Palladium/copper-catalyzed sila-Sonogashira reactions of aryl iodides with alkynylsilanes via a direct C-Si bond activation

Article abstract of DOI:10.1016/j.tetlet.2009.05.112

The palladium-catalyzed cross-coupling reactions of aryl iodides with alkynylsilanes in the presence of a substoichiometric amount (50 mol %) of copper(I) chloride as an activator in DMF under strictly non-basic reaction conditions afford the corresponding unsymmetrical diarylethynes in moderate to excellent yields. A wide range of substrates bearing an electron-donating or an electron-withdrawing substituent on the aromatic ring are compatible.

Full text of DOI:10.1016/j.tetlet.2009.05.112

Tetrahedron Letters 50 (2009) 4643–4646
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Palladium/copper-catalyzed sila-Sonogashira reactions of aryl iodides
with alkynylsilanes via a direct C–Si bond activation
*
Yasushi Nishihara , Eiji Inoue, Daisuke Ogawa, Yoshiaki Okada, Shintaro Noyori, Kentaro Takagi
Division of Chemistry and Biochemistry, Graduate School of Natural Science and Technology, Okayama University, 3-1-1 Tsushimanaka, Kita-ku, Okayama 700-8530, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
The palladium-catalyzed cross-coupling reactions of aryl iodides with alkynylsilanes in the presence of a
substoichiometric amount (50 mol %) of copper(I) chloride as an activator in DMF under strictly non-basic
reaction conditions afford the corresponding unsymmetrical diarylethynes in moderate to excellent
yields. A wide range of substrates bearing an electron-donating or an electron-withdrawing substituent
on the aromatic ring are compatible.
Received 7 May 2009
Revised 26 May 2009
Accepted 28 May 2009
Available online 6 June 2009
Ó 2009 Elsevier Ltd. All rights reserved.
The palladium-catalyzed cross-coupling reaction of aryl halides
resulting to protodesilylation to yield the terminal alkynes that
are often employed for the typical Sonogashira reactions.
To the best of our knowledge, there are no general methods to
synthesize unsymmetrical diarylethynes by the coupling of stable,
functionally rich alkynylsilanes with aryl iodides under neutral
conditions. We herein describe the Pd-catalyzed ‘sila’-Sonogash-
ira-type cross-coupling reaction of unactivated aryl iodides with
alkynylsilanes, which proceeds in the presence of CuCl under
strictly non-basic reaction conditions, leading to various unsym-
metrical diarylethynes.
or pseudo-halides with terminal alkynes in the presence of a base
–4
(
Sonogashira reaction)1 has been proven to be a most efficient
method for the synthesis of unsymmetrical diarylethynes, and be-
5
sides the original protocol using the palladium/copper(I) system,
6
7
copper-free or silver-cocatalyzed method to retard the formation
of undesired conjugate diynes from terminal alkynes, has been
8
9
developed.
Numerous reports describe the coupling of trimethylsilylacety-
1
0,11
lene with aryl halides in Sonogashira-type reactions.
The
C(sp)–Si bond is generally not affected by these reaction condi-
tions. The silyl group can, therefore, if desired, subsequently be re-
moved to furnish a structurally modified terminal alkyne. The
trimethylsilyl group is thereby used as a protective group.
We first carried out the sila-Sonogashira coupling reaction of
trimethyl(phenylethynyl)silane (1a, 0.3 mmol) with an electron-
rich 4-methoxyphenyl iodide (2a, 0.2 mmol). The results are
summarized in Table 1. In the presence of CuCl (150 mol %) as
In a sharp contrast, we have reported ‘sila’-Sonogashira reac-
tion, in which aryl triflates12 or chlorides can be coupled directly
with alkynylsilanes via the Si–C bond activation by using the Pd/Cu
co-catalyst system, in DMF as solvent to yield unsymmetrical diar-
ylethynes in good to high yields. This process avoids totally the for-
an activator and 5 mol % of Pd(PPh
3 4
) as the catalyst in DMF
13
(1 mL), which is the best solvent for the homocoupling of alky-
1
7
nylsilanes, the desired cross-coupled product 3a was obtained
in 90% yield, based on 2a (run 1). However, the reaction mix-
ture contained 1,4-diphenyl-1,3-butadiyne (4; 7%, based on
1a), presumably derived from homocoupling of 1a (confirmed
by GC and NMR). Thus, we screened other catalysts for the
reactions of 1a with 2a and found that the catalysts employed
had a considerable influence on the yield of 3a. The reason why
1
4,15
mation of the alkyne homocoupling Glaser-type product.
In
these reactions, the transmetalation from silicon to copper has
been proposed when copper(I) co-catalyst was used.12 This process
has been applied more recently to the copper-free sila-Sonogash-
ira-type cross-coupling of electron-poor aryl and heteroaryl bro-
mides or iodides, with 1-aryl-2-(trimethylsilyl)acetylenes, giving
the cross-coupled product in the presence of tetra-n-butylammo-
nium chloride, with use of microwave.16 However, in the afore-
mentioned reactions, the yields of the desired products from
electron-rich aryl iodides are rather lower and they required the
addition of a base to activate the carbon–silicon bond, indeed
Pd(PPh
catalyst by coordination of excess PPh
tion metal catalysts examined in the coupling of 1a with 2a,
PdCl (PPh combined with PPh (P/Pd = 2.0; 5 mol %) gave a
71% yield of 3a (run 2). Other catalytic systems with Pd(OAc)
or Pd(dba) gave poor results (runs 5–8). In addition, Ni(cod)
3
)
4
was so effective may be due to stabilization of the
3
ligands. Among transi-
2
3
)
2
3
2
2
2
was completely ineffective as the catalyst (run 10). Deletion
of Pd(PPh yielded no product, confirming the requirement
3 4
)
for a palladium catalyst. Increasing the 1a:2a ratio had a signif-
icant effect on yield of 3a; 94% (2:1), 90% (1.5:1), and 60% (1:1)
yield, respectively.
*
Corresponding author. Tel./fax: +81 86 251 7855.
E-mail address: ynishiha@cc.okayama-u.ac.jp (Y. Nishihara).
0
040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.05.112

4
644
Y. Nishihara et al. / Tetrahedron Letters 50 (2009) 4643–4646
reportedly effective for coupling reactions using organoboron18
and organotin reagents hardly accelerated the present reaction.
Table 1
a
CuCl-mediated cross-coupling of 1a with 2a using various catalysts
19
It is noteworthy that the present reaction does not require any
catalyst
CuCl (150 mol%)
2
0,21
activator such as a fluoride ion
or an added base, which is nor-
Ph
OMe
Ph
SiMe3
+
I
OMe
DMF
mally required for activation of alkynylsilicon compounds in Hiy-
ama cross-coupling reaction.
1
a
2a
(0.2 mmol)
3a
(
0.3 mmol)
With the optimized 2:1 ratio of 1a:2a for the cross-coupling in
hand, we next screened the amount of CuCl in the reactions of 2a
Run
Catalyst
Yield (%)
2
2
with 1a. The results are summarized in Table 2. Although the
use of a stoichiometric amount (100 mol %) of CuCl afforded 3a
in 99% GC yield (run 1), the use of a catalytic amount was found
to retard the reaction progress (run 2). Searching the decreased
amounts of CuCl (runs 3–5), we employed a substoichiometric
3
a
4b
1
2
3
4
5
6
7
8
9
Pd(PPh
3
)
4
90
71
19
<1
12
<1
10
8
7
11
6
<1
5
<1
2
<1
10
0
PdCl
PdCl
PdCl
2
2
2
(PPh
(PPh
(NCPh)
3
)
)
2
/2PPh
3
3
2
2
2
0a
Pd(OAc)
Pd(OAc)
2
/2PPh
3
amount (50 mol %) of CuCl as the optimal condition.
2
The generality of the reaction of alkynylsilanes 1a–j with aryl
iodides 2a–n was studied under the optimum conditions, that is,
50 mol % of CuCl and 5 mol % of Pd(PPh ) . The results are also
3 4
Pd(dba)
Pd(dba)
2 3
/2PPh
2
(dppf)c
PdCl
2
46
0
summarized in Table 2. The reactions of various aryl iodides 2
bearing the substituents in the para-position with 1a proceeded
to give the corresponding unsymmetrical diarylethynes 3b–g in
good to excellent yields (runs 6–11). However, a sterically hin-
dered aryl iodides 2h–k partially retarded reaction of 1a and gave
the lower yield of the desired products (runs 12–15). Reaction of
iodides 2l and 2m having heteroaromatics such as 2-pyridyl and
2-thienyl groups also afforded the corresponding cross-coupled
products 3l and 3m in 69% and 68% isolated yields, respectively
(runs 16 and 17). On the other hand, the reactions of alkynylsilanes
1b–h bearing various functional groups similarly underwent the
1
0
Ni(cod)
2
a
Reaction conditions: DMF (1 mL), 1a (0.3 mmol), 2a (0.2 mmol), catalyst
5 mol %), and CuCl (0.3 mmol) at 80 °C. Reaction time is 12 h.
Yields of 1,4-diphenyl-1,3-butadiyne (4) were based on 1a.
dppf = 1,1 -bis(diphenylphosphino)ferrocene.
(
b
c
0
Counter ions of a halogenated copper salt dramatically affected
the yield of 3a. CuCl, which is the effective copper salt for the cross-
coupling of alkynylsilanes with aryl triflates12 and aryl chlorides,
was also suitable for the present reaction. Whereas CuI that was
13
Table 2
Cross-coupling reaction of alkynylsilanes 1 with aryl iodides 2a
Pd(PPh ) (5 mol%)
3
4
CuCl (50 mol%)
R1
R2
R1
+
R2
SiMe3
a-1j
Halide 2, R2=
I
DMF
8
0 ºC, 1-6 h
1
2a-2n
3a-3y
Run
Alkynylsilane 1, R1=
–(1a)
Time (h)
CuCl (mol %)
Products
Yieldb (%)
1
2
3
4
5
6
7
8
9
C
6
H
5
4-MeO–C
6
H
4
(2a)
3
3
3
3
3
3
1
1
3
1
1
3
6
3
6
3
3
6
1
1
6
6
6
6
3
3
6
1
1
100
10
200
150
50
3a
(99)
(>1)
(90)
(96)
(94) 84
75
89
90
92
92
93
49
66
77
36
69
68
73
89
78
71
88
83
59
77
47
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
4-Me–C
4-NC–C
4-MeCO–C
4-O N–C
4-Cl–C
4-EtO C–C
2-MeO–C
2-Me–C
6
H
4
(2b)
(2c)
50
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
3o
3p
3q
3r
6
H
4
6
H
4
(2d)
(2e)
(2f)
2
6
H
4
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
6
H
4
2
6
H
4
(2g)
(2h)
(2i)
H
6 4
6
H
4
2-MeCO–C
6
H
4
(2j)
(2k)
2,4,6-Me –C
3
6
H
2
2-Pyridyl (2l)
2-Thienyl (2m)
2b
2d
2c
2a
4-MeO–C
6
H
4
–(1b)
1b
1b
4-NC–C
4-MeCO–C
6
H
4
(1c)
(1d)
6
H
4
C
H
6 5
(2n)
1d
2a
2a
2n
2a
2a
2n
2n
3s
3t
4-O
4-CF
2
N–C
–C
6
H
4
(1e)
(1f)
3
6
H
4
3u
3v
3w
3x
3y
2-Pyridyl (1g)
2-Thienyl (1h)
75
48
49
t
4- BuMe
2
SiO–C
6
H
4
(1i)
n-C
6
H
13 (1j)
a
Conditions: 1 (2.0 mmol); 2 (1.0 mmol); CuCl (50 mol %); Pd(PPh
Isolated yields based on aryl iodides 2 and GC yields were shown in parentheses.
3
)
4
(5 mol %); DMF (5 mL) unless otherwise noted.
b

Y. Nishihara et al. / Tetrahedron Letters 50 (2009) 4643–4646
4645
cross-coupling reactions and gave the corresponding unsymmetri-
cal diarylethynes 3n–w in up to 89% yield (runs 18–27). The pres-
ent reaction is applied to the coupling reaction of alkynylsilane 1i
having t-butyldimethylsilyl (TBDMS) group as a protecting group.
Indeed, 1i selectively coupled with 2n to give 3x with the TBDMS
group remaining intact (run 28). Alkyl-substituted alkynylsilane
In conclusion, we have successfully developed a preparative
process for the generation of unsymmetrical diarylethynes from
the cross-coupling reactions of aryl iodides with alkynylsilanes in
the presence of CuCl in DMF. Because the presented method is car-
ried out using the alkynes protected with the silicon moiety, side
reactions leading to the butadiynes by the Pd-catalyzed homocou-
pling reactions of terminal alkynes can be avoided. This reaction is
synthetically useful in the sense of being straightforward carbon–
carbon bond formation via a direct activation of carbon–silicon
bond using a stable, nontoxic, and functional group tolerant
organosilicon compound. It is a highly applicable novel transfor-
mation that occurs in the absence of a fluoride ion and a base as
an activator.
1
j underwent the reaction to give 3y in 49% yield (run 29).
Noteworthy is that this reaction using aryl iodides 2 consider-
ably surpasses those of aryl triflates and bromides, probably owing
to the efficiency of oxidative addition of Pd(0) to the C–I bond of 2.
For instance, the reactions of 1c with 2a, 1d with 2n, and 1d with
2
a furnished cross-coupled products 3q, 3r, and 3s in 71%, 88%, and
8
3% yields, respectively, whereas the corresponding substrates of
aryl triflates and bromides decreased the yields (27, 30, and 19%
Further studies on application of the present system to other
base-free carbon–carbon bond forming reactions of organosilicon
compounds toward new organic molecules bearing a carbon–car-
bon triple bond are currently ongoing and will be published in
due course.
1
2
for triflates, and 8%, 17%, and 10% for bromides, respectively).
Therefore, the use of aryl iodides 2 is important for the synthesis
of 3 from alkynylsilanes 1 in higher yields.
Because these reactions disclosed the direct transmetalation of
an alkynyl group from silicon to copper, bis(trimethylsilyl)ethyne
(4) can be used as an alternative to acetylene. Indeed, the reaction
Acknowledgments
of 4 with 4-cyanophenyl iodide (2c, 2 mol equiv) furnished 1,2-
bis(4-cyanophenyl)ethyne (5) in 72% yield (Eq. 1). This method
for the synthesis of symmetrical diarylethynes is also useful be-
cause precise control of the amount of acetylene is considerably
difficult due to its gaseous form although acetylene is usually used
for the synthesis of symmetrical diarylethynes through Sonogash-
ira reaction.
This work was supported by a Grant-in-Aid for Scientific Re-
search on Priority Areas ‘Advanced Molecular Transformations of
Carbon Resources’ from the Ministry of Education, Culture, Sports,
Science and Technology, Japan. Y.N. is grateful to Chisso Petro-
chemical Corporation for generous donation of trimethylsilylacet-
ylene to prepare 1.
Me Si
SiMe3
+
2
I
3
CN
References and notes
4
2
c
1
.
.
Examples of recent reviews, see: (a) Sonogashira, K. J. Organomet. Chem. 2002,
53, 46; (b) Negishi, E.; Anastasia, L. Chem. Rev. 2003, 103, 1979; (c) Nicolaou, K.
C.; Bulger, P. G.; Sarlah, D. Angew. Chem., Int. Ed. 2005, 44, 4442; (d) Yin, L.;
Liebscher, J. Chem. Rev. 2007, 107, 133; (e) Chinchilla, R.; Nájera, C. Chem. Rev.
6
ð1Þ
5
mol% Pd(PPh3)4
0 mol% CuCl
DMF, 80 ºC, 6 h
2%
5
2007, 107, 874.
NC
CN
2
(a) Sonogashira, K. In Metal-Catalyzed Cross-Coupling Reactions; Diederich, F.,
Stang, P. J., Eds.; Wiley-VCH: New York, 1998. Chapter 5; (b) Brandsma, L.;
Vasilevsky, S. F.; Verkruijsse, H. D. Application of Transition Metal Catalysts in
Organic Synthesis; Springer: Berlin, 1998; (c) Rossi, R.; Carpita, A.; Bellina, F. Org.
Prep. Proced. Int. 1995, 27, 127–160; (d) Sonogashira, K.. In Comprehensive Organic
Synthesis; Trost, B. M., Ed.; Pergamon: New York, 1991; Vol. 3,. Chapter 2.4.
(a) Ames, D. E.; Bull, D.; Takundwa, C. Synthesis 1981, 364; (b) Menchi, G.;
Scrivanti, A.; Matteoli, U. J. Mol. Chem. A: Chem. 2000, 152, 77.
7
5
In addition, the utility of the cross-coupling reaction with alky-
nylsilanes is demonstrated by a one-pot synthesis of unsymmetri-
cal diarylethynes 3, starting from 4. The one-pot double sila-
Sonogashira reactions of 4 with R -I and another aryl iodide R –I
allow the synthesis of unsymmetrical diarylethynes 3 as shown
in Scheme 1. Subsequent addition of two kinds of aryl iodides re-
sulted in three-component coupling of bis(trimethylsilyl)ethyne
4), R –I, and R –I to give R –C„C–R 3p and 3z in 49% and 93%
yields, respectively.
3
.
1
2
4. (a) Mori, A.; Shimada, T.; Kondo, T.; Sekiguchi, A. Synlett 2001, 649; (b) Mori, A.;
Mohamed Ahmed, M. S.; Sekiguchi, A.; Masui, K.; Koike, T. Chem. Lett. 2002, 756;
(
c) Mohamed Ahmed, M. S.; Mori, A. Tetrahedron 2004, 60, 9977; (d) Mohamed
Ahmed, M. S.; Sekiguchi, A.; Masui, K.; Mori, A. Bull. Chem. Soc. Jpn. 2005, 78, 160.
5. (a) Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett. 1975, 4467; (b)
Cassar, L. J. Organomet. Chem. 1975, 93, 253; (c) Dieck, H. A.; Heck, R. F. J.
Organomet. Chem. 1975, 93, 259.
1
2
1
2
(
6
.
(a) Bohm, V. P. W.; Herrmann, W. A. Eur. J. Org. Chem. 2000, 3679; (b) Pal, M.;
Parasuraman, K.; Gupta, S.; Yeleswarapu, K. R. Synlett 2002, 14, 1976; (c)
Fukuyama, T.; Shinmen, M.; Nishitani, S.; Sato, M.; Ryu, I. Org. Lett. 2002, 4,
1691; (d) Uozumi, Y.; Kobayashi, Y. Heterocycles 2003, 59, 71; (e) Urgaonkar, S.;
Verkade, J. G. J. Org. Chem. 2004, 69, 5752; (f) Heuzé, K.; Méry, D.; Gauss, D.;
Blais, J.-C.; Astruc, D. Chem. Eur. J. 2004, 10, 3936; (g) Liang, B.; Dai, M.; Chen, J.;
Yang, Z. J. Org. Chem. 2005, 70, 391; (h) Liang, B.; Huang, M.; You, Z.; Xiong, Z.;
Lu, K.; Fathi, R.; Chen, J.; Yang, Z. J. Org. Chem. 2005, 70, 6097; (i) Cwik, A. A.;
Hell, Z.; Figueras, F. Tetrahedron Lett. 2006, 47, 3023; (j) Kawanami, H.;
Matsushima, K.; Sato, M.; Ikushima, Y. Angew. Chem., Int. Ed. 2007, 46, 5129; (k)
Shi, S.; Zhang, Y. Synlett 2007, 1843; (l) Ljungdahl, T.; Bennur, T.; Dallas, A.;
Emtenaes, H.; Maartensson, J. Organometallics 2008, 27, 2490.
R1
I
5
mol% Pd(PPh3)
4
5
0 mol% CuCl
R1
SiMe3
Me Si
SiMe3
3
DMF, 80 ºC, 1 h
4
R2
mol% Pd(PPh3)4
0 mol% CuCl
I
7
.
.
(a) Mori, A.; Kawashima, J.; Shimada, T.; Suguro, M.; Hirabayshi, K.; Nishihara,
Y. Org. Lett. 2000, 2, 2935; (b) Kobayashi, K.; Sugie, A.; Takahashi, M.; Masui, K.;
Mori, A. Org. Lett. 2005, 7, 5083.
5
5
R1
R2
8
Siemsen, P.; Livingston, R. C.; Diederich, F. Angew. Chem., Int. Ed. 2000, 39, 2632.
DMF, 80 ºC, 1 h
9. Examples of recent reviews, see: (a) Littke, A. F.; Fu, G. C. Angew. Chem., Int. Ed.
002, 41, 4176; (b) Sonogashira, K. J. Organomet. Chem. 2002, 653, 46; (c)
3
2
Negishi, E.-I.; Anastasia, L. Chem. Rev. 2003, 103, 1979.
10. Rossi, R.; Carpita, A.; Lezzi, A. Tetrahedron 1984, 40, 2773.
R1
R2
GC Yield
3p: 49%
3z: 93%
I
I
1
1. Pd(0)-catalyzed couplings of aryl iodides with alkynylsilanes in the presence of
an equivalent of Ag
CO and several other silver salts: see Koseki, Y.; Omino, K.;
Anzai, S.; Nagasaka, T. Tetrahedron Lett. 2000, 41, 2377.
R1 = 4-NCC H -(2c)
R1 = 4-NCC H - (2c)
R2 = 4-MeOC6H4- (2a)
R2 = 4-ClC6H4- (2f)
2
3
6
4
6
4
12. (a) Nishihara, Y.; Ikegashira, K.; Mori, A.; Hiyama, T. Chem. Lett. 1997, 1233; (b)
Nishihara, Y.; Ikegashira, K.; Hirabayashi, K.; Ando, J.; Mori, A.; Hiyama, T. J.
Org. Chem. 2000, 65, 1780.
Scheme 1.

4
646
Y. Nishihara et al. / Tetrahedron Letters 50 (2009) 4643–4646
1
1
3. Nishihara, Y.; Inoue, E.; Okada, Y.; Takagi, K. Synlett 2008, 3041.
Mori, A.; Kawashima, J.; Suguro, M.; Nishihara, Y.; Hiyama, T. J. Org. Chem.
2000, 65, 5342.
21. Chang, S.; Yang, S. H.; Lee, P. H. Tetrahedron Lett. 2001, 42, 4833.
4. The copper-free version of ‘sila’-Sonogashira cross-coupling reaction has been
achieved using a palladium/imidazolium salt system, see: (a) Yang, C.; Nolan, S.
P. Organometallics 2002, 21, 1020; (b) Hillier, A. C.; Grasa, G. A.; Viciu, M. S.; Lee,
H. M.; Yang, C.; Nolan, S. P. J. Organomet. Chem. 2002, 653, 69.
22. Typical procedure for sila-Sonogashira reactions of aryl iodides
alkynylsilanes (Table 2, run 19): To solution of Pd(PPh
0.05 mmol, 5 mol %) and CuCl (50 mg, 0.5 mmol, 50 mol %) in dry DMF
(5 mL) were added 1-(4-methoxypheny)-2-trimethylsilylethyne (1b) (432 L,
2
with
1
a
3 4
) (58 mg,
1
5. Examples for modified ‘sila’-Sonogashira cross-coupling reaction under the
basic conditions, see: (a) Marshall, J. A.; Chobanian, H. R.; Yanik, M. M. Org. Lett.
l
2
001, 3, 4107; (b) Mio, M. J.; Kopel, L. C.; Braun, J. B.; Gadzikwa, T. L.; Hull, K. L.;
2.0 mmol) and 4-acetylphenyl iodide (2d) (246 mg, 1.0 mmol) at room
temperature. The dark yellow suspension was heated for 1 h at 80 °C and
monitored by GC and TLC. After completion of the reaction, the reaction
mixture was quenched with 1 M HCl and extracted with diethyl ether
(20 mL, 3 times). The combined organics were washed with brine and dried
Brisbois, R. G.; Markworth, C. J.; Grieco, P. A. Org. Lett. 2002, 4, 3199; (c) Gil-
Moltóa, J.; Nájera, C. Adv. Synth. Catal. 2006, 348, 1874.
1
1
6. Sørensen, U. S.; Pombo-Villar, E. Tetrahedron 2005, 61, 2697.
7. Nishihara, Y.; Takemura, M.; Mori, A.; Osakada, K. J. Organomet. Chem. 2001,
6
20, 282.
over anhydrous MgSO
evaporator gave a viscous oil. The residue was purified by flash column
chromatography on silica gel (hexane/ethyl acetate = 9:1, = 0.21), 3o
(224 mg, 0.89 mmol, 89%) was obtained as white solid.
(300 MHz, CDCl ): d 2.61 (s, 3H), 3.84 (s, 3H), 6.88–6.91 (m, 2H), 7.48–
7.50 (m, 2H), 7.57–7.60 (m, 2H), 7.92–7.94 (m, 2H).
4
.
Filtration and concentration with
a
rotary
1
8. Nishihara, Y.; Okamoto, M.; Inoue, Y.; Miyazaki, M.; Miyasaka, M.; Takagi, K.
Tetrahedron Lett. 2005, 46, 8661.
9. Kang, S.-K.; Kim, J.-S.; Choi, S.-C. J. Org. Chem. 1997, 62, 4208.
0. (a) Hatanaka, Y.; Hiyama, T. J. Org. Chem. 1988, 53, 918; (b) Hatanaka, Y.;
Matsui, K.; Hiyama, T. Tetrahedron Lett. 1989, 30, 2403; (c) Hirabayashi, K.;
R
f
1
1
2
a
H NMR
3