Highly regio- and stereoselective three-component nickel-catalyzed syn-hydrocarboxylation of alkynes with diethyl zinc and carbon dioxide

Article abstract of DOI:10.1002/anie.201007128

Hydrocarboxylation of alkynes: The first example of nickel-catalyzed hydrozincation of alkynes that form stereodefined hydrocarboxylation products is presented (see scheme; cod=cycloocta-1,5-diene). This catalytic system is efficient for the activation of CO₂ and the three-component reaction produces products that could be converted into important oxindole or γ-butyrolactam derivatives. Copyright

Full text of DOI:10.1002/anie.201007128

Communications
DOI: 10.1002/anie.201007128
CO2 Activation
Highly Regio- and Stereoselective Three-Component Nickel-Catalyzed
syn-Hydrocarboxylation of Alkynes with Diethyl Zinc and Carbon
Dioxide**
Suhua Li, Weiming Yuan, and Shengming Ma*
In memory of Xinwei Ma
Activation of carbon dioxide and converting it into useful
chemical feedstock have attracted much attention owing to
the fact that CO2 is abundant, inexpensive, nontoxic, and
environmentally benign.[1] However, the challenges still to be
overcome are its lack of thermodynamic and kinetic stability.
For the reaction of allylic tin species,[2] aryl boronates, and
limited examples of 1-alkenyl boronates,[3] aryl or alkyl zinc
substrates[4,5] can react with carbon dioxide, usually under
palladium, nickel, copper, or rhodium catalysis [Scheme 1,
Eq. (1)]. In addition, a stoichiometric amount of Ni or Ti
reagents have been used to mediate the reaction of CO2 with
alkene,[6] diene,[7] alkyne,[8] or allene[9] substrates to form five-
membered metallaoxacyclic intermediates A, which may
undergo further reactions to afford carboxylation products
[Scheme 1, Eq. (2)]. There are very limited reports on the
catalytic reactions of alkyne[10] or allene[11] substrates involv-
ing A-type intermediate using 20 mol% of [Ni(cod)2] and
10 equivalents of 1,8-diazabicyclo[5.4.0]undec-7-ene.
Furthermore, there are a few reports on substituted
alkynes that can undergo a stereoselective titanium- or
rhodium-catalyzed syn-hydrozincation[12,13] or rhodium- or
nickel-catalyzed carbozincation[13,14] reaction [Scheme 1,
Eq. (3)]. With this notion in mind, we envisioned that
organozinc reagents generated in situ from hydro- or carbo-
zincation of unsaturated hydrocarbon species may react with
CO2 to afford the corresponding carboxylic acids in a
convenient manner [Scheme 1, Eq. (3)]. Rovis and co-work-
ers reported the hydrozincation/carboxylation of styrenes
with [Ni(acac)2] (10 mol%; acac = acetylacetonate) and
Cs2CO3 (20 mol%).[15] Takaya and Iwasawa reported such a
hydrocarboxylation of allene with 1–2.5 mol% of a silyl
pincer-type palladium complex.[16] However, it should be
noted that both reports involve the reaction of very reactive
allylic or benzylic metallic species with CO2. So far, there are
no such reports on alkyne substrates; the challenge here
would be the lower reactivity of the 1-alkenylic zinc generated
in situ towards CO2[17] and the regioselectivity of the alkynes.
Herein, we report the concise highly regio- and stereoselec-
tive three-component nickel-catalyzed (1–3 mol%) syn-
hydrocarboxylation of alkynes[11,12] with diethyl zinc and the
subsequent efficient reaction with carbon dioxide mediated
by CsF to afford stereodefined and synthetically useful 2-
alkenoic acids. This reaction has been applied to the highly
regio- and stereoselective synthesis of 3-alkylideneoxindole[18]
and a-alkylidene-g-butyrolactam.[19]
Scheme 1. Previous work and our concept for CO2 activation.
[*] S. Li, Prof. Dr. S. Ma
State Key Laboratory of Organometallic Chemistry, Shanghai
Institute of Organic Chemistry, Chinese Academy of Sciences
354 Fenglin Lu, Shanghai 200032 (P.R. China)
Fax: (+86)21-6260-9305
E-mail: masm@sioc.ac.cn
Initially, diphenylacetylene (1a) was treated with CO2 in
the presence of 10 mol% of [Ni(cod)2], 20 mol% of PCy3, and
3 equivalents of ZnEt2. Pleasingly, 10% of the expected syn-
hydrocarboxylation product, that is, (E)-2,3-diphenylacrylic
acid 2a, was formed together with 25% of the hydrolysis
product 4a (Table 1, entry 1). Various bases were then
screened with no obvious improvement (Table 1, entries 2–
4). Then we tested the effect of inorganic salts such as ZnBr2,
LiCl, KF, and CsF as the ligand[3a,b,20] (Table 1, entries 5–8).
We observed that when 3 equivalents of CsF were used, the
hydrocarboxylation product 2a was formed in 59% yield
together with 11% of the ethylcarboxylation product, that is,
W. Yuan, Prof. Dr. S. Ma
Shanghai Key Laboratory of Green Chemistry and Chemical Process,
Department of Chemistry, East China Normal University
3663 North Zhongshan Lu, Shanghai 200062 (P.R. China)
[**] Financial support from the Major State Basic Research and
Development Program (2009CB825300), National Natural Science
Foundation of China (20732005), and Project for Basic Research in
Natural Science Issued by Shanghai Municipal Committee of
Science (08dj1400100) is greatly appreciated. We thank Mr. Guobin
Chai of our research group for reproducing the results presented in
entry 2 of Table 2, entry 4 of Table 3, and Equation 1 in Scheme 3.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201007128.
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Angew. Chem. Int. Ed. 2011, 50, 2578 –2582

Table 1: Optimization of reaction conditions.[a]
Table 2: Nickel-catalyzed hydrocarboxylation of alkynes.[a]
Entry [Ni(cod)2] Ligand/additive
T
t
Yield [%][b]
Entry
R1
R2
[Ni(cod)2]
[mol%]
Yield of 2 [%][b]
[mol%]
(equiv)
[8C] [h]
2a
10
3a
4a
25
1
Ph
Ph
1
1
1
1
3
3
3
3
3
3
3
81 (2a)
89 (2b)
89 (2c)
91 (2d)
62 (2e)
79 (2 f)
91 (2 f)
88 (2g)
81 (2h)
77 (2i)
68 (2j)
1
2
3
4
5
6
7
8
10
10
10
10
10
10
10
10
1
PCy3 (0.20)
K3PO4 (3)
Cs2CO3 (3)
K2CO3 (3)
ZnBr2 (3)
LiCl (3)
KF (3)
CsF (3)
CsF (3)
CsF (1)
RT 31
RT 31
RT 25
RT 29
RT 31
RT 31
RT 31
RT 22
4
2[c]
3
p-MeOC6H4
p-MeC6H4
m-MeC6H4
nPr
Ph
Ph
a-naphthyl
p-MeOC6H4
p-FC6H4
2-thienyl
p-MeOC6H4
p-MeC6H4
m-MeC6H4
nPr
tBu
tBu
tBu
tBu
tBu
tBu
11 <1.6
<1 n.d.
<1
26
13
46
27
12
16
4
<1
2
11
4
6
5[d]
6
3
53
35
59
80
84
52
3
3
1.4
11
4
3
1
7[e]
8
9
10
11
9
60
60
60
1.5
1.5
1.5
10[c]
11[c]
1
1
CsF (0.5)
[a] Reaction conditions: The reaction was carried out with 0.5 mmol of
alkyne, the indicated amount of [Ni(cod)2], 0.5 mmol of CsF, 1.5 mmol of
ZnEt2 (1.5m in toluene, 1.0 mL), and about 1 L of CO2 (a balloon) in
CH3CN (3 mL) at 608C. [b] Yields of isolated products. [c] 1.5 equiv of
CsF. [d] 3 equiv of CsF. [e] 7.0 mmol of the alkyne, 0.21 mmol of
[Ni(cod)2], 7.0 mmol of CsF, and 21 mmol of ZnEt2 (1.5m in toluene,
14.0 mL), and about 1.5 L of CO2 (a balloon) in CH3CN (42 mL).
[a] Reaction conditions: The reaction was carried out with 0.5 mmol of
1a, the indicated amount of [Ni(cod)2], ligand or additive, 3 equiv of
ZnEt2 (1m in hexane, 1.5 mL), and about 1 L of CO2 (a balloon) in
CH3CN (3 mL) at the indicated temperature. [b] Yield based on 1H NMR
analysis. [c] 3 equiv of ZnEt2 (1.5m in toluene, 1.0 mL). cod=cycloocta-
l,5-diene, Cy=cyclohexyl, n.d.=not determined.
(E)-2,3-diphenylpent-2-enoic acid (3a; Table 1, entry 8).
Screening of the temperature and the catalyst loading led to
the observation that when the reaction was conducted at
608C, the yield of 2a was improved to 80% while the amount
of 3a was lowered to 3% with using just 1 mol% of [Ni(cod)2]
(Table 1, entry 9). Furthermore, when the solution of ZnEt2 in
toluene was used instead of that in hexane, the yield was
further improved even with just 1 equivalent of CsF (Table 1,
entry 10). Lowering the amount of CsF to 0.5 equivalents led
to a much lower yield of 2a (Table 1, entry 11).
With the optimized conditions in hand, we began to
explore the scope of this reaction (Table 2). Symmetrical
alkyne substrates were studied first (Table 2, entries 1–5).
Electron-rich aryl alkynes gave higher yields (Table 2,
entries 2–4 vs. entry 1). The reaction of dialkyl-substituted
alkynes was less effective with a yield of 62% for product 2e
(Table 2, entry 5). When R1 is an aryl (Table 2, entries 6–10)
or 2-thienyl group (Table 2, entry 11) and R2 is the tertiary
butyl group, the product is regiospecific. The structure of 2h
was confirmed by its X-ray crystal structure analysis
(Figure 1).[21] The reaction was conducted on a 7.0 mmol
scale to afford the product in a higher yield (Table 2, entry 6
vs. entry 7).
Figure 1. ORTEP plot of 2h shown with ellipsoids at the 30%
probability level.
The reaction of 1-phenyl-1-propyne was also regioselec-
tive ( ꢀ 4:1), and afforded the syn-products 5k and 2k with
the CO2H group connected to the sp2 carbon atom bearing the
methyl group; The product 2k is the major product
(Scheme 2).
To improve the regioselectivity, an amino group was
introduced to the starting alkyne substrates. Pleasingly, (E)-
amino acid 9a was produced exclusively in an excellent yield
and with high regio- and stereoselectivity when 1-phenyl-2-(o-
(N-tosylphenyl))acetylene 8a was used (Table 3, entry 1).
Scheme 2. Nickel-catalyzed hydrocarboxylation of 1k. Yields of 2k and
5k were determined by 1H NMR analysis. TMS=trimethylsilyl.
Further study shows that exclusive regio- and stereoselectivity
was also observed when R is tBu, nBu, cyclopropyl, or 2-
thienyl (Table 3, entries 2–5).
The amino acid 9b could be easily transformed into 3-
alkylideneoxindole 10b[18] with the aid of EDCI (Scheme 3,
Eq. (1)). Notably, in this reaction the crude product 9b was be
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Communications
Table 3: Nickel-catalyzed highly regio- and stereoselective hydrocarbox-
ylation of alkynyl amines.[a]
standard reaction conditions failed to afford the expected
hydrocarboxylation product, with 1h being recovered in 86%
yield (based on 1H NMR analysis) after 11 hours followed by
quenched with HCl (Scheme 3, Eq. (3)). This result indicates
that the reaction should proceed via a different mechanism.
To further probe the mechanism, a couple of control
experiments of alkyne 1h were conducted in the absence of
CO2. The reaction of 1h with ZnEt2 in the presence of
3 mol% of [Ni(cod)2] and 1 equivalent of CsF for 5 minutes
followed by quenching with D2O provided cis-alkene 12 in
Entry
R
[Ni(cod)2]
[mol%]
t [h]
Yield of 9 [%][b]
1
2
3
4
5
Ph (8a)
tBu (8b)
nBu (8c)
cyclopropyl (8d)
2-thienyl (8e)
1
3
3
3
1
6
3
3
3
6
95 (9a)
94 (9b)
95 (9c)
91 (9d)
96 (9e)
1
46% yield (based on H NMR analysis) with a D incorpo-
ration of 34% (Table 4, entry 1). We reasoned that the low D
Table 4: Mechanistic study.
[a] Reaction conditions: The reaction was carried out with 0.5 mmol of
alkyne, the indicated amount of [Ni(cod)2], 0.5 mmol of CsF, and
1.5 mmol of ZnEt2 (1.5m in toluene, 1.0 mL), and about 1 L of CO2 (a
balloon) in DMSO (3 mL) at RT (25–308C). [b] Yield of isolated product.
DMSO=dimethyl sulfoxide, Ts=4-toluenesulfonyl.
Entry Solvent t [min] Yield of 12 [%][b] %D in 12 Yield of 1h [%][b]
1
CH3CN
5
46
25
43
43
78
34
78
92
91
32
48
67
49
47
11
2[a]
3
CH3CN 30
CD3CN 30
CD3CN 90
4
5[c]
CH3CN
5
[a] The reaction was carried out in the absence of CsF. [b] Yield
1
determined by H HMR analysis. [c] 7 mol% of [Ni(cod)2].
incorporation was caused by the deprotonation of the hydro-
zincation intermediate B1 with the solvent (MeCN) in the
absence of CO2, and this was confirmed by running the
reaction in CD3CN (Table 4, entry 3). In addition, it is
interesting to observe that the reaction stopped at 53%
conversion even after 90 minutes (Table 4, entry 4). Finally it
was observed that the reaction with 7 mol% of [Ni(cod)2]
Scheme 3. Synthesis of 3-alkylideneoxindole 10b and a-alkylidene-g-
butyrolactam 11 f. a) [Ni(cod)2] (3 mol%), CsF (1.0 equiv), ZnEt2
(3 equiv), CO2 (a balloon), DMSO, RT, 3 h; b) EDCI (1.2 equiv),
CH2Cl2, ꢁ108C!RT; c) 1. [Ni(cod)2] (1.0 equiv), CsF (1.0 equiv), CO2
(a balloon), CH3CN, 608C, 11 h; 2. HCl. Amount of 1h recovered was
1
afforded 12 in 78% yield (based on H NMR analysis) with
32% of D incorportation, however, in the absence of CsF, the
D incorportation in 12 increased to 78% with a yield of 25%
(based on 1H NMR analysis) for 12 (compare Table 4, entry 1
vs. entry 2). This outcome indicates that CsF is not only
accelerating the hydrozincation reaction but also increasing
the reactivity of the alkenyl zinc intermediate B1.[17a,22] Here,
it should be noted that the subsequent reaction of intermedi-
ate B1 with CO2 also increase the efficiency of the nickel
catalyst (compare the results in Table 4 with that presented in
Table 2, entry 9).
1
determined by H NMR analysis. EDCI=(1-ethyl-3-(3-dimethylamino-
propyl) carbodiimide HCl.
directly transformed into 10b without purification. Such a
strategy can also be applied to 4-phenylbut-3-ynyl N-tosyla-
mide 8 f for the efficient and highly regio- and stereoselective
synthesis of a-alkylidene-g-butyrolactam 11 f[19] (Scheme 3,
Eq. (2)).
According to Yamamoto and co-workers[8a] if the reaction
proceeds via the cyclometalation mechanism (Scheme 1,
Eq. (2)), the reaction of an alkyne, CO2, and a stoichiometric
amount of [Ni(cod)2] would directly afford the hydrocarbox-
ylation product upon protonlysis. However, the reaction of
alkyne 1h and CO2 with 1 equivalent of [Ni(cod)2] under the
Based on these findings, a possible mechanism was
proposed and is shown Scheme 4. The Ni0 species first
reacts with the alkyne to form Int 1, which was followed by
transmetalation with ZnEt2 to form Int 2, and this explains
the syn stereoselectivity. The beta hydride elimination and
subsequent reductive elimination generates the B1-type
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Angew. Chem. Int. Ed. 2011, 50, 2578 –2582

activation and the potential usefulness of the products, this
reaction will be of much interest. Further studies in this area
are being pursued in our laboratory.
Experimental Section
Procedure for the preparation of (E)-4,4-dimethyl-2-phenylpent-2-
enoic acid (2 f; Table 2, entry 7): To an oven dried 250 mL Schlenk
flask were added [Ni(cod)2] (57.8 mg, 0.21 mmol), CsF (1.0638 g,
7.0 mmol), alkyne 1 f (1.1080 g, 7.0 mmol), and CH3CN (42 mL)
under argon. The mixture was then frozen in a liquid nitrogen bath
with the argon being completely replaced by CO2 ( ꢀ 1.5 L, and the
CO2 gas was dried by passing it through two gas washing bottles with
conc. H2SO4), and then the reaction flask was allowed to stand until
the mixture thawed with the CO2 balloon still attached. To the
resulting suspension was added ZnEt2 (1.5m in toluene, 14.0 mL) via a
syringe. After being stirred at 608C for 1.5 h (until reaction was
complete as evident by TLC), the resulting mixture was quenched
with 3m HCl (30 mL). The aqueous layer was extracted with EtOAc
(20 mL ꢁ 5) and the organic layer was washed with brine, and
concentrated in vacuo. The crude residue was purified by column
chromatography on silica gel (petroleum ether/acetone = 10/1!5/1)
to afford 2 f (91%, 1.3071 g) as a white solid. M.p. = 132–1338C
(petroleum ether/ethyl ether); 1H NMR (300 MHz, CDCl3): d = 11.96
(bs, 1H, COOH), 7.38–7.26 (m, 3H, ArH), 7.20–7.08 (m, 3H, ArH
Scheme 4. A plausible mechanism. RL =large group, RS =small group.
intermediate of (E)-alkenyl zinc Int 4 and regenerates Ni0. At
the same time, Ni0 reacts with CO2 to produce Arestaꢀs
complex Int 5,[23] which undergoes transmetalation with Int 4
and subsequent reductive elimination to yield the zinc
carboxylate and Ni0 to complete the catalytic cycle.[24]
Arnold et al. have reported that Fꢁ may react with CO2 to
ꢁ
=
form FCO2 , which increases the reactivity of the C O bond
in CO2.[25] This outcome explains the high catalytic activity of
[Ni(cod)2].
and HC C), 0.92 ppm (s, 9H, tBu); 13C NMR (75 MHz, CDCl3): d =
=
The origin of exclusive regioselectivity of hydrocarbox-
ylation of alkynyl amines comes from the directing ability of
the amino group (Table 3, Scheme 3, and Scheme 5). Depro-
tonation with ZnEt2 would readily form Int 7, followed by the
nickel(0)-catalyzed intramolecular hydrozincation of alkynes
and insertion of carbon dioxide to produce the amino acids.
173.8, 155.8, 135.6, 130.7, 130.1, 127.6, 127.5, 34.4, 30.2 ppm; MS
+
~
(m/z): 204 (M , 76.2), 143 (100); IR (KBr): n = 3400–2500 (br), 1675,
1631, 1598, 1496, 1421, 1366, 1269, 1208, 926, 785, 703 cmꢁ1; Elemental
analysis calcd for C13H16O2: C 76.44, H 7.90; found: C 76.34, H 8.18.
Received: November 12, 2010
Revised: December 12, 2010
Published online: February 17, 2011
Keywords: alkynes · carbon dioxide · hydrocarboxylation ·
.
inorganic fluoride · nickel
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Communications
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[21] Crystal data for compound 2h: C14H18O3, MW= 234.28, mono-
clinic, space group P21/c, final R indices [I > 2s(I)], R1 = 0.0451,
wR2 = 0.1230; R indices (all data), R1 = 0.0518, wR2 = 0.1308;
a = 5.9511(2) ꢄ, b = 8.4184(3) ꢄ, c = 26.7119(9) ꢄ, a = 908, b =
96.1180(10)8, g = 908, V= 1330.61(8) ꢄ3, T= 296(2) K, Z = 4,
reflections collected/unique 14842/2331 (Rint = 0.0206), number
of observations [ > 2s(I)] 2331, parameters: 154. CCDC 799934
(2h) contains the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.
uk/data_request/cif.
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[18] This is a class of important compounds with a broad range of
biological activities. For related references, see: a) L. Sun, N.
Tran, F. Tang, H. App, P. Hirth, G. McMahon, C. Tang, J. Med.
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[24] When Ph2Zn was treated with CO2 mediated by 2.0 equivalents
of CsF in CH3CN under the catalysis of 1 mol% of [Ni(cod)2],
93% yield (based on 1H NMR analysis) of PhCOOH was
produced based on Ph2Zn.
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