Electrochemical intramolecular cyclisation of propargyl bromoethers catalysed by nickel complexes

Article abstract of DOI:10.1039/b415920a

The electrochemical intramolecular cyclisation of propargyl 2-bromoethers 1-3 in N,N′-dimethylformamide at constant current in a diaphragmless cell has been developed using Ni^II complexes as electron-transfer mediators. During controlled-current electrolyses of solutions of Ni ^II complexes in the presence of bromoethers, catalytic reduction of the latter proceeds via one-electron cleavage of the carbon-bromine bond to form a radical intermediate that undergoes cyclisation to afford the cyclic compounds in moderate to good yields. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2005.

Full text of DOI:10.1039/b415920a

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P A P E R
Electrochemical intramolecular cyclisation of propargyl
bromoethers catalysed by nickel complexes
Elisabet Dun
˜
ach,ab Ana Paula Esteves,c Maria J. Medeiros*c and Sandra Oliverob
a
b
c
´
Laboratoire de Chimie Bioorganique (CNRS UMR 6001), Universite de Nice-Sophia
Antipolis, Parc Valrose, 06108 Nice cedex 2, France
` ´
Laboratoire Aroˆmes, Syntheses et Interactions, Universite de Nice-Sophia Antipolis,
Parc Valrose, 06108 Nice cedex 2, France
´
Centro de Quımica/IBQF, Universidade do Minho, Largo do Pac¸o, 4700-320 Braga, Portugal.
E-mail: medeiros@quimica.uminho.pt; Fax: þ351-253-678983
Received (in Toulouse, France) 13th October 2004, Accepted 6th January 2005
First published as an Advance Article on the web 10th March 2005
The electrochemical intramolecular cyclisation of propargyl 2-bromoethers 1–3 in N,N0-dimethylformamide
at constant current in a diaphragmless cell has been developed using NiII complexes as electron-transfer
mediators. During controlled-current electrolyses of solutions of NiII complexes in the presence of
bromoethers, catalytic reduction of the latter proceeds via one-electron cleavage of the carbon–bromine bond
to form a radical intermediate that undergoes cyclisation to afford the cyclic compounds in moderate to
good yields.
clopentane, as well as give dimeric species arising from cou-
Introduction
pling of both cyclic and acyclic radicals.7 In our previous
investigations of radical cyclisation reactions promoted by
electrogenerated nickel(I) complexes8–11 it was reported that
electrogenerated nickel(I) complexes catalyse the intramolecu-
lar cyclisation of unsaturated halides to yield alkenyl radical
type intermediates, which can react intramolecularly to give
carbocyclic products.
The radical methodology has emerged as one of the leading
methods in synthetic organic chemistry for the construction of
complex carbocyclic as well as heterocyclic compounds.1 The
versatility of this technology has aroused great interest in
developing novel reagents and procedures for the generation
of radicals. In addition to various synthetic methods, electro-
chemical radical-type cyclisations catalysed by transition metal
complexes have been studied. This process is based on an
EC0 mechanism:
In our previous work, we reported the electrochemical
studies of intramolecular cyclisation of ethyl 2-bromo-3-
(30,40-methylenedioxophenyl)-3-propargyloxypropanoate and
ethyl 2-bromo-3-(30,40-dimethoxyphenyl)-3-allyloxypropano-
ate catalysed by the macrocyclic Ni(II) complexes 4–6, shown
in Scheme 1.14 It was demonstrated that the electroreduction
reaction of bromopropargyl and allyl esters (Scheme 2) carried
out in DMF in a single-compartment cell was catalysed by the
Ni(I) complexes electrogenerated from 4–6. Only cyclisation to
the five-membered ring ethers as the main products was
observed, in moderate to good yields. This encouraged us to
undertake further studies on analogues and derivatives of
bromopropargyl esters as substrates.
Following this approach, we have investigated the use of
electrogenerated nickel(I) complexes for the catalytic reduction
of three propargyl bromoethers {[1-bromo-2-methoxy-2-(prop-
20-ynyloxy)ethyl]benzene (1), 1-[2-bromo-2-phenyl-1-(prop-20-
ynyloxy)ethyl]-4-methoxybenzene (2), and 2-bromo-3-(30,40-di-
methoxyphenyl)-3-propargyloxy-propanamide (3)} in dimethy-
lformamide (DMF) containing tetraalkylammonium salts.
Apart from being environmentally less aggressive, this techni-
que makes the purification of the products much easier.
NiIIL þ eꢀ " NiIL
kcat
(1)
NiIL þ RBr ꢀ! NiIIL þ products
ð2Þ
in which NiIIL, with a reduction potential less negative than
the substrate (RBr), is reduced at the electrode. The reduced
mediator diffuses into the solution and reduces the substrate.
The catalytic reaction recycles the mediator back to the
electrode according to the kinetics of the system and the
relative concentration of the substrate and the mediator (called
the excess factor, g ¼ [RBr]/[NiIIL]).2
Several reports on the electrogenerated NiI catalysed indirect
reductive radical cyclisation of various organic halides,3 bro-
moacetals possessing electron-deficient olefinic moieties,4 and
2-haloaryl ethers containing unsaturated side chains5 have
shown that the electrochemical method is an expedient alter-
native to the use of organometallic reagents. This nickel-
mediated radical cyclisation has also been applied to the
synthesis of substituted tetrahydrofurans;6–11 the furofuran
moiety is an important subunit in a wide range of biologically
active natural products.12–13 Mubarak and Peters6 have shown
that reductive intramolecular cyclisation of 6-iodo- and 6-
bromo-1-phenyl-1-hexyne, catalysed by electrogenerated nickel(I)
salen, leads to the production of benzylidenecyclopentane in 64
–95% yield. More recently, the nickel(I) salen catalysed reduc-
tion of 6-bromo-1-hexene has been shown to entail cleavage of
the carbon–bromine bond to give the 5-hexen-l-yl radical,
which undergoes intramolecular cyclisation to form methylcy-
Experimental
Methods and materials
Published procedures were employed for the preparation of
[Ni(tmc)]Br2,15 [Ni(cyclam)](ClO4)2,15 [Ni(CR)]Cl2,16 1,17 2,18
and 3.18 Synthesis of 2-methoxy-4-methylene-3-phenyltetrahy-
drofuran (7), 2-(40-methoxyphenyl)-4-methylene-3-phenyltetra-
T h i s j o u r n a l i s & T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 5
N e w J . C h e m . , 2 0 0 5 , 2 9 , 6 3 3 – 6 3 6
633

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Controlled-current electrolyses were carried out with the aid
of a stabilised constant-current supply (Sodilec, EDL 36.07).
Identification and quantification of products
The DMF solvent was evaporated under vacuum, the reaction
mixture was hydrolysed 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 was evaporated and the
residue analysed by GC, GC-MS and 1H NMR. The crude
residue was submitted to flash chromatography over silica gel
(230–400 mesh) using diethyl ether–light petroleum 1 : 2 (for 7),
diethyl ether–light petroleum 1 : 3 (for 9) and ethyl acetate–
methanol 13 : 1 (for 11) as eluants. The 1H NMR spectra of
different fractions collected were compared to those of authen-
tic samples.
Scheme 1
hydrofuran (9) and 2-(3040-dimethoxyphenyl)-3-carbamoyl-4-
methylenetetrahydrofuran (11) was based on the method pub-
lished by McCague et al.19
1
We identified the compounds by means of H NMR spec-
trometry with a Varian Unity Plus 300-MHz instrument
(CDCl3). 7: d 3.41 (3H, s, OCH3); 3.81 (1H, br s, 3-H); 4.61
(2H, app q, J 2.0 Hz, 5-H2); 4.99 (1H, app q, J 2.0 Hz,
C
C
CHH); 5.02 (1H, br s, 2-H); 5.12 (1H, app q, J 2.0 Hz,
Q
Results and discussion
CHH); 7.20–7.40 (5H, m, C H ); 9: d 3.64–3.70 (1H, m, 3-
Q
H); 3.78 (3H, s, OCH3); 4.66 (1H, dq, J 13.2 and 2.4 Hz, 5-Ha);
6
5
The use of sacrificial anodes has enabled new and efficient
processes in electrosynthetic reactions.20 The cationic nickel
complexes 4–6 (Scheme 1), having shown an efficient catalytic
activity for the electrochemical cleavage of carbon–bromine
bond of unsaturated halides,8–11,14 were used as the catalysts in
the current work. The catalytic electroreduction of propargyl
bromoethers 1–3 by electrogenerated nickel(I) complexes was
performed at constant current and the results are summarised
in Table 1.
Cyclic voltammetric studies established that the formal
electrode potential of Ni(II) complexes 4–6 occurred at
ꢀ1.36, ꢀ0.91 and ꢀ0.56 V vs. Ag/AgCl, 3 M KCl, respec-
tively,9,11 with further electron transfer from the electrogener-
ated Ni(I) complexes to the bromoethers, according to eqns.
(3)–(5) below. Cyclic voltammetric studies of solutions of
bromoethers 1–3 in N,N0-dimethylformamide containing 0.1
M n-Bu4NBF4 at a glassy carbon electrode show two well-
defined reduction peaks at potentials ꢀ1.78 and ꢀ2.84 V vs. Ag
/AgCl for compound 1, three well-defined reduction peaks at
potentials ꢀ1.65, ꢀ2.23 and ꢀ2.49 V vs. Ag/AgCl for com-
pound 2 and three well-defined reduction peaks at potentials
ꢀ1.73, ꢀ2.01 and ꢀ2.25 V vs. Ag/AgCl for compound 3. We
attribute the first reduction peak to the irreversible reduction
cleavage of the carbon–bromine bond. When voltammetric
curves were reported for complexes 4–6 in the presence of
bromoethers 1–3, the reversible 1-electron reduction peak of
4.72 (1H, app q, J 2.4 Hz, C CHH); 4.79 (1H, d, J 9.3 Hz, 2-
Q
H); 4.84 (1H, br d, J 13.2 Hz, 5-Hb); 5.09 (1H, app q, J 2.1 Hz,
C
CHH); 6.81 (2H, app d, J 8.5 Hz, 20-H and 60-H); 7.14 (2H,
Q
app d, J 8.5 Hz, 30-H and 50-H); 7.22–7.34 (5H, m, Ph); 11: d
3.38 (1H, br app dd, J 7.2 and 1.8 Hz, 3-H); 3.88 (3H, s,
OCH3); 3.89 (3H, s, OCH3); 4.54 (1H, app dq, J 13.2 and 2.4
Hz, 5-Ha); 4.63 (1H, br app d, J 13.2 Hz, 5-Hb); 5.19 (1H, app
q; J 2.4 Hz, C CHH); 5.20 (1H, d, J 6.9 Hz, 2-H); 5.22 (1H,
Q
Q
app q, J 2.4 Hz, C CHH); 5.66 (1H, br s, NH); 5.73 (1H, br s,
NH); 6.82–6.95 (3H, m, 20-H, 50-H and 60-H).
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. 7:
m/z (70 eV) 290, M1 (0.5); 159, [M ꢀ CH3O]1 (8); 129,
[M ꢀ CH3OCHOH]1 (100); 91, [C6H5CH2]1 (32); 77,
[M ꢀ C6H5]1 (11); 8: m/z (70 eV) 158, M1 (100); 129,
[M ꢀ CHO]1 (65); 77, [C6H5]1 (17); 9: m/z (70 eV) 266, M1
(10); 129, [M ꢀ CH3OC6H4CHOH]1 (100); 91, [C6H5CH2]1
(8); 77, [C6H5]1 (18); 10: m/z (70 eV) 210, M1 (100); 115,
[C6H5C3H2]1 (10); 89, [C6H5C]1 (10); 11: m/z (70 eV) 263, M1
(73); 218, [M ꢀ H2NCOH]1 (65); 165, [(CH3O)2C6H3CO]1
(100); 77, [C6H5]1 (38).
Electrochemical cells and instrumentation
The constant-current electrolyses were carried out in a single-
compartment cell (capacity 50 ml), such as described in ref. 19,
with a Zn, Al or Mg rod as the sacrificial anode (diameter 1 cm)
and a carbon felt cathode (apparent surface, 20 cm2), which
was formed into a cylinder around the counter electrode.
Freshly distilled DMF (50 ml), n-Bu4NBF4 (6 ꢁ 10ꢀ3 M),
NiIIL (2.0 ꢁ 10ꢀ3 M) and bromoether (2.0 ꢁ 10ꢀ2 M) 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 1.5 mA cmꢀ2 and 2–5 V
between the rod anode and the carbon felt cathode) until
complete disappearance of the bromoether (checked by GC
analysis of aliquots). Generally, 4–6 F molꢀ1 of starting
material were necessary to achieve a complete conversion.
The excess of electricity is accounted for mainly by the presence
of a Zn anode, which can be easily oxidised and further
reduced.
Table 1 Electrochemical intramolecular cyclisationa of propargyl
ethers1–3 catalysed by complexes 4–6
% Yield of
cyclised
Entry Substrate Catalyst Anode Products (ratio) products
1
1
1
1
1
1
1
1
2
2
2
2
3
3
3
4
4
5
6
5
5
5
5
5
4
5
5
5
5
Mg
Zn
Zn
Zn
Al
7, 8 (56 : 44)
7
7, 8 (64 : 36)
63
48
89
49
52
100
36
35
76
49
98
13
83
76
2
3
4
7
5
7
7, 8 (50 : 50)
6b
7c
8
Zn
Zn
Al
7
9
9
Zn
Zn
Zn
Al
9
10
11b
12
13
14b
a
9
9
11
11
11
Zn
Zn
Conditions: 10 mM of 1–3; 20 mol % of 4–6; carbon fibre cathode in
DMF containing 0.006 M of Bu4NBF4; j ¼ 0.15 A dmꢀ2
.
[RBr]/
b
c
[Ni(II)] ¼ 10. Cathode ¼ Zn.
Scheme 2
634
N e w J . C h e m . , 2 0 0 5 , 2 9 , 6 3 3 – 6 3 6

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the Ni complexes became irreversible and the reduction peak
increased in height.9,11
Experimental parameters such as the ratio of bromoethers to
catalyst, the influence of the nature of the electrodes and the
nature of the ligand of the catalyst were examined in order to
evaluate their effect on the product distribution.
Next, the influence of the ratio of the concentration of the
bromoether and that of the catalyst was examined. No sig-
nificant change was observed for the yield of the cyclised
product 7 when the catalyst concentration was decreased from
20% to 10% (entry 6).
Finally, the influence of the cathode nature was also studied
using a zinc anode and it was observed that the use of a zinc
cathode led, for the electrolysis of 1, to the formation of 7 in
36% yield (entry 7). These results indicated that the radical-
type cyclisation of 1 could be affected by the nature of the
cathode electrode. Thus, a carbon fibre cathode seemed to be
more effective compared to a zinc electrode. The same phe-
nomenon was observed in a previous investigation in our
laboratory in the study of the intramolecular cyclisation of
propargyl derivatives by electrogenerated nickel(I) com-
plexes.10
The study was initiated by carrying out the electrolysis of 1
in DMF with Mg/C couple electrodes [eqn. (3)] in the presence
of [Ni(cyclam)](ClO4)2] (4, 20 mol %; cyclam ¼ 1,4,8,11-
tetraazacyclotetradecane). Hence, the electrolysis of 1 led to
the expected cyclised product 7 as the main product in 35%
yield, along with the furan derivative 8 in 28% yield (Table 1,
entry 1). These results are in agreement with those reported by
Olivero et al.21 under the same experimental conditions where
intramolecular cyclisation of propargyl 2-bromophenyl ethers
led to the cyclic furan derivatives in moderate yields (31–32%).
Compound 8 is probably obtained from compound 7 via an
Ec1B mechanism where a carbanion (resulting from deproto-
nation and possessing a poor leaving group) is stabilised by
resonance,22 followed by a [1,3] sigmatropic rearrangement.
This process affords the aromatic compound 8, which is
particularly stable.
The electroreduction of propargyl bromoether 2 by the
nickel(I) complex electrogenerated from 5 at a carbon fibre
cathode using a sacrificial aluminium anode was also carried
out [eqn. (4)].
ð4Þ
The reaction proceeded through the cleavage of the carbon–
bromine bond to afford the cyclic ether 9 in 35% yield together
with the alkene 10 in 47% yield (entry 8). Compound 10 can
issue from a two-electron reduction of 2 and a further elimina-
tion of the propargylic moiety. The latter process is commonly
encountered as part of the so-called E1cB mechanism.22 The
influence of the nature of the anode was examined using a
carbon fibre cathode and the electrolysis of 2 with a Zn anode
led to the formation of 9 in 76% yield (entry 9). These results
indicate that the cyclisation of 2 is strongly affected by the
nature of the anode material. Using Zn/C couple electrodes
with complex 4, the electrolysis of 2 afforded cyclised 9 in 49%
yield as the main product (entry 10), along with 18% of alkene
10. These results demonstrate that the [Ni(cyclam)](ClO4)2
system catalyses the cyclisation of 2 but also the cleavage of
the propargyl carbon–oxygen bond. Hence, when [Ni(tmc)]Br2
is used as the catalyst precursor in the electrolysis of 2, the
process involves a more selective radical-type reactivity of the
reduced carbon–halogen bond of the substrate, and enables a
more selective formation of 5-membered heterocycles. Finally,
the reaction was performed under the same experimental
conditions but in the presence of 10 mol % of catalyst 5 with
respect to the concentration of the bromoether 2. The cyclised
compound 9 was obtained in 98% yield as the only product of
the reaction (entry 11). This result is in agreement with that
presented above (entry 6) and shows that the ratio [RBr]/
[Ni(II)] does not influence the reaction mechanism.
ð3Þ
No reaction occurred in the absence of current; in the
absence of the nickel catalyst a mixture of cyclic and acyclic
products was formed. The presence of the nickel complex as
the catalytic system seems essential to control the selectivity of
the process. Similar results were obtained in our previous
investigation on the direct electrochemical reduction of a
bromo propargyloxy ester at vitreous carbon cathodes in
DMF;23 the results corresponded to an anionic nature of the
intermediates, also proposed in other electrochemical dehalo-
genation processes.24
The influence of the nature of the anode on the electroreduc-
tion of 1 was examined. When this reaction was performed
under the same experimental conditions but in the presence of
a zinc anode the cyclised compound 7 was formed as the major
product in 48% yield (entry 2).
The influence of the nature of the ligand on the reaction
selectivity was also investigated in reactions using a Zn/C
couple of electrodes in DMF. Hence, when [Ni(tmc)]Br2 (5;
tmc ¼ 1,4,8,11-tetramethylcyclam) was used as the catalyst, the
electrolysis of 1 afforded the expected cyclised product 7 in
57% yield along with 32% of 8 (entry 3). The electroreduction
of 1 was also performed in the presence of [Ni(CR)]Cl2 (6) as
the catalytic system; the electrolysis gave rise to the formation
of 7 in 49% (entry 4). These results indicate that all the tested
Ni(II) complexes having macrocyclic ligands efficiently catalyse
the cyclisation of 1. The process involves a selective radical-
type reactivity of the reduced carbon–halogen bond of the
substrate and enables the cyclisation with exclusive formation
of 5-membered heterocyclic rings. Ozaki et al.25 in the study of
indirect electrochemical radical cyclisation of halogeno ethers
using nickel(II) complexes as electron-transfer catalysts,
showed that various halides cyclised preferentially via the 5-
exo-trig mode to the corresponding tetrahydrofuran deriva-
tives as sole products in good yields. These results are similar to
those obtained in the present work.
The electroreduction of 3 was also performed in the presence
of 5 as the catalyst using Al/C couple of electrodes [eqn. (5)]
and the cyclic compound 11 was obtained in 13% yield (entry
12).
The nature of the anode was also investigated in reactions
a carbon fibre cathode and in the presence of
[Ni(tmc)]Br2 as the catalytic system. It was observed that the
change of the anode from zinc to aluminium had a strong effect
on the product yields (entry 5).
using
ð5Þ
The influence of the nature of the anode on this reaction was
studied: the change from an aluminium to a zinc anode led to
N e w J . C h e m . , 2 0 0 5 , 2 9 , 6 3 3 – 6 3 6
635

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the formation of cyclic compound 11 in 83% yield (entry 13).
Here again, the nature of the anode material strongly influ-
enced the results, as was observed with bromoether 1 (entries 3
and 5) and bromoether 2 (entries 8 and 9). Similar results were
obtained in a previous investigation in our laboratory in the
study of the intramolecular cyclisation of bromo propargyl and
allyl derivatives by electrogenerated nickel(I) complexes.10,14
Interestingly, the role of the sacrificial anode was found to be
important in this electroreduction, as in the present work.
Moreover, it was also demonstrated that, under comparable
experimental conditions, the unsaturated a-bromoesters cyclise
via the exo mode to a five-membered ring to give the corres-
ponding tetrahydrofuran derivatives as the main products.
When the amount of catalyst was decreased from 20% to
10% the yield of the cyclic compound 11 was 76% (entry 14).
Again, this result is in agreement with those presented above
(entries 6 and 11) and shows that the change in the yield of
cylised product 11 is not significant when the catalyst concen-
tration decreases.
In summary, an effort has been done to extend the scope of
the work on nickel-catalysed electrochemical reactions, by
further developing the intramolecular cyclisation of bro-
moethers 1–3 catalysed by NiII complexes. [Ni(cyclam)]
(ClO4)2, [Ni(tmc)]Br2 and [Ni(CR)]Cl2 have been used as the
catalyst precursors. The nature of the organometallic catalytic
system and, particularly, of the ligand on nickel can strongly
determine the reactivity, the selectivity, and the mechanism of
the electrochemical reaction. The presence of Zn21, Mg21 and
Al31 ions during the electrolyses displays an important effect in
controlling the reactivity and the selectivity of the system.
Thus, it seems that from the results presented in this work,
the [Ni(tmc)]Br2 system is the best catalyst, zinc is the best
anode material and the ratio [RBr]/[Ni(II)] has no effect on the
reaction mechanism.
logia/FEDER (POCTI/QUI/37808/01) for financial support of
this work.
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Acknowledgements
Most of this research was conducted while M. J. M. was a
Visiting Scholar at the University of Nice. In addition, we are
grateful to the CRUP and Fundac¸ ao para a Ciencia e Tecno-
636
N e w J . C h e m . , 2 0 0 5 , 2 9 , 6 3 3 – 6 3 6