EtAlCl2/2,6-Disubstituted Pyridine-Mediated Carboxylation of Alkenes with Carbon Dioxide

Article abstract of DOI:10.1021/acs.orglett.6b00918

α-Arylalkenes and trialkyl-substituted alkenes undergo carboxylation with CO₂ in the presence of EtAlCl₂ and 2,6-dibromopyridine to afford the corresponding α,β- and/or β,γ-unsaturated carboxylic acids. This reaction is suggested to proceed via the electrophilic substitution of EtAlCl₂ with the aid of the base, followed by the carbonation of the resulting ate complex. This reaction can be applied to terminal dialkylalkenes by using a mixture of 2,6-di-tert-butylpyridine and 2,6-dibromopyridine.

Full text of DOI:10.1021/acs.orglett.6b00918

Letter
pubs.acs.org/OrgLett
EtAlCl2/2,6-Disubstituted Pyridine-Mediated Carboxylation of
Alkenes with Carbon Dioxide
Shinya Tanaka,* Kota Watanabe, Yuuki Tanaka, and Tetsutaro Hattori*
Department of Biomolecular Engineering, Graduate School of Engineering, Tohoku University, 6-6-11 Aramaki-Aoba, Aoba-ku,
Sendai 980-8579, Japan
S
* Supporting Information
ABSTRACT: α-Arylalkenes and trialkyl-substituted alkenes
undergo carboxylation with CO2 in the presence of EtAlCl2
and 2,6-dibromopyridine to afford the corresponding α,β-
and/or β,γ-unsaturated carboxylic acids. This reaction is
suggested to proceed via the electrophilic substitution of
EtAlCl2 with the aid of the base, followed by the carbonation
of the resulting ate complex. This reaction can be applied to
terminal dialkylalkenes by using a mixture of 2,6-di-tert-
butylpyridine and 2,6-dibromopyridine.
Lewis acid.12,13 In this study, we have found such a base and
ecently, increasing attention has been paid to the
Rdevelopment of methods to prepare carboxylic acids achieved the carboxylation of alkenes.
First, the carboxylation of 1,1-diphenylethene (1a) was
investigated using various bases (1.0 molar equiv) under CO2
pressure (3.0 MPa) in the presence of EtAlCl2 (1.0 molar equiv)
in toluene at 60 °C (Table 1). The use of Hunig’s base, DBU,
N,N-dimethylaniline, and triphenylamine gave no carboxylic
acid. The use of a pyridine-type base, 2,6-di-tert-butylpyridine
(B1), which is often used as a proton scavenger under Lewis
acidic conditions,14 also failed (entry 1). However, less sterically
bulky 2,6-lutidine (B2) afforded a small amount (3%) of the
desired acid 2a (entry 2). We then investigated the suitability of
2-mono- and 2,6-disubstituted pyridines, as well as related
heterocyclic amines, using the pKa of the conjugate acid15,16 and
the sum of the A values of the substituents as the clues (Figure 1);
A value is used to estimate the steric bulkiness of substituents.17
Reducing the basicity of 2,6-lutidine (B2) by replacing one of the
methyl groups of B2 with a chloro group (B3) slightly improved
the yield of the acid (entry 3). However, the removal of the other
methyl group (B4) completely inhibited the reaction (entry 4),
indicating that 2,6-disubstituted pyridines are promising. In fact,
the use of 2,6-dichloropyridine (B5) significantly improved the
yield (entry 5). Better yields were obtained by using 2,6-
dibromopyridine (B6) and 2,6-diiodopyridine (B7), which have
a similar steric bulk as dichloride B5 but a slightly higher basicity
(entries 6 and 7). On the other hand, less bulky and less basic
difluoride B8 decreased the yield and worsened the mass balance
(entry 8). A similar trend for the effects of the steric bulkiness and
basicity on the yield of the acid was also observed for other
heterocyclic amines. A good yield was obtained for 2,3-
dichloroquinoxaline (B13), whose bulkiness and basicity are
similar to those of dihalopyridines B5−B7 (entry 13). On the
using CO2. Remarkable advances have been made in the
transition-metal-catalyzed carboxylation of unsaturated com-
pounds.1−7 Although the carboxylation of alkenes with CO2 is
one of the most attractive methods to prepare vinylcarboxylic
acids from the perspective of atom economy, such a trans-
formation via the cleavage of a vinyl C−H bond with a transition
metal catalyst is difficult. This is because a CC bond is easily
inserted into various M−X bonds of transition metal complexes,
whereas it is difficult to add a vinyl C−H bond oxidatively to low-
valent transition metals.6 Therefore, α,β-unsaturated carboxylic
acids have been prepared via addition reactions to alkynes, which
allow only limited access to multisubstituted vinylcarboxylic
acids.7 Ethylene is converted into sodium acrylate using a Ni
complex via Ni-mediated oxidative coupling of ethylene with
CO2 to form a nickelalactone, followed by the abstraction of an
α-hydrogen of the carboxy group with sodium tert-butoxide.8,9
Recently, this reaction was applied to some other alkenes;
however, the yields of the corresponding acids based on the
starting alkenes were moderate (<40%).8c
In our continuing efforts to develop Lewis-acid-mediated
carboxylation reactions of aromatic compounds with CO2,10,11
we succeeded in the carboxylation of 1-methylindoles using
Me2AlCl. Mechanistic studies revealed that the reaction proceeds
via the electrophilic aromatic substitution of a substrate with
Me2AlCl, followed by the carbonation of the resulting
indolylaluminum ate complex.10d Because alkenes are more
susceptible to electrophilic attacks than aromatic compounds, an
Al-based Lewis acid such as Me2AlCl adds to an alkene to form a
zwitterionic species. Although the species should rather react
with nucleophiles (e.g., the substrate) than eliminate a proton at
the α-position to the Al,11 we considered that the latter
substitution pathway would predominate if the α-proton is
abstracted by the addition of a base without deactivating the
Received: March 31, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b00918
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
substituents, with the sum of A values in an approximate range of
0.7−1.8, and exhibit a basicity (pKa) of approximately −2; they
fall within the area shown by a rounded rectangle in Figure 1. The
reaction conditions were investigated further using dibromide B6
as the base. An increase in the amount of EtAlCl2 to 2 molar equiv
increased the yield to 96% (entry 14). Interestingly, a reduction
in the amount of the base to 0.5 molar equiv did not significantly
affect the yield (entry 15). The acid was still obtained in a
moderate yield even by reducing the amount to 0.1 molar equiv;
however, the conversion of the alkene to the acid decreased
(entry 16). These results indicate that the amine can serve as a
catalyst, but its stoichiometric use is effective in achieving a high
yield by suppressing side reactions. We also investigated the
combined use of dibromide B6 with other Al-based Lewis acids
under the conditions used in entry 6, which was adopted as a
standard thereafter, but the acid yield did not increase that
obtained in entry 6.18
Table 1. Carboxylation of 1,1-Diphenylethene (1a) in the
Presence of Various Bases
a
b
b
entry
base
x
2a (%)
1a (%)
1
2
B1
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.5
0.1
nd
3
24
B2
91
3
B3
7
93
The carboxylation of diverse α-arylalkenes was investigated
under standard conditions (Table 2). An internal alkene, 1,1-
4
B4
nd
58
87
71
19
5
>99
42
5
B5
6
B6
5
Table 2. Carboxylation of α-Arylalkenes and
a
7
B7
28
Heteroaromatics
8
B8
48
9
B9
95
10
11
12
13
B10
B11
B12
B13
B6
1
99
nd
nd
71
>99
>99
29
c
14
96 (95)
83
2
15
16
B6
trace
nd
B6
53
a
Reaction conditions: 1a (0.50 mmol), EtAlCl2 (1.0 molar equiv, used
as a 1.0 M hexane solution), base (x molar equiv), CO2 (3 MPa),
b
toluene (1.0 mL), 60 °C, 3 h. 1H NMR yield determined using
CH2Br2 as the internal standard. Isolated yield is shown in parentheses.
c
EtAlCl2 (2.0 molar equiv) was used.
a
Reaction conditions: 1 (0.50 mmol), EtAlCl2 (1.0 molar equiv, used
as a 1.0 M hexane solution), B6 (1.0 molar equiv), CO2 (3 MPa),
b
c
toluene (1.0 mL), 60 °C, 3 h. Isolated yield. 1f (E/Z = 95:5) was
d
e
f
used. 24 h. E only. 1-Methylene-1,2,3,4-tetrahydronaphthalene-2-
carboxylic acid (3h, 2%) was also isolated. 1 h.
Figure 1. Correlation chart between the properties of base B and the
yield of acid 2a in the carboxylation of alkene 1a (Table 1). The x- and y-
axes indicate the pKa of the conjugate acid of base B and the steric
bulkiness of base B estimated from the sum of the A values of its
substituents, respectively. The yield of acid 2a is indicated in
parentheses.
g
diphenylpropene (1b), afforded the corresponding vinylcarbox-
ylic acid 2b in a yield lower than that of 2a obtained in the
reaction of 1,1-diphenylethane (entry 1 vs entry 6 in Table 1),
indicating that the steric hindrance caused by the methyl group
prevented the proton abstraction from the zwitterionic
intermediate generated by the addition of EtAlCl2 to the alkene.
Indeed, less bulky cyclic diarylalkenes, 1c and 1d, were converted
to the corresponding acids in good to excellent yields (entries 2
and 3). Styrene gave cinnamic acid in a low yield (5%) because of
other hand, less bulky (B9−11) or more basic (B12)
heterocycles were ineffective, even though either the bulkiness
or basicity was similar to that of the dihalopyridines (entries 9−
12). On the basis of these observations, it can be concluded that
the suitable amines for this reaction should have two ortho
B
DOI: 10.1021/acs.orglett.6b00918
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
many unidentified byproducts. However, introduction of a
methyl group to the α-position (1e) furnished the product in a
moderate yield (entry 4). In this reaction, both α,β- and β,γ-
unsaturated acids (2e and 3e, respectively) were obtained in
similar yields, but they converged to the former by prolonging
the reaction time (entry 5). The time course of the change in the
yield indicated that nonconjugated acid 3e was formed initially
and then isomerized to more stable conjugated acid 2e.18 The
initial formation of 3e is rationalized by the selective proton
abstraction from the zwitterionic intermediate at the γ-position
to the Al that is less crowded than the α-position (vide infra).
Under the standard conditions, α,β-dimethyl-substituted styrene
(1f) and 1-phenylcyclohexene (1g) selectively produced the
corresponding β,γ-unsaturated acids (entries 6 and 7), whereas
dihydronaphthalene 1h and indenes 1i and 1j afforded the
corresponding α,β-unsaturated acids (entries 8−10). The
product selectivity seems to depend on the easiness of the
isomerization from the nonconjugated to conjugated acids.
Indeed, in the reaction of dihydronaphthalene 1h, shortening the
reaction time to 1 h provided the corresponding β,γ-unsaturated
acid in 27% yield at the expense of α,β-unsaturated acid 2h
(51%). Furthermore, this reaction could be applied to
heteroaromatics. We previously reported that 1-substituted
indoles and thiophenes were carboxylated using Me2AlCl and
EtAlCl2, respectively.10d,b Indole 1k was carboxylated in a yield
comparable to that obtained by the reaction using Me2AlCl
(86%), instead of EtAlCl2 and B6 under the same conditions
(entry 11). Thiophenes 1l and 1m were carboxylated in high
yields (entries 12 and 13); notably, the addition of B6 made the
reaction conditions milder and increased the yields of the
acids.10b Moreover, furan (1n), which could not be carboxylated
using any Al-based Lewis acid in the absence of a base, afforded
the corresponding 2-carboxylic acid in a good yield (entry 14).
However, a vinyl ether, 2,3-dihydrofuran, was not carboxylated
under the standard conditions.
Table 3. Carboxylation of Aliphatic Alkenes
a
Reaction conditions: 1 (0.50 mmol), EtAlCl2 (1.0 molar equiv, used
as a 1.0 M hexane solution), B6 (1.0 molar equiv), CO2 (3 MPa),
toluene (1.0 mL), 60 °C, 3 h. Isolated yield. The ratio was
determined by H NMR. EtAlCl2 (2.5 molar equiv), B6 (0.5 molar
equiv), B1 (1.4 molar equiv) were used. E/Z = 32:68. E/Z = 58:42.
E/Z = 69:31. E/Z = 57:43. E/Z could not be determined.
b
c
d
1
e
f
g
h
i
of an excess of EtAlCl2, the desired β,γ-unsaturated acid 3q was
obtained as the major product from the terminal alkene 1q, along
with its α,β-unsaturated isomer 2q and a trace amount of
undesired 4qa (entry 4). The molar equivalences of B1, B6, and
EtAlCl2 to the substrate were optimized to be 1.4, 0.5, and 2.5,
respectively.18 Under the same conditions, another terminal
alkene 1r was also successfully carboxylated (entry 5).
A feasible mechanism for the carboxylation is shown in
Scheme 1. First, EtAlCl2, which is in an equilibrium state with its
In general, the electrophilic substitution of alkenes is difficult,
except when a cationic intermediate formed by the addition of an
electrophile benefits from the resonance stabilization by a
substituent such as an aryl, amino, or alkoxy group attached to
the β-position to the newly introduced substituent, and/or the
intermediate has a structure that can easily release a proton at the
α-position.19 Gratifyingly, this reaction could be applied to
trialkyl-substituted alkenes 1o and 1p (entries 1 and 2 in Table
3); the products were only β,γ-unsaturated acids 3. On the other
hand, terminal alkene 1q underwent isomerization prior to the
carboxylation to afford β,γ-unsaturated acids 4qa and 4qb,
corresponding to the resulting internal alkene, in preference to
β,γ-unsaturated acid 3q, corresponding to the original alkene
(entry 3). To investigate this isomerization, alkene 1q was heated
with 1.0 molar equiv each of EtAlCl2 and B6 in toluene-d8 at 60
°C. Under the conditions, the terminal alkene was gradually
isomerized to the internal alkene; the molar ratio reached 98:2
(internal/terminal) after 3 h, as evidenced by 1H NMR
analysis.18 This indicates that the isomerization proceeds via
the electrophilic addition of EtAlCl2 to the alkene, followed by
the abstraction of a hydrogen at the γ-position to the Al by the
base and subsequent protonolysis of the resulting allylaluminum
ate complex with the conjugate acid of B6. We envisioned that
the isomerization would be suppressed by the addition of a
strong and bulky base that can receive a proton from the
conjugate acid of B6 and tightly hold it without deactivating the
Lewis acid. We found that 2,6-di-tert-butylpyridine (B1) is such a
suitable base. By the combined use of B1 and B6 in the presence
Scheme 1. Feasible Mechanism for the Carboxylation of
Alkenes
pyridine salt, electrophilically attacks an alkene to generate
zwitterion A. Then, the base abstracts a proton at the α- or γ-
position to the Al (A and A′, respectively), affording ate complex
B or B′. Apparently, a sterically bulky and less basic amine favors
the dissociation of the Al-pyridine salt, whereas a sterically
undemanding and highly basic amine favors the proton
abstraction. The balance between these conflicting demands
seems to determine the amines suitable for this reaction, as
shown in Figure 1. The ate complex B or B′ is then carbonated to
C
DOI: 10.1021/acs.orglett.6b00918
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Int. Ed. 2012, 51, 11487. (d) Zhang, L.; Cheng, J.; Carry, B.; Hou, Z. J.
Am. Chem. Soc. 2012, 134, 14314.
(8) (a) Lejkowski, M. L.; Lindner, R.; Kageyama, T.; Bod
afford aluminum carboxylate C or C′, respectively. Because the
pKa of the conjugate acid of the pyridine base (approximately
−2) is far less than that of the carboxylic acid (∼5), the aluminum
carboxylate liberates the free acid, regenerating the Al-pyridine
salt. Hence, the base can serve as a catalyst. On the other hand, it
is necessary to use EtAlCl2 stoichiometrically because it is
deactivated by coordinating to the carboxylic acid.
In conclusion, alkenes were carboxylated with CO2 by the
combined use of EtAlCl2 and 2,6-dibromopyridine. This is the
first example of the displacement of vinyl hydrogen atoms with
the carboxy group in practical yields. This will provide useful
insights for the development of methodologies to react alkenes
with a strong Lewis acid and to compatibly use a Lewis acid and
Lewis base. Further studies along these lines are in progress.
́
́
izs, G. E.;
Plessow, P. N.; Muller, I. B.; Schafer, A.; Rominger, F.; Hofmann, P.;
̈
̈
Futter, C.; Schunk, S.; Limbach, M. Chem. - Eur. J. 2012, 18, 14017.
(b) Hendriksen, C.; Pidko, E. A.; Yang, G.; Schaffner, B.; Vogt, D. Chem.
̈
- Eur. J. 2014, 20, 12037. (c) Huguet, N.; Jevtovikj, I.; Gordillo, A.;
Lejkowski, M. L.; Lindner, R.; Bru, M.; Khalimon, A. Y.; Rominger, F.;
Schunk, S. A.; Hofmann, P.; Limbach, M. Chem. - Eur. J. 2014, 20, 16858.
(9) (a) Bruckmeier, C.; Lehenmeier, M. W.; Reichardt, R.; Vagin, S.;
Rieger, B. Organometallics 2010, 29, 2199. (b) Plessow, P. N.; Weigel, L.;
Lindner, R.; Schafer, A.; Rominger, F.; Limbach, M.; Hofmann, P.
̈
Organometallics 2013, 32, 3327.
(10) (a) Nemoto, K.; Yoshida, H.; Egusa, N.; Morohashi, N.; Hattori,
T. J. Org. Chem. 2010, 75, 7855. (b) Nemoto, K.; Onozawa, S.; Konno,
M.; Morohashi, N.; Hattori, T. Bull. Chem. Soc. Jpn. 2012, 85, 369.
(c) Konno, M.; Chiba, M.; Nemoto, K.; Hattori, T. Chem. Lett. 2012, 41,
913. (d) Nemoto, K.; Tanaka, S.; Konno, M.; Onozawa, S.; Chiba, M.;
Tanaka, Y.; Sasaki, Y.; Okubo, R.; Hattori, T. Tetrahedron 2016, 72, 734.
(11) For an extension to the hydrocarboxylation of arylalkenes, see:
Tanaka, S.; Tanaka, Y.; Chiba, M.; Hattori, T. Tetrahedron Lett. 2015, 56,
3830.
(12) For base-assisted electrophilic substitution of alkenes, see:
(a) Cook, A. G.; Alt, G. H. In Enamines: Synthesis, Structure, and
Reactions, 2nd ed.; Cook, A. G., Ed.; Marcel Dekker: New York, 1988; pp
181−246. (b) Hojo, M.; Masuda, R.; Kokuryo, Y.; Shioda, H.; Matsuo, S.
Chem. Lett. 1976, 499. (c) Hojo, M.; Masuda, R.; Kamitori, Y.
Tetrahedron Lett. 1976, 17, 1009. (d) Hojo, M.; Masuda, R.; Okada, E.
Tetrahedron Lett. 1986, 27, 353.
(13) (a) Olah, G. A.; Bach, T.; Parakash, G. K. S. New J. Chem. 1991, 15,
571. (b) Munshi, P.; Beckman, E. J.; Padmanabhan, S. Ind. Eng. Chem.
Res. 2010, 49, 6678.
(14) (a) Brown, H. C.; Kanner, B. J. Am. Chem. Soc. 1966, 88, 986.
(b) Cheon, C. H.; Yamamoto, H. Chem. Commun. 2011, 47, 3043 and
references cited therein.
(15) The pKa values were cited from ref 17 or calculated using ACE and
JChem acidity and basicity calculator (https://epoch.uky.edu/ace/
public/pKa.jsp).
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.6b00918.
Supplemental data, experimental procedures, character-
ization data, and NMR spectral charts (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: tanaka@orgsynth.che.tohoku.ac.jp.
*E-mail: hattori@orgsynth.che.tohoku.ac.jp.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
T.H. thanks Kazuo Tanaka (the Koheleth private foundation) for
financial support.
■
(16) (a) Brown, H. C.; McDaniel, D. H.; Haflinger, O. In Determination
̈
REFERENCES
■
of Organic Structures by Physical Methods; Braude, E. A., Nachod, F. C.,
Eds.; Academic Press: New York, 1955; Vol. 1, pp 567−662. (b) Albert,
A.; Phillips, J. N. J. Chem. Soc. 1956, 1294. (c) Clarke, K.; Rothwell, K. J.
Chem. Soc. 1960, 1885.
(1) Recent reviews: (a) Huang, K.; Sun, C.-L.; Shi, Z.-J. Chem. Soc. Rev.
2011, 40, 2435. (b) Cokoja, M.; Bruckmeier, C.; Rieger, B.; Herrmann,
W. A.; Kuhn, F. E. Angew. Chem., Int. Ed. 2011, 50, 8510. (c) Tsuji, Y.;
̈
Fujihara, T. Chem. Commun. 2012, 48, 9956. (d) Zhang, L.; Hou, Z.
Chem. Sci. 2013, 4, 3395. (e) Yu, B.; Diao, Z.-F.; Guo, C.-X.; He, L.-N. J.
CO2 Util. 2013, 1, 60. (f) Cai, X.; Xie, B. Synthesis 2013, 45, 3305.
(g) Liu, Q.; Wu, L.; Jackstell, R.; Beller, M. Nat. Commun. 2015, 6, 5933.
(2) Carboxylation of alkenes: (a) Williams, C. M.; Johnson, J. B.; Rovis,
T. J. Am. Chem. Soc. 2008, 130, 14936. (b) Greenhalgh, M. D.; Thomas,
S. P. J. Am. Chem. Soc. 2012, 134, 11900. (c) Ostapowicz, T. G.; Schmitz,
M.; Krystof, M.; Klankermayer, J.; Leitner, W. Angew. Chem., Int. Ed.
2013, 52, 12119. (d) Wu, L.; Liu, Q.; Fleischer, I.; Jackstell, R.; Beller, M.
Nat. Commun. 2014, 5, 3091.
(3) Carboxylation of allenes: (a) Takaya, J.; Iwasawa, N. J. Am. Chem.
Soc. 2008, 130, 15254. (b) Tani, Y.; Fujihara, T.; Terao, J.; Tsuji, Y. J.
Am. Chem. Soc. 2014, 136, 17706. (c) Zhu, C.; Takaya, J.; Iwasawa, N.
Org. Lett. 2015, 17, 1814.
(17) (a) Hirsch, J. A. Topics in Stereochemistry 1967, 1, 199. (b) Jensen,
F. R.; Bushweller, C. H. Adv. Alicyclic Chem. 1971, 3, 139. (c) Eliel, E. L;
Wilen, S. H.; Mander, L. N. Stereochemistry of Organic Compounds;
Wiley: New York, 1994; pp 696−697. (d) Eliel, E. L.; Wilen, S. H.;
Doyle, M. P. Basic Organic Stereochemistry; Wiley: New York, 2001; pp
443−445.
(18) See Supporting Information for details.
(19) For the applicability of Friedel−Crafts acylation toward alkenes,
see: (a) Groves, J. K. Chem. Soc. Rev. 1972, 1, 73. (b) Nenitzescu, C. D.;
Balaban, A. T. In Friedel−Crafts and Related Reactions; Olah, G. A., Ed.;
Wiley: New York, 1964; Vol. 3, pp 1033−1152. (c) Hojo, M. Yuki Gosei
Kagaku Kyokaishi 1978, 36, 473 (Chem. Abstr. 1978:562584).
(d) Nenajdenko, V. G.; Balenkova, E. S. Arkivoc 2011, i, 246.
(4) Carboxylation of dienes: (a) Takimoto, M.; Mori, M. J. Am. Chem.
Soc. 2001, 123, 2895. (b) Takaya, J.; Sasano, K.; Iwasawa, N. Org. Lett.
2011, 13, 1698. (c) Mori, Y.; Mori, T.; Onodera, G.; Kimura, M.
Synthesis 2014, 46, 2287.
(5) Carboxylation of bisdienes: (a) Takimoto, M.; Mori, M. J. Am.
Chem. Soc. 2002, 124, 10008. (b) Takimoto, M.; Nakamura, Y.; Kimura,
K.; Mori, M. J. Am. Chem. Soc. 2004, 126, 5956.
(6) Carboxylation of o-hydroxystyrenes: Sasano, K.; Takaya, J.;
Iwasawa, N. J. Am. Chem. Soc. 2013, 135, 10954.
(7) Hydro-, sila-, and boracarboxylation of alkynes: (a) Fujihara, T.;
Xu, T.; Semba, K.; Terao, J.; Tsuji, Y. Angew. Chem., Int. Ed. 2011, 50,
523. (b) Li, S.; Yuan, W.; Ma, S. Angew. Chem., Int. Ed. 2011, 50, 2578.
(c) Fujihara, T.; Tani, Y.; Semba, K.; Terao, J.; Tsuji, Y. Angew. Chem.,
D
DOI: 10.1021/acs.orglett.6b00918
Org. Lett. XXXX, XXX, XXX−XXX