Enol Ester Synthesis via Cobalt-Catalyzed Regio- and Stereoselective Addition of Carboxylic Acids to Alkynes

Article abstract of DOI:10.1021/acs.orglett.8b02824

A cobalt-catalyzed highly regio- and stereoselective hydro-oxycarbonylation of alkynes is reported. Both terminal and internal alkynes can react with carboxylic acids to afford enol esters in high yields. The catalyst generated from Co(BF₄)₂, tridentate phosphine ligand L5, and zinc in situ exhibits much higher reactivity than the corresponding cobalt/diphosphine complex.

Full text of DOI:10.1021/acs.orglett.8b02824

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Enol Ester Synthesis via Cobalt-Catalyzed Regio- and Stereoselective
Addition of Carboxylic Acids to Alkynes
Jia-Feng Chen and Changkun Li*
Shanghai Key Laboratory for Molecular Engineering of Chiral Drugs, School of Chemistry and Chemical Engineering Shanghai Jiao
Tong University, 800 Dongchuan Road, Shanghai 200240, China
S
* Supporting Information
ABSTRACT: A cobalt-catalyzed highly regio- and stereo-
selective hydro-oxycarbonylation of alkynes is reported. Both
terminal and internal alkynes can react with carboxylic acids to
afford enol esters in high yields. The catalyst generated from
Co(BF4)2, tridentate phosphine ligand L5, and zinc in situ
exhibits much higher reactivity than the corresponding cobalt/diphosphine complex.
nol esters are very versatile building blocks in organic
chemistry that can be used as intermediates in polymer-
and a reducing hydride reagent exists to keep the high
reactivity of the cobalt catalysts (Scheme 1b). However, the
addition of an acidic molecule HX to alkynes is very rare
because of the decomposition of Co(I)−H in the presence of a
proton.9 Herein we report a syn addition of carboxylic acids to
both terminal and internal alkynes catalyzed by a cobalt/
triphosphine ligand complex to prepare enol esters (Scheme
1c).
Recently, the complexes of cobalt salts and tridentate ligands
have exhibited especially high reactivities in different catalytic
organic reactions.10 Although different tridentate NNN, NNP,
PNP, and CXC ligands have been developed for cobalt, in
which two carbon atoms are always used as the linker between
the coordinating atoms, the reactivity of cobalt/triphos
complexes has not been well-explored (Figure 1). Elsevier
and de Bruin reported the cobalt/1,1,1-tris-
(diphenylphosphinomethyl)ethane-catalyzed hydrogenation
of carboxylic acids and esters,11 while Beller’s group realized
the hydrogenation of quinoline with cobalt and a tetraphos-
E
ization, hydrogenation, and recently developed cross-coupling
reactions.1 The atom-economical addition of carboxylic acids
to alkynes catalyzed by transition metals under mild conditions
provides the most efficient method of preparing enol esters.2
Although mercury salts were first used as catalysts in this
transformation, their high toxicity limits its applications.
Different transition metals, including Ru, Pd, Rh, Ir, Re, Au,
and Ag, can be utilized in this reaction by different mechanisms
and have resulted in diverse selectivities in the products
(Scheme 1a).3 However, all of these metals are expensive and
Scheme 1. Transition-Metal-Catalyzed
Hydrofunctionalization of Alkynes
unsustainable. Thus, it would be very appealing if the same
products could be synthesized using earth-abundant first-row
transition-metal catalysts. In the past decade, cobalt-catalyzed
organic reactions have experienced significant advances.4 Not
only a simple replacement of precious metals but also better
reactivities and different selectivities could also be expected.
For the functionalization of alkynes, cobalt-catalyzed hydro-
genation,5 hydroboration,6 and hydrosilylation7 have been
well-studied, in which a Co(I)−H intermediate8 is involved
Figure 1. Selected tridentate ligands for cobalt.
Received: September 4, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b02824
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
phine ligand.12 We found that the triphos ligand bis(diphenyl-
phosphinoethyl)phenylphosphine plays an essential role in this
enol ester synthesis.
We commenced our studies with 1-decyne (1a) and benzoic
acid (2a) as model substrates (Table 1). Different phosphine
reactivity (entry 7), probably because of the lack of a d orbital
on the nitrogen atom or the high energy of the antibonding
orbital that prevents it from accepting electrons from cobalt,
which suggests the key role of the middle phosphorus atom.
Control reactions without cobalt salts or reducing zinc did not
give any products (entries 8 and 9).
Simply extending the reaction time to 40 h resulted in higher
conversion, and an 88% isolated yield was obtained (entry 10).
With the same ligand L5, Rh(cod)2BF4 is less reactive and
selective than cobalt, while [Ir(cod)Cl]2 is inactive (entries 10
and 11). Cobalt nanoparticles were also examined, and no
product was detected under the same conditions.13 It is worth
mentioning that no trimerization of terminal alkyne 1a was
observed in all cases.
Table 1. Optimization of the Co-Catalyzed Addition of
Benzoic Acid to 1-Decyne
a
With the optimized conditions in hand, we then examined
the scope of the reaction (Table 2). A 10 mmol scale reaction
was conducted to check the application of this reaction, and an
80% yield of 3aa was obtained at 60 °C. Carboxylic acids with
electron-withdrawing or -donating groups on the aromatic ring
(3ab, 3ac, and 3ad) or carbon−carbon double bonds (3ae,
3af, and 3ah) participate in this reaction smoothly to give high
b
c
entry
catalyst
ligand
yield (%)
3aa/(3aa′+3aa″)
1
2
3
4
5
6
7
Co(BF4)2
Co(BF4)2
Co(BF4)2
Co(BF4)2
Co(BF4)2
Co(BF4)2
Co(BF4)2
Co(BF4)2
−
L1
L2
L3
L4
L5
L6
L7
L5
L5
L5
L5
L5
<5
25
5
−
3.5/1
3/1
−
92/1
−
−
−
−
<5
72
<5
<5
<5
<5
88
60
<5
Table 2. Scope of Hydro-oxycarbonylation of Terminal
a
Alkynes
d
8
9
e
10
Co(BF4)2
Rh(cod)2BF4
[Ir(cod)Cl]2
92/1
13/1
−
11
12
f
a
Conditions: 1a (0.5 mmol, 1.0 equiv), 2a (0.65 mmol, 1.3 equiv),
b
c
CH3CN (1.5 mL). Isolated yield. The 3a/(3a′+3a″) ratio was
d
e
1
determined by crude H NMR analysis. Without Zn. The reaction
f
time was 40 h. 2.5 mol % [Ir(cod)Cl]2.
ligands were tested under the conditions of 5 mol % Co(BF4)2
and 5 mol % Zn in acetonitrile at room temperature for 20 h.
With the monodentate ligand PPh3 (L1) (entry 1), no product
could be detected. Among the diphosphine ligands with
different bite angles, only dppe (L2) and dppp (L3) gave low
product yields and moderate regioselectivity (entries 2 and 3),
while smaller- and larger-bite-angle ligands (dppm, dppb, and
DPEphos) gave no products (see the Supporting Information
(SI)). Furthermore, we examined the reactivities of several
multiphosphorus ligands. A complex of cobalt with L4 as the
ligand showed no reactivity at all (entry 4), probably because
of the long distance between the two phosphorus atoms, which
does not match with the small cobalt atom. With tridentate
ligand L5, in which there are two carbons between phosphorus
atoms as in L2, the Markovnikov addition product 3aa was
obtained smoothly in 72% yield along with trace amounts of
regiomers 3aa′ and 3aa″ (92/1) (entry 5). The extra
coordination site on tetradentate ligand L6 inhibited the
same reaction (entry 6). Replacing the middle P atom by
nitrogen in ligand L7 also had a negative effect on the
a
Conditions: alkyne (0.5 mmol), carboxylic acid (0.65 mmol),
Co(BF4)2 (0.025 mmol), L5 (0.025 mmol), Zn (0.025 mmol),
CH3CN (1.5 mL), rt, 40 h. The ratios of isomers were determined by
crude H NMR analysis, and the yields are isolated yields. 10 mmol
scale at 60 °C. At 80 °C.
b
1
c
B
DOI: 10.1021/acs.orglett.8b02824
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Organic Letters
Letter
yields of products. Aliphatic carboxylic acids also work well to
form enol esters (3ag and 3ai). Different functional groups
(cyano, free hydroxyl, and amide) in the acids do not affect the
reaction (3aj, 3ak, and 3al). On the other side, functionalized
terminal alkynes were tested. A phenyl ring, free hydroxyl
group, protected amine, and ester can all be tolerated in this
reaction (3ba to 3ga). Besides aliphatic alkynes, aromatic
alkynes were also evaluated. When phenylacetylene was added
to the reaction, only a 35% yield of the product (3ha) was
obtained, along with the enyne generated by homocoupling of
the terminal alkyne. We assume that the more acidic aromatic
alkynes can have an exchange reaction with the carboxylate
intermediate. This assumption was proven by applying less
acidic substrates, as 3ia and 3ja were isolated in 80% and 88%
yield, respectively.
internal alkynes can also help exclude the possible mechanism
involving metal vinylidenes.15
Tridentate phosphine ligand L5 exhibits much higher
reactivity than diphosphines. Its cobalt complex CoCl2·L5
can be easily prepared by mixing CoCl2 and L5 in
dichloromethane. The crystal structure reveals that all three
phosphorus atoms coordinate to the cobalt center (Figure 2
Encouraged by the cobalt-catalyzed addition of carboxylic
acids to terminal alkynes, we further investigated whether more
challenging internal alkynes can also work in this reaction
(Table 3).14 Under the same reaction conditions, no product
Figure 2. Structures of CoCl2·L5 and CoCl·L5.
Table 3. Scope of Hydro-oxycarbonylation of Internal
Alkynes
a
left). The crystal geometry is distorted square-pyramidal, and
the τ5 value is 0.20 (the two greatest bond angles are 165.60°
for P1−Co−P3 and 153.34° for P2−Co−Cl2). The CoCl2·L5
complex itself is not active in this addition reaction. Upon
reduction with Zn, the Markovnikov addition product 3aa was
obtained in 92% yield.
Control experiments were conducted to address two
questions for the mechanism (Scheme 2): (1) Is the reaction
Scheme 2. Mechanistic Studies
a
Conditions: alkyne (0.5 mmol), carboxylic acid (0.65 mmol),
Co(BF4)2 (0.025 mmol), L5 (0.025 mmol), Zn (0.025 mmol),
CH3CN (1.5 mL), 80 °C, 40 h. The ratios of the isomers were
1
determined by crude H NMR analysis, and the yields are isolated
yields. The product configuration was confirmed by previous reports.
b
At 100 °C.
was detected. However, when the temperature was increased
to 80 °C, 5aa, the addition product from diphenylacetylene
(4a), was obtained in 95% yield with exclusive syn selectivity.
To the best of our knowledge, only two Ru complexs can
catalyze the formation of (E)-enol esters by syn hydro-
oxycarbonylation of internal alkynes.13 Cinnamic acid and
pivalic acid also reacted, giving 5ae and 5am, respectively, in
high yields. Symmetric aliphatic internal alkynes also easily
reacted to form 5ba to 5ea. Compound 5fa was generated with
exclusive addition of the acid to the electropositive β-position,
which is controlled by the electron distribution on the alkyne.
Asymmetric 2-hexyne afforded the products 5ga and 5ga′
without good regioselectivity, while phenylethylacetylene (4h)
reacted to give a 6/1 ratio favoring 5ha. The high reactivity of
catalyzed by a Co(0) or Co(I) complex?16 (2) Is a radical
process involved in this addition reaction? When 5 mol %
CoCl(PPh3)3 was used in this reaction, both substrates 1a
and 2a were recovered (eq 1). However, the addition of 5 mol
% L5 led to the formation of 3aa in 51% yield (eq 2). These
results suggest that triphos ligand L5 replaces PPh3 and that an
active Co(I) complex catalyzes the addition reaction. CoCl·L5
was prepared by mixing CoCl2, L5, and NaBH4 in EtOH. The
crystal structure was obtained (Figure 2 right), showing that
the complex adopts a distorted tetrahedral geometry with τ4 =
0.78 (the two greatest bond angles are 125.56° for P3−Co−Cl
17
C
DOI: 10.1021/acs.orglett.8b02824
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Letter
and 124.45° for P2−Co−Cl). CoCl·L5 exhibits similar
efficiency as the in situ-generated Co(I) catalyst, giving an
85% yield under the standard conditions (eq 3). This result
further supports that triphos/Co(I) complex is able to catalyze
the addition reaction. For the second question, 1 equiv of the
radical scavenger BHT was added to the standard conditions,
and no inhibition effect was observed (eq 4). Furthermore,
cyclopropyl alkyne 1k was tested as a radical clock (eq 5). The
normal addition product 3ka was isolated in 83% yield, and no
allene was detected (see the SI).18 These two reactions support
our hypothesis that a vinyl radial intermediate is probably not
involved in the catalysis.
The cobalt-catalyzed addition of carboxylic acids to alkynes
can be used for the gram-scale synthesis of enol esters (Scheme
4). Two grams of compound 5ai was obtained in 87% yield
Scheme 4. Gram-Scale Synthesis and Transformations of
Enol Ester 5ai
On the basis of the above experiments and literature reports,
we proposed two plausible mechanisms (Scheme 3). The
Scheme 3. Proposed Mechanism
from 4a and 2a under the catalysis of 2.5 mol % cobalt at 80
°C. The enol ester 5ai could be transformed into
triphenylethene (6) via a nickel-catalyzed cross-coupling
reaction.22 Ketones 7 and 8 were obtained smoothly under
hydrolysis and oxidation conditions, respectively.23
In summary, we have reported a practical cobalt/triphos-
catalyzed method for the synthesis of enol esters from
carboxylic acids and alkynes in high yield with high
stereoselectivity. Both the cobalt salt and the ligand are
commercially available and nonexpensive. This is another
example that earth-abundant first-row transition metals can
take the place of precious late transition metals. The use of
tridentate phosphorus ligand L524 is essential for the reactivity
of the cobalt catalysis. Further applications of the cobalt/
triphos complex are under investigation in our group.
ASSOCIATED CONTENT
cobalt(II)/L5 complex is reduced in situ by zinc to form a
reactive [Co(I)] catalyst. In path a, [Co(I)] can undergo
oxidative addition with carboxylic acid 2 to generate Co(III)
intermediate A. This mechanism was proposed in the iridium-
catalyzed reactions of carboxylic acids and supported by the
isolation of an Ir(III)−H complex.19 Coordination of A with
alkyne 1 forms B, and subsequent migratory insertion in B
leads to intermediate C. Direct reductive elimination from C
releases the product enol ester 3 and regenerates the Co(I)
catalyst. In path b, [Co(I)] coordinates with alkyne 1 to form
Co(I) complex D. Oxidative addition of complex D with
carboxylic acid 2 forms the same vinyl Co(III) carboxylate C.
Stoichiometric rhodium chemistry by Wolf and Werner20a and
rhenium chemistry by Casey et al.20b gave the precedents. A
similar mechanism was proposed and calculated by the Breit
group in the catalytic reaction by rhodium, in which the barrier
for intramolecular protonation was lower than that in path a.20c
CpCo/alkyne complexes can also be synthesized from Co(I)
and alkynes.21 A stoichiometric reaction of CoCl·L5 and
benzoic acid was conducted to check the possibility of path a.
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b02824.
Detailed experimental procedures, characterization data,
1
copies of H and 13C NMR spectra, and X-ray crystal
structures of CoCl2·L5 and CoCl·L5 (PDF)
Accession Codes
CCDC 1826753 and 1865904 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
e-mailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, U.K.; fax: +44 1223 336033.
AUTHOR INFORMATION
■
1
However, none of the new species was observed by H NMR
Corresponding Author
*E-mail: chkli@sjtu.edu.cn
ORCID
spectroscopy (see the SI). This result may suggest that
intermediate A is not formed without the help of the alkyne.
Although no Co/alkyne intermediate was isolated directly,
path b is more likely because CoCl·L5 is able to catalyze the
homocoupling of phenylacetylene (1h) in 75% yield at room
temperature without carboxylic acids, in which one molecule of
acidic phenylacetylene may take the place of benzoic acid.
Changkun Li: 0000-0002-4277-830X
Notes
The authors declare no competing financial interest.
D
DOI: 10.1021/acs.orglett.8b02824
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Alkynes with Molecular Hydrogen. Chem. Commun. 2017, 53,
4612−4615.
ACKNOWLEDGMENTS
■
This work was supported by the Thousand Youth Talents Plan,
the National Natural Science Foundation of China (Grant
21602130), and Shanghai Jiao Tong University.
(6) For selected examples, see: (a) Obligacion, J. V.; Neely, J. M.;
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Cheng, B.; Shen, X.; Lu, Z. Cobalt-Catalyzed Asymmetric Sequential
Hydroboration/Hydrogenation of Internal Alkynes. J. Am. Chem. Soc.
2017, 139, 15316−15319.
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