CÀH functionali-
zation of primary aliphatic amines giving rise to 1,2-disubstituted benzimida-
zoles.
[1] For representative recent reviews, see: a) L. Que Jr, W. B. Tolman, Nature
2008, 455, 333–340; b) J. Piera, J.-E. Bäckvall, Angew. Chem. Int. Ed.
2008, 47, 3506–3523; Angew. Chem. 2008, 120, 3558–3576; c) S.-I. Mur-
ahashi, D. Zhang, Chem. Soc. Rev. 2008, 37, 1490–1501; d) J. C. Lewis,
P. S. Coelho, F. H. Arnold, Chem. Soc. Rev. 2011, 40, 2003–2021.
[2] For reviews, see: a) M. Mure, S. A. Mills, J. P. Klinman, Biochemistry 2002,
41, 9269–9278; b) M. Mure, Acc. Chem. Res. 2004, 37, 131–139; c) J. P.
Klinman, F. Bonnot, Chem. Rev. 2014, 114, 4343–4365.
[3] For recent reviews, see: a) M. T. Schümperli, C. Hammond, I. Hermans,
ACS Catal. 2012, 2, 1108–1117; b) R. D. Patil, S. Adimurthy, Asian J. Org.
Chem. 2013, 2, 726–744; c) R. J. Angelici, Catal. Sci. Technol. 2013, 3,
279–296; d) W. Qing, S. Long, M. Panunzio, S. Biondi, Molecules 2013,
18, 12264–12289; e) M. Largeron, Eur. J. Org. Chem. 2013, 5225–5235;
f) X. Lang, W. Ma, C. Chen, H. Ji, J. Zhao, Acc. Chem. Res. 2014, 47, 355–
363; g) X. Lang, X. Chen, J. Zhao, Chem. Soc. Rev. 2014, 43, 473–486;
h) B. L. Ryland, S. S. Stahl, Angew. Chem. Int. Ed. 2014, 53, 8824–8838;
Angew. Chem. 2014, 126, 8968–8983.
[4] a) A. E. Wendlandt, S. S. Stahl, Org. Lett. 2012, 14, 2850–2853; b) M.
Largeron, M.-B. Fleury, Science 2013, 339, 43–44; c) D. V. Jawale, E.
Gravel, E. Villemin, N. Shah, V. Geertsen, I. N. N. Namboothiri, E. Doris,
Chem. Commun. 2014, 50, 15251–15254.
Scheme 3. Control experiments starting from N-benzylidenebenzylamine.
azole 4a was isolated. This result indicated that CuBr2 also
mediated the ultimate oxidation step that led to the benzimi-
dazole product.[11b,c]
In conclusion, we have developed a bioinspired catalytic
aerobic oxidative CÀH functionalization of primary amines that
leads to 1,2-disubstituted benzimidazoles under environmen-
tally benign reaction conditions. Notably, low loadings of bio-
compatible CuBr2 electron transfer mediator and topaquinone-
like organocatalyst are sufficient to activate, under ambient air,
the a-CÀH bond of non-activated primary amines that are
known to be challenging substrates for non-enzymatic catalyt-
ic aerobic systems. This biomimetic approach should be useful
for other synthetic applications involving unstable alkylimines
as the intermediate.
[5] Y. Qiang, L. Zhang, J. Lv, S. Luo, J.-P. Cheng, Org. Lett. 2015, 17, 1469–
1472.
[6] a) H. Yuan, W.-J. Yoo, H. Miyamura, S. Kobayashi, J. Am. Chem. Soc. 2012,
134, 13970–13973; b) A. E. Wendlandt, S. S. Stahl, J. Am. Chem. Soc.
2014, 136, 506–512; c) A. E. Wendlandt, S. S. Stahl, J. Am. Chem. Soc.
2014, 136, 11910–11913; d) D. V. Jawale, E. Gravel, N. Shah, V. Dauvois,
H. Li, I. N. N. Namboothiri, E. Doris, Chem. Eur. J. 2015, 21, 7039–7042.
[7] a) M. Largeron, M.-B. Fleury, Angew. Chem. Int. Ed. 2012, 51, 5409–5412;
Angew. Chem. 2012, 124, 5505–5508; b) M. Largeron, M.-B. Fleury,
Chem. Eur. J. 2015, 21, 3815–3820.
[8] For some recent reviews, see: a) Y. Bansal, O. Silakari, Bioorg. Med.
Chem. 2012, 20, 6208–6236; b) G. Yadav, S. Ganguly, Eur. J. Med. Chem.
2015, 97, 419–443; c) R. S. Keri, A. Hiremathad, S. Budagumpi, B. M. Na-
garaja, Chem. Biol. Drug Des. 2015, 86, 799–845.
[9] For selected recent reviews on transition metal-catalyzed CÀH function-
alization reactions, see: a) A. E. Wendlandt, A. M. Suess, S. S. Stahl,
Angew. Chem. Int. Ed. 2011, 50, 11062–11087; Angew. Chem. 2011, 123,
11256–11283; b) S. E. Allen, R. R. Walvoord, R. Padilla-Salinas, M. C. Ko-
zlowski, Chem. Rev. 2013, 113, 6234–6458; c) J. Yuan, C. Liu, A. Lei,
Chem. Commun. 2015, 51, 1394–1409; d) X.-X. Guo, D.-W. Gu, Z. Wu, W.
Zhang, Chem. Rev. 2015, 115, 1622–1651.
[10] For a recent review, see: a) L. C. R. Carvalho, E. Fernandes, M. M. B. Mar-
ques, Chem. Eur. J. 2011, 17, 12544–12555. For selected recent exam-
ples, see also: b) N. Zheng, K. W. Anderson, X. Huang, H. N. Nguyen,
S. L. Buchwald, Angew. Chem. Int. Ed. 2007, 46, 7509–7512; Angew.
Chem. 2007, 119, 7653–7656; c) B. Zou, Q. Yuan, D. Ma, Angew. Chem.
Int. Ed. 2007, 46, 2598–2601; Angew. Chem. 2007, 119, 2652–2655;
d) G. Brasche, S. L. Buchwald, Angew. Chem. Int. Ed. 2008, 47, 1932–
1934; Angew. Chem. 2008, 120, 1958–1960; e) P. Saha, T. Ramana, N.
Purkait, Md Ashif Ali, R. Paul, T. Punniyamurthy, J. Org. Chem. 2009, 74,
8719–8725; f) P. Saha, Md Ashif Ali, P. Ghosh, T. Punniyamurthy, Org.
Biomol. Chem. 2010, 8, 5692–5699; g) D. Zhao, J. Hu, N. Wu, X. Huang,
Experimental Section
General experimental procedure for the synthesis of 1,2-disub-
stituted benzimidazoles: Equimolar amounts of benzylamine 2
(1.25 mmol) and o-amino aniline 3 (1.25 mmol) with reduced orga-
[16]
nocatalyst 1red
(0.05 mmol, 4 mol%) and copper(II) bromide
(0.005 mmol, 0.4 mol%) were mixed in methanol (25 mL) in an air
atmosphere. The reaction mixture was stirred at 458C for 6 h.
Then, an additional aliquot of 1red (0.025 mmol, 2 mol%) was intro-
duced into the reaction mixture and the reaction was continued
for 18 h. The solvent was then removed by evaporation under re-
duced pressure and the residue was purified by column chroma-
tography on silica gel (eluent: dichloromethane/methanol 99:1 v/v)
to afford the desired benzimidazole 4 (see the Supporting Informa-
tion).
Chem. Eur. J. 2015, 21, 12606 – 12610
www.chemeurj.org
12609 ꢀ 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
X. Qin, J. Lan, J. You, Org. Lett. 2011, 13, 6516–6519; h) J. li, S. BØnard, L.
Neuville, J. Zhu, Org. Lett. 2012, 14, 5980–5983; i) M. Pizzetti, E.
De Luca, E. Petricci, A. Porcheddu, M. Taddei, Adv. Synth. Catal. 2012,
354, 2453–2464; j) N. T. Jui, S. L. Buchwald, Angew. Chem. Int. Ed. 2013,
52, 11624–11627; Angew. Chem. 2013, 125, 11838–11841; k) J.-P. Lin, F.-
H. Zhang, Y.-Q. Long, Org. Lett. 2014, 16, 2822–2825; l) M. Sun, C. Chen,
W. Bao, RSC Adv. 2014, 4, 47373–47376.
[12] For selected examples, see: a) T. B. Nguyen, L. Ermolenko, W. A. Dean, A.
Al-Mourabit, Org. Lett. 2012, 14, 5948–5951; b) T. B. Nguyen, L. Ermolen-
ko, A. Al-Mourabit, Green Chem. 2013, 15, 2713–2717; c) G. Naresh, R.
Kant, T. Narender, J. Org. Chem. 2014, 79, 3821–3829; d) M. Reddy Marri,
S. Peraka, A. Kumar Marchala, N. Mameda, S. Kodumuri, N. Nama, Tetra-
hedron Lett. 2014, 55, 6520–6525.
[13] M. Largeron, A. Chiaroni, M.-B. Fleury, Chem. Eur. J. 2008, 14, 996–1003.
[14] D. B. Ushakov, M. B. Plutschack, K. Gilmore, P. H. Seeberger, Chem. Eur. J.
2015, 21, 6528–6534.
[15] B. Xu, J.-P. Lumb, B. A. Arndtsen, Angew. Chem. Int. Ed. 2015, 54, 4208–
4211; Angew. Chem. 2015, 127, 4282–4285.
[16] Reduced catalyst 1red can be easily prepared in two steps from com-
mercially available 2-nitroresorcinol, according to our published proce-
dure: D. Xu, A. Chiaroni, M.-B. Fleury, M. Largeron, J. Org. Chem. 2006,
71, 6374–6381.
[11] For selected examples, see: a) F. Su, S. C. Mathew, L. Mçhlmann, M. An-
tonietti, X. Wang, S. Blechert, Angew. Chem. Int. Ed. 2011, 50, 657–660;
Angew. Chem. 2011, 123, 683–686; b) Y. Endo, J.-E. Bäckvall, Chem. Eur.
J. 2012, 18, 13609–13613; c) T. Xiao, S. Xiong, Y. Xie, X. Dong, L. Zhou,
RSC Adv. 2013, 3, 15592–15595; d) H. Wang, J. Zhang, Y.-M. Cui, K.-F.
Yang, Z.-J. Zheng, L.-W. Xu, RSC Adv. 2014, 4, 34681–34686; e) R. Rama-
chandran, G. Prakash, S. Selvamurugan, P. Viswanathamurthi, J. G. Male-
cki, V. Ramkumar, Dalton Trans. 2014, 43, 7889–7902; f) C. Su, R. Tandi-
ana, J. Balapanuru, W. Tang, K. Pareek, C. T. Nai, T. Hayashi, K. P. Loh, J.
Am. Chem. Soc. 2015, 137, 685–690; g) K. Gopalaiah, S. N. Chandrudu,
RSC Adv. 2015, 5, 5015–5023; h) X. Shi, J. Guo, J. Liu, M. Ye, Q. Xu,
Chem. Eur. J. 2015, 21, 9988–9993.
Received: June 26, 2015
Published online on July 23, 2015
Chem. Eur. J. 2015, 21, 12606 – 12610
www.chemeurj.org
12610 ꢀ 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim