Pd-catalyzed multidehydrogenative cross-coupling between (Hetero)arenes and nitroethane to construct β-aryl nitroethylenes

Article abstract of DOI:10.1021/ol400507u

The Pd-catalyzed multidehydrogenative cross-coupling reactions of arenes with nitroethane are described. The established methods afford β-aryl nitroethylenes that are an important class of synthetic intermediates and biologically active compounds in an atom- and step-economical fashion. The reactions were applicable to substituted benzenes and a variety of heterocycles such as benzothiophenes, benzofurans, and indoles. Mechanistic experiments indicated that β-nitroethylbenzene might be the intermediate in this transformation.

Full text of DOI:10.1021/ol400507u

ORGANIC
LETTERS
2013
Vol. 15, No. 7
1718–1721
Pd-Catalyzed Multidehydrogenative
Cross-Coupling between (Hetero)Arenes
and Nitroethane to Construct β‑Aryl
Nitroethylenes
Min Zhang, Peng Hu, Jun Zhou, Ge Wu, Shijun Huang, and Weiping Su*
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the
Structure of Matter, Chinese Academy of Sciences, Fuzhou, 350002, China
wpsu@fjirsm.ac.cn
Received February 23, 2013
ABSTRACT
The Pd-catalyzed multidehydrogenative cross-coupling reactions of arenes with nitroethane are described. The established methods afford β-aryl
nitroethylenes that are an important class of synthetic intermediates and biologically active compounds in an atom- and step-economical fashion.
The reactions were applicable to substituted benzenes and a variety of heterocycles such as benzothiophenes, benzofurans, and indoles.
Mechanistic experiments indicated that β-nitroethylbenzene might be the intermediate in this transformation.
β-Aryl nitroethylenes and the compounds with β-aryl
nitroethylene structural units are versatile intermediates in
organic syntheses owing to various transformations of
such structural units,1 such as selective reductions to
diverse functional groups,2 the DielsÀAlder cycloaddition
reactions,3 and the Michael addition reactions to construct
carbonÀcarbon or carbonÀheteroatom bonds.4 Yet,
many β-aryl nitroethylenes and their derivatives have been
documented to display diverse biological activities includ-
ing antibacterial, molluscicidal,5 and anticancer.6
The most commonly used method for the preparation of
β-aryl nitroethylenes is the Henry condensation reaction of
aromatic aldehydes with nitromethane,7 which is usually
carried out under basic conditions (Scheme 1). Given that
aromatic aldehydes are generally prepared from the corre-
sponding arenes by way of the GattermannÀKoch formyla-
tion8 or the VilsmeierÀHaack formylation9 process, a signi-
ficant improvement would be made if β-aryl nitroethylenes
could be directly synthesized from arenes. Herein, we
(1) Barrett, A. G. M.; Graboski, G. G. Chem. Rev. 1986, 86, 751.
(2) (a) Ono, N. The Nitro Group in Organic Synthesis; Wiley-VCH:
New York, 2001. (b) Bhattacharjya, A.; Mukhopadhyay, R.; Pakrashi, S. C.
Synthesis 1985, 886. (c) Kabalka, G. W.; Laila, G. M. H.; Varma, R. S.
Tetrahedron 1990, 46, 7443. (d) Varma, R. S.; Varma, M.; Kabalka,
G. W. Tetrahedron Lett. 1985, 26, 3777. (e) Corma, A.; Serna, P.; Garcıa,
H. J. Am. Chem. Soc. 2007, 129, 6358. (f) Mourad, M. S.; Varma, R. S.;
Kabalka, G. W. J. Org. Chem. 1985, 50, 133. (g) Yao, C.-F.; Chu, C.-M.;
Liu, J.-T. J. Org. Chem. 1998, 63, 719. (h) Tamura, R.; Kamimura, A.;
Ono, N. Synthesis 1991, 423. (i) Mukaiyama, T.; Hoshino, T. J. Am.
Chem. Soc. 1960, 82, 5339. (j) Kamlet, M. J.; Kaplan, L. A.; Dacons,
J. C. J. Org. Chem. 1961, 26, 4371.
(5) (a) Schales, O.; Graefe, H. A. J. Am. Chem. Soc. 1952, 74, 4486.
(b) Milhazes, N.; Calheiros, R.; Marques, M. P.; Garrido, J.; Cordeiro,
M. N.; Rodrigues, C.; Quinteira, S.; Novais, C.; Peixe, L.; Borges, F.
Bioorg. Med. Chem. 2006, 14, 4078.
(6) (a) Carter, K. C.; Finnon, Y. S.; Daeid, N. N.; Robson, D. C.;
Waddell, R. Immunopharmacol. Immunotoxicol. 2002, 24, 187. (b) Kaap,
S.; Quentin, I.; Tamiru, D.; Shaheen, M.; Eger, K.; Steinfelder, H.
J. Biochem. Pharmacol. 2003, 65, 603. (c) Kim, J. H.; Kim, J. H.; Lee,
G. E.; Lee, J. E.; Chung, I. K. Mol. Pharmacol. 2003, 63, 1117. (d) Cheng,
P.; Jiang, Z.-Y.; Wang, R.-R.; Zhang, X.-M.; Wang, Q.; Zheng, Y.-T.;
Zhou, J.; Chen, J.-J. Bioorg. Med. Chem. Lett. 2007, 17, 4476. (e) Reddy,
M. A.; Jain, N.; Yada, D.; Kishore, C.; Vangala, J. R.; P. Surendra, R.;
Addlagatta, A.; Kalivendi, S. V.; Sreedhar, B. J. Med. Chem. 2011,
54, 6751.
ꢀ
ꢀ
(3) (a) Sunden, H.; Rios, R.; Xu, Y.; Eriksson, L.; Cordova, A. Adv.
Synth. Catal. 2007, 349, 2549. (b) Xu, D.-Q.; Xia, A.-B.; Luo, S.-P.;
Tang, J.; Zhang, S.; Jiang, J.-R.; Xu, Z.-Y. Angew. Chem., Int. Ed. 2009,
48, 3821. (c) Jia, Z.-J.; Zhou, Q.; Zhou, Q.-Q.; Chen, P.-Q.; Chen, Y.-C.
Angew. Chem., Int. Ed. 2011, 50, 8638. (d) Wu, W.; Min, L.; Zhu, L.; Lee,
C.-S. Adv. Synth. Catal. 2011, 353, 1135.
(7) (a) Henry, L. C. R. Acad. Sci. Ser. C 1895, 120, 1265. (b) Henry, L.
Bull. Soc. Chim. Fr. 1895, 13, 999.
(8) Gattermann, L.; Koch, J. A. Chem. Ber. 1897, 30, 1622.
(4) Berner, O. M.; Tedeschi, L.; Enders, D. Eur. J. Org. Chem. 2002,
1877.
r
10.1021/ol400507u
Published on Web 03/25/2013
2013 American Chemical Society

demonstrate that this goal could be achieved through
Pd-catalyzed cross-coupling reactions of (hetero)arenes
with nitroethane via 4-fold CÀH bond cleavages.
Scheme 1. Methods for Construction of β-Aryl Nitroethylene
In the past few years, substantial progress has been
achieved in the development of catalytic methods for the
dehydrogenative cross-coupling between two Csp2ÀH
bonds10,11 or between a Csp2ÀH bond and a Csp3ÀH bond
adjacent to heteroatoms or aromatic rings.12 Recently,
when investigating the decarboxylative cross-couplings
of arene carboxylic acids with saturated ketones or nitro-
ethane,13 we found that they afforded the decarboxyla-
tive Heck-type products. On the basis of these findings,
we have developed the Pd-catalyzed multidehydrogena-
tive cross-coupling of (hetero)arenes and the saturated
ketones.14 To expand such a new type of multidehydro-
genative cross-coupling further, we considered whether
the reaction of (hetero)arenes with nitroethane under a Pd
catalyst affords β-aryl nitroethylenes. The expected tan-
dem transformation, which just involves multiple succes-
sive CÀH cleavages and CÀC formation steps in one
pot,15 would provide a fundamentally new approach that
enables the facile synthesis of β-aryl nitroethylenes in an
atom- and step-economical fashion. However, achieve-
ment of this target reaction remains a great challenge
because the successive cleavage of the CÀH bond and the
formation of the CÀC bond would be required in this
process.
Taking into account the above problems, we screened a
variety of reaction parameters by reacting mesitylene (1a,
l mL) with nitroethane 2 as a model system (Table 1). The
desired product 3a was obtained in 7% yield from the
reaction carried out in DMSO/DME (DME = 1,2-
dimethoxyethane) at 100 °C with 10 mol % Pd(TFA)2 as
a catalyst and Ag2CO3 (2.0 equiv) as an oxidant (entry 2).
In the previous studies on the dehydrogenative cross-
coupling reactions involving the dehydrogenation of satu-
rated ketones to olefins,13b,14 we observed that additional
bases were required to promote the olefin formation from
saturated ketones and accept the protons from the reac-
tion. However, the additional bases such as carboxylate
salts and carbonates totally shut down the reaction of 1a
with 2, probably because these bases competed for coordi-
nation to the Pd center preferentially over substituted
benzene or a reaction intermediate and therefore impeded
the desired catalytic process (entries 5À7). In line with this
speculation, weakly polar ethers provided better results
than strongly polar solvents such as DMA and NMP
(entries 13À16). Nevertheless, control experiments showed
that 35À70 equiv of DMSO were indispensable for this
reaction to occur (entry 8). These observations suggested
that a Pd complex with DMSO as the ligand may be
involved in the catalytic process.16 The use of AgOAc
(4.0 equiv) to replace Ag2CO3 significantly improved the
yield (entries 3À4). Other silver salts such as AgTFA,
Ag2O, and AgOTf were observed to be inferior to AgOAc.
The beneficial effect of AgOAc may arise from either the
liberation of acetate anion at a suitable rate through the
reduction of AgOAc or the formation of a reactive catalyst
from Pd(TFA)2 and AgOAc.11b Other commonly used
oxidants for the oxidation of Pd(0) to Pd(II) such as BQ,
Cu(OAc)2, and O2 were totally invalid (entries 10À12).
Interestingly, the reaction could be carried out under air
without a loss in yield (entry 17).
(9) Vilsmeier, A.; Haack, A. Chem. Ber. 1927, 60, 119.
(10) For selected reviews, see: (a) Chen, X.; Engle, K. M.; Wang,
D. H.; Yu, J. Q. Angew. Chem., Int. Ed. 2009, 48, 5094. (b) Cho, S. H.;
Kim, J. Y.; Kwak, J.; Chang, S. Chem. Soc. Rev. 2011, 40, 5068. (c) Liu,
C.; Zhang, H.; Shi, W.; Lei, A. Chem. Rev. 2011, 111, 1780. (d) Yeung,
C. S.; Dong, V. M. Chem. Rev. 2011, 111, 1215. (e) Zhao, D.; You, J.; Hu,
C. Chem.;Eur. J. 2011, 17, 5466. (f) Bugaut, X.; Glorius, F. Angew.
Chem., Int. Ed. 2011, 50, 7479. (g) Li, B. J.; Shi, Z. J. Chem. Soc. Rev.
2012, 41, 5588. (h) Karimi, B.; Behzadnia, H.; Elhamifar, D.; Akhavan,
P.; Esfahani, F.; Zamani, A. Synthesis 2010, 1399. (i) Le Bras, J.; Muzart,
J. Chem. Rev. 2011, 111, 1170.
(11) For selected examples, see: (a) Stuart, D. R.; Fagnou, K. Science
2007, 316, 1172. (b) Stuart, D. R.; Villemure, E.; Fagnou, K. J. Am.
Chem. Soc. 2007, 129, 12072. (c) Dwight, T. A.; Rue, N. R.; Charyk, D.;
Josselyn, R.; DeBoef, B. Org. Lett. 2007, 9, 3137. (d) Hull, K. L.;
Sanford, M. S. J. Am. Chem. Soc. 2007, 129, 11904. (e) Xia, J.-B.;
You, S.-L. Organometallics 2007, 26, 4869. (f) Li, B.-J.; Tian, S.-L.;
Fang, Z.; Shi, Z.-J. Angew. Chem., Int. Ed. 2008, 47, 1115. (g) Cho, S. H.;
Hwang, S. J.; Chang, S. J. Am. Chem. Soc. 2008, 130, 9254. (h) Zhao, X.;
Yeung, C. S.; Dong, V. M. J. Am. Chem. Soc. 2010, 132, 5837. (i) Wei, Y.;
Su, W. J. Am. Chem. Soc. 2010, 132, 16377. (j) He, C.-Y.; Fan, S.; Zhang,
X. J. Am. Chem. Soc. 2010, 132, 12850. (k) Xi, P.; Yang, F.; Qin, S.;
Zhao, D.; Lan, J.; Gao, G.; Hu, C.; You, J. J. Am. Chem. Soc. 2010, 132,
1822. (l) Karthikeyan, J.; Cheng, C.-H. Angew. Chem., Int. Ed. 2011, 50,
9880. (m) Gong, X.; Song, G.; Zhang, H.; Li, X. Org. Lett. 2011, 13,
1766. (n) Han, W.; Mayer, P.; Ofial, A. R. Angew. Chem., Int. Ed. 2011,
50, 2178. (o) Wang, Z.; Li, K.; Zhao, D.; Lan, J.; You, J. Angew. Chem.,
Int. Ed. 2011, 50, 5365. (p) Yamaguchi, A. D.; Mandal, D.; Yamaguchi,
J.; Itami, K. Chem. Lett. 2011, 40, 555. (q) Wu, G.; Zhou, J.; Zhang, M.;
Hu, P.; Su, W. Chem. Commun. 2012, 48, 8964. (r) Li, Z.; Ma, L.; Xu, J.;
Kong, L.; Wu, X.; Yao, H. Chem. Commun. 2012, 48, 3763.
(s) Pankajakshan, S.; Xu, Y.-H.; Cheng, J. K.; Low, M. T.; Loh, T.-P.
Angew. Chem., Int. Ed. 2012, 51, 5701.
(12) (a) Li, Z.; Li, C.-J. J. Am. Chem. Soc. 2005, 127, 3672. (b) Li, Z.;
Li, C. J. J. Am. Chem. Soc. 2005, 127, 6968. (c) Li, Z.; Li, C. J. J. Am.
Chem. Soc. 2006, 128, 56. (d) Li, C. J. Acc. Chem. Res. 2009, 42, 335. (e)
Qian, B.; Xie, P.; Xie, Y.; Huang, H. Org. Lett. 2011, 13, 2580. (f) Wei,
Y.; Deb, I.; Yoshikai, N. J. Am. Chem. Soc. 2012, 134, 9098. (g) Xie, Y.;
Hu, J.; Wang, Y.; Xia, C.; Huang, H. J. Am. Chem. Soc. 2012, 134,
20613. (h) Moon, Y.; Kwon, D.; Hong, S. Angew. Chem., Int. Ed. 2012,
51, 11333.
(13) (a) Zhang, M.; Zhou, J.; Kan, J.; Wang, M.; Su, W.; Hong, M.
Chem. Commun. 2010, 46, 5455. (b) Zhou, J.; Wu, G.; Zhang, M.; Jie, X.;
Su, W. Chem.;Eur. J. 2012, 18, 8032.
(14) Shang, Y.; Jie, X.; Zhou, J.; Hu, P.; Huang, S.; Su, W. Angew.
Chem., Int. Ed. 2013, 52, 1299.
With the optimized reaction conditions in hand
(10 mol % Pd(TFA)2, 4.0 equiv of AgOAc in DMSO/
(15) (a) Xiong, T.; Li, Y.; Bi, X.; Lv, Y.; Zhang, Q. Angew. Chem., Int.
Ed. 2011, 50, 7140. (b) Hu, P.; Huang, S.; Xu, J.; Shi, Z. J.; Su, W. Angew.
Chem., Int. Ed. 2011, 50, 9926. (c) Leskinen, M. V.; Yip, K.-T.;
Valkonen, A.; Pihko, P. M. J. Am. Chem. Soc. 2012, 134, 5750.
(16) (a) Tanaka, D.; Romeril, S. P.; Myers, A. G. J. Am. Chem. Soc.
2005, 127, 10323. (b) Steinhoff, B. A.; Stahl, S. S. J. Am. Chem. Soc.
2006, 128, 4348. (c) He, C.-Y.; Fan, S.; Zhang, X. J. Am. Chem. Soc.
2010, 132, 12850.
Org. Lett., Vol. 15, No. 7, 2013
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Table 1. Optimization Studies of Reaction Conditionsa
Table 2. Pd-Catalyzed Cross-Coupling of Simple Arenes with
Nitroethanea
1a
oxidant/additive
(equiv)
yield
(%)b
entry
(mL)
solvent
1
2
3
4
5
0.5
1.0
0.2
0.5
0.5
Ag2CO3 (2.0)
Ag2CO3 (2.0)
AgOAc (4.0)
AgOAc (4.0)
AgOAc (4.0)/
LiOAc (20 mol %)
AgOAc (4.0)/
K2CO3 (20 mol %)
AgOAc (4.0)/
PivOk (20 mol %)
AgOAc (4.0)
AgTFA (4.0)
BQ (2.0)
DME
DME
DME
DME
DME
trace
7
20
75 (81)
7
6
7
0.5
0.5
DME
DME
0
3
8c
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
DME
DME
DME
DME
DME
DEE
6
9
trace
0
10
11
12
13
14
15
16
17d
Cu(OAc)2 (2.0)
O2 (1 atm)
0
0
AgOAc (4.0)
AgOAc (4.0)
AgOAc (4.0)
AgOAc (4.0)
AgOAc (4.0)
69
73
23
18
72
dioxane
DMA
NMP
DME
a Reaction conditions: mesitylene 1a (excess), nitroethane
2
(0.2 mmol), Pd(TFA)2 (10 mol %), oxidant, DMSO (0.1 mL), solvent
(0.5 mL), 100 °C, 24 h. b GC yields with dodecane as internal standard
(values in parentheses refers to the isolated yield). c In absenceof DMSO.
d Under air. DEE = 1,2-diethyoxylethane. DMA = N,N-dimethylace-
tamide. NMP = N-methyl-2-pyrrolidone.
a Reaction conditions: arene (0.5 mL), nitroethane (0.2 mmol), Pd-
(TFA)2 (10 mol %), AgOAc (4.0 equiv), DMSO (0.1 mL), DME (0.5 mL),
100 °C, 24 h. b Isolated yield; the value given in parentheses denotes the
relative ratio of isomers determined by GC-MS. c 1,3,5-Trimethoxyben-
zene (3.0 equiv). d The ratio of isomers was determined by 1H NMR.
e DMSO (0.05 mL).
DME, 100 °C, 24 h), we next evaluated the scope of
substituted benzenes. As depicted in Table 2, an array of
substituted benzenes smoothly underwent the reaction
with nitroethane to produce substituted β-aryl nitroethy-
lenes exclusively. Since the size of nitroethane is relatively
small, the reactivity and regioselectivity of the reactions
were predominantly governed by the electronic effect of
the substituents rather than steric hindrance. Electron-
rich arenes such as 1,3,5-trimethoxybenzene, even though
sterically encumbered, provided the desired product in
good yield (3b), while benzene, toluene, and xylene under-
went the reaction in reduced yields (3c, 3d, 3e, 3h). Note
that only 3.0 equiv of electron-rich 1,3,5-trimethoxyben-
zene were required for a good yield. For m-xylene, the
cross-coupling mainly occurred at the more electron-rich
C2- and C4-position (3d). In the case of o-xylene, in which
the two different aromatic CÀH bonds are electronically
similar but have different steric environments, the reac-
tion preferred the less hindered C4-position over the
C3-position (3c). The monosubstituted benzenes gave a
mixture of ortho, meta, and para regioisomers (3e, 3f, and
3g), and the ratio of the three regioisomers depended on the
nature of the substituents. Toluene and chlorobenzene
offered the mixture with the ortho isomer slightly more than
the other two, while the small fluoro substitutent obviously
favored the reaction at its adjacent CÀH bond. 1,4-Difluor-
obenzene could also take part in this reaction to afford the
corresponding product in a good yield (3i). Other electron-
withdrawing substituents such as the ester, nitro, and cyano
group were ineffective. When other nitroalkanes such as
1-nitropropane, 2-nitropropane, and nitrocyclohexane were
induced to react with 1a under the established conditions,
the desired products were not obtained.
Given that the electron-donating group shows a bene-
ficial influence on the reactivity of substituted benzenes, we
speculated that the electron-rich heteroarene 4 might also
be suitable for the cross-coupling with nitroethane. We
were pleased to found that, when the ratio of 4/2 reversed
to 1:7 and the amount of solvent was increased to 2 mL, the
reaction was also applicable to a range of substituted ben-
zothiophenes, benzofurans, and indoles (Scheme 2). The
variety of substituents tolerated in these cross-couplings,
such as fluoro, chloro, bromo, methyl, and methoxyl,
provide opportunities for further synthetic elaboration.
Interestingly, in the cases of benzothiophenes and benzo-
furans, a substituent, at either the C2 or C3 position, was
1720
Org. Lett., Vol. 15, No. 7, 2013

required to obtain good yields, of which the cause is under
investigation (5aÀ5g, 5lÀ5s).
directed β-arylation of nitroethane,18 might be the inter-
mediate in this transformation. As nitroethylene was
reported to be unstable because of its facile polymeriza-
tion,19 the in situ dehydrogenation of nitroethane to a
nitroethylene intermediate cannot be completely ruled out
at present. Detailed studies are needed for insight into the
reaction mechanism.
Scheme 2. Pd-Catalyzed Cross-Coupling of Aromatic Hetero-
cycles with Nitroethanea
Scheme 3. Mechanistic Experiments
In conclusion, a Pd-catalyzed multidehydrogenative
cross-coupling reaction of (hetero)arenes with nitroethane
has been developed for the synthesis of β-aryl nitroethy-
lenes that are an important class of synthetic intermediates
and biologically active compounds. Importantly, our find-
ings further encouraged us to design and explore such a
multidehydrogenation based cross-coupling because it has
great potential in enabling the rapid construction of func-
tionalized compounds from simple starting materials in an
atom- and step-economical fashion. Further studies to
gain a deeper understanding of the reaction mechanism
and expand the scope of this transformation are ongoing in
our group.
a Reaction conditions: heterocycles (0.2 mmol), nitroethane (7.0 equiv),
Pd(TFA)2 (10 mol %), AgOAc (4.0 equiv), DMSO (0.1 mL), DME
(2.0 mL), 100 °C, 24 h. b DMSO (0.05 mL).
In our previous work on Pd-catalyzed 4-fold dehydro-
genative cross-coupling reaction of heterocycles with sat-
urated ketones,14 we presented detailed mechanistic
investigations for the possible reaction pathways of such
a catalytic process; finally in situ dehydrogenation of
ketones to vinyl ketones12h,13b,17 followed by olefination
of heterocycles was supported. Analogously, to probe the
potential intermediates in this transformation, we exam-
ined the reaction of mesitylene as well as benzene with
freshly prepared nitroethylene under standard conditions.
However, neither gave the expected 3a and 3h, respectively
(eqs 1 and 2, Scheme 3). Yet, we checked the behavior of
β-nitroethylbenzene under our standard conditions, which
afforded β-nitrostyrene 3h in 73% yield (eq 3, Scheme 3).
These observations demonstrated another possibility that
β-nitroethylbenzene, which may be generated via nitro
Acknowledgment. Financial support from the 973 Pro-
gram (2011CB932404, 2011CBA00501), NSFC (20925102)
is greatly appreciated.
Supporting Information Available. Detailed experimen-
tal procedures and characterization for products. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(18) (a) Zhang, X.; Emge, T. J.; Ghosh, R.; Krogh-Jespersen, K.;
Goldman, A. S. Organometallics 2006, 25, 1303. (b) Caron, L.; Campeau,
L.-C.; Fagnou, K. Org. Lett. 2008, 10, 4533.
(19) (a) Buckley, G. D.; Scaife, C. W. J. Chem. Soc. 1947, 1471.
(b) Ranganathan, S.; Ranganathan, D.; Mehrotra, A. K. J. Am. Chem.
Soc. 1974, 96, 5261. (c) Wiesner, M.; Revell, J. D.; Tonazzi, S.;
Wennemers, H. J. Am. Chem. Soc. 2008, 130, 5610.
(17) (a) Gao, W. M.; He, Z. Q.; Qian, Y.; Zhao, J.; Huang, Y. Chem.
Sci. 2012, 3, 883. (b) Diao, T.; Wadzinski, T. J.; Stahl, S. S. Chem. Sci.
2012, 3, 887. (c) Dobereiner, G. E.; Crabtree, R. H. Chem. Rev. 2010,
110, 681.
The authors declare no competing financial interest.
Org. Lett., Vol. 15, No. 7, 2013
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