Lewis acid-mediated β-selective hydrocarboxylation of α,α-diaryl- and α-arylalkenes with R3SiH and CO2

Article abstract of DOI:10.1016/j.tetlet.2015.04.090

α,α-Diarylalkenes are successfully hydrocarboxylated with Et₃SiH and CO₂ with the aid of Et₃SiB(C₆F₅)₄ or EtAlCl₂/Ph₃SiCl to give carboxylic acids with a carboxy group at the β-position to the aryl groups. The EtAlCl₂/Ph₃SiCl-mediated reaction is also applicable to various α-arylalkenes. ¹H NMR analysis of a mixture of EtAlCl₂, Ph₃SiCl, and Et₃SiH strongly suggests the formation of a μ-H complex, [Ph₃Si-H-SiEt₃]⁺ AlEtCl₃^-, which is an equivalent of R₃Si⁺ ions, while Et₃SiB(C₆F₅)₄ is an ion pair with a Et₃Si⁺ ion. Therefore, in these reaction systems, a siloxycarbonylium, R₃SiOCO⁺, is considered to be a common electrophile, the addition of which to the substrate, followed by trapping of the resulting cationic species with Et₃SiH seems to afford the desired acid after aqueous workup.

Full text of DOI:10.1016/j.tetlet.2015.04.090

Accepted Manuscript
Lewis acid-mediated β-Selective hydrocarboxylation of α,α-diaryl-α and -ary-
lalkenes with R3SiH and CO2
Shinya Tanaka, Yuuki Tanaka, Masafumi Chiba, Tetsutaro Hattori
PII:
S0040-4039(15)00737-6
DOI:
http://dx.doi.org/10.1016/j.tetlet.2015.04.090
TETL 46230
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
3 March 2015
16 April 2015
21 April 2015
Please cite this article as: Tanaka, S., Tanaka, Y., Chiba, M., Hattori, T., Lewis acid-mediated β-Selective
hydrocarboxylation of α,α-diaryl-α and -arylalkenes with R3SiH and CO2, Tetrahedron Letters (2015), doi: http://
dx.doi.org/10.1016/j.tetlet.2015.04.090
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Lewis Acid-Mediated β-Selective
Hydrocarboxylation of α,α-Diaryl- and
α-Arylalkenes with R3SiH and CO2
Shinya Tanaka,a, * Yuuki Tanaka,a Masafumi Chiba,a and Tetsutaro Hattori a, *

1
Tetrahedron Letters
journal homepage: www.els evier.com
Lewis Acid-Mediated β-Selective Hydrocarboxylation of α,α-Diaryl- and
α-Arylalkenes with R3SiH and CO2
Shinya Tanaka,a,∗ Yuuki Tanaka,a Masafumi Chiba,a Tetsutaro Hattori a,∗
aDepartment of Biomolecular Engineering, Graduate School of Engineering, Tohoku University, 6-6-11 Aramaki-Aoba, Aoba-ku, Sendai 980-8579, Japan
ARTICLE INFO
ABSTRACT
Article history:
Received
α,α-Diarylalkenes are successfully hydrocarboxylated with Et3SiH and CO2 with the aid of
Et3SiB(C6F5)4 or EtAlCl2/Ph3SiCl to give carboxylic acids with a carboxy group at the β-position
to the aryl groups. The EtAlCl2/Ph3SiCl-mediated reaction is also applicable to various α-
arylalkenes. 1H NMR analysis of a mixture of EtAlCl2, Ph3SiCl, and Et3SiH strongly suggests the
formation of a µ-H complex, [Ph3Si–H–SiEt3]+ AlEtCl3−, which is an equivalent of R3Si+ ions,
while Et3SiB(C6F5)4 is an ion pair with a Et3Si+ ion. Therefore, in these reaction systems, a
siloxycarbonylium, R3Si–O–C≡O+, is considered to be a common electrophile, the addition of
which to the substrate, followed by trapping of the resulting cationic species with Et3SiH seems
to afford the desired acid after aqueous workup.
Received in revised form
Accepted
Available online
Keywords:
Carbon dioxide
Lewis acid
Hydrocarboxylation
Alkene
2009 Elsevier Ltd. All rights reserved.
Chlorosilane
Carbon dioxide (CO2) is an inexpensive and renewable carbon
source and, therefore, the development of methods to use CO2 as
a C1 feedstock is highly demanded. In recent years, the
development of transition metal-catalyzed carboxylation has
advanced remarkably, which improved the substrate applicability
of aromatic compounds.1 However, successful examples for the
carboxylation of alkenes are still limited,2 except for specific
alkenes such as allenes,3 dienes,4 bisdienes,5 and o-
hydroxystyrenes,6 because alkenes are readily decomposed,
polymerized, and/or isomerized under the reaction conditions.7
methods to functionalizing alkenes and succeeded in the
hydrocarboxylation of α-aryl- and α,α-diarylalkenes, in which a
carboxy group is introduced at the β-position to the aryl group(s).
Although the α-selective hydrocarboxylation of α-arylalkenes
has been reported,2a,b this is, to the best of our knowledge, the
first example to achieve β-selectivity.
Initially, 1,1-diphenylpropene (1a) was hydrocarboxylated by
treating with Et3SiH under CO2 pressure (3.0 MPa) in the
13
presence of a benzene complex of Et3SiB(C6F5)4 in benzene at
room temperature (entry 1 in Table 1).14 The reaction proceeded
with complete regioselectivity to give 2-methyl-3,3-
diphenylpropanoic acid (2a) in a good yield (65%). Other
trialkylsilyliums exhibited similar performances (entries 2 and 3),
while triphenylsilylium was less effective (entry 4). Changing the
solvent to hexane slightly improved the yield (entry 5). As for the
reductant, i-Pr3SiH could be used without appreciable loss of the
acid yield (entry 6), while Ph3SiH reduced the yield (entry 7).
Lowering the temperature to 0 °C was effective (entry 8). Finally,
the best yield (78%) was achieved by using a small excess of
Et3SiH over the triethylsilylium borate in hexane at 0 °C (entry
9). It has been reported that silylium ions readily form µ-H
complexes in the presence of hydrosilanes.15 We then prepared a
µ-H complex beforehand and subjected it to the
hydrocarboxylation using no external reductant. The reaction
gave acid 2a although in a reduced yield (entry 10 as compared
with entry 8).
It is known that aromatic compounds are carboxylated with
CO2 in the presence of an aluminum-based Lewis acid to give
aromatic carboxylic acids.8 The reaction is believed to proceed
via the SEAr mechanism, in which CO2 is activated by a Lewis
acid and the resulting active CO2 species serves as an
electrophile.8d However, the yields of carboxylic acids are
generally poor because of the low electrophilicity of CO2 and/or
side reactions caused by the strong Lewis acidity of aluminum-
based compounds.8d–f In this respect, we previously reported that
the combined use of AlBr3 and R3SiCl efficiently promotes the
carboxylation.9 Mechanistic studies revealed that
a silyl
haloformate-like species is generated in situ from CO2 and
R3SiCl with the aid of AlBr3. Therefore, a siloxycarbonylium,
≡O+
R3Si–O–C
, is considered to be the actual electrophile in this
reaction. This consideration led us to investigate the direct
activation of CO2 with silylium ions, which resulted in the
development of R3SiB(C6F5)4-mediated carboxylation.10–12 In this
study, we investigated the application of these CO2 activation
———
The hydrocarboxylation of various α,α-diarylalkenes was
conducted under the optimal conditions (Scheme 1). The reaction
∗ Corresponding author. Tel./fax: +81-22-795-7263; e-mail: tanaka@orgsynth.che.tohoku.ac.jp
∗ Corresponding author. Tel./fax: +81-22-795-7262; e-mail: hattori@orgsynth.che.tohoku.ac.jp

2
Tetrahedron
Table 1. Hydrocarboxylation of 1,1-diphenylpropene with the aid of
silylium boratesa
other than AlBr3 were employed. Changing the solvent to
chlorobenzene–hexane and hexane did not affect much on the
acid yield (entries 5 and 6). Increasing the temperature to 80 °C
improved the yield (entry 7). The molar ratio of Ph3SiCl to
EtAlCl2 (y/x) was varied within the range of 0–2.0, among which
the ratio of 1.5 was most effective (entries 7–11). Finally, the
best yield (80%) was achieved by increasing the molar
equivalence of EtAlCl2 to the substrate (x) to 2.5 with keeping
the molar ratio y/x (entry 12).
entry
1
R
reductant
Et3SiH
solvent
temp. (°C)
r.t.
yield (%)b
65
Et
benzene
2
3
4
5
6
7
8
9c
i-Pr
Hex
Ph
Et
Et3SiH
Et3SiH
Et3SiH
Et3SiH
i-Pr3SiH
Ph3SiH
Et3SiH
Et3SiH
–
benzene
benzene
benzene
hexane
hexane
hexane
hexane
hexane
hexane
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
0
62
59
22
71
69
56
73
78
51
Table 2. Hydrocarboxylation of 1,1-diphenylethylene by combined use of
aluminum-based Lewis acids and Ph3SiCla
Et
Et
Et
entry
Lewis
acid
x
y/x
Solvent
temp.
(°C)
60
yield
(%)b
8d
Et
0
10d,e
a
–
0
1
2
3
4
5
AlBr3
1.25
1.25
1.25
1.25
1.25
1.5
1.5
1.5
1.5
1.5
toluene
Reaction conditions: 1a (0.30 mmol), silylium borate (0.20 mmol),
reductant (0.20 mmol), CO2 (3.0 MPa), solvent (1.0 mL).
b Isolated yield based on the quantity of silylium borate.
c Et3SiH (0.24 mmol) was used.
AlCl3
toluene
toluene–hexanec
toluene–hexanec
60
60
60
60
43
53
10
50
EtAlCl2
Et2AlCl
Et2AlCl
chlorobenzene–
hexanec
hexanec
d (Et3Si)2(µ-H)B(C6F5)4 was used as a silylium borate-cum-reductant.
e Compound 1a (0.60 mmol) was used.
6
Et2AlCl
EtAlCl2
EtAlCl2
EtAlCl2
EtAlCl2
EtAlCl2
EtAlCl2
1.25
1.25
1.25
1.25
1.25
1.25
2.5
1.5
1.5
0
60
80
80
80
80
80
80
50
7
toluene–hexanec
toluene–hexanec
toluene–hexanec
toluene–hexanec
toluene–hexanec
toluene–hexanec
66
of terminal alkene 1b resulted in the decomposition of the
substrate, while internal alkenes bearing a phenyl group (1c) and
two methyl groups (1d) at the β-position gave the corresponding
acids 2c and 2d, respectively, although in poor yields. Better
yields were obtained for cyclic α,α-diarylalkenes 1e–g. The
stereochemistries of the resulting acids 2e–g were assigned to be
cis by NOESY analysis. This is probably because the attack of
the hydrosilane to a cationic intermediate, generated in situ by
the addition of an active CO2 species, i.e. a siloxycarbonylium
(vide infra), to the substrate occurred from the opposite side to
the ester group to avoid steric repulsion.
8
54
9
0.5
1.0
2.0
1.5
49
10
11
12
53
63
80 (77)
a Reaction conditions: 1a (0.40 mmol), Lewis acid (x mol equiv), Ph3SiCl
(y mol equiv), Et3SiH (0.60 mmol), CO2 (3.0 MPa), solvent (1.5–2.5 mL).
b
1H NMR yield determined using CH2Br2 as an internal standard. Isolated
yield is shown in the parenthesis.
c Lewis acid was used as a hexane solution and, therefore, hexane was
included in the reaction mixture.
d Toluic acid (0.032 mmol, o- : p- = 1:4) was also obtained.
The hydrocarboxylation of various arylalkenes was then
conducted under the optimized conditions (Scheme 2). Acyclic
α,α-diarylalkene 1a with a methyl group at the β-position and
cyclic α,α-diarylalkenes 1e–1g with no substituent at the β-
position gave the corresponding acids 2a and 2e–2g, respectively,
in good yields; the stereochemistries of the cyclic acids 2e–2g
were determined to be cis by NOESY analysis. On the other
hand, acid yields were considerably reduced for more bulky α,α-
diarylalkenes with a phenyl group (1c) and two methyl groups
(1d) at the β-position. It should be noted that both acyclic (1j)
and cyclic alkenes 1h and 1i with only one aryl substituent at the
α-position could be hydrocarboxylated in good yields; each of
acids 2h and 2i was obtained as a cis/trans mixture containing
the cis isomer as the major component. It seems that in the
reaction of compounds 1h–1j, the attack of the hydrosilane to a
cationic intermediate, generated in situ by the addition of an
active CO2 species or EtAlCl2 to the substrate (vide infra),
occurred rather unselectively from both sides of the alkene plane
due to the low stability of the cationic intermediate bearing an
alkyl group at the α position instead of a phenyl group. As
Scheme 1. Hydrocarboxylation of 1,1-diarylalkenes with the aid of
Et3SiB(C6F5)4. Reaction conditions: 1 (0.30 mmol), Et3Si(C6H6)B(C6F5)4
(0.20 mmol), Et3SiH (0.24 mmol), CO2 (3.0 MPa), hexane (0.50 mL). Yields
are based on the quantity of the silylium borate. a 1H NMR yield determined
using CH2Br2 as an internal standard.
Next, we investigated the hydrocarboxylation of arylalkenes
by combined use of an aluminum-based Lewis acid and
Ph3SiCl.16 Contrary to the silylium borate-mediated reaction,
terminal alkene 1b gave the corresponding acid 2b (8%), when
treated with Et3SiH under CO2 pressure (3.0 MPa) in the
presence of AlBr3 and Ph3SiCl in toluene at 60 °C (entry 1 in
Table 2). The reaction was accompanied by the formation of
toluic acid, apparently due to the carboxylation of the solvent
with the aid of AlBr3 and Ph3SiCl. Interestingly, EtAlCl2 was
most effective among the Lewis acids tested (entries 1–4). This
shows a sharp contrast to the carboxylation of arenes,9 in which
AlBr3 with stronger Lewis acidity exhibited better performance
than EtAlCl2. Toluic acid was not obtained when Lewis acids
mentioned
hydrocarboxylation exhibited wide applicability toward not only
above,
the
EtAlCl2/Ph3SiCl-mediated
α,α-diarylalkenes but also α-aryllalkenes.
A feasible mechanism for the silylium borate-mediated
hydrocarboxylation is illustrated in Scheme 3 as represented by
the reaction using Et3Si(C6H6)B(C6F5)4 and Et3SiH. As in the
case of the carboxylation of aromatic compounds,10,12 the

3
benzene complex of Et3SiB(C6F5)4 or the µ-H complex generated
in situ from the benzene complex and Et3SiH activates CO2 to
form siloxycarbonylium A. This active species electrophilically
adds to an alkene, and the resulting dibenzyl cation is trapped by
Et3SiH to afford a silyl ester, which is decomposed by aqueous
workup to give the corresponding carboxylic acid. The formation
of the stable dibenzyl cation intermediate determines the
regiochemistry of the reaction.
The resulting Ph3SiH was disproportionated into Ph4Si and
Ph2SiH2 through µ-H complexes C and D with the aid of
1
Ph3SiCl.17 In contrast, H NMR spectrum of a mixture of Ph3SiCl
and Et3SiH after stirring for 3 h at room temperature showed only
the signal of Et3SiH as the Si–H signal (Figure 1e), indicating
that EtAlCl2 is indispensable for the ligand exchange.
Interestingly, 1H NMR spectrum of a mixture of EtAlCl2,
Et3SiCl, and Ph3SiH measured after stirring for 1 h at room
temperature (Figure 1f) exhibited the Si–H signals of Et3SiH and
Ph2SiH2, indicating the formation of Et3SiH and Ph3SiCl by the
ligand exchange between Et3SiCl and Ph3SiH. Indeed, the
hydrocarboxylation of compound 1b conducted under the same
conditions as employed in entry 12 (Table 2), except Et3SiCl and
Ph3SiH were used instead of Ph3SiCl and Et3SiH, afforded acid
2b in a comparable yield (72%) to that achieved in entry 12. On
the other hand, under the same conditions, the combined use of
Et3SiCl and Et3SiH gave 54% yield, which is lower than the yield
obtained only by using Et3SiH (63%), that is, no beneficial effect
of Et3SiCl was observed. On the basis of these results, a feasible
mechanism for the EtAlCl2/R3SiCl- mediated hydrocarboxylation
is proposed as follows: When either the chlorosilane or
R1
R1
CO2
EtAlCl2, Ph3SiCl
R3
or
Ph
Et3SiH
toluene–hexane
80 ºC, 24 h
R2
n
R2
1a
1c
1d
: n = 1, R1 = Ph, R2 = H
: R1 = Ph, R2 = Me, R3 = H
: R1 = R2 = Ph, R3 = H
: R1 = Ph, R2 = R3 = Me
1e
1f
1g
1h
1i
: n = 1, R1 = Ph, R2 = Br
: n = 2, R1 = Ph, R2 = H
: n = 2, R1 = Me, R2 = H
1j a
:
R1 = R3 = Me, R2 = H
: n = 1, R1 = i-Pr, R2 = H
Ph
Ph
Me
CO2H
CO2H
Ph
CO2H
Ph
2a
Ph
Me Me
R
Me
b,c
2d
,
, 27%
2j
, 74%
: R = Me 82%
2c
: R = Ph, 24%
Ph
n
R
CO2H
CO2H
R2
n
b,d
2e
2h
: R = Me, n = 2, 75%
: n = 1, R = H, 82%
b,e
2i
2f
: R = i-Pr, n = 1, 82%
: n = 1, R = Br, 77%
b
2g
: n = 2, R = H, 85%
Scheme 2. Hydrocarboxylation of alkenes with the aid of EtAlCl2 and
Ph3SiCl. Reaction conditions: 1 (0.40 mmol), EtAlCl2 (1.0 mmol), Ph3SiCl
(1.5 mmol), Et3SiH (0.60 mmol), CO2 (3.0 MPa), toluene (1.5 mL). Yields
are based on the quantity of substrate. a 1j (E : Z = 95:5) was used. b 1H NMR
yield
determined
using
CH2Br2
as
an
internal
standard.
c (2RS,3RS):(2RS,3SR) = 17:83. d cis : trans = 70:30. e cis : trans = 77:23.
Figure 1. Expanded 1H NMR spectra of (a) a mixture of EtAlCl2, Ph3SiCl,
and Et3SiH (1.0:1.5:0.6), (b) Et3SiH, (c) Ph3SiH, (d) Ph2SiH2, (e) a mixture of
Ph3SiCl, and Et3SiH, and (f) a mixture of EtAlCl2, Et3SiCl, and Ph3SiH
(1.0:1.5:0.6) in toluene-d8–hexane (3:2).
Scheme 3. Feasible mechanism for the Et3SiB(C6F5)4-mediated
hydrocarboxylation.
On the other hand, in the EtAlCl2/Ph3SiCl-mediated
hydrocarboxylation, a beneficial effect of Ph3SiCl was observed
(entries 7 and 9–11 as compared with entry 8 in Table 2). This
indicates that CO2 was activated with the aid of Ph3SiCl. In order
to obtain better insight into the activation process, 1H NMR
analysis was carried out. Figure 1a shows 1H NMR spectrum of a
mixture of EtAlCl2, Ph3SiCl, and Et3SiH (1.0:1.5:0.6) measured
after stirring for 3 h at room temperature. The spectrum exhibited
two singlets at 5.50 and 4.92 ppm, which are assignable to
hydrogen atoms on silicon. The former and latter signals are
different from the Si–H signal of Et3SiH in both chemical shift
and splitting pattern (Figure 1b), but are very close to the Si–H
signals of Ph3SiH and Ph2SiH2, respectively (Figure 1c and 1d).
Moreover, stirring the solution for a further 9 h gave Ph4Si as a
colorless precipitate. These observations strongly suggests that µ-
H complex B was generated from Ph3SiCl, Et3SiH, and EtAlCl2
through a five-coordinated transition state, and was fragmented
into Ph3SiH, Et3SiCl, and EtAlCl2 through a similar transition
state; the overall reaction results in the ligand exchange between
the chlorine of Ph3SiCl and the hydrogen of Et3SiH (Scheme 4).15
Ph
Ph
Et
Et
+
EtAlCl2
Ph3SiCl + Et3SiH
Al Cl Si
H
Si
Cl
Et
Ph
Cl
Et
Ph
Et
Ph
Ph
Cl
Cl
Et
+
Ph
AlEtCl3
Si
H
Si
Si
H
Si Cl Al
Et
Et
Ph
Ph
Et
Et
Et
B
Ph3SiH
+ Et3SiCl
– EtAlCl2
Ph
Ph
Ph
Ph3SiH
– Ph4Si
EtAlCl2
Ph3SiCl + Ph3SiH
AlEtCl3
Si
H
Si
Ph
Ph
Ph
C
Ph
H
Ph
AlEtCl3
Ph3SiCl + Ph2SiH2
Si
H
Si
Ph
Ph
– EtAlCl2
Ph
D
Scheme 4. Feasible mechanism for the ligand exchange between Ph3SiCl and
Et3SiH and the disproportionation of Ph3SiH.

4
Tetrahedron
5. (a) Takimoto, M.; Mori, M. J. Am. Chem. Soc. 2002, 124, 10008–
10009; (b) Takimoto, M.; Nakamura, Y.; Kimura, K.; Mori, M. J.
Am. Chem. Soc. 2004, 126, 5956–5957.
hydrosilane has a Ph3Si group, the relevant Si–X bond (X = Cl
or H) is highly polarized, so that a µ-H complex such as B can be
formed. However, not only the electronic effects but also the
steric effects of the substituents on silicon seem to affect the
efficiency of producing a µ-H complex, as indicated by the fact
that the combined use of Ph3SiCl/Ph3SiH and Ph3SiCl/i-Pr3SiH
gave 67% and 41% yields, respectively. The resulting µ-H
complex activates CO2 to form siloxycarbonylium E (path a in
Scheme 5), as in the case of the Et3SiB(C6F5)4-mediated
hydrocarboxylation. The active CO2 species adds to the substrate
and the resulting dibenzyl cation is trapped with a hydrosilane to
deliver the hydrocarboxylated product after aqueous workup. In
the reaction conducted in the absence of a chlorosilane (entry 8 in
Table 2), EtAlCl2 seems to activate CO2 instead of the µ-H
complex (path b). An alternative mechanism is that an alkene is
electrophilically metalated with EtAlCl2 and the resulting
cationic species is reduced by Et3SiH to give an
organoaluminum, which is carbonated to afford a carboxylic acid
(path c).18 The latter two paths may be involved also in the
reaction conducted in the presence of Ph3SiCl.
6. Sasano, K.; Takaya, J.; Iwasawa, N. J. Am. Chem. Soc. 2013, 135,
10954–10957.
7. For indirect routes via hydroboration of alkenes, see: (a) Ohishi,
T.; Zhang, L.; Nishiura, M.; Hou, Z. Angew. Chem. Int. Ed. 2011,
50, 8114–8117; (b) Ohmiya, H.; Tanabe, M.; Sawamura, M. Org.
Lett. 2011, 13, 1086–1088.
8. (a) Friedel, C.; Crafts, J. M. Compt. Rend. 1878, 86, 1368–1371;
(b) Norris, J. F.; Wood, III, J. E. J. Am. Chem. Soc. 1940, 62,
1428–1432; (c) Ito, T.; Sugahara, N.; Kindaichi, Y.; Takami, Y.
Nippon Kagaku Kaishi 1976, 353–355 (Chem. Abstr. 1976,
421589); (d) Olah, G. A.; Török, B.; Joschek, J. P.; Bucsi, I.;
Esteves, P. M.; Rasul, G.; Prakash, G. K. S. J. Am. Chem. Soc.
2002, 124, 11379–11391; (e) Munshi, P.; Beckman, E. J. Ind. Eng.
Chem. Res. 2009, 48, 1059–1062; (f) Munshi, P.; Beckman, E. J.;
Padmanabhan, S. Ind. Eng. Chem. Res. 2010, 49, 6678–6682.
9. (a) Suzuki, Y.; Hattori, T.; Okuzawa, T.; Miyano, S. Chem. Lett.
2002, 102–103; (b) Hattori, T.; Suzuki, Y.; Miyano, S. Chem. Lett.
2003, 454–455; (c) Nemoto, K.; Yoshida, H.; Suzuki, Y.;
Morohashi, N.; Hattori, T. Chem. Lett. 2006, 820–821; (d)
Nemoto, K.; Yoshida, H.; Egusa, N.; Onozawa, S.; Morohashi, N.;
Hattori, T. J. Org. Chem. 2010, 75, 7855–7862.
10. Konno, M.; Chiba, M.; Nemoto, K.; Hattori, T. Chem. Lett. 2012,
41, 913–914.
11. At nearly the same time as we reported the R3SiB(C6F5)4-mediated
carboxylation, Müller et al. also reported the carboxylation of
benzene in the presence of Ph3CB(C6F5)4 and Et3SiH: Schäfer, A.;
Saak, W.; Haase, D.; Müller, T. Angew. Chem. Int. Ed. 2012, 51,
2981–2984.
12. In our previous report on the carboxylation of aromatic
compounds,10 Et3SiB(C6F5)4 was prepared from Et3SiH and
Ph3CB(C6F5)4, according to Lambert’s method.13b Later on, we
realized that it was
a
µ-H complex.15b We then prepared
Et3Si(C6H6)B(C6F5)4 according to Müller’s method,11 and
confirmed that it mediates the carboxylation of aromatic
compounds.
13. (a) Lambert, J. B.; Zhang, S. J. Chem. Soc., Chem. Commun.
1993, 383–384; (b) Lambert, J. B.; Zhang, S.; Ciro, S. M.
Organometallics 1994, 13, 2430–2443; (c) Lambert, J. B.; Zhang,
S.; Stern, C. L.; Huffman, J. C. Science 1993, 260, 1917–1918.
Scheme 5. Feasible mechanism for the EtAlCl2/R3SiCl-mediated
hydrocarboxylation.
14. Typical
procedure
for
the
Et3SiB(C6F5)4-mediated
hydrocarboxylation of alkenes (entry 9 in Table 1): In a 50 mL
autoclave equipped with a glass inner tube and a magnetic stirring
bar were charged freshly prepared Et3Si(C6H6)B(C6F5)4 (175 mg,
0.20 mmol), dry hexane (1.0 mL), compound 1a (58.0 mg, 0.30
mmol), and Et3SiH (d 0.73; 38 µL, 0.24 mmol) under nitrogen,
and the apparatus was purged with CO2 by repeated pressurization
and subsequent expansion; the final pressure was adjusted to 3.0
MPa. After the mixture was stirred at 0 °C for 1 h, the reactor was
depressurized, and the mixture was quenched with 2 M HCl (10
mL) and extracted with ethyl acetate. The organic layer was
extracted with 0.5 M Na2CO3 and the extract was acidified with
concentrated HCl to liberate the free acid, which was extracted
with ethyl acetate. The extract was dried over MgSO4 and
evaporated to leave a residue, which was purified by column
chromatography with chloroform/methanol (10:1) containing 1
vol% of acetic acid as an eluent to afford acid 2a (37.8 mg, 78%).
15. Silylium ions are unstable and readily form benzene complexes
and µ-H complexes: (a) Hoffmann, S. P.; Kato, T.; Tham, F. S.;
Reed, C. A. Chem. Commun. 2006, 767; (b) Nava, M.; Reed, C. A.
Organometallics 2011, 30, 4798–4800; (c) Connelly, S. J.;
Kaminsky, W.; Heinekey, D. M. Organometallics 2013, 32, 7478–
7481.
In conclusion, we have shown that arylalkenes are
hydrocarboxylated with Et3SiH and CO2 in the presence of
Et3SiB(C6F5)4 or EtAlCl2/Ph3SiCl. The present methods provide
the first example for the β-selective hydrocarboxylation of
arylalkenes. It is expected that the method to activate CO2 with
EtAlCl2, Ph3SiCl, and Et3SiH is applicable to the carboxylation
of other unsaturated compounds, which are intolerant of strong
Lewis acids such as AlBr3. Further investigation along this line is
in progress.
References and notes
1. For recent reviews: (a) Huang, K.; Sun, C.-L.; Shi, Z.-J. Chem.
Soc. Rev. 2011, 40, 2435–2452; (b) Tsuji, Y.; Fujihara, T. Chem.
Commun. 2012, 48, 9956–9964; (c) Zhang, L.; Hou, Z. Chem. Sci.
2013, 4, 3395–3403; (d) Yu, B.; Diao, Z.-F.; Guo, C.-X.; He, L.-
N. J. CO2 Utilization 2013, 1, 60–68; (e) Cai, X.; Xie, B. Synthesis
2013, 45, 3305–3324.
2. (a) Williams, C. M.; Johnson, J. B.; Rovis, T. J. Am. Chem. Soc.
2008, 130, 14936–14937; (b) Greenhalgh, M. D.; Thomas, S. P. J.
Am. Chem. Soc. 2012, 134, 11900–11903; (c) Shirakawa, E.;
Ikeda, D.; Masui, S.; Yoshida, M.; Hayashi, T. J. Am. Chem. Soc.
2012, 134, 272–279; (d) Ostapowicz, T. G.; Schmitz, M.; Krystof,
M.; Klankermayer, J.; Leitner, W. Angew. Chem. Int. Ed. 2013,
52, 12119–12123; (e) Wu, L.; Liu, Q.; Fleischer, I.; Jackstell, R.;
Beller, M. Nat. Commun. 2014, 5, 3091.
16. Typical
procedure
for
the
EtAlCl2/Ph3SiCl-mediated
hydrocarboxylation of alkenes (entry 12 in Table 2): In a 50 mL
autoclave equipped with a glass inner tube and a magnetic stirring
bar were charged Ph3SiCl (442 mg, 1.50 mmol), toluene (1.5 mL),
Et3SiH (d 0.73; 96 µL, 0.60 mmol), compound 1b (d 1.03; 70 µL,
0.40 mmol), and EtAlCl2 (1.0 M solution in hexane; 1.0 mL, 1.00
mmol) in this order under nitrogen. The apparatus was purged
with CO2 by repeated pressurization and subsequent expansion;
the final pressure was adjusted to 3.0 MPa. After the mixture was
stirred at 80 °C for 24 h, the reactor was depressurized and the
mixture was quenched with 2 M HCl (10 mL). The mixture was
worked up and purified as mentioned above to afford acid 2b
(70.0 mg, 77%).
3. Takaya, J.; Iwasawa, N. J. Am. Chem. Soc. 2008, 130, 15254–
15255.
4. (a) Takimoto, M.; Mori, M. J. Am. Chem. Soc. 2001, 123, 2895–
2896; (b) Takaya, J.; Sasano, K.; Iwasawa, N. Org. Lett. 2011, 13,
1698–1701; (c) Mori, Y.; Mori, T.; Onodera, G.; Kimura, M.
Synthesis 2014, 46, 2287–2292.

5
17. For R3SiB(C6F5)4–mediated disproportionation of R3SiH, see:
Labbow, R.; Reiß, F.; Schulz, A.; Villinger, A. Organometallics
2014, 33, 3223.
18. Nemoto, K.; Onozawa, S.; Egusa, N.; Morohashi, N.; Hattori, T.
Tetrahedron Lett. 2009, 50, 4512–4514.