Stereoselective synthesis of α,β-unsaturated carboxylic acids from alkynes using the Fe(CO)5/t-BuOK/AcOH/CH2Cl 2 reagent system

Article abstract of DOI:10.1016/j.jorganchem.2012.01.005

Reactive iron carbonyl species generated in situ using the Fe(CO) ₅/t-BuOK/CH₃COOH/CH₂Cl₂ reagent system reacts with alkynes to give the corresponding α,β-unsaturated carboxylic acids after CuCl₂·2H₂O oxidation with some regio and stereoselectivity.

Full text of DOI:10.1016/j.jorganchem.2012.01.005

Journal of Organometallic Chemistry 705 (2012) 30e33
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Stereoselective synthesis of a,b-unsaturated carboxylic acids from alkynes using
the Fe(CO)5/t-BuOK/AcOH/CH2Cl2 reagent system
*
Mallesh Beesu, Mariappan Periasamy
School of Chemistry, University of Hyderabad, Hyderabad 500046, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Reactive iron carbonyl species generated in situ using the Fe(CO)5/t-BuOK/CH3COOH/CH2Cl2 reagent
Received 13 August 2011
Received in revised form
3 January 2012
system reacts with alkynes to give the corresponding
oxidation with some regio and stereoselectivity.
a,b
-unsaturated carboxylic acids after CuCl2$2H2O
Ó 2012 Elsevier B.V. All rights reserved.
Accepted 9 January 2012
Keywords:
Hydrocarboxylation
Iron carbonyls
Alkynes
a,b-Unsaturated carboxylic acids
1. Introduction
easy accessibility and commercial availability of the inexpensive
Fe(CO)5 reagent [22e24]. In recent years, several organic trans-
formations have been reported using reactive iron carbonyl
species prepared in situ via the reaction of Fe(CO)5 reagent with
various additives [25e31]. Interesting reactivity pattern has been
uncovered in these transformations using iron carbonyl reagents.
For example, whereas the NaHFe(CO)4 prepared from Fe(CO)5
reacts with CH3I and alkynes 1 to give the corresponding cyclo-
butenediones 3 and carboxylic acids 2 after CuCl2$2H2O oxidation,
the reaction using the NaHFe(CO)4/CH2Cl2 reagent system gives
Hydrocarboxylation of alkynes is an industrially important
reaction for large scale production of
derivatives [1]. These acrylic acid derivatives are versatile synthons
in organic synthesis. The -ethylenic carboxyl moiety is also
present widely in honeybee [2], caffeic acid [3] and in biologically
active molecules [4,5].
Methods based on transition metal complexes have been
widely reported for hydrocarboxylation of alkynes [6e21]. The
chemistry of metal carbonyls, especially the iron carbonyl
reagents, has been the subject of considerable interest in view of
a,b-ethylenic carboxylic acid
a,b
mainly the corresponding
a,b-unsaturated carboxylic acids as
outlined in Scheme 1 [32e34].
CH3COOH
25 oC
Na/Naphthalene
THF, 25 oC
NaHFe(CO)4
Fe(CO)5
Na2Fe(CO)4
CH2Cl2
R2
CH3I
R1
CuCl2. 2H2O
R1
O
O
R2
H
R1
CuCl2. 2H2O
R2
1
R1
+
COOH
R2
2
H
COOH
3
2
Scheme 1.
* Corresponding author. Tel.: þ91 40 23134814; fax: þ91 40 23012460.
E-mail address: mpsc@uohyd.ernet.in (M. Periasamy).
0022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2012.01.005

M. Beesu, M. Periasamy / Journal of Organometallic Chemistry 705 (2012) 30e33
31
Table 1
O
Fex(CO)y
Fe(CO)3
R2
R1
R2
O
O
R1C CR2
Regio and stereoselective hydrocarboxylation of alkynes using novel Fe(CO)5/
CuCl2.2H2O
R1
1
t-BuOK/AcOH/CH2Cl2 reagent system.a
Fe(CO)5
t-BuOK +
Entry Alkyne
Productb
Yieldc
%
4
3
R1
R2
Scheme 2.
Ph
H
Ph
75
1
2
Ph
Ph
60d
COOH
2a
In these transformations, the NaHFe(CO)4 reagent is prepared
from Na2Fe(CO)4, which in turn needs to be prepared from the
Fe(CO)5/Na/Naphthalene reagent system. More recently, we have
reported that the Fe(CO)5/t-BuOK reagent system gives reactive
iron carbonyl species that converts the alkynes to the corre-
sponding cyclobutenediones [35]. This transformation probably
2-H3CC6H4
C6H4CH3-2
COOH
2-CH3C6H4
2-CH3C6H4
65
60
H
2b
2c
C6H4OCH3-4
COOH
4-CH3OC6H4
goes through the intermediacy of an
a,b-unsaturated carbonyl
complex 4 (Scheme 2).
3
4-CH3OC6H4 4-CH3OC6H4
H
We have envisioned that the intermediates formed in this
transformation could be used to achieve hydrocarboxylation of
alkynes. Indeed, this objective has been realized by carrying out the
reaction in the presence of acetic acid and CH2Cl2. The results are
described here.
n-C3H7
C3H7-n
4
5
n-C3H7
n-C4H9
n-C3H7
n-C4H9
65
68
H
COOH
2d
2e
2. Results and discussions
n-C4H9
C4H9-n
2.1. Stereoselective hydrocarboxylation of alkynes using the
Fe(CO)5/t-BuOK/AcOH/CH2Cl2 reagent system
H
COOH
Ph
CH3
H
We have observed that the iron carbonyl intermediates,
prepared in situ using the Fe(CO)5/t-BuOK reagent system in the
presence of CH3COOH and CH2Cl2 lead to hydrocarboxylation of
alkynes (Scheme 3). In these reactions, the Fe(CO)5 (6 equiv.),
t-BuOK (6 equiv.) and CH3COOH (6 equiv.) were used in larger
quantities as the initially formed “Fe(CO)4” would also form dimeric
and trimeric iron carbonyl species.
HOOC
2fa (71%)
6
Ph
CH3
78e
Ph
CH3
H
COOH
2f b (29%)
Initially, we have carried out this reaction with diphenylacety-
lene 1a and Fe(CO)5/t-BuOK/CH3COOH/CH2Cl2 reagent system at
25 ꢀC to obtain the corresponding (E)-2,3-diphenylacrylic acid in
55% yield. When the reaction was carried out at 60 ꢀC for 45 min
and stirred further at 25 ꢀC for 12 h, the (E)-2,3-diphenylacrylic acid
was obtained in 75% yield. In a run without using the CH2Cl2, the
(E)-2,3-diphenylacrylic acid 2a was obtained only in 30% yield.
Several symmetrical and unsymmetrical alkynes were converted to
H
C6H13-n
H
COOH
2ga
(75%)
7
H
n-C6H13
62e
H
C6H13-n
HOOC
2gb
H
the corresponding a,b-unsaturated carboxylic acids in good yields
(25%)
(60e78%) using the Fe(CO)5/t-BuOK/CH3COOH/CH2Cl2 reagent
system (Table 1). We have also observed that the use of Fe2(CO)9
reagent in the place of Fe(CO)5 gave the (E)-2,3-diphenylacrylic acid
2a in 60% yield.
Symmetrical alkynes undergo stereoselective hydrocarbox-
ylation to give the (E)-a,b-unsaturated carboxylic acids (entries
1e5). In the case of 1-phenyl-1-propyne (entry 6), the (E)-2-
phenylbut-2-enoic acid 2fa and (E)-2-methyl-3-phenylacrylic acid
2fb are formed in 71:29 ratio. With 1-octyne (entry 7), the
2-methyleneoctanoic acid 2ga and (E)-non-2-enoic acid 2gb
were obtained in 75:25 ratio. Clearly, unsymmetrical alkynes
undergo carbonylation predominantly at the highly substituted
a
All the reactions were carried out using Fe(CO)5 (7.5 mmol, 6 equiv.), t-BuOK
(7.5 mmol, 6 equiv.) and alkyne (1.25 mmol, 1 equiv.) in THF (25 mL).
b
The products were formed after CuCl2$2H2O oxidation, identified by spectral
data (IR, 1H NMR, 13C NMR and Mass) and comparison with the reported data
[21,33,37e39].
c
Yields reported are for the isolated products and based on the amount of alkynes
used.
d
The reaction was carried out using Fe2(CO)9 (3 mmol, 3 equiv.), t-BuOK (6 mmol,
6 equiv.) and diphenylacetylene (1 mmol, 1 equiv.) in THF (25 mL).
e
The ratio of regio isomers was determined by 1H NMR analysis of the crude
product mixture.
acetylenic carbon atom but the regio selectivity observed for the
hydrocarboxylation of these unsymmetrical alkynes is slightly
lower (e.g. for 1-phenyl-1-propyne, 71:29 and for 1-octyne, 75:25,
entries 6 and 7) compared to that observed in the previously
reported methods (e.g. for 1-phenyl-1-propyne, 80:20 [21] and
75:25 [36] and for 1-octyne 96:4 [40]). However, this reagent
system is also useful for the conversion of the relatively less reac-
1). THF/60 oC/45min.
2). CH3COOH/ 25 oC/1 h
3). DCM/25 oC/1 h
R1
H
R2
Fe(CO)5 + t-BuOK
O
R
R2
1
HO
/25 oC/10h
4).
tive aliphatic internal alkynes to the corresponding a,b-unsaturated
2
CuCl2.2H2O
carboxylic acids in reasonably good yields (65e68%, entries 4
and 5).
Scheme 3.

32
M. Beesu, M. Periasamy / Journal of Organometallic Chemistry 705 (2012) 30e33
O
C
H
- +
ROM
CH2Cl2
CH3COOH
MOAc
+
-
ROM
Fe(CO)4
CH3Cl
R2
MHFe(CO)4
R2CO3
R
M
O
Fe(CO)4
R
O
C
O
Fe(CO)4
Fe(CO)5
THF
R2
O
O
ROM + CO
CuCl2. 2H2O
Fe2(CO)9
Fe(CO)4
MHFe(CO)5
Fe(CO)3
Fe(CO)3
R1
R1
O
4
(< 5% )
2 Fe(CO)4
1
R2
MHFe(CO)4
MHFe3(CO)11
+
R
AcOH
R2
R1
H
R1
R2
CuCl2. 2H2O
R'= H, CH3, Aryl, Alkyl
R''= Aryl, Alkyl
R = t-Bu, Me
M= Na, K
H
O
O
HO
Fex(CO)Y
5
2
60-78%
Scheme 4.
In all these cases, along with
a
,
b
-unsaturated carboxylic acids,
reference. Mass spectral analysis was carried out on VG 7070H mass
spectrometer using EI technique at 70 eV. Fe(CO)5, and t-BuOK
were supplied by Fluka and Loba respectively. The alkynes used in
the reactions were supplied by Aldrich. THF was distilled over
sodium benzophenone ketyl system. Chromatographic purification
was conducted by column chromatography using 100e200 mesh
silica gel obtained from Acme Synthetic Chemicals, India. All
reactions and manipulations were carried out under nitrogen
atmosphere.
the corresponding cyclobutenediones were also formed as side
products (<5%) but the acids could be easily separated. We
have also examined the use of NaOMe in the place of t-BuOK in
the reaction using diphenylacetylene. In this run, the (E)-2,3-
diphenylacrylic acid 2a was obtained in 65% yield.
The formation of a,b-unsaturated carboxylic acids in the reac-
tion of alkynes and with the Fe(CO)5/ROM/AcOH/CH2Cl2 and
the Fe2(CO)9/t-BuOK/CH3COOH/CH2Cl2 reagent systems may be
considered by a tentative mechanism outlined in Scheme 4.
Initially, the Fe(CO)5 would react with alkoxide, CH3COOH and
CH2Cl2 to provide the Fe(CO)4 intermediates, which could further
4.2. Synthesis of (E)-2,3-diphenylacrylic acid 2a by using novel
Fe(CO)5/t-BuOK/AcOH/CH2Cl2 reagent system
react with MHFe(CO)4 to give the [HFe3(CO)11]
ꢁ species as outlined
in Scheme 4. Reaction of these species with alkynes could lead
to the acyl complex 5 or iron complex 4 (Scheme 4) which are
The Fe(CO)5 (1.0 mL, 7.5 mmol) was added dropwise to a solu-
tion of anhydrous t-BuOK (0.84 g, 7.5 mmol) in THF (25 mL) at 25 ꢀC
under dry nitrogen. The contents were stirred for 45 min at 60 ꢀC
and brought slowly to 25 ꢀC. Acetic acid (0.43 mL, 7.5 mmol) was
added and the contents were stirred for 1 h at 25 ꢀC. Dichloro-
methane (5 mL) was added and the contents were stirred for
another 1 h. Then diphenylacetylene (0.22 g, 1.25 mmol) was added
and further stirred for 10 h. The metal carbonyl complexes were
oxidized using CuCl2$2H2O (2.55 g, 15 mmol) in acetone (10 mL).
Saturated NaCl solution was added and the contents were extracted
with ether (2 ꢂ 40 mL), dried over anhydrous Na2SO4 and
concentrated under reduced pressure. The residue was subjected to
column chromatography (silica gel, hexaneeEtOAc). Ethyl acetate
(10%) in hexane eluted the (E)-2,3-diphenylacrylic acid 2a.
expected to give the
a,b-unsaturated carboxylic acids after
CuCl2$2H2O oxidation [33]. Previously, we have explained the
hydrocarboxylation of alkynes by considering the hydrometalla-
ꢁ
tion pathway involving the [HFe3(CO)11
]
species [33]. However,
the formation of traces of cyclobutenediones along with
unsaturated carboxylic acids in this transformation indicates that
the mechanism involving the intermediacy of an -unsaturated
a,b-
a,b
carbonyl complex 4 [35] and subsequent protonolysis to give
the corresponding acyl complex 5 cannot be also ruled out
(Scheme 4) [41].
3. Conclusion
We have developed a simple, one-pot method for the stereo-
selective synthesis of a,b-unsaturated carboxylic acids using reac-
4.3. Synthesis of (E)-2,3-diphenylacrylic acid 2a by using novel
Fe2(CO)9/t-BuOK/AcOH/CH2Cl2 reagent system
tive iron carbonyl species generated in situ using readily accessible
iron carbonyl reagents. Though, an excess of Fe(CO)5 is used in this
transformation, it has considerable potential for applications in
organic synthesis as the procedure described here for generation of
reactive iron carbonyl species involves very simple readily acces-
sible reagents.
To a solution of Fe2(CO)9 (1.10 g, 3 mmol) in THF (25 mL),
t-BuOK (0.67 g, 6 mmol) was added under dry nitrogen and stirred
for 0.5 h at 25 ꢀC and for 0.5 h at 60 ꢀC. The contents were brought
slowly to 25 ꢀC and acetic acid (0.34 mL, 6.0 mmol) was added and
stirred for 1 h. Then dichloromethane (5 mL) was added and stir-
red for 1 h. Diphenylacetylene (0.178 g, 1.0 mmol) was added and
the contents were further stirred for 10 h. The metal carbonyl
complexes were oxidized using CuCl2$2H2O (2.55 g, 15 mmol) in
acetone (10 mL). Saturated NaCl solution was added and the
contents were extracted with ether (2 ꢂ 40 mL), dried over
anhydrous Na2SO4 and concentrated under reduced pressure. The
residue was subjected to column chromatography (silica gel,
hexaneeEtOAc). Ethyl acetate (10%) in hexane eluted the (E)-2,3-
diphenylacrylic acid 2a.
4. Experimental
4.1. General
1H NMR (400 MHz) and 13C NMR (100 MHz) spectra were
recorded in CDCl3, (CD3)2SO and TMS was used as reference
(d
¼ 0 ppm). Melting points are uncorrected. IR spectra were
recorded on a JASCO FT-5300 instrument with polystyrene as

M. Beesu, M. Periasamy / Journal of Organometallic Chemistry 705 (2012) 30e33
33
2a: mp 168e170 ꢀC (Lit. [33] mp 172e173 ꢀC); IR (KBr): nmax
1678 cmꢁ1; 1H NMR (CDCl3):
11.16 (bs, 1H), 7.95 (s, 1H), 7.35e7.05
(m, 10H). 13C NMR (CDCl3):
173.2, 142.5, 135.3, 134.3, 131.7, 130.9,
Appendix. Supplementary material
d
d
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jorganchem.2012.01.005.
129.8, 129.5, 128.7, 128.3, 128.1. MS (EI): m/z 225 (M þ 1).
2b: mp 164e165 ꢀC; IR (KBr): nmax 1674 cmꢁ1; 1H NMR (CDCl3):
d
11.77 (bs, 1H), 8.16 (s, 1H), 7.20e6.60 (m, 8H), 2.38 (s, 3H), 2.09
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6.76 (d, J ¼ 8.8 Hz, 2H), 3.77 (s, 3H), 3.69 (s, 3H). 13C NMR
((CD3)2SO):
d 168.9, 159.9, 158.6, 138.6, 132.0, 130.8, 130.4, 128.7,
127.1, 114.1, 113.9, 55.2, 55.1. MS (EI): m/z 283 (M ꢁ 1).
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d
11.67 (bs, 1H),
6.90 (t, J ¼ 7.2 Hz, 1H), 2.26 (t, J ¼ 8 Hz, 2H), 2.17 (q, J ¼ 7.4 Hz, 2H),
1.51e1.39 (m, 4H), 0.94 (t, J ¼ 7.6 Hz, 3H), 0.90 (t, J ¼ 7.2, Hz, 3H).
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d 173.9, 145.5, 131.8, 30.8, 28.4, 22.4, 21.9, 13.9,
13.8. MS (EI): m/z 155 (M ꢁ 1).
2e: IR (neat): nmax 1682 cmꢁ1; 1H NMR (CDCl3):
d 11.40 (bs, 1H),
6.90 (t, J ¼ 7.2 Hz, 1H), 2.28 (t, J ¼ 7.2 Hz, 2H), 2.20 (q, J ¼ 7.2 Hz, 2H),
1.43e1.31 (m, 8H), 0.93e087 (m, 6H). 13C NMR (CDCl3):
d
173.8,
145.4, 131.9, 31.5, 30.8, 28.4, 26.2, 22.7, 22.4, 13.9, 13.8. MS (EI): m/z
185 (M þ 1).
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138e139 ꢀC); 1H NMR (CDCl3):
d 12.37 (bs, 1H), 7.46e7.25 (m, 6H),
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d 10.91 (bs,1H),
6.27 (s, 1H), 5.64 (s, 1H). 2.30 (t, J ¼ 7.2 Hz, 2H), 1.50e1.44 (m, 2H),
1.34e1.26 (m, 6H), 0.89 (t, J ¼ 6.8 Hz, 3H). 13C NMR (CDCl3):
d 173.1,
140.3, 126.9, 31.6, 31.5, 28.9, 28.3, 22.6, 14.1. 2gb 1H NMR (CDCl3):
d
7.11 (dt, J ¼ 15.6, 7.2 Hz, 1H), 5.84 (d, J ¼ 15.6 Hz, 1H). 2.27e2.22
(m, 2H), 1.50e1.45 (m, 2H), 1.36e1.28 (m, 6H), 0.90 (t, J ¼ 6.8 Hz,
3H). 13C NMR (CDCl3):
172.3, 152.6, 120.6, 32.3, 31.6, 28.8, 27.8,
d
22.5, 14.1, MS (EI): m/z 155 (M ꢁ 1).
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We thank the CSIR (New Delhi) for research fellowship to MB.
DST support through a J. C. Bose National Fellowship grant to MP is
gratefully acknowledged. We also thank the DST for the 400 MHz
NMR facility under FIST program. This research work is also
supported by the UGC under the “University of Potential for
Excellence”, the Center for Advanced Study (CAS) and the UGC-
MHRD Chemistry Networking Resource Center programs.
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