Selectivity in the tandem cyclization - Carboxylation reaction of unsaturated haloaryl ethers catalyzed by electrogenerated nickel complexes

Article abstract of DOI:10.1002/(sici)1099-0690(199908)1999:8<1885::aid-ejoc1885>3.0.co;2-u

The electrochemical reduction of a series of 2-haloaryl ethers containing allyl and propargyl groups under CO₂ allows the synthesis of benzofuranacetic acid derivatives. This novel intramolecular cyclization- carboxylation reaction is carried out in single-compartment cells and is catalyzed by [Ni(cyclam)Br₂].

Full text of DOI:10.1002/(sici)1099-0690(199908)1999:8<1885::aid-ejoc1885>3.0.co;2-u

FULL PAPER
Selectivity in the Tandem Cyclization – Carboxylation Reaction of Unsaturated
Haloaryl Ethers Catalyzed by Electrogenerated Nickel Complexes
[a]
[a][°]
˜
S. Olivero and E. Dunach*
Keywords: Electrochemistry / Carbon dioxide fixation / Cyclizations / Nickel / Dihydrobenzofuran
The electrochemical reduction of a series of 2-haloaryl ethers
containing allyl and propargyl groups under CO2 allows the
intramolecular cyclization–carboxylation reaction is carried
out in single-compartment cells and is catalyzed by
synthesis of benzofuranacetic acid derivatives. This novel [Ni(cyclam)Br2].
cyclizationϪCO2 incorporation processes have, to the best
Introduction
of our knowledge, yet been described.[9]
We have recently explored the possibilities of direct syn-
thesis of substituted dihydrobenzofuran and dihydrobenzo-
pyran derivatives, and we have described a nickel-catalyzed
electrochemical method involving the intramolecular re-
ductive cyclization of o-haloaryl ethers containing unsatu-
rated side chains.[1] The possibility of intramolecular cas-
cade cyclization reactions involving trapping with CO2
should allow the construction of carboxylic acids (or their
derivatives) in polycyclic structures. Carboxylic acids in
benzofuran-, benzopyran-, and indole-type derivatives con-
stitute products of interest in the field of pharmacology[2]
and agrochemicals.[3] Moreover, the direct and selective in-
corporation of CO2 into cyclic compounds may widen the
field of its utilization as a carbon source for organic com-
pounds.[4]
The electrochemical CO2 fixation into organic substrates
deals with the problem of the direct CO2 reduction. It is
worth noting that NiII complexes, and particularly
[Ni(cyclam)2ϩ · 2 ClO4Ϫ] have been reported to be efficient
catalysts for the CO2 electroreduction to carbon monoxide
with high selectivities, in water-containing solvents.[10]
According to literature data on electrochemical carbon
dioxide incorporation into aromatic halides or into unsatu-
rated hydrocarbons, different reaction products can be ex-
pected from the electroreductive heterocoupling reaction
between substrates 1 and CO2 (Scheme 1). On one hand,
as illustrated by path a, the direct electrocarboxylation of
aromatic halides to afford benzoic acid derivatives may con-
stitute a possible coupling reaction of substrates such as 1
with carbon dioxide.[11] This Ar Ǟ ArCOOH electrochemi-
cal reaction has been reported to afford good yields with
the use of consumable anodes[12] or in the presence of
(phosphane)nickel complexes as the catalysts[13] in the case
of phenyl bromide.
We present here our results on the tandem cyclizationϪ
carboxylation reaction of substrates such as 1 in the pres-
ence of CO2, for the electrosynthesis of a series of dihydro-
benzofuran- and dihydrobenzopyran-containing carboxylic
acids (Equation 1).
On the other hand, the direct CO2 fixation on the double
(or triple) bond of the unsaturated side chain of 1 (path b)
should also be considered as a possible reaction pathway,
in the light of the examples of electrochemical car-
boxylation of unsaturated hydrocarbons (alkynes,[14] ole-
fins[15]) in the presence of NiII associated to catalytic 2,2Ј-
bipyridine systems.
Intramolecular reductive cyclizations of ether derivatives
such as 1 can be effected in the presence of tin hydrides[5]
or SmII species[6] as stoichiometric reagents, and require
generally the more activated iodo (or sometimes bromo) de-
rivatives. Electrochemical methods with Co[7] or Ni[8] com-
plexes have also been reported for related reactions. We
have shown that [Ni(cyclam)Br2] (cyclam ϭ 1,4,7,11-tetra-
azacyclotetradecane) is an efficient catalyst for the electro-
chemical cyclization of substrates 1.[1] However, no tandem
´
´
[a] Laboratoire de Chimie Moleculaire, Associe au CNRS, Univer-
´
site de Nice-Sophia Antipolis,
Parc Valrose, F-06108 Nice Cedex 2, France
Fax: (internat.) ϩ 33-4/92076144
E-mail: dunach@unice.fr
Scheme 1. Possibilities of electrocarboxylation of substrates 1
Our own results on the [Ni(cyclam)Br2]-catalyzed electro-
chemical cyclization of substrates 1 leading to 3-methyldi-
hydrobenzofuran led us to explore a third and not yet re-
[°]
´
Present adress: Laboratoire de Chimie Bio-Organique, Associe
´
au CNRS, Universite de Nice-Sophia Antipolis,
Parc Valrose, 06108 Nice Cedex 2, France
Eur. J. Org. Chem. 1999, 1885Ϫ1891
WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1999
1434Ϫ193X/99/0808Ϫ1885 $ 17.50ϩ.50/0
1885

S. Olivero, E. Dun˜ach
FULL PAPER
ported possibility, involving a tandem cyclizationϪCO2 up-
take process, as illustrated by path c.
In the absence of the catalyst, a nonselective reaction oc-
curred and a mixture of compounds was formed in low
yield. No reaction occurred in the absence of electricity.
The use of [Ni(cyclam)Br2][18] as the catalyst in the elec-
trochemical reduction of 1a occurred with 85% faradic yield
and afforded dihydrobenzofuran-3-ylacetic acid (2a), fur-
nished by the cascade cyclizationϪcarboxylation reaction,
in 33% chemical yield (Table 1, entry 1). 3-Methyldihydro-
benzofuran (4a) was also formed in 49% yield. Car-
boxylated, noncyclized 3a, issued from the direct car-
boxylation of the aryl moiety, was obtained in only 4%
yield. The selectivity of carboxylated compounds 2a/3a
was 8.3:1.
Results and Discussion
1. Influence of the Catalytic System on the
Electrochemical Cyclization؊Carboxylation of 1a
In order to study the possibilities of an electrochemical
tandem cyclizationϪcarboxylation reaction, we first exa-
mined the influence of the nickel catalyst in the electrored-
uction of 1a (Equation 2), by varying the nature of the li-
gand associated to the nickel complex.
When cationic [Ni(cyclam)2ϩ · 2 BF4Ϫ] was used as the
catalyst (entry 2), 2a was formed in 50% yield with a 2a/3a
selectivity of 5:1.
We can conclude that in the presence of the catalytic (cy-
clam)nickel system, the cyclizationϪcarboxylation process
is preferred to the direct carboxylation of the arylϪhal-
ogen bond.
Interestingly, the good faradic yield obtained in the elec-
trolysis of 1a with [Ni(cyclam)Br2] (Rf of 85%) indicates
that in anhydrous DMF, [Ni(cyclam)Br2]-catalyzed intra-
molecular cyclizationϪcarboxylation under CO2 can be
performed, and that the direct CO2 electroreduction to give
CO[10] can be inhibited in the presence of the substrate. The
catalyst thus effects a very selective substrate selectivity able
to orient the reactivity towards the organic halide and not
towards the carbon dioxide reduction.
The results are presented in Table 1. Carboxylic acids 2a
and 3a, in cyclized and noncyclized structures, respectively,
were obtained, together with cyclized, noncarboxylated 3-
methyldihydrobenzofuran (4a).
The electrochemical methodology was based on the use
of single-compartment cells fitted with a consumable mag-
nesium anode. This methodology has already shown a wide
applicability in several electrosynthetic processes.[16][17] The
reactions were carried out in freshly distilled DMF, under
constant current intensity, at room temperature and atmos-
pheric CO2 pressure, with a 10% molar ratio of nickel com-
plex to substrate and a magnesium/carbon fiber couple of
electrodes.
With [Ni(cyclam)2ϩ · 2 BF4Ϫ], the faradic yield was of
48% (consumption of 4.1 F/mol for complete conversion),
the process involving some parallel CO2 electroreduction.
In sharp contrast to these results, when the electrolysis of
1a was run with the analogous tetramethylcyclam (tmc) as
the ligand of nickel, the (tmc)Ni system resulted in a less
efficient and less selective carboxylation reaction (entry 3).
The (Ph3P)Ni system seemed more efficient towards the
cyclization, and afforded 2a in 45% yield (entry 4). The use
of (diphenylphosphanyl)ethane (dppe) as the ligand lowered
the yield of 2a to 16% (entry 5). The presence of phos-
phanes involved a difficult and low-yield purification of the
reaction products. The low faradic yield in these cases indi-
cates that the CO2 reduction process takes place simul-
taneously to the ArϪBr reduction.
Table 1. Influence of the catalyst system on the electrocarboxyla-
tion of 1a (CO2, 1 atm, 20 °C)
Products (% yield) Ratio
Entry Nickel complex[a]
F/mol[b] 2a
3a
4a
2a/3a
1
2
3
4
5
6
7
Ni(cyclam)Br2
Ni(cyclam)2ϩ · 2 BF4
Ni(tmc)Br2
2.3
4.1
3.6
4.4
4.6
2
33
50
8
45
16
Ϫ
Ϫ
4
10
5
Ϫ
Ϫ
Ϫ
Ϫ
49
40
19
38
48
6
8.3:1
5:1
1.5:1
Ϫ
Ϫ
Ϫ
Ϫ
By changing the ligand to 2,2Ј-bipyridine (entry 6) or to
PMDTA (entry 7, PMDTA ϭ pentamethyldiethylenetriam-
ine), the entire chemoselectivity of the process was modi-
fied: No cyclization nor carboxylation products 2a and 3a
were obtained, and no CO2 incorporation into the unsatu-
rated double bond of that side chain could be observed
[c]
Ni(dme)Br2 ϩ 4 PPh3
Ni(dme)Br2 ϩ 2 dppe
Ϫ[d]
Ni(bipy)32ϩ · 2 BF4
Ni(dme)Br2 ϩ 2 PMDTA[e]
5
8
Ϫ
[a]
A 10:1 molar ratio of 1a/NiII was used with Mg/C as couple of
electrodes. Ϫ [b] F/mol needed for complete conversion. Ϫ [c] dme ϭ
dimethoxyethane. Ϫ
[d]
With 2,2Ј-bipyridine as the ligand, phenol (Scheme 1, path b).
Ϫ [14]
With [Ni(bipy)32ϩ · 2 BF4
]
or catalytic (PMDTA)Ni
and 2-bromophenol were obtained in 45% and 40% yield, respec-
tively. Ϫ [e] With PMDTA as the ligand, phenol and 2-bromophenol
were obtained in 35% and 35% yield, respectively.
systems and 1a, deallylation and dehalogenation occurred;
the cleavage of the carbonϪoxygen bond of the allyl group
The results in Table 1 show the important influence of was carried out selectively to afford a mixture of phenol
the nickel complex and particularly of the nature of the and 2-bromophenol in 85 and 70% yield, respectively. The
ligand, which can determine the nature of the reaction deallylation reaction could be run more selectively in the
products as well as the selectivity of the process.
absence of CO2.[19]
1886
Eur. J. Org. Chem. 1999, 1885Ϫ1891

Selectivity in the Tandem Cyclization
FULL PAPER
Our results in Table 1 clearly indicate that the nature of 3. Influence of the Carbon Dioxide Pressure
the ligand associated to the nickel center (phosphane, bi-
pyridine, or cyclam-type ligand) plays an important role in
The electrocarboxylation of 1a catalyzed by [Ni(cy-
clam)Br2] was carried out in DMF at a CO2 pressure of 3
determining the chemoselectivity of the carboxylation pro-
atm, with a magnesium/stainless steel couple of electrodes.
cess.
Among the products, 2a was obtained in 48% yield, to-
gether with 3a (28%) and 4a (20%). The CO2 pressure also
influenced the results, and an increase of the pressure from
1 to 3 atm improved the carboxylation yield of 1a by 18%
2. Influence of the Nature of the Electrodes
(Table 2, entries 4, 5). However, the ratio 2a/3a was lowered
to 1.7:1. Under 3 atm of CO2 the rate of the direct CO2
electroreduction was enhanced, thus decreasing the faradic
yield to 20%.
The reaction yields and selectivities were shown to be de-
pendent on the reaction conditions, particularly on the nat-
ure of the electrodes. Table 2 presents the results obtained
with the use of Mg and Zn as the anodes, and of carbon
fiber, nickel foam or stainless steel as the cathode materials
for the electrocarboxylation of 1a with [Ni(cyclam)Br2] as
the catalyst.
4. Tandem Cyclization؊Carboxylation Reaction
The catalytic (cyclam)Ni system, which afforded the best
carboxylation selectivity, was chosen for the cyclizationϪ
carboxylation of other substrates in electrolyses with Mg/C
or Mg/Ni electrodes at 1 atm of CO2. The results concern-
ing the carboxylic acid electrosyntheses are presented in
Table 3.
Table 2. Influence of the nature of the electrodes and of the CO2
pressure in the electrocarboxylation of 1a with Ni(cyclam)Br2 as
the catalyst
Entries 1Ϫ3 involve the cyclization of allyl 2-halophenyl
ethers with either iodo, bromo, or chloro substituents. In
the electrochemical reductive carboxylation of the 2-(al-
lyloxy)-1-iodobenzene (1b) the expected dihydrobenzofu-
ran-3-ylacetic acid (2a) was obtained in 41% yield, together
with 3a (13%). The cyclized dimerization product 5a was
the major compound (46% yield, see Scheme 3). The forma-
tion of dimer 5a was specific of the iodo derivative, thus
indicating a possible radical reactivity of the intermediate
Products (% yield) Ratio
Entry Anode Cathode
CO2 [atm]
2a
3a
4a
2a/3a
1
2
3
4
5
Mg
Zn
Mg
Mg
Mg
C
C
Ni
1
1
1
1
3
33
3
50
30
48
4
3
36
10
28
49
67
14
10
20
8.3:1
1:1
1.4:1
3:1
Stainless steel
Stainless steel
1.7:1
At 1 atm of CO2 with a magnesium/carbon fiber couple cyclized species in this case (entry 2).
of electrodes, 33% of 2a and 49% of 4a were obtained, with The possibility of activation of the CϪCl bond (entry 3)
a 2a/3a ratio of 8.3:1 (entry 1). The nature of the anodic in 1c is interesting, considering that the less available iodo
material strongly determined the yields or reactivity in car- (or bromo) derivatives are required in the R3SnϪH[5] or
bon dioxide fixation. Thus, the replacement of the mag- SmI2[6]-mediated cyclizations. In the case of the electrocar-
nesium by a zinc anode (with a carbon fiber cathode, entry boxylation of 1c, a mixture of two carboxylic acids was
2) made the global carboxylation yield of 1a drop from 37 formed in 85% yield (with a 64% conversion): the bicyclic
to 6%. With a zinc anode the faradic yield dropped to 30%; carboxylic acid 2c (70%) and the benzoic acid derivative
the reduction of the electrogenerated zinc ions affording 3c (15%).
metallic zinc, at potentials similar to those of the reduction
The cyclization of the analogous bromo methallyl deriva-
of the nickel(II) complex, took place preferentially.
tive 1d (entry 4) led to the formation of the dihydrobenzofu-
The nature of the cathodic material could also strongly ranylacetic acid derivative 2c with a quaternary carbon
influences the carboxylation selectivity, as shown by com- center, in 40% yield together with 50% of cyclized com-
paring entries 1 and 3. With an Mg/Ni couple of electrodes, pound 4c.
1a afforded 50% yield of the expected carboxylic acid 2a
The synthesis of the dihydropyran-2-ylacetic acid 2e
and 36% of carboxylated, noncyclized 3a. However, the ra- (24%) could be carried out by the cyclizationϪcar-
tio of carboxylation/cyclization to 2a versus direct aryl car- boxylation of an allyl benzyl ether derivative (1e, entry 5).
boxylation to 3a went down to 1.4:1 (entry 3).
Direct debromination to the corresponding allyl benzyl
The use of a magnesium/stainless steel couple of elec- ether also took place in 33% yield.
trodes (entry 4) led to 30% of 2a with a 2a/3a ratio of 3:1.
Naphthyl derivative 1f, electrolyzed with a carbon fiber
A carbon fiber cathode seems to favor the cyclization as the cathode (entry 6), mainly afforded cyclization to 4f
and/or cyclizationϪcarboxylation processes (entry 1), (47%), together with cyclizationϪcarboxylation to 2f (8%)
whereas a nickel foam cathode offers a high carboxylation and direct carboxylation of the aromatic position (3f, 8%).
yield but tends to orient the reaction towards the direct car- The same reaction with a nickel foam cathode (entry 7) led
boxylation of the ArϪBr bond (entry 3). The observed dif- to a preferential CO2 fixation in the aromatic position, for-
ferences in yield and selectivities could partially be due to ming 2-allyloxy-1-naphthoic acid (3f) in 60% yield. Here
a different current density.
again, the nature of the cathodic material strongly influ-
Eur. J. Org. Chem. 1999, 1885Ϫ1891
1887

S. Olivero, E. Dun˜ach
FULL PAPER
Table 3. Tandem intramolecular cyclizationϪcarboxylation of substrates 1 with the catalytic [Ni(cyclam)2ϩ] system (CO2 pressure,
1 atm, 20 °C)
[Ni(cyclam)Br2] as the catalyst and Mg/Ni as the electrodes. Ϫ [Ni(cyclam)2ϩ · 2 BF4Ϫ] as the catalyst and Mg/Ni as the electrodes.
[a]
[b]
Conversion of 64% after 8 F/mol. Ϫ [Ni(cyclam)Br2] as the catalyst and Mg/C as the electrodes. Ϫ [Ni(cyclam)2ϩ · 2 BF4Ϫ] as
[c]
[d]
[e]
Ϫ
the catalyst and Mg/C as the electrodes.
enced the carboxylation selectivity; the nickel foam cathode 5. Electrochemical Studies
selectively orients the process towards the direct car-
boxylation of the ArϪBr bond.
The carboxylative cyclization could also be carried out
Some mechanistic aspects of this tandem reaction were
with propargyl derivatives such as o-bromophenyl propar- investigated. Cyclic voltammetry experiments showed that
gyl ether (1g, entry 8). The [Ni(cyclam)2ϩ]-catalyzed car- a 1eϪ reversible reduction of Ni(cyclam)2ϩ to Ni(cyclam)ϩ
boxylation led to three different cyclized carboxylic acids in occurs at Ϫ1.45 V vs Ag/AgCl in DMF solutions contain-
a global 67% yield: the expected benzofuran-3-ylacetic acid ing tetrabutylammonium tetrafluoroborate, as shown in
(2g) its reduced analog 2a and a dicarboxylic acid 2gЈ.
Figure 1, curve a), in agreement with literature data.[20][21]
1888
Eur. J. Org. Chem. 1999, 1885Ϫ1891

Selectivity in the Tandem Cyclization
FULL PAPER
ratio) were introduced into the cathodic compartment un-
der CO2 bubbling and could be electrolyzed up to the pas-
sage of 1.6 F/mol of nickel, after which passivation oc-
curred. Starting compound 1a could be recovered in 75%
yield and the reaction led to 10% of 2a (40% according to
conversion), 6% of 4a and 5% of dimer 5a. Under these
experimental conditions, the tandem cyclizationϪcar-
boxylation reaction takes place, but the NiII species are not
efficiently recycled. However, when the electrolysis was car-
ried out in a single-compartment cell with a Mg anode, no
passivation occurred and the reaction could be run cata-
lytically with a NiII/1a molar ratio of 1:10. These results
give evidence of the important role of the magnesium ions
(issued from the anodic oxidation process) in the recycling
of the nickel species and therefore in allowing the reaction
to be catalytic.
Figure 1. Cyclic voltammetry on a carbon graphite electrode (v ϭ
Ϫ
100 mVsϪ1) of a DMF/nBu4NϩBF4 solution containig: curve a:
[Ni(cyclam)Br2] (2.5 ); curve b: [Ni(cyclam)Br2] (2.5 ) and CO2
(saturated solution at 1 atm); curve c: [Ni(cyclam)Br2] (2.5 ) and
CO2 (saturated solution at 1 atm) ϩ 1a (3 equiv.)
Upon addition of CO2 (saturated solution at 1 atm), the
NiII/NiI reduction potential was slightly shifted to Ϫ1.40 V
(curve b) with the same initial intensity, the process being
irreversible. This is compatible with a CO2 coordination to
the NiII and to the NiI species, as illustrated in Scheme 2.
6. Proposed Catalytic Cycle
A catalytic cycle is proposed in Scheme 3, illustrating
plausible pathways to the different reaction products that
have been isolated from the carboxylation reaction of 1a.
The electrochemical reduction of Ni(cyclam)2ϩ affords
NiI species. In the tandem cyclizationϪcarboxylation pro-
cess, we have a competition of the electrogenerated NiI spec-
ies, either for the reaction with the aryl halide[21] or for the
direct CO2 reduction.[10] According to the results of the
electrosynthesis and to the electrochemical behavior shown
by cyclic voltammetry, we propose a preferred oxidative
addition of the NiI species (coordinated to CO2) to the C-
(aryl)Ϫhalogen bond of 1a. After oxidative addition, the
formation of NiIII intermediate species A is proposed. This
NiIII intermediate induces a radical character to the aryl
moiety, resulting in a rapid intramolecular cyclization on
the double bond in the side chain, leading to C. The oxidat-
ive addition of the electrogenerated NiI species to alkyl hal-
ides followed by a radical-type reaction has already been re-
ported.[20][21]
Scheme 2. Reduction of Ni(cyclam)2ϩ complexes
The cyclic voltammetric behavior of Ni(cyclam)2ϩ in the
presence of CO2 in anhydrous DMF (Figure 1) is in con-
trast to the catalytic wave of CO2 reduction to CO reported
in water.[10] In DMF, no catalytic current was observed.
When one or more equivalents of 1a with respect to NiII
were added to the electrolytic solution (3 equiv., curve c),
the NiII/NiI reduction peak at Ϫ1.40 V doubled its initial
intensity and remained irreversible. A similar behavior was
observed in the absence of CO2.[21] We can propose a
chemical reaction between the electrogenerated NiI species
and 1a, with formation of a new intermediate, according to
Equation 3, which in turn undergoes a one-electron re-
duction at the same potential.
Further one-electron reduction of NiIII intermediate
species C should form an alkylnickel(II) species D, able to
undergo CO2 uptake, forming a nickel(II) carboxylate. The
overall reaction is a two-electron process. Competitive pro-
tonation of D by the electrolytic medium (solvent) affords
cyclized, noncarboxylated 4a.
In the one-compartment cell procedure with a mag-
nesium anode, the presence of the magnesium ions issued
from the anodic oxidation process, enables the formation of
a magnesium carboxylate E liberating the nickel species for
further recycling. Magnesium carboxylate E is stable under
the reaction conditions, it accumulates during electrolysis
and it is simply hydrolyzed (or esterified to the correspond-
ing methyl esters) at the end of the reaction.
Curve c indicates that the process consumes 2 F/mol of
In a competitive reaction, intermediate A can also be di-
nickel, and that it is not catalytic. We carried out some pre- rectly reduced and carboxylated to afford the corresponding
parative-scale electrolyses at the controlled potential of benzoic acid derivatives via intermediate B.
Ϫ1.5 V in two-compartment cells in DMF containing
Intermediate C, an alkylnickel(III) species possessing
nBu4NϩBF4Ϫ (0.1 ). Ni(cyclam)Br2 and 1a (in a 1:3 molar some radical character, can also undergo a radical-type di-
Eur. J. Org. Chem. 1999, 1885Ϫ1891
1889

S. Olivero, E. Dun˜ach
FULL PAPER
Scheme 3. Proposed catalytic cycle
merization leading to product 5. This dimer was only ob- ring or the direct reduction of CO2 may become less favored
tained in reactions in the absence of Mg2ϩ ions, or when processes. The ligand was shown to be dominant in con-
starting with the iodo substrate 1b.
trolling the reactivity and the chemoselectivity of the pro-
As shown by our experimental results, slight modifi- cesses. The nature of the electrodes may also strongly influ-
cations of the reaction conditions or of the nature of the ence the selectivity of the CO2 fixation.
substrates may direct the main catalytic cycle towards the
different reaction products.
Experimental Section
General: All solvents were dried and degassed by standard meth-
Conclusions
ods. DMF was freshly distilled from calcium hydride before electro-
In conclusion, we studied a new example of a tandem
lyses. Allyl 2-bromophenyl ether (1a) (and other allyl ethers) was
cyclizationϪcarboxylation reaction under electrochemical
prepared from 2-bromophenol by treatment with allyl chloride and
Ϫ
conditions. [Ni(cyclam)Br2] and [Ni(cyclam)2ϩ · 2 BF4
]
potassium carbonate in DMF.
were shown to be efficient and selective catalysts for the
incorporation of carbon dioxide in a sequential electro-
chemical cyclizationϪcarboxylation reaction of a series of
allyl o-haloaryl ethers and related compounds. The direct,
one-step synthesis of carboxylic acid in bicyclic structures
of benzofuran and benzopyran-type was achieved under
very mild conditions (room temperature, atmospheric CO2
pressure).
Instrumentation and Cells: NMR: Bruker AC-200 spectrometer
(200 MHz and 50.3 MHz for 1H and 13C, respectively), with CDCl3
as solvent, TMS as internal standard. Ϫ IR: KBr disks; Nicolet
520 FT-IR spectrometer. Ϫ MS: Finnigan MAT INCOS 500E
spectrometer (GC/MS, 70 eV). Ϫ Cyclic voltammetry experiments
and controlled potential electrolyses were performed with the aid
of P.A.R. Scanning Potentiostat model 362 equipment, and were
carried out at 25°C by utilizing Pt or carbon fiber microelectrodes
(Tacussel). All potentials were quoted with respect to Ag/AgCl
By careful control of the reaction conditions, competitive
reactions such as the direct carboxylation of the aromatic electrode at room temperature, which correspond to a potential
1890 Eur. J. Org. Chem. 1999, 1885Ϫ1891

Selectivity in the Tandem Cyclization
FULL PAPER
[1]
[2]
Ϫ
difference from that of Fc/Fcϩ of Ϫ0.55 V in DMF/nBu4NϩBF4
.
S. Olivero, E. Dun˜ach, Tetrahedron Lett. 1995, 36, 4429Ϫ32.
T. Eicher, S. Hauptmann, in The Chemistry of Heterocycles, Thi-
eme, Organic Chemistry, Monograph Series, Stuttgart, 1995.
X. Guan, R. T. Borchardt, Tetrahedron Lett. 1994, 35,
3013Ϫ3016.
Controlled constant intensity electrolyses were carried out with a
stabilized constant current supply (Sodilec, EDL 36.07). The elec-
trochemical one-compartment cell is a cylindrical glass vessel of
approx. 40 mL volume, equipped with a carbon fiber cathode
(20 cm2) and a magnesium rod anode immersed to 3 cm. In the
two-compartment cell, the two compartments are separated by a
sintered glass frit (no. 4); the anodic compartment has a Pt wire as
the anode, and the cathodic compartment is equipped with a car-
bon fiber cathode and a Ag/AgCl electrode.
[3]
[4]
[4a] S. Inoue, N. Yamazaki, in Organic and Bio-Organic Chemis-
[4b]
try of Carbon Dioxide, Kodansha Ltd., Tokyo, 1992. Ϫ
A.
Behr, in Carbon Dioxide Activation by Metal Complexes, VCH,
Weinheim, 1988.
[5] [5a]
K. Shankaran, C. P. Sloan, V. Sniekus, Tetrahedron Lett.
[5b]
1985, 26, 6001Ϫ6004. Ϫ
A. J. L. Beckwith,W. B. Gara, J.
Chem. Soc., Perkin Trans. 2 1975, 795Ϫ802.
[6] [6a]
D. P. Curran, M. J. Totleben, J. Am. Chem. Soc. 1992, 114,
6050Ϫ6058. Ϫ [6b] G. A. Molander, L. S. Harring, J. Org. Chem.
General Procedure for One-Compartment Cell Electrolyses: A DMF
solution (40 mL) containing [Ni(cyclam)Br2] (0.3 mmol, 1a) (or the
[6c]
1990, 55, 6171. Ϫ
J. Inanaga, O. Ujikawa, M. Yamaguchi,
Tetrahedron Lett. 1991, 32, 1737Ϫ1740.
Ϫ
other ether derivatives, 3 mmol) and nBu4NϩBF4 (0.2 mmol) was
[7] [7a]
V. F. Patel, G. Pattenden, J. Chem. Soc., Chem. Commun.
[7b]
1987, 871Ϫ872. Ϫ
T. Inokuchi, H. Kawafuchi, K. Aoki, A.
placed in the cell and stirred at room temperature under nitrogen.
A current of 60 mA was applied between the electrodes connected
Yoshida, S. Torii, Bull. Chem. Soc. Jpn. 1994, 67, 595Ϫ598. Ϫ
[7c]
S. Busato, R. Scheffold, Helv. Chim. Acta 1994, 77, 92Ϫ99.
to a DC power supply (apparent current density of 0.3 A·dmϪ2
,
[8a] S. Ozaki, H. Matsushita, H. Ohmori,. J. Chem. Soc., Chem.
[8]
[9]
[8b]
applied voltage ca. 3Ϫ15 V). The consumption of 1 was monitored
by GC analysis of aliquots withdrawn from the reaction mixture,
and the electrolysis was continued until the starting material was
almost depleted, e.g. about 4Ϫ6 h. Generally, 2Ϫ5 F/mol of 1 was
necessary to achieve a complete conversion. The electrolytic solu-
tions were directly esterified by adding K2CO3 and methyl iodide,
stirring at 50°C for 10 h, and then hydrolyzing with aqueous HCl
(0.1  up to pH ϭ 1Ϫ2) and extracting with Et2O. Methyl esters
of the corresponding carboxylic acids were prepared for ease of
purification. The organic layers, dried with MgSO4, were filtered
and concentrated. The products were purified by column chroma-
tography on silica gel with pentane/Et2O mixtures as eluent. Most
of the products are known compounds[22Ϫ24] and their spectral data
were compared to authentic samples. Spectral data for other com-
pounds:
Commun. 1992, 1120Ϫ1121. Ϫ
S. Ozaki, I. Horiguchi, H.
Matsushita, H. Ohmori, Tetrahedron Lett. 1994, 35, 725Ϫ728.
A Pd-catalyzed electroreduction of N-alkenyl-2-bromoanilines
under CO2 has been reported in a symposium: H. Tanaka, O.
Ren, S. Torii, 2nd Intern. Symposium on Electroorganic Syn-
thesis, Kurashiki, Japan, 1994.
[10]
[11]
M. Beley, J. P. Collin, R. Ruppert, J. P. Sauvage, J. Am. Chem.
Soc. 1986, 108, 7461Ϫ7467.
B. P. Sullivan, in Electrochemical and Electrocatalytical Reac-
tions of Carbon Dioxide, Elsevier, Amsterdam, chapter 5, 1993.
[12] [12a]
´
O. Sock, M. Troupel, J. Perichon, Tetrahedron Lett. 1985,
26, 1509Ϫ1512. Ϫ [12b] G. Silvestri, S. Gambino, G. Filardo, G.
Greco, A. Gulotta, Tetrahedron Lett. 1984, 25, 4307Ϫ4310.
[13] [13a]
C. Amatore, A. Jutand, J. Am. Chem. Soc. 1991, 113,
[13b]
2819Ϫ2825. Ϫ
J. F. Fauvarque, C. Chevrot, A. Jutand, M.
Francois, J. Perichon, J. Organomet. Chem. 1984, 264, 273Ϫ281.
[14]
[15]
[16]
[17]
[18]
´
´
S. Derien, E. Dun˜ach, J. Perichon, J. Am. Chem. Soc. 1991,
113, 8447Ϫ8454.
´
´
S. Derien J. C. Clinet, E. Dun˜ach, J. Perichon, Tetrahedron
1,2-Bis(dihydrobenzofuran-3-yl)ethane (5a): 1H NMR (CDCl3,
TMS): δ ϭ 7.15Ϫ7.00 (4 H, m), 6.85Ϫ6.65 (4 H, m), 4.56 (2 H, t;
J ϭ 0.7), 4.14 (2 H, dd, J ϭ 0.7, 0.4), 3.29Ϫ3.45 (2 H, m),
1.40Ϫ1.75 (4 H, m). Ϫ 13C NMR (CDCl3, TMS): δ ϭ 160.1, 130.5,
128.3, 124.3, 124.2, 120.5, 76.6, 41.9, 32.3. Ϫ MS (70 eV); m/z (%):
266 [Mϩ], 133, 132, 119, 91(100), 65.
1992, 48, 5235Ϫ5248.
´ ´
´
J. Chaussard, J. C. Folest, J. Y. Nedelec, J. Perichon, S. Sibille,
M. Troupel, Synthesis 1990, 5, 369Ϫ387.
G. Silvestri, S. Gambino, G. Filardo, Acta Chem. Scand. 1991,
987Ϫ992.
The [Ni(cyclam)Br2] catalyst, a stable pink complex, was easily
prepared from nickel bromide and cyclam in absolute ethanol:
B. Bosnich, C. K. Poon, M. L. Tobes, Inorg. Chem. 1965, 4,
1102Ϫ1104.
Methyl 3-Methyldihydrobenzofuran-3-carboxylate (2c): 1H NMR
(CDCl3, TMS): δ ϭ 7.23Ϫ6.83 (4 H, m), 4.63 (1 H, d, J ϭ 9.5),
4.32 (1 H, d, J ϭ 9.5), 3.49 (3 H, s); 2.70 (2 H, d, J ϭ 3.8), 1.45 (3
H, s). Ϫ 13C NMR (CDCl3, TMS): δ ϭ 172.8, 159.7, 129.1, 128.5,
124.6, 121.6, 109.7, 76.7, 51.8, 45.3, 43.6, 38.3). Ϫ MS (70 eV); m/z
(%): 206 [Mϩ], 175, 146, 131, 120, 92, 86, 77, 63, 55(100).
[19]
S. Olivero, E. Dun˜ach, J. Chem. Soc., Chem. Commun. 1995,
2497Ϫ2498.
[20] [20a]
C. Gosden, K. P. Healy, D. Pletcher, J. Chem. Soc., Dalton
[20b]
Trans. 1978, 972Ϫ976. Ϫ
C. Gosden, D. Pletcher, J. Or-
[20c]
ganomet. Chem. 1980, 186, 401Ϫ409. Ϫ
K. P. Healy, D.
Pletcher, J. Organomet. Chem. 1978, 161, 109Ϫ120.
S. Olivero, J. P. Rolland, E. Dun˜ach, Organometallics 1998, 17,
3747Ϫ3753.
[21]
General Procedure for Two-Compartment Cell Electrolyses: Both
[22]
[23]
R. Rai, D. B. Collum, Tetrahedron Lett. 1994, 35, 6221Ϫ6224.
S. C. Zimmerman, K. D. Cramer, A. A. Galan, J. Org. Chem.
1989, 54, 1256Ϫ1264.
compartments were filled with a DMF solution (30 mL each) of
Ϫ
nBu4NϩBF4 (1 g, 3 mmol) under inert atmosphere. The desired
[24]
[25]
(cyclam)NiII complex (0.1 mmol) and 1a (0.3 mmol) were added to
the cathodic compartment, saturated with CO2. The electrolyses
were run at 20°C at the controlled potential of Ϫ1.5 V and were
stopped when the current was negligible. The workup was the same
as described above, the reaction being followed by GC.
A. N. Abeywickrema, A. L. J. Beckwith, S. Gerba, J. Org.
Chem. 1987, 52, 4072Ϫ4078.
A. R. Deshpande, M. V. Paradkar, Synth. Commun. 1990, 20,
809Ϫ816.
Received January 18, 1999
[O99019]
Eur. J. Org. Chem. 1999, 1885Ϫ1891
1891