Copper-catalyzed enantioselective intramolecular N -arylation, an efficient method for kinetic resolutions

Article abstract of DOI:10.1021/ol401449b

For the first time, copper-catalyzed intramolecular N-arylation was successfully applied to the kinetic resolution strategy. Under the catalysis of CuI-BINOL derived ligands, the kinetic resolution of rac-2-amino-3-(2-iodoaryl) propionates and rac-2-amino-4-(2-iodoaryl)butanoates afforded chiral intramolecular coupling products and recovered the starting materials in high enantioselectivity (s factors up to 245).

Full text of DOI:10.1021/ol401449b

ORGANIC
LETTERS
2013
Vol. 15, No. 14
3598–3601
Copper-Catalyzed Enantioselective
Intramolecular N‑Arylation, an Efficient
Method for Kinetic Resolutions
Wenqiang Yang,†,‡ Yan Long,§ Shasha Zhang,† Youlin Zeng,§ and Qian Cai*,†,‡
Guangzhou Institutes of Biomedicine and Health, Chinese Academy of Sciences,
No.190 Kaiyuan Avenue, Guangzhou Science Park, Guangzhou 510530, China, State
Key Laboratory of Elemento-organic Chemistry, Nankai University, No. 94 Weijin
Road, Tianjin 300071,China, and College of Chemistry and Chemical Engineering,
Hunan Normal University, No. 36 Lushan Road, Changsha, Hunan 410081, China
cai_qian@gibh.ac.cn
Received May 22, 2013
ABSTRACT
For the first time, copper-catalyzed intramolecular N-arylation was successfully applied to the kinetic resolution strategy. Under the catalysis of
CuIꢀBINOL derived ligands, the kinetic resolution of rac-2-amino-3-(2-iodoaryl)propionates and rac-2-amino-4-(2-iodoaryl)butanoates afforded
chiral intramolecular coupling products and recovered the starting materials in high enantioselectivity (s factors up to 245).
Functionalized aromatic and heteroaromatic amines
have found extensive applications as key players in the
synthesis of bioactive natural products, important phar-
maceuticals, and materials.1 Because of their widespread
importance, many methods have been developed for the
formation of the aryl CꢀN bond, in which transition
metals, such as Pd,2 Cu,3 Ni,4 etc., catalyzed coupling
reactions of aryl halides with amines may be the most
powerful methods. However, enantioselective N-arylation
remains a remarkable challenge. In such reactions, the
bond formation is between a sp2 planar geometric carbon
and a nitrogen atom which is usually not a configuration-
ally stable chiral center, although NꢀC axially chiral
compounds may be formed through Pd-catalyzed N-aryla-
tion reactions, as has been successfully achieved by the
prominent work of Kitagawa and Taguchi since 2005.5
Besides, for substrates bearing prochiral or achiral carbon
centers which did not directly participate in the bond
† Guangzhou Institutes of Biomedicine and Health.
‡ Nankai University.
§ Hunan Normal University.
(1) (a) Lawrence, S. A. Amines: Synthesis Properties and Application;
Cambridge University: Cambridge, 2004. (b) Breuer, M.; Ditrich, K.;
Habicher, T.; Hauer, B.; Keßeler, M.; St€urmer, R.; Zelinski, T. Angew.
Chem., Int. Ed. 2004, 43, 788.
(2) For selected reviews and books about Pd-catalyzed CꢀN bond
formation, see: (a) Yudin, A. K.; Hartwig, J. F. Catalyzed Carbonꢀ
Heteroatom Bond Formation; Wiley-VCH: Weinheim, 2010. (b) Maiti, D.;
Fors, B. P.; Henderson, J. L.; Nakamura, Y.; Buchwald, S. L. Chem. Sci.
2011, 2, 57. (c) Hartwig, J. F. Nature 2008, 455, 314. (d) Marion, N.;
Nolan, S. P. Acc. Chem. Res. 2008, 41, 1440. (e) Hartwig, J. F. Acc.
Chem. Res. 2008, 41, 1534. (f) Surry, D. S.; Buchwald, S. L. Angew.
Chem., Int. Ed. 2008, 47, 6338. (g) Corbet, J.-P.; Mignani, G. Chem. Rev.
2006, 106, 2651.
(3) For recent reviews about copper-catalyzed CꢀN bond formation,
see: (a) Ley, S. V.; Thomas, A. W. Angew. Chem., Int. Ed. 2003, 42, 5400.
(b) Beletskaya, I. P.; Cheprakov, A. V. Coord. Chem. Rev. 2004, 248,
2337. (c) Evano, G.; Blanchard, N.; Toumi, M. Chem. Rev. 2008, 108,
3054. (d) Monnier, F.; Taillefer, M. Angew. Chem., Int. Ed. 2009, 48,
6954. (e) Ma, D.; Cai, Q. Acc. Chem. Res. 2008, 41, 1450. (f) Surry, D. S.;
Buchwald, S. L. Chem. Sci. 2010, 1, 13.
(4) For books and examples of Ni-catalyzed N-arylation, see: (a)
Modern Organonickel Chemistry; Tamaru, Y., Ed.; Wiley-VCH: Weinheim,
2005. (b) Wolfe, J. P.; Buchwald, S. L. J. Am. Chem. Soc. 1997, 119, 6054.
(c) Omar-Amrani, R.; Thomas, A.; Brenner, E.; Schneider, R.; Fort, Y.
Org. Lett. 2003, 5, 2311. (d) Gao, C. Y.; Cao, X. B.; Yang, L. M. Org.
Biomol. Chem. 2009, 7, 3922. (e) Hie, H.; Ramgren, S. D.; Mesganaw, T.;
Garg, N. K. Org. Lett. 2012, 14, 4182.
(5) For examples of Pd-catalyzed enantioselective N-Arylation for
the formation of optically active atropisomeric compounds with NꢀC
chiral axis, see: (a) Kitagawa, O.; Takahashi, M.; Yoshikawa, M.;
Taguchi, T. J. Am. Chem. Soc. 2005, 127, 3676. (b) Kitagawa, O.;
Yoshikawa, M.; Tanabe, H.; Morita, T.; Takahashi, M.; Dobashi, Y.;
Taguchi, T. J. Am. Chem. Soc. 2006, 128, 12923. (c) Kitagawa, O.;
Kurihara, D.; Tanabe, H.; Shibuya, T.; Taguchi, T. Tetrahedron Lett.
2008, 49, 471. (d) Takahashi, M.; Tanabe, H.; Nakamura, T.; Kuribara,
D.; Yamazaki, T.; Kitagawa, O. Tetrahedron 2010, 66, 288. (e) Takahashi,
I.; Morita, F.; Kusagaya, S.; Fukaya, H.; Kitagawa, O. Tetrahedron
Asymmetry 2012, 23, 1657.
r
10.1021/ol401449b
Published on Web 07/05/2013
2013 American Chemical Society

formation at the reactive site, little attention has been
focused on asymmetric N-arylation reactions. So far, only
sporadic examples have been reported in this field. Gen-
erally, two strategies may be utilized to achieve enan-
tioselectivity: asymmetric desymmetrization6 and kinetic
resolution,7 both of which have been successfully implemen-
ted in Pd-catalyzed asymmetric N-arylation reactions,8,9
albeit only low to moderate enantioselectivity was obtained
in most cases.
Based on the asymmetric desymmetrization strategy,
our laboratory has developed the first copper-catalyzed
highly enantioselective intramolecular N-arylation reac-
tion, which afforded chiral indolines and 1,2,3,4-tetra-
hydroquinolines in high yields and excellent enantio-
selectivity.10 However, such desymmetrization reactions
have some disadvantages: the substrate scope is limited
and the products bear two aryl rings with similar substi-
tuents, which may cause problems in later selective trans-
formations. To overcome such problems and increase
the applicability of asymmetric N-arylation reactions,
we envisioned that the kinetic resolution strategy may be
a good choice for achieving enantioselectivity through
copper-catalyzed intramolecular aryl CꢀN coupling.
According to this strategy, the kinetic resolution of
racemic 2-(2-iodophenyl)ethanamine derivative (1) by
copper-catalyzed intramolecular N-arylation reaction,
would deliver indolines (2) and recover the starting materi-
al 1 in high enantioselectivities (Scheme 1). The kinetic
resolutions of amines have been extensively studied,11,12
however, mainly through acylations. The reported exam-
ples for resolutions of amines by enantioselective N-aryl-
tionarerareand the selectivitiesarepoor tomodestinmost
cases.9 To the best of our knowledge, it is the first time for
the kinetic resolution of amines to be achieved through
copper-catalyzed coupling reactions. Herein we report the
details of this research.
Scheme 1. Copper-Catalyzed Enantioselective Intramolecualr
N-Arylation via Kinetic Resolution Strategy
In our previous work, we have realized that an ester
group in the prochiral center is beneficial to the enantio-
selectivity of copper-catalyzed desymmetric N-arylation
reactions, and it is also an important group for further
functionalization.10 Thus, in this work, we keep the ester
group as an important directing group in the substrates
and our investigation was started with the kinetic resolu-
tion of racemic methyl 2-amino-2-benzyl-3-(2-iodophenyl)-
propanoate 1a under the catalysis of 5 mol % of CuI and
6 mol % of BINOL-derived ligands and with 100 mol %
of Cs2CO3 as the base.13,14 As shown in Table 1, by using
(R)-BINOL (L1) as the ligand, 1a was recovered in 32% ee
and 2awas isolated in 57% ee (36% conversion, s-factor =
4.9)15 after 0.5 h in 1,4-dioxane at room temperature
(Table 1, entry 1). Next, several 3,30-biaryl-substituted
(R)-BINOL ligands were examined, and better selectivities
were obtained with bulkier aryl substituents in the 3,30-
positions of the BINOL backbone (Table 1, entries 2ꢀ5),
which is consistent with our previous results in copper/
BINOL-catalyzed asymmetric desymmetric N-arylation
reactions. As observed, the CuIꢀL5-catalyzed reaction
showed the best result, which afforded the coupling pro-
duct 2a in 89% ee and recovered 1a in 94% ee (51%
conversion, s = 70.0) in 4 h (Table 1, entry 5). Further,
a variety of solvents such as THF, MeCN, DMF, and
toluene were screened and all gave inferior results under
the similar reaction conditions (Table 1, entries 6ꢀ9).
With the optimized conditions in hand, we then inves-
tigated the scope of substrates for kinetic resolution by
copper-catalyzed coupling reactions and the results are
shown in Table 2. First, the substrates bearing different
(6) For some important reviews about asymmetric desymmetriza-
tion, see: (a) Garcıa-Urdiales, E.; Alfonso, I.; Gotor, V. Chem. Rev.
´
2005, 105, 313. (b) Willis, M. C. J. Chem. Soc., Perkin Trans. 1 1999,
1765. (c) Studer, A.; Schleth, F. Synlett 2005, 3033. (d) Rovis, T. In New
Frontiers in Asymmetric Catalysis; Mikami, K., Lautens, M., Eds.; John
Wiley & Sons, Inc.: New York, 2007; pp 275ꢀ309. (e) Atodiresei, I.;
Schiffers, I.; Bolm, C. Chem. Rev. 2007, 107, 5683.
(7) For some important reviews about kinetic resolution, see: (a)
Vedejs, E.; Jure, M. Angew. Chem., Int. Ed. 2005, 44, 3974. (b) Huerta,
€
F. F.; Minidis, A. B. E.; Backwall, J.-E. Chem. Soc. Rev. 2001, 30, 321. (c)
Noyori, R.; Tokunaga, M.; Kitamura, M. Bull. Chem. Soc. Jpn. 1995,
68, 36.
(8) For examples of the Pd-catalyzed enantioselective Buchwaldꢀ
Hartwig N-arylation via desymmetrization strategy, see: (a) Takenaka,
K.; Itoh, N.; Sasai, H. Org. Lett. 2009, 11, 1483. (b) Porosa, L.; Viirre,
R. D. Tetrahedron Lett. 2009, 50, 4170.
(9) For selective examples of the BuchwaldꢀHartwig reaction using
kinetic resolution of racemic substrates, see: (a) Rossen, K.; Pye, P. J.;
Maliakal, A.; Volante, R. P. J. Org. Chem. 1997, 62, 6462. (b) Tagashira,
J.; Imao, D.; Yamamoto, T.; Ohta, T.; Furukawa, I.; Ito, Y. Tetrahe-
€
dron: Asymmetry 2005, 16, 2307. (c) Kreis, M.; Friedmann, C. J.; Brase,
S. Chem.;Eur. J. 2005, 11, 7387.
(10) Zhou, F.; Guo, J.; Liu, J.; Ding, K.; Yu, S.; Cai, Q. J. Am. Chem.
Soc. 2012, 134, 14326.
(11) Selected reviews on enzymatic resolutions of amines: (a) Lee,
J. H.; Han, K.; Kim, M.-J.; Park, J. Eur. J. Org. Chem. 2010, 99. (b)
Gotor-Fernandez, V.; Gotor, V. Curr. Opin. Drug. Discovery Dev. 2009,
€
12, 784. (c) Hohne, M.; Bornscheuer, U. T. ChemCatChem 2009, 1, 42.
(12) For examples of kinetic resolution or dynamic kinetic resolution
of amines or amides by acylations or other reacitons, see: (a) Reetz,
M. T.; Schimossek, K. Chimia 1996, 50, 668. (b) Parvulescu, A.; De Vos,
€
D.; Jacobs, P. Chem. Commun. 2005, 5307. (c) Paetzold, J.; Backvall,
J. E. J. Am. Chem. Soc. 2005, 127, 17620. (d) Kim, M.-J.; Kim, W.-H.;
Han, K.; Choi, Y. K.; Park, J. Org. Lett. 2007, 9, 1157. (e) De, C. K.;
Klauber, E. G.; Seidal, D. J. Am. Chem. Soc. 2009, 131, 17060. (f)
Klauber, E. G.; De, C. K.; Shah, T. K.; Seidal, D. J. Am. Chem. Soc.
2010, 132, 13624. (g) De, C. K.; Seidel, D. J. Am. Chem. Soc. 2011, 133,
14538. (h) Mittal, N.; Sun, D. X.; Seidel, D. Org. Lett. 2012, 14, 3084. (i)
Binanzer, M.; Hsieh, S. Y.; Bode, J. W. J. Am. Chem. Soc. 2011, 133,
19698. (j) Birman, V. B.; Jiang, H.; Li, X.; Guo, L.; Uffman, E. W. J. Am.
Chem. Soc. 2006, 128, 6536. (k) Yang, X.; Bumbu, V. D.; Liu, P.; Li, X.;
Jiang, H.; Uffman, E. W.; Guo, L.; Zhang, W.; Jiang, X.; Houk, K. N.;
Birman, V. B. J. Am. Chem. Soc. 2012, 134, 17605. (l) Fowler, B. S.;
Mikochik, P. J.; Miller, S. J. J. Am. Chem. Soc. 2010, 132, 2870. (m) Arai,
S.; Bellemin-Laponnaz, S.; Fu, G. C. Angew. Chem., Int. Ed. 2001, 40,
234. (n) Arp, F. O.; Fu, G. C. J. Am. Chem. Soc. 2006, 128, 14264.
(13) For reviews about BINOL as the ligands in asymmetric syn-
thesis, see: (a) Brunel, J. M. Chem. Rev. 2005, 105, 857. (b) Chen, Y.;
Yekta, S.; Yudin, A. K. Chem. Rev. 2003, 103, 3155.
(14) For BINOL used as the ligand in copper-catalyzed Ullmann-
type coupling reactions, see: (a) Zhu, D.; Wang, R.; Mao, J.; Xu, L.; Wu,
F.; Wan, B. J. Mol. Catal. A: Chem. 2006, 256, 256. (b) Jiang, D.; Fu, H.;
Jiang, Y.; Zhao, Y. J. Org. Chem. 2007, 72, 672.
(15) s-factorswere calculated according to: Kagan, H. B.; Fiaud, J. C.
Top. Stereochem. 1988, 18, 249.
Org. Lett., Vol. 15, No. 14, 2013
3599

Furthermore, the absolute configuration of the coupling
product 2b was assigned to be R by comparing with
our reported compound through one-step chemical
transformation.16 Correspondingly, the absolute configura-
tions of the recovered starting material 1b were assigned to be
S. The absolute configurations of other coupling products
and recovered starting materials were assigned by analogue
to that of 2b and 1b, respectively.
Table 1. Screening Reaction Conditionsa
Table 2. Substrate Scope for the Kinetic Resolution of
rac-2-Amino-3-(2-iodoaryl)propionatesa
eec (%)
entry L*
solvent
time (h) convb (%)
1a
2a
s
1
2
3
4
5
6
7
8
9
L1 dioxane
L2 dioxane
L3 dioxane
L4 dioxane
L5 dioxane
L5 THF
0.5
2
36
38
34
44
51
37
23
37
8
32
37
40
63
94
50
15
24
7
57
60
77
74
89
87
52
42
89
4.9
5.8
time
(h)
convb
(%)
ee of 1
ee of 2
entry rac-1
(yield,%)c
(yield, %)c
s
1.5
1
11.7
16.8
70.0
20.4
3.5
1
1a
1b
1c
1d
1e
1f
4
51
48
49
50
50
52
51
48
47
54
47
49
53
94 (49)
85 (51)
87 (50)
90 (49)
86d (48)
98 (46)
92 (49)
86 (46)
80 (52)
96 (46)
80 (53)
86 (50)
96 (46)
89 (51)
93 (46)
90 (49)
91 (50)
87 (49)
92 (51)
90 (50)
93 (46)
91 (45)
83 (50)
91 (46)
90 (49)
86 (52)
70.0
65.9
57.2
58.5
40.7
91.3
53.1
78.7
47.7
38.6
47.7
50.0
48.4
4
2
8
2
3
2.5
2.5
4
L5 MeCN
L5 DMF
2
4
2
3.0
5
L5 toluene
4
9.9
6
3.5
1.5
1.5
2
7
1g
1h
1i
a Reagents and reaction conditions: 1a (0.20 mmol, 1.0 equiv), CuI
(0.01 mmol, 5 mol %), ligand (0.012 mmol, 6 mol %), base, (0.2 mmol,
1.0 equiv), solvent (1.5 mL), rt. b Calculated conversion. c Determined by
HPLC analysis (Chirapak AD-H column).
8
9
10
11
12
13
1j
3
1k
1l
2
1.5
2
1m
ester groups at the quaternary chiral carbon center were
tested and all gave excellent enantioselectivities with
s-factors from 57.2 to 70.0, well above the threshold
for practical utility (Table 2, entries 1ꢀ4). Next, by con-
trolling the conversion ratios between 45 and 55%, a variety
of substrates bearing electron-withdrawing or electron-
donating substituents on the 2-iodophenyl ring were tested
and excellent enantioselectivities (s = 38ꢀ91) were also
obtained in all cases tested (Table 2, entries 5ꢀ11). As an
example to elucidate the applicability of such a kinetic
resolution process, the recovered enriched enantiomer 1a
was transformed into 2a under the catalysis of CuI/rac-
BINOL14 (97% yield, ꢀ94% ee).
a Reagents and reaction conditions: 1 (0.2 mmol, 1.0 equiv), CuI
(0.01 mmol, 5 mol %), L5 (0.012 mmol, 6 mol %), Cs2CO3, (0.2 mmol,
1.0 equiv), 1,4-dioxane (1.5 mL), rt. b Calculated conversion. c Isolated
yields are given in parentheses; the enantiomeric excesses were deter-
mined by HPLC analysis (Chirapak AD-H or OD-H column). d The ee
of 1e was determined after one-step reaction.
Encouraged by the above success, we then explored the
kinetic resolution of racemic 2-amino-4-(2-iodophenyl)-
butanoates. As shown in Table 3, although they were less
reactive than the corresponding 2-amino-3-(2-iodoaryl)-
propionates, the reactions of 2-amino-4-(2-iodoaryl)-
butanoates proceeded well under the catalysis of CuI and
ligand (R)-L4,17 affording the 1,2,3,4-tetrahydroquinoline
coupling products and recovering the starting materials in
even better enantioselectivities than their one-carbon-
shorter couterparts (s factors up to 245, Table 3, entries
1ꢀ7).18 It is noteworthy that, in these reactions, the size of
R2 group showed important influence to the efficiency of
Further efforts to increase the variations on the phenyl
ring of benzyl also successfully afforded the coupling
products and recovered the starting materials in excellent
selectivities, as in the case of compounds 1l and 1m
(Table 2, entries 12 and 13). Noticeably, in the case of
1m, an iodo atom was introduced on the phenyl ring of the
benzyl group, which could be a good reactive site for
further transformations.
(17) The electron-withdrawing ꢀCF3 groups on the aryl rings of the
ligand L4 have obvious accelerating effect to the reactions; see ref 10.
(18) The absolute configuration of recovered 3d was determined by
X-ray experiments; see the Supporting Information.
(16) For the assignment of absolute configuration of the products
and recovered starting materials, see the Supporting Information.
3600
Org. Lett., Vol. 15, No. 14, 2013

Table 3. Kinetic Resolution of rac-2-Amino-4-(2-iodoaryl)-
butanoatesa
convb
(%)
ee of 3
ee of 4
entry rac-1 time (h)
(yield, %)c (yield, %)c
s
1
2
3
4
5
6
7
3a
3b
3c
3d
3e
3f
5
36
17
10
27
29
24
48
48
48
47
52
53
48
89 (50)
86 (50)
88 (51)
85 (49)
88 (47)
98 (46)
90 (50)
95 (45)
95 (47)
96 (46)
97 (50)
80 (47)
85 (48)
96 (46)
165
79
122
129
28
65
3g
245
a Reagents and reaction conditions: 3 (0.2 mmol, 1.0 equiv), CuI
(0.02 mmol, 10 mol %), L4 (0.024 mmol, 12 mol %), Cs2CO3, (0.2 mmol,
1.0 equiv), 1,4-dioxane (1.5 mL), rt. b Calculated conversion. c Isolated
yields are given in parentheses; the enantiomeric excesses were deter-
mined by HPLC analysis (Chirapak AD-H or OD-H column).
the kinetic resolutions. As observed, better selectivities
were obtained with bulkier R2 groups (Table 3, entries 1
and 5ꢀ7).
Figure 1. Proposed mechanism for kinetic resolution.
to the formation of the coupled chiral indoline and 1,2,3,4-
tetrahydroquinoline products in good yields and high ee
values and recovered the starting materials in high ee values.
Such a method overcomes some shortcomings in our pre-
vious research of asymmetric desymmetric N-arylation and
provides a more general tool for asymmetric N-arylation.
Further exploration and applications of this reaction in
organic synthesis is underway in our laboratory.
Although the mechanistic details are not clear at this
point, based on the literature reports19 and our experi-
mental results, a plausible transition-state model was
proposed for the excellent enantioselectivity achieved
in our reaction system. As shown in Figure 1, the CuI
may coordinate with the racemic substrate and the chiral
ligand to form two different tetrahedral CuI complexes.
Obviously, the aryl CꢀI bond in the R enantiomer of 1 can
preferentially interact with the Cu center (TS-1), while the
aryl CꢀI bond in the S enantiomer of 1 may suffer with the
steric hindrance from the aryl moiety of the chiral ligand
(TS-2). Thus, the R enantiomer of1 would react faster than
the S enantiomer to produce enantiomerically enriched
R indoline products and recover enantiomerically enriched
S starting materials in excellent selectivity.
Acknowledgment. We are grateful to the 100-talent
program of CAS, National Natural Science Foundation
(Grant No. 21272234), and National S&T Major Special
Project on Major New Drug Innovation (Grant No.
2011ZX09102-010), Guangdong Natural Science Founda-
tion (Grant S2011010003705) for their financial support.
We thank Prof. Jinsong Liu in Guangzhou Institutes of
Biomedicine and Health, Chinese Academy of Sciences
(GIBH,CAS), for the X-ray experiment. We also thank
Dr. Shouyun Yu in Nanjing University and Prof. Fayang
Qiu (GIBH, CAS) for the helpful discussions.
In summary, for the first time, copper-catalyzed N-aryla-
tion has been successfully applied in the kinetic resolution
of the rac-2-amino-3-(2-iodoaryl)propionates and rac-2-
amino-4-(2-iodoaryl)butanoates. Excellent enantioselectiv-
ities have been achieved in such a process. The reactions led
Supporting Information Available. Full experimental
procedures, characterization data for all the compounds,
and crystal structure (CIF) of 3d. This material is avail-
able free of charge via the Internet at http://pubs.acs.org.
(19) (a) Huang, Z.; Hartwig, J. F. Angew. Chem., Int. Ed. 2012, 51,
1028. (b) Chen, B.; Hou, X.-L.; Li, Y.-X.; Wu, Y.-D. J. Am. Chem. Soc.
2011, 133, 7668. (c) Huffmann, L. M.; Stahl, S. S. Dalton Trans. 2011, 40,
8959. (d) Casitas, A.; King, A. E.; Parella, T.; Costas, M.; Stahl, S. S.;
Ribas, X. Chem. Sci. 2010, 1, 326. (e) Strieer, E. R.; Bhayana, B.;
Buchwald, S. L. J. Am. Chem. Soc. 2009, 131, 78. (f) Zhang, S.-L.;
Liu, L.; Fu, Y.; Guo, Q.-X. Organometallics 2007, 26, 4546.
The authors declare no competing financial interest.
Org. Lett., Vol. 15, No. 14, 2013
3601