Solvent-induced selectivity switching: Intermolecular allylsilylation, arylsilylation, and silylation of alkynes over montmorillonite catalyst

Article abstract of DOI:10.1016/j.tetlet.2011.09.062

Proton-exchanged montmorillonite showed catalytic activity for intermolecular allylsilylation, arylsilylation, and terminal silylation of alkynes with allylsilanes. The reaction selectivity greatly depended on the solvent used. Reactions proceeded with va

Full text of DOI:10.1016/j.tetlet.2011.09.062

Tetrahedron Letters 52 (2011) 6687–6692
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Solvent-induced selectivity switching: intermolecular allylsilylation,
arylsilylation, and silylation of alkynes over montmorillonite catalyst
Ken Motokura a, Shigekazu Matsunaga a, Akimitsu Miyaji a, Tatsuaki Yashima b, Toshihide Baba a,
⇑
a Department of Environmental Chemistry and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8502, Japan
b Advanced Research Institute for the Sciences and Humanities, Nihon University, 12-5 Gobancho, Chiyoda-ku, Tokyo 102-8251, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Proton-exchanged montmorillonite showed catalytic activity for intermolecular allylsilylation, arylsilyla-
tion, and terminal silylation of alkynes with allylsilanes. The reaction selectivity greatly depended on the
solvent used. Reactions proceeded with various terminal alkynes and allylsilanes in good to moderate
yields. The reaction pathways involving cationic Si species on the montmorillonite surface were also
investigated.
Received 8 August 2011
Revised 3 September 2011
Accepted 14 September 2011
Available online 21 September 2011
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Proton-exchanged montmorillonite
Carbosilylation
Silylation
Alkyne
Heterogeneous catalysis
Introduction
methodologies to introduce allyl, aryl, and silyl groups to alkynes
and (ii) the effect of solvents on the reaction pathways.
Carbosilylation of alkynes is a method to create both C–C and
C–Si bonds on the triple bonds of alkynes to give substituted vinyl
silanes, which are useful intermediates for organic transforma-
tions.1 Among the carbosilylation reactions of alkynes, allyl- and
arylsilylations are known reactions which are catalyzed by several
homogeneous Lewis acids.2–4 For example, AlCl3, EtAlCl2, and HfCl4
have been reported as catalysts for the intermolecular allylsilyla-
tion of alkynes.2 HfCl4 and EtAlCl2 have also been found to be a cat-
alyst for intramolecular arylsilylation.4a In almost all cases, these
Lewis acids activate the carbon–carbon triple bond for nucleophilic
addition of allyl and aryl groups.
Recently, our group has reported allylsilylation of alkenes using
proton-exchanged montmorillonite (H+-montmorillonite) as a het-
erogeneous catalyst.5 The allylsilylation of alkenes is initiated by
the formation of a cationic silyl species on the montmorillonite sur-
face ([Si]+-montmorillonite) from a protonic acid site and allylsi-
lane.5,6 If the [Si]+-montmorillonite can react not only with alkenes
but also alkynes to give b-silyl cations, catalytic allylsilylation of
alkynes should be possible, as shown in Scheme 1. Herein, allylsily-
lation of alkynes with allylsilanes was examined using H+-montmo-
rillonite (Scheme 2A). In addition, during the course of our study on
allylsilylations, wefoundthatarylsilylationandsilylationproceeded
selectively in the presence of solvents having relatively high nucleo-
philicity (Scheme 2B and C). In this Letter, we describe (i) novel
Results and discussion
Reactions of alkyne with allylsilane in various solvents
To investigate the effect of solvent on the product selectivity of
the reaction between alkynes and allylsilane, the reaction of phenyl-
acetylene (1a) with allyltrimethylsilane (2a) in various solvents was
examined using H+-montmorillonite, as shown in Table 1. trans-
Allylsilylation of 1a selectively proceeded to afford the product
3aa in non-polar solvents, such as n-heptane and toluene (entries
1–3). In the case of aromatic solvents with relatively high nucleophi-
licity due to the presence of electron-donating functions, such as
mesitylene and anisole, both the silyl group and the aromatic
solvent were introduced to the carbon–carbon triple bond of 1a
(entries 4–6). For example, the reaction of 1a with mesitylene and
2a afforded the corresponding trans-arylsilylation product (4aa) in
66% yield (entry 5). In the presence of solvents with ether functions,
terminal silylation of 1a proceeded selectively, giving the product
5aa (entries 7 and 8). In the case of other solvents having high polar-
ity, such as DMF and DMSO, the reactions hardly occurred.
Allylsilylation of alkynes
The allylsilylation of alkynes with allylsilanes in toluene solvent
⇑
Corresponding author.
E-mail address: tbaba@chemenv.titech.ac.jp (T. Baba).
is summarized in Table 2.7 The reaction of crotyltrimethylsilane (2c:
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.09.062

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K. Motokura et al. / Tetrahedron Letters 52 (2011) 6687–6692
SiMe3
SiMe3
[Si]+
R
(A)
(B)
R
R
O
O
SiMe3
[Si]+-montmorillonite
SiMe3
[Si]+
R
SiMe3
O
R
R
O
SiMe3
[Si]+-montmorillonite
Scheme 1. Allylsilylations of (A) alkenes and (B) alkynes catalyzed by [Si]+-montmorillonite.
nonpolar solvent
(A)
R
SiMe3
H+-montmorillonite
aromatic solvent with nucleophilicity
R'
+
SiMe3
(B)
R
R
SiMe3
SiMe3
ether solvent
(C)
R
Scheme 2. Solvent-induced selectivity switching.
R2 = Me, R3 = H, R4 = Me) afforded (Z)-trimethyl(3-methyl-2-phe-
nylpenta-1,4-dienyl)silane (3ac) in 51% yield (entry 6). This result
suggests that the reaction proceeded through acidic activation of
the alkyne and nucleophilic addition of the allylsilane. As shown
in Scheme 1, it can be expected that the [Si]+-montmorillonite,
which is formed from H+-montmorillonite and allylsilane, reacts
with alkynes. To confirm the direct activation of the alkyne by the
cationic silyl species on montmorillonite, the reaction mixture of
1a with 2a was quenched with H2O (Scheme 3). If H+ or another me-
tal species such as Al3+ initially activates the alkyne, diene com-
pound 6 should be obtained via hydration of the intermediate
(Scheme 3, dotted arrows).2b However, the formation of 6 was not
detected. This result indicates direct activation of the cationic Si spe-
cies on the montmorillonite surface. It can be expected that the acti-
vation of alkynes by a silyl cation induces the formation of b-silyl
alkenyl cation 7.8 A proposed allylsilylation pathway involving 7 is
shown in Scheme 4(A). Because an electron-withdrawing group on
the para-position of phenylacetylene enhanced the reactivity
(entries 1–3), increasing the electrophilicity of the b-silyl cationic
intermediate 78 is necessary to promote the reaction effectively.
Arylsilylation of alkynes
The reaction of 1a with 2a in mesitylene solvent was examined
using various protonic acids, as shown in Table 3. Only protonic
montmorillonites showed catalytic activity for the arylsilylation
(entries 1 and 2). Other typical protonic acids, such as Nafion and
H2SO4, did not show any catalytic activity for allylsilylation, arylsi-
lylation, or silylation (entries 3–5). Because the reaction did not pro-
ceed at all with Na+-montmorillonite (entry 6), the H+ site on the
montmorillonite surface must be necessary for the arylsilylation.
As shown in Eq. 1, the para-substitution effect on the arylsilyla-
tion was similar to that on the allylsilylation. The complete trans-
selectivity of the Si and mesityl groups in the arylsilylation product
indicates stepwise addition of the silyl and aryl groups to the triple
bond. These results suggest that the reaction also proceeded
through the b-silyl cation intermediate 7 as in the allylsilylation
(Scheme 4). The proposed pathway for the arylsilylation is shown
in Scheme 4(B). The reaction ceased completely by the addition
of 2,6-di-tert-butyl pyridine. This result supports the formation of
H+ in the catalytic cycle.9
H+-montmorillonite
(0.10 g)
SiMe3
+
mesitylene (1 mL)
ð1Þ
X
SiMe3
50 °C
X
X=Cl: 79% yield (1.5 h)
X=H: 69% yield (8 h)
X=Me: 31% yield (32 h)

K. Motokura et al. / Tetrahedron Letters 52 (2011) 6687–6692
6689
Table 1
Reactions between phenylacetylene (1a) and allylsilane (2a) using H+-montmorillonite in various solventsa
X
H+-montmorillonite
SiMe3
SiMe3
2a
+
+
+
solvent (1 mL)
SiMe3
3aa
SiMe3
4aa
1a
5aa
Entry
Solvent
Time (min)
Conv. of 1a (%)b
Yield (%)b
3aa
4aa
5aa
1
2
240
300
39
73
12
57
—
4
Cl
<1
<1
13
Me
3
4
180
300
>99
>99
62
52
10
16
Me
Me
12
Me
Me
Me
5
6
90
60
>99
>99
9
8
66
9
1
OMe
55c
O
O
7
8
150
180
91
87
<1
<1
–
–
76
58
O
a
b
c
Reaction conditions: 1a (1.0 mmol), 2a (3.0 mmol), H+-montmorillonite (0.10 g), solvent (1 mL), 100 °C.
Determined by GC and 1H NMR analysis using an internal standard. Based on 1a used.
(o/p = 23/77). In the reaction using aromatic solvent, the formation of small amount of hydroarylation product was detected.
Table 2
Allylsilylation of alkynes (1) with allylsilanes (2)a
R2
R1
R3
R3
H+-montmorillonite
R2
SiR4
+
3
R1
toluene (1 mL)
100 °C
SiR43
2
1
3
Entry
R1
R2
R3
R4
Time (h)
Yield (%)b
Selectivity (%)b
1
2
3
4
5
6
7
p-Cl-Ph (1b)
Ph (1a)
p-Me-Ph (1c)
n-C6H13 (1d)
Ph (1a)
H
H
H
H
H
Me
H
H
H
H
H
Me
H
H
Me (2a)
Me (2a)
Me (2a)
Me (2a)
Me (2b)
Me (2c)
Et (2d)
0.5
3
12
0.5
1.5
4
66 (64c)
62
26
67
62
30
30
74
53
35
30
72 (61c)
51
Ph (1a)
Ph (1a)
4
34
a
b
c
Reaction conditions: 1 (1.0 mmol), 2 (3.0 mmol), H+-montmorillonite (0.10 g), toluene (1 mL), 100 °C.
Determined by GC and 1H NMR analysis using an internal standard. Based on 1 used.
Isolated yield.
Terminal silylation of alkynes
and montmorillonite acted as catalysts. A proposed reaction path-
way of the terminal silylation is shown in Scheme 4(C). The cat-
ionic intermediate 7 may react easily with Lewis basic ethers to
form the silylated product and H+.
The generality of alkynes for the terminal silylation was inves-
tigated by using 1,4-dioxane solvent and H+-montmorillonite as a
catalyst, as shown in Table 4. Good to moderate yields were ob-
tained in the case of both aromatic and aliphatic terminal alkynes.
It is known that the terminal silylation of alkynes is catalyzed
by bases, which abstract the terminal proton of the alkyne.10 As
shown in Table 1, terminal silylation of 1a proceeded selectively
in ether solvents.11 Even though the oxygen atom of ethers acts
as a weak Lewis base, the reaction did not proceed in the absence
of the H+-montmorillonite. From these facts, both the ether solvent

6690
K. Motokura et al. / Tetrahedron Letters 52 (2011) 6687–6692
H+-montmorillonite
H2O (4 mL)
(0.10 g)
SiMe3
+
toluene (1 mL)
50 °C, 1 h
H
1a
2a
6
0% yield
H2O
SiMe3
SiMe3
Mn+
X
Mn+
Scheme 3. Quench experiment.
SiMe3
R
H+
SiMe3
O
SiMe3
O
R
[Si]+
O
mont surface
(A)
SiMe3
2
SiMe3
R'
R'
R
[Si]+
O
SiMe3
O
R
R
R'
H+
SiMe3
O
SiMe3
R
(B)
(C)
O
7
R"
SiMe3
O
R"
R"
R"
R"
O
O
R"
R"
R
R"
O
H
H+
O
SiMe3
O
R
R
SiMe3
O
Scheme 4. Proposed reaction pathways using (A) nonpolar solvent, (B) aromatic solvent with relatively high nucleophilicity, and (C) ether solvent.
In the terminal silylation, phenylacetylene having electron-donat-
ing group (entry 3) showed higher reactivity than that of the elec-
tron-withdrawing group (entry 1). On the other hand, substrates
having electron-withdrawing group showed high reactivity in al-
lyl- and arylsilyaltion (Table 2 and Eq. 1). It can be expected that
the electron-donating effect of the substituent enhances adsorp-
tion of the alkyne on the cationic Si species even in the presence
of ether solvent which may also adsorb on the active cationic site.
Experimental section
Proton nuclear magnetic resonance (1H NMR) and carbon nucle-
ar magnetic resonance (13C NMR) spectra were recorded in CDCl3
with a JNM-AL300 spectrometer operating at 300 and 75 MHz,
respectively. Analytical GLC was measured using a Shimadzu GC-
8A equipped with a Silicon SE-30 column and a flame ionization
detector. A Shimadzu QP5000 GC-MS equipped with DB-1 column
was used. The products were confirmed by comparison with the
reported NMR and MS data. To determine yields and conversions,
n-propylbenzene and p-xylene were used as internal standards
for GC and 1H NMR analysis.
Summary
Solvent-induced selectivity switching of montmorillonite-cata-
lyzed allylsilylation, arylsilylation, and silylation of alkynes was
discovered. These three reaction pathways included the same b-si-
lyl cation as an intermediate, which was generated by the reaction
between the cationic silyl species on the montmorillonite surface
and the alkyne. The reaction selectivity changed depending on
the reactivity between the intermediate and the solvent used.
Na+-montmorillonite
[Na0.66(OH)4Si8(Al3.34Mg0.66Fe0.19)O20;
Kunipia F] was obtained from Kunimine Industry. Proton-ex-
changed montmorillonite was prepared from Na+-montmorillonite
using the reported ion exchange procedure with aqueous hydrogen
chloride.12 The prepared H+-montmorillonite was stored under 30%
humidity. H+-montmorillinite was dried under vacuum (ca.

K. Motokura et al. / Tetrahedron Letters 52 (2011) 6687–6692
6691
Table 3
Reactions between phenylacetylene (1a) and allylsilane (2a) in the presence of mesitylene using various acidsa
Me
SiMe3
Me
Me
H+-montmorillonite
+
+
SiMe3
2a
+
mesitylene (1 mL)
SiMe3
3aa
SiMe3
4aa
5aa
1a
Entry
Acid
Conv. of 1a (%)b
Yield (%)b
3aa
4aa
5aa
1c
2
3
4
5
6
7
H+-montmorillonite
Mont K10
Amberlyst
Nafion
98
79
26
16
9
4
69
35
<1
<1
<1
<1
<1
3
<1
<1
<1
<1
<1
<1
<1
<1
<1
<1
<1
<1
H2SO4
Na+-montmorillonite
None
15
8
a
b
c
Reaction conditions: 1a (1.0 mmol), 2a (3.0 mmol), H+-montmorillonite (0.10 g), mesitylene (1 mL), 50 °C, 8 h.
Determined by GC and 1H NMR analysis using an internal standard. Based on 1a used.
5% Yield of 1,3,5-trimethyl-2-(1-phenylvinyl)-benzene (8) was obtained via hydroarylation of 1a with mesitylene.
8
Table 4
Terminal silylation of alkynes (1) with allylsilanes (2)a
H+-montmorillonite
1,4-dioxane (1 mL)
SiR4
+
3
R1
SiR43
R1
1
2
5
Entry
R1
R4
Time (h)
Yield of 5 (%)b
Selectivity of 5 (%)b
1
2
3
4
5
6
p-Cl-Ph (1b)
Ph (1a)
p-Me-Ph (1c)
PhCH2 (1e)
n-C6H13 (1d)
Ph (1a)
Me (2a)
Me (2a)
Me (2a)
Me (2a)
Me (2a)
Et (2d)
3
2.5
5
3
3
65
76
93
84
91
99
71
92
87 (78c)
40
62
9
82 (77c)
a
b
c
Reaction conditions: 1 (1.0 mmol), 2 (3.0 mmol), H+-montmorillonite (0.10 g), 1,4-dioxane (1 mL), 100 °C.
Determined by GC and 1H NMR analysis using an internal standard. Based on 1 used.
Isolated yield.
1 mmHg) at 120 °C for 1 h before the catalytic reaction. Amberlyst
was purchased from Organo Co. as AmberlystÒ 15DRY. Nafion was
purchased from Aldrich Inc. as NafionÒ NR50. Unless otherwise
noted, all other materials were purchased from Wako Pure Chem-
icals, Tokyo Kasei Co., Kanto Kagaku Co., and Aldrich Inc.
phy using silica (n-hexane as eluant) to afford a pure product. The
product was identified by comparison with the reported 1H and 13
NMR and mass spectral data.
C
Arylsilylation of alkynes
Allylsilylation of alkynes
The typical procedure for arylsilylation of phenylacetylene
(1a) with allyltrimethylsilane (2a) and mesitylene is as follows.
The typical procedure for allylsilylation of phenylacetylene (1a)
with allyltrimethylsilane (2a) is as follows. Into a glass reactor
were placed the H+-montmorillonite (0.10 g), toluene (1.0 mL), 1a
(1.0 mmol), and 2a (3.0 mmol) under a dry Ar atmosphere using
Schlenk techniques. The resulting mixture was vigorously stirred
at 100 °C. After 180 min, the catalyst was separated by filtration.
GC analysis of the filtrate showed a 62% yield of (Z)-trimethyl
(2-phenylpenta-1,4-dien-1-yl)silane (3aa). The filtrate was evapo-
rated and the crude product was purified by column chromatogra-
Into a
glass reactor were placed the H+-montmorillonite
(0.10 g), mesitylene (1.0 mL), 1a (1.0 mmol), and 2a (3.0 mmol)
under a dry Ar atmosphere using Schlenk techniques. The result-
ing mixture was vigorously stirred at 50 °C. After 8 h, the catalyst
was separated by filtration. GC analysis of the filtrate showed a
69% yield of (E)-(2-mesityl-2-phenylvinyl)trimethylsilane (4aa).
The filtrate was evaporated and the crude product was purified
by column chromatography using silica (n-hexane as eluant) to
afford a pure product: 1H NMR (400 MHz, CDCl3): 0.03 (s, 9H),

6692
K. Motokura et al. / Tetrahedron Letters 52 (2011) 6687–6692
2. Intermolecular allylsilylation: (a) Asao, N.; Yoshikawa, E.; Yamamoto, Y. J. Org.
Chem. 1996, 61, 4874; (b) Yoshikawa, E.; Gevorgyan, V.; Asao, N.; Yamamoto, Y.
J. Am. Chem. Soc. 1997, 119, 6781; (c) Yeon, S. H.; Han, J. S.; Hong, E.; Do, Y.;
Jung, I. N. J. Organomet. Chem. 1995, 499, 159; (d) Asao, N.; Tomeba, H.;
Yamamoto, Y. Tetrahedron Lett. 2005, 46, 27.
3. Intramolecular allylsilylation: (a) Imamura, K.-i.; Yoshikawa, E.; Gevorgyan, V.;
Yamamoto, Y. J. Am. Chem. Soc. 1998, 120, 5339; (b) Matsuda, T.; Kadowaki, S.;
Yamaguchi, Y.; Murakami, M. Chem. Commun. 2008, 2744; (c) Horino, Y.;
Nakashima, Y.; Hashimoto, K.; Kuroda, S. Synlett 2010, 2879.
2.15 (s, 6H), 2.27 (s, 3H), 5.65 (s, 1H), 6.84 (s, 2H), 7.22 (m, 5H);
13C{1H} NMR (100.61 MHz. CDCl3): 0.39, 20.5, 21.0, 127.4, 127.7,
128.2, 128.5, 133.1, 134.9, 136.0, 142.2, 142.8, 156.7; MS (EI) m/z
(%): 59, 73(100), 135, 177, 207, 220, 279, 294(M+). The trans rela-
tionship of the mesityl and silyl groups was confirmed by H–H
NOESY.
4. Arylsilylation: (a) Asao, N.; Shimada, T.; Shimada, T.; Yamamoto, Y. J. Am. Chem.
Soc. 2001, 123, 10899; (b) Chatani, N.; Inoue, H.; Ikeda, T.; Murai, S. J. Org. Chem.
2000, 65, 4913; (c) Agenet, N.; Mirebeau, J.-H.; Petit, M.; Thouvenot, R.;
Gandon, V.; Malacria, M.; Aubert, C. Organometallics 2007, 26, 819.
5. Motokura, K.; Matsunaga, S.; Miyaji, A.; Sakamoto, Y.; Baba, T. Org. Lett. 2010,
12, 1508.
6. Formation of silyl cation on the montmorillonite surface: (a) Higuchi, K.;
Onaka, M.; Izumi, Y. Bull. Chem. Soc. Jpn. 1993, 66, 2016; (b) Wang, J.; Masui, Y.;
Watanabe, K.; Onaka, M. Adv. Synth. Catal. 2009, 351, 553.
7. The reactivities of internal alkynes were much lower than those of the terminal
alkynes. For example, the allylsilylation of 1-phenyl-1-propyne afforded less
than 10% yield of product for 24 h.
8. b-Silyl alkenylcations have been reported: (a) Siehl, H.-U.; Kaufmann, F.-P.;
Apeloig, Y.; Braude, V.; Danovich, D.; Berndt, A.; Stamatis, N. Angew. Chem., Int.
Ed. Engl. 1991, 30, 1479; (b) Kresge, A. J.; Tobin, J. B. Angew. Chem., Int. Ed. Engl.
1993, 32, 721.
Terminal silylation of alkynes
The typical procedure for terminal silylation of phenylacetylene
(1a) with allyltrimethylsilane (2a) is as follows. Into a glass reactor
were placed the H+-montmorillonite (0.10 g), 1,4-dioxane (1.0 mL),
1a (1.0 mmol), and 2a (3.0 mmol) under a dry Ar atmosphere using
Schlenk techniques. The resulting mixture was vigorously stirred
at 100 °C. After 150 min, the catalyst was separated by filtration.
GC analysis of the filtrate showed a 76% yield of trimethyl(phenyl-
ethynyl)silane (5aa). The product was identified by comparison
with the reported 1H and 13C NMR and mass spectral data.13
9. Hara, K.; Akiyama, R.; Sawamura, M. Org. Lett. 2005, 7, 5621.
10. Baba, T.; Kato, A.; Yuasa, H.; Toriyama, F.; Handa, H.; Ono, Y. Catal. Today 1998,
44, 271.
11. The selectivity of silylated alkyne (5) based on the alkyne (1) used was high.
However, the formation of silylated ether solvent, such as 1,2-
bis(trimethylsiloxy)ethane, was also detected.
Acknowledgments
This work was supported by The Noguchi Institute (NJ200909).
The authors thank Center for Advanced Materials Analysis
(Suzukakedai), Technical Department, Tokyo Institute of Technol-
ogy, for NMR and elemental analysis.
12. Motokura, K.; Fujita, N.; Mori, K.; Mizugaki, T.; Ebitani, K.; Kaneda, K. Angew.
Chem., Int. Ed. 2006, 45, 2605.
13. For silylation product data, see: 5aa and 5ba: (a) Cai, M.; Sha, J.; Xu, Q.
Tetrahedron 2007, 63, 4642; 5ca: (b) Sakai, N.; Komatsu, R.; Uchida, N.; Ikeda,
R.; Konakahara, T. Org. Lett. 2010, 12, 1300; 5ea: (c) Hameury, T.; Guillemont, J.;
Hijfte, L. V.; Bellosta, V.; Cossy, J. Org. Lett. 2009, 11, 2397; 5da: (d) Shimizu, K.;
Takimoto, M.; Sato, Y.; Mori, M. Org. Lett. 2005, 7, 195; 5ad: (e) Batail, N.;
Bendjeriou, A.; Lomberget, T.; Barret, R.; Dufaud, V.; Djakovitch, L. Adv. Synth.
Catal. 2009, 351, 2055.
References and notes
1. Recent reports on the transformation of substituted vinyl silanes: (a) Källström,
K.; Munslow, I. J.; Hedberg, C.; Andersson, P. G. Adv. Synth. Catal. 2006, 348,
2575; (b) Parker, K. A.; Denton, R. W. Tetrahedron Lett. 2011, 52, 2115; (c)
Greedy, B.; Gouverneur, V. Chem. Commun. 2001, 233; (d) Warren, J. D.; Shi, Y. J.
Org. Chem. 1999, 64, 7675.