Catalytic boracarboxylation of alkynes with diborane and carbon dioxide by an N-heterocyclic carbene copper catalyst

Article abstract of DOI:10.1021/ja3063474

By the use of an N-heterocyclic carbene copper(I) complex as a catalyst, the boracarboxylation of various alkynes (e.g., diaryl alkynes, aryl/alkyl alkynes, and phenylacetylene) with a diborane compound and carbon dioxide has been achieved for the first time, affording the α,β-unsaturated β-boralactone derivatives regio- and stereoselectively via a borylcupration/carboxylation cascade. Some important reaction intermediates were isolated and structurally characterized to clarify the reaction mechanism.

Full text of DOI:10.1021/ja3063474

Communication
pubs.acs.org/JACS
Catalytic Boracarboxylation of Alkynes with Diborane and Carbon
Dioxide by an N‑Heterocyclic Carbene Copper Catalyst
Liang Zhang, Jianhua Cheng, Beatrice Carry,† and Zhaomin Hou*
́
Organometallic Chemistry Laboratory and Advanced Catalyst Research Team, RIKEN Advanced Science Institute, 2-1 Hirosawa,
Wako, Saitama 351-0198, Japan
S
* Supporting Information
compound and CO2 using an N-heterocyclic carbene (NHC)
copper catalyst (eq 1). Both a boryl moiety and a carboxylate
ABSTRACT: By the use of an N-heterocyclic carbene
copper(I) complex as a catalyst, the boracarboxylation of
various alkynes (e.g., diaryl alkynes, aryl/alkyl alkynes, and
phenylacetylene) with a diborane compound and carbon
dioxide has been achieved for the first time, affording the
α,β-unsaturated β-boralactone derivatives regio- and
stereoselectively via a borylcupration/carboxylation cas-
cade. Some important reaction intermediates were isolated
and structurally characterized to clarify the reaction
mechanism.
group are added regio- and stereoselectively to the C−C triple
bond via a borylcupration/carboxylation sequence to afford
α,β-unsaturated β-boralactone derivatives. Several reaction
intermediates were isolated and structurally characterized,
thus providing important insight into the mechanistic aspects
of the catalytic cycle.
We previously demonstrated that NHC−copper(I) com-
plexes such as [(IPr)Cu(OtBu)], where IPr = 1,3-bis(2,6-
diisopropylphenyl)imidazol-2-ylidene, can serve as excellent
catalysts for the carboxylation of various nucleophiles.3f,g,j,q To
see whether an NHC−Cu catalyst is suitable for the
boracarboxylation of alkynes, we examined first the sequential
reactions of [(IPr)Cu(OtBu)] with bis(pinacolato)diboron
[B2(pin)2], diphenylacetylene, and CO2 (Scheme 1). As
reported previously,12 the reaction of [(IPr)Cu(OtBu)] with
B2(pin)2 at room temperature instantly gave the borylcopper
complex [(IPr)CuB(pin)]. The subsequent reaction of [(IPr)-
CuB(pin)] with diphenylacetylene afforded β-boryl alkenyl-
he use of CO2 as a low-cost and renewable C1 building
Tblock for the synthesis of value-added chemicals is of great
importance and has attracted increasing interest.1 The direct
carboxylation of carbon nucleophiles with CO2 as an
electrophile is a straightforward protocol for the synthesis of
carboxylic acids.2,3 It was recently demonstrated that α,β-
unsaturated carboxylic acids and their derivatives can be
prepared by transition-metal-catalyzed carboxylations (e.g.,
hydrocarboxylation, alkylative/arylative carboxylation, and
double carboxylation) of alkynes with CO2.4−7 In principle,
the heterocarboxylation of alkynes (such as boracarboxylation)
with CO2, which would lead to the addition of both a
heteroatom unit (e.g., boryl) and a carboxylate group to the C−
C triple bond could serve as a useful method for the synthesis
of multifunctionalized alkenes, which are potentially versatile
building blocks for various synthetic applications. However,
there have been only a few reports on stoichiometric
heterocarboxylation of highly reactive substrates. Previously,
aminative carboxylation8a and hydroxycarboxylation8b of allenes
were achieved by the use of a stoichiometric amount of a Ni
complex. The insertion of CO2 into copper intermediates
formed by silylcupration of dienes was reported to give the
corresponding silacarboxylation products.8c Three-component
coupling reactions of arynes, imines, and CO2 were reported to
afford benzoxazinone derivatives.8d In contrast, the catalytic
heterocarboxylation of simple unsaturated C−C bonds with
CO2 has remained almost unexplored to date.9
Scheme 1. Sequential Reactions of a Copper Alkoxide with a
Diboron Compound, an Alkyne, CO2, and a Lithium
Alkoxide
Boracarboxylation of alkynes with a boron compound and
CO2 is of particular interest and importance in view of the wide
utility of C−B bonds in various chemical transformations.10
Moreover, the catalytic regio- and stereoselective synthesis of
multifunctionalized alkenes remains one of the most difficult
tasks in synthetic organic chemistry.11 Herein we report the
first catalytic boracarboxylation of alkynes with a diborane
Received: June 29, 2012
Published: August 21, 2012
© 2012 American Chemical Society
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Communication
copper complex 1 in 92% isolated yield through insertion of the
C−C triple bond into the Cu−B bond in a syn fashion
(Scheme 1 and Figure 1). Exposure of 1 to a CO2 atmosphere
Figure 3. ORTEP drawing of 3 with thermal ellipsoids set at 30%
probability. H atoms have been omitted for clarity. Selected bond
lengths (Å) and angles (deg): B1−O3, 1.552(6); B1−C3, 1.633(6);
O4−Li1, 1.873(8); Li1−O2i, 1.949(8); O3−C1−O4, 122.4(3).
With these well-established stoichiometric reactions in mind,
we then attempted the catalytic boracarboxylation of
diphenylacetylene with B2(pin)2 and CO2. However, when
[(IPr)CuCl] (5 mol %) was used as the catalyst in the presence
of LiOtBu (1.1 equiv), only a trace amount of the CO2
insertion product was observed (Table 1, entry 1). To our
Figure 1. ORTEP drawing of 1 with thermal ellipsoids set at 30%
probability. H atoms have been omitted for clarity. Selected bond
lengths (Å) and angles (deg): Cu1−C1, 1.910(3); Cu1−C28,
1.924(4); C28−C35, 1.356(5); C35−B1, 1.557(5); C1−Cu1−C28,
166.4(1); Cu1−C28−C35, 130.2(3).
gave carboxylation product 2 in 95% isolated yield. X-ray
crystallographic analysis revealed that complex 2 adopts a cyclic
structure through a bonding interaction between the boron
atom and the carboxylate unit, and the NHC−copper moiety is
bonded to an O atom of the boronic ester group (Figure 2).
Table 1. Catalytic Boracarboxylation of Diphenylacetylene
with B2(pin)2 and CO2
a
b
entry
[Cu]
solvent
T (°C)
yield (%)
1
2
3
4
5
6
7
(IPr)CuCl
THF
80
80
trace
73
(IMes)CuCl
(SIMes)CuCl
(SIMes)CuCl
(SIMes)CuCl
(SIMes)CuCl
(SIMes)CuCl
THF
THF
80
81
c
THF
80
76
dioxane
toluene
THF
100
100
rt
80
86
26
a
Reaction conditions: [Cu] (5 mol %), diphenylacetylene (0.5 mmol),
B2(pin)2 (1.0 equiv), CO2 (1 atm), LiOtBu (1.1 equiv), solvent (3.0
b
c
mL), 80 °C, 14 h, unless otherwise noted. Isolated yields. 5 h.
Figure 2. ORTEP drawing of 2 with thermal ellipsoids set at 30%
probability. H atoms have been omitted for clarity. Selected bond
lengths (Å) and angles (deg): Cu1−C1, 1.862(2); Cu1−O1, 1.872(2);
O4−C48, 1.206(3); O3−C48, 1.336(3); O3−B1, 1.504(3); O1−B1,
1.509(3); O2−B1, 1.443(3); C35−B1, 1.619(4); C1−Cu1−O1,
173.3(1); O3−C48−O4, 122.6(2).
Treatment of 2 with 1 equiv of LiOtBu in tetrahydrofuran
(THF) quantitatively yielded the copper alkoxide complex
[(IPr)Cu(OtBu)] and lithium boralactone derivative 3 through
a transmetalation reaction. An X-ray crystallographic study
showed that 3 possesses a dimeric structure resulting from an
“intermolecular” interaction between the Li atom and the
carbonyl group (Scheme 1 and Figure 3). To the best of our
knowledge, complexes 2 and 3 represent the first examples of
structurally characterized α,β-unsaturated boralactone com-
pounds.
delight, replacing the IPr ligand with the less sterically
demanding 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene
(IMes) ligand in the copper complex (i.e., using [(IMes)CuCl]
instead of [(IPr)CuCl]) resulted in a significant increase of the
catalytic activity, giving the boracarboxylation product 4a in
73% isolated yield (entry 2).13 Using the saturated NHC ligand
1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene (SIMes)
further increased the product yield to 81% under the same
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Journal of the American Chemical Society
Communication
conditions (entry 3). The catalyst remained similarly active
even at 100 °C (entries 5 and 6). At room temperature, the
catalyst was not very efficient, and a yield of only 26% was
observed (entry 7).
We then used [(SIMes)CuCl] as a catalyst to examine the
boracarboxylation reactions of various alkynes. Some repre-
sentative results are summarized in Table 2. Diaryl alkynes with
amount of the carboxylation product 4j was obtained, probably
because of steric hindrance by the bulky tert-butyl group.15
On the basis of the above experimental observations, a
possible mechanism for the current boracarboxylation reactions
is shown in Scheme 2. At first, the metathesis reaction between
Scheme 2. Possible Mechanism of Catalytic
Boracarboxylation
Table 2. Catalytic Boracarboxylation of Various Alkynes with
a
B2(pin)2 and CO2
[(SIMes)CuCl] and LiOtBu would afford [(SIMes)Cu-
(OtBu)], which upon reaction with B2(pin)2 could generate
the borylcopper complex [(SIMes)CuB(pin)]. Subsequent
insertion of an alkyne into the Cu−B bond in a syn fashion
would give β-boryl alkenylcopper complex A. Nucleophilic
addition of the alkenylcopper species to CO2 followed by
replacement of the Cu moiety in the resulting carboxylate with
the B atom would yield β-boralactone derivative B. Finally, the
transmetalation reaction between copper complex B and
LiOtBu should regenerate [(SIMes)Cu(OtBu)] and release
the final product 4.
In an attempt to check the synthetic utility of the present
boracarboxylation products, the Suzuki−Miyaura cross-cou-
pling reaction of 4h with iodobenzene was carried out under
standard conditions (unoptimized), and it afforded tetrasub-
stituted alkene 5 in 45% isolated yield (Scheme 3).16
Scheme 3. Suzuki−Miyaura Cross-Coupling Reaction Using
4h
a
Reaction conditions: [(IMes)CuCl] (5 mol %), alkyne (0.5 mmol),
B2(pin)2 (1.0 equiv), CO2 (1 atm), LiOtBu (1.1 equiv), THF (3.0
mL), 80 °C, 14 h. Isolated yields are shown.
various substituents on the aromatic ring were generally
suitable for this reaction, affording the expected products in
good to excellent yields. Aromatic C−Cl bond (4e) and ester
(4f) functional groups could survive the reaction conditions.
Dithienylacetylene was also transformed into the corresponding
α,β-unsaturated boralactone derivative 4g in high yield. In the
case of 1-phenyl-1-propyne and 1-phenyl-1-butyne, the
carboxylation took place exclusively at the carbon atom having
the phenyl substituent, and the borylation occurred at the
carbon atom having an alkyl group, selectively giving the α-
phenylacrylic acid derivatives 4h and 4i, respectively.
Remarkably, phenylacetylene could also undergo the boracar-
boxylation reaction with B2(pin)2 and CO2 to afford α-
phenylacrylic acid derivative 4k with high selectivity.14 In the
case of 2-tert-butyl-substituted phenylacetylene, only a trace
In summary, we have developed the first catalytic
boracarboxylation reaction of alkynes with diborane and CO2,
which gives a novel family of α,β-unsaturated β-boralactone
derivatives with high regio- and stereoselectivity under mild
conditions. Some key reaction intermediates have been isolated
and structurally characterized, thus offering important insight
into the mechanism of the catalytic cycle. The present reaction
may serve as an attractive method for the synthesis of
multifunctionalized alkenes, as it uses CO2 and easily available
alkynes as building blocks and a relatively cheap copper
complex as a catalyst.
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Journal of the American Chemical Society
Communication
Chem. Soc. 2008, 130, 15254. (g) Greenhalgh, M. D.; Thomas, S. P. J.
Am. Chem. Soc. 2012, 134, 11900.
ASSOCIATED CONTENT
* Supporting Information
Experimental details, spectroscopic and analytical data for new
compounds, and crystallographic data for 1, 2, 3, and 4k (CIF).
This material is available free of charge via the Internet at
http://pubs.acs.org.
■
S
(6) Selected examples of alkylative and arylative carboxylation:
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M.; Mori, M. J. Am. Chem. Soc. 2002, 124, 10008.
(7) Double carboxylation of allenes: Takimoto, M.; Kawamura, M.;
Mori, M.; Sato, Y. Synlett 2005, 2019.
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AUTHOR INFORMATION
Corresponding Author
houz@riken.jp
Notes
■
The authors declare no competing financial interest.
N. Org. Lett. 2007, 9, 1251. (c) Liepins, V.; Backvall, J.-E. Eur. J. Org.
̈
†B.C. is an intern at RIKEN from Universite
ENSIACET, UPS, France.
́
de Toulouse, INP-
Chem. 2002, 3527. (d) Yoshida, H.; Fukushima, H.; Ohshita, J.; Kunai,
A. J. Am. Chem. Soc. 2006, 128, 11040.
(9) During the preparation of this manuscript, Tsuji and coworkers
briefly reported the silacarboxylation of alkynes with CO2 and
silylborane. See: Tani, Y.; Semba, K.; Fujihara, T.; Terao, J.; Tsuji,
Y. 92nd Annual Meeting of the Chemical Society of Japan, Tokyo,
Japan, March 25−28, 2012; Abstract 3L1-44.
ACKNOWLEDGMENTS
■
Financial support by Grants-in-Aid for Scientific Research (S)
(21225004) and (B) (24750047) from the Ministry of
Education, Culture, Sports, Science and Technology of Japan
is gratefully acknowledged. L.Z. thanks RIKEN for a special
postdoctoral fellowship. We thank Drs. Masayoshi Nishiura and
Daisuke Hashizume for help in the preliminary X-ray analysis of
3 and Ms. Akiko Karube for elemental analysis.
(10) (a) Boronic Acids; Hall, D. G., Ed.; Wiley-VCH: Weinheim,
Germany, 2005. (b) Miyaura, N. Bull. Chem. Soc. Jpn. 2008, 81, 1535.
(c) Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457.
(11) Reviews: (a) Flynn, A. B.; Ogilvie, W. W. Chem. Rev. 2007, 107,
4698. (b) Negishi, E.; Huang, Z.; Wang, G.; Mohan, S.; Wang, C.;
Hattori, H. Acc. Chem. Res. 2008, 41, 1474.
(12) (a) Laitar, D. S.; Muller, P.; Sadighi, J. P. J. Am. Chem. Soc. 2005,
127, 17196. (b) Laitar, D. S.; Tsui, E. Y.; Sadighi, J. P. Organometallics
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(13) Recrystallization of 4a in THF gave 3 quantitatively. The THF
ligands in 3 could be removed under vacuum to give 4a, as shown by
NMR analyses.
(14) An X-ray diffraction study showed that 4k adopts a tetrameric
structure. See the Supporting Information for details.
(15) GC−MS analysis showed that monoborylated intermediates
remained in the reaction mixture.
̈
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