Catalytic cyclization of propargyl bromoethers via electrogenerated nickel(I) tetramethylcyclam in ionic liquids: Water effects

Article abstract of DOI:10.1149/2.1171904jes

Cyclic voltammetry and controlled-potential electrolysis have been employed to investigate the reductive intramolecular cyclization of propargyl bromoethers derivatives, catalyzed by electrogenerated (1,4,8,11-tetramethyl-1,4,8,11-tetraaza-cyclotetradecan

Full text of DOI:10.1149/2.1171904jes

Journal of The Electrochemical Society, 166 (6) G17-G24 (2019)
G17
0
013-4651/2019/166(6)/G17/8/$38.00 © The Electrochemical Society
Catalytic Cyclization of Propargyl Bromoethers via
Electrogenerated Nickel(I) Tetramethylcyclam in Ionic Liquids:
Water Effects
1
2
3
1,z
4
J. P. Mendes, E. Duñach, J. M. S. S. Esperança,
M. J. Medeiros,
J. F. Ribeiro,
1
2
M. M. Silva, and S. Olivero
1
Centro de Química, Universidade do Minho, Campus de Gualtar, 4710-057 Braga, Portugal
2
3
Université Côte d’Azur, Institut de Chimie de Nice, CNRS, UMR 7272, Parc Valrose, 06108 Nice Cedex 2, France
LAQV, REQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade NOVA de Lisboa
FCT NOVA), 2829-516 Caparica, Portugal
(
4
CMEMS-UMinho, University of Minho, Campus Azurém, 4804-533 Guimarães, Portugal
Cyclic voltammetry and controlled-potential electrolysis have been employed to investigate the reductive intramolec-
ular cyclization of propargyl bromoethers derivatives, catalyzed by electrogenerated (1,4,8,11-tetramethyl-1,4,8,11-
+
tetraaza-cyclotetradecane)nickel(I), [Ni(tmc)] , as the catalyst, in N,N,N-trimethyl-N-(2-hydroxyethyl)ammonium
bis(trifluoromethylsulfonyl)imide, [N1 1 1 2(OH)][NTf2] and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide,
[
C2mim][NTf2] in the absence and in the presence of water. The results show that the reaction leads to the formation of the expected
heterocyclic compounds, in moderate to good yields. These compounds are important intermediates in the synthesis of natural
products with possible biological activities.
©
2019 The Electrochemical Society. [DOI: 10.1149/2.1171904jes]
Manuscript submitted December 17, 2018; revised manuscript received March 14, 2019. Published April 8, 2019. This was Paper
1671 presented at the Chicago, Illinois, Meeting of the Society, May 24–28, 2015.
Lignans have attracted considerable attention from organic
chemists due to their widespread presence in nature as well as their
environmental problems caused by volatile organic solvents. More-
over, provided that both cation and anion are chosen appropriately, a
medium with a large electrochemical window can be obtained which
make it to be desirable solvent for organic electrosynthesis reactions,
as for example, in direct or nickel-catalyzed electroreductive coupling
1
wide range of biological activity, e.g., antipyretic, anti-rheumatic,
2
analgesic, anti-tumor, diuretic, anti-HIV. A major subgroup of lig-
nans is comprised by tri- and tetra-substituted tetrahydrofurans, which
are very useful precursors for the synthesis of a variety of natural
products with biological activity such as antibiotics, C-glycosides and
7
,8
9
of organic halides and in electrochemical reductive coupling.
RTILs are also promising solvents due to its inherent ionic nature
which can avoid the addition of supporting electrolyte. However, it
should be noting that some properties of ionic liquids, such as, high
viscosity and low ion conductivity, may be consider their major draw-
2
furanolignans.
The formation of carbon-carbon bonds by radical cyclization has
become an important tool for the synthesis of natural products con-
3
10
taining heterocyclic rings. The majority of radical cyclizations in
back when used as electrolytic medium. The RTILs conductivities
heterocyclic chemistry are still carried out using tributyltin hydride
as the reducing agent. For example, this methodology has been ap-
plied to stereoselective total synthesis of the furanolignan, dimethoxy
(in the range 0.1 – 20 mS/cm) are considerably lower and their viscosi-
5
11
ties (ranging from 10 to 10 cP at room temperature) are much higher
than those of conventional electrolyte solutions. The high viscosity of
the RTILs has the effect to decrease the rate of mass transport within
the solution and the conductivity of the salts because the conductivity
is inversely related to the viscosity. However, these disadvantages can
be easily overcome by heating the solution to 40–50°C or by adding a
small amount of a co-solvent (5-10% v/v) or if necessary, combining
4
analogue of (± ) samin. A key step of this synthesis involves the in-
tramolecular radical cyclization of the bromo propargyl ester 1c. This
ꢀ
allows the formation of the heterocyclic ring precursor of type 2c
(
Figure 1).
However, this method has disadvantages, as the tributylin hydride
12
is toxic and it is difficult to completely remove the tin species from
the reaction mixture.
these two methods. Many RTILs are hygroscopic and, even if they
are hydrophobic, they are also able to absorb a considerable amount
of water from the atmosphere resulting in changes in the physical and
chemical properties. So, the reactions carried out in these media pro-
ceed under the influence of water unless we remove it. However, water
impurities have shown the advantage to decrease the viscosity and to
The electrochemical version of similar intramolecular processes
in reactions involving electrogenerated nickel(I) complexes as cat-
−
alysts for the radical cyclization of aryl or vinyl bromides, aliphatic
5
halides, α-bromoesters and α-bromoamides with double or triple
10
bonds − has been shown to be highly efficient. The reaction can be
performed under mild conditions using relatively simple equipment.
It also avoids the hazardous or toxic reagents used in stoichiometric
amounts in the tributyltin hydride method.
Although most of the reactions have been reported in aprotic or-
ganic solvents and organic electrolytes, their toxicity, hazards and
cost make them unattractive. This makes the search for eco-friendly
reaction media alternatives highly desirable and it would inevitably
increase the value of the electrosynthetic methods.
increase the conductivity of RTILs . Hence, addition of water to the
RTIL as a co-solvent have been the focus of recent research in this
13
area.
As an extension of our previous research, we undertook fur-
ther studies on analogs/derivatives of bromopropargyl esters as
substrates. In the present work, we investigated the reductive
intramolecular cyclization of propargyl bromoethers derivatives
1a–1c, catalyzed by electrogenerated (1,4,8,11-tetramethyl-
+
1,4,8,11-tetraaza-cyclotetradecane)nickel(I), [Ni(tmc)] , as the
Room−temperature ionic liquids (RTILs) represent a class of alter-
catalyst,
bis(trifluoromethylsulfonyl)imide, [N1 1 1 2(OH)][NTf
ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide,
[C mim][NTf ], in the absence and in the presence of added water,
in
N,N,N-trimethyl-N-(2-hydroxyethyl)ammonium
native solvents currently receiving serious consideration, as they have
2
]
and 1-
6
environmental and technological benefits. Their characteristics ren-
derthempromisingreplacementsforvolatileorganicsolvents(VOCs).
RTILs are stable compounds that exhibit no detectable vapor pres-
sure under ambient temperature conditions, in a large contrast to the
2
2
by cyclic voltammetry and controlled-potential electrolysis.
Our main goal, in this investigation, was to study electrolytic media
for the intramolecular radical cyclization of unsaturated halides which
avoids the use of supporting electrolyte and organic volatile solvents.
In addition, we were also interested to examine the influence of added
zE-mail: medeiros@quimica.uminho.pt
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G18
Journal of The Electrochemical Society, 166 (6) G17-G24 (2019)
1
a: R = phenyl; R = 4-methoxyphenyl
R1
Br
O
1 2
1
b: R = phenyl; R = methoxy
1
2
1
c: R = ethoxycarbonyl; R = 3,4-dimethoxyphenyl
R2
1
2
CO Et
CH3
CH
2
2
EtO C
EtO C
2
2
MeO
MeO
MeO
MeO
MeO
MeO
O
O
2c
2c'
3c
Figure 1. Chemical structures of compounds 1a–1c, 2c, 2c’, 3c,
Ni(tmc)]Br2 complex, [C2mim][NTf2] and [N1 1 1 2(OH)][NTf2]
ionic liquids.
Me
Me
N
[
N
N
Ni
Ni(tmc)Br2
,
2Br-
N
Me
Me
+
-NTf
N
[N1 1 1 2(OH)][NTf2]
2
N,N,N-Trimethyl-N-(2-hydroxyethyl)ammonium
bis(trifluoromethylsulfonyl)imide
OH
N +
[
C mim][NTf ]
2 2
-NTf
2
N
1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide
1
water as the co-solvent on the reaction selectivity. Adding water as the
co-solvent instead of organic volatile solvents makes this methodology
more environmentaly friendly.
3
For compounds 1b and 2b the H NMR spectra (CDCl ) are pre-
sented below for comparison purposes:
(a) for [1-bromo-2-methoxy-2-(prop-2’-ynyloxy)ethyl]benzene
1
(
(
1b): H NMR (CDCl
0.8H, t, J = 2.4 Hz, C≡CH); 3.33 (2.4H, s, OCH
); 4.14 (0.4H, dqAB, J = 2.4 and 16.0 Hz, OCH
d, J = 2.4 Hz, OCH
(1H, d, J = 6.4 Hz; 1-H ou 2-H); 7.31-7.38 (3H, m, C
(2H, m, C );
3
): δ
H
2.42 (0.2H, t, J = 2.4 Hz, C≡CH); 2.50
); 3.55 (0.6H, s,
); 4.37 (1.6H,
3
Experimental
OCH
3
2
2
)
;
4.97 (1H, d, J = 6.4 Hz; 1-H ou 2-H); 5.03
H
6 5
); 7.44-7.48
6 5
H
2
1
(b) for 2-methoxy-4-methylene-3-phenyltetrahydrofuran (2b): H
(
tetramethylcyclam, tmc, Fluka, 97%), and cyclohexane (C
6
H
12
)
NMR (CDCl
(2H, app q, J = 2.0 Hz, 5-H
5.02 (1H, broad s, 2-H); 5.12 (1H, app q, J = 2.0 Hz, C=CHH);
7.20-7.40 (5H, m , C ).
3
): δ
H
3.41 (3H, s, OCH
3
); 3.81 (1H, broad s, 3-H); 4.61
(Aldrich, 99.5%). Deaeration procedures were carried out with
2
); 4.99 (1H, app q, J = 2.0 Hz, C=CHH);
zero-grade argon (Air Products). Preparation of N,N,N-trimethyl-
N-(2-hydroxyethyl)ammonium bis(trifluoromethylsulfonyl)imide,
] was based on a published method. All RTILs
were dried under vacuum and at moderate temperature for at least
6
H
5
14
[N1 1 1 2(OH)][NTf
2
Electrodes and cells.—Working electrodes for cyclic voltamme-
try experiments 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
1
4
8 h. H NMR were performed for all samples and indicated a purity
of the samples prepared in-house of better than 99 % and confirmed
the supplier indication for all the other samples.
2
Published procedures were employed for the preparation of
working electrodes with areas of 0.071 cm . Before use, the elec-
15
[
4
Ni(tmc)]Br
-methoxybenzene
2
,
1-[2-bromo-2-phenyl-1-(prop-2’-ynyloxy)ethyl]-
trodes were cleaned with an aqueous suspension of 0.05-μm alumina
(Buehler) on a Master-Tex (Buehler) polishing pad. Counter elec-
trode was a Pt spiral in the same compartment. Working electrodes
for controlled-potential electrolysis were reticulated vitreous carbon
(RVC) or graphite felt (2 cm × 2 cm × 0.5 cm) (China Yangzhou
Guo Tai Fiberglass Co.,LTD) while a carbon rod was the counter elec-
trode. Procedures for the cleaning and handling of these electrodes
16
(1a),
[1-bromo-2-methoxy-2-(prop-2’-
16
ꢀ ꢀ
and ethyl 2-bromo-3-(3 ,4 -
ynyloxy)ethyl]benzene (1b)
dimethoxyphenyl)-3-propargyloxy-propanoate (1c).
Synthesis of 2-(4’-methoxyphenyl)-4-methylene-3-
phenyltetrahydrofuran (2a), 2-methoxy-4-methylene-3-
phenyltetrahydrofuran (2b) and 2-(3 ,4 -dimethoxyphenyl-3-
ethoxycarbonyl-4-methyl-2,5-dihydrofuran (2c) was based on
16
ꢀ
ꢀ
1
9
have been described previously. For constant-current electrolyses
working electrodes were carbon felt (apparent geometric surface area,
17
the method published by McCague et al.
1
2
H NMR data were recorded on a Bruker 300-MHz spectrome-
ter in CDCl ; δ ppm was measured versus residual peak of the sol-
20 cm ) and a metal rod (Mg) as the sacrificial anode.
3
Cyclic voltammograms were recorded in a three-electrode, two-
20
vent. Gas chromatography-mass spectrometry data were recorded on
a Hewlett-Packard 5890 Series II gas chromatograph coupled to a
Hewlett-Packard 5971 mass-selective detector.
compartment cell, as described in earlier publications. All solutions
were deoxygenated with a fast stream of argon before each experiment.
For controlled-potential electrolysis and product analysis, a divided
cell with an anodic and a cathodic compartment, each 13 cm , sepa-
rated by a glass sinter (as have been described in earlier publication )
3
Identities of the major products (2a−2c) were confirmed by com-
1
21
parison of gas chromatographic retention times as well as H NMR
18
and mass spectra with those of the authentic compounds reported.
2
was used. A mixture of bromoether 1 (5.0 mM), [Ni(tmc)]Br (0.5 mM
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Journal of The Electrochemical Society, 166 (6) G17-G24 (2019)
G19
2 2 2
or 1.0 mM) and [N1 1 1 2(OH)][NTf ] or [C mim][NTf ] was introduced
in the cathodic compartment. All preparative electrolyses were carried
out in an atmosphere of argon, due to the extreme sensitivity of Ni(I)
22
complexes to oxygen, and the catholyte solutions were stirred with
a magnetic bar. The constant-current electrolyses were performed in
23
a single-compartment cell (capacity 50 mL), with a Mg rod as the
anode (diameter 1 cm) and a carbon felt cathode (apparent surface,
2
2
0 cm ) which was formed into a cylinder around the counter elec-
−3
trode. A mixture of Ni(tmc)Br
2
(2.0 × 10 M) and bromoester (1.0
−
2
3
×
10 M) and [N1 1 1 2(OH)][NTf
2
] (20 cm ) were introduced in the
cell under argon flow. The solution was stirred and electrolysed at
room temperature, at a constant current of 30 mA (current density of
.5 mA cm and 7 - 9 V between the rod anode and the carbon felt
cathode).
All potentials are quoted with respect to Ag/AgCl/3 M KCl in water
−
2
1
reference electrode (−0.036 vs SCE).
Instrumentation.—Cyclic voltammograms were obtained and
controlled-potential electrolyses were performed with the aid of an
AUTOLAB model PGSTAT12 potentiostat–galvanostat. Data from
the above experiments were acquired and stored by GPES 4.9 soft-
ware, which controlled a data acquisition board installed in a personal
computer. Controlled-current electrolyses were carried out with the
aid of a stabilized constant current supply (Sodilec, EDL 36.07).
The ionic conductivity of the RTILs, at ambient pressure, and
at a temperature of 26.0°C for [C
2 2
mim][NTf ] and 30.4°C for
[N1 1 1 2(OH)][NTf
2
], was measured by electrochemical impedance
spectroscopy using a GAMRY potentiostat/galvanostat reference 600.
The experiments were carried out in two electrodes configuration. An
Ag/AgCl electrode was used as reference and a platinum wire with
1
1
0
mm diameter was used as working electrode. An AC voltage with
0 mV rms was used within a frequency ranging from 100 kHz to
.5 Hz. The temperature was measured with a PT100 temperature
sensor in a four probe measurement configuration.
Identification and quantification of products.—After the elec-
trolysis, the reaction mixture was allowed to stand under stirring with
cyclohexane 30 min and then it was extracted with cylohexane (3 ×
Figure 2. Nyquist plots: (A) [N1 1 1 2(OH)][NTf2] at 30.4°C and (B) [C2mim][
NTf2] at 26.0°C
1
5 mL portions). For the second independent experiment carried out
in the experimental conditions of run 1, after the extraction of the
reaction mixture with cylohexane, H O was added to the resultant
ionic liquid and then extracted with Et
O (3 × 15 mL portions). In
both cases, the combined extract was washed in succession with three
5 mL portions of brine and 25 mL of water. The organic phase was
2
2
where d is the distance between the electrodes (23.0 mm), A is the
2
electrode contact area (12.56 mm ), and R
b
is the bulk resistance of
2
b
the sample. R was obtained from the intercept of the imaginary part of
dried over anhydrous magnesium sulfate and the solvent was evapo-
rated under reduced pressure to afford the products of the reaction.
the impedance (minimum value of Z”) with the slanted line in the real
part of the impedance (Z’). Figures 2A and 2B represents the Nyquist
curves of the ILs determined by AC impedance at 30.4°C and 26.0°C,
respectively. Independently of the frequency, an inclined straight-line
is observed and this behavior represents the diffusion of the ions that
1
We analyzed the residue by means of GC, GC-MS and H NMR and
compared with spectra of authentic samples. On the other hand, the
resultant ionic liquid containing the catalyst dissolved was filtered (if
necessary) and dried under vacuum to be reused.
24
favors ion migration which results in a low impedance.
Incorporation of water into ILs increases the diffusion of ions due
b
, shift to
Results and Discussion
3
Water influence on the performance of [N1 1 1 2(OH)][NTf
mim][ NTf ] ionic liquids.—In order to investigate water effects
2
on the ionic conductivity of [N1 1 1 2(OH)][NTf ] and [C mim][ NTf ]
2
] and
[
C
2
2
2
], at the initial stage, we can observe
2
2
ionic liquids, different amounts of water were added to both ionic
liquids at 30.4 and 26.0°C, respectively. The component ions of ILs
interact strongly with water via ion-dipole interactions. The transport
properties depend on many factors, such as the degree of IL dissocia-
tion, IL concentration, the degree of ionic association and ion mobility.
The effect of water in the transport properties was determined through
electrochemical impedance spectroscopy.
decrease in viscosity (and increase in conductivity) of the ILs with
increasing water content. This suggests that water molecules decrease
the interactions among ions and these lower interactions results in less
stiff ions aggregates than those in a dry system, resulting in a less
The ionic conductivity (σ
Equation 1:
i
) was determined according to
d
σ
i
= R
,
[1]
26
viscous mixture.
b
A
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G20
Journal of The Electrochemical Society, 166 (6) G17-G24 (2019)
plex in [N1 1 1 2(OH)][NTf
[
2
] ionic liquid. We can observed that the
Ni(tmc)] complex undergoes a reversible one-electron reduction
to a stable product. These observations are in agreement with the re-
2
+
30
sults found in an earlier investigation as well as those of reactions
18
carried out in other media. Similar results were obtained for the re-
2
+
duction of [Ni(tmc)] in the [C
2
mim][NTf
2
] ionic liquid. The formal
2
+
+
electrode potential of [Ni(tmc)] −[Ni(tmc)] redox couple occurred
at −0.89 V vs an aqueous Ag/AgCl/3M KCl in [N1 1 1 2(OH)][NTf
ionic liquid and at −0.93 V vs an aqueous Ag/AgCl/3M KCl in
mim][NTf ] ionic liquid.
2
]
[
C
2
2
Hence, all the data obtained indicate that the electrode reaction is:
[
Ni (tmc)]2 + e− ꢀ [Ni (tmc)]+.
+
Further, we have explored the influence of the added water to
the ILs on the cyclic voltammetric behavior of [Ni(tmc)]Br com-
plex. Figure 4, curve B shows the cyclic voltammogram obtained at
a vitreous carbon electrode for a solution of 1.0 mM [Ni(tmc)]Br in
2
2
[
N
1 1 1 2(OH)][NTf
2
] ionic liquid after addition of water.
We can observe that addition of 15% (v/v) of H
] ionic liquid leads to an increase for both cathodic
complex, which indicates
2
O to the
[
N1 1 1 2(OH)][NTf
2
and anodic peak currents of [Ni(tmc)]Br
2
that there is a change in its diffusion rate, although the peak poten-
tials for the anodic and cathodic processes remain approximately con-
stant. Similar behavior was observed when these experiments were
2 2 2
performed in [C mim][NTf ] ionic liquid in the presence of 15% H O.
31
Schroder et al. found similar results in the investigation of the in-
fluenceofwateronthevoltammetryofseveraldissolvedredoxsystems
in the [BMIM][BF
that the presence of water in the ionic liquids have a dramatic effect
4 6
] and [BMIM][PF ] ionic liquids. They reported
ꢀ
ꢀ
leading to an increase of the rate of diffusion of N,N,N ,N -tetramethyl-
paraphenylenediamine(TMPD)andofmethylviologendication. They
attributed the increase in the rate of diffusion to the decrease of the
viscosity of the ionic liquid.
If we compare our results presented above with those reported by
Schroder et al., we believe that there is an increase in the diffusion rate
of [Ni(tmc)]Br complex in [N1 1 1 2(OH)][NTf ] and [C mim][NTf ]
2 2 2 2
ionic liquids in the presence of added water and this should be due to
the increase of ionic conductivity of these media.
Figure 3. Water influence on conductivities of ionic liquids: (A)
[
N1 1 1 2(OH)][NTf2] at 30.4°C and (B) [C2mim][ NTf2] at 26.0°C.
Cyclic voltammetry of [Ni(tmc)]Br
gyl bromoethers derivatives 1.—The cyclic voltammetric behav-
ior of a solution of 1.0 mM [Ni(tmc)]Br was also performed in
the presence of an excess of substrates 1 in [N1 1 1 2(OH)][NTf ] and
mim][NTf ] ionic liquids. In all cases, we observed that the addi-
2
in the presence of propar-
2
The conductivity started to decrease at concentrations of water
2
higherthan18%(v/v). Owingtothehydrophobicnatureof[N1 1 1 2(OH)
NTf ], H O began to separate. A clear interface between the organic
solution and aqueous solution could be observed and these phenomena
]
[
C
2
2
[
2
2
tion of 1 to the [Ni(tmc)]Br
of [Ni(tmc)] to [Ni(tmc)] increased as we increased the concen-
2
solution, the peak current for the reduction
2+
+
27
was also observed by other authors.
Also, maybe, as the concentration of water increases, the associa-
tion of water molecules increases and spreads throughout the ILs, in
which small ionic clusters are formed.
Many of these ionic clusters result in the aggregation of ions, be-
cause of the inherent amphiphilicity of the component ions of ILs.
For [C mim][ NTf ] ionic liquid, we can also observe in Figure 3B
tration of 1 and the anodic wave disappeared. For example, Figure 5
(
curves B and C) present the cyclic voltammograms recorded for so-
lutions of 1.0 mM [Ni(tmc)]Br ] ionic liquid in
the presence of different concentrations of 1a.
2
in [N1 1 1 2(OH)][NTf
2
28
The large catalytic current observed is due to the fast regeneration
29
2+
of [Ni(tmc)] complex and leads to the possibility of quick synthetic
2
2
electrolyses using only a small amount of the nickel(II) complex ac-
that, at the initial stage, the conductivity increases as we increase the
water concentration. When the water concentration is approximately
32,33
cording to the EC’ mechanism presented in Figure 6.
On the basis of these data we can conclude that the extent of the cat-
alytic reaction increases when increasing [RBr] and the regeneration
3
0 % (v/v), the conductivity reaches a maximum value of 0.033 S/cm
and keeps almost constant until water concentration of 40 % (v/v).
In the case of this [C mim][ NTf ] ionic liquid the conductivity
value remains practically constant with a higher percentage of added
water compared to the [N1 1 1 2(OH)][NTf ] ionic liquid. This is probably
due to the more hydrophilic character of the [C mim] [NTf ].
2+
of [Ni(tmc)] after the chemical reaction is a fast process.
2+
2
2
In the absence of [Ni(tmc)] , propargyl bromoethers derivatives
1
a−1c are reduced directly at potentials below −1.54 V vs an aque-
2
ous Ag/AgCl/3M KCl under the same experimental conditions, cor-
responding to the reductive cleavage of the carbon–bromine bond.
2
2
Water influence on cyclic voltammetric behavior of [Ni(tmc)]Br
2
Controlled-potential electrolysis of [Ni(tmc)]Br
ence of propargyl bromoethers 1.—A series of controlled-potential
electrolyses for mixtures of [Ni(tmc)]Br and each of propargyl
bromoethers 1 in the presence of [Ni(tmc)]Br were performed
at carbon felt or reticulated vitreous carbon cathodes (RVC) in
] and [C mim][NTf ] ionic liquids. The experimen-
2
in the pres-
in [N1 1 1 2(OH)][NTf
2
] and [C
voltammetric behavior of [Ni(tmc)]Br
and in [C mim][NTf
curve A, shows a typical voltammogram obtained at a glassy car-
bon disc electrode for a solution of 1.0 mM of [Ni(tmc)]Br com-
2
mim][ NTf
2
] ionic liquids.—The cyclic
2
complex in [N1 1 1 2(OH)][NTf
2
]
2
2
2
] ionic liquids was first investigated. Figure 4,
2
2
[N1 1 1 2(OH)][NTf
2
2
2
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Journal of The Electrochemical Society, 166 (6) G17-G24 (2019)
G21
E / V vs Ag / AgCl
-2.00
-1.50
-1.00
-0.50
0.00
5
3
0
Figure 4. Cyclicvoltammogramsrecordedforthereductionofasolution1.0mM
2
of [Ni(tmc)]Br2 at a glassy carbon electrode (area = 0.071 cm ) at a scan rate of
–1
1
00 mV s in [N1 1 1 2(OH)][NTf2] ionic liquid: (A) under dry conditions; (B) in
-3
the presence of 15% (v/v) of water.
A
-5
B
-8
-10
E / V vs Ag / AgCl
-1.00
cases, leads to the formation of the expected cyclic compound, namely,
2
-(4’-methoxyphenyl)-4-methylene-3-phenyltetrahydrofuran (2a) as
-2.00
-1.50
-0.50
0.00
the major product in 57% yield (Table I, entries 1) and 97% yield
5
(Table I, entries 2), respectively.
0
Ph
CH2
A
-
5
B
C
O
-
10
15
20
Figure 5. Cyclic voltammograms recorded at a glassy carbon electrode (area
MeO
2
a
-
We can observe that when the catalyst to substrate ratio was
-
changed from 20 to 10 mol %, the reaction afforded a more efficient
cyclization (Table I, entries 1 and 2). These results are in agreement
with those found in our previous investigation of the intramolecular
radical cyclization of analogous bromo propargyloxy ester 1c in room
temperature ionic liquids, where it was also observed that the product
yields seems to be dependent on the ratio [RBr]/[Ni(II)].
2
–1
=
0.070 cm ) at a scan rate of 100 mV s in [N1 1 1 2(OH)][NTf2] ionic liquid:
A) 1.0 mM [Ni(tmc)]Br2; (B) 1.0 mM [Ni(tmc)]Br2 and 5.0 mM 1a; (C)
.0 mM [Ni(tmc)]Br2 and 10.0 mM 1a.
(
30
1
Next, the nature of the ionic liquid was also investigated. Hence,
+
when the catalytic reduction of 1a by electrogenerated [Ni(tmc)] was
2 2
carried out in [C mim][NTf ] ionic liquid, the cyclized compound 2a
tal results obtained are reported in Table I. Product yields are tabulated
as the percentage of starting material incorporated into a particular
product. The potential was set at approximately 100 mV more nega-
was obtained in 47% yield as the only product (Table I, entry 5). These
results show that the nature of ionic liquid did not exert any major
effect neither on the yields of the product 2a nor on the coulometric
n values. Here again, the results also agree with those obtained in the
intramolecular radical cyclization of 1c, where the product yields seem
2+
tive than the peak potential for the reduction wave of [Ni(tmc)] in
the presence of the substrate 1. This potential ensured the complete
reduction of each starting material. For each controlled-potential elec-
trolysis, the current was monitored as a function of the charge passed,
until all of the starting material was consumed and the current-time
data were used to determine the number of electrons (n) involved in the
reduction of the substrate 1a−1b. On the basis of the results presented
in Table I, we can observe that the number of electrons involved per
molecule of starting material 1 lies between 0.9 and 1.3. Experimental
conditions, such as, the ratio of substrate to catalyst concentration and
the presence of water were examined to evaluate their effect on the
products yields.
30
to be independent on the nature of ionic liquid.
In order to get more insight onto the influence of added wa-
ter as a co-solvent in the yields and selectivity of the reaction, we
have examined the catalytic reduction of 1a in both ionic liquids,
[
N
1 1 1 2(OH)][NTf
2
2 2
] and [C mim][NTf ], respectively, in the presence
of 15% (v/v) of H O. It shoud be noted that it was not possible to
2
use an amount of water higher than 15% in the ionic liquid mixture
because the bromoethers 1 were not soluble.
We observed that the reaction was very efficient affording the cyclic
compound 2a in yields ranging from 91−97% (Table I, entries 3 and 4)
The study was initiated by performing the catalytic reduction
+
in [N
1 1 1 2(OH) 2 2 2
][NTf ] and 81% (Table I, entry 7) in [C mim][NTf ]. It
of 1a in the presence of 20% of electrogenerated [Ni(tmc)] in
] ionic liquid and comparing the results with those
obtained in the presence of 10% of catalyst. The reaction, in both
is worth noting that, in the presence of water, the electrolysis afforded
an almost quantitative yield of cyclization and the product yields were
[N1 1 1 2(OH)][NTf
2
R1
R2
Br
O
.
[
Ni(tmc)]+
+
1e-
Figure 6. Mechanism for the Ni(II)-catalyzed cycliza-
tion of compounds of type 1.
.
R1
R1
CH2
R1
H+ / or H.
Ni(tmc)]2+
[
R2
O
R
2
O
R2
Br-
O
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G22
Journal of The Electrochemical Society, 166 (6) G17-G24 (2019)
+
Table I. Coulometric data and product yields for catalytic reduction of solutions of 1 by [Ni(tmc)] electrogenerated at carbon felt or reticulated
vitreous carbon cathode in ionic liquids, under controlled-potential electrolysis.
[Ni(tmc)]2 , mM
+
[RBr], mM
na
Product yield, %b
Entry
Substrate
Medium
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1b
1b
1b
1b
1b
1c
[N1 1 1 2(OH)][NTf2]
[N1 1 1 2(OH)][NTf2]
[N1 1 1 2(OH)][NTf2]c
[N1 1 1 2(OH)][NTf2]c
[C2mim][NTf2]
[C2mim][NTf2]
[C2mim][NTf2]c
DMF/0.10 M Et4NBF4
1.2
0.6
1.2
0.6
1.1
1.2
0.6
0.4
0.3
0.3
0.7
0.4
0.3
0.26
1.0
1.0
6.0
5.8
5.9
5.8
5.9
5.8
5.8
4.0
2.7
2.5
6.0
4.0
2.5
2.5
10.0
5.0
0.9
1.1
1.2
1.2
0.9
1.2
1.1
1.1
1.0
1.3
1.3
1.1
1.0
1.1
-
57
97
91
97
47
38
81
98
79
98
84
84
99
80
100
d
e
f
EtOH/0.10 M Et4NBr
g
h
0
1
2
3
4
5
6
MTAB /Tetradecane/H2O/1-pentanol (17.5/12.5/35/35)
[N1 1 1 2(OH)][NTf2]
e
DMF/0.10 M Et4NBF4
f
EtOH/0.10 M Et4NBr
i
h
CTAB /Dodecane/H2O/1-pentanol (17.5/12.5/35/35
j
DMF/0.006 M Bu4NBF4
l
m
[N1 1 1 2(OH)][NTf2]
1.1
65 (82:18)
a
Number of electrons per molecule of starting material.
b
c
d
e
f
%
= yield expressed as the percentage of 1 incorporated into each product.
Controlled potential electrolysis carried out in the presence of 15% (v/v) of water.
The IL was reused.
Data from Ref. 36.
Data from Ref. 18.
g
Tetradecyltrimethylammonium bromide (MTAB).
Data from Ref. 41.
h
i
Cetyltrimethylammonium bromide (CTAB).
Data from Ref. 40 (Contant-current electrolysis, Zn sacrificial anode).
Data from Ref. 30.
j
l
m
ꢀ
Product ratio (2c, 2c ).
not sensitive to the [RBr]/[Ni(II)] ratio (Table I, entries 3 and 4), as
it was observed in the absence of water (Table I, entries 1 and 2).
CO Et
2
CH3
EtO C
2
CH
2
EtO C
2
MeO
MeO
Hence, these results indicate that 15% H
2
O/ionic liquid mixture could
MeO
MeO
MeO
MeO
O
O
be a good electrolytic medium for the intramolecular radical cycliza-
tion of propargyl bromoethers derivatives. Besides, these results also
demonstrate that the presence of water not only did not interfere on
the overall cyclization process but could also improve the product
yields and the selectivity of the reaction by inhibiting the formation of
by-products.
2c
2c'
3c
ꢀ
If we compare the total yield of the cyclic compound 2c and its
ꢀ
endocyclic isomer 2c (Table I, entry 16: 65%, 2c/2c = 82:18) ob-
9
This behavior is similar to that reported by A. Jones et al. in the in-
tained in our previous investigation by means of controlled-potential
30
vestigation of the electrochemical reductive coupling of 2-cyclohexen-
electrolysis in dry [N1 1 1 2(OH)][NTf
2
]
with the yield of the cyclic
1
-one in a mixture of [BMIM][BF
4
] ionic liquid and water. They ob-
compound 2c (69%, see entry 1 of Table II, footnote a) obtained in the
present investigation, we see that water has no influence on the yield
but does influence the selectivity of the reaction as only the endocyclic
compound 2c was produced.
served that there was a significant improvement of the yield of the cou-
pling product, 1,6-diketone, upon addition of water as the co-solvent
in the ionic liquid and with the increase of volume % of H
product yield was also increased. The best results were obtained with
0% H O/ [BMIM][BF ] mixture.
The studies were extended to the bromoether derivative 1b. As
2
O, the
2
2
4
Table II. Ionic liquid recycling: Electrochemical intramolecular
+
cyclization of 1c (6.0 mM) catalyzed by [Ni(tmc)] (20 mol %) at
a carbon fiber cathode in recycled 15% H2O/[N1 1 1 2(OH)][NTf2]
mixture, under constant-current electrolysis.
shown in Table I, entry 11, the catalytic reduction of 1b by electroge-
narated [Ni(tmc)] performed in [N1 1 1 2(OH)][NTf
+
2
] ionic liquid gave
rise to 2-methoxy-4-methylene-3-phenyltetrahydrofuran (2b) as the
major product in a good yield, as it was observed for 1a and 1c
30
Product distribution (%)
when the catalyst to substrate ratio was 10 mol %.
Run
Anode
2c
3c
1c
Ph
CH2
1a
2
3
4
5
Mg
Mg
Mg
Mg
Mg
30
33
36
33
38
-
-
4
6
13
4
13
16
10
-
MeO
O
2b
The electroreductive intramolecular cyclization of compound 1c
was also performed at constant-current electrolysis using a sacrificial
magnesium anode in [N1 1 1 2(OH)][NTf ]/H O mixture leading to the
2 2
aA second independent experiment in the experimental conditions of
run 1 was performed. After the extraction of reaction mixture with
cyclohexane, we carried out a second extraction of the resultant IL
formation of the cyclic compound 2c as the major product (see details
mixture with Et O (see details in the Experimental section) and the
2
in Table II).
total yield of the endocyclic compound 2c increased from 30% to 69%.
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Journal of The Electrochemical Society, 166 (6) G17-G24 (2019)
G23
In dry [N1 1 1 2(OH)][NTf
group but, more probably, it is the H-CH(OH)CH
this C-H bond is weaker than that of the OH) giving formaldehyde
reduced subsequently to methanol) and trimethylamine by Hofmann
2
], the source of protons could be the OH
reported electrochemical studies performed in the same experimental
conditions (technique used, cell design and nature of the electrodes) in
+
2
N (CH
3
)
3
bond
26,40
(
(
DMF containing 0.10 M Et
4
NBF
4
(Table I, entries 8,12 and 15)
as
well as with those obtained in green alternative media such as EtOH
18
elimination. So the medium becomes more basic. But, adding water
to the IL should favor the exchange of protons (higher mobility of
water molecules and of OH anions – diffusion). The based catalyzed
4
containing 0.10 M Et NBr (Table I, entries 9 and 13) and microemul-
41
sions (Table I, entries 10 and 14). The overall yields and selectivity
of the reactions obtained in ionic liquids are similar or slightly lower
than those obtained in DMF, EtOH and in microemulsions. However,
the advantage of using RTILs is that they avoid the expensive use of
supporting electrolytes or surfactants in microemulsions due to the in-
trinsic ionic conductivity. Another advantage of using an ionic liquid
is that it makes the separation step after the electrolysis much easier.
Besides, the ionic liquid can be recovered efficiently because ILs are
not volatile and can be recycled and reused in future reactions.
Finaly, it should be mentioned that after the extraction of products
ꢀ
isomerization of 2c to 2c would then be faster in the presence of water.
Reuse of ionic liquid.—Afterwards, the study of the reuse of
ionic liquid system was performed. Hence, the catalytic electroreduc-
tion of the bromoester 1c was carried out at constant-current den-
−
2
sity of 1.5 mA cm in the presence of [Ni(tmc)]Br
at carbon fiber cathode using a sacrificial magnesium anode in 15%
].
The reaction proceeded through the cleavage of both carbon-
2
(20 mol%),
2 2
H O/[N1 1 1 2(OH)][NTf
2
+
from ionic liquids, the [Ni(tmc)] totally remained in the ionic liquid
and the system ionic liquid/catalyst may be dried and reused. The
reuse of the ionic liquid system was performed and the results obtained
indicate the same efficiency in the cyclization (Tables II). In addition,
the extraction and the purification of the products is much easier in
the absence of supporting electrolyte.
halogen and carbon-(propargyl)oxygen bonds to afford the cyclised
rearranged ester 2c, as the main product, along with the aryl propi-
onic ester 3c, as a by-product. Similar results were obtained for the
electrolysis of 1c carried out at constant-current electrolysis in DMF
34
using Mg anode.
Table II presents the product distribution of the reaction and we
can observe that the yields of the endocyclic compound 2c obtained
in each experiment are similar in the five runs. This indicates that the
ionic liquid/water mixture used in the reactions can be recyclable.
It is evident from the results presented that controlled-potential
electrolysis performed in ionic liquids or in ionic liquids/water mix-
Conclusions
The experimental results presented suggest that the catalytic elec-
troreductive intramolecular cyclization of the bromoethers 1 catalized
by electrogenerated Ni(I) complex can afford cyclic compounds under
mild conditions in ionic liquids or in a mixture of ionic liquid/water
as solvents. The mechanism of the catalytic electroreductive cycliza-
tion of the bromoethers 1 in ionic liquids is similar to that proposed
in DMF, EtOH and microemulsions. Moreover, an advantage of this
method is that it avoids the use of volatile organic solvents and any ad-
dition of supporting electrolyte as well as the use of toxic n-tributyltin
hydride as the classical radical source (and its by-products).
2
+
tures in the presence of a catalytic amount of [Ni(tmc)] complex is a
selective and convenient method to convert the propargyl bromoethers
1a–1b in the corresponding tetrahydrofuran derivatives 2a–2b. More-
over, the reactions were also carried out at room temperature using an
environmentaly friendly methodology.
The electrolyses results also show that the reaction involves a first
2
+
electron-transfer of [Ni(tmc)] complex at the electrode to produce
+
[
Ni(tmc)] complex which can transfer one electron to the propargyl
bromoether derivative 1 via an inner-sphere mechanism to form a radi-
35
Acknowledgments
calintermediate(Figure6). Infact, theproductsobtainedinthisstudy
are consistent with formation of radical intermediates. Once formed,
the radical intermediates undergoes intramolecular cyclization on the
triple bond affording the carbocyclic radicals with regeneration of the
We acknowledge the referees for their helpful comments to im-
prove the manuscript. The authors thank the Institut de Chimie de
Nice of the University Côte d’Azur (France) for the Mass Spec-
trometry facilities. Thanks are due to Fundação para a Ciência e
Tecnologia and Associate Laboratory for Green Chemistry-LAQV
2
+
starting [Ni(tmc)] complex. The coulometric n values (0.9 < n ≤
1
.3) would suggest that the cyclic radical intermediates can be re-
duced and / or to abstract of a hydrogen radical atom from a hydrogen
radical atom donor in the medium to afford the corresponding cyclic
compounds.
(
UID/QUI/50006/2019) for partial financial support of this work.
Similar results were obtained in our previous work on catalytic
reduction of analogous propargyloxy bromo derivatives by electro-
ORCID
+
30,32,33,36
J. M. S. S. Esperança https://orcid.org/0000-0001-9615-8678
M. J. Medeiros https://orcid.org/0000-0002-0919-1822
J. F. Ribeiro https://orcid.org/0000-0003-1454-2778
generated [Ni(tmc)] complex.
Besides, we believe that the
beneficial effect of adding water to the RTILs as a co-solvent can be
due to the fact that water is a good H-proton donor. Hence, probably it
can accelerate the last step of the reductive protonation of the cyclized
radical formed during the reaction to afford the corresponding cyclic
compound, avoiding the formation of by-products.
With respect to the stereochemistry of the two starting materi-
als and of the cyclic products, it should be noted that the original
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