Rhodium-catalyzed cross-coupling of aryl carbamates with arylboron reagents

Article abstract of DOI:10.1016/j.tet.2015.02.088

A new method has been developed for the rhodium-catalyzed cross-coupling of aryl carbamates with organoboron reagents. The use of an NHC ligand bearing a 2-adamantyl group, i.e., I(2-Ad), is essential to the success of the reaction. The reaction involves the rhodium-mediated activation of the relatively inert C(aryl)-O bond of aryl carbamates.

Full text of DOI:10.1016/j.tet.2015.02.088

Tetrahedron xxx (2015) 1e6
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Rhodium-catalyzed cross-coupling of aryl carbamates with
arylboron reagents
Keisuke Nakamura , Kosuke Yasui , Mamoru Tobisu ,*, Naoto Chatani
a
a
a
,
b
a,
*
a
Department of Applied Chemistry, Faculty of Engineering, Osaka University, Suita, Osaka 565-0871, Japan
Center for Atomic and Molecular Technologies, Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan
b
a r t i c l e i n f o
a b s t r a c t
Article history:
A new method has been developed for the rhodium-catalyzed cross-coupling of aryl carbamates with
organoboron reagents. The use of an NHC ligand bearing a 2-adamantyl group, i.e., I(2-Ad), is essential to
the success of the reaction. The reaction involves the rhodium-mediated activation of the relatively inert
C(aryl)-O bond of aryl carbamates.
Received 15 January 2015
Received in revised form 24 February 2015
Accepted 25 February 2015
Available online xxx
Ó 2015 Elsevier Ltd. All rights reserved.
Keywords:
Rhodium catalyst
CeO bond activation
Suzuki-Miyaura reaction
Cross-coupling
Carbamate
1. Introduction
Cross-coupling reactions between organic halides and organo-
metallic reagents are recognized as one of the most powerful
methods for the formation of carbonecarbon bonds, especially
1
C(aryl)-C(aryl) bonds. Among them, the Suzuki-Miyaura reaction
represents one of the most useful processes, largely because of the
functional group compatibility and stability of organoboron nu-
cleophiles.2 Although this reaction has seen significant advance-
2
a,b
ment since it was first discovered in 1979,
the scope of the
electrophilic coupling partner still remains rather limited to or-
ganic halides and sulfonates (Scheme 1a). In 2004, the use of ani-
soles containing an ortho directing group in the cross-coupling
reaction with organoboron nucleophiles was reported to proceed
3
with a ruthenium catalyst. In this reaction, an inert C(aryl)-OMe
bond is activated through a chelation-assisted oxidative addition.
The scope of the electrophile used in Suzuki-Miyaura type reactions
was significantly expanded by the use of nickel catalysts, which
4
Scheme 1. Aryl electrophiles for use in the Suzuki-Miyaura reaction.
allowed for the cross-coupling of simple unactivated phenol de-
5
6
7
8
rivatives such as aryl ethers, esters, carbamates, carbonates, and
reactivity towards C(aryl)-O bonds that could not otherwise be
activated under conventional palladium-catalyzed conditions.
9
naphtholate (Scheme 1b). The success of these reactions indicated
that low valent nickel species possess exceptionally high levels of
10
From a fundamental perspective, it would be intriguing to know
whether transition metals other than nickel could be used to pro-
mote the Suzuki-Miyaura reaction of such unactivated phenol de-
rivatives. It has also been reported that the C(aryl)-O bonds of aryl
*
Corresponding authors. Tel.: þ81 6 6879 7395 (M.T.); tel.: þ81 6 6879 7397; fax:
þ81 6879 7396 (N.C.); e-mail addresses: tobisu@chem.eng.osaka-u.ac.jp
M. Tobisu), chatani@chem.eng.osaka-u.ac.jp (N. Chatani).
11
12
esters and pivalates can be activated by iron and cobalt cata-
lysts, although these reactions require the addition of more than
6
(
http://dx.doi.org/10.1016/j.tet.2015.02.088
040-4020/Ó 2015 Elsevier Ltd. All rights reserved.
0
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2
K. Nakamura et al. / Tetrahedron xxx (2015) 1e6
a stoichiometric amount of Grignard reagents.13,14 Furthermore,
these catalysts cannot be used to affect cross-coupling reactions
with organoboron reagents. Our group recently found that the
rhodium-catalyzed reaction of aryl pivalates with a diboron reagent
resulted in the formation of borylated products. The use of
a diboron reagent in this reaction was found to be essential to
promote the cleavage of the C(aryl)-O bond of aryl pivalates under
donor would provide a better ligand candidate, and we therefore
preceded to examine a series of NHC ligands. As expected, the use
of IMes as a ligand led to the formation of 5 in 25% yield (Entry 4).
Although the use of an NHC ligand bearing bulkier aryl groups (i.e.,
15
t
IPr) inhibited the arylation (Entry 5), the use of a Bu-substituted
t
NHC ligand (I Bu) led to a significant increase in the yield of 5e77%
(Entry 6). Furthermore, the use of an NHC ligand bearing adamantyl
groups led to even higher catalytic activity, with the 2-adamantyl
derivative I(2-Ad) (Entry 9) performing much more effectively
than the 1-adamantyl isomer I(1-Ad) (Entry 8). In terms of the base,
NaOEt could be replaced with CsF, although this led to a slight
decrease in the yield of 5 from 98 to 74% (Entry 10). The boronic acid
these rhodium-catalyzed conditions (i.e., [RhCl(cod)]
2
/P(4-
MeOC ), and the catalyst system was therefore only applica-
6 4 3
H )
ble to carboneboron bond-forming reactions. Based on these re-
sults, it was envisaged that the Suzuki-Miyaura reaction of inert
phenol derivatives could be realized by identifying an appropriate
rhodium catalyst system. Ozerov et al.16 reported that a rhodium
complex bearing a PNP-pincer ligand could be used to mediate the
oxidative addition of phenyl pivalate and carbamate, and this ob-
servation also encouraged us to investigate the development of
a new rhodium-catalyzed reaction. Pleasingly, our investigative
efforts in this area culminated in the development of a rhodium-
catalyzed cross-coupling of aryl carbamates with arylboronic es-
ters, which we report herein (Scheme 1c).
2
PhB(OH) could also be used as an arylating reagent under these
conditions, without any discernible decrease in the efficiency of the
reaction (Entry 11), whereas the use of the bulkier PhB(pin) resul-
ted in a much lower yield of 5 (Entry 12). The cross-coupling of aryl
pivalate 2 with 4a did not occur under these conditions, but
resulted instead in the exclusive formation of a hydrolyzed naph-
thol product (Entry 13). The use of the corresponding tert-butyl
carbonate also led to the formation of 2-naphthol (see ESI for de-
tails). Aryl methyl ether 3 was found to be completely unreactive
under the current conditions (Entry 14).
2
. Results and discussion
Having identified I(2-Ad) as the optimal ligand for the reaction
(
Table 1, Entry 9), we preceded to examine the scope of aryl car-
At the outset of our studies, we decided to investigate the re-
bamates. Although 2-naphthyl carbamate 1 underwent the cross-
coupling with 4a to form 5 in excellent yield, use of less reactive
substrates led to a significant reduction in the yield. For example,
phenyl carbamate 6 underwent the cross-coupling with 4a to afford
action of 2-naphthyl carbamates
1
with boronic ester 4a
as a cat-
(
nep¼neopentylglycolate) in the presence of [RhCl(cod)]
2
alyst and NaOEt as a base (Table 1). However, virtually none of the
desired cross-coupling product 5 was formed in the absence of
a ligand (Entry 1) and in the presence of phosphine ligands, such as
8
in only 38% yield, with the hydrolyzed compound 9 being formed
as the major product in 56% yield (Scheme 2). This undesired hy-
drolysis could be suppressed completely by increasing the steric
bulk of the carbamate moiety, as exemplified by the use of an
PPh
3 3
(Entry 2) and PCy (Entry 3). Based on our experience of
5
b,17
nickel-catalyzed C(aryl)-O bond activation processes
related reports from others it was assumed that a stronger
as well as
18
s
-
i
OCON Pr
2
group, which delivered 8 in 86% yield. Based on these
i
2
results, the bulky diisopropyl carbamate group (-OCON Pr ) was
selected as the best group for further exploration. It is noteworthy
that this carbamate moiety can be readily introduced at a phenolic
Table 1
Optimization studies
a
hydroxyl group by the reaction of the appropriate phenol with
i
ClCON Pr
2
, which is commercially available (See Experimental
Section for details).
Entry
Substrate
Ligand
None
Base
Yield/%b
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
1
1
2
3
NaOEt
NaOEt
NaOEt
NaOEt
NaOEt
NaOEt
NaOEt
NaOEt
NaOEt
CsF
8
0
0
25
3
77
5
54
94
74
73
16
3
PPh
PCy
3
3
IMes$HCl
IPr$HCl
t
I Bu$HCl
Scheme 2. Effect of the substituent of the carbamoyl group.
ICy$HCl
I(1-Ad)$HCl
I(2-Ad)$HCl
I(2-Ad)$HCl
I(2-Ad)$HCl
I(2-Ad)$HCl
I(2-Ad)$HCl
I(2-Ad)$HCl
The scope of the rhodium/I(2-Ad)-catalyzed cross-coupling of
aryl carbamates with boronic ester 4a is shown in Table 2. Phenyl
carbamate 12 (Entry 2) proved to be significantly less reactive than
the naphthyl substrate 10 (Entry 1), as is often observed in nickel-
10
11
12
13
14
c
NaOEt
NaOEt
NaOEt
NaOEt
d
0
19
catalyzed C(aryl)-O bond activation reactions. Interestingly, the
a
Reaction conditions: substrate (0.30 mmol), 4a (0.45 mmol), [RhCl(cod)]
2
introduction of a phenyl group to the substrate led to a significant
improvement in the yield of the product, as evidenced by biphenyl
carbamates 14 (Entry 3) and 16 (Entry 4), which reacted success-
fully under the optimized conditions to give 15 and 17 in 86 and
(
1
0.015 mmol), ligand (0.060 mmol), and base (0.60 mmol) in toluene (1.0 mL) at
ꢀ
30 C for 20 h in a sealed tube.
b
GC yield of 5 based on the substrate.
c
p-TolylB(OH)
2
was used instead of 4a.
d
p-TolylB(pin) was used instead of 4a.
82% yields, respectively. Several biaryl derivatives bearing CF
3
and
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K. Nakamura et al. / Tetrahedron xxx (2015) 1e6
3
Table 2
a
Rh(I)/NHC-catalyzed cross-coupling of aryl carbamates with 4a
Entry
Aryl carbamate
Product
Yield/%b
1
80
t
2
3
4
5
6
R¼p- Bu 12
13
15
17
19
21
18
86
82
0
R¼p-Ph 14
R¼m-Ph 16
R¼o-Ph 18
R¼p-CF
3
20
79
7c
8c
9d
97
61
63
1
0e
60
82
81
40
1 f
1
1
1
2e
3
4
1
90
a
Reaction conditions: aryl carbamates (0.30 mmol), 4a (0.45 mmol), [RhCl(cod)]
2
(0.015 mmol), I(2-Ad)$HCl (0.060 mmol), and NaOEt (0.60 mmol) in toluene (1.0 mL) at
ꢀ
1
30 C for 20 h in a sealed tube.
b
Isolated yield after column chromatography.
c
Toluene (1.5 mL) was used.
d
e
f
t
NaO Bu (0.060 mmol) and CsF (0.60 mmol) was used instead of NaOEt.
Toluene (2 mL) was used.
NaOEt (0.45 mmol) was used.
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4
K. Nakamura et al. / Tetrahedron xxx (2015) 1e6
MeO groups also performed effectively as substrates for the cou-
pling reactions (Entries 7 and 8), whereas the o-phenyl-substituted
substrate 18 (Entry 5) was found to be completely unreactive, most
likely because of its steric bulk. Given the minor inductive effect of
found to have a profound impact on the outcome of the reaction.
Arylboronic esters bearing an electron-donating group, such as
a Me (Entry 1), NMe (Entry 2) or an OMe (Entry 3) group, per-
2
formed as excellent aryl donors to efficiently form the corre-
sponding biaryl derivatives. In contrast, arylboronic esters
2
0
a phenyl group (
observed in the reactivity of the biaryl carbamates could be at-
tributed to the extension of their -conjugation. Indeed, stryrenyl
Entry 10) and alkynyl (Entry 11) groups also exerted a positive
s
p
¼0.05,
s
m
¼0.05), the significant enhancement
3
bearing an electron-deficient CF group on its phenyl ring, i.e., 4e
p
reacted poorly to afford the desired biaryl product in only 17%
yield (Entry 4). The presence of ester and Boc groups was also
well tolerated under these rhodium/I(2-Ad)-catalyzed conditions
(Entries 5 ad 6). The sterically hindered o-tolylboronic ester 4h
(Entry 7) and 1-naphthylboronic ester 4i (Entry 8) also reacted
smoothly with 10 to form the corresponding congested biaryl
frameworks. Heteroarylboronic esters, including those contain-
ing a pyridine or a indole ring, could also be used in this aryla-
tion, although the product was formed in a relatively low yield
(Entries 9 and 10).
(
effect on the efficiency of the reaction and exhibited higher levels of
reactivity compared with the simple alkyl-substituted substrate 12
(
Entry 2). It is noteworthy that carbamate 30 could not be arylated
under the previously reported nickel-catalyzed conditions (see ESI
for details). Inductive effects can also have an important impact on
this cross-coupling, as exemplified by the electron-deficient aryl
carbamate 20 with no extended
arylated product 21 in 79% yield (Entry 6). Notably, several heter-
oaromatic substrates, including -deficient quinoline (Entry 12)
and pyridine (Entry 13) rings and a -rich carbazole (Entry 14), all
p-system, which gave rise to the
p
Based on the results reported by Ozerov,16 we have proposed
a mechanism for the current rhodium-catalyzed cross-coupling of
aryl carbamates with aryl boronic esters, which is depicted in
Scheme 3 The catalyst precursor A would initially react with NaOEt
to give the catalytically competent rhodium ethoxide complex B,
which would subsequently undergo transmetalation with an aryl
p
reacted smoothly to provide the corresponding phenylated prod-
ucts. Aryl bromides and chlorides were found to be incompatible
with these conditions and reacted instead to give the arylated
products.21
The scope with respect to boronic esters is shown in Table 3.
The electronic nature of the aryl group in the boronic ester was
2
boronic ester Ar’B(OR) to give the arylrhodium species C. The ox-
idative addition of a carbamate substrate to the arylrhodium C
would activate the C(aryl)-O bond of the carbamate, leading to the
formation of a rhodium(III) intermediate D, which would release an
arylated product through reductive elimination. The resulting
rhodium carbamate E would then be converted to rhodium eth-
oxide B via ligand exchange with NaOEt. A direct transmetalation
pathway from E to C was considered to be unlikely because the use
of excess NaOEt was essential for an efficient cross-coupling under
Table 3
a
Scope of the boronic ester in the Rh(I)/NHC-catalyzed cross-coupling of 10
2
2
these conditions. The key to achieving the difficult oxidative ad-
dition of the C(aryl)-O bond of the aryl carbamate (i.e., C/D) could
be attributed to the electron-rich nature of the rhodium center
Entry
Boronic ester
Product
Yield/%b
2
3
generated through the addition of the I(2-Ad) ligand. The elec-
tronic effects observed for the boronic ester most likely originated
from the electronic effect of the oxidative addition step (C/D)
rather than the effect of the transmetalation step (B/C), because it
is well known that the latter of these two steps can proceed with
1
2
3
4
5
6
R¼Me 4b
38
39
40
41
42
43
98
81
82
17
62
73
R¼NMe
2
4c
24
a wide range of electronically different organoboron compounds.
R¼OMe 4d
The use of electron-deficient boronic esters would lead to a de-
crease in the electron density of the rhodium center in C, which
would in turn lead to reduction in the efficiency of the oxidative
R¼CF
R¼CO
3
4e
Et 4f
R¼NMeBoc 4g
2
2
5
addition.
7
8
9
81
79
49
93
1
0c
a
Reaction conditions: 10 (0.30 mmol), arylboronic ester (0.45 mmol),
[
RhCl(cod)]
2
(0.015 mmol), I(2-Ad)$HCl (0.060 mmol), and NaOEt (0.60 mmol) in
ꢀ
toluene (1.0 mL) at 130 C for 20 h in a sealed tube.
b
Isolated yield after column chromatography.
c
NaOEt (0.45 mmol) was used.
Scheme 3. Possible mechanism.
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K. Nakamura et al. / Tetrahedron xxx (2015) 1e6
5
3
. Conclusion
128.7, 138.0, 140.6, 150.8, 153.7; HRMS (FAB): Calcd for
þ
C
19
2
H23NO þH 298.1802, Found 298.1801.
In summary, we have developed a new rhodium-catalyzed
0
0
00
cross-coupling reaction of aryl carbamates with organoboron re-
agents, which involves the cleavage of a relatively inert C(aryl)-O
bond. Although C(aryl)-O bonds of this type can be activated us-
4.4. 1,1 :4 ,1 -Terphenyl (15)
[RhCl(cod)]2 (7.4 mg, 0.015 mmol), I(2-Ad)$HCl (22 mg,
0.060 mmol), NaOEt (41 mg, 0.60 mmol) and toluene (0.40 mL)
were added to a 10 mL sample vial with a Teflon sealed screwcap in
a glovebox filled with nitrogen, and the resulting mixture was
5
e9
ing nickel-based catalysts,
the reaction described herein repre-
sents the first use of a rhodium-based catalyst in a Suzuki-Miyaura
type reaction. The choice of an appropriate ligand was found to be
essential for efficient catalysis, and an NHC ligand bearing a 2-
adamantyl group was found to be optimal. It is envisaged that
this rhodium-mediated C(aryl)-O bond activation reaction could be
applied to a range of other catalytic transformations, when con-
sidering a diverse reactivity of organorhodium intermediates, such
as CeH activation.26 Studies aimed at a better understanding of the
scope and limitations of this rhodium-mediated C(aryl)-O bond
activation strategy are currently underway in our laboratory.
0
stirred for 10 min [1,1 -Biphenyl]-4-yl diisopropylcarbamate (14,
89 mg, 0.30 mmol), 5,5-dimethyl-2-phenyl-1,3,2-dioxaborinane
(4a, 86 mg, 0.45 mmol) and toluene (0.60 mL) were then added
to the vial, and the resulting mixture was sealed in the vessel and
ꢀ
heated at 130 C for 20 h on an aluminum block. The mixture was
then cooled to room temperature and purified directly by flash
column chromatography over silica gel (eluent: hexane/
EtOAc¼10:1) to give the title compound 15 as a white solid (59 mg,
1
8
(
4
6%); R
f
(hexane/EtOAc¼3/1) 0.75; H NMR (CDCl
3
, 400 MHz): 7.37
t, J¼7.4 Hz, 2H), 7.47 (t, J¼7.4 Hz, 4H), 7.65 (d, J¼7.6 Hz, 4H), 7.68 (s,
4
4
. Experimental section
13
H); C NMR (CDCl
3
, 100 MHz): 127.0, 127.3, 127.5, 128.8, 140.0,
14 230.1096, Found 230.1089.
140.7; HRMS (EI): Calcd for C18
H
.1. General
1H NMR and 13C NMR spectra were recorded on a JEOL JMTC-
Acknowledgements
4
3
00/54/ss spectrometer in CDCl with tetramethylsilane as an in-
This work was supported by a Grant-in-Aid for Scientific Re-
search on Innovative Areas “Molecular Activation Directed toward
Straightforward Synthesis” from MEXT, Japan and ACT-C from JST,
Japan. We also thank the Instrumental Analysis Center, Faculty of
Engineering, Osaka University, and Professors Mitsuhiro Arisawa,
Tetsuya Satoh and Hayato Tsurugi (Osaka University) for assistance
with HRMS.
ternal standard. Data have been reported as follows: chemical shift
in parts per million (
t¼triplet, q¼quartet, and m¼multiplet), coupling constant (Hertz),
and integration. Infrared spectra (IR) were obtained on a JASCO FT/
IR-4000; absorptions have been reported in reciprocal centimeters
with the following relative intensities: s (strong), m (medium), or w
d
), multiplicity (s¼singlet, d¼doublet,
(
weak). Mass spectra were recorded on a Shimadzu GCeMS-QP
010 instrument with an ionization voltage of 70 eV. Melting points
were determined using a Yamato melting point apparatus. Column
chromatography was performed with SiO [Merck SilicaGel 60
230e400 mesh) or Silycycle Silica Flash F60 (230e400 mesh)]. Gel
permeation chromatography (GPC) was performed using LC-
210NEXT HPLC or LC9225NEXT HPLC system. All of the reactions
2
Supplementary data
2
(
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.tet.2015.02.088.
9
were carried out in 10 mL sample vials with Teflon-sealed screw
caps. All of the chemicals used in the current study were manipu-
lated in a glovebox filled with nitrogen.
References and notes
1. Reviews on cross-coupling: (a) Cross-Coupling Reactions: A Practical Guide;
Miyaura, N., Ed.; Springer: Berlin, Germany, 2002; (b) Metal-Catalyzed Cross-
Coupling Reactions, 2nd ed.; de Meijere, A., Diederich, F., Eds.; Wiley-VCH:
New York, NY, 2004; A recent themed issue on cross-coupling:Chem. Lett. 2011,
4
.2. Materials
40, 894.
2
. Seminal works: (a) Miyaura, N.; Yamada, K.; Suzuki, A. Tetrahedron Lett. 1979,
20, 3437; (b) Miyaura, N.; Suzuki, A. J. Chem. Soc., Chem. Commun. 1979, 866
Reviews; (c) Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457; (d) Suzuki, A.;
Brown, H. C. Organic Synthesis via Boranes; Aldrich: Milwaukee, WI, 2003; (e)
Hall, D. G. Boronic Acids; Wiley-VCH: Weinheim, Germany, 2005; (f) Miyaura, N.
Bull. Chem. Soc. Jpn. 2008, 81, 1535; (g) Suzuki, A. Angew. Chem., Int. Ed. 2011, 50,
I(1-Ad)$HCl were purchased from Strem Chemicals and used as
t
received. NaO Bu, NaOEt, IMes$HCl, IPr$HCl and diisopropylcarba-
moyl chloride were purchased from TCI and used as received. Tol-
uene (for Organic Synthesis), [RhCl(cod)]
purchased from Wako Chemicals and used as received. PCy
2
, PPh
3
, 4a, 4b and 4j were
and
6
722.
. (a) Kakiuchi, F.; Usui, M.; Ueno, S.; Chatani, N.; Murai, S. J. Am. Chem. Soc. 2004,
26, 2706; (b) Ueno, S.; Mizushima, E.; Chatani, N.; Kakiuchi, F. J. Am. Chem. Soc.
2006, 128, 16516; (c) Zhao, Y.; Snieckus, V. J. Am. Chem. Soc. 2014, 136, 11224.
3
3
CsF were purchased from Aldrich and used as received. NaH was
purchased from nacalai tesque and used as received. Compounds
1
27
28
29
5b
30
5b
31
5b
5b
5b
32
1
2, 16, 34, 4c, 4d, 4e, 4f, 4g, 4h, 4i and 4k were
4. Reviews: (a) Yu, D.-G.; Li, B.-J.; Shi, Z.-J. Acc. Chem. Res. 2010, 43, 1486; (b) Rosen,
B. M.; Quasdorf, K. W.; Wilson, D. A.; Zhang, N.; Resmerita, A.-M.; Garg, N. K.;
Percec, V. Chem. Rev. 2011, 111, 1346; (c) Li, B.-J.; Yu, D.-G.; Sun, C.-L.; Shi, Z.-J.
Chem.dEur. J. 2011, 17, 1728; (d) Mesganaw, T.; Garg, N. K. Org. Process Res. Dev.
synthesized according to the reported procedures.
2
013, 17, 29; (e) Tobisu, M.; Chatani, N. Top. Organomet. Chem. 2013, 44, 35; (f)
0
4
.3. [1,1 -Biphenyl]-4-yl diisopropylcarbamate (14)
Yamaguchi, J.; Muto, K.; Itami, K. Eur. J. Org. Chem. 2013, 19; (g) Han, F.-S. Chem.
Soc. Rev. 2013, 42, 5270; (h) Cornella, J.; Zarate, C.; Martin, R. Chem. Soc. Rev.
2014, 43, 8081.
0
[
1,1 -Biphenyl]-4-ol (1.7 g, 10 mmol), diisopropylcarbamic
chloride (2.5 g, 15 mmol) and K CO (2.1 g, 15 mmol) were refluxed
2 3
5
. (a) Tobisu, M.; Shimasaki, T.; Chatani, N. Angew. Chem., Int. Ed. 2008, 47, 4866;
(b) Tobisu, M.; Yasutome, A.; Kinuta, H.; Nakamura, K.; Chatani, N. Org. Lett.
in MeCN for 12 h. The resulting mixture was cooled to room tem-
perature, and the solvent was removed in vacuo. The purification by
flash column chromatography over silica gel (hexane/EtOAc¼2/1)
2014, 16, 5572.
6. (a) Quasdorf, K. W.; Tian, X.; Garg, N. K. J. Am. Chem. Soc. 2008, 130, 14422; (b)
Guan, B.-T.; Wang, Y.; Li, B.-J.; Yu, D.-G.; Shi, Z.-J. J. Am. Chem. Soc. 2008, 130,
14468; (c) Molander, G. A.; Beaumard, F. Org. Lett. 2010, 12, 4022; (d) Xu, M.; Li,
gave the title compound 14 as a white solid (2.7 g, 91%); R
f
(hexane/
, 400 MHz): 1.31 (br s,
H), 1.36 (br s, 6H), 3.97 (br s, 1H), 4.13 (br s, 1H), 7.20 (d, J¼8.8 Hz,
H), 7.34 (t, J¼7.4H, 1H), 7.43 (t, J¼7.8 Hz, 2H), 7.56e7.58 (m, 4H);
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