Intramolecular carbostannylation of alkynes catalyzed by silver(I) species

Article abstract of DOI:10.1002/anie.200605041

Joined at the hip: The intramolecular carbostannylation of alkynes catalyzed by silver(I) stereoselectively gives E-alkenyl-stannanes as single isomers (see scheme, Z = C(CO₂Me)₂, C(SO ₂Ph)₂, C(CH₂OR)

Full text of DOI:10.1002/anie.200605041

Communications
DOI: 10.1002/anie.200605041
Cyclization
Intramolecular Carbostannylation of Alkynes Catalyzed by Silver(I)
Species**
Susana Porcel and Antonio M. Echavarren*
Cyclizations of a,w-enynes catalyzed by transition- or main-
group metals provide highly functionalized carbo- and
heterocycles under mild conditions in atom-economical
processes.[1] We have previously shown that reactions of
allylsilanes or allylstannanes 1 with different metal catalysts
proceed to form hetero- or carbocycles 2 (Scheme 1).[2,3] The
selectivity), in the radical cyclization of substrates 1 per-
formed in the presence of azobisisobutyronitrile (5 mol%)
and Bu3SnH (10 mol%).[5] Conversely, when the reaction of 1
was promoted by GaCl3 (1 equiv), products 2 were obtained
exclusively, whereas a similar reaction with InCl3 (1 equiv) led
to 2 as the major products along with mixtures of 3 and 4 in
very low yields.[6–8]
Reactions of simple 1,6-enynes with platinum(II)[9] or
gold(I)[10] species have been shown to proceed through
cyclopropyl metal–carbene intermediates. In many of the
gold(I)-catalyzed cyclizations of enynes and other alkynes, a
silver salt such as AgOTf, AgBF4, or AgSbF6 (Tf = trifluor-
omethanesulfonyl) is used to generate in situ a cationic
gold(I) species from [AuCl(L)] complexes (L = phosphine
or a related ligand).[10–12] The silver salt has been shown to be
catalytically inert for many of these processes,[10,11] although
in a few instances silver(I) species can catalyze cyclization
reactions.[13] Herein we report that, in contrast to gold(I)
complexes, silver(I) salts and complexes are catalytically
active in the cyclization of 1 and afford stannanes 4 as the
exclusive stereoisomers. We also report the first examples of
skeletal rearrangements and intramolecular cyclopropanation
reactions of 1,6-enynes catalyzed by silver(I) complexes,
which suggest that metal–carbene intermediates are also
involved in these silver(I)-catalyzed transformations.[14,15]
When the reaction of 1a with AgOTf (10 mol%) was
carried out in toluene at 708C, stannane 4a was obtained in
29% yield, along with 2a (Table 1, entry 1). The use of AgBF4
led to 4a and dimer 5 in low yield, whereas complex 6 was
ineffective (Table 1, entries 2 and 3). Remarkably, AgSbF6 or
AgSbF6/PPh3 led to a very fast cyclization of (Z)-1a, and
yielded 4a (83%) and 2a (5–12%) (Table 1, entries 4 and 5).
The reaction also proceeded satisfactorily in the presence of a
variety of silver(I) complexes [Ag(OTf)L] bearing phos-
phines as the ligands (L = PPh3, (o-tolyl)3P, (naphthyl)3P, 2-
biphenyldicyclohexylphosphine) or [(AgOTf)2(L–L)] (L–L =
ethane-l,2-diylbis(diphenylphosphane) (dppe), 9,9-dimethyl-
4,5-bis(diphenylphosphino)xanthene (xantphos), 2,2’-bis(di-
phenylphosphanyl)-1,1’-binaphthyl (binap)),[16,17] which gave
4a in 85–91% yield. Consistent results were obtained with the
preformed [{Ag(OTf)(PPh3)}3] complex[18] as catalyst
(Table 1, entry 6). No reaction was observed when the ratio
of L to Ag was higher than 1:1.
Scheme 1. Cyclizations of allylsilanes or allylstannanes 1 with different
II
metal catalysts. M=PtII, P d, CuI, RuII, AgI, AuIII; L=ligand; Y=SiMe3,
SnBu3; Z=C(CO2Me)2, C(SO2Ph)2, C(CH2OR)2.
best results were obtained by using PtCl2 or [Pt(MeCN)2Cl2]
as the catalyst and methanol or acetone as the solvent. The
reaction proceeds by exo attack of the allyl nucleophile to the
alkyne to form carbocycles with five- or six-membered rings.
Interestingly, when the reactions of the substrates 1 (Y=
SnBu3) were catalyzed by palladium(0) complexes, stannyl
derivatives 3 were obtained stereoselectively in a process that
involves a very different mechanism to that seen in the
oxidative addition of the allylstannane to Pd0.[4] Stannanes 4
have been obtained, along with the Z isomers (ca. 9:1 E/Z
[*] S. Porcel, Prof. Dr. A. M. Echavarren
Institute of Chemical Research of Catalonia (ICIQ)
Av. Països Catalans 16, 43007 Tarragona (Spain)
and
Department of Organic Chemistry
Universidad Autónoma de Madrid
Cantoblanco, 28049 Madrid (Spain)
Fax: (+34)97-792-0225
E-mail: aechavarren@iciq.es
[**] We are grateful to the MEC (project CTQ2004-02869 and Consolider
Ingenio 2010, Grant CSD2006-0003), the UAM (predoctoral fellow-
ship to S.P.), the AGAUR (2005 SGR 00993), and the ICIQ
Foundation for financial support. We also thank E. Escudero-Adµn
and Dr. J. Benet-Buchholz (X-ray diffraction unit, ICIQ) for the
determination of the structure of complex 8.
The reaction of substrates similar to 1a proceeds satisfac-
torily regardless of their E/Z configuration. For example, (E)-
1a (Table 2, entry 1) reacts similarly to (Z)-1a, and both (E)-
and (Z)-1d gave 4d in 90–91% yields after 30 minutes
(Table 2, entries 4 and 5). This silver(I)-catalyzed reaction
tolerates protection of the hydroxy groups with acetate and
tert-butyldiphenylsilyl (TBDPS) groups (Table 2, entries 6
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Table 1: Silver(I)-catalyzed carbostannylation of (Z)-1a.
cyclization of diols with alkynes has been reported by
GenÞt and co-workers with gold(I) catalysts.[19] Substrate
1h, which is substituted at the alkyne with a phenyl group,
reacted with the catalyst [Ag(2-biphenyldicyclohexylphos-
phine)(thf)]SbF6 (8)[20] to give a 1:1 ratio of 4h and 2h in
quantitative yield. In this case, extensive destannylation of 4h
was observed. The structure of the cationic silver(I) complex 8
is similar to the related gold(I) complexes.[10c,20,21] In contrast,
the radical reaction of (E)-1h reported by Hosomi and co-
workers occurred exclusively by a 6-endo-dig pathway.[5]
Alkenyl stannanes 4a–h were obtained as single stereoiso-
mers, whose configuration was assigned as E by comparison of
their NMR spectra with those of the Z isomers 3[4] as well as
by reaction of 4c with DCl in CD3OD.[22] Reaction of the
analogous allylsilanes under these conditions proceeded
rather sluggishly to give only products 2.
Entry
Cat. (mol%)
t
Yield
Yield of
2a [%]
of 4a [%]
1[a]
2[a]
3
AgOTf (10)
AgBF4 (10)
6 (5)
12 h
12 h
12 h
29
29
–
24
[b]
–
–
12
5
4
5
6
AgSbF6 (10)
AgSbF6 (10)+PPh3 (10)
[Ag OTf(PPh3)]3 (3)
1 min
1 min
30 min
83
83
90
–
[a] Conversion of 85%. [b] Dimer 5 (7%) was obtained. Tf=trifluoro-
methanesulfonyl, Mes=2,4,6-trimethylphenyl.
The reaction can be extended for the preparation of six-
and seven-membered-ring compounds (Scheme 2). For these
cyclizations, complex 8 was found to be the best catalyst
Table 2: Silver(I)-catalyzed carbostannylation of 1b–h.[a]
Entry Substrate
t [h] Product(s) (yields,[%])
0.5 4a (87)+2a (3)
1
(E)-1a:
Z=C(CO2Me)2, R1 =H, R2 =Me
(E)-1b:
2
5
4b (71)+2b (6)
Z=C(CO2Me)2, R1 =R2 =H
(Z)-1c:
3
3.5 4c (72)+2c (9)
0.5 4d (90)
Z=C(SO2Ph)2, R1 =R2 =H
(E)-1d:
4
Z=C(SO2Ph)2, R1 =H, R2 =Me
(Z)-1d:
5
0.5 4d (91)
Z=C(SO2Ph)2, R1 =H, R2 =Me
(E)-1e:
Scheme 2. Silver(I)-catalyzed carbostannylations for the synthesis of
six- and seven-membered-ring systems. Ts=toluene-4-sulfonyl.
6
2.5 4e (69)+2e (12)
Z=C(CH2OAc)2, R1 =R2 =H
(E)-1 f:
7
2
4 f (93)
Z=C(CH2OTBDPS)2, R1 =R2 =H
(E)-1g:
8
0.2 4g (31)+2g (11)
+7 (30)
Z=C(CH2OH)2, R1 =R2 =H
(E)-1h:
(Figure 1). Substrate 9, which has an additional methylene
group at the stannane chain, afforded the six-membered-ring
derivatives 10 and 11. A 7-endo-dig cyclization was the
predominant pathway in the reaction of tosylamide 12, which
afforded heterocycle 13 as the major product after treatment
of the mixture of stannanes with I2. A seven-membered ring
16 was also obtained in the 7-exo-dig cyclization of 15.
Although sterically hindered, the alkenylstannanes 4a
and 4d undergo Stille reactions with iodobenzene in the
presence of [Pd(PPh3)4] (10 mol%), CuI (10 mol%), and CsF
(4 equiv) in THF[23] to give stereospecifically 18 and 19,
respectively (Scheme 3). Compound 19 is the E diastereomer
of 2h (Table 2, entry 9), thus providing further confirmation
of the configuration assigned for 4a–h.
9[b]
3
4h (50)+2h (50)
Z=C(SO2Ph)2, R1 =Ph, R2 =Me
[a] Reaction with [AgOTf(PPh3)]3 (3 mol%) in toluene at 708C. [b] Reac-
tion carried out with catalyst 8 (10 mol%) in toluene at 908C. Cy=
cyclohexyl.
and 7). The free hydroxy groups in substrate 1g compete in
the reaction with the alkyne, leading to a 1:1 ratio of bicyclic
acetal 7 and stannane 4g (Table 2, entry 8). A similar
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Table 3: Enantioselective cyclization of (E)-1d.[a]
Entry
Catalyst
T [8C]
Yield [%]
e.r.[b]
1
2
3
[(AgSbF6)2Tol-binap]
[(AgOTf)2binap]
[(AgOTf)2Tol-binap]
[(AgOTf)2Tol-binap]
[(AgOTf)2Tol-binap]
30
70
70
50
30
57
87
87
91
74
50:50
86:13
87:12
89:11
89:11
4
5[c]
[a] Reactions with 5 mol% catalyst for 30 min. [b] Determined by HPLC
(Daicel Chiralpack AD column). [c] Reaction time: 100 min. Tol-binap=
2,2’-bis(di-p-tolylphosphanyl)-l,l’-binaphthyl.
Figure 1. X-ray crystal structure of the cation of silver(I) complex 8.
product 2a was obtained. Reaction of (Z)-1d under the same
conditions as in entry 4 led to stannane 4d with 80:19 e.r.
Yamamoto and co-workers have suggested that a trans-
metalation between the silver(I) complex and allyltrimethoxy-
silanes may take place.[16a–c] However, in the reactions of
allylstannanes with aldehydes, the silver(I) complex was
proposed to act as a chiral Lewis acid rather than forming
an allyl–silver(I) species.[16d] A transmetalation of the allyl-
stannane with silver(I) would lead to the products 3, after
insertion of the allyl–silver(I) species into the alkyne followed
by reductive elimination. To exclude this pathway, we
prepared the Z isomer of 1a from the cyclization of (E)-1d
with [Pd2(dba)3·dba] (dba = trans,trans-dibenzylideneace-
tone) as the catalyst.[4] However, no isomerization of this
substrate into 4d was observed after the substrate was heated
with complex 8 (10 mol%) in toluene at 708C for 14 h.
The formation of silver(I)–acetylide complexes is not a
major pathway under these reaction conditions.[26] Instead,
the isolation of the products 4 is consistent with a mechanism
in which the silver(I) complex selectively activates the alkyne
of 1 to form the cyclopropyl carbene–silver(I) complex 20,
followed by cleavage of the cyclopropane to form the alkenyl–
silver(I) complex 21 (Scheme 4).[2,3,10] Reaction of 21 with
Scheme 3. Stille coupling reactions of stannanes 4a and 4d.
A similar reaction of substrates 1 with gold(I) catalysts[10]
led only to destannylated products 2. For example, reaction of
(Z)-1c with [AuCl(PPh3)] (5 mol%) was very sluggish and led
only to 2c in around 20% yield (toluene, 708C, 14 h).
Conversely, reaction of (Z)-1c with the cationic complex
[20,21]
[Au{P[C6H4(o-Ph)](tBu)2}{NCMe}]SbF6
was complete in
30 minutes at 708C to give 2c in 97% yield. Reaction of (Z)-
1a with Lewis acids such as Et2AlCl and ZrCl4 (toluene, À78
to 08C, 12–24 h) led only to the partial destannylation of the
starting material.[24]
The enantioselective cyclization of allylstannanes with
alkynes was examined with substrate (E)-1d using the
complexes [(AgOTf)2(L–L)] with chiral bidentate ligands.[25]
Bidentate ligands such us (+)-(2S,3S)-bis-(diphenylphosphi- Bu3SnOTf (or a similar electrophile in the case of AgSbF6 or
no)bicyclo[2,2,llhept-5-ene ((+)-(S,S)-norphos), (À)-(R,R)-
P,Pꢀ-[2,2-dimethyl-1,3-dioxolane-4,5-bis(methylene)]bis(di-
phenylphosphane) ((À)-(R,R)-diop), (R,R)-l-benzyl-3,4-bis-
(diphenylphosphino)pyrrolidine ((R,R)-deguphos), (2S,4S)-
N-tert-butoxycarbonyl-4-diphenylphospanyl-2-diphenylphos-
phanylmethylpyrrolidine ((2S,4S)-bppm) in combination with
AgOTf (20 mol% of AgOTf, 9% mol of ligand) gave low
enantiomeric ratios (e.r.), ranging from 53:47 to 63:37. The
best results were obtained when isolated complexes
[(AgOTf)2(R)-binap] or [(AgOTf)2(R)-Tol-binap][16b] were
employed (Table 3). The nature of the counteranion plays a
crucial role; for example, no enantioselectivity was achieved
with complex [(AgSbF6)2(R)-Tol-binap] (Table 3, entry 1),
but use of the OTf, which is a better coordinating anion, led to
a 89:11 e.r. under the same reaction conditions (Table 3,
entry 5). The corresponding complex formed between (R)-
Tol-binap and AgPF6 led to lower enantioselectivity
(68:32 e.r.), whereas with AgBF4, only the destannylated
catalyst 8) gives the stannanes 4. Alternatively, formation of
21 might take place in a single step as shown in Scheme 4.
To support the formation of silver carbenes in these
cyclizations, we examined the reactions of simple enynes with
silver(I) catalysts (Scheme 5). Thus, 22 afforded in quantita-
tive yield diene 23, the product of a skeletal rearrangement
with a single cleavage, as shown in the reaction of deuterated
substrate [D1]-22 to give [D1]-23.[10c] Dienyne 24 led to 25 in
66% yield, along with the skeletal-rearrangement products as
minor compounds. Tetracycle 25 is identical to that obtained
before using gold(I) species as the catalyst.[10d] Conversely, 26
gave rise to a 15:1 mixture of the skeletal-rearrangement
derivative 27 and the product of an intramolecular cyclo-
propanation 28 (94% yield, Scheme 5).[27]
In summary, we have reported the first intramolecular
carbostannylation of alkynes catalyzed by silver(I) species,
which gave (E)-alkenylstannanes stereoselectively as single
isomers in a reaction that appears to be mechanistically
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[1] For reviews, see: a) G. C. Lloyd-Jones, Org. Biomol. Chem. 2003,
1, 215 – 236; b) C. Aubert, O. Buisine, M. Malacria, Chem. Rev.
2002, 102, 813 – 834; c) S. T. Diver, A. J. Giessert, Chem. Rev.
2004, 104, 1317 – 1382; d) A. M. Echavarren, C. Nevado, Chem.
Soc. Rev. 2004, 33, 431 – 436; e) E. JimØnez-Nfflæez, A. M.
Echavarren, Chem. Commun. 2006, 333 – 346.
[2] a) C. Fernµndez-Rivas, M. MØndez, A. M. Echavarren, J. Am.
Chem. Soc. 2000, 122, 1221 – 1222; b) C. Fernµndez-Rivas, M.
MØndez, C. Nieto-Oberhuber, A. M. Echavarren, J. Org. Chem.
2002, 67, 5197 – 5201.
[3] M. MØndez, A. M. Echavarren, Eur. J. Org. Chem. 2002, 15 – 28.
[4] a) B. Martín-Matute, E. Buæuel, D. J. Cµrdenas, M. MØndez, C.
Nieto-Oberhuber, A. M. Echavarren, J. Organomet. Chem.
2003, 687, 410 – 419; b) S. Shin, T. V. RajanBabu, J. Am. Chem.
Soc. 2001, 123, 8416 – 8417.
Scheme 4. Mechanistic hypotheses for the silver(I)-catalyzed
carbostannylation of alkynes.
[5] K. Miura, N. Fujisawa, H. Saito, H. Nishikori, A. Hosomi, Chem.
Lett. 2002, 32 – 33.
[6] K. Miura, N. Fujisawa, A. Hosomi, J. Org. Chem. 2004, 69, 2427 –
2430.
[7] The intramolecular addition of allylsilanes to alkynes promoted
by indium(III) takes place by a mechanism similar to that
proposed in Ref. [2]. See: K. Miura, N. Fujisawa, S. Toyohara, A.
Hosomi, Synlett 2006, 1883 – 1886.
[8] For the cyclization of allyl bromides with alkynes catalyzed by
InCl3 that involves a halogen transfer, see: G. R. Cook, R.
Hayashi, Org. Lett. 2006, 8, 1045 – 1048.
[9] a) M. MØndez, M. P. Muæoz, A. M. Echavarren, J. Am. Chem.
Soc. 2000, 122, 11549 – 11550; b) M. MØndez, M. P. Muæoz, C.
Nevado, D. J. Cµrdenas, A. M. Echavarren, J. Am. Chem. Soc.
2001, 123, 10511 – 10520.
[10] a) C. Nieto-Oberhuber, M. P. Muæoz, E. Buæuel, C. Nevado,
D. J. Cµrdenas, A. M. Echavarren, Angew. Chem. 2004, 116,
2456 – 2460; Angew. Chem. Int. Ed. 2004, 43, 2402 – 2406; b) C.
Nieto-Oberhuber, M. P. Muæoz, S. López, E. JimØnez-Nfflæez, C.
Nevado, E. Herrero-Gómez, M. Raducan, A. M. Echavarren,
Chem. Eur. J. 2006, 12, 1677 – 1693; c) C. Nieto-Oberhuber, S.
López, M. P. Muæoz, D. J. Cµrdenas, E. Buæuel, C. Nevado,
A. M. Echavarren, Angew. Chem. 2005, 117, 6302 – 6304; Angew.
Chem. Int. Ed. 2005, 44, 6146 – 6148; d) C. Nieto-Oberhuber, S.
López, E. JimØnez-Nfflæez, A. M. Echavarren, Chem. Eur. J.
2006, 12, 5916 – 5923.
[11] See, for example: a) D. J. Gorin, N. R. Davis, F. D. Toste, J. Am.
Chem. Soc. 2005, 127, 11260 – 11261; b) J. J. Kennedy-Smith,
S. T. Staben, F. D. Toste, J. Am. Chem. Soc. 2004, 126, 4526 –
4527; c) J. J. Kennedy-Smith, S. T. Staben, F. D. Toste, J. Am.
Chem. Soc. 2004, 126, 4526 – 4527; d) B. D. Sherry, F. D. Toste, J.
Am. Chem. Soc. 2004, 126, 15978 – 15979.
Scheme 5. Skeletal-rearrangement and cyclopropanation reactions
catalyzed by silver(I). DCE=1,2-dichloroethane.
similar to that of the reaction of enynes with other electro-
philic transition-metal complexes. In this case, the alkenyl–
silver(I) intermediate is able to react with the tin electrophile
generated in situ, thus leading to stannanes of structure 4 with
total control of the stereoselectivity. A 78% ee has been
achieved by using [(AgOTf)2(R)-Tol-binap] as the catalyst.
We have also reported the first examples of skeletal
rearrangements and intramolecular cyclopropanation reac-
tions of 1,6-enynes catalyzed by silver(I) species, an observa-
tion that indicates that silver–carbene species are probably
involved in these reactions.
[12] S. Ma, S. Yu, Z. Gu, Angew. Chem. 2006, 118, 206 – 209; Angew.
Chem. Int. Ed. 2006, 45, 200 – 203.
[13] For leading references of silver(I)-catalyzed cyclizations of
alkynes, see: a) R. F. Sweis, M. P. Schramm, S. A. Kozmin, J.
Am. Chem. Soc. 2004 126, 7442 – 7443; b) T. J. Harrison, G. R.
Dake, Org. Lett. 2004, 6, 5023 – 5026; c) T. J. Harrison, J. A.
Kozak, M. Corbella-PanØ, G. R. Dake, J. Org. Chem. 2006, 71,
4525 – 4529; d) J. Sun, S. A. Kozmin, Angew. Chem. 2006, 118,
5113 – 5115; Angew. Chem. Int. Ed. 2006, 45, 4991 – 4993; e) J.
Zhao, C. O. Hughes, F. D. Toste, J. Am. Chem. Soc. 2006, 128,
7436 – 7437.
[14] For leading references on silver(I)–carbene complexes, see:
a) J. C. Garrison, W. J. Youngs, Chem. Rev. 2005, 105, 3978 –
4008; b) P. de FrØmont, N. M. Scott, E. D. Stevens, T. Ramnial,
O. C. Lightbody, C. L. B. Macdonald, J. A. C. Clyburne, C. D.
Abernethy, S. P. Nolan, Organometallics 2005, 24, 6301 – 6309;
c) R. R. Julian, J. A. May, B. M. Stoltz, J. L. Beauchamp, J. Am.
Chem. Soc. 2003, 125, 4478 – 4486; d) A. J. Arduengo, H. V. R.
Received: December 13, 2006
Published online: March 6, 2007
Keywords: cyclization · enantioselectivity · enynes · silver ·
.
stannanes
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Communications
Dias, J. C. Calabrese, F. Davidson, Organometallics 1993, 12,
3405 – 3409.
Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif..
[21] C. Nieto-Oberhuber, S. López, A. M. Echavarren, J. Am. Chem.
Soc. 2005, 127, 6178 – 6179.
[22] For details, see the Supporting Information.
[23] a) S. P. H. Mee, V. Lee, J. E. Baldwin, Angew. Chem. 2004, 116,
1152 – 1156; Angew. Chem. Int. Ed. 2004, 43, 1132 – 1136;
b) S. P. H. Mee, V. Lee, J. E. Baldwin, Chem. Eur. J. 2005, 11,
3294 – 3308.
[24] M. Yasuhisa, A. Naoki, K. Haruo, Yoshinori, Y. Tetrahedron
1999, 55, 3779 – 3790.
[25] For enantioselective cyclizations of enynes initiated by the
coordination of the metal to the alkyne function of enynes, see:
platinum(III) catalysis: a) L. Charruault, V. Michelet, R. Taras,
S. Gladialli, J.-P. GenÞt, Chem. Commun. 2004, 850 – 851;
silver(I) catalysis: b) M. P. Muæoz, J. Adrio, J. C. Carretero,
A. M. Echavarren, Organometallics 2005, 24, 1293 – 1300.
[26] a) U. LØtinois-Halbes, P. Pale, S. Berger, J. Org. Chem. 2005, 70,
9185 – 9190; b) A. VitØrisi, A. Orsini, J.-M. Weibel, P. Pal,
Tetrahedron Lett. 2006, 47, 2779 – 2781, and references therein.
[27] For closely related examples of intramolecular cyclopropanation
reactions catalyzed by ruthenium(II) or platinum(II), see: B. P.
Peppers, S. T. Diver, J. Am. Chem. Soc. 2004, 126, 9524 – 9525.
[15] For discussions on p-back bonding in silver(I)–carbene com-
plexes, see: a) X. Hu, I. Castro-Rodriguez, K. Olsen, K. Meyer,
Organometallics 1993, 12, 3405 – 3409; b) D. Nemcsok, K.
Wichmann, G. Frenking, Organometallics 2004, 23, 3640 – 3646.
[16] a) M. Wadamoto, H. Yamamoto, J. Am. Chem. Soc. 2005, 127,
14556 – 14557; b) N. Momiyama, H. Yamamoto, J. Am. Chem.
Soc. 2004, 126, 5360 – 5361; c) M. Wadamoto, N. Ozasa, A.
Yanagisawa, H. Yamamoto, J. Org. Chem. 2003, 68, 5593 – 5601;
d) A. Yanagisawa, H. Nakashima, A. Ishiba, H. Yamamoto, J.
Am. Chem. Soc. 1996, 118, 4723 – 4724.
[17] S. E. Denmark, J. Fu, Chem. Rev. 2003, 103, 2763 – 2793.
[18] M. Bardají, O. Crespo, A. Laguna, A. K. Fischer, Inorg. Chim.
Acta 2000, 304, 7 – 16.
[19] S. Antoniotti, E. Genin, V. Michelet, J.-P. GenÞt, J. Am. Chem.
Soc. 2005, 127, 9976 – 9977.
[20] a) E. Herrero-Gómez, C. Nieto-Oberhuber, S. López, J. Benet-
Buchholz, A. M. Echavarren, Angew. Chem. 2006, 118, 5581 –
5585; Angew. Chem. Int. Ed. 2006, 45, 5455 – 5459; b) CCDC-
634526 contains the supplementary crystallographic data for
complex 8. These data can be obtained free of charge from The
2676 www.angewandte.org
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2007, 46, 2672 –2676