Selectivity control by silver catalysts in the cycloisomerization of 1,6-enynes derived from propiolamides

Article abstract of DOI:10.1016/j.tetlet.2012.11.067

Silver-catalyzed cycloisomerizations of 1,6-enynes derived from propiolamides led to a selective formation of Alder-ene type 1,4-dienes. Interestingly, AgNTf₂ outperformed gold or platinum catalysts in terms of selectivity and reactivity, providing the 1,4-dienes at room temperature. The presence of C(5) carbonyl group in combination with Ag salts is key to the selectivity and the β-oxo coordinated silver carbenoids were proposed as an intermediate based on the reaction profiles.

Full text of DOI:10.1016/j.tetlet.2012.11.067

Tetrahedron Letters 54 (2013) 834–839
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Selectivity control by silver catalysts in the cycloisomerization
of 1,6-enynes derived from propiolamides
⇑
Jaeyoung Koo, Hyun-Sub Park, Seunghoon Shin
Department of Chemistry, Hanyang University, Seoul 133-791, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Silver-catalyzed cycloisomerizations of 1,6-enynes derived from propiolamides led to a selective forma-
tion of Alder-ene type 1,4-dienes. Interestingly, AgNTf2 outperformed gold or platinum catalysts in terms
of selectivity and reactivity, providing the 1,4-dienes at room temperature. The presence of C(5) carbonyl
group in combination with Ag salts is key to the selectivity and the b-oxo coordinated silver carbenoids
were proposed as an intermediate based on the reaction profiles.
Received 21 October 2012
Revised 14 November 2012
Accepted 19 November 2012
Available online 28 November 2012
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
1,6-Enyne
Cycloisomerization
Silver catalysis
Propiolamide
Silver carbenoid
Gold catalysis
1,6-Enynes have served as prototypical substrates in the identi-
fication of numerous new reactivities catalyzed by transition-
metals.1 These processes have allowed for an access to polyfunc-
tional molecular frameworks through atom-economical C–C bond
formations. Among them, Alder-ene reactions are typically cata-
lyzed by transition metal salts, such as Pd, Ru, Co, Rh, Ir, Pt and
Ti, and proceed through a metallocyclopentene intermediates or
via hydrometallation/carbometallation mechanism to provide for
a general entry to 1,4-dienes with E-configuration at the exocyclic
double bond.1,2 Electrophilic activation of alkynes could also lead
to Alder-ene type 1,4-dienes (Scheme 1). For example, Echavarren
et al. first reported PtCl2 catalyzed cycloisomerization employing
tri-substituted olefins at an elevated temperature (70 °C) in polar
solvents. However, the 1,4-diene (a) was proposed to arise via
cyclometallation (I) pathway based on d-isotope experiments,
whereas hydroxyl(alkoxy)cyclization (b) in alcohols occurs via dis-
tinct cyclopropyl metal carbenoid (II/III, Scheme 1A).3 Change of
substrates to incorporate allylsilanes or allylstannanes4 facilitates
the nucleophilic attack on the activated alkyne and a variety of
metals could be employed for the formation of 1,4-dienes, includ-
ing Ag (Scheme 1B).4c Since then, only two examples of selective
5-exo-dig formation of Alder-ene type 1,4-diene have been
reported in electrophilic metal catalysis,5 employing cationic
Au(I) catalysts in highly polar solvent (DMSO) or Pt(II) catalysts.6,7
Lack of selectivity toward 1,4-diene (d) via 5-exo-dig pathway un-
der electrophilic metal catalysis results from multiple available
pathways for the enyne that depend on the subtle structural
change, leading to a diverse range of products as represented by
d–i among others (Scheme 1C).8
To address this selectivity issue, we adopted to use acceptor-
substituted alkynes based on the favorable reactivity studied by
us9 and others.2b–d,6b,10 The interaction of polarized triple bonds
with the
p-acidic metals could be markedly different from those
of unpolarized alkynes and synergistic interactions of the metal
catalysts and the acceptor-substituents could lead to an enhanced
reactivity and/or change of the selectivity. With this prospect, we
explored the cycloisomerizations of propiolamide-derived 1,6-eny-
nes under silver catalysis and found that the presence of carbonyl
group is a key for the selective formation of 5-exo-dig 1,4-diene (j)
(vs 6-endo-dig and vs alkoxycyclization and skeletal reorganiza-
tions) and that simple silver salts11–13 outperformed expensive cat-
ionic Au(I) or Pt(II) complexes in terms of reactivity as well
(Scheme 1D). In addition, we also present the formation of k14
and l15 that has not been reported with Ag catalysts.
We set out to examine the feasibility of the process with sub-
strate 1a employing various metal catalysts (Table 1). Employment
of Au(tBu-XPhos)NTf2 led to a mixture of 1,4-enyne 2a,5a skeletal
rearrangement product 3a and hydroxycyclization 4a (entry 1, Ta-
ble 1).16 Repeating the reaction in a flame-dried tube suppressed
the formation of 4a somehow, but the selectivity between 2a and
3a remained mediocre (entry 2). Change of counter-anion to BF4
⇑
Corresponding author. Tel.: + 82 2 2220 0948; fax: +82 2 2299 0762.
E-mail address: sshin@hanyang.ac.kr (S. Shin).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.11.067

J. Koo et al. / Tetrahedron Letters 54 (2013) 834–839
A: Alder-ene type cycloisomerization by PtCl2 (ref. 3)
835
D
D
D
PtCl2
X
X
X
MCl2
acetone
I
a
+M
D
H
M
H
H
PtCl2
H
H
H
X
+
X
X
X
X
CD3OD
R
H
H
H
R
b
OCD3
R
R
R
DOCD3
II
III
a'
B: Alder-ene type cycloisomerization with allylsilanes(stannanes)
+M
(Sn)
M =
HOMe
-MeOY
R
Pt, Pd, Ru, Ag
R
X
R
Y
X
MeOH
X
Y
(R' = H, Me
R'
II
R'
Y = Si, Sn)
H
c
R'
C: Electrophilic metal-catalyzedr eaction manifolds
+M
R
R
R
R
R
X
X
X
X
II
Nu
R
d
e
H
f
X
M = Au, Pt
M+
R
R
X
R
X
IV
X
X
g
h
i
Nu
D: This report
M
M+
H
O
O
O
X
R
R
M = Ag
RT
R
X
X
H
II'
III'
R
1
(X = NR'')
H
O
O
O
R'
R
X
X
X
j
k
l
(R = CH2R')
(R = Ph)
Scheme 1. Electrophilic metal-catalyzed Alder-ene type 1,4-diene formation.
led to a satisfactory selectivity toward 2a but required heating at
100 °C with 10 mol % of catalyst for an effective conversion (entry
3). Among the gold catalysts screened (entries 4–7),17
Au(PPh3)NTf2 showed a clean formation of 2a, but significant
amount of 4a competitively formed (entry 6). Surprisingly at this
point, we found that silver salts alone were competent to effect a
clean and selective conversion to 2a (entry 8).13 Prompted by this
result, we screened a variety of Ag-salts17 and found that AgNTf2 is
the most effective for the desired cyclization (entries 8–11). Fur-
ther optimization with regard to solvents led us to choose 1,2-
dichloroethane (DCE) as solvent and 2a was obtained in 91% yield
at room temperature in one hour (entry 13). It is noteworthy that
we did not observe any isomerization of double bond from 2a into
the internal position under this mild condition.5a Control experi-
ments with HNTf2 or PtCl2 turned out to be far less efficient.
With a simple but exceptionally mild and selective catalytic
system in hand, we examined various substrates to define its scope
(Table 2). The reactions of N-arylamides having a prenyl unit
smoothly afforded the respective 1,4-diene products, irrespective
of the electronic density on the N-aryl groups (entries 1–3). The
N-alkylamides turned out to be excellent substrates as well,
including N-allyl 1f where a complete chemoselectivity was ob-
served (entries 4–7).18 Secondary amides 1h, on the other hand, re-
quired a longer reaction time because of unfavorable rotameric
population (entry 8). We further examined the effect of allyl sub-
stitution (entries 9–11). The reaction with a cyclic substrate 1i
proved efficient. Unfortunately, an allyl chloride 1j failed to pro-
vide any product, presumably due to the reduced electron density
on the alkene. However, the reaction of an allyl acetate 1k slowly
gave the hydroxycyclization adduct 4k at an elevated temperature

836
J. Koo et al. / Tetrahedron Letters 54 (2013) 834–839
temperature (entry 12).16a The reactions of cyclohexenyl amide
Table 1
Test reactions with 1a
substrate 1l, on the other hand, gave only 22% of 2l even at 80 °C
with AgNTf2 (5 mol %) catalyst. AgSbF6 performed slightly better
but required 15 mol % of catalyst to yield 48% of 2l (entry 13). In
the case of the cycloalkenyl amines 1l–n, a change of catalyst into
Au(tBuXPhos)SbF6 resulted in a marked increase in the yields,
delivering fused azacycles 2l–n in 50–99% yield. The dependence
of efficiency on the ring size is consistent with a congested polycy-
clic reaction intermediate (vide infra, Scheme 2).
Substrates 1o–p having alkynyl substituents deserve a com-
ment (Table 3). The reaction of methyl substituted 1o gave a mix-
ture of 2o and an allene 5o14,17 in approximately equal amounts
under the standard reaction condition (AgNTf2 in DCE). Interest-
ingly, the ratio of 2o and 5o was highly dependent on the solvent:
AgOTf as catalyst in DCE predominantly gave 2o (2o/5o = 13:1),
while AgOTf in nitromethane preferred the formation of 5o (44%)
(2o/5o = 1:1.8). The formation of the allene in 5o is accompanied
by reduction in N-allyl functionality and its mechanism is consis-
tent with 1,5-hydride shift as reported previously.14 Substrate
1q–r having aryl substituent on alkyne favored the formation of
previously reported fused cyclic products 6q–r as major products
with silver catalysts (entries 5 and 6).15
O
O
O
O
see
below
PhN
+
+
PhN
PhN
PhN
OH
1a
2a
3a
4a
Entry
Catalysta (5 mol %)
Conditionsb
Yieldc (%) (2a:3a:4a)
1d
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Au(tBu-XPhos)NTf2
Au(tBu-XPhos)NTf2
rt, CHCl3, 1 h
rt, CHCl3, 2 h
100 °C, CHCl3, 3 h
rt, DCE, 1 h
rt, DCE, 1 h
28:14:45
37:22:23
76:0:18
30:17:33
34:15:34
58:0:38
43:13:41
59:0:7
e
Au(tBu-XPhos)BF4
Au(tBu-XPhos)NTf2
Au(SPhos)NTf2
Au(PPh3)NTf2
Au(IPr)NTf2
AgSbF6
rt, DCE, 1 h
rt, DCE, 1 h
100 °C, CHCl3, 1 h
100 °C, CHCl3, 1 h
100 °C, CHCl3, 1 h
100 °C, CHCl3, 1 h
rt, CH2Cl2, 1 h
rt, DCE, 1 h
AgOTf
AgBF4
AgNTf2
AgNTf2
AgNTf2
HNTf2
PtCl2
36:0:0
13:0:4
76:0:8
87:0:10
91:0:8
100 °C, DCE, 18 h
60 °C, DCE, 20 h
5:0:0
43:0:0
a
Au(I)-catalysts were prepared in situ from equimolar amount of Au(L)Cl and
To interrogate the origin of the above selectivity under silver
catalysis, the following substrates (1s and 1t) were examined
under the AgNTf2 catalyst. An N-Ts-tethered 1,6-enyne 1s devoid
of C(5)-carbonyl group afforded a 6-endo-dig Alder-ene type
product 8s7a in low yield instead of a 5-exo-dig product along with
extensive decomposition of starting material (Eq. 1). On the other
hand, an 1,6-enyne 1t with a carbonyl at C8 delivered an intramo-
lecular [4 + 2] annulation product 9t20 concomitant with the
hydrolysis of the ethyl ester and no Alder-ene type 1,4-diene was
observed (Eq. 2).
AgX; Flame-dried glassware was used for reactions unless otherwise noted.
b
DCE: 1,2-dichloroethane.
c
Based on crude 1H NMR spectra (1,3,5-tri-MeO-C6H3 as an internal standard).
d
The vessel was not pre-dried.
10 mol % of catalyst.
e
which presumably formed through anchimeric assistance by ace-
tate group.19 Likewise, the reaction of 1a in methanol (instead of
DCE) smoothly gave the methoxycyclization adduct 4a0 at room
Table 2
Scope of cycloisomerization leading to 1,4-dienesa
Entry
Substrate
Conditions
Yieldb (%)
O
O
R
N
AgNTf2 (5 mol %)
DCE
RN
2a-h
1a-h
1
2
3
4
5
6
7
8
R = Ph (1a)
rt, 1 h
rt, 1 h
rt, 1.5 h
rt, 3 h
rt, 2.5 h
rt, 5 h
82
76
71
92
89
96
82
76
R = 4-MeO-C6H4 (1b)
R = 4-F-C6H4 (1c)
R = Bn (1d)
R = PMB (1e)c
R = allyl (1f)
R = n-Bu (1g)
R = H (1h)
rt, 6 h
rt, 22 h
O
O
O
Ph
AgNTf2 (5 mol %)
DCE
N
PhN
PhN
or
OR
R2
R2
R1
R1
R2
R1
2
4
1i-k
9
R1,R2 = (CH2)4 (1i)
R1 = Me, R2 = Cl (1j)
rt, 3 h
2i,75
10
60 °C, 41 h
NR
O
R1 = Me, R2 = OAc (1k)
60 °C, 18 h
PhN
H
11
OAc
4k, 78 %
OH

J. Koo et al. / Tetrahedron Letters 54 (2013) 834–839
837
Table 2 (continued)
Entry
Substrate
Conditions
Yieldb (%)
O
PhN
12
1a
rt, 3 h in MeOH
OMe
4a', 74 %
O
O
AgSbF6 (15 mol %)
or
Ph
N
1l
(n = 1)
(n = 2)
(n = 3)
PhN
Au(tBu-XPhos)SbF6
1m
1n
2l-n
(5 mol %)
DCE
n
1l
1m
1n
n
13
14
15
80 °C, 1 hd
80 °C, 0.7 hd
80 °C, 3 hd
48 (92)e
75 (99)e
25 (50)e
a
The reactions were conducted in 0.1 mmol scale (0.1 M in DCE).
Isolated yield after chromatography.
PMB: p-methoxybenzyl.
AgSbF6 (15 mol %) was used as a catalyst.
Yields in parenthesis were obtained with Au(tBu-XPhos)SbF6 (5 mol %, formed in situ) as a catalyst (3 h at 100 °C).
b
c
d
e
AgNTf2
Ts
Ag
H
Ts
+ H+
N
N
M+
M+
H
(5 mol %)
Ts
N
(1)
O
O
DCE
R
R
R
60o C, 1.5 h
R'N
R'N
Ag+
1s
8s, 21 %
O
H
R
II
II'
R'N
+Ag
Ag
OEt
CO2Et
AgNTf2
(5 mol %)
CO2Et
TsN
O
M
M
TsN
TsN
O
O
DCE
rt, 6 h
R
H
1t
R'N
R'N
III'
III
O
O
C-O
TsN
(2)
then
H
hydrolysis
O
9t, 61 %
-Ag+
R
R'N
D0.50
D0.75
O
O
AgNTf2
(5 mol %)
DCE
1a
PhN
PhN
+
(3)
Scheme 2. Proposed mechanism for the Ag-catalyzed cycloisomerization.
D2O (10 equiv.)
rt, 24 h
OH
4a
(42 %)
2a
d- (27 %)
d-
Based on these experiments, the following mechanism in
Scheme 2 was proposed. The preference of 5-exo-dig pathway in
1 having C(5) carbonyl group compared to 1s without such group
indicated that a significant stabilization of the resulting species ex-
ists, for which three mesomeric species (II0/III/III0) could be envi-
sioned. Significant amount of formal metathesis products (3)
observed with gold catalysts (Table 1) indicated that a highly elec-
trophilic cyclopropyl gold carbenoid such as II is a main contribu-
Craft arylation leads to 5 and 7, respectively.
Finally, the current Ag-catalyzed 1,4-diene formation seems to
be differentiated from the mechanism with PtCl2 (Scheme 1A).
The reaction of 1a in the presence of D2O (10 equiv) results in a sig-
nificant D-atom incorporation for both 1,4-diene formation (d-2a)
and hydroxycyclization (d-4a), justifying our exclusion of metallo-
cyclopentenes from possible catalytic intermediates (Eq. 3).
In summary, we have reported herein a silver catalyzed cyclo-
isomerization of 1,6-enynes derived from propiolamides. Selective
Alder-ene type 1,4-diene formation has not been common with
electrophilic metal catalysis.5,6 We have shown that the reactions
of propiolamides derived 1,6-enynes with Ag(I) catalysts occur at
much milder conditions than with Au(I) or Pt(II) catalysts. In con-
trast to earlier report on the reaction of similar substrates under
Au(I) complexes where 6-endo-dig mode is preferred (DFT-compu-
tation),6b all the current reaction profile with silver catalyst is con-
tor with Au, resulting in
r-bond reorganization. Compared to Au(I)
complex generally having linear bi-dentate coordination and thus
precluding carbonyl coordination,21,22 silver metal can adopt more
than two coordinating ligands and structures II0, III and III0 could
become main contributing resonance structures.23,24 Coordination
of carbonyl oxygen with Ag metal in II0 tempers its electrophilicity
and thus leads to less tendency for
r
-bond reorganization (3).23 In-
stead, proton loss from III and III0 results in a selective formation of
Alder-ene type 1,4-diene, while 1,5-hydride transfer or Friedel–

838
J. Koo et al. / Tetrahedron Letters 54 (2013) 834–839
Table 3
Further scope with a substituent at alkyne
Entry
Substrate
Conditions
Products (%)a
R
O
O
O
O
Ph
N
R
R
Ag-salts
R
PhN
PhN
+
+
PhN
DCE, 80 oC
2
5
6
1
1
2
3
4
1o (R = H)
1o
1p (R = Me)
1p
AgOTf (10 mol %)
AgOTf (10 mol % in CH3NO2)
AgOTf (10 mol %)
2o (76), 5o (6)
2o (24), 5o (44)
2p (48), 5p (10)b, 6p (22)
2p (11), 5p (31)b, 6p (28)
AgOTf (10 mol % in CH3NO2)
O
O
O
Ph
N
Ar
Ag-salts
PhN
PhN
+
Ar
DCE, 80 oC
2
7
1
5
6
1q (R = Ph)
1r (R = 4-MeO-C6H4)
AgNTf2 (5%), 48 h
AgNTf2 (5%), 12 h
2q (14), 7q (72)
2r (20), 7r (67)
a
Isolated yield after chromatography.
The allene 5p was obtained as 2:1 dr (by 13C NMR).
b
7. For Bi(III)-catalyzed 1,4-diene formation: Fang, S.; Wang, Z. Eur. J. Org. Chem.
2009, 5505.
sistent with 5-exo-dig cyclization. The generality of this control
element will be further explored in subsequent studies.
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3180; (b) Corma, A.; Leyva-Perez, A.; Sabataer, M. J. Chem. Rev. 2011, 111, 1657;
(c) Sohel, S. Md. A.; Liu, R. S. Chem. Soc. Rev. 2009, 38, 2269; (d) Fürstner, A.
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Acknowledgments
This work was supported by the National Research Foundation
of Korea (NRF-2011-0023686 and NRF-2012-015662). J.K. and
H.S.P. thank BK21 program for a fellowship.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2012.
11.067.
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12. Recently, silver salts are increasingly recognized as
p-acids for alkynes
(selected examples): (a) Harrison, T. J.; Dake, G. R. Org. Lett. 2004, 6, 5023;
(b) Ji, K.-G.; Shu, X.-Z.; Zhao, S.-C.; Zhu, H.-T.; Niu, Y.-N.; Liu, X.-Y.; Liang, Y.-M.
Org. Lett. 2009, 11, 3206; (c) Sweis, R. F.; Schramm, M. P.; Kozmin, S. A. J. Am.
Chem. Soc. 2004, 126, 7442; (d) Godet, T.; Belmont, P. Synlett 2008, 2513; (e)
Wang, M.-Z.; Zhou, C.-Y.; Che, C.-M. Chem. Commun. 2011, 47, 1312; (f) Kikuchi,
S.; Sekine, K.; Ishida, T.; Yamada, T. Angew. Chem., Int. Ed. 2012, 51, 6989; (g)
Yoshida, S.; Fukui, K.; Kikuchi, S.; Yamada, T. J. Am. Chem. Soc. 2010, 132, 4072;
(h) Oh, C. H.; Karmakar, S.; Park, H.; Ahn, Y.; Kim, J. W. J. Am. Chem. Soc. 2010,
132, 1792; (i) Susanti, D.; Koh, F.; Kusuma, A.; Kothandaraman, P.; Chan, P. W.
H. J. Org. Chem. 2012, 77, 7166.
13. For effects of silver salts in gold catalysis, see: Wang, D.; cai, R.; Sharma, S.;
Jirak, J.; Thummanapelli, S. K.; Akhmedov, N. G.; Zhang, H.; Liu, X.; Petersen, J.
L.; Shi, X. J. Am. Chem. Soc. 2012, 134, 9012.
14. Harrak, Y.; Simonneau, A.; Malacria, M.; Gandon, V.; Fensterbank, L. Chem.
Commun. 2010, 46, 865.
15. Nieto-Oberhuber, C.; Lopez, S.; Echavarren, A. M. J. Am. Chem. Soc. 2005, 127,
6178.
16. (a) Nieto-Oberhuber, C.; Muñoz, M. P.; Buñuel, E.; Nevado, C.; Cárdenas, D. J.;
Echavarren, A. M. Angew. Chem., Int. Ed. 2004, 43, 2402; (b) Nieto-Oberhuber,
C.; Lopez, S.; Muñoz, M. P.; Cárdenas, D. J.; Buñuel, E.; Nevado, C.; Echavarren,
A. M. Angew. Chem., Int. Ed. 2005, 44, 6146.
17. For a full optimization study, see Supplementary data. The geometry of 2o–q
was determined by 1D-NOE experiments. To confirm the connectivity of 2a (5-
exo or 6-endo cyclization), 2a (an oil) was hydrogenated with Pd/C and the
obtained S4 was analyzed by COSY spectra. The connectivity of 2o and 5o was
determined similarly.

J. Koo et al. / Tetrahedron Letters 54 (2013) 834–839
839
O
H2 (5 psi)
Pd/C (20 %)
MeOH
O
PhN
O
2o or 5o
H2 (5 atmi)
Pd/C (10 %)
PhN
PhN
MeOH
S5
67 % (dr 1/1) from 2o
5o
2a
S4 72 % (dr 1/1)
69 % (dr 12/1) from
18. No product resulting from the participation of N-allyl group (metathesis or
hydroalkoxylation) was observed.
19. Trace amount of extraneous water presumably hydrolyzed the following
intermediate formed by the anchimeric assistance of acetate.
Engelhardt, L. M.; Pakawatchai, C.; White, A. H. J. Chem. Soc., Dalton Trans. 1987,
1089; (b) Ito, H. l.; Takagi, K.; Miyahara, T.; Sawamura, M. Org. Lett. 2005, 7,
3001.
22. Recently, tri-coordinate Au(I) complex was also implied as
a catalytic
Ag
Ag+
O
intermediate (a) Zhan, J.-H.; Lv, H.; Yu, Y.; Zhang, J.-L. Adv. Synth. Catal. 2012,
354, 1529; (b) Luo, Y.; Ji, K.; Li, Y.; Zhang, L. J. Am. Chem. Soc. 2012. http://
dx.doi.org/10.1021/ja307948m.
O
R
H2O
(trace)
PhN
1k
4k
23. Yam, V. W. W.; Cheng, E. C. C. In Comprehensive Organometallic Chemistry III;
Crabtree, R. H., Mingos, D. M. P., Eds.; Elsevier: Oxford, 2007; Vol. 2,. Chapter
2.04.
X
OAc
O
H
H
O
I'
24. A similar type of intermediate to III0 has been proposed in the Ru-catalyzed
redox isomerization of propargyl alcohol: Trost, B. M.; Breder, A.; O’Keefe, M.;
Rao, M.; Franz, A. W. J. Am. Chem. Soc. 2011, 133, 4766. In the Ag-catalyzed
cycloisomerization of allylsilanes/allylstannanes, the authors proposed a Ag-
carbenoid as an intermediate. (Scheme 1B, Ref. 4c).
20. For intermolecular version, see Ref. 4a.
21. Yet several tri-coordinate Au(I) complexes have been isolated, although their
role as catalytically active species is unclear due to possible
disproportionation: (a) Bowmaker, G. A.; Dyason, J. C.; Healy, P. C.;