Palladium-catalyzed trifluoromethylation of aromatic C-H bond directed by an acetamino group

Article abstract of DOI:10.1021/ol302814x

The first palladium-catalyzed ortho-trifluoromethylation of the aromatic C-H bond directed by an acetamino group is reported. This method provides an efficient and green approach to synthesize the highly biological potential key structure of ortho-CF₃ acetanilides and anilines.

Full text of DOI:10.1021/ol302814x

ORGANIC
LETTERS
XXXX
Vol. XX, No. XX
000–000
Palladium-Catalyzed Trifluoromethylation
of Aromatic CꢀH Bond Directed by an
Acetamino Group
Li-Sheng Zhang,† Kang Chen,† Guihua Chen,† Bi-Jie Li,† Shuang Luo,†
Qing-Yun Guo,† Jiang-Bo Wei,† and Zhang-Jie Shi*,†,‡
Beijing National Laboratory of Molecular Sciences (BNLMS) and Key Laboratory of
Bioorganic Chemistry and Molecular Engineering of Ministry of Education, College of
Chemistry and Green Chemistry Center, Peking University, Beijing 100871, China, and
State Key Laboratory of Natural and Biomimetic Drugs, School of Pharmaceutical
Sciences, Peking University, Xue Yuan Road 38, Beijing 100191, China
zshi@pku.edu.cn
Received October 16, 2012
ABSTRACT
The first palladium-catalyzed ortho-trifluoromethylation of the aromatic CꢀH bond directed by an acetamino group is reported. This method
provides an efficient and green approach to synthesize the highly biological potential key structure of ortho-CF3 acetanilides and anilines.
Presently, the trifluoromethyl group has been incorpo-
rated into significant structural motifs to improve the
physical, chemical, and biological properties of a large
number of organic compounds, such as pharmaceutical
molecules and drug candidates, because of its electronic
properties, special size, and great metabolic stability.1
Especially trifluoromethyl-substituted arenes have played
an important role in many synthetically valuable organic
compounds.2 Recently several Pd- and Cu-catalyzed tri-
fluoromethylation reactions of aryl halides3 and aryl boronic
acids4 have been reported, providing available methods
to synthesize trifluoromethyl-substituted arenes.
† Beijing National Laboratory of Molecular Sciences (BNLMS) and Key
Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry
of Education.
‡ State Key Laboratory of Natural and Biomimetic Drugs.
(1) (a) William, K.; Hagmann, W. K. J. Med. Chem. 2008, 51, 4359.
(b) Purser, S.; Moore, P. R.; Swallow, S.; Gouverneur, V. Chem. Soc.
Rev. 2008, 37, 320. (c) Muller, K.; Faeh, C.; Diederich, F. Science 2007,
317, 1881. (d) Kirk, K. L. Org. Process Res. Dev. 2008, 12, 305.
(e) Schlosser, M. Angew. Chem., Int. Ed. 2006, 45, 5432.
(2) (a) Ye, Y.; Sanford, M. S. Synlett 2012, 23, 2005. (b) Furuya, T.;
Kamlet, A. S.; Ritter, T. Nature 2011, 473, 470. (c) Tomashenko, O. A.;
Grushin, V. V. Chem. Rev. 2011, 111, 4475. (d) Wu, X.-F.; Neumann, H.;
Beller, M. Chem.;Asian J. 2012, 7, 1744. (e) Studer, A. Angew. Chem.,
Int. Ed. 2012, 51, 8950.
For a direct transformation from the aromatic CꢀH
bond to the CꢀCF3 bond, various efficient methods, involv-
ing electrophilic substitution and a Minisci type reaction,
have also been developed.5 Furthermore, to achieve the
(3) (a) Oishi, M.; Kondo, H.; Amii, H. Chem. Commun. 2009, 1909.
(b) Cho, E. J.; Senecal, T. D.; Kinzel, T.; Zhang, Y.; Watson, D. A.;
Buchwald, S. L. Science 2010, 328, 1679. (c) Morimoto, H.; Tsubogo, T.;
Litvinas, N. D.; Hartwig, J. F. Angew. Chem., Int. Ed. 2011, 50, 3793.
(4) (a) Chu, L.; Qing, F.-L. Org. Lett. 2010, 12, 5060. (b) Xu, J.; Luo,
D.-F.; Xiao, B.; Liu, Z.-J.; Gong, T.-J.; Fu, Y.; Liu, L. Chem. Commun.
2011, 47, 4300. (c) Liu, T.; Shen, Q. Org. Lett. 2011, 13, 2342. (d) Senecal,
T. D.; Parsons, A. T.; Buchwald, S. L. J. Org. Chem. 2011, 76, 1174.
(e) Novak, P.; Lishchynskyi, A.; Grushin, V. V. Angew. Chem., Int. Ed.
2012, 51, 7767. (f) Ye, Y.; Sanford, M. S. J. Am. Chem. Soc. 2012, 134,
9034. (g) Qi, Q.; Shen, Q.; Lu, L. J. Am. Chem. Soc. 2012, 134, 6548.
(5) (a) Wiehn, M. S.; Vinogradova, E. V.; Togni, A. J. Fluorine Chem.
2010, 131, 951. (b) Mejıa, E.; Togni, A. ACS Catal. 2012, 2, 521. (c) Ji, Y.;
Brueckl, T.; Baxter, R. D.; Fu-jiwara, Y.; Seiple, I. B.; Su, S.; Blackmond,
D. G.; Baran, P. S. Proc. Natl. Acad. Sci. U.S.A. 2011, 108, 14411.
(d) Nagib, D. A.; MacMillan, D. W. C. Nature 2011, 480, 224.
€
(d) Knauber, T.; Arikan, F.; Roschenthaler, G.-V.; Gooßen, L. Chem.;
Eur. J. 2011, 17, 2689. (e) Zhang, C.-P.; Wang, Z.-L.; Chen, Q.-Y.;
Zhang, C.-T.; Gu, Y.-C.; Xiao, J.-C. Angew. Chem., Int. Ed. 2011, 50,
1896. (f) Zanardi, A.; Novikov, M. A.; Martin, E.; Benet-Buchholz, J.;
Grushin, V. V. J. Am. Chem. Soc. 2011, 133, 20901. (g) Tomashenko,
O. A.; Escudero-Adan, E. C.; Belmonte, M. M.; Grushin, V. V. Angew.
Chem., Int. Ed. 2011, 50, 7655. (h) Dobele, M.; Wiehn, M. S.; Brase, S.
Angew. Chem., Int. Ed. 2011, 50, 11533. (i) Popov, I.; Lindeman, S.;
Daugulis, O. J. Am. Chem. Soc. 2011, 133, 9286. (j) Samant, B. S.;
Kabalka, G. W. Chem. Commun. 2011, 47, 7236.
r
10.1021/ol302814x
XXXX American Chemical Society

selective transformation of the aromatic CꢀH bond to the
CꢀCF3 bond through direct CꢀH activation via transition
metal catalysis, several effective methods have also been
reported. Sanford et al. reported Ag-mediated trifluoro-
methylation of arenes using TMSCF36b and Pd-catalyzed
CꢀH perfluoroalkylation of arenes.6c Brase et al. devel-
oped the Ag-mediated selective ortho-triluoromethylation
of functionalized aromatic triazenes.7 Yu et al. reported
the Pd(II)-catalyzed ortho-trifluoromethylation of 2-phe-
nylpyridines and N-arylbenzamides derived from readily
available benzoic acids.8a,b Hartwig and Shen simulta-
neously reported the highly selective trifluoromethyla-
tion of 1,3-disubstituted arenes through the sequence of
Ir-catalyzed borylation of arenes and subsequent trifluoro-
methylation.9 Qing et al. developed Cu-catalyzed direct
CꢀH oxidative trifluoromethylation of heteroarenes.10
Liu et al. reported palladium-catalyzed oxidative trifluor-
omethylation of indoles at rt.11b Herein we report the
Pd(II)-catalyzed trifluoromethylation of the aromatic
CꢀH bond directed by an acetamino group, which pro-
vides an efficient and green approach to produce the highly
biological potential key structure of ortho-CF3 acetanilides
and anilines such as Dutasteride,12a Mabuterol,12b and
Triflumizole.12c Notably, the utility of the acetamino
group provided other opportunities to transform it into
different functional groups through existing methods
(Scheme 1).
reactivity in our previous studies (Table 1).13 The first
reaction was performed in dichloroethane with Pd(OAc)2
as the catalyst and Togni’s reagent14 as the trifluoromethy-
lation reagent, which has been broadly used and shows
high reactivity (entry 1). Although we observed a trace
amount of desired product of ortho-trifluoromethylation,
the transformation could also occur in the absence of any
transition metal catalyst, which confirmed that Togni’s
reagent might decompose and generate a CF3 radical
under relatively high temperature.5b We next tried to
explore the TESCF3/Fꢀ system11 with PhI(OAc)2 as the
oxidant to achieve the conversion via the Pd(IV) pathway
(entry 2). Unfortunately, several products with ortho-,
meta-, and para- trifluoromethylation were observed in a
trace amount detected by GC-MS, but we failed to pro-
mote the selectivity to approach a sole product. Subse-
quently, we tested Umemoto’s reagent 2,15 combined with
different oxidants such as PhI(OAc)2, K2S2O8, BQ, N-
fluorobenzenesulfonimide, and Ce(SO4)2 as well as Ag and
Cu salts in the presence of Pd(OAc)2 as the catalyst (entreis
3ꢀ7). Actually, the yield could be slightly improved with
Cu(OAc)2, CuF2, or Cu(TFA)2 in the system compared
with other various oxidants. 15% of desired product 3a (by
GC with n-dodecane as the internal standard) was ob-
tainedin the presenceof Cu(OAc)2 (1.0 equiv) (entry 7). To
our delight, PivOH (10.0 equiv) improved the yield to 49%
(by GC), while either other acids or bases were not
efficient. Increasing or decreasing the amount of PivOH
slightly diminished the yield (entries 10ꢀ15). To our de-
light, the yield of product was promoted to 65% when the
amount of Cu(OAc)2 was increased to 2.0 equiv (entries
16ꢀ17). Moreover, the yield was further enhanced by
increasing the amount of Umemoto’s reagent to 1.5 equiv
(entry 18). Finally, we found the yield reached an optimal
level when 2.2 equiv of Cu(OAc)2 were used (entry 19). A
further increase of Cu(OAc)2 did not affect the efficiency
(entries 19ꢀ20). Other transition metal catalysts, such as
[Cp*Rh(MeCN)3](SbF6)2 or [Cp*RhCl2]2, did not pro-
mote the transformation.
Scheme 1. Drug Molecules and Possible Transformations of the
Acetamino Moiety into a Series of Functional Groups
With the above optimized conditions in hand, we further
investigated various acetanilides to explore the application
of the reaction (Scheme 2). To explore the influence of
directing groups, we first tested different substrates
with carbonyl groups. Other than the acetyl group,
pivaloyl and benzoyl groups showed partial reactivity
With this in mind, we initially chose acetanilide 1a as the
starting material, which had shown relatively high
(12) (a) Satyanarayana, K.; Srinivas, K.; Himabindu, V.; Reddy,
G. M. Org. Process Res. Dev. 2007, 11, 842. (b) Krueger, G.; Keck, J.;
Noll, K.; Pieper, H. Arzneim.-Forsch. 1984, 34, 1612. (c) Watanabe, E.;
Watanabe, S.; Ito, S.; Hayashi, M.; Watanabe, T.; Yuasa, Y.; Nakazawa,
H. J. Agric. Food Chem. 2000, 48, 5124.
(13) (a) Wan, X.; Ma, Z.; Li, B.-J.; Zhang, K.; Cao, S.; Zhang, S.; Shi,
Z.-J. J. Am. Chem. Soc. 2006, 128, 7416. (b) Yang, S.-D.; Li, B.-J.; Wan,
X.; Shi, Z.-J. J. Am. Chem. Soc. 2007, 129, 6066. (c) Li, B.-J.; Tian, S.-L.;
Fang, Z.; Shi, Z.-J. Angew. Chem., Int. Ed. 2008, 47, 1115. (d) Shi, Z.; Li,
B.-J.; Wan, X.; Cheng, J.; Fang, Z.; Cao, B.; Qin, C.; Wang, Y. Angew.
Chem., Int. Ed. 2007, 46, 5554.
(14) (a) Eisenberger, P.; Gischig, S.; Togni, A. Chem.;Eur. J. 2006,
12, 2579. (b) Kieltsch, I.; Eisenberger, P.; Stanek, K.; Togni, A. Chimia
2008, 62, 260.
(15) (a) Umemoto, T.; Ishihara, S. Tetrahedron Lett. 1990, 31, 3579.
(b) Umemoto, T.; Ishihara, S. J. Am. Chem. Soc. 1993, 115, 2156.
(c) Umemoto, T.; Ishihara, S. J. Fluorine Chem. 1998, 92, 181.
(6) For highlights, see: (a) Pan, F.; Shi, Z.-J. Acta Chimica Sinica
2012, 70, 1679. (b) Ye, Y.; Lee, S. H.; Sanford, M. S. Org. Lett. 2011, 13,
5464. (c) Loy, R. N.; Sanford, M. S. Org. Lett. 2011, 13, 2548.
€
(7) Hafner, A.; Brase, S. Angew. Chem., Int. Ed. 2012, 51, 3713.
(8) (a) Wang, X.; Truesdale, L.; Yu, J.-Q. J. Am. Chem. Soc. 2010,
132, 3648. (b) Zhang, X.-G.; Dai, H.-X.; Wasa, M.; Yu, J.-Q. J. Am.
Chem. Soc. 2012, 134, 11948.
(9) (a) Litvinas, N. D.; Fier, P. S.; Hartwig, J. F. Angew. Chem., Int.
Ed. 2012, 51, 536. (b) Liu, T.; Shao, X.; Wu, Y.; Shen, Q. Angew. Chem.,
Int. Ed. 2012, 51, 540.
(10) Chu, L.; Qing, F.-L. J. Am. Chem. Soc. 2012, 134, 1298.
(11) (a) Mu, X.; Wu, T.; Wang, H.-Y.; Guo, Y.-L.; Liu, G. J. Am.
Chem. Soc. 2012, 134, 878. (b) Mu, X.; Chen, S.; Zhen, X.; Liu, G.
Chem.;Eur. J. 2011, 17, 6039.
B
Org. Lett., Vol. XX, No. XX, XXXX

Table 1. Optimization of Reaction Conditions
Scheme 2. Palladium-Catalyzed ortho-Trifluoromethylation of
Different Acetanilides 1 Using Umemoto’s Reagent 2a,b
entrya
oxidant (equiv)
additive (equiv)
yieldb
1
ꢀ
ꢀ
tracec
2
PhI(OAc)2 (1.0)
PhI(OAc)2 (1.0)
K2S2O8 (1.0)
BQ (l.0)
CsF (2.0)
ꢀ
traced
3
0%
4
ꢀ
trace
trace
trace
15%
5
ꢀ
6
AgOAc (l.0)
ꢀ
7
Cu(OAc)2 (1.0)
Cu(OAc)2 (1.0)
Cu(OAc)2 (1.0)
Cu(OAc)2 (1.0)
Cu(OAc)2 (1.0)
Cu(OAc)2 (1.0)
Cu(OAc)2 (1.0)
Cu(OAc)2 (1.0)
Cu(OAc)2 (1.0)
Cu(OAc)2 (1.5)
Cu(OAc)2 (2.0)
Cu(OAc)2 (2.0)
Cu(OAc)2 (2.2)
Cu(OAc)2 (2.5)
Cu(OAc)2 (2.2)
Cu(OAc)2 (2.2)
ꢀ
8
TFA (10.0)
AcOH (10.0)
PivOH (10.0)
PivOH (20.0)
PivOH (50.0)
ꢀ
16%
9
15%
10
11
12
13
14
15
16
17
18f
19f
20f
21f
22f
49%
46%
trace
0%e
PivOH (5.0)
PivOH (2.0)
PivOH (5.0)
PivOH (5.0)
PivOH (5.0)
PivOH (5.0)
PivOH (5.0)
PivOH (5.0)
PivOH (5.0)
53%
50%
58%
65%
72%
76% (69%)
76%
0%g
43%h
a Reaction conditions: 0.10 mmol of substrate, 0.15 mmol of
Umemoto’s reagent, 0.01 mmol of Pd(OAc)2 as the catalyst, 0.22 mmol
of Cu(OAc)2, 0.50 mmol of PivOH, 1.0 mL of ClCH2CH2Cl, 24 h.
bIsolated yield. The details for reaction conditions are described in the
Supporting Information.
a Reaction conditions: 0.10 mmol of acetanilide, 0.12 mmol of
Umemoto’s reagent, 0.01 mmol Pd(OAc)2 as the catalyst, 1.0 mL of
ClCH2CH2Cl, 24 h. b Yields are determined by GC analysis using
n-dodecane as an internal standard. Yields of isolated product are given
within parentheses. c 0.12 mmol of Togni’s reagent was used. d 0.20 mmol
of TESCF3 was used. e PivOH was used as the solvent. f 0.15 mmol of
Umemoto’s reagent. g Without Pd(OAc)2 as the catalyst. h At 90 °C.
directing acetamino group, was also accommodated in
our method (3m). The presence of a phenyl group at the
para-position decreased the yield to 46% due to its incom-
plete conversion within 24 h (3d). ortho-Methyl substituted
acetanilide was submitted, and an isolated yield of 51%
was achieved (3u). The test of the substituents at the meta-
position indicated that, in most cases, only a sole product
was isolated at the less hindered position in 47ꢀ66%
yields, which may arise from steric effects (3nꢀr). Unfor-
tunately, a methoxyl group or a trifluoromethyl group at
the meta-position obviously inhibited the reaction (3sꢀt).
(3wꢀx). Methyl-N-phenylacetamide is not a proper sub-
strate, and no conversion was observed (3v). Later on, we
tested various substituted acetanilides. To our interest,
para-substituents did not obviously affect the efficacy.
For instance, with alkyl groups at the para-position, such
asmethyland tert-butylgroups, the desiredproduct 3band
3c were obtained in 63% and 77% yields, respectively. To
our delight, chloro-, bromo-, and fluoro- groups at the
para-position were compatible and afforded the desired
products in 72%, 66%, and 71% yields (3eꢀg). Surpris-
ingly, the reaction tolerated the existence of the most active
CꢀI bond at the para-position, affording the desired
product in 48% yield (3h). Such group tolerances provided
great potential for orthogonal functionalization.16 No
observation of the trifluoromethylation of CꢀX bonds
indicated that such a transformation did not go through
the sequence of halogenation/trifluoromethylation though
the halogenation is possible under this condition. In fact, a
trace amount of chlorination product was observed with
1,2-dichloroethane as the solvent. Moreover, electron-
withdrawing groups such as ꢀCO2Et, ꢀCO2Me, and
ꢀCOPh were also well tolerated under the reaction condi-
tions and provided the product in 83%, 64%, and 72%
yields, respectively (3iꢀk). Comparably, the acetyl group
gave a lower yield with starting material remaining (3l).
Notably, the acetoxyl group, which is similar to the
Scheme 3. Transformations of the Acetamino Moiety through
Existing Methods
Furthermore, starting from the ortho-trifluoromethyl
product 3a, the acetamino moiety could be easily
(17) (a) Wang, G.-B.; Wang, L.-F.; Li, C.-Z.; Sun, J.; Zhou, G.-M.;
Yang, D.-C. Res. Chem. Intermed. 2012, 38, 77. (b) Satyanarayana, K.;
Srinivas, K.; Himabindu, V.; Reddy, G. M. Org. Process Res. Dev. 2007,
11, 842. (c) Sidney, D. R.; Markarian, M.; Schwarz, M. J. Am. Chem.
Soc. 1953, 75, 4967.
(16) Lyons, T.; Sanford, M. S. Chem. Rev. 2010, 110, 1147.
Org. Lett., Vol. XX, No. XX, XXXX
C

transformed into an amino group which could be trans-
formed into different functional groups through existing
methods successfully (Scheme 3).17,21
39% (eq 5). Based on these studies, a plausible mechanism
is proposed in Scheme 5.
Scheme 5. A Plausible Mechanism
Scheme 4. Experiments for Mechanism Research
First, directed by the acetamino group, the reaction was
initiated by the palladation of acetanilide 1a to afford the
palladacycle 4a. In the presence of Cu(OAc)2 and Ume-
moto’s reagent 2, a Pd(IV) intermediate 5a was supposedly
generated in the system. Finally, the desired product of
trifluoromethylation was afforded after reductive elimþina-
tion. Alternatively, the electrophilic attack from CF3 to
the PdꢀC bond16 or the formation of the bis-Pd(III)
complex as a key intermediate18a could not be excluded
at this stage. Since the coordination ability of the amide
carbonyl group was not strong enough, the addition of a
stoichiometric amount of a Cu salt could coordinate with
dibenzothiophene generated by Umemoto’s reagent to
maintain the activity of the Pd species.8b Other functions
of a Cu salt in this system are still unclear, requiring further
study.
In summary, we have developed an efficient Pd(II)-
catalyzed trifluoromethylation of the aromatic CꢀH bond
directedbyanacetaminogroup. Thebroadsubstratescope
makes this method potentially useful for the synthesis of
many biologically active molecules. The downstream ex-
periments from the product highly explored the applica-
tion of this method. Furthermore, a series of experiments
were performed to support the proposed catalytic cycle.
Further application of this method is underway.
To unveil the catalytic mechanism, a series of experi-
ments were conducted (Scheme 4).18,19 The KIE for inter-
molecular, intramolecular, and parallel experiments22 (1.8,
2.3, and 4.6, respectively) suggested that the CꢀH cleavage
was involved in the rate-determining step (rds) at the
catalytic cycle (eqs 1ꢀ3). To obtain more details about
the reaction mechanism, we performed the reaction by
using the palladacycle complex 4a, which was prepared
according to ref 20. To our delight, the desired product 3a
or 3i was obtained in a comparable yield of 72% and 68%,
respectively, when 4a was used as the catalyst (eq 4). The
stoichiometric reaction from 4a under the same conditions
also afforded the desired product 3a, albeit in a GC yield of
(18) (a) Powers, D. C.; Ritter, T. Acc. Chem. Res. 2012, 45, 840.
(b) Powers, D. C.; Geibel, M. A. L.; Klein, J. E. M. N.; Ritter, T. J. Am.
Chem. Soc. 2009, 131, 17050. (c) Powers, D. C.; Lee, E.; Ariafard, A.;
Sanford, M. S.; Yates, B. F.; Canty, A. J.; Ritter, T. J. Am. Chem. Soc.
2012, 134, 12002. (d) Powers, D. C.; Benitez, D.; Tkatchouk, E.; God-
dard, W. A.; Ritter, T. J. Am. Chem. Soc. 2010, 132, 14092. (e) Furuya,
T.; Benitez, D.; Tkatchouk, E.; Strom, A. E.; Tang, P.; Goddard, W. A.;
Ritter, T. J. Am. Chem. Soc. 2010, 132, 3793. (f) Powers, D. C.; Xiao,
D. Y.; Geibel, M. A. L.; Ritter, T. J. Am. Chem. Soc. 2010, 132, 14530.
(19) (a) Ball, N. D.; Gary, J. B.; Ye, Y.; Sanford, M. S. J. Am. Chem.
Soc. 2011, 133, 7577. (b) Nicholas, D. B.; Kampf, J. W.; Sanford, M. S.
J. Am. Chem. Soc. 2010, 132, 2878. (c) Ye, Y.; Ball, N. D.; Kampf, J. W.;
Sanford, M. S. J. Am. Chem. Soc. 2010, 132, 14682. (d) Racowski, J. M.;
Ball, N. D.; Sanford, M. S. J. Am. Chem. Soc. 2011, 133, 18022. (e) Ball,
N. D.; Sanford, M. S. J. Am. Chem. Soc. 2009, 131, 3796.
Acknowledgment. Support of this work by the “973”
Project from the MOST of China (2009CB825300)
and NSFC (Nos. 20925207 and 21002001) is gratefully
acknowledged.
(20) (a) Horino, H.; Inoue, N. J. Org. Chem. 1981, 46, 4416. (b) Ueda,
S.; Nagasawa, H. Angew. Chem., Int. Ed. 2008, 47, 6411.
(21) Huang, A.; Moretto, A.; Janz, K.; Lowe, M.; Bedard, P. W.;
Tam, S.; Di, L.; Clerin, V.; Sushkova, N.; Tchernychev, B.; Tsao,
D. H. H.; Keith, J. C.; Shaw, G. D.; Schaub, R. G.; Wang, Q.; Kaila,
N. J. Med. Chem. 2010, 53, 6003.
Supporting Information Available. Experimental pro-
cedures and spectral data for products and mechanistic
study experiments. This material is available free of
charge via the Internet at http://pubs.acs.org.
(22) Wang, H.; Grohmann., C.; Nimphius., C.; Glorius, F. J. Am.
Chem. Soc. 2012, 134, 19592.
The authors declare no competing financial interest.
D
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