Rhodium-catalyzed tandem cyclization/Si-C activation reaction for the synthesis of siloles

Article abstract of DOI:10.1002/anie.201400828

Siloles represent an important emerging class of photoluminescent materials. Reported herein is a new synthetic strategy involving a tandem cyclization/Si-C activation reaction featuring high efficiency, wide substrate scope, and practical utility. This m

Full text of DOI:10.1002/anie.201400828

Angewandte
Chemie
DOI: 10.1002/anie.201400828
Synthetic Methods
À
Rhodium-Catalyzed Tandem Cyclization/Si C Activation Reaction for
the Synthesis of Siloles**
Qing-Wei Zhang, Kun An, and Wei He*
Abstract: Siloles represent an important emerging class of
photoluminescent materials. Reported herein is a new synthetic
a valuable alternative, Xi and co-workers discovered that Pd0
underwent oxidative addition to aryl bromides, thus leading
À
À
strategy involving a tandem cyclization/Si C activation reac-
to efficient activation of Si C bonds in both an inter- and
tion featuring high efficiency, wide substrate scope, and
practical utility. This method enabled the first synthesis of
benzofuran siloles as well as rapid access to conjugated siloles.
intramolecular fashion (Scheme 1b).[12] Despite this progress,
À
additional investigations on silole synthesis through Si C
bond activation would not only expand the synthetic toolbox,
but also provide insight into the mechanistic aspects associ-
ated with this useful yet less explored process.
During the course of the study we also uncovered an unusual
yet general Si C(sp ) activation in the presence of p acids.
2
À
Inspired by these pioneering works, we were intrigued by
À
S
iloles have found wide applications in light-emitting
the possibility of accessing siloles by tandem cyclization/Si C
bond activation reactions (Scheme 2). At the heart of this
materials, fluorescent probes, thin-film transistors and solar
cells because of their unique optical and electronic proper-
ties.[1–6] Introduction of a heteroatom or an aryl group is
frequently explored to modify the light-absorbing and light-
emitting behavior of the parent siloles.[2b] In this context,
facile synthesis and easy derivation of silole scaffolds remain
important both in synthetic and material chemistry. Tradi-
tional syntheses of siloles often require stoichiometric organic
lithium or magnesium reagents, and thus limited the choice of
functional groups on the substrates. In answer to this
challenge, remarkable advances have been made toward the
catalytic synthesis of siloles.[7–14] Among them, the most recent
À
Scheme 2. The tandem cyclization/Si C activation strategy. TMS=tri-
methylsilyl.
2
À
À
syntheses involving catalytic activation of Si C bonds were
design is the new strategy to generate a M C(sp ) species
capable of Si C bond activation, which is distinct from the
previous transmetalation or oxidative insertion pathways
(Scheme 1). Briefly, we anticipated that the intramolecular
cyclization[15] of the aniline derivative 1 would generate such
an organometallic intermediate (A). Subsequent selective
activation of the Si C(sp ) bond would provide the desired
indole [3,2-b] silole 2. However enticing this proposal might
appear, it was conceived to have three major challenges:
1) the cyclization reaction requires a Lewis-acidic (electron-
À
conceptually inspiring, thus featuring high efficiency and
intriguing reactivity.[11–12] Specifically, Chatani and co-workers
À
reported facile Si C bond activation reactions catalyzed by in
2
À
situ generated Rh C(sp ) species by transmetalation between
aryl boronic acids or esters and alkynes (Scheme 1a).[11b,c] As
3
À
À
deficient) metal catalyst, while subsequent Si C bond acti-
vation is best achieved with an electron-rich metal species;[16]
2) the protodemetalation of A (undesired) and [Rh]-Me
(desired for catalyst turnover) are two competing pathways.
The protonation of A might prevail because of its inherent
À
higher basicity relative to that of the intermediate [Rh]
3
À
Scheme 1. Recent breakthrough in catalytic Si C(sp ) activation.
Me;[17] 3) very little is known about the selectivity in rhodium-
[11]
À
catalyzed activation of different Si C bonds.
In this
[*] Dr. Q.-W. Zhang, K. An, Prof. Dr. W. He
School of Medicine and Tsinghua-Peking Joint Centers for Life
Science
particular case, the relatively electron-rich nature of A
2
[18]
À
might favor oxidative addition of the Si C(sp ) bond,
thus leading to the undesired silicon-group-transfer product
3 upon reductive elimination.
Tsinghua University, Beijing 100084 (China)
E-mail: whe@mail.tsinghua.edu.cn
With these considerations in mind, we commenced our
study using the methylsulfonyl-protected aniline derivative
1a as the model substrate (Table 1). It is important to point
out that one advantage of this strategy lies in the easy access
to the substrate of type 1a by standard Sonogashira coupling
reactions between widely available alkynes and aryl halides.
[**] We gratefully acknowledge generous financial support from the
Tsinghua-Peking Joint Centers for Life Sciences (CLS). Q.-W.Z. is
supported by the Postdoctoral Fellowship of CLS and the China
Postdoctoral Science Foundation (2013M540083).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201400828.
Angew. Chem. Int. Ed. 2014, 53, 5667 –5671
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5667

.
Angewandte
Communications
Table 1: Screening of the reaction conditions.[a]
obtained product was the protodemetalation byproduct 4a
(entry 9). Since the reaction pathway had no clear depend-
ence on the pKa value of the bases, factors governing the
À
competing protodemetalation/Si C bond activation are likely
2
À
more complicated than the rate of the Rh C(sp ) protolysis.
In other words, the bases could also act as a ligand to the
Yield (2a/3a)[b]
À
rhodium and influence the rate of the Si C bond activation in
Entry Cat.
Base
Solvent
terms of both electronic and steric effects.[20]
1
CuOTf
DABCO 1,4-dioxane
DABCO 1,4-dioxane
DABCO 1,4-dioxane
DABCO 1,4-dioxane
n.d.
n.d.
n.d.
During the screening process, we observed that the yield
of product 2a was closely related to the catalyst loading, thus
hinting at a difficult catalyst turnover. Accordingly, we
attempted to use external proton sources to facilitate the
2
PdCl2
3
4
[{Ir(cod)Cl}2]
[Ru(cod)Cl2]
n.r.
5
6
7
[{Rh(cod)Cl}2] DABCO 1,4-dioxane
[{Rh(cod)Cl}2] DABCO toluene
[{Rh(cod)Cl}2] tBuONa 1,4-dioxane
32 (3:1)
23 (5:1)
trace
À
protonation of the Rh Me intermediate (Scheme 2; Table 1,
entries 10–14). We were cautious that this might lead to
escalated protodemetalation of A. However, as indicated in
our experiments with the base additive, the key lies in
alternating the competition between the desired catalyst
turnover and the undesired protodemetalation of A. Water
has been known to help rhodium catalyst turnover, however
in our case it resulted in a diminished yield (entry 10). After
extensive experimentation, we identified that addition of
ethanol could improve the yield to 44% without apparent
change in the ratio of 2a/3a (4:1; entry 11). Satisfyingly, both
the yield (65%) and the selectivity (2a/3a = 7:1) were further
improved when n-octanol was employed (entry 12). A similar
result was obtained in the less-polar solvent mesitylene
(entry 13). Very interestingly, when the p acid 3,3-dimethyla-
crolein (5.0 equiv) was used instead of alcohols, the selectivity
8
9
[{Rh(cod)Cl}2] DMAP
[{Rh(cod)Cl}2] DBU
1,4-dioxane
1,4-dioxane
<10
0
<10
44 (4:1)
65 (7:1)
60 (8:1)
40 (<5:95)
10
11
12
13
14[c]
[{Rh(cod)Cl}2] DABCO toluene/H2O
[{Rh(cod)Cl}2] DABCO toluene/ethanol
[{Rh(cod)Cl}2] DABCO toluene/octanol
[{Rh(cod)Cl}2] DABCO mesitylene/octanol
[{Rh(cod)Cl}2] DABCO toluene
[a] 2a (0.1 mmol), base (2.0 equiv), catalyst (10 mol%) and degassed
solvent (0.5 mL, for entries 10–13, 100 uL of the proton source was
added) in a N2 flushed glove box, 808C for 24 h. [b] Yield of isolated
1
products. 2a/3a determined by H NMR spectroscopy. [c] 3,3-dimethyl-
acrolein (5.0 equiv) was added. cod=1,5-cyclooctadiene, DABCO=1,4-
diazobicyclo[2.2.2]octane, DBU=1,8-diazabicyclo[5.4.0]undec-7-ene,
DMAP=4-(N,N-dimethylamino)pyridine, Ms=methanesulfonyl,
n.d.=not determined, n.r.=no reaction, Tf=trifluoromethanesulfonyl.
3
2
À
À
for Si C(sp ) over Si C(sp ) bond activation was reversed,
thus leading exclusively to the silicon transfer product 3a
(entry 14; see the Supporting Information). We further
identified that 2-trifluoromethyl phenylsulfonyl is the optimal
protecting group of the indole nitrogen atom after extensive
screening (see the Supporting Information), such that the
reaction proceeded smoothly under milder reaction condi-
tions (5 mol% catalyst at 808C) to afford 3a in 82% yield.
With the optimized reaction conditions and protecting
group, we investigated the substrate scope as summarized in
Table 2. First, we studied the substituent effect on ring A
(entries 1–7). Both electron-withdrawing (entry 3) and elec-
tron-donating (entries 1, 2, and 4–6) groups on ring A were
well tolerated, thus giving good yields of the desired silole
products with high selectivity. The piperonyl substrate 1g also
reacted smoothly to furnish 2g in 66% yield with excellent
selectivity (16:1; entry 7). In addition, the structure of 2g was
unambiguously determined by single-crystal X-ray diffrac-
tion.[21] Second, we examined the silicon moiety with ethyl, n-
butyl, and isopropyl substitutions (entries 8–10). To our great
We first tested a number of catalysts (entries 1–5) using
DABCO (2.0 equiv) as the base and 1, 4-dioxane as the
solvent. Disappointingly, the more cationic CuOTf and PdCl2
only gave the byproduct 4a (entries 1 and 2). In these two
cases, the protodemetalation took place before any Si C
activation could be observed, probably because of the
À
aforementioned lability of A. In light of this, heavy late-
À
transition metals, which are thought to form more stable M C
bonds,[19] were then examined (entries 3–5). However, [{Ir-
(cod)Cl}2] only gave a complex mixture with no desired
product detected (entry 3), and no reaction took place with
[Ru(cod)Cl2] (entry 4). To our delight, with [{Rh(cod)Cl}2]
À
the desired Si C bond activation products (e.g. 2a and 3a)
could be obtained in a low combined yield (32%). The
selectivity for Si C(sp ) over Si C(sp ) bond activation was
also moderate (2a/3a = 3:1; entry 5). When the solvent was
changed to the less-polar toluene, the selectivity improved to
5:1 but the yield decreased to 23% (entry 6). In both cases the
3
2
À
À
À
À
low yields of Si C bond activation products were due to the
undesired protodemetalation of A, a process which generated
the hydroamination product 4a predominantly.
delight, in all three cases the Si Me bond was preferentially
cleaved to give the corresponding unsymmetrical siloles in
good yields. We then turned our attention to the substitution
on ring B (entries 11–14). Notably, regardless of their stereo-
electronic difference, all the four cases produced the desired
siloles in moderate to good yields with uncompromised
selectivity. It is worth mentioning that our method enabled
the first synthesis of benzofuran siloles, which are predicted to
possess interesting properties.[22] Thus, when tBuONa
(1.0 equiv) was used in place of n-octanol, the phenolic
substrates 1o (entry 15) and 1p (entry 16) were successfully
To circumvent the undesired protodemetalation, we
tested various bases with the notion that decreasing the
acidity of the reaction mixture might favor the Si C bond
activation pathway (Table 1, entries 7–9). With the strong
base tBuONa, very low conversion of the substrate was
observed (entry 7). A similar observation was made with
DMAP (entry 8). In the presence of another strong base
DBU, full conversion of 1a was achieved, however, the only
À
5668 www.angewandte.org
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 5667 –5671

Angewandte
Chemie
Table 2: Substrate scope.[a]
fully converted into the silole product 2b in 76% yield upon
isolation with a greater than 20:1 selectivity. After screening
a panel of reaction conditions, the 2-trifluoromethyl phenyl-
sulfonyl group was cleanly cleaved using Mg foil with the
assistance of NH4Cl, thus furnishing the unprotected silole 5
in 91% yield (for the structure of 5, see 2b but with R = H)),
which could serve as a good starting point to further
functionalize the N atom. Our preliminary data suggested
that the representative silole products (2d, 2e, 2g, 2o, 2p, 3,
5) exhibit interesting light-emitting properties which warrant
further investigation (see Table S1 in the Supporting Infor-
mation).
Entry Substrate
Product
Yield[b]
2/3[c]
1
2
3
4
5
6
R1 =H, R=Ms (1a) 2a
65
7:1
R1 =H (1b)
2b
2c
2d
2e
2 f
82(50)[d] 10:1(0:1)[d]
The p-acid effect observed with 1a (entry 14, Table 1) was
proven to be a general phenomenon as illustrated by various
other substrates (1b, 1h, 1o and 1p). As shown in Table 2
R1 =3-F (1c)
69
67
75
54
8:1
10:1
11:1
5:1
R1 =3-Me (1d)
R1 =3-OMe (1e)
R1 =4-Me (1 f)
2
À
(entries 2, 8, 15 and 16), the selective Si C(sp ) activation led
to the products 3b, 3h, 3o, and 3p with excellent selectivity
and in moderate yields (34–61%). The single-crystal X-ray
structure of 3b was also obtained to unambiguously deter-
mine its identity.[21] To the best of our knowledge this
represents the first case wherein selective cleavage of both
Si C(sp ) and Si C(sp ) could be achieved with the same
metal catalyst, thus leading to the synthesis of two types of
valuable fluorescent materials under subtly different reaction
conditions.
3
2
À
À
7
1g
2g
66
19:1
One potential advantage of this method is its compati-
bility with aryl halides as it does not involve an oxidative-
addition pathway under rhodium catalysis.[23] We demon-
strated such an advantage in the synthesis of the conjugated
siloles 6–8 as depicted in Scheme 3. Again, the substrate 1q,
8
9
10
R2 =Et (1h)
R2 =nBu (1i)
R2 =iPr (1j)
2h
2i
2j
77(34)[d] 8:1(0:1)[d]
79
61
9:1
8:1
11
12
13
R3 =Me, (1k)
R3 =tBu, (1l)
2k
2l
71
65
52
57
5:1
5:1
11:1
8:1
R3 =C(O)nBu, (1m) 2m
14[e] R3 =Cl, (1n)
2n
Scheme 3. Syntheses of conjugated indole siloles. Reagents and con-
ditions: a) Standard conditions, 10 mol% cat.; b) phenylacetylene,
Et3N, [Pd(PPh3)2Cl2] (5 mol%), CuI (1 mol%), 508C; c) PhB(OH)2
(1.5 equiv), Pd(OAc)2 (10 mol%), Sphos (20 mol%), K3PO4 (3 equiv),
toluene, 508C; d) 4-tBuPhB(OH)2 (1.5 equiv), Pd(OAc)2 (10 mol%),
Sphos (20 mol%), K3PO4 (3 equiv), toluene, 908C.
15[f] R1 =H (1o)
2o
2p
54(50)[d] 18:1(0:1)[d]
41(61)[d] 16:1(0:1)[d]
16[f] R1 =3-OMe (1p)
[a] Standard conditions, R=2-trifluoromethanesulfonyl except for 1a
where R=Ms [b] Yields of isolated products [%]. [c] Determined by
1H NMR analysis. [d] 3,3-dimethylacrolein (1.0 equiv) were added
instead of n-octanol, the yields and ratios of 2/3 are shown within the
parentheses. [e] 10 mol% catalyst was used. [f] tBuONa (1.0 equiv) was
added, the reaction was carried out in toluene (1 mL), please see the
Supporting Information. M.S.=molecular sieves.
containing preinstalled halogens (Br and Cl), was readily
synthesized. Satisfyingly, the desired product 2q was obtained
in good yield with high selectivity under the reaction
conditions on a 1 mmol scale. Subsequent Sonogashira
coupling provided the phenylethynyl-substituted silole 6 in
96% yield with the aryl chloride intact. Alternatively,
sequential Suzuki–Miyaura coupling reactions first with
phenylbronic acid then with para-tert-butyl phenylbronic
acid elaborated the product 2q into the functionalized siloles
7 and 8 in excellent yield and chemoselectivity.[24,8b] Given the
versatility of the preinstalled halogen functionality, this
converted into the siloles 2o and 2p, respectively, with
excellent selectivities (18:1 and 16:1).
To prove the utility of our method in fluorescent material
research, larger scale preparation as well as the removal of the
protecting group was pursued (see the Supporting Informa-
tion). As an illustration, 5 mmol of 1b (2.37 g) was success-
Angew. Chem. Int. Ed. 2014, 53, 5667 –5671
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 5669

.
Angewandte
Communications
method should find applications in the syntheses of other
conjugated siloles.
well as light emitting siloles are currently ongoing in our
group.
We propose the mechanism of the selective activation of
À
Si C bonds based on our experimental findings and recent
work[25] (Figure 1). As shown in the central box, ligand Experimental Section
The substrate 1 (0.1 mmol), DABCO (0.1 mmol), [{Rh(cod)Cl}2]
exchange of the precatalyst [{Rh(cod)Cl}2] (D1) with
(5 mol%), and degassed solvent (toluene/1-octanol, 0.5:0.1 mL) were
added to a screw-capped tube in a N2 flushed glove box. The tube was
capped and removed from the glove box and the reaction mixture was
heated at 808C for 12 h. The reaction mixture was filtered, washed
with ethyl ether, and concentrated under vacuum. The resulting
mixture was directly subjected to silica gel column chromatography
(eluent: hexanes/EtOAc 100:1).
Received: January 25, 2014
Published online: April 8, 2014
Keywords: heterocycles · luminescence · rhodium · silicon ·
.
synthetic methods
À
Figure 1. Proposed mechanism for the selective Si C activation.
[1] a) S. Yamaguchi, K. Tamao, Silicon-containing Polymers: The
Science and Technology of their Synthesis and Applications
(Eds.: R. G. Jones, W. Ando, J. Chojnowski), Kluwer Academic
Publishers, Dordrecht, 2000, pp. 461 – 498; b) J. Y. Corey in
Advances in Organometallic Chemistry, Vol. 59 (Eds.: F. H.
Anthony, J. F. Mark), Academic Press, New York, 2011, pp. 1 –
328.
[2] a) M. Shimizu, K. Mochida, T. Hiyama, J. Phys. Chem. C 2011,
115, 11265; b) M. Shimizu, K. Mochida, M. Katoh, T. Hiyama, J.
Phys. Chem. C 2010, 114, 10004; c) K. Geramita, J. McBee, Y.
Shen, N. Radu, T. D. Tilley, Chem. Mater. 2006, 18, 3261; d) E.
Pusztai, I. S. Toulokhonova, N. Temple, H. Albright, U. I. Zakai,
S. Guo, I. A. Guzei, R. Hu, R. West, Organometallics 2013, 32,
2529; e) J. Liu, Y. Zhong, J. W. Y. Lam, P. Lu, Y. Hong, Y. Yu, Y.
Yue, M. Faisal, H. H. Y. Sung, I. D. Williams, K. S. Wong, B. Z.
Tang, Macromolecules 2010, 43, 4921.
[3] a) S. J. Toal, K. A. Jones, D. Magde, W. C. Trogler, J. Am. Chem.
Soc. 2005, 127, 11661; b) S. J. Toal, D. Magde, W. C. Trogler,
Chem. Commun. 2005, 5465.
[4] G. Lu, H. Usta, C. Risko, L. Wang, A. Facchetti, M. A. Ratner,
T. J. Marks, J. Am. Chem. Soc. 2008, 130, 7670.
[5] a) J. Chen, Y. Cao, Macromol. Rapid Commun. 2007, 28, 1714;
b) R. C. Coffin, J. Peet, J. Rogers, G. C. Bazan, Nat. Chem. 2009,
1, 657; c) J. Hou, H. Y. Chen, S. Zhang, G. Li, Y. Yang, J. Am.
Chem. Soc. 2008, 130, 16144; d) T. Y. Chu, J. Lu, S. Beaupre, Y.
Zhang, J. R. Pouliot, S. Wakim, J. Zhou, M. Leclerc, Z. Li, J.
Ding, Y. Tao, J. Am. Chem. Soc. 2011, 133, 4250.
DABCO (annotated as [N]) gave rise to [Rh(cod)-
(DABCO)Cl] (D2). Species D2 was successfully detected by
HRMS ([M+H]+ = 359.0757; see the Supporting Informa-
tion) after careful experimentation. Dissociation of a ClÀ
from D2 would lead to the cationic, coordinative unsaturated
[Rh(cod)(DABCO)]+ (D3) which serves as the active catalyst
for the cyclization. Depending on the additive (e.g., n-octanol
or 3,3-dimethylacrolein), the resulting indole Rh intermedi-
ate A or Ap acid could then undergo oxidative insertion to
either Si C bond as depicted, thus leading to intermediates B
or C, respectively. Reductive elimination of B would produce
the final product 2 as well as [Rh(cod)(DABCO)Me] (D4).
Protodemetalation of D4 with the aid of n-octanol then
regenerates the active catalyst D3 and concludes the catalytic
cycle. Based on our observations, this step was the turnover-
limiting step of the tandem reactions. In contrast, reductive
elimination of C would give the silicon-transfer product 3 and
À
À
À
the corresponding Rh Ar species (not shown). Unlike [Rh-
À
(cod)(DABCO)Me], the more liable Rh Ar could undergo
facile protonation possibly by the DABCO conjugated acid to
regenerate the active catalyst D3.
In conclusion, we have developed a new method for the
rapid synthesis of structurally diverse siloles, including the
first examples of benzofuran siloles, through a rhodium(I)-
[6] a) X. Zhan, S. Barlow, S. R. Marder, Chem. Commun. 2009,
1948; b) S. Yamaguchi, T. Goto, K. Tamao, Angew. Chem. 2000,
112, 1761; Angew. Chem. Int. Ed. 2000, 39, 1695; c) K. Tamao, S.
Yamaguchi, M. Shiro, J. Am. Chem. Soc. 1994, 116, 11715.
[7] a) W. Rauf, J. M. Brown, Angew. Chem. 2008, 120, 4296; Angew.
Chem. Int. Ed. 2008, 47, 4228; b) Y. Nakao, M. Takeda, T.
Matsumoto, T. Hiyama, Angew. Chem. 2010, 122, 4549; Angew.
Chem. Int. Ed. 2010, 49, 4447; c) M. Shindo, K. Matsumoto, K.
Shishido, Angew. Chem. 2004, 116, 106; Angew. Chem. Int. Ed.
2004, 43, 104.
À
catalyzed cyclization/Si C bond activation sequence. During
our efforts to reconcile the different demands of these two
processes, we discovered that external proton source is
beneficial for the catalyst turnover. Divergent selectivity for
3
2
À
À
Si C(sp ) and Si C(sp ) bond activation was achieved
depending on subtly different additives, thus underscoring
[8] a) M. Shimizu, K. Mochida, T. Hiyama, Angew. Chem. 2008, 120,
9906; Angew. Chem. Int. Ed. 2008, 47, 9760; b) K. Mochida, M.
Shimizu, T. Hiyama, J. Am. Chem. Soc. 2009, 131, 8350; c) R.
Shintani, H. Otomo, K. Ota, T. Hayashi, J. Am. Chem. Soc. 2012,
134, 7305.
À
interesting mechanistic aspects of rhodium-catalyzed Si C
activation.[25] We further demonstrated the advantage of this
method in the synthesis of halogenated siloles which allows
rapid derivation. Further work concerning the mechanistic
implications, the enantioselective synthesis of chiral siloles, as
[9] a) T. Ureshino, T. Yoshida, Y. Kuninobu, K. Takai, J. Am. Chem.
Soc. 2010, 132, 14324; b) Y. Kuninobu, K. Yamauchi, N. Tamura,
5670 www.angewandte.org
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 5667 –5671

Angewandte
Chemie
T. Seiki, K. Takai, Angew. Chem. 2013, 125, 1560; Angew. Chem.
Int. Ed. 2013, 52, 1520.
[14] I. Fleming, A. Barbero, D. Walter, Chem. Rev. 1997, 97, 2063,
and references therein.
[10] a) T. Matsuda, Y. Suda, Y. Fujisaki, Synlett 2011, 813; b) T.
Ohmura, K. Masuda, M. Suginome, J. Am. Chem. Soc. 2008, 130,
1526; c) T. Matsuda, Y. Ichioka, Org. Biomol. Chem. 2012, 10,
3175.
[15] a) N. Isono, M. Lautens, Org. Lett. 2009, 11, 1329; b) D. Shu, W.
Song, X. Li, W. Tang, Angew. Chem. 2013, 125, 3319; Angew.
Chem. Int. Ed. 2013, 52, 3237; c) X. Li, W. Song, W. Tang, J. Am.
Chem. Soc. 2013, 135, 16797.
[11] a) I. Ojima, D. A. Fracchiolla, R. J. Donovan, P. Banerji, J. Org.
Chem. 1994, 59, 7594; b) M. Tobisu, M. Onoe, Y. Kita, N.
Chatani, J. Am. Chem. Soc. 2009, 131, 7506; c) M. Onoe, K.
Baba, Y. Kim, Y. Kita, M. Tobisu, N. Chatani, J. Am. Chem. Soc.
2012, 134, 19477; d) M. Yu, X. Fu, J. Am. Chem. Soc. 2011, 133,
15926.
[16] T. E. Mꢀller, K. C. Hultzsch, M. Yus, F. Foubelo, M. Tada, Chem.
Rev. 2008, 108, 3795.
[17] C. M. So, S. Kume, T. Hayashi, J. Am. Chem. Soc. 2013, 135,
10990.
[18] T. Seiser, N. Cramer, Angew. Chem. 2010, 122, 10361; Angew.
Chem. Int. Ed. 2010, 49, 10163.
[12] a) Y. Liang, W. Geng, J. Wei, Z. Xi, Angew. Chem. 2012, 124,
1970; Angew. Chem. Int. Ed. 2012, 51, 1934; b) Y. Liang, S.
Zhang, Z. Xi, J. Am. Chem. Soc. 2011, 133, 9204; c) T. Meng, K.
Ouyang, Z. Xi, RSC Adv. 2013, 3, 14273.
[13] a) T. Matsuda, S. Kadowaki, T. Goya, M. Murakami, Org. Lett.
2007, 9, 133; b) K. Ikenaga, K. Hiramatsu, N. Nasaka, S.
Matsumoto, J. Org. Chem. 1993, 58, 5045; c) Y. Yabusaki, N.
Ohshima, H. Kondo, T. Kusamoto, Y. Yamanoi, H. Nishihara,
Chem. Eur. J. 2010, 16, 5581; d) W. S. Palmer, K. A. Woerpel,
Organometallics 1997, 16, 1097; e) T. Matsuda, S. Kadowaki, M.
Murakami, Chemm. Commun. 2007, 2627; f) T. Matsuda, Y.
Yamaguchi, M. Shigeno, S. Sato, M. Murakami, Chem. Commun.
2011, 47, 8697.
[19] P. J. Davidson, M. F. Lappert, R. Pearce, Acc. Chem. Res. 1974, 7,
209.
[20] K. Ouyang, Z. Xi, Acta Chim. Sin. 2013, 71, 13.
[21] CCDC 948123 (2g) and 979513 (3b) contain 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.ac.uk/data_request/cif. (Also see SI)
[22] X.-K. Chen, L.-Y. Zou, A.-M. Ren, J.-X. Fan, Phys. Chem. Chem.
Phys. 2011, 13, 19490.
[23] Y. Yamanoi, H. Nishihara, J. Org. Chem. 2008, 73, 6671.
[24] T. E. Barder, S. D. Walker, J. R. Martinelli, S. L. Buchwald, J.
Am. Chem. Soc. 2005, 127, 4685.
[25] Z. Yu, Y. Lan, J. Org. Chem. 2013, 78, 11501.
Angew. Chem. Int. Ed. 2014, 53, 5667 –5671
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5671