Site-selective mono-oxidative addition of active zinc into carbon-bromine bond of dibrominated-thiophenes: Preparation of thienylzinc reagents and their applications

Article abstract of DOI:10.1016/j.tetlet.2012.12.024

A facile protocol for the preparation of 3-bromo-2-thienylzinc bromide A and 5-bromo-2-thienylzinc bromide B has been developed. It has been successfully accomplished by a site-selective oxidative addition of active zinc into a chemically pseudo-equivalent or equivalent carbon-bromine bond, respectively. The subsequent cross-coupling reactions of the organozincs were also successfully carried out under mild conditions providing the corresponding products in moderate to high yields.

Full text of DOI:10.1016/j.tetlet.2012.12.024

Tetrahedron Letters 54 (2013) 960–964
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Site-selective mono-oxidative addition of active zinc into carbon–bromine
bond of dibrominated-thiophenes: preparation of thienylzinc reagents and
their applications
⇑
Hye-Soo Jung, Hyun-Hee Cho, Seung-Hoi Kim
Department of Chemistry, Dankook University, 119 Dandaero, Cheonan 330-714, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
A facile protocol for the preparation of 3-bromo-2-thienylzinc bromide A and 5-bromo-2-thienylzinc bro-
mide B has been developed. It has been successfully accomplished by a site-selective oxidative addition
of active zinc into a chemically pseudo-equivalent or equivalent carbon–bromine bond, respectively. The
subsequent cross-coupling reactions of the organozincs were also successfully carried out under mild
conditions providing the corresponding products in moderate to high yields.
Received 16 November 2012
Revised 4 December 2012
Accepted 7 December 2012
Available online 19 December 2012
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Active zinc
Site-selective oxidative addition
Cross-coupling
Thienylzinc bromide
Poly-functionalized thiophene derivatives have been utilized for
the preparation of valuable compounds in various fields such as
optoelectrochemical materials, pharmaceuticals, and biologically
active molecules.1 Consequently, a variety of synthetic protocols
for functionalized thiophene derivatives, especially 2,5-
disubstituted thiophenes, has been developed. Among those meth-
odologies, a transition-metal-catalyzed ring-closure methodology
has been successfully employed.2 Along with this method, another
approach to the synthesis of poly-functionalized thiophene
molecules is to utilize a further functionalization of the preformed
thiophenes. Most such approaches have been performed by the
cross-coupling reactions of organometallics with multiple
halogen-substituted thiophene derivatives, which are mostly
disubstituted thiophenes such as 2,3-, 2,4-, 2,5-, and 3,4-dihalogen-
ated thiophenes.3 Of those, 2,3- and 2,5-dibromothiophenes are
mostly frequently utilized in the cross-coupling reactions of
organometallics since these molecules are of special interest for
both academic and industrial applications. The regio-selectivity in
the cross-coupling reactions of 2,3- and 2,5-dibromothiophene
molecules attracts more academic attention due to the chemical
similarity of the two C–Br bonds presented in those compounds.
Christophersen et al. reported a protocol for the synthesis
of 2,3-substituted thienylboronic acids and esters using a combina-
tion of halogen-magnesium exchange and borylation.4 Site-
selective lithiation of 2,3-dibromothiophene and subsequent coupling
reaction were utilized for the construction of a building block for
polymeric systems.5 2,3-Dibromothiophene was also successfully
employed for the site-selective coupling reactions of Negishi,6a
Stille, Suzuki, and Sonogashira.6b Selectivity between the
a-position
C–Br and the b-position C–Br of the thiophene ring was studied in
the coupling reactions using 2,3,5-tribromothiophene and 2,3,4,5-
tetrabromothiophene, respectively. As expected, the coupling reac-
tions preferred to take place at the
a-position C–Br affording
mono- or disubstituted thiophene products.7
In contrast, a relatively limited number of examples of the site-
selective coupling reaction of 2,5-dibromothiophene bearing two
chemically equivalent C–Br bonds was reported, presumably, due
to the low selectivity.8 Instead of being used as a coupling partner,
2,5-dibromothiophene was generally used for the preparation of
the corresponding Grignard reagent via
a selective mono-
bromine-magnesium exchange reaction.9 The resulting Grignard
reagents were successfully applied to the preparation of unsymmet-
rical 2,5-disubstituted thiophene derivatives. From a synthetic
point of view, it is of significance that this approach can be utilized
as a somewhat different synthetic protocol for introducing more un-
ique functional groups onto the thiophene ring. In addition, to the
best of our knowledge, very limited number of research for the di-
rect preparation of thienyl metallic reagents via a site-selective oxi-
dative addition of a metal into the dihalogenated thiophenes has
been performed.10
In our continuing study on the preparation and application of
new organozinc reagents, we found that a very selective oxidative
addition of active zinc into the C–Br bond of 2,3-dibromo- and
⇑
Corresponding author.
E-mail address: kimsemail@dankook.ac.kr (S.-H. Kim).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.12.024

H.-S. Jung et al. / Tetrahedron Letters 54 (2013) 960–964
961
oxidative addition reaction was carried out at room temperature
using 1.2 equiv of active zinc. It was of interest that the oxidative
Br
Br
E'
Br
Br
E+
THF
rt
addition occurred exclusively at the
a-position leaving the b-
+
Zn*
ZnBr
bromine atom intact and yielding the corresponding organozinc
reagent, 3-bromo-2-thienylzinc bromide (A). This result could be
rationalized by the NMR study performed by Handy and Zhang,12
and was also confirmed by GC–MS analysis of the reaction mixture
in our study (Scheme 1).
S
S
S
A
1.0 eq
1.2 eq
Scheme 1. Preparation of 3-bromo-2-thienylzinc bromide (A).
2,5-dibromothiophene took placed yielding 3-bromo-2-thienylzinc
bromide (A) and 5-bromo-2-thienylzinc bromide (B), respectively.
The resulting organozinc reagents (A, B) were also utilized in var-
ious different types of coupling reactions under mild conditions.
As is well known, organozinc reagents have been widely used in
synthetic organic chemistry mainly owing to their relatively high
functional group tolerance and great efforts have been made to ex-
pand their applicability.11 We, herein, report a facile protocol for
the synthesis and application of a new type of organozinc reagent
prepared by a site-selective oxidative addition of active zinc into a
chemically equivalent or pseudo-equivalent C–Br bond.
With this reagent, we attempted the cross-coupling reaction
with a variety of coupling partners and the results are summarized
in Table 1.
The formation of the organozinc reagent (A) was further con-
firmed by the cross-coupling reactions. Initial study was carried
out with acid chlorides in the presence of Pd-catalyst at room tem-
perature. Benzoyl chlorides containing an electron-withdrawing
group required 24 h-stirring to complete the coupling reaction
yielding the corresponding ketones (1a and b) in good yields (en-
tries 1 and 2, Table 1). In the case of trimethylbenzoyl chloride,
the coupling reaction was completed in a shorter reaction time
yielding the ketone (1c) in 90% isolated yield (entry 3, Table 1). A
less reactive acid chloride, 4-morpholinecarbonyl chloride, was
also successfully employed for the coupling reaction at refluxing
In order to accomplish our challenge, we first tried the oxidative
addition of active zinc with 2,3-dibromothiophene which
possesses two chemically pseudo-equivalent C–Br bonds. The
Table 1
Cross-coupling reaction of 3-bromo-2-thienylzinc bromide (A)
Entry
Electrophile
Conditions
Product
Yielda (%)
80
Br
COCl
Br
1
Pd(PPh3)2Cl2 rt/24 h
1a
1b
Br
S
O
F
F
Br
COCl
2
Pd(PPh3)2Cl2 rt/24 h
85
S
S
S
S
F
O
Br
F
COCl
3
4
Pd(PPh3)2Cl2 rt/2 h
1c
90
61
O
COCl
Br
Br
N
O
O
N
Pd(PPh3)2Cl2 reflux/24 h
1d
O
I
5
Pd(PPh3)2Cl2 rt/24 h
1e
92
Br
Br
Br
Br
6
7
Pd(PPh3)2Cl2 rt/24 h
Pd(PPh3)2Cl2 reflux/24 h
Pd(PPh3)2Cl2 rt/24 h
1f
86
52
S
Br
Br
Br
Br
Br
S
1g
1h
S
N
Br
Br
S
S
8
9
66
N
Br
I
Pd(OAc)2/SPhos/reflux
Pd(PPh3)2Cl2/reflux/24 h
—
Nrb
H2N
a
Isolated yeild (based on electrophile).
No reaction occurred.
b

962
H.-S. Jung et al. / Tetrahedron Letters 54 (2013) 960–964
E+
THF
+
Zn*
Br
Br
Br
E'
Br
ZnBr
rt
S
S
S
B
1.0 eq
1.1 eq
Scheme 2. Preparation of 5-bromo-2-thienylzinc bromide (B).
Table 2
Coupling reaction of B with acid chlorides
conditions
+
electrophile
(0.8 eq)
2a 2l
)
Br
ZnBr
B (1.0 eq)
Products (
~
S
THF
Entry
1
Electrophile
Conditions
Product
Yielda (%)
73
Br
COCl
COCl
Cul/LiCl rt/2 h
Cul/LiCl rt/2 h
Cul/LiCl rt/2 h
2a
2b
2c
Br
Cl
Br
S
O
O
O
Cl
Cl
2
3
79
63
Br
Br
S
S
COCl
Cl
COCl
COCl
CF3
CN
4
5
Cul/LiCl rt/1 h
2d
2e
90
75
F3C
NC
Br
Br
Br
S
S
S
O
O
Cul/LiCl 0 °C/2 h
COCl
OCH3
6
7
Cul/LiCl rt/2 h
Cul/LiCl rt/2 h
2f
85
75
H3CO
O
O
COCl
2g
Br
Br
S
S
COCl
COCl
8
9
Cul/LiCl rt/2 h
Cul/LiCl rt/2 h
2h
2i
93
84
S
S
O
Br
O
O
O
S
COCl
Cl
N
O
10
Cul/LiCl rt/1 h
2j
66
Cl
N
Br
Br
Br
S
S
S
O
O
COCl
11
12
Cul/LiCl rt/2 h
2k
2l
79
68
COCl
N
N
O
Pd(PPh3)2Cl2 reflux/24 h
O
a
Isolated yield (based on electrophile).

H.-S. Jung et al. / Tetrahedron Letters 54 (2013) 960–964
963
Table 3
Coupling of B with aryl halides
conditions
electrophile
(0.8 eq)
+
Products
Br
ZnBr
THF
S
3a 3f
(
~
)
B (1.0 eq)
Entry
1
Electrophile
Conditions
Product
Yielda (%)
I
Br
Br
S
S
Pd(PPh3)2Cl2 rt/24 h
Pd(PPh3)2Cl2 rt/24 h
3a
3b
63
84
Br
Br
I
2
EtO2C
CO2Et
Br
3
4
Pd(PPh3)4Cl2 rt/12 h
Pd(PPh3)2Cl2 rt/2 h
3c
53
62
Br
Br
Br
S
S
CO2Et
3d
Br
CO2Et
Br
S
S
S
5
6
Pd(PPh3)2Cl2 reflux/24 h
Pd(PPh3)2Cl2 reflux/24 h
3e
3f
61
72
N
N
N
Br
Br
S
I
N
Br
a
Isolated yield (based on electrophile).
temperature to give rise to the corresponding product (1d) in 61%
isolated yield (entry 4, Table 1).13 By employing the aryl halides,
more examples of 2-aryl-substituted thiophenes were obtained.
Under the identical catalytic system, a successful C–C bond was
formed to yield 2-(40-bromophenyl)-substituted thiophene (1e)
and 2-(20-naphthyl)-substituted thiophene (1f) in 92% and 86% iso-
lated yields, respectively (entries 5 and 6, Table 1). Interestingly,
heteroaryl halides (entries 7 and 8, Table 1) were coupled well
with the organozinc (A) in the presence of Pd-catalyst providing
bisheteroaryl compounds (1g and h, Table 1) in moderate yields.
Unfortunately, no desired coupling product was obtained from
the reaction with 4-iodoaniline bearing an acidic proton, either
using Pd(OAc)2 or Pd(PPh3)2Cl2 (entry 9, Table 1).
products. Once again, the formation of the organozinc reagent (B)
was further confirmed by the transition-metal-catalyzed cross-
coupling reactions with various electrophiles and aryl halides.
The results are summarized in Tables 2 and 3.
On the basis of the promising results obtained from the
previous work utilizing A, we examined the coupling reaction of
B with acid chlorides in the presence of a catalytic amounts of
Pd(PPh3)2Cl2. In contrast to the previous results, however, this cat-
alytic system was not effective for the formation of the desired
product. Interestingly, an efficient route for the completion of the
coupling reaction was attained simply by switching the catalyst
to copper. As depicted in Table 2, a copper catalytic system worked
more effectively to give the corresponding ketone products in
moderate to good yields. Regardless of the substituents on the ben-
zoyl chloride such as bromo-, chloro-, trifluoromethyl-, cyano-, and
methoxy-, the corresponding ketones (2a, b, d–f, Table 2) were suc-
cessfully obtained in moderate to good isolated yields (entries 1, 2,
4–6, Table 2), respectively. 4-Chloromethylbenzoyl chloride was
coupled effectively with B to produce the ketone 2c (entry 3,
Table 2) in moderate yield, leaving a sensitive active site (benzyl
chloride) intact. The coupling reaction of B with a bulky acid chlo-
ride resulted in the formation of the ketone (2g) in 75% isolated
yield (entry 7, Table 2). In addition, heteroaryl acid chlorides were
also employed to investigate the facility of this methodology under
the same conditions. As shown in Table 2, the corresponding cou-
pling products (2h–j, Table 2) were obtained in good to excellent
isolated yields (entries 8–10, Table 2). The copper catalyst system
also showed good reactivity in the coupling reaction of B with
cyclohexanecarbonyl chloride to provide the heteroaryl alkyl
ketone (2k) in 79% isolated yield (entry 11, Table 2). Interestingly,
in the case of 4-morpholinecarbonyl chloride, the Pd-catalyst sys-
tem (2 mol % of Pd(PPh3)2Cl2 at refluxing temperature) was more
efficient to complete the coupling reaction affording the product
2l in 68% isolated yield (entry 12, Table 2).
With these promising results obtained from employing
2,3-dibromothiophene in hand, our studies were expanded to
assess whether this protocol could be used for a much more chal-
lenging molecule, 2,5-dibromothiophene possessing two chemi-
cally equivalent C–Br bonds.
Symmetrically disubstituted-thiophene was treated with
1.1 equiv of active zinc at room temperature in THF. Surprisingly,
the oxidative addition of active zinc into C–Br bond occurred at
one of the two C–Br bonds in a highly selective manner. From GC
and GC–MS analyses of the reaction mixture, it was observed that
less than 5% of diorganozinc reagent was formed in the reaction
mixture. Prior to the study of this reagent on the coupling reac-
tions, an aliquot of the solution was treated with iodine and ana-
lyzed by GC and GC–MS. Both analyses clearly showed the
formation of 2-bromo-5-iodothiophene, which suggested the for-
mation of 5-bromo-2-thienylzinc bromide (B). All of the spectro-
scopic data of the compound were consistent with the literature
values (Scheme 2).
The minor product (diorganozinc bromide) obtained during the
procedure of organozinc synthesis did not interfere with the subse-
quent cross-coupling reaction that provided the desired coupling

964
H.-S. Jung et al. / Tetrahedron Letters 54 (2013) 960–964
4. Christophersen, C.; Begtrup, M.; Ebdrup, S.; Petersen, H.; Vedsø, P. J. Org. Chem.
In continuation of ongoing coupling reaction of a new organo-
2003, 68, 9513.
zinc reagent B, more thiophene derivatives were prepared under
the typical Pd-catalyzed coupling reaction conditions and the re-
sults are summarized in Table 3. In a standard experiment, iodin-
ated aryl compounds were coupled with B in the presence of
2 mol % of Pd(PPh3)2Cl2 in THF and the reaction was completed
in 24 h at room temperature. Analytically pure products (3a and
b, Table 3) were obtained in 63% and 84% yields, respectively. How-
ever, this catalyst system did not work well for the brominated aryl
compound. Instead, as described in Table 3, the reaction of
2-bromoindene with B was completed at room temperature in
12 h using 1 mol % of Pd(PPh3)4 affording the coupling product 3c
in 43% isolated yield (entry 3, Table 3). Surprisingly, Pd(II)-catalyst
worked very efficiently for both brominated and iodinated hetero-
aryl compounds. The reaction with brominated-thiophene resulted
successfully in the formation of unsymmetrically disubstituted
bithiophene 3d in 62% yield (entry 4, Table 3). A similar reaction
with 3-bromoquinoline was executed under the same conditions
giving rise to the desired product 3e in 61% yield (entry 5, Table
3). A selective coupling reaction of B, as expected, was observed
from the reaction with a pyridine containing both iodine and bro-
mine atoms (entry 6, Table 3). The product 3f with two bromine
atoms was obtained in 72% isolated yield.
In conclusion, a site-selective oxidative addition of active
zinc14 into both chemically equivalent and pseudo-equivalent
carbon–bromine bonds has been developed and the subsequent
coupling reactions of the resulting organozinc reagents were
successfully demonstrated with a variety of electrophiles and aryl
halides under mild conditions.15 The desired products were
obtained in good to excellent isolated yields. Most of the coupling
products obtained in this study may potentially be further trans-
formed to the highly substituted thiophene derivatives due to the
presence of a bromine atom on the thiophene ring. Such applica-
tions of the strategy proposed herein are presently being
investigated.
5. Ogawa, K.; Rasmussen, S. C. J. Org. Chem. 2003, 68, 2921.
6. (a) Tamao, K.; Nakamura, K.; Ishii, H.; Yamaguchi, S.; Shiro, M. J. Am. Chem. Soc.
1996, 118, 12469; (b) Pereira, R.; Iglesias, B.; de Lera, A. R. Tetrahedron 2001, 57,
7871.
7. (a) Yamazaki, T.; Murata, Y.; Komatsu, K.; Furukawa, K.; Morita, M.; Maruyama,
N.; Yamao, T.; Fujita, S. Org. Lett. 2004, 6, 4865; (b) Amb, C. M.; Rasmussen, S. C.
Eur. J. Org. Chem. 2008, 801; (c) Dang, T. T.; Rasool, N.; Dang, T. T.; Reinke, H.;
Langer, P. Tetrahedron Lett. 2007, 48, 845.
8. Tranchier, J.-P.; Chavignon, R.; Prim, D.; Auffrant, A.; Plyta, Z. F.; Rose-Munch,
F.; Rose, E. Tetrahedron Lett. 2000, 41, 3607.
9. (a) Iida, T.; Wada, T.; Tomimoto, K.; Mase, T. Tetrahedron Lett. 2001, 42, 4841;
(b) Abarbri, M.; Thibonnet, J.; Berillon, L.; Dehmel, F.; Rottlander, M.; Knochel,
P. J. Org. Chem. 2000, 65, 4618; (c) Abarbri, M.; Dehmel, F.; Knochel, P.
Tetrahedron Lett. 1999, 40, 7449.
10. For the direct preparation of thienylmanganese bromide, see: (a) Kim, S. H.;
Rieke, R. D. Tetrahedron Lett. 1997, 38, 993; (b) Rieke, R. D.; Kim, S. H.; Wu, X. J.
Org. Chem. 1997, 62, 6921.
11. Meijere, A.; Diederich, F. Metal-Catalyzed Cross-Coupling Reactions, 2nd ed.;
Wiley-VCH Gmbh & Co, 2004. Vol. 2.
12. Handy, S. T.; Zhang, Y. Chem. Commun. 2006, 299.
13. For the coupling reaction of organozinc reagents with carbamoyl chloride, see:
Rieke, R. D.; Kim, S. H. Tetrahedron Lett. 2012, 53, 3478.
14. Prepared by the literature: Rieke, R. D.; Hanson, M. V.; Brown, J. D. J. Org. Chem.
1996, 61, 2726.
15. A typical procedure: (a) Preparation of 5-bromo-2-thienylzinc bromide (B); In
an oven-dried 50 mL round-bottomed flask equipped with a stir bar was added
1.40 g of active zinc (Zn⁄, 22.0 mmol). 2,5-Dibromothiophene (4.82 g,
20.0 mmol) dissolved in 20 mL of THF was then cannulated neat into the
flask at room temperature. The resulting mixture was stirred for 1 h at room
temperature. The whole mixture was settled down and then the supernatant
was used for the subsequent coupling reactions; (b) Cu-catalyzed cross-
coupling reaction; Into a 25 mL round-bottomed flask were placed CuI (0.05 g,
10 mol %) and LiCl (0.02 g, 20 mol %). 5-Bromo-2-thienylzinc bromide (B)
(5.0 mL, 0.5 M in THF, 2.5 mmol) was added into the flask under an argon
atmosphere. Next, 6-chloronicotinoyl chloride (0.35 g, 2.0 mmol) was slowly
added via a syringe while being stirred at room temperature. The resulting
mixture was stirred at room temperature for 1 h. Quenched with saturated
NH4Cl solution, then extracted with ethyl ether (10 mL ꢀ 3). Washed with
saturated NaHCO3, Na2S2O3 solution and brine, then dried over anhydrous
MgSO4. Purification by column chromatography on silica gel (2% ethyl acetate/
98% heptane) afforded 0.40 g of 2j in 66% isolated yield as a light yellow solid
(mp 117 °C); 1H NMR (CDCl3, 400 MHz) d = 8.84 (d, J = 2.4 Hz, 1H), 8.10 (dd,
J = 2.4; 2.4 Hz, 1H), 7.51 (d, J = 8.0 Hz, 1H), 7.40 (d, J = 4.0 Hz, 1H), 7.20 (d,
J = 4.0 Hz, 1H); 13C NMR (CDCl3, 100 MHz) d = 183.7, 155.2, 149.9, 144.2, 139.1,
135.2, 131.8, 131.6, 124.9, 124.6; FT-IR (solid) 3096, 1624, 1558, 1415 cmꢁ1
(c) Pd-catalyzed cross-coupling reaction procedure; Into 25 mL round-
;
a
References and notes
bottomed flask were added Pd(PPh3)2Cl2 (0.035 g, 2.0 mol %), ethyl
5-bromothiophene-2-carboxylate (0.47 g, 2.0 mmol), and 5.0 mL of 5-bromo-
2-thienylzinc bromide (B) (0.5 M in THF, 2.5 mmol) under an argon
atmosphere at room temperature. The resulting mixture was stirred at room
temperature for 2 h. Quenched with 3 M HCl solution, then extracted with
ethyl ether (10 mL ꢀ 3). Washed with saturated NaHCO3, Na2S2O3 solution and
brine, then dried over anhydrous MgSO4. Purification by column
chromatography on silica gel (2% ethyl acetate/98% heptane) afforded 0.39 g
of ethyl 50-bromo-2,20-bithiophene-5-carboxylate (3d) in 62% isolated yield as
a yellow solid (mp 69–70 °C) ; 1H NMR (CDCl3, 400 MHz) d = 7.70 (d, J = 4.0 Hz,
1H), 7.08 (d, J = 3.6 Hz, 1H), 7.03 (dd, J = 3.6; 4.0 Hz, 2H), 4.37 (q, J = 7.2 Hz, 2H),
1.40 (t, J = 7.2 Hz, 3H); 13C NMR (CDCl3, 100 MHz) d = 161.9, 142.8, 137.8,
134.0, 132.3, 130.9, 125.2, 124.1, 112.9, 61.3, 14.4; FT-IR (solid) 2978, 1699,
1. (a) Handbook of Conducting Polymers; Skotheim, T. A., Reynolds, J. R., Eds., 3rd
ed.; CRC Press: Boca Raton, FL, 2007; (b) Handbook of Oligo- and Polythiophenes;
Fichou, D., Ed.; Wiley-VCH: Weinheim, 1999; (c) Bohlmann, F.; Zdero, C.
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Gronowitz, S., Ed.; Wiley & Sons: New York, 1985; Vol. 44 (part 1), pp 261–323;
(d) Press, J. B. Thiophene and its derivatives In The Chemistry of Heterocyclic
Compounds; Gronowitz, S., Ed.; Wiley & Sons: New York, 1991; Vol. 44, pp 397–
502.
2. A recent example: Jiang, H.; Zeng, W.; Li, Y.; Wu, W.; Huang, L.; Fu, W. J. Org.
Chem. 2012, 77, 5179 and references cited therein.
3. For reviews: (a) Wang, J.-R.; Manabe, K. Synthesis 2009, 9, 1405; (b) Fairlamb, I.
J. S. Chem. Soc. Rev. 2007, 1036, 36; (c) Schröter, S.; Stock, C.; Bach, T.
Tetrahedron 2005, 2245.
1558 cmꢁ1
.