Selectivities in Nickel-Catalyzed Hydrocarboxylation of Enynes with Carbon Dioxide

Article abstract of DOI:10.1021/acscatal.7b00556

A three-component hydrocarboxylation of enynes with ZnEt₂ and CO₂ is realized. Highly selective cyclizative carboxylation of the C-C triple bond was observed. Preliminary mechanistic studies indicated both the concerted cyclometalation mechanism and the stepwise C=C bond-directed carboxylation were possible in this reaction.

Full text of DOI:10.1021/acscatal.7b00556

Article
Selectivities in Nickel-Catalyzed
Hydrocarboxylation of Enynes with Carbon Dioxide
Tao Cao, Zheng Yang, and Shengming Ma
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Selectivities in Nickel-Catalyzed Hydrocarboxylation of Enynes with
Carbon Dioxide
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Tao Cao,a Zheng Yang,a and Shengming Ma*a,b
a State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of
Sciences, 345 Lingling Lu, Shanghai 200032, P. R. China
b Department of Chemistry, Fudan University, 220 Handan Lu, Shanghai 200433, P.R. China
Supporting Information Placeholder
ABSTRACT: A three component hydrocarboxylation of enynes with ZnEt2 and CO2 is realized. Highly selective cyclizative
carboxylation of the CꢀC triple bond was observed. Preliminary mechanistic studies indicated both the concerted cyclometalation
mechanism and the stepwise C=C bondꢀdirected carboxylation were possible in this reaction.
KEYWORDS: nickel, enyne, diethyl zinc, carbon dioxide, cyclic hydrocarboxylation
As one of the most important fundamental organic
compounds, carboxylic acid plays an irreplaceable role in life
science,1 chemical industry,2 and daily life. Traditionally,
carboxylic acids are prepared mainly through oxidation of
primary alcohols3 or aldehydes,2 oxidative cleavage of CꢀC
bonds,4 or hydrolysis of nitriles, esters, and amides. In recent
years, carboxylation of alkynes or alkenes with carbon dioxide
under mild conditions has become a powerful method to
prepare synthetically useful alkenoic or alkanoic acids.5ꢀ7 We
envisioned that carboxylation of enynes may provide cyclic
alkenoic or alkanoic acids, providing that the selectivity issue
between CꢀC double and triple bonds may be addressed. Y.
Sato and M. Mori et al. reported the stoichiometric
nickelꢀmediated cyclizative carboxylation of enyne S1 with a
methoxycarbonyl group connected to the C=C bond8 resulting
in malonate derivative P1 (Scheme 1, Eq. 1). However, the
simple enyne substrate S2 with a terminal C=C bond did not
react at all (Scheme 1, Eq. 2).8 Herein, we report a
nickelꢀcatalyzed cyclizative carboxylation of enynes S3
providing 2ꢀalkenoic acids P3 (Scheme 1, Eq. 3).9,10
Scheme 1. Cyclizative carboxylation of enynes.
Scheme 2. Initial results of cyclizative carboxylation of
enyne 1a.
Initially, enyne with a nitrogen tether 1a was treated with 1
mol% of Ni(cod)2 and 3 equiv of ZnEt2 with a balloon of CO2
o
in CH3CN at 60 C. Hydrocarboxylation product Eꢀ2a was
obtained in 40% NMR yield accompanied with 30% of the
protonolysis product Zꢀ3a. The structure of Eꢀ2a was
confirmed by Xꢀray single crystal study (Figure 1).11 In
addition, based on the analysis of crude NMR spectra, the
stereoisomer Zꢀ2a and P1ꢀtype product with the C=C bond
being carboxylated (2a’) was not observed (Scheme 2).
Figure 1. ORTEP representation of E-2a.
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Encouraged by these initial results, we turned to optimize
Table 2. Cyclizative carboxylation of alkyl- or
aryl-substituted enynes 1.a
the selectivity between Eꢀ2a and Zꢀ3a (Table 1). Surprisingly,
when CsF was added to the reaction, the yield of Eꢀ2a was
remarkably improved (Table 1, Entries 1ꢀ4). With 2 equiv of
CsF, the yield of Eꢀ2a reached 83% and only 8% of Zꢀ3a was
formed (Table 1, Entry 4). Further increasing the amount of
CsF to 3 equiv did not affect the yield of Eꢀ2a (Table 1, Entry
5). However, when 2 equiv of ZnEt2 were used, Eꢀ2a was
formed only in 64% yield with 16% of 1a recovered (Table 1,
Entry 6). Ligands such as PPh3 and IPr would more or less
suppress the reaction (Table 1, Entries 7,8). Finally, 1 mol %
of Ni(cod)2, 2 equiv of CsF, 3 equiv of ZnEt2 and a balloon of
entry
X
R
yield of 2
or 4 (%)b
77 (Eꢀ2a)
1
2c
3
TsN
TsN
TsN
TsN
O
Me (1a)
Me (1a)
77 (Eꢀ2a)
72 (Eꢀ2b)
66 (Eꢀ2c)
60 (Eꢀ2d)
73 (Zꢀ2e)
43 (Zꢀ2f)
42 (Eꢀ2g)
60 (Eꢀ2h)
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60
Et (1b)
4
5
nBu (1c)
Me (1d)
o
CO2 in CH3CN at 60 C have been defined as the standard
6d
7d
8e
9
C(CO2Me)2
C(CO2Me)2
TsN
Me (1e)
conditions for further study.
nBu (1f)
Table 1. Optimization of cyclizative carboxylation of enyne
NC(CH2)3 (1g)
AcO(CH2)2 (1h)
1a.a
TsN
Ni(cod)2 (1 mol %)
CO2H
ZnEt2 (X equiv), CsF (Y equiv) HCl
TsN
10
TsN
50 (Eꢀ2i)
TsN
+
(1i)
Ph (1j)
4ꢀMeOC6H4 (1k)
4ꢀFC6H4 (1l)
nBu (1m)
TsN
CO2 balloon, CH3CN, 60oC, 3 h
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12
13
TsN
TsN
TsN
CH2
42 (Eꢀ4j)
42 (Eꢀ4k)
37 (Eꢀ4l)
0
1a
E-2a
Z-3a
entry
X
Y
yield of Eꢀ2a yield of Zꢀ3a
(%)b
40
(%)b
30
27
17
8
14f
1c
2
3
3
3
3
3
2
3
3
0
0.5
1
2
3
2
2
2
a
Condition A for the syntheses of 2a-i and the reaction of
1m: 1 mmol of 1, 1 mol% of Ni(cod)2, 2 equiv of CsF, 3 equiv
of ZnEt2 (1.5 M in toluene), and a balloon of CO2 (about 1 L)
57
74
3
4c
5
83 (77d)
83
o
in 10 mL of CH3CN at 60 C for 3 h followed by addition of
8
HCl (3 M, 10 mL). Condition B for the syntheses of 4j-l: 1
mmol of 1, 5 mol% of Ni(cod)2, 2 equiv of CsF, 3 equiv of
ZnEt2 (1.5 M in toluene), and a balloon of CO2 (about 1 L) in
10 mL of DMSO at 80 oC for 3 h followed by addition of HCl
(3 M, 10 mL), and the crude products were reacted with 4
equiv of TMSCHN2 in 1 mL of MeOH and 4 mL of Et2O at
room temperature for 1 h. b Isolated yields. c The reaction was
6e
7f
8g
64
8
6
0
61
9
a The reaction was carried out with 1a (0.5 mmol), Ni(cod)2,
CsF, ZnEt2 (1.5 M in toluene), and a balloon of CO2 (about 1
L) in 5 mL of CH3CN at 60 oC. b Yields are determined by 1H
c
d
NMR analysis with CH2Br2 as the internal standard. The
carried out with 5 mmol of 1a to afford 1.1962 g of Eꢀ2a.
e
reaction was carried out with 1 mmol of 1a in 10 mL of
CsF was not used. The reaction was carried out with 0.3
d
e
mmol of 1g with 3 mol% of Ni(cod)2. f Recovery of 1m (74%)
CH3CN. Isolated yield. Recovery of 1a in 16% yield was
f
observed. PPh3 (2 mol%) was added and recovery of 1a in
86% yield was observed. IPr (2 mol%) was added and
was observed.
g
Scheme 3. Hydrocarboxylation of 1n.
recovery of 1a in 19% yield was observed.
With the optimized conditions in hand, various alkylꢀ or
arylꢀsubstituted enynes have been investigated (Table 2). With
elongated alkyl chain that substituted to CꢀC triple bond, the
yields were slightly decreased (Eꢀ2a-c). The substrates with an
oxygen tether (Eꢀ2d) or dimethyl malonate (Zꢀ2e,f) were also
suitable for the reaction. Moreover, the reaction could tolerate
varies synthetically useful functional groups, such as cyano
(Eꢀ2g), acetoxy (Eꢀ2h), and acetal (Eꢀ2i). In addition, phenyl
substituted enynes could be reacted smoothly in this reaction
with 5 mol% of Ni(cod)2 in DMSO at 80 oC (Eꢀ4j-l). It is easy
to conduct the reaction on one gram scale to afford Eꢀ2a in
77% yield (Table 2, Entry 2). However, 1m with a tether of
CH2 was not suitable for the reaction, indicating that the
ThorpeꢀIngold effect was critical for this cyclization (Table 2,
Notably, when the substrates bearing a homopropargylic
hydroxyl (1o and 1p) or amino moiety (1q) were applied, as
expected the hydroxyl and amino groups could react further
with the inꢀsitu formed carboxylic group to produce lactones
(Scheme 4, Eq. 4 and 5) or lactam (Scheme 4, Eq. 6).
Scheme
4.
Hydrocarboxylation-lactonization
or
lactamization.
Entry 14). The reaction of 1n yielded
a
pair of
diastereoisomers of Eꢀ2n (dr = 1:1), indicating that there is no
1,2ꢀchiral induction (Scheme 3). In all of these cases, the
major byꢀproducts were the protonolysis products (for details,
see the Supporting Information).
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Interestingly, when substrate 1r that has a nonꢀterminal
3
4
5
6
7
8
9
methylꢀsubstituted C=C bond was reacted under standard
conditions, the cyclic hydrocarboxylation product Eꢀ2r was
formed in only 13% yield, accompanied with nonꢀcyclic
regioisomeric CꢀC triple bond hydrocarboxylation products 7r
and 8r, indicating the importance of the terminal C=C bond
(Scheme 5, Eq. 7). Similarly, the reaction of the 1,7ꢀenyne 1s
produced only 8% of the sixꢀmembered product (Scheme 5,
Eq. 8). Surprisingly, when the S1ꢀtype enyne 1t was reacted
under standard conditions, an ethyl carboxylation product Eꢀ9t
with the C=C bond being carboxylated was formed as the
major product, accompanied with the formation of the P1ꢀtype
product Zꢀ10t (Scheme 5, Eq. 9).
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To further study the nature of the dizinc intermediate Int 1,
1a was reacted with 1 mol% of Ni(cod)2, 2 equiv of CsF and 3
equiv of ZnEt2 in CD3CN under CO2. After being quenched
with D2O, the methylꢀdeuterated carboxylation product
Eꢀ2aꢀD was obtained in 78% yield with a Dꢀincorporation of
41%, accompanied with 7% of the protonolysis product
Zꢀ3aꢀD1, in which the methyl group directly attached to the
fiveꢀmembered ring was deuterated in 60% (Scheme 7, Eq.
14). The double carboxylation product C was not observed,
indicating that the alkylꢀzinc was not able to react with CO2.
Interestingly, when the amount of ZnEt2 was reduced to 1
equiv, the yield of Eꢀ2a was 30% with no deuteration at the
methyl group and 41% of starting material 1a was recovered
(Scheme 7, Eq. 15).
Scheme 5. Carboxylation of enynes 1r-t.
Scheme 7. Detection of the dizinc species Int 1.
Furthermore, in order to capture any organometallic
intermediates, 1a was treated with 1 mol% of Ni(cod)2, 2
equiv of CsF, and 3 equiv of ZnEt2 in CH3CN under Ar in the
absence of CO2, followed by quenching with D2O (Scheme 6,
Eq. 10). To our surprise, no deuteration occurred to the
olefinic position, while the methyl group directly attached to
the fiveꢀmembered ring was deuterated in 29%. As a
comparison, when the same reaction of 1a was conducted in
CD3CN and quenched with HCl (Scheme 6, Eq. 11),
Dꢀincorporations of the olefinic position and the methyl group
directly attached to the fiveꢀmembered ring are 65% and 10%,
respectively. These results indicated that both olefinic
carbonꢀzinc bond and sp3 CꢀZn bond may exist and both of
them are so reactive that it may abstract a proton from with the
solvent, CH3CN, however, the olefinic carbonꢀzinc bond is
more reactive than the sp3 CꢀZn bond. Thus, we proposed the
formation of a dizinc species Int 1 (Scheme 6, Eq. 13).12 In
addition, when 1a was reacted in CH3CN without CsF and
CO2 and quenched with D2O (Scheme 6, Eq. 12),
Dꢀincorporations of the olefinic position and the methyl group
directly attached to the fiveꢀmembered ring are 41% and 27%,
respectively, indicating that CsF could enhance the reactivity
of olefinic carbonꢀzinc bond.
Thus, if the reaction started with concerted cyclometalation
of 1a and Ni(cod)2 to form Int 1’ (Scheme 8),12,13 the
subsequent transmetalation with ZnEt2 would form an
isomeric mixture of Int 2 and Int 2’, which may produce three
zinc species Int 3, Int 1, and Int 3’, leading to the formation
of Eꢀ2a, Eꢀ2aꢀD, and Zꢀ3aꢀD1, respectively. Since these three
products were all observed in the reaction of Scheme 7, Eq. 14,
this pathway may be a possible mechanism for the reaction.
Scheme 8. The cyclometalation mechanism.
However, when 1a was reacted with 1 equiv of Ni(cod)2,
either in the presence or in the absence of CO2, only
complicated mixtures were obtained, respectively (Scheme 9),
Scheme 6. Deuteration experiments in the absence of CO2.
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1
Experimental procedure, spectroscopic data, and the H and 13C
NMR spectra of all the products; cif files of Eꢀ2a. This material is
available free of charge via the Internet at http://pubs.acs.org.
indicating that the concerted cyclometalation mechanism may
be less likely.
Scheme 9. Reactions of 1a with stoichiometric amount of
Ni(cod)2.
AUTHOR INFORMATION
7
Corresponding Author
8
9
masm@sioc.ac.cn
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Notes
The authors declare no competing financial interests.
On the other hand, another stepwise mechanism may also be
possible or even predominant for this reaction as supported by
the results shown in eqs 14 and 15 (Scheme 10): At first, Ni(0)
would react with the CꢀC triple bond of 1a to form a
nickelacyclopropene Int 4 with the C=C bond coordinating to
nickel,14 which is critical since the substrate with the
methylꢀsubstituted C=C bond 1r failed to perform the reaction
smoothly. Subsequent transmetalation with ZnEt2 highly
selectively formed Int 5. The C=C bond was then inserted into
the C(sp2)ꢀNi bond to form Int 2. There were two possible
pathways to finish the observed reaction: when the loading of
ZnEt2 is one equiv (Scheme 7, Eq. 15), Int 2 could undergo
ACKNOWLEDGMENT
Financial support from National Basic Research Program
(2015CB856600) and National Natural Science Foundation of
China (21232006) are greatly appreciated.
REFERENCES
(1) Nelson, D. L.; Cox, M. M. In Lehninger Principles of
Biochemistry, 6th ed.; W. H. Freeman and Company: New York,
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βꢀH elimination and reductive elimination to form
a
hydrozincation product Int 3 (path A). After being reacted
7d,15,16
with CO2
followed by quenching with D2O, Int 3 could
produce the nonꢀdeuterated product Eꢀ2a; In the presence of
excess of ZnEt2, further transmetalatation with ZnEt2 would
form the dizinc intermediate Int 1 and NiEt2, which underwent
reductive elimination to regenerate Ni(0). Int 1 would
generate Eꢀ2aꢀD upon reaction with CO2 and quenching with
D2O (see Eq. 14).
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Scheme 10. The C=C bond-directed carboxylation
mechanism.
In conclusion, we have developed a highly selective
cyclizative hydrocarboxylation of enynes. Two possible
mechanisms have been proposed to rationalize the formation
of Int 1ꢀtype dizinc and monozinc Int 3 species. In Int 1, the
reactivity of the sp3 CꢀZn bond toward carbon dioxide was
very low, thus alkanoic acid was not obtained, resulting in
highly chemoselective carboxylation of the CꢀC triple bond.
Related research including more detailed mechanism studies
and further utilization of such intermediates is undertaken in
this laboratory actively.
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ASSOCIATED CONTENT
Supporting Information
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Klankermayer, J.; Leitner, W. Angew. Chem. Int. Ed. 2013, 52,
12119ꢀ12123.
(8) a) Takimoto, M.; Mizuno, T.; Sato, Y.; Mori, M. Tetrahedron
Lett. 2005, 46, 5173ꢀ5176. b) Takimoto, M.; Mizuno, T.; Mori, M.;
Sato, Y. Tetrahedron 2006, 62, 7589ꢀ7597.
Table of contents Graphic
(9) For examples on nickelꢀcatalyzed cyclic carboxylation of
bis(1,3ꢀdiene)s with ZnR2, see: a) Takimoto, M.; Mori, M. J. Am.
Chem. Soc. 2002, 124, 10008ꢀ10009. b) Takimoto, M.; Nakamura, Y.;
Kimura, K.; Mori, M. J. Am. Chem. Soc. 2004, 126, 5956ꢀ5957.
(10) Recently, we have reported cyclic hydroꢀ and
alkylꢀcarboxylation of diynes with Ni(cod)2 and ZnR2, see: a) Cao, T.;
Ma, S. Org. Lett. 2016, 18, 1510ꢀ1513. b) Cao, T.; Ma, S. Org. Chem.
Front. 2016, 3, 1711ꢀ1715.
(11) Crystal data for Eꢀ2a:C15H19NO4S, MW = 309.37, Monoclinic,
space group P 21/c, final R indices [I> 2σ(I)], R1 = 0.0647, wR2 =
0.1737; R indices (all data), R1 = 0.0839, wR2 = 0.1887; a =
12.6954(14) Å, b = 5.5859(6) Å, c = 22.534(2) Å, α = 90.00°, β =
102.103(3)°, γ = 90.00°, V = 1562.5(3) Å3, T = 293(2) K, Z = 4,
reflections collected/unique 8969/2255 (Rint = 0.0396), number of
observations [> 2σ(I)] 3062, parameters: 195. Supplementary
crystallographic data have been deposited at the Cambridge
Crystallographic Data Centre, CCDC 1417855.
(12) For a recent example on detection of dizinc species, see: Xi, T.;
Chen, X.; Zhang, H.; Lu, Z. Synthesis 2016, 48, 2837ꢀ2844.
(13) For selected examples on nickelꢀ or ironꢀcatalyzed reductive
cyclizations of enynes in the presence of ZnR2 that involve
metelacyclopentenes, see: a) Chen, M.; Weng, Y.; Guo, M.; Zhang,
H.; Lei, A. Angew. Chem. Int. Ed. 2008, 47, 2279ꢀ2282. b) Lin, A.;
Zhang, Z.ꢀW.; Yang, J. Org. Lett. 2014, 16, 386ꢀ389.
(14) For selected examples on C=C bond directed reactions of
enynes, see: a) Miller, K. M.; Luanphaisarnnont, T.; Molinaro, C.;
Jamison, T. F. J. Am. Chem. Soc. 2004, 126, 4130ꢀ4131. b) Miller, K.
M.; Jamison, T. F. J. Am. Chem. Soc. 2004, 126, 15342ꢀ15343. c)
Moslin, R. M.; Jamison, T. F. Org. Lett. 2006, 8, 455ꢀ458.
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(15) Aresta, M.; Nobile, C. F.; Albano, V. G.; Forni, E.; Manassero,
M. J. Chem. Soc. Chem. Commun. 1975, 636ꢀ637.
(16) For selected examples on carboxylation of organozincs, see: a)
Yeung, C. S.; Dong, V. M. J. Am. Chem. Soc. 2008, 130, 7826ꢀ7827.
b) Ochiai, H.; Jang, M.; Hirano, K.; Yorimitsu, H.; Oshima, K. Org.
Lett. 2008, 10, 2681ꢀ2683. c) Kobayashi, K.; Kondo, Y. Org. Lett.
2009, 11, 2035ꢀ2037.
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