Catalytic asymmetric conjugate allylation of coumarins

Article abstract of DOI:10.1021/ol201312y

A catalytic asymmetric conjugate allylation was successfully developed to synthesize potential pharmacologically active 4-allyl-2-oxochroman skeletons. A dual activation strategy was employed by using N,N′-dioxide-Yb(OTf) ₃ to activate coumarins and using (CuOTf)₂?C ₇H₈ to activate tetraallyltin via transmetalation, respectively. Good yields and enantioselectivities were obtained under mild conditions.

Full text of DOI:10.1021/ol201312y

ORGANIC
LETTERS
2011
Vol. 13, No. 15
3814–3817
Catalytic Asymmetric Conjugate
Allylation of Coumarins
Yulong Kuang, Xiaohua Liu, Lu Chang, Min Wang, Lili Lin, and Xiaoming Feng*
Key Laboratory of Green Chemistry & Technology, Ministry of Education,
College of Chemistry, Sichuan University, Chengdu 610064, P. R. China
xmfeng@scu.edu.cn
Received May 16, 2011
ABSTRACT
A catalytic asymmetric conjugate allylation was successfully developed to synthesize potential pharmacologically active 4-allyl-2-oxochroman
skeletons. A dual activation strategy was employed by using N,N0-dioxide-Yb(OTf)3 to activate coumarins and using (CuOTf)2•C7H8 to activate
tetraallyltin via transmetalation, respectively. Good yields and enantioselectivities were obtained under mild conditions.
The catalytic asymmetric allylation reaction is among
the most important of CꢀC bond-forming reactions which
constructs a stereocenter with an allyl group.1 The asym-
metric allylation of CdO and CdN bonds has been widely
developed to afford homoallylic alcohols or amines.2
Although the preferential conjugate addition of soft
nucleophiles has been realized,3 the asymmetric conjugate
allylation reaction is still a challenge due to the relatively
low nucleophilic nature of allyl species to the CdC bond.4
Until now, only two examples of the catalytic asymmetric
conjugate allylation reaction have been reported. The
Morken group pioneered the nickel- or palladium-cata-
lyzed enantioselective conjugate addition of allylboronic
acid pinacol ester to dialkylidene ketones. The reaction
proceeded via oxidative addition to form an unsaturated
π-allyl complex, followed by transmetalation and reduc-
tive elimination steps.5 The Snapper group realized the
enantioselective HosomiꢀSakurai conjugate allylation of
cyclic unsaturated ketoesters.6 Good to excellent enantio-
selectivities have been obtained in both cases; nonetheless,
the yield of the product and variety of substrate remain
questionable. Herein, we describe an effective N,N0-diox-
ide-Yb(III) catalyst system for the conjugate allylation of
coumarins. A dual activation strategy was employed to
improve the yield by using a cocatalyst (CuOTf)2•C7H8 to
(1) For selected application of catalytic asymmetric allylation in
^
ꢀ
^
natural product syntheses, see: (a) de Fatima, A.; Kohn, L. K.; Antonio,
M. A.; de Carvalho, J. E.; Pilli, R. A. Bioorg. Med. Chem. 2005, 13, 2927.
(b) Sanchez, C. C.; Keck, G. E. Org. Lett. 2005, 7, 3053. (c) Itoh, T.;
Miyazaki, M.; Fukuoka, H.; Nagata, K.; Ohsawa, A. Org. Lett. 2006, 8,
1295. (d) Miyazaki, M.; Ando, N.; Sugai, K.; Seito, Y.; Fukuoka, H.;
Kanemitsu, T.; Nagata, K.; Odanaka, Y.; Nakamura, K. T.; Itoh, T. J.
Org. Chem. 2011, 76, 534.
(2) For selected examples of catalytic asymmetric allylation, see: (a)
Shimada, T.; Kina, A.; Ikeda, S.; Hayashi, T. Org. Lett. 2002, 4, 2799. (b)
Lu, J.; Ji, S. J.; Teo, Y. C.; Loh, T. P. Org. Lett. 2005, 7, 159. (c)
Kiyohara, H.; Nakamura, Y.; Matsubara, R.; Kobayashia, S. Angew.
Chem., Int. Ed. 2006, 45, 1615. (d) Wooten, A. J.; Kim, J. G.; Walsh, P. J.
Org. Lett. 2007, 9, 381. (e) Massa, A.; Acocella, M. R.; De Sio, V.;
Villano, R.; Scettri, A. Tetrahedron: Asymmetry 2009, 20, 202. (f) Shi,
S. L.; Xu, L. W.; Oisaki, K.; Kanai, M.; Shibasaki, M. J. Am. Chem. Soc.
2010, 132, 6638. (g) Chakrabarti, A.; Konishi, H.; Yamaguchi, M.;
Schneider, U.; Kobayashi, S. Angew. Chem., Int. Ed. 2010, 49, 1838. (h)
Boyd, D. R.; Sharma, N. D.; Sbircea, L.; Murphy, D.; Malone, J. F.;
James, S. L.; Allen, C. C. R.; Hamilton, J. T. G. Org. Biomol. Chem.
2010, 8, 1081. (i) Li, J.; Lutz, M.; Spek, A. L.; van Klink, G. P. M.; van
Koten, G.; Klein Gebbink, R. J. M. Organometallics 2010, 29, 1379. (j)
Qiao, X. C.; Zhu, S. F.; Chen, W. Q.; Zhou, Q. L. Tetrahedron:
Asymmetry 2010, 21, 1216. (k) Wu, Q. F.; He, H.; Liu, W. B.; You,
S. L. J. Am. Chem. Soc. 2010, 132, 11418. (l) Zhu, S. F.; Qiao, X. C.;
Zhang, Y. Z.; Wang, L. X.; Zhou, Q. L. Chem. Sci. 2011, 2, 1135.
(3) For the character of nucleophiles to Michael reaction, see: Poon,
T. J. Chem. Educ. 2002, 79, 264.
(4) For the studies of character of allyl anion, see: (a) Brickhouse,
M. D.; Squires, R. R. J. Am. Chem. Soc. 1988, 110, 2706. (b) Linares, M.;
Humbel, S.; Braıda, B. J. Phys. Chem. A 2008, 112, 13249.
(5) (a) Sieber, J. D.; Liu, S.; Morken, J. P. J. Am. Chem. Soc. 2007,
129, 2214. (b) Sieber, J. D.; Morken, J. P. J. Am. Chem. Soc. 2008, 130,
4978. (c) Brozek, L. A.; Sieber, J. D.; Morken, J. P. Org. Lett. 2011, 13,
995.
(6) Shizuka, M.; Snapper, M. L. Angew. Chem., Int. Ed. 2008, 47,
5049.
r
10.1021/ol201312y
Published on Web 06/21/2011
2011 American Chemical Society

generate a more active organometallic agent from tetra-
allyltin through transmetalation.7
two-carbonic linkage, 4-allyl chroman-2-one derivative 3a
was obtained with 67% yield, 76% ee, and an up to 95:5
anti/syn ratio in ClCH2CH2Cl (Table 1, entry 1). With
ligand L2 derived from 2-adamantyl amine, both the yield
and the enantioselectivity of the reaction dropped (Table 1,
entry 2). When ligand L3 with a three-carbonic linkage was
used, the enantioselectivity of the reaction deteriorated
sharply (26% ee, Table 1, entry 3). It indicated that the
compact structure of the catalyst benefited the enantioselec-
tivity in the asymmetric transformation which was also in
accord with the sterical influence of the ester group of
coumarins. The substrate with a bulky ester group could
afford higher stereoselectivity, and substrate 1a was the
best candidate.13 Further optimization of the reaction con-
ditions showed that THF benefited the reaction with a
higher ee value and increasing the reaction temperature to
40 °C could lead to a moderate improvement of the yield
We chose 3-carboxylate-coumarin as the model substrate
which presents a kind of privileged scaffold of biological and
pharmaceutical interest.8 The introduction of a 3-carbox-
ylate group would enhance the activity and chelating cap-
ability. Initially, allyl reagents such as allylbromide/zinc,9
allyltrichlorosilane, and allyltrimethylsilane10 with various
Lewis acids were examined. Unfortunately, no desired
conjugate addition product was obtained. Only when tetra-
allyltin was used could the product 3a be detected. Then we
extended the scope to chiral Lewis acid catalysts of N,N0-
dioxide to promote the asymmetric conjugate allylation
reaction, which have been used in the allylation of carbonyl
compounds11 and other nucleophilic additions12 in our
group. A variety of metalꢀligand combinations showed
that chiral rare earth metal complexes of N,N0-dioxide with
sterically hindered aliphatic amide moieties could give
moderate results.13 In the presence of 10 mol % of Yb(OTf)3
and ligand L1 which contains a 1-adamantyl group and a
Table 1. Optimization of the Reaction Conditionsa
(7) For the precedent example of generating active organometallic
agents from organotin through transmetalation in a conjugate allylation
reaction, see: Shibata, I.; Kano, T.; Kanazawa, N.; Fukuoka, S.; Baba,
A. Angew. Chem., Int. Ed. 2002, 41, 1389.
(8) For the biological and pharmaceutical activity of coumarins, see:
(a) Takechi, M.; Tanaka, Y.; Takehara, M.; Nonaka, G. I.; Nishioka, I.
Phytochemistry 1985, 24, 2245. (b) Iinuma, M.; Tanaka, T.; Mizuno, M.;
Katsuzaki, T.; Ogawa, H. Chem. Pharm. Bull. 1989, 37, 1813. (c) Iinuma,
M.; Ohyama, M.; Tanaka, T.; Mizuno, M.; Hong, S. K. Phytochemistry
1991, 30, 3153. (d) Vilain, A. C.; Okochi, V.; Vergely, I.; Reboud-
Ravaux, M.; Mazaleyrat, J. P.; Wakselman, M. Biochim. Biophys. Acta
1991, 1076, 401. (e) Chang, Y. C.; Nair, M. G. J. Nat. Prod. 1995, 58,
ꢀ
1901. (f) Hoult, J. R. S.; Paya, M. Gen. Pharmacol. 1996, 27, 713. (g)
Boyle, F. T.; Costello, G. F. Chem. Soc. Rev. 1998, 27, 251. (h)
Tillekeratne, L. M. V.; Sherette, A.; Grossman, P.; Hupe, L.; Hupe,
D.; Hudson, R. A. Bioorg. Med. Chem. Lett. 2001, 11, 2763. (i) Bailly, C.;
Bal, C.; Barbier, P.; Combes, S.; Finet, J. P.; Hildebrand, M. P.; Peyrot,
V.; Wattez, N. J. Med. Chem. 2003, 46, 5437. (j) Roelens, F.; Huvaere,
K.; Dhooge, W.; Van Cleemput, M.; Comhaire, F.; De Keukeleire, D.
Eur. J. Med. Chem. 2005, 40, 1042. (k) Kumar, A.; Singh, B. K.; Tyagi,
R.; Jain, S. K.; Sharma, S. K.; Prasad, A. K.; Raj, H. G.; Rastogi, R. C.;
Watterson, A. C.; Parmar, V. S. Bioorg. Med. Chem. 2005, 13, 4300. (l)
Menasria, F.; Azebaze, A. G. B.; Billard, C.; Faussat, A. M.; Nkengfack,
A. E.; Meyer, M.; Kolb, J. P. Leuk. Res. 2008, 32, 1914.
(9) For examples of allylbromide/zinc in an allylation reaction, see:
(a) Li, H.; Cheng, H. S.; Seow, A. H.; Loh, T. P. Tetrahedron Lett. 2007,
48, 2209. (b) Kong, W.; Fu, C.; Ma, S. Org. Biomol. Chem. 2008, 6, 4587.
(10) For selected examples of allyltrimethylsilane as the nucleophilic
reagent in a conjugate allylation reaction, see: (a) Hosomi, A.; Sakurai,
H. J. Am. Chem. Soc. 1977, 99, 1673. (b) Majetich, G.; Casares, A.;
Chapman, D.; Behnke, M. J. Org. Chem. 1986, 51, 1745. (c) Hayashi,
M.; Mukaiyama, T. Chem. Lett. 1987, 289. (d) Lee, P. H.; Lee, K.; Sung,
S.; Chang, Y.; S. J. Org. Chem. 2001, 66, 8646.
T
yield
[%]b
ee
entry
L
Cocatalyst
none
Solvent
[°C]
[%]c
1
L1
L2
L3
L1
L1
L1
L1
L1
L1
L1
L1
L1
DCE
DCE
DCE
THF
THF
THF
THF
THF
THF
THF
THF
THF
25
25
25
25
40
40
40
40
40
40
40
40
67
58
60
22
77
40
21
15
NRd
50
63
99
76
50
26
86
86
89
90
89
2
none
3
none
4
none
5
none
6e
7e
8e
9e
10e
11e
12f
none
CuCl
CuBr
CuI
CuOAc
(CuOTf)2•C7H8
(CuOTf)2•C7H8
90
90
91
(11) For N,N0-dioxide in an asymmetric allylation reaction of carbo-
nyl, see: (a) Zhang, X.; Chen, D. H.; Liu, X. H.; Feng, X. M. J. Org.
Chem. 2007, 72, 5227. (b) Zheng, K.; Qin, B.; Liu, X. H.; Feng, X. M. J.
Org. Chem. 2007, 72, 8478. (c) Huang, J. L.; Wang, J.; Chen, X. H.; Wen,
Y. H.; Liu, X. H.; Feng, X. M. Adv. Synth. Catal. 2008, 350, 287.
(12) For selected examples of N,N0-dioxide-metal complexes in nu-
cleophilic addition reactions, see: (a) Xie, M. S.; Chen, X. H.; Zhu, Y.;
Gao, B.; Lin, L. L.; Liu, X. H.; Feng, X. M. Angew. Chem., Int. Ed. 2010,
49, 3799. (b) Cai, Y. F.; Liu, X. H.; Hui, Y. H.; Jiang, J.; Wang, W. T.;
Chen, W. L.; Lin, L. L.; Feng, X. M. Angew. Chem., Int. Ed. 2010, 49,
6160. (c) Chang, L.; Kuang, Y. L.; Qin, B.; Zhou, X.; Liu, X. H.; Lin,
L. L.; Feng, X. M. Org. Lett. 2010, 12, 2214. (d) Li, W.; Wang, J.; Hu,
X. L.; Shen, K.; Wang, W. T.; Chu, Y. Y.; Lin, L. L.; Liu, X. H.; Feng,
X. M. J. Am. Chem. Soc. 2010, 132, 8532. (e) Wang, W. T.; Liu, X. H.;
Cao, W. D.; Wang, J.; Lin, L. L.; Feng, X. M. Chem.;Eur. J. 2010, 16,
1664. (f) Wang, W. T.; Liu, X. H.; Cao, W. D.; Wang, J.; Lin, L. L.; Feng,
X. M. Chem.;Eur. J. 2010, 16, 1664. (g) Zheng, K.; Yin, C. K.; Liu,
X. H.; Lin, L. L.; Feng, X. M. Angew. Chem., Int. Ed. 2011, 50, 2573.
(13) For details, see the Supporting Information.
a Unless otherwise noted, the reactions were performed with 1a (0.1
mmol), 2a (0.1 mmol), Yb(OTf)3 (10 mol %), L (10 mol %), cocatalyst
(10 mol %) in 0.4 mL of solvent for 24 h under a nitrogen atmosphere.
Up to 95:5 dr values were observed which were determined by 1H NMR
or HPLC on a chiral stationary phase. b Isolated yield. c Determined by
HPLC on a chiral stationary phase (chiralcel OD-H). d NR = No
Reaction. e THF (0.8 mL) was used. f THF (1.2 mL) was used, and the
reaction time was prolonged to 48 h.
(77% yield, 86% ee, Table 1, entries 4 and 5). Lowering the
concentration of the reaction system resulted in the enan-
tioselectivity slightly increasing to 89% ee, whereas a low
yield was obtained even with a prolonged reaction time
(40% yield, Table 1, entry 6).
Org. Lett., Vol. 13, No. 15, 2011
3815

Further attemptstoincreasethe yield werecarried out. It
was found that the reaction was sensitive to the amount of
water which might becrucialforthe processofprotonation
to form the desired product. Other additives were also
tested with negative results. Organotin compounds are
liable to participate in transmetalation to generate more
active organometallic reagents.7,14 Therefore, cocatalysts
were employed to accelerate the reaction. Among the
copper salts surveyed in Table 1, (CuOTf)2•C7H8 was
beneficial for the yield of the reaction, and other Cu(I)
salts such as CuCl, CuBr, CuI, and CuOAc gave lower
yields (Table 1, entries 7ꢀ10 vs 11). This disparity might be
due to the exchanging counterion with Yb(OTf)3 which
would affect the activation of coumarin 1a. In the presence
of 10 mol % of (CuOTf)2•C7H8, the yield of the reaction
was elevated from 40% to 63% without affecting the
enantioselectivity (Table 1, entry 11 vs 6). Further dilution
of the reaction system and a prolonged reaction time
resulted in a 99% yield and 91% ee (Table 1, entry 12).
Under the optimized reaction conditions, the outcomes of
the reaction became insensitive to moisture.
(>95:5, detected by 1H NMR) were achieved for both the
racemic products and the asymmetric catalytic products.
Substituted coumarin derivatives were very tolerant with
up to a 99% yield and 93% ee. As shown in Table 2, there
was no pronounced distinctness of the enantioselectivity
between electron-withdrawing and -donating substituents.
The yields were closely related to the position of the
substituent. Generally, 7- and 8-substituted coumarins
resulted in lower yields compared with 6-substituted sub-
strates (Table 2, entries 3, 4, 6, 7, 8, 10, 11 vs entries 2, 5, 9, 12).
Moreover, fused ring coumarin derivative 1m was also
tolerable, affording the desired product with a 94% yield
and 92% ee (Table 2, entry 13). Besides, the absolute
configuration of 3a was determined to be (3S,4S) by
X-ray crystallography.15
Table 3. Control Experimentsa
Table 2. Substrate Scope of Coumarin Derivatives in the Cata-
lytic Asymmetric Conjugate Allylationa
Allyl
yield
[%]b
ee
[%]c
entry
source
M1
M2
1
2a
2a
2a
2a
2b
2c
2d
2d
2a
2a
2a
none
none
Cu(OTf)2
NRd
NRd
Trace
Trace
NRd
NRd
30
2
(CuOTf)2•C7H8
3e
4f
5
Yb(OTf)3, (CuOTf)2•C7H8
Yb(OTf)3
Yb(OTf)3
Yb(OTf)3
Yb(OTf)3
Yb(OTf)3
Yb(OTf)3
Yb(OTf)3
Yb(OTf)3
(CuOTf)2•C7H8
(CuOTf)2•C7H8
(CuOTf)2•C7H8
(CuOTf)2•C7H8
none
6
7g
8g
9
86
80
9
28
LiBrþAgOTf
FeCl2þ2AgOTf
PdCl2þ2AgOTf
65
10
11
67
85
88
81
a General conditions: 1a (0.1 mmol), 2 (0.1 mmol), prepared M1/L1
(1:1, 10 mol %), M2 (10 mol %), THF (1.2 mL) at 40 °C for 48 h under a
nitrogen atmosphere. b Isolated yield. c Determined by HPLC (chiralcel
OD-H). d NR = No Reaction. e M1 and M2 were mixed together with L1
for 1 h before the reactants were added. f Without nitrogen replacement.
g The reaction time was prolonged to 96 h.
a The reactions were performed with 1 (0.1 mmol), 2a (0.1 mmol),
prepared Yb(OTf)3/L1 (1:1, 10 mol %), (CuOTf)2•C7H8 (10 mol %),
THF (1.2 mL) at 40 °C for 48 h under a nitrogen atmosphere. All dr
values were >95:5, detected by 1H NMR. b Isolated yield. c Determined
by HPLC on a chiral stationary phase.
To shed light on the function of (CuOTf)2•C7H8 in
catalytic asymmetric conjugate allylation, comparative ex-
periments were performed. The reaction did not proceed
using the complex of Cu(OTf)2 or (CuOTf)2•C7H8 (Table 3,
entries 1 and 2), which implied that the chiral copper complex
could not provide sufficient Lewis acidity for the activation
of coumarin 1a. When the mixture of (CuOTf)2•C7H8,
Yb(OTf)3 and L1 was served as the catalyst, only trace
amounts of product 3a was detected with the disappearance
of tetraallyltin (Table 3, entry 3). Thus, the formation of a
After identifying the optimal reaction conditions, a
range of substrates with different substituents on the
aromatic ring were examined. High diastereoselectivities
(14) For primary studies of the copper effect of organotin, see: (a)
Han, X.; Stoltz, B. M.; Corey, E. J. J. Am. Chem. Soc. 1999, 121, 7600.
(b) Mee, S. P. H.; Lee, V.; Baldwin, J. E. Angew. Chem., Int. Ed. 2004, 43,
1132.
(15) CCDC822331 (1a) contains the supplementary crystallographic
data for this paper. These data can be obtained free from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_requst/cif.
3816
Org. Lett., Vol. 13, No. 15, 2011

heterobimetallic catalyst could be ruled out, and copper(I)
might serve as the cocatalyst to react with tetraallyltin.
An HRMS spectrum was obtained to identify the intermedi-
ates generated from equivalent (CuOTf)2•C7H8 and tetra-
allyltin.13 The spectrum of the mixture revealed an ion signal
at m/z 104.9766 which corresponds to allyl copper(I)
[CH2dCHCH2ꢀCu(I)þHþ], and the characteristic ion
peak of tetraallyltin was not detected. The actual structure
determining whether the allyl group is σ- or π-bonding to
Cu(I) was not confirmed, while the role of (CuOTf)2•C7H8
could be deduced to generate a more active allyl reagent by
transmetalation. In the presence of (CuOTf)2•C7H8, the
reaction system became sensitive to oxygen rather than
moisture, and trace amounts of product but depleted tetra-
allyltin were observed in the absence of a nitrogen atmo-
sphere (Table 3, entry 4). The reason might be that the newly
formed allyl Cu(I) intermediate was too active to undergo
oxidation. Additionally, the transmetalation role of other
Lewis acids and allyl reagents was checked under the
optimized conditions. The reaction could not take place with
allyl bromide or allyltricholrosilane with (CuOTf)2•C7H8 as
the cocatalyst (Table 3, entries 5 and 6). Besides, the tributyl-
allyltin also did not show obvious efficiency (Table 3, entries
7 and 8). Among other metal salts investigated such as Li(I),
Fe(II), and Pd(II), only Pd(II) could raise the yield to some
degree (Table 3, entries 9ꢀ11).
charge transfer from cuprate(I) species to the substrate16
to form the intermediate A3, following cleavage of the
allylꢀCuIII bond. Then, the interaction between tetraal-
lyltin and the intermediate A4 would release the catalyst
and regenerate the active allyl copper(I) species. The
desired product 3a was obtained by protonation in the
workup procedure.
Figure 2. The synthetic utility of the reaction.
To show the prospect of the methodology in synthesis, the
scaled-up reaction was performed with subgram quantities
of coumarin 1a (1.23 g, 5 mmol), and 91% yield with 92% ee
could be achieved (Figure 2a). The enantiopure product
could be obtained by a simple recrystallization. The 4-allyl-
dihydrocoumarin which has a similar skeleton with cou-
marin-based drugs could be produced by decarboxylation
from 3a by p-toluenesulfonic acid in toluene at 80 °C for 5 h
with the maintained enantioselectivity (Figure 2b).17
In summary, a dual activation strategy has been devel-
oped in the asymmetric conjugate allylation reaction to
synthesize opticallyactive4-allyl-2-oxochromanskeletons.
A chiral N,N0-dioxide-Yb(OTf)3 complex was responsible
for the activation of coumarins, and (CuOTf)2•C7H8
operated to form an active allyl nucleophile by transmeta-
lation. Excellent results were obtained under mild reaction
conditions. Further efforts should be devoted to exploring
the method to the asymmetric conjugate allylation of
simple enones.
Figure 1. Proposed catalytic cycle.
Acknowledgment. We appreciate the National Natural
Science Foundation of China (Nos. 20732003 and
21021001), and the Basic Research Program of China
(973 Program: 2010CB833300) for financial support. We
alsothank Sichuan UniversityAnalytical & Testing Center
for NMR and X-ray analysis.
Next, the relationship between the ee value of ligand L1
and the product 3a showed a linear effect,13 which implied
that the monomeric catalyst might be the main catalyti-
cally active species. A possible catalytic cycle was raised
to explain the asymmetric induction and the special func-
tion of (CuOTf)2•C7H8 (Figure 1). The L1-Yb(OTf)3
catalyst first coordinated with the coumarin 1a to form
intermediate A2. The allyl copper(I) reagent generated
from (CuOTf)2•C7H8 and tetraallyltin would undergo
Supporting Information Available. Experimental pro-
cedures, spectral and analytical data for the products.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(16) Mori, S.; Nakamura, E. Morden Organocopper Chemistry;
Wiley: 2002; Vol. 10, pp 315ꢀ346.
(17) (a) Biddle, M. M.; Lin, M.; Scheidt, K. A. J. Am. Chem. Soc.
2007, 129, 3830. (b) Liu, X.; Lu, Y. Org. Lett. 2010, 12, 5592.
Org. Lett., Vol. 13, No. 15, 2011
3817