Gold(I)-catalyzed cycloisomerization of arylvinylcyclopropenes: An efficient synthetic protocol for the construction of indene skeletons

Article abstract of DOI:10.1002/chem.200801370

An efficient gold(I)-catalyzed intra-molecular cycloisomerization of arylvinylcyclopropenes to produce indene derivatives in good to excellent yields was reported. Arylvinylcyclopropanes, AgSbF₆, PPh₃AuCl, DBU and DCE were added into

Full text of DOI:10.1002/chem.200801370

COMMUNICATION
DOI: 10.1002/chem.200801370
Gold(I)-Catalyzed Cycloisomerization of Arylvinylcyclopropenes:
An Efficient Synthetic Protocol for the Construction of Indene Skeletons
Zhi-Bin Zhu[a] and Min Shi*[a, b]
As the smallest of cycloolefins, the cyclopropenes[1] are
highly strained[2] but readily accessible substances, which
have been serving as useful building blocks in many organic
reactions.[3] It can bee seen that during the last several de-
cades, thermal and photochemical skeleton rearrangements
as well as metal-catalyzed/-mediated reactions of cyclopro-
pene-containing compounds have attracted much attention
from both synthetic and mechanistic viewpoints and numer-
ous interesting transformations have been disclosed.[4] For
example, we recently have explored a new type of arylvinyl-
cyclopropenes from the corresponding arylvinylidenecyclo-
propanes under basic conditions and have reported that the
choice of Lewis acid catalyst can result in dramatic differen-
ces in the chemoselectivity of the rearrangement reactions
of these vinylcyclopropenes, leading to naphthalene and
indene derivatives (Scheme 1).[5a] This is because that differ-
ent Lewis acids can form the key carbon cationic intermedi-
ates at the different positions of cyclopropane. In fact, trace
of water in these systems plays a very important role to re-
lease H+ as the real catalyst if using BF3·OEt2 as the cata-
lyst.[5b] Gold, which has emerged as the most efficient cata-
lyst for the activation of alkynes, allenes and alkenes, has re-
cently been a rising star of transition-metal catalysts.[6] Since
there is the similarity on the chemical behavior between
LAu+ and H+,[7] we envisaged that intramolecular rear-
rangement may take place in an interesting manner in the
presence of gold(I) under mild conditions. In this communi-
cation, we wish to report an efficient gold(I)-catalyzed intra-
molecular cycloisomerization of arylvinylcyclopropenes to
produce indene derivatives in good to excellent yields.[8] To
the best of our knowledge, this is the first example of gold-
catalyzed intramolecular reaction of cyclopropenes.
Scheme 1. Lewis acid promoted rearrangement of arylvinylcyclopro-
penes.
Using (2-(3,3-dimethyl-2-phenylcycloprop-1-enyl)ethene-
1,1-diyl)dibenzene (1a) as the substrate, we examined the
reaction in the presence of Au+ (5 mol%). We found that 2-
(2,2-diphenylvinyl)-1,1-dimethyl-1H-indene (2a), which was
unambiguously determined by X-ray diffraction (Figure 1),[9]
and 1-(2-methyl-1-phenylprop-1-enyl)-3-phenyl-1H-indene
(3a) were obtained in excellent yield and moderate selectiv-
ity (2a/3a 75:25) at 508C in 1,2-dichloroethane (DCE)
(Table 1, entry 1). Interestingly, indene derivative 3a was
formed as the minor product in this reaction, but it could be
obtained as the sole product in the presence of CuACTHNUTRGNEUNG
(OTf)2.[5a]
[a] Z.-B. Zhu, Prof. M. Shi
Lab for Advanced Materials and Institute of Fine Chemicals
East China University of Science and Technology
130 Mei Long Road, Shanghai 200237 (China)
Fax : (+86)21-64166128
E-mail: mshi@mail.sioc.ac.cn
[b] Prof. M. Shi
State Key Laboratory of Organometallic Chemistry
Shanghai Institute of Organic Chemistry
354 Fenglin Road, Shanghai, 200032 (China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200801370.
Figure 1. ORTEP drawing of compound 2a.
Chem. Eur. J. 2008, 14, 10219 – 10222
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
10219

Table 1. Optimization of the reaction conditions.[a]
During the further examina-
tion, we also found that 2a
could not be obtained upon
treatment of 1a with the other
Lewis acids (5 mol%) or
Brønsted acids (5 mol%) such
Entry
Catalyst
Additive
Solvent
T [oC]
t [h]
Yield [%][b] (2a/3a)
as LnACHTUNGTRENNUNG(OTf)3 or HOTf (Tf=
1
2
3
PPh3AuCl/AgOTf
PPh3AuCl/AgOTf
PPh3AuCl
–
–
–
DCE
DCE
DCE
50
20
50
24
24
24
99 (75:25)
trace
–
CF3SO3) under identical condi-
tions. Moreover, it was found
that this reaction gave 2a in
trace at room temperature
(Table 1, entry 2). In the ab-
sence of AgOTf, no reaction
occurred, suggesting that the
in situ generated Au+ species
is the real catalyst (Table 1,
entry 3). Using AgOTf as the
catalyst afforded 2-isopropyl-
1,4-diphenylnaphthalene as the
single product. This result was
very similar as that of
BF3·OEt2-catalyzed result, sug-
gesting that H+ might be the
real catalyst in this reaction
(Table 1, entry 4).[5a] Adding
methanol to the reaction
system did not improve the
yield and selectivity (Table 1,
entry 5).[10] To clarify the influ-
ence of trace of H+ in this re-
action, we added acetic acid
(50 mol%) or various bases
4
AgOTf
–
DCE
50
24
5
6
PPh3AuCl/AgOTf
PPh3AuCl/AgOTf
PPh3AuCl/AgOTf
PPh3AuCl/AgOTf
PPh3AuCl/AgOTf
PPh3AuCl/AgOTf
AgOTf
PPh3AuCl/AgOTf
PPh3AuCl/AgOTf
PPh3AuCl/AgOTf
PPh3AuCl/AgOTf
PPh3AuCl/AgOTf
PPh3AuCl/AgSbF6
PPh3AuCl/AgBF4
PPh3AuCl/AgOTf[d]
PPh3AuCl/AgOTf[e]
MeOH
CH3CO2H
Et3N
DCE
DCE
DCE
DCE
DCE
DCE
DCE
toluene
THF
DCM
MeOH
MeCN
DCE
DCE
DCE
DCE
50
50
50
50
50
50
50
50
50
20
50
50
50
50
50
50
10
10
10
10
10
10
10
10
10
24
24
24
10
10
10
10
94 (76:24)
90 (82:18)
13 (100/0)
90 (83:17)
90 (76:24)
97 (100:0)
–
trace of 2a
–
14 (100:0)
–
7[c]
8
TTBP
tBuOK
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
9
10
11
12
13
14
15
16
17
18
19
20
–
94 (100:0)
88 (100:0)
99 (77:23)
96 (84:16)
[a] All reactions were carried out using 1 (0.2 mmol), additive (50 mol%) in the presence of catalyst (5 mol%)
in various solvents (1.0 mL). [b] Isolated yield, the ratio of 2a/3a was determined by 1H NMR spectroscopic
data. [c] 80% of 1a was recovered. [d] 10 mol% of DBU was added. [e] 20 mol% of DBU was added.
(50 mol%) such as Et3N, 2,4,6-tri-tert-butylpyridine (TTBP)
and KOtBu as the additives to examine the reaction out-
come. Adding acetic acid did not cause significant alteration
(Table 1, entry 6). In the presence of Et3N, most of starting
materials were recovered and 2a was obtained in 13% yield
(Table 1, entry 7).[11] As for TTBP or KOtBu, which only can
act as a base to neutralize the trace of H+ in the reaction
system, the similar result was obtained as that of PPh3AuCl/
AgOTf, suggesting again that Au+ is the real catalyst in this
reaction (Table 1, entries 8 and 9). To our delight, using 1,8-
With these optimal conditions in hand, we next examined
a variety of arylvinylcyclopropenes 1 in this reaction and the
results of these experiments are shown in Table 2. When
electron-rich arylvinylcyclopropenes 1b and 1c were used as
the substrates, the corresponding products 2b and 2c were
produced in excellent yields in the presence of PPh3AuCl/
AgOTf (Table 2, entries 1 and 2). However, in the cases of
arylvinylcyclopropenes 1d–f having electron-withdrawing
groups on the benzene rings, traces of 2d–f were obtained
under the standard conditions. Fortunately, if using
PPh3AuCl/AgSbF6 (Table 2, entries 3–5) or PPh3AuCl/
AgBF4 (Table 2, entry 6) as the catalyst, products 3d–f could
diazabicycloACHTUNGTRENNUNG[5.4.0]undec-7-ene (DBU) as an additive afford-
ed 2a as a single product in 97% yield, indicating that weak
coordinating amine can adjust the catalytic ability of gold(I)
(Table 1, entry 10).[11] In the presence of DBU, AgOTf did
not promote the rearrangement of 1a, suggesting clearly
that H+ is the real catalyst in entry 4 of Table 1 (Table 1,
entry 11). The examination of solvent effects revealed that
DCE is the best one for the reaction (Table 1, entries 12–
16). Other silver salts such as AgSbF6 and AgBF4 could also
produce 2a in excellent yields as a sole product in the pres-
ence of DBU (Table 1, entries 17 and 18). We also examined
10 mol% and 20 mol of DBU in this reaction and found
that 2a and 3a were both formed in the ratios of 77:23 and
84:16, respectively (Table 1, entries 19 and 20).
be produced in good to excellent yields. As for spiro
ACHTUNGTRENNUNG[2.5]oct-
ene (1g) and spiro[2.4]hept-ene (1h) as well as (2-(2-(2-
AHCTUNGTRENNUNG
chlorophenyl)-3,3-dimethylcycloprop-1-enyl)ethene-1,1-di-
yl)dibenzene (1i) which has an ortho-substituted chloro
atom on the aromatic ring of R2, the reactions could also
proceed smoothly to produce indene products 2g, h and i in
good yields in the presence of PPh3AuCl/AgSbF6 (Table 2,
entries 7–9). When there was a meta group on the aromatic
ring of R2, the product mixtures of 5-chloro-2-(2,2-diphenyl-
vinyl)-1,1-dimethyl-1H-indene and 7-chloro-2-(2,2-diphenyl-
vinyl)-1,1-dimethyl-1H-indene were obtained in a 1:1 ratio
and in excellent yields (Table 2, entry 10). Either R3 or R4
was hydrogen atom or both of them were hydrogen atoms,
10220
www.chemeurj.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 10219 – 10222

Gold(I)-Catalyzed Cycloisomerization of Arylvinylcyclopropenes
COMMUNICATION
Table 2. Scope of Au-catalyzed rearrangement of 1.[a]
notable that electron-with-
drawing groups on the R1 or
R2 aromatic ring slow down
the reaction rate when AgOTf
was used as the source of
counter ion of gold(I), but if
using AgSbF6 as the source of
counter ion, the reactions can
easily take place. According to
the recent DFT calculation
Entry
1 (R1/R2/R3/R4)
Catalyst
t [h]
Yield [%][b]
1
2
3
4
(C6H5/p-MeC6H4/Me/Me), 1b
(p-MeC6H4/C6H5/Me/Me), 1c
(C6H5/p-ClC6H4/Me/Me), 1d
(p-ClC6H4/C6H5/Me/Me), 1e
(p-FC6H4/C6H5/Me/Me), 1 f
(p-ClC6H4/C6H5/Me/Me), 1e
PPh3AuCl/AgOTf
PPh3AuCl/AgOTf
PPh3AuCl/AgSbF6
PPh3AuCl/AgSbF6
PPh3AuCl/AgSbF6
PPh3AuCl/AgSbF6
10
12
15
15
15
15
>99, 2b
94, 2c
89, 2d
90, 2e
99, 2 f
86, 2e
5[c]
6
data,[13]
PPh3AuBF4
or
PPh3AuSbF6 has lower dissoci-
ation energy than that of
7
8
PPh3AuCl/AgSbF6
10
86, 2g
85, 2h
PPh3AuOTf.
Therefore,
PPh3AuSbF6 can more easily
generate PPh3Au+ to coordi-
nate with 1a than that of
PPh3AuOTf under identical
conditions.
In summary, we have devel-
oped an efficient cycloisomeri-
zation of (2-vinylcycloprop-1-
enyl)benzene 1 catalyzed by
gold(I). This synthetic protocol
PPh3AuCl/AgSbF6
10
9
(C6H5/o-ClC6H4/Me/Me), 1i
(C6H5/m-ClC6H4/Me/Me), 1j
(C6H5/p-MeC6H4/Me/H), 1k
(C6H5/p-MeC6H4/H/H), 1l
PPh3AuCl/AgSbF6
PPh3AuCl/AgSbF6
PPh3AuCl/AgSbF6
PPh3AuCl/AgSbF6
24
10
10
10
98, 2i
10
11
12
94, 2j/2j[d]
complex
complex
[a] All reactions were carried out using 1 (0.2 mmol) in the presence of catalyst (0.01 mmol), DBU (0.1 mmol)
in DCE (1.0 mL) at 508C for 10 h. [b] Isolated yield. [c] The reaction was carried out at 658C. [d] Ratio of 2j/
2j’ 1:1.
furnishes
2-vinyl-1H-indene
complex product mixtures were formed under the standard
conditions (Table 2, entries 11 and 12).
straightforwardly from simple
A plausible mechanism for the formation of these indene
derivatives is outlined in Scheme 2. Activation of cyclopro-
pene 1a by gold(I) forms intermediate A,[12] which can pro-
duce intermediate B or B’ via the addition of LAu+ to the
double bond of cyclopropene at different position. Inter-
À
mediate B undergoes cleavage of C1 C2 bond in cyclopro-
pane cation to form p-allyl cationic intermediate C-1 or its
resonance-stabilized Au-carbene intermediate C-2, which
produces intermediate D either via intramolecular Friedel–
Crafts reaction of C-1 or intramolecular Michael addition of
C-2. Deprotonation of intermediate D followed by replace-
ment of LAu+ with proton provides indene derivative prod-
uct 2a. On the other hand, intermediate B’ undergoes cleav-
À
age of C1 C3 bond in cyclopropane cation to form p-allyl
cationic intermediate C’-1 or its resonance-stabilized cation-
ic intermediate C’-3 as well as Au–carbene intermediate C’-
2. From p-allyl cationic intermediate C’-3, intramolecular
Friedel–Crafts reaction takes place to give intermediate D’,
which produces 3a via deprotonation and replacement of
Au+ with proton (Scheme 2). The mechanism on the forma-
tion of 3a is very similar to that of CuACTHNUTRGNEUNG(OTf)2-catalyzed rear-
rangement.[5a] The addition of DBU eliminates trace of
HOTf or HSbF6, which is generated by trace water and
AgOTf or AgSbF6 and also might have weak coordination
effect with gold(I) that results in perfect chemoselectivity in
the addition step.[11] If either R3 or R4 is hydrogen atom or
both of them are hydrogen atoms, intermediate C-1 will be
a secondary or a primary carbocationic intermediate, which
is not as stable as those of R3 and R4 are alkyl substituents
and can hardly produce the rearrangement products. It is
Scheme 2. Gold(I)-catalyzed cycloisomerization of arylvinylcyclopro-
penes.
Chem. Eur. J. 2008, 14, 10219 – 10222
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
10221

M. Shi and Z.-B. Zhu
f) S. Ma, S. Yu, Z. Gu, Angew. Chem. 2007, 119, 206–209; Angew.
Chem. Int. Ed. 2006, 45, 200–203; g) A. H. -Rçder, N. Krause, Org.
Biomol. Chem. 2005, 3, 387–391; h) A. Arcadi, S. Di Giuseppe,
Curr. Org. Chem. 2004, 8, 795–812; i) A. M. Echavarren, C. Nevado,
Chem. Soc. Rev. 2004, 33, 431–436; j) A. S. K. Hashmi, Gold Bull.
2004, 37, 51–65.
starting materials in good to excellent yields under mild con-
ditions, substantially enriching gold chemistry. A plausible
mechanism has been proposed that is based on intramolecu-
lar Friedel–Crafts reaction or Michael addition reaction
pathway. Clarification of the reaction mechanism and fur-
ther application of this chemistry are in progress.
[7] Differences between proton (H+) with the isolobal fragments LAu+
and Hg2+ see: R. Hoffmann, Angew. Chem. 1982, 94, 725–739;
Angew. Chem. Int. Ed. Engl. 1982, 21, 711–724.
[8] For an example of photochemical rearrangement of cyclopropenes
to indene derivatives, see: a) H. E. Zimmerman, D. J. Kreil, J. Org.
Chem. 1982, 47, 2060–2075; for an example of thermal rearrange-
ment of cyclopropenes to indene derivatives, see b) M. A. Battiste,
B. Halton, R. H. Grubbs, Chem. Commun. 1967, 907–909; for an ex-
ample of silver-catalyzed rearrangement of cyclopropenes to indene
derivatives, see: c) A. Padwa, T. J. Blacklock, R. Loza, J. Am. Chem.
Soc. 1981, 103, 2404–2405; For an example of Ru-catalyzed rear-
rangements of cyclopropenes, see: d) A. Padwa, J. M. Kassir, S. L.
Xu, J. Org. Chem. 1991, 56, 6971–6972.
Experimental Section
General procedure of gold(I)-catalyzed rearrangement of arylvinylcyclo-
propene: Under an argon atmosphere, arylvinylcyclopropenes
1
(0.2 mmol), AgSbF6 (0.01 mmol), PPh3AuCl (0.01 mmol), DBU
(0.1 mmol) and DCE (1.0 mL) were added into a Schlenk tube. The reac-
tion mixture was stirred at 508C until the reaction completed. Then, the
solvent was removed under reduced pressure and the residue was puri-
fied by a flash column chromatography (SiO2).
[9] CCDC 678993 (2a) contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.a-
c.uk/data_request/cif. Empirical formula: C25H22; formula weight:
322.43; crystal color, habit: colorless, prismatic; crystal dimensions:
0.490”0.457”0.291 mmÀ3; crystal system: monoclinic; lattice type:
primitive; lattice parameters: a=19.953(8), b=10.396(4), c=
9.247(4) Š, a=90, b=93.274(9), g=908, V=1914.8(14) Š3; space
Acknowledgements
Financial support from the Shanghai Municipal Committee of Science
and Technology (04 JC14083, 06XD14005), National Basic Research Pro-
gram of China (973-2009CB825300), and the National Natural Science
Foundation of China (20472096, 203900502, 20672127 and 20732008) are
greatly acknowledged.
group: Cc; Z=4; 1calcd =1.118 gcmÀ3
Rigaku AFC7R; residuals: R; Rw: 0.0554, 0.1005. 2-(2,2-Diphenyl-
vinyl)-1,1-dimethyl-1-indene (2a): yellow solid; m.p. 87–898C;
; F000 =688; diffractometer:
AHCTUNGTRENNUNG
1H NMR (300 MHz, CDCl3, TMS): d = 1.37 (s, 6H, CH3), 5.76 (s,
1H, CH), 6.65 (s, 1H, CH), 7.01–7.04 (m, 1H, Ar), 7.10–7.13 (m,
2H, Ar), 7.21–7.40 ppm (m, 11H, Ar); 13C NMR (75 MHz, CDCl3,
TMS): d = 24.4, 50.7, 119.8, 121.1, 121.3, 124.9, 126.5, 126.7, 127.2,
127.4, 127.5, 128.2, 128.3, 128.8, 129.6, 141.1, 142.4, 142.8, 144.6,
152.5, 152.9 ppm; IR (CH2Cl2): n˜ = 3058, 3020, 2959, 2922, 2862,
1598, 1491, 1467, 1443, 1294, 1174, 1154, 1118, 1098, 1073, 890, 749,
699, 639 cmÀ1; MS (EI): m/z (%): 322 (100) [M+], 323 (27.59), 307
(44.77), 292 (26.90), 291 (29.40), 229 (38.18), 215 (24.31), 145
(24.43); elemental analysis calcd (%) for C25H22: C 93.12, H 6.88;
found: C 92.72, H 6.77
Keywords: cycloisomerization
homogeneous catalysis · indenes
· cyclopropene · gold ·
[1] For a review, see: M. S. Baird, in Cyclopropenes: Synthesis by Con-
struction of the System, Vol. E17d/2 (Ed.: A. de Meijere), Houben-
Weyl, Thieme, Stuttgart, 1996, pp. 2695–2744.
[2] a) R. D. Bach, O. Dmitrenko, J. Am. Chem. Soc. 2006, 128, 4598–
4611; b) P. R. Khoury, J. D. Goddard, W. Tam, Tetrahedron 2004, 60,
8103–8112; c) A. Fattahi, R. E. McCarthy, M. R. Ahmad, S. R. Kass,
J. Am. Chem. Soc. 2003, 125, 11746–11750.
[3] a) P. Binger, H. M. Bꢁch, Top. Curr. Chem. 1987, 135, 77–151;
b) P. W. Jennings, L. L. Johnoson, Chem. Rev. 1994, 94, 2241–2290;
c) M. S. Baird, in Cyclopropenes: Transformations, Vol. E17d/2 (Ed.:
A. de Meijere), Houben-Weyl, Thieme, Stuttgart, 1996, pp. 2781–
2860; d) M. Nakamura, H. Isobe, E. Nakamura, Chem. Rev. 2003,
103, 1295–1326.
[10] a) M. H. Chisholm, H. C. Clark, Inorg. Chem. 1971, 10, 2557–2563;
b) M. H. Chisholm, H. C. Clark, Acc. Chem. Res. 1973, 6, 202–209;
c) C. S. Browning, D. H. Farrer, Inorg. Chim. Acta 1988, 144, 105–
108.
[11] 31P NMR of AuPPh3Cl/AgSbF6, AuPPh3Cl/AgSbF6/Et3N and
AuPPh3Cl/AgSbF6/DBU suggested the coordination between Au+
and bases, which may affect the activity of Au+, see Supporting In-
formation.
[12] For activation of alkenes by gold, see: a) A. S. K. Hashmi, L.
Schwarz, J. H. Choi, T. M. Frost, Angew. Chem. 2000, 112, 2382–
2385; Angew. Chem. Int. Ed. 2000, 39, 2285–2288; b) C. Brouwer, C.
He, Angew. Chem. 2006, 118, 1776–1779; Angew. Chem. Int. Ed.
2006, 45, 1744–1747; c) X. Han, R. A. Widenhoefer, Angew. Chem.
2006, 118, 1779–1781; Angew. Chem. Int. Ed. 2006, 45, 1747–1749;
d) C. F. Bender, R. A. Widenhoefer, Chem. Commun. 2006, 4143–
4144; e) C. F. Bender, R. A. Widenhoefer, Org. Lett. 2006, 8, 5303–
5305; f) J. Zhang, C.-G Yang, C. He, J. Am. Chem. Soc. 2006, 128,
1798–1799; g) C.-G Yang, C. He, J. Am. Chem. Soc. 2005, 127,
6966–6967.
[4] For reviews, see: a) M. Rubin, M. Rubina, V. Gevorgyan, Chem.
Rev. 2007, 107, 3117–3179; b) M. Rubin, M. Rubina, V. Gevorgyan,
Synthesis 2006, 1221–1245; c) J. M. Fox, N. Yan, Curr. Org. Chem.
2005, 9, 719–732; d) R. Walsh, Chem. Soc. Rev. 2005, 34, 714–732;
e) W. R. Dolbier, Jr., M. A. Battiste, Chem. Rev. 2003, 103, 1071–
1098; f) A. Sekiguchi, V. Y. Lee, Chem. Rev. 2003, 103, 1429–1448;
g) K. C. Chen, G. A. Lee, Huaxue 2006, 64, 73–95.
[5] a) L.-X Shao, Y.-P Zhang, M.-H Qi, M. Shi, Org. Lett. 2007, 9, 117–
120; b) L.-X. Shao, B. Xu, J.-W. Huang, M. Shi, Chem. Eur. J. 2006,
12, 510–517.
[6] a) A. S. K. Hashmi, Chem. Rev. 2007, 107, 3180–3211; b) A. Fꢁrst-
ner, P. W. Davies, Angew. Chem. 2007, 119, 3478–3519; Angew.
Chem. Int. Ed. 2007, 46, 3410–3449; c) D. J. Gorin, F. D. Toste,
Nature 2007, 446, 395–403; d) N. Marion, S. P. Nolan, Angew. Chem.
2007, 119, 2806–2809; Angew. Chem. Int. Ed. 2007, 46, 2750–2752;
e) E. J. -NfflÇez, A. M. Echavarren, Chem. Commun. 2007, 333–346;
[13] G. Kovµcs, G. Ujaque, A. Lledós. J. Am. Chem. Soc. 2008; 130,
853–864.
Received: July 7, 2008
Published online: October 16, 2008
10222
www.chemeurj.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 10219 – 10222