Rhodium-mediated decarboxylative conjugate addition of fluorinated benzoic acids: stoichiometric and catalytic transformations

Full text of DOI:10.1002/anie.200901097

Communications
DOI: 10.1002/anie.200901097
Synthetic Methods
Rhodium-Mediated Decarboxylative Conjugate Addition of
Fluorinated Benzoic Acids: Stoichiometric and Catalytic
Transformations**
Zhong-Ming Sun and Pinjing Zhao*
Transition-metal-catalyzed conjugate addition is a powerful
[1]
À
strategy for C C bond formation. In general, a stoichio-
metric amount of a main-group organometallic reagent is
required, and the desired organometallic nucleophile con-
taining a transition metal is generated through transmetala-
tion. Rhodium-catalyzed conjugate addition reactions of
organoboron reagents proceed under mild conditions with
high regio- and stereoselectivities.[2] To further advance
rhodium-catalyzed conjugate addition, it is desirable to find
alternatives to the conventional transmetalation method for
the formation of organorhodium nucleophiles.[3] We herein
report the development of a rhodium-catalyzed decarboxyla-
Scheme 1. Proposed catalytic cycle for the rhodium-catalyzed conju-
gate addition of benzoic acids through decarboxylation.
tive conjugate addition of benzoic acids on the basis of
mechanistic observations for the corresponding stoichiomet-
ric reactions.
Decarboxylative transformations of benzoic acids under
the catalysis of late transition metals have generated much
interest in the past few years.[4] Typically, reactive aryl metal
intermediates were formed by the release of CO2 from
benzoate species.[5c] In this way, benzoic acids could be used as
readily available and easy-to-handle building blocks in
transition-metal catalysis. Pioneered by the Myers research
group in the palladium-catalyzed decarboxylative Heck–
Mizoroki reaction,[5] this decarboxylation strategy has been
expanded to several new reactions. For example, Gooßen and
co-workers developed palladium-catalyzed decarboxylative
cross-coupling reactions for the synthesis of biaryl com-
pounds,[6a–e] and similar reactions were reported by a few
other research groups.[6f–j] Other important examples include
palladium- and copper-catalyzed reductive decarboxylation[7]
and rhodium- and iridium-catalyzed decarboxylative alkyne
arylation.[8,9]
rhodium(I) aryl nucleophile C would be generated by
decarboxylation from a rhodium(I) benzoate intermediate
À
B. Subsequent olefin insertion would form the desired C C
bond at the b position to give a rhodium(I) enolate D, and
protonolysis would then release the product of conjugate
addition and regenerate the rhodium(I) hydroxide catalyst
A.[2e]
Besides the typical issues encountered with reported
decarboxylative cross-coupling reactions (e.g. reaction tem-
peratures generally above 1508C, limited substrate scope, and
stoichiometric amounts of heavy-metal salts required as
additives),[4] the following challenges are expected from a
mechanistic viewpoint: First, CO2 release from rhodium(I)
carboxylates may be thermodynamically disfavored, as sug-
gested by observations of CO2 insertion into rhodium(I) aryl
species to generate rhodium(I) benzoates in stoichiometric
reactions.[10] In fact, this reverse reactivity has recently been
exploited in the rhodium-catalyzed carboxylation of organo-
boron reagents and in related studies.[11] Second, carboxyl-
Our decarboxylative conjugate addition of benzoic acids
is based on a proposed catalytic cycle (Scheme 1) that has
similarities with the rhodium-catalyzed conjugate addition of
aryl boronic acids.[2d,e] In place of transmetalation, the
À
directed aromatic C H activation at the ortho positions may
interfere with decarboxylation, as demonstrated with rho-
dium and other late transition metals.[5d,6h,12] Third, attack on
the olefin by carboxylic acids as oxygen-based nucleophiles
may also occur as a competitive pathway.[13] Lastly, the
selectivity issue of the desired conjugate-addition products
(Michael-type) versus oxidative-arylation products (Heck–
Mizoroki-type) may arise if b-hydride elimination occurs
after the decarboxylation and olefin insertion steps
(Scheme 1, formation of E).[5,14]
[*] Dr. Z.-M. Sun, Prof. Dr. P. Zhao
Department of Chemistry and Molecular Biology
North Dakota State University
1231 Albrecht Avenue, P.O. Box 6050, Fargo, ND 58105-6050 (USA)
Fax: (+1)701-231-8831
http://www.chem.ndsu.nodak.edu/people/faculty/zhao.html
E-mail: pinjing.zhao@ndsu.edu
[**] Financial support for this research was provided by an ND EPSCoR
seed grant (EPS-0447679) and the NDSU startup fund. We thank Dr.
Angel Ugrinov for assistance with X-ray crystallographic analysis.
With these possible obstacles in mind, we began our
investigation by the design and preparation of phosphine-
ligated rhodium(I) carboxylates and tested their reactivity
towards stoichiometric decarboxylative conjugate addition.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200901097.
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Chemie
We focused our attention on chelating bisphosphine ligands as
a result of their successful application in rhodium-catalyzed
conjugate addition reactions of boronic acids.[2] For the
benchmark carboxylate group, we chose 2,6-difluorobenzoate
for the following reasons: 1) the favorable combination of an
electron-deficient fluorinated aryl group and an electron-rich
RhI center should facilitate decarboxylation from a thermo-
dynamic viewpoint (product stabilization);[15] 2) substitution
at both ortho positions would prevent the undesirable ortho
characterized by spectroscopic methods, and both structures
À
C H activation pathway; 3) the moderate steric hindrance of
ortho fluoro substituents would probably promote decarbox-
were determined by single-crystal X-ray diffraction
(Figure 1).[19] In the solid state, the chelating carboxylato
ligand forced 1 into a significantly distorted square-planar
geometry, as also observed for the related structure of [Rh(k2-
O2CCH3)(PiPr3)2].[19a] Complex 6 adopts a near-square-planar
geometry, with apparent p–p stacking between the difluoro-
phenyl plane and a phenyl group on the adjacent phosphorus
atom (centroid distance: ca. 3.53 ꢀ; see the Supporting
Information).[20]
ylation through ground-state destabilization, but should not
significantly hinder subsequent C C bond formation through
olefin insertion. Furthermore, fluorinated aryl groups are
themselves highly useful building blocks in biomedical
studies.[16]
À
The highest reactivity was observed for a bidentate
rhodium(I) carboxylato complex, [(biphep)Rh{k2-O2C(2,6-
F2C6H3)}] (1; biphep = 2,2’-bis(diphenylphosphanyl)-1,1’-
biphenyl), which was prepared from 2,6-difluorobenzoic
acid (2a) and [{(cod)Rh(OH)}2] [Eq. (1)].[17] The treatment
of 1 with excess n-butyl acrylate (3a, 6 equiv) in dry toluene at
1208C gave a mixture of the conjugate-addition product 4a
and a Heck–Mizoroki product 5 (1:6) in 69% combined yield
[Eq. (2)]. In contrast, when the same mixture of 1 and 3a was
heated in a 10:1 mixture of toluene and H2O, 4a was formed
selectively in near-quantitative yield [Eq. (3)]. These results
suggested that the decarboxylation of 1 did occur to generate
an aryl rhodium(I) intermediate; subsequent olefin insertion
into the rhodium–aryl linkage, followed by competitive
hydrolysis/b-hydride elimination, generated 4a or 5, respec-
tively (see Scheme 1).[2c,d]
Figure 1. ORTEP diagrams of [Rh(biphep){k2-O2C(2,6-F2C6H3)}] (1, left)
and [Rh(biphep)(2,6-F2C6H3)(pyridine)] (6, right). Thermal ellipsoids
are set at the 30% probability level; hydrogen atoms are omitted for
clarity.
To further elucidate the decarboxylation step, we sought
to identify the rhodium(I) aryl intermediate before the olefin-
insertion step. The thermal decomposition of 1 in the absence
The stoichiometric investigation guided our efforts to
develop a catalytic process. Some key results are shown in
Table 1. The catalytic decarboxylative conjugate addition of
2a (1 equiv) to 3a (1.5 equiv) proceeded smoothly at 1208C
with
a catalyst system composed of [{(cod)Rh(OH)}2]
(1.5 mol%), the biphep ligand (3 mol%), and NaOH as an
additive (1.0 equiv); the desired conjugate-addition product
4a was formed in near-quantitative yield (Table 1, entry 1).
Notably, this reaction occurred in a common solvent system
composed of toluene and water (10:1), whereas polar
solvents, such as 1-methyl-2-pyrrolidinone, dimethyl sulfox-
ide, and N,N-dimethylformamide, which are used in typical
palladium-catalyzed decarboxylative Heck–Mizoroki reac-
tions and cross-coupling reactions were not suitable.[4] The
less expensive racemic binap ligand gave equally good results
(Table 1, entry 2), whereas the use of other phosphine ligands
led to a lower yield and lower selectivity for 4a over the
Heck–Mizoroki by-product 5 (Table 1, entries 4–12). As
expected, the aqueous reaction medium was beneficial for
optimal yield and selectivity (see Table 1, entry 14). The
choice of NaOH as an inorganic additive was also critical for
satisfactory results, although its role remains unclear at this
stage (Table 1, entries 15–20).[21] Interestingly, the ligand
(R,R)-diop[22] promoted the selective formation of 5 over 4a
of an olefin substrate failed to generate detectable organo-
rhodium complexes, presumably as a result of the instability
of the proposed [(biphep)Rh(2,6-F2C6H3)] complex as the
direct decarboxylation product. However, when 1 was heated
at 808C with pyridine, clean formation of a discrete aryl
rhodium(I) complex, [(biphep)Rh(2,6-F2C6H3)(pyridine)]
(6), was observed [Eq. (4)].[18] Complexes 1 and 6 were
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Communications
Table 1: Development of the catalytic reaction conditions.[a]
Table 2: Scope of the decarboxylative conjugate addition.[a]
Entry ArCOOH
Olefin
Product
Yield
[%][b]
1
2
87 (99)
70 (75)
Entry
Ligand
Additive
Yield [%][b]
(4a and 5)
4a/5
1
2
biphep
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
none
99
99
96
0
77
73
53
68
0
0
0
<5
0
72
<3
83
91
95
0
0
62
95
>99:1
>99:1
3a
3a
3a
3a
rac-binap
(R,R)-diop
dpppentane
dppb
dppp
diphos
dppf
xantphos
PEt3
PPh3
PtBu3
3[c]
4
1:19
–
1.4:1
1.3:1
3:1
1.2:1
–
–
–
3:1
–
20:1
10:1
>99:1
>99:1
>99:1
–
3
4
5
80 (90)
56 (72)
75 (89)
5
6
7
8
9
10
11
12
13
14[d]
15
16
17
18
19
20
21[e]
22[f]
none
rac-binap
rac-binap
rac-binap
rac-binap
rac-binap
rac-binap
rac-binap
rac-binap
rac-binap
KOH
LiOH
6
3a
3a
48 (61)
55
Na2CO3
LiCl
pyridine
NaOH
NaOH
–
7[c]
49:1
>99:1
[a] Reaction conditions: 2a (1 equiv), 3a (1.5 equiv), [{(cod)Rh(m-OH)}2]
(0.015 equiv), phosphine ligand (0.03 equiv for chelating phosphines,
0.06 equiv for monophosphines), additive (1.0 equiv), toluene/H2O
(10:1), 1208C, 24 h. [b] The combined yield of 4a and 5 was determined
by GC. [c] The reaction was carried out with [{(cod)Rh(m-OH)}2]
(0.025 equiv), (R,R)-diop (0.05 equiv), and 3a (3.0 equiv). [d] The
reaction was carried out in dry toluene. [e] RhCl3 (0.03 equiv) was used
instead of [{(cod)Rh(m-OH)}2]. [f] [{(coe)2Rh(m-Cl)}2] (0.015 equiv) was
used instead of [{(cod)Rh(m-OH)}2] (coe=cis-cyclooctene). Binap=2,2’-
bis(diphenylphosphanyl)-1,1’-binaphthyl, dpppentane=1,5-bis(diphe-
8[d]
3a
3a
(18)
0
9[e]
10
11
12
13
2a
2a
2a
2a
65 (74)
nylphosphanyl)pentane,
dppb=1,4-bis(diphenylphosphanyl)butane,
dppp=1,3-bis(diphenylphosphanyl)propane, dppf=1,1’-bis(diphenyl-
63 (82)
56 (68)
(20)
phosphanyl)ferrocene,
dimethylxanthene.
xantphos=4,5-bis(diphenylphosphanyl)-9,9-
(19:1) in 96% combined yield (Table 1, entry 3). In this case, a
larger excess of the olefin 3a (3.0 equiv) was required: this
substrate presumably also serves as a sacrificial hydrogen
acceptor. Thus, the chemoselectivity of the reaction could be
reversed with a different phosphine ligand, and our catalyst
system could potentially serve as an alternative for the
palladium-catalyzed decarboxylative Heck–Mizoroki reac-
tion described by Myers and co-workers, who required a
stoichiometric amount of a silver salt.[5]
Having established the standard reaction conditions, we
tested 2a with other electron-poor olefin substrates in
conjugate addition reactions. Ethyl- and tert-butyl acrylate,
as well as N,N-dimethylacrylamide, reacted with 2a to give
the corresponding conjugate-addition products 4j–l in good
yields (Table 2, entries 10–12). Methyl vinyl ketone showed
poor reactivity (Table 2, entry 13), and more-hindered sub-
strates, such as 2-cyclohexen-1-one and ethyl (E)-2-crotonate,
were unreactive.
[a] Reaction conditions: 2 (0.23 mmol), 3 (1.5 equiv), [{(cod)Rh(OH)}2]
(0.015 equiv), rac-binap (0.030 equiv), NaOH (1.0 equiv), toluene/H2O
(1.0 mL/100 mL), 1208C, 24 h. [b] Average yield of the isolated product
from two reactions. The yield determined by GC is given in parentheses.
[c] The reaction was carried out with H2O (150 mL) in toluene (1.0 mL)
and with n-butyl acrylate in greater excess (5.0 equiv). The Mizoroki–
Heck product (11%) and 1,3,5-trimethoxybenzene (25%) were also
formed. [d] The reaction was carried out at 1508C for 24 h. [e] Substrate
2i was recovered unchanged.
In terms of the benzoic acid substrate, high reactivity was
limited to derivatives with fluoro substituents at both ortho
positions. These compounds underwent the conjugate addi-
tion to form the corresponding products 4a–f in good yields
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Katoh, T. Hayashi, M. Sakai, H. Hayashi, Angew. Chem. 2007,
and without detectable amounts of Mizoroki–Heck by-
products (Table 2, entries 1–6). 2,4,6-Trimethoxybenzoic
acid was less reactive: the conjugate-addition product 4g
was formed in 55% yield together with by-products from the
Mizoroki–Heck reaction and reductive decarboxylation when
a larger excess of n-butyl acrylate (5 equiv) was used (Table 2,
entry 7). 2-Fluorobenzoic acid displayed significantly reduced
reactivity (Table 2, entry 8), and the parent compound
benzoic acid was inert under the reaction conditions
(Table 2, entry 9). Other less substituted benzoic acids
showed little or no decarboxylation reactivity.[23] Reduced
reactivity towards decarboxylation has been a common
observation for benzoic acids that lack ortho substituents[4]
119, 5025 – 5027; Angew. Chem. Int. Ed. 2007, 46, 4937 – 4939;
b) T. Nishimura, T. Katoh, K. Takatsu, R. Shintani, T. Hayashi, J.
Am. Chem. Soc. 2007, 129, 14158 – 14159; c) R. Shintani, K.
Takatsu, T. Katoh, T. Nishimura, T. Hayashi, Angew. Chem.
2008, 120, 1469 – 1471; Angew. Chem. Int. Ed. 2008, 47, 1447 –
1449.
[4] For a recent review, see: L. J. Gooßen, N. Rodrꢃguez, K. Gooßen,
Angew. Chem. 2008, 120, 3144 – 3164; Angew. Chem. Int. Ed.
2008, 47, 3100 – 3120.
[5] For palladium-catalyzed decarboxylative Heck–Mizoroki olefi-
nation reactions, see: a) A. G. Myers, D. Tanaka, M. R. Man-
nion, J. Am. Chem. Soc. 2002, 124, 11250 – 11251; b) D. Tanaka,
A. G. Myers, Org. Lett. 2004, 6, 433 – 436; c) D. Tanaka, S. P.
Romeril, A. G. Myers, J. Am. Chem. Soc. 2005, 127, 10323 –
10333; d) A. Maehara, H. Tsurugi, T. Satoh, M. Miura, Org.
Lett. 2008, 10, 1159 – 1162.
À
and is proposed to result partly from competitive ortho C H
activation.[5a,7]
[6] For palladium-catalyzed decarboxylative biaryl synthesis, see:
a) L. J. Gooßen, G. Deng, L. M. Levy, Science 2006, 313, 662 –
664; b) L. J. Gooßen, N. Rodrꢃguez, M. Bettina, C. Linder, G.
Deng, L. M. Levy, J. Am. Chem. Soc. 2007, 129, 4824 – 4833;
c) L. J. Gooßen, F. Rudolphi, C. Oppel, N. Rodrꢃguez, Angew.
Chem. 2008, 120, 3085 – 3088; Angew. Chem. Int. Ed. 2008, 47,
3043 – 3045; d) L. J. Gooßen, B. Zimmermann, T. Knauber,
Angew. Chem. 2008, 120, 7211 – 7214; Angew. Chem. Int. Ed.
2008, 47, 7103 – 7106; e) L. J. Gooßen, N. Rodrꢃguez, C. Linder,
J. Am. Chem. Soc. 2008, 130, 15248 – 15249; f) P. Forgione, M.-C.
Brochu, M. St-Onge, K. H. Thesen, M. D. Bailey, F. Bilodeau, J.
Am. Chem. Soc. 2006, 128, 11350 – 11351; g) J.-M. Becht, C. D.
Catala, C. Le Drian, A. Wagner, Org. Lett. 2007, 9, 781 – 784;
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10, 1851 – 1854; i) J.-M. Becht, C. Le Drian, Org. Lett. 2008, 10,
3161 – 3164; j) A. Voutchkova, A. Coplin, N. E. Leadbeater,
R. H. Crabtree, Chem. Commun. 2008, 6312 – 6314.
[7] For copper- and palladium-catalyzed reductive decarboxylation,
see: a) L. J. Gooßen, W. R. Thiel, N. Rodrꢃguez, C. Linder, B.
Melzer, Adv. Synth. Catal. 2007, 349, 2241 – 2246; b) J. S. Dick-
stein, C. A. Mulrooney, E. M. OꢄBrien, B. J. Morgan, M. C.
Kozlowski, Org. Lett. 2007, 9, 2441 – 2444.
[8] For rhodium- and iridium-catalyzed decarboxylative alkyne
insertion, see: K. Ueura, T. Satoh, M. Miura, J. Org. Chem.
2007, 72, 5362 – 5367.
In conclusion, a new method for catalytic conjugate
addition reactions has been developed on the basis of the
decarboxylative generation of rhodium(I) aryl intermediates
from fluorinated benzoic acids. Current efforts are focused on
ligand modification and further mechanistic probing. We
envision that an improved catalytic system would enable us to
overcome the current limitation in terms of substrate scope
and to develop applications in asymmetric catalysis.
Experimental Section
General procedure: Compound 2 (0.225 mmol), 3 (0.34 mmol), and
NaOH (0.225 mmol) were placed in a 4 mL screw-cap vial equipped
with a magnetic stirrer bar in a nitrogen-filled glovebox. Degassed
H2O (100 mL) and a stock solution of [{(cod)Rh(OH)}2] (0.034 mmol)
and binap (0.068 mmol) in toluene (1.0 mL) were added with a
syringe. The reaction vessel was sealed with a silicone-lined screw cap
and removed from the glove box, and the reaction mixture was stirred
at 1208C for 24 h. The mixture was then cooled to room temperature,
and all volatile materials were removed under reduced pressure. The
residue was extracted into EtOAc (30 mL), washed with brine (3 ꢁ
20 mL), dried over anhydrous MgSO4, filtered, and concentrated.
Purification by flash column chromatography (SiO2, 2–15% EtOAc/
hexane) yielded the corresponding product 4.
[9] For recent examples of palladium-catalyzed decarboxylative
allylation and the decarboxylative protonation of activated
allylic esters, see: a) D. E. White, I. C. Stewart, R. H. Grubbs,
B. M. Stoltz, J. Am. Chem. Soc. 2008, 130, 810 – 811; b) C. Wang,
J. A. Tunge, J. Am. Chem. Soc. 2008, 130, 8118 – 8119.
[10] a) A. D. English, T. Herskovitz, J. Am. Chem. Soc. 1977, 99,
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Quaranta, I. Tommasi, J. Organomet. Chem. 2000, 605, 143 – 150.
[11] a) K. Ukai, M. Aoki, J. Takaya, N. Iwasawa, J. Am. Chem. Soc.
2006, 128, 8706 – 8707; b) C. S. Yeung, V. M. Dong, J. Am. Chem.
Soc. 2008, 130, 7826 – 7827.
Received: February 26, 2009
Revised: June 25, 2009
Published online: August 5, 2009
Keywords: carboxylic acids · conjugate addition ·
.
decarboxylation · Heck–Mizoroki reaction · rhodium
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Daugulis, J. Am. Chem. Soc. 2007, 129, 9879 – 9884.
[13] For the rhodium-catalyzed conjugate addition of carboxylic
acids and alcohols as oxygen-based nucleophiles, see: M. V.
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7441 – 7443.
[14] G. Zou, J. Guo, Z. Wang, W. Huang, J. Tang, Dalton Trans. 2007,
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[15] For early studies on the stoichiometric decarboxylation of PPh3-
ligated RhI and IrI perfluorinated benzoates, see: a) G. B.
Deacon, S. J. Faulks, J. M. Miller, Transition Met. Chem. 1980,
5, 305 – 313; b) G. B. Deacon, S. J. Faulks, J. Organomet. Chem.
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[2] For reviews, see: a) K. Yoshida, T. Hayashi in Modern Rhodium-
Catalyzed Organic Reactions (Ed.: P. A. Evans), Wiley-VCH,
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4231; e) T. Hayashi, M. Takahashi, Y. Takaya, M. Ogasawara, J.
Am. Chem. Soc. 2002, 124, 5052 – 5058.
[3] For the generation of rhodium(I) aryl and alkynyl nucleophiles
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Communications
[17] V. V. Grushin, V. F. Kuznetsov, C. Bensimon, H. Alper, Organo-
tional, ligand-dependent thermodynamic driving force for
decarboxylation.
metallics 1995, 14, 3927 – 3932; cod = 1,5-cyclooctadiene.
[18] Preliminary results from kinetic studies showed inhibition of
decarboxylation by a larger excess of pyridine (4 equiv). This
observation is consistent with a pathway involving reversible
pyridine dissociation from a pyridine-coordinated rhodium(I)
carboxylate, rate-limiting CO2 release, and subsequent pyridine
coordination to stabilize the final product.
[19] For examples of relevant reported structures, see: a) H. Werner,
M. Schꢅfer, O. Nꢆrnberg, J. Wolf, Chem. Ber. 1994, 127, 27 – 38;
b) C. Krug, J. F. Hartwig, J. Am. Chem. Soc. 2004, 126, 2694 –
2695. CCDC 704653 (1) and 704654 (6) contain the supplemen-
tary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[21] We suspect that reversible decarboxylation/carboxylation may
occur for certain substrates and that NaOH may act as a CO2
scavenger to promote decarboxylation. Additionally, the for-
mation of sodium benzoate may promote the decarboxylation
process as suggested in Ref. [6e].
[22] (R,R)-Diop = (4R,5R)-(À)-4,5-bis(diphenylphosphanylmethyl)-
2,2-dimethyl-1,3-dioxolane.
[23] Since the rhodium(I)-catalyzed conjugate addition of aryl
boronic acids displays much broader substrate scope,[2] this
limitation in terms of the type of benzoic acids that can be used
appears to originate from limited decarboxylation reactivity. In
particular, the observation of the highest reactivity with 2,6-
difluoro or 2,6-dimethoxy substituents is consistent with calcu-
lations by Gooßen et al., the results of which indicated highest
decarboxylation reactivities for copper(I) benzoates with two
strongly s withdrawing substituents.[7a]
[20] Such p–p stacking was not observed in the related structure of
[(diphos)Rh(p-tolyl)(pyridine)] (diphos = 1,2-bis(diphenylphos-
phanyl)ethane),[19b] which suggests the existence of an addi-
6730 www.angewandte.org
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2009, 48, 6726 –6730