Nickel-catalysed electrochemical carboxylation of allylic acetates and carbonates

Article abstract of DOI:10.1016/j.electacta.2010.12.066

The electrochemical carboxylation of a series of allylic acetates and carbonates was carried out in the presence of CO₂ under atmospheric pressure, with a catalytic amount of nickel-bipyridine complex, to afford the corresponding β,γ-unsaturated carboxylic acids. In the absence of nickel catalyst, alcohols were obtained.

Full text of DOI:10.1016/j.electacta.2010.12.066

Electrochimica Acta 56 (2011) 4384–4389
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Nickel-catalysed electrochemical carboxylation of allylic acetates and carbonates
M.J. Medeirosa, C. Pintaricb, S. Oliverob, E. Dunachb,∗
a Centro de Química, Universidade do Minho, Largo do Pac¸ o, 4704-553 Braga, Portugal
b Laboratoire de Chimie des Molécules Bioactives et des Arômes, CNRS, UMR 6001, Institut de Chimie de Nice, Université de Nice-Sophia Antipolis, Faculté des Sciences, Parc Valrose,
06108 Nice Cedex 2, France
a r t i c l e i n f o
a b s t r a c t
Article history:
The electrochemical carboxylation of a series of allylic acetates and carbonates was carried out in the
presence of CO2 under atmospheric pressure, with a catalytic amount of nickel-bipyridine complex, to
afford the corresponding ␤,␥-unsaturated carboxylic acids. In the absence of nickel catalyst, alcohols
were obtained.
Received 19 October 2010
Received in revised form
16 December 2010
Accepted 18 December 2010
Available online 25 December 2010
© 2011 Elsevier Ltd. All rights reserved.
Keywords:
Carboxylation
Allylic acetates
Carboxylic acids
Carbon dioxide
Electrosynthesis
1. Introduction
2. Experimental
In the context of carbon–carbon bond formation by electro-
chemical methods, carboxylation reactions using carbon dioxide
are particularly interesting. Carbon dioxide is an easily available,
cheap and abundant starting material which is difficult to activate
by conventional organometallic chemistry [1].
2.1. Preparation of starting compounds
2.1.1. Allylic acetates (1a–d)
Acetic anhydride (10 mL) is slowly added during 15 min to the
alcohol (30 mmol) in pyridine (10 mL). After addition, the solution is
stirred at room temperature during 15 h. The solution is hydrolysed
with ice-water and extracted with petroleum ether. The organic
layer is washed with water, with a 5% solution of sulfuric acid
and with a saturated sodium carbonate solution, dried over MgSO4
and evaporated under vacuum. The acetates (1a–e) are obtained in
yields of about 90–95%.
The electrochemical reduction of organic halides in the pres-
ence of CO2 has been shown to afford the corresponding carboxylic
acids under mild conditions [2]. This reaction has been reported
to occur directly (without catalyst) in the case of aryl [3], ben-
zyl [4–6] and allyl [7,8] halides in the presence of consumable
magnesium or aluminum anodes. Electrochemical carboxylation of
halides in the presence of metal complexes has also been reported,
for example in the Ni(II)/phosphine catalysed carboxylation of
bromobenzene [9,10], or in the Pd(II)/phosphine catalysed car-
boxylation of aryl triflates [11,12]. CO2 incorporation into carbonyl
compounds to afford ␣-hydroxyacids [13,14] or to alkynes and
olefins has also been reported [15,16]. Although, the electrochemi-
cal carboxylation of organic halides has been thoroughly examined
[2], that of allylic acetates or carbonates had not been previously
reported.
2.1.2. Allylic carbonates (5a–d)
The alcohol (4 mmol), pyridine (8 mmol) and ethyl chlorofor-
mate (6 mmol) are added in a flask with a cooler. The solution is
stirred at room temperature during 30 mn. The solution is hydrol-
ysed with water and acidic water (HCl 0.1 M) and then extracted
with petroleum ether. The organic layer is washed with a satu-
rated sodium carbonate solution, with water, dried over MgSO4 and
evaporated under vacuum. The carbonates (5a–d) are obtained in
yields of about 90–95%.
2.1.3. Catalyst
∗
Ni(bipy)Br2 is prepared following the procedure described
in Ref [17]. A mixture of Ni(DME)Br2 (30 mmol) (DME = 1,2
Corresponding author.
E-mail address: dunach@unice.fr (E. Dunach).
0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2010.12.066

M.J. Medeiros et al. / Electrochimica Acta 56 (2011) 4384–4389
4385
dimethoxyethane) and 2,2ꢀ-bipyridine (30 mmol) in absolute
ethanol is stirred overnight at room temperature. Ni(bipy)Br2 is
formed by precipitation. The complex is filtered on a Büchner,
washed with acetone and dried at 70 ◦C under vacuum. The com-
plex is obtained with 95% yield.
m); 0.89 (3H, m). MS: 170 (M+); 128; 110; 81; 67; 54; 43
(100%).
(E)-2,6-Heptadien-1-ol 2,4-dimethyl 1-acetate (1c): 1H NMR
5.74 (1H, m); 5.26 (1H, Dd, 8.3 Hz, 1.14 Hz); 5.01 (2H, m); 4.44
(2H, s); 2.48 (1H, m); 2.08 (5H, m); 1.65 (3H, D, 1.4 Hz); 0.96 (3H,
D, 6.7 Hz). 13C NMR: 171.28, 137.24, 135.59, 129.2, 116.15, 70.59,
41.84, 32.58, 21.37, 14.49. MS: 182 (M+); 141; 99; 81; 43 (100%).
(E,E)-2,6,10-Dodecatrien-1-ol 3,7,11-trimethyl 1-acetate (1d):
1H NMR 5.33 (1H, m); 5.1 (1H, m); 4.58 (2H, D, 6.8 Hz); 2.06, (10H,
m); 1.76 (3H, s); 1.68 (3H, s); 1.65 (3H, s); 1.60 (3H, s).
(E)-4,8-Dimethyl-3,7-nonadienoic acid (2a): 1H NMR 5.24 (1H,
Td, 5.95 Hz, 1.3 Hz); 5.03 (1H, m); 3.02 (2H, Dd, 6.2 Hz, 1.3 Hz); 1.69
(3H, D, 1.3 Hz); 1.61 (3H, s); 1.53 (3H, s). 13C NMR: 178.14, 140.25,
132.52, 124.11, 116.04, 33.53, 32.51, 26.64, 26.08, 23.79, 18.02. MS:
182 (M+), 139, 69 (100%), 41.
2.2. Electrodes
Electrodes for cyclic voltammetry were fabricated from 3 mm-
diameter glassy carbon rods (Tokai Electrode Manufacturing
Company, Tokyo, Japan, Grade GC-20) press-fitted into Teflon
shrouds to provide planar, circular working electrodes with areas
of 0.077 cm2. Before use, the electrodes were cleaned with an aque-
ous suspension of 0.05 ␮m alumina (Buehler) on a Master-Tex
(Buehler) polishing pad. All potentials are quoted with respect to
an Ag/AgCl/3 M KCl in water reference electrode (0.036 V vs SCE).
(E)-3-Nonenoic acid (2b): 1H NMR 5.48 (2H, m); 3.01 (2H, d,
5.5 Hz); 1.96 (2H, m); 1.22 (6H, m); 0.81 (3H, t, 6.5 Hz). 13C NMR
178.03, 135.97, 121.08, 38.11, 32.83, 31.75, 29.2, 22.9, 14.43. MS:
156 (M+), 138, 96, 67, 55, 41 (100%).
2.3. Cells and instrumentation
Cells for cyclic voltammetry were identical to those described
in publication [18]. Cyclic voltammograms were obtained with the
aid of an AUTOLAB model PGSTAT12 potentiostat–galvanostat. The
data from the above experiments were acquired and stored by
locally written software, which controlled a data acquisition board
installed in a personal computer.
(E)-3,5-Dimethyl-3,7-octadienoic acid (2c): 1H NMR: 5.7 (1H,
m); 4.8 (3H, m); 3.02 (2H, d, 6.5 Hz); 2.3 (1H, m); 1.96 (2H, t, 6.5 Hz);
1.6 (3H, m); 0.88 (3H, d, 6.5 Hz). 13C NMR: MS: 168 (M+), 127, 85,
85 (100%), 67
(E,E)-4,8,12-Trimethyl-3,7,11-tridecatrienoic acid (2d): 1H
NMR: 5.2 (1H, m); 5.03 (1H, m); 3.02 (2H, d, 7.2 Hz); 1.98 (8H, m);
1.69 (3H, d, 1.1 Hz); 1.61 (3H, s); 1.57 (3H, s); 1.57 (3H, s); 1.53 (3H,
s). MS: +250 (M+), 207, 121, 81, 69, 41 (100%).
2.4. General electrolysis procedure
(E)-3-Heptenoic acid (2e): 1H NMR 5.49 (2H, m); 3.01 (2H, d,
5.75 Hz); 1.95 (2H, m); 1.35 (2H, m); 0.83 (3H, t, 7.3 Hz). MS: 128
(M+), 110, 68, 55, 45, 39 (100%).
The constant-current electrolyses were carried out in a single
compartment cell (capacity 50 mL), such as described in Ref. [18],
generally with Mg rod as the sacrificial anode (diameter 1 cm) and a
carbon fibre cathode (apparent surface, 20 cm2) around the counter
electrode. Freshly distilled DMF (50 mL), n-Bu4NBF4 (6 × 10−3 M),
Ni(bipy)Br2 (3.0 × 10−3 M) and allylic acetate or carbonate 1 or 5
(1.5 × 10−2 M) were introduced into the cell under CO2 flow at
atmospheric pressure. The solution was stirred and electrolysed
at room temperature, at a constant current of 30 mA (current den-
sity of 0.15 A dm−2 and 2–5 V between the rod anode and the carbon
fibre cathode) until disappearance of the starting material (checked
by GC analysis of aliquots). Generally, 2–7 F mol−1 of starting mate-
rial were necessary to achieve a complete conversion. Controlled
current electrolyses were carried out with the aid of a stabilized
constant current supply (Sodilec, EDL 36.07).
(E)-3,7-Dimethyl-2,6-octadien-1-yl carbonic acid ethyl ester
(5a): 1H NMR 5.38 (1H, m); 5.09 (1H, m); 4.63 (2H, Dd, 6.6 Hz, 0.76);
1.2 (2H, q, 7.2 Hz); 2.11 (4H, m); 1.76 (3H, s); 1.68 (3H, s); 1.6 (3H,
s); 1.3 (3H, t, 7.2 Hz). 155.85, 150.14, 143.78, 136.82, 132.82, 124.1,
119.31, 32.77, 27.19, 26.24, 18.22, 14.86. MS: 226 (M+), 121, 107,
93, 69 (100%), 43, 41.
(E)-2-Octenyl carbonic acid ethyl ester (5b): 1H NMR 5.7 (1H,
m); 5.54 (1H, m); 4.48 (2H, d, 5.5 Hz); 4.11 (2H, q, 7.2 Hz); 1.97 (5H,
m); 1.23 (9H, m); 0.8 (3H, m). 13C NMR: 154.07, 136.35, 122.28,
67.42, 62.79, 31.19, 30.34, 27.5, 21.48, 13.25. MS: 200 (M+), 110,
81, 67, 54, 41.
(E)-2,6-Heptadien 2,4-dimethyl carbonic acid ethyl ester (5c):
1H NMR 5.72 (1H, m); 5.30 (1H, Dd, 8.3 Hz, 1.14 Hz); 5.01 (2H, m);
4.5 (2H, s); 4.2 (2H, q, 7.15 Hz); 2.48 (1H, m); 2.03 (2H, m); 1.67 (3H,
s); 1.31 (3H, t, 7.2 Hz); 0.96 (3H, D, 6.7 Hz). 13C NMR: 155.45, 137.1,
136.19, 128.75, 116.1, 73.82, 64.08, 41.72, 32.56, 20.44, 14.57, 14.33.
MS: 212 (M+), 171, 99, 81, 43 (100%).
(E,E)-3,7,11-Trimethyl 2,6,10-dodecatrien 1-yl carbonic acid
ethyl ester (5d): 1H NMR 5.31 (1H, m); 5.02 (1H, m); 4.57 (2H, d,
6 Hz), 4.1 (2H, Td, 6 Hz, 1.3 Hz); 1.95 (10H, m); 1.69 (3H, s); 1.65
(3H, s); 1.61 (3H, s); 1.52 (3H, s); 1.23 (3H, Td, 6 Hz, 1.3 Hz). MS:
294 (M+), 204, 161, 133, 107, 93, 69, 41 (100%).
2.5. Identification and quantification of products
After electrolysis, the crude solution is introduced in a round-
bottom flask with K2CO3 and heated at 60 ◦C during 1 h. Methyl
iodide is introduced and heated during 5 h. The mixture is hydrol-
ysed with 0.1 M HCl saturated with NaCl, up to pH 1–2, extracted
with CH2Cl2 and washed with H2O. The dried (MgSO4) organic layer
is evaporated and the residue analysed by GC, GC-MS and 1H NMR,
and the compounds characterised and compared with spectra of
authentic samples.
3. Results and discussion
1H NMR data were recorded on a Bruker 200 MHz spectrome-
ter in CDCl3; ␦ ppm was measured vs residual peak of the solvent.
Identities of the electrolysis products were confirmed by means of
a Hewlett-Packard 5890 Series II gas chromatograph coupled to a
Hewlett-Packard 5971 mass-selective detector.
3.1. Electrocarboxylation of allylic acetates
We were interested in the electrochemical reactivity of allylic
acetates 1, easily available from the corresponding alcohols, in
the presence of CO2, in order to obtain the corresponding ␤,␥-
unsaturated carboxylic acids 2 and/or 2ꢀ, according to Eq. (1).
2.5.1. 1e, commercial compound
For other compounds:(E)-2,6-Octadien-1-ol 3,7-dimethyl 1-
acetate (1a): 1H NMR 5.36 (1H, T, 7.3 Hz); 5.09 (1H, m); 4.56 (2H,
D, 7.2 Hz); 2.08 (4H, m); 1.77 (3H, s); 1.68 (3H, s); 1.61 (3H, s)
(E)-2-Octen-1-ol 1-acetate (1b): 1H NMR 5.74 (1H, m); 5.59
(1H, m); 4.5 (2H, Dd, 5.7 Hz, 0.76 Hz); 2.06 (5H, m); 1.29 (6H,
COOH
R
R
1) e-
+
+
CO2
COOH
OAc
2) H+
R
2'
2
1
(1)

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M.J. Medeiros et al. / Electrochimica Acta 56 (2011) 4384–4389
Table 1
Preliminary electrochemical carboxylation tests of allylic acetate 1a with CO2 (1 atm) at constant current density 0.15 A dm−2
.
F mol−1
Entry
Selectivity
Catalyst (10 mol%)
Anode/cathode
Consumption of 1a
Carboxylic acids 2a + 2ꢀa
Alcohol 3a
Alkene by-product
1
2
3
4
5
6
–
–
–
Mg/C
3
3
3
7
4.5
2
63%
66%
94%
100%
80%
42%
29%
–
7%
41%
45%
60%
47%
68%
31%
–
–
–
24%
32%
62%
59%
55%
40%
Mg/stainless steel
Mg/nickel foam
Mg/C
Mg/C
Mg/C
Ni(bipy)Br2
Ni(bipy)Br2
Ni(bipy)Br2
When the model geranyl acetate 1a was reacted in the presence
of carbon dioxide at atmospheric pressure in a one-compartment
cell fitted with a Mg anode and a carbon fibre cathode in DMF at
20 ◦C, the direct electrolysis led to 63% consumption of 1a after
3 F mol−1, as summarized in Table 1, entry 1. However, the main
product obtained was not the expected carboxylic acids 2a and/or
2aꢀ (selectivity 29%) but the alcohol 3a formed in 47% selectivity, in
an unprecedented electrochemical process (Eq. (2)).
of regioisomers (2a:2ꢀa) (Eq. (3) and Table 1, entry 4). The car-
boxylation reaction reached 100% conversion of 1a after 7 F mol−1
.
After 2 or 4.5 F mol−1, the conversions of 1a were of 42% and 80%
respectively (Table 1, entries 5, 6). Interestingly, in the presence
of Ni(bipy)Br2 as the catalyst, no formation of alcohol 3a was
observed. For ease of treatment and analysis, the carboxylic acids
were isolated in the form of their corresponding methyl esters 4a
and 4aꢀ after in situ esterification of the carboxylates in the presence
of K2CO3 and methyl iodide.
2a + 2a'
H3O+
OAc
2 e-, Ni(bipy)Br2 (10 mol%)
COOMe
+
CO2
DMF, Mg/C
20 °C
COOMe
+
1 atm
K2CO3
MeI
1a
4'a
4a
41% (91:9)
(3)
The Ni(II)-catalysed carboxylation was then extended to several
other allylic acetates, as presented in Table 2.
OAc
OH
The electrocarboxylation of several allylic acetates 1, gave the
corresponding carboxylic acids or esters in moderate to good yields.
(E)-2-Octen-1-ol acetate, 1b (entry 2), afforded 3-nonene car-
boxylic acid in 54% yield as two regioisomers in 86:14 ratio. Allylic
acetate 1c (entry 3) afforded exclusively the ester 4c in 55% yield.
The highly hindered regioisomer 4ꢀc was not observed in this case.
The farnesyl acetate 1d (entry 4) gave the corresponding ester in
30% yield and a regioisomeric ratio 4d:4ꢀd of 84:16. The carboxylic
esters 4e and 4ꢀe were obtained in 63% yield and a ratio of 89:11
was observed when the allylic acetate 1e (entry 5) was electrol-
ysed in the presence of carbon dioxide and a catalytic amount of
Ni(bipy)Br2. The faradaic consumption was of 4–7 F mol−1, indicat-
ing a concomitant CO2 electroreduction under these conditions.
-
2 e
+
CO2
DMF, Mg/C
20 °C
1 atm
1a
3a
(2)
Alcohol 3a was obtained in up to 68% selectivity when the elec-
trolysis was run with stainless steel cathode and a Mg anode. This
alcohol was issued from a hydrolysis process from 1a. In these pro-
cesses, the reaction involved also the electrochemical reduction of
CO2, mainly to oxalate as well as the formation of alkene reduc-
tion by-products. To obtain carboxylic acids 2 and/or 2ꢀ without
any catalyst, a Mg/C couple gave the best results, with a combined
selectivity of 29%. The temperature was changed from 20 to 0 ◦C
but no significant change occurred.
3.2. Electrocarboxylation of allylic carbonates
Several organometallic complexes were used as the catalysts,
to get a more selective cleavage of the carbon-acetate bond.
Among the complexes tested, Co(bipy)Cl2 (bipy = 2,2ꢀ-bipyridine),
The electrochemical carboxylation was also examined with a
series of allylic carbonates 5. As indicated in Eq. (4), and in Table 3,
the corresponding homoallylic carboxylic esters 4 and 4ꢀ were
obtained in 32–71% yields in the presence of the Ni(bipy)Br2 as
the catalyst. In the absence of the metal catalyst, the formation of
the alcohol 3 was again observed as the main reaction product.
Co(cyclam)Br2
(cyclam = 1,4,8,11-tetraazacyclotetradecane),
Co(OTf)2 (OTf = CF3SO3), Ni(cyclam)(BF4)2 or Ni(OTf)2 afforded low
yields (<10%) of expected acids 2 and/or 2ꢀ. The best conditions
were obtained with Ni(bipy)Br2 as the catalyst, used in 10 mol%
COOMe
1) 2 e-, Ni(bipy)Br2 (10 mol%)
OCO2Et
+ CO2
1 atm
COOMe
+
R
R
DMF, Mg/C
20 °C
2) K2CO3, MeI
R
5
4
4'
(32-71 %)
(4)
with respect to 1a. Under these conditions, the acetate 1a could
When the allylic ethyl carbonates 5 were electrolysed in the
be carboxylated with an overall yield of 41%, as a 91:9 mixture
presence of a catalytic amount of Ni(bipy)Br2 at 1 atm of carbon

M.J. Medeiros et al. / Electrochimica Acta 56 (2011) 4384–4389
4387
Table 2
Electrochemical carboxylation of allylic acetates 1 with CO2 (1 atm) in the presence of Ni(bipy)Br2 as the catalyst (10 mol%) in DMF at 20 ◦C with Mg/C electrodes, at constant
current density of 0.15 A dm−2
.
Entry
Substrate
F mol−1
Consumption of 1
Yield of esters 4 + 4ꢀ
Regioisomer ratio 4:4ꢀ
OAc
1
7
100%
41%
91:9
1a
OAc
2
3
4
7
96%
85%
54%
55%
86:14
100:–
1b
1c
OAc
OA
4
5
7
4
78%
30%
63%
86:14
89:11
1d
OAc
100%
1e
dioxide, the corresponding carboxylic acids were formed. Geranyl
carbonate 5a (entry 1) was carboxylated and the corresponding car-
boxylic esters were obtained in 71% yield of with a regioisomer ratio
4a:4ꢀa of 91:1. Allylic carbonate 5b (entry 2) afforded carboxylic
esters 4b and 4ꢀb in 32% yield in a ratio of 57:43. The carbonate 5c
gave 66% yield of carboxylic ester 4c, the regioisomer 4ꢀc not being
observed (entry 3). The farnesyl carbonate 5d (entry 4) afforded
46% yield of carboxylic acids with a ratio of regioisomers 4d:4ꢀd of
84:16. The isomeric ratio of carboxylic acids issued from starting
acetates or carbonates is sensibly the same.
3.3. Cyclic voltammetry studies
The cyclic voltammetry of this electrochemical system was car-
ried out in the particular case of geranyl acetate, 1a. Fig. 1 indicates
the electrochemical behaviour of geranyl acetate 1a alone (curve
Table 3
Electrochemical carboxylation of allylic carbonates 5 with CO2 (1 atm) in the presence of Ni(bipy)Br2 as the catalyst (10 mol%) in DMF at 20 ◦C with Mg/C electrodes at constant
current density of 0.15 A dm−2
.
Entry Substrate
F mol−1
Consumption of 5
Yield of esters 4 and 4ꢀ
Regioisomer ratio 4:4ꢀ
OCO2Et
1
4
100%
71%
91:9
5a
OCO2Et
2
3
5
7
88%
32%
66%
57:43
100:–
5b
OCO2Et
100%
5c
OCO2Et
4
4
90%
46%
84:16
5d

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M.J. Medeiros et al. / Electrochimica Acta 56 (2011) 4384–4389
[NiIIL]2+
2e-
L = bipy
R
AcO-
e-
OAc
-
[Ni0L]
2
CO2
R
[NiIL]
OAc
1
2'
R
-
CO2
R
Ni(II)L
OAc
-
R
AcO
A
NiI L
CO2
e-
-
R
NiI L
OAc
C
CO2
Scheme 1.
B
Fig. 1. Cyclic voltammograms recorded on
a
glassy carbon electrode
(area = 0.07 cm2) at 100 mV s−1 in: (A) DMF containing 0.1 M of tetrabutylam-
monium bromide; (B) solution (A) after addition of 10 mM of geranyl acetate; (C)
solution (A) after addition of 4 mM of Ni(bipy)Br2; (D) solution (C) with 20 mM of
geranyl acetate; (E) solution (D) saturated with CO2.
R
NiII(bipy)
OAc
B). No reduction occured between 0 and −2.5 V vs Ag/AgCl. Curves
C and D of the Fig. 1 show the behaviour of Ni(bipy)Br2 alone and
in the presence of 1a, respectively. NiII(bipy)Br2 shows a reversible
two-electron reduction at −1.1 V vs Ag/AgCl, which corresponds to
the formation of Ni0(bipy) (curve C). Upon addition of 1–5 equiv-
alents of 1a (curve D, 5 equiv.), the reduction occurs at −1.2 V vs
Ag/AgCl and becomes irreversible. Its intensity corresponds to a
three-electron wave independently of the concentration of 1a (1
equiv. or more). A reversible reduction is also observed in curve D
at −1.75 V vs Ag/AgCl, for the further reduction of the Ni-bipy com-
plex [19]. Curve E corresponds to curve D in the presence of CO2
and indicates a first three-electron reduction wave at −1.2 V and a
further irreversible catalytic wave at −1.75 V.
Ni0(bipy) +
OAc
R
A
(6)
A one-electron reduction of A may occur at −1.2 V vs Ag/AgCl to
give intermediate B (Eq. (7)). This step explains the three-electron
wave observed at this potentiel in curve (D).
-
1e-
R
R
NiI bipy
OAc
NiIIbipy
OAc
-1.2 V vs Ag/AgCl
3.4. Mechanistic proposal
B
A
(7)
According to these electrochemical data and to the redox
behaviour of Ni(bipy)Br2, we propose that the first step of the car-
boxylation reaction is the reduction of the NiII complex to Ni0 at
−1.1 V vs Ag/AgCl (Eq. (5)), in agreement with literature data [19].
In the presence of the eletrogenerated Ni0 complex, an oxidative
The coordination of CO2 to the Ni-complex B forms an inter-
mediate C that can evolve to the carboxylates 2 (and/or 2ꢀ) (Eq.
(8)), releasing a NiI-complex which can be further reduced to Ni0
to restart the catalytic cycle.
-
-
R
NiI L
OAc
R
NiI bipy
+ CO2
2 and/or 2'
CO2
- NiIL
OAc
OAc
C
B
(8)
The proposed catalytic cycle is presented in Scheme 1.
After hydrolysis, the carboxylic acids 2 and 2ꢀ are formed. The
ratio of carboxylated regioisomers 2 and 2ꢀ mainly depends on the
steric hindrance of the intermediates A, B and C.
addition on the acetate group of 1a takes place, generating a ␲-allyl
NiII intermediate complex, A (Eq. (6)).
In the absence of CO2, intermediate B may also undergo dimeri-
sation or protonation by the solvent (Eq. (9)). These dimers or
reduced compounds (according to the GC-MS data) constitute the
2e-
Ni0(bipy)
+
2Br-
NiII(bipy)Br
2
-1.1 V vs Ag/AgCl
(5)

M.J. Medeiros et al. / Electrochimica Acta 56 (2011) 4384–4389
4389
by-products of the reaction.
Dimerisation
R
(+ isomers)
-
R
R
NiI bipy
OAc
and/or
H
B
Solvent
R
+
R
H
(9)
It is to note that carbon dioxide can also be reduced, and its
reduction can also be catalysed by the NiII complex [20,21]. The
Ni-catalysed reduction of CO2 affords mainly oxalate dianion, and
may also form carbon monoxide and carbonate [22,23].
[5] O. Sock, M. Troupel, J. Périchon, Tetrahedron Lett. 26 (1985) 1509.
[6] M. Ohkoshi, J.-Y. Michinishi, S. Hara, H. Senboku, Tetrahedron 66 (2010) 7732.
[7] M. Tokuda, T. Kabuki, Y. Katoh, H. Suginome, Tetrahedron Lett. 36 (1995) 3345.
[8] M. Tokuda, J. Nat. Gas Chem. 15 (2006) 275.
[9] C. Amatore, A. Jutand, F. Khalil, M.F. Nielsen, J. Am. Chem. Soc. 114 (1992) 7076.
[10] J.F. Fauvarque, C. Chevrot, A. Jutand, M. Franc¸ ois, J. Perichon, J. Organomet.
Chem. 264 (1984) 273.
4. Conclusions
[11] A. Jutand, S. Négri, Eur. J. Org. Chem. (1998) 1811.
[12] A. Jutand, S. Negri, A. Mosleh, Chem. Commun. (1992) 1729.
[13] G. Silvestri, S. Gambino, G. Filardo, Tetrahedron Lett. 27 (1986) 3429.
[14] O. Scialdone, A. Galia, A.A. Isse, A. Gennaro, M.A. Sabatino, R. Leone, G. Filardo,
J. Electroanal. Chem. 609 (2007) 8.
[15] S. Derien, J.-C. Clinet, E. Dunach, J. Périchon, J. Org. Chem. 58 (1993) 2578.
[16] S. Derien, J.-C. Clinet, E. Dunach, J. Périchon, Tetrahedron 48 (1992) 5235.
[17] R.J. Maldanis, J.S. Wood, A. Chandrasekaran, M.D. Rausch, J.C.W. Chien, J.
Organomet. Chem. 645 (2002) 158.
In conclusion, the electrochemical reductive carbon dioxide
incorporation into a series of allylic acetates and carbonates pro-
ceeded in moderate to good yields under mild conditions in the
presence of Ni(bipy)Br2 as the catalyst. In the absence of the cata-
lyst, the main reaction products were the corresponding alcohols.
[18] K.L. Vieira, D.G. Peters, J. Electroanal. Chem. 196 (1985) 93.
[19] S. Olivero, J.P. Rolland, E. Dunach, E. Labbe, Organometallics 19 (2000) 2798.
[20] M. Isaacs, J.C. Canales, M.J. Aguirre, G. Estiu, F. Caruso, G. Ferraudi, J. Costamagna,
Inorg. Chim. Acta 339 (2002) 224.
[21] J.C. Canales, J. Ramirez, G. Estiu, J. Costamagna, Polyhedron 19 (2000) 2373.
[22] C. O’ Connell, S.I. Hommeltoft, R. Einsenberg, in: M. Aresta, G. Forti (Eds.), Carbon
Dioxide as a Source of Carbon, D. Reidel Publ. Co., 1986, p. 33.
[23] G. Silvestri, in: M. Aresta, G. Forti (Eds.), Carbon Dioxide as a Source of Carbon,
D. Reidel Publ. Co., 1986, p. 339.
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