Carboxylation of alkylboranes by N-heterocyclic carbene copper catalysts: Synthesis of carboxylic acids from terminal alkenes and carbon dioxide

Article abstract of DOI:10.1002/anie.201101769

Caught in the act: N-Heterocyclic carbene copper(I) complexes (1; IPr=1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) serve as an excellent catalyst for the carboxylation of alkylboranes (2; R=alkyl) with CO₂ to afford a variety of functionalized carboxylic acids (3) in high yields. A novel copper methoxide/alkylborane adduct (A) and its subsequent CO₂ insertion product (B) have been isolated and shown to be true active catalyst species.

Full text of DOI:10.1002/anie.201101769

Communications
DOI: 10.1002/anie.201101769
Carbon Dioxide Fixation
Carboxylation of Alkylboranes by N-Heterocyclic Carbene Copper
Catalysts: Synthesis of Carboxylic Acids from Terminal Alkenes and
Carbon Dioxide**
Takeshi Ohishi, Liang Zhang, Masayoshi Nishiura, and Zhaomin Hou*
À
The use of carbon dioxide (CO2) as a C1 building block for C
insight into the mechanistic aspect of the catalytic process.
Some preliminary results of this work were disclosed in
2010.[8] During the preparation of this manuscript, the
carboxylation of alkylboranes catalyzed by a CuOAc/1,10-
phenanthroline system was reported by Sawamura and co-
workers.[8,9]
C
bond-formation reactions such as carboxylation has
recently received much attention.[1–4] Among the most
promising and attractive substrates for the carboxylation
reaction with CO2 are alkylboron compounds given their ease
of availability, functional group tolerance, and substrate
scope. However, the catalytic carboxylation of alkylboranes
with CO2 has hardly been explored to date, which is in
contrast with recent successful carboxylation of aryl- and
alkenylboronic esters catalyzed by various transition-metal
catalysts.[2g–i] The fewer number of reports on carboxylation of
alkylboranes could probably be because of the difficulty in
generating a transition metal–alkyl species that has the
appropriate stability and high activity (toward CO2) from an
alkylborane compound. Indeed, although alkylboron com-
pounds have been used for various transition-metal-catalyzed
cross-coupling reactions, information on the interaction
between an alkylboron compound and a transition-metal
species has remained very limited.[5,6] The isolation of a
structurally characterizable intermediate in the transmetala-
tion of an organoboron compound to a transition metal is
scarce.[7]
We report herein that N-heterocyclic carbene (NHC)
copper complexes can serve as excellent catalyst systems for
the carboxylation of alkylboron compounds with CO2. The
reaction can be carried out easily in one pot by hydroboration
of terminal alkenes with 9-borabicyclo[3.3.1]nonane (9-BBN-
H) and subsequent carboxylation of the resulting alkylbor-
anes with CO2 in the presence of a NHC–copper complex.
More remarkably, a novel copper methoxide/alkylborane
adduct and its subsequent CO2 insertion product have been
isolated and structurally characterized, thus providing new
At first, the reaction of an alkylborane compound 1a,
which was generated in situ from the hydroboration reaction
of 3-(4- methoxyphenyl)propylene with 9-BBN-H, was exam-
ined using [(IPr)CuCl] (IPr= 1,3-bis(2,6-diisopropylphenyl)i-
midazol-2-ylidene) as a catalyst.[2i] In the presence of 1 mol%
[(IPr)CuCl], 1.05 equivalents of tBuOK, and CO2 (1 atm) in
THF at 708C, the carboxylation product 4-(4-methoxyphe-
nyl)butanoic acid (2a) was obtained in 35% yield after
24 hours (Table 1, entry 1). This yield is much lower than
those (almost quantitative) obtained in the analogous car-
boxylation of aryl- and alkenylboronic esters under similar
reaction conditions,[2i] thus showing that alkylborane is less
effective for the carboxylation with CO2. The use of MeOLi, a
less bulky base, instead of tBuOK gave a notably higher yield
(Table 1, entry 4). To our delight, when 3 mol% [(IPr)CuCl]
was used in the presence of 1.05 equivalents of LiOMe, the
carboxylation product 2a was obtained in almost quantitative
yield (Table 1, entry 5). MeONa and MeOK are also as
effective as LiOMe (Table 1, entries 6 and 7). The use of a less
Table 1: Carboxylation of alkylborane 1a with carbon dioxide.[a]
[*] Dr. T. Ohishi,[+] Dr. L. Zhang,[+] Dr. M. Nishiura, Prof. Dr. Z. Hou
Organometallic Chemistry Laboratory and Advanced Catalyst
Research Team, RIKEN Advanced Science Institute
2-1 Hirosawa, Wako, Saitama 351-0198 (Japan)
E-mail: houz@riken.jp
Entry
[Cu] (mol%)
Base
Yield [%][b]
1
2
3
4
5
6
7
8
9
[(IPr)CuCl] (1)
[(IPr)CuCl] (1)
–
[(IPr)CuCl] (1)
[(IPr)CuCl] (3)
[(IPr)CuCl] (3)
[(IPr)CuCl] (3)
[(IMes)CuCl] (3)
[(IPr)Cu(OAc)2](3)
[(IPr)CuCl] (3)
tBuOK
–
35
–
–
42
97
97
81
41
97
37
[+] These authors contributed equally to this work.
tBuOK
MeOLi
MeOLi
MeONa
MeOK
MeOLi
MeOLi
MeOLi[c]
[**] This work was partly supported by a Grant-in-aid for Scientific
Research (S) (No. 21225004) from the Ministry of Education,
Culture, Sports, Science and Technology of Japan and by the Key
Project of International Cooperation of the NSFC (20920102030).
L.Z. thanks JSPS for a postdoctoral fellowship. We are grateful to
Drs. Jianhua Cheng and Shihui Li for help in X-ray analyses, Ms.
Karube for elemental analysis, and Ms. Hongo for high resolution
mass spectroscopic analysis.
10
[a] Reaction conditions: 1a (1 mmol), [Cu] (mol%), base (1.05 mmol),
CO2 (1 atm), THF (5 mL), 708C, 24 h, unless otherwise noted. [b] Yield of
isolated product. [c] 0.5 mmol of MeOLi was used.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201101769.
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Angew. Chem. Int. Ed. 2011, 50, 8114 –8117

bulky copper catalyst such as [(IMes)CuCl] (IMes = 1,3-
bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) instead of
[(IPr)CuCl] led to a much lower yield (41%) of 2a (Table 1,
entry 8), probably owing to lower stability (or shorter life-
time) of the resultant active species. The CuII complex
[(IPr)Cu(OAc)2] was also effective (Table 1, entry 9). The
use of 0.5 mmol of MeOLi did not result in completion of the
reaction (Table 1, entry 10). In a control experiment, the
carboxylation reaction was not observed in the absence of
either [(IPr)CuCl] or the base under the same reaction
conditions (Table 1, entries 2 and 3, respectively).
reactive functional groups, such as propargyl, carbonyl, halide
(I, Br, Cl, F), and vinyl bromide survived the reaction
conditions (Table 2, entries 2–8 and 12–13). Synthetically
useful protecting groups, such as silyl ethers, N-tert-but-
oxycarbonyl, benzyl ether, and isopropylidene can also be
utilized under the present reaction conditions (Table 2,
entries 8–13). Alkylboranes with heteroaromatic motifs are
also applicable (Table 2, entries 9–11). In all the cases, the
reaction took place selectively to give the corresponding
carboxylation products in high yields (81–99%) upon iso-
lation. In the case of a tertiary substituent (Table 2, entry 14),
the carboxylation product was obtained in 60% yield at 708C,
but was increased to 70% at a higher temperature (1008C).
The one-pot reaction of styrene with 9-BBN-H and CO2
afforded the b-carboxylated product 2o in 65% yield upon
isolation (Table 2, entry 15). 1,1-Diphenylethylene was not
suitable for this reaction probably owing to steric bulk
(Table 2, entry 16).
We then examined the reactions of various functionalized
alkylboranes with CO2 by using [(IPr)CuCl] as the catalyst.
The results are summarized in Table 2. A wide variety of
Table 2: [(IPr)CuCl]-catalyzed carboxylation of various alkylboranes with
CO2.
To elucidate the mechanism of the current catalytic
process, several stoichiometric reactions were examined
(Scheme 1). When [IPrCu(OMe)],[10a] which was prepared
Entry
Alkene
Product 2[a]
2a (97)
1
2
3
4
2b (92)
2c (94)
2d (91)
5
6
7
2e (90)
2 f (91)
2g (82)
Scheme 1. Some stoichiometric reactions.
by the reaction of [IPrCuCl] with MeOLi, was mixed with
alkylborane 1a in THFat room temperature, the adduct 3 was
obtained almost instantly. Single crystals suitable for X-ray
crystallographic studies were obtained from THF/n-hexane
(1:1) at À308C. It was determined that 3 is formed by the
interaction between the oxygen atom of the methoxy group in
8
2h (99)
2i (97)
2j (81)
9
10
À
[IPrCu(OMe)] and the boron atom in 1a (Figure 1). The Cu
O bond distance in 3 (1.859(2) ꢀ) is significantly longer than
those in [IPrCu(OEt)] (1.799(3) ꢀ)[5e] and [IPrCu(OtBu)]
(1.8104(13) ꢀ),[10b] apparently owing to the coordination of
11
12
2k (92)
2l (95)
À
the MeO group to the boron atom in 3. The B O bond
distance (1.572(3) ꢀ) in 3 is shorter than that found in a 1-
boraadamantane·THF adduct (1.637(2) ꢀ).[10c] When 3 was
heated to 708C in [D6]C6H6, 3-(4-methoxyphenyl)propylene
was formed as shown by 1H NMR spectroscopy.[11] However,
when 3 was exposed to a CO2 atmosphere at 708C, the
carboxylate complex 4 was obtained in 90% yield (Scheme 1
and Figure 2), thus suggesting that a 3-(4-methoxyphenyl)-
propyl copper intermediate might be formed by transfer of
the alkyl group from the B atom to the Cu atom in 3.[12]
Alternatively, the carboxylate complex 4 could also be
obtained in 92% yield by the reaction of [IPrCu(OMe)] and
1a under a CO2 atmosphere. [IPrCu(OMe)], 3, and 4 all
13
2m (87)
14
15
16
2n (70)[b]
2o (65)[c]
–
[a] Yield of isolated product. [b] Hydroboration was carried out at 708C,
and the subsequent carboxylation was carried out at 1008C in toluene.
The carboxylation at 708C in THF gave a lower yield (60%). [c] The
hydroboration reaction was carried out at 708C in THF. Boc=tert-
butoxycarbonyl.
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Communications
Scheme 2. A possible mechanism for [(IPr)CuCl]-catalyzed carboxyla-
tion of alkylboranes with CO2.
complex [(IPr)Cu(OMe)], which on reaction with an alkyl-
borane gives the adduct 3 through interaction between the
methoxy group and the boron atom. The subsequent trans-
metalation of the alkyl group to the copper atom would
generate the alkylcopper complex [(IPr)CuR]. Nucleophilic
addition of the alkyl group to CO2 should yield the
carboxylate [(IPr)Cu(OCOR)], which upon metathesis with
MeOLi could regenerate [(IPr)Cu(OMe)] and release the
lithium carboxylate RCO2Li. The latter would yield the
carboxylic acid RCO2H after hydrolysis.
Figure 1. ORTEP structure of 3. Thermal ellipsoids are shown at 30%
probability. Hydrogen atoms have been omitted for clarity. Selected
bond lengths [ꢀ] and angles [8]: Cu(1)–O(1) 1.859(2), O(1)–B(1)
1.572(3), B(1)–C(37) 1.631(4); Cu(1)-O(1)-B(1) 126.4(2), Cu(1)-O(1)-
C(1) 116.2(2), C(1)-O(1)-B(1) 117.0(2).[14]
In summary, we have demonstrated that the combination
of [(IPr)CuCl] with MeOLi can serve as an excellent catalyst
system for the carboxylation of alkylboranes with CO2,
leading to the efficient synthesis of various functionalized
carboxylic acids. The isolation of the copper methoxide/
alkylborane adduct complex 3 and its CO2 insertion product 4
has provided important information for understanding the
mechanistic details. These results may also offer new insight
into other catalytic cross-coupling reactions involving metal/
organoboron compounds; in particular reactions for the
transmetalation of organoboron compounds.
Received: March 11, 2011
Published online: July 7, 2011
Figure 2. ORTEP structure of 4. Thermal ellipsoids are shown at 30%
probability. Hydrogen atoms have been omitted for clarity. Selected
bond lengths [ꢀ] and angles [8]: Cu(1)–C(1) 1.857(4), Cu(1)–O(1)
1.836(3), O(1)–C(28) 1.286(5), O(2)–C(28) 1.216(5), C(1)–Cu(1)-O(1)
174.2(2); Cu(1)-O(1)-C(28) 122.0(3), O(1)-C(28)-O(2) 124.5(4).[14]
Keywords: alkenes · carbon dioxide · carboxylation · copper ·
N-heterocyclic carbene
.
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