Activation of aryl and vinyl triflates by palladium and electron transfer - Electrosynthesis of aromatic and αβ-unsaturated carboxylic acids from carbon dioxide

Article abstract of DOI:10.1002/(sici)1099-0690(199809)1998:9<1811::aid-ejoc1811>3.0.co;2-v

The electrochemical reduction of aryl and vinyl triflates in the presence of CO₂ and a catalytic amount of palladium results in the formation of aromatic and αβ-unsaturated carboxylic acids. Aryl and vinyl triflates usually undergo palladium-catalysed cross-coupling reactions with nucleophiles. Their reactivity has been reversed in the presence of an electron source, so that they react with electrophiles such as CO₂. The reaction proceeds through an activation of the C-O bond of the aryl or vinyl triflate by oxidative addition to a palladium(0) complex, followed by an activation by electron transfer of the thus formed aryl- or vinylpalladium(II) complexes.

Full text of DOI:10.1002/(sici)1099-0690(199809)1998:9<1811::aid-ejoc1811>3.0.co;2-v

FULL PAPER
Activation of Aryl and Vinyl Triflates by Palladium and Electron Transfer –
Electrosynthesis of Aromatic and ␣,β-Unsaturated Carboxylic Acids from
Carbon Dioxide
´
Anny Jutand* and Serge Negri
´
´
Ecole Normale Superieure, Departement de Chimie, CNRS URA 1679,
24 Rue Lhomond, F-75231 Paris Cedex 5, France
Fax: (internat.) ϩ 33-1/44323325
E-mail: Anny.Jutand@ens.fr
Received March 20, 1998
Keywords: Aryl triflates / Vinyl triflates / Palladium(0) catalyst / Carbon dioxide / Electron transfer
The electrochemical reduction of aryl and vinyl triflates in
presence of an electron source, so that they react with
the presence of CO2 and a catalytic amount of palladium electrophiles such as CO2. The reaction proceeds through an
results in the formation of aromatic and α,β-unsaturated
carboxylic acids. Aryl and vinyl triflates usually undergo
activation of the C–O bond of the aryl or vinyl triflate by
oxidative addition to a palladium(0) complex, followed by an
palladium-catalysed
cross-coupling
reactions
with activation by electron transfer of the thus formed aryl- or
nucleophiles. Their reactivity has been reversed in the
vinylpalladium(II) complexes.
•Ϫ
Introduction
ϪSO2CF3 formed by the first electron transfer (Eq. 1), is
highly favoured compared to the cleavage of the ArϪO
bond (Eq. 3).
Aryl and vinyl triflates usually undergo cross-coupling re-
actions with nucleophiles in the presence of catalytic
amounts of palladium complexes, affording substituted ar-
enes or alkenes.[1] The reactions proceed by nucleophilic at-
tack on the aryl- or vinylpalladium(II) complexes formed
by oxidative addition of the aryl[2] or vinyl[3] triflates to
the palladium(0) complex. In 1992, we reported that it was
possible to invert the reactivity of aryl triflates and to make
them to react with electrophiles in the presence of an elec-
tron source and a palladium catalyst.[4][5] We reported a
similar reactivity for vinyl triflates in 1997.[6] In the absence
of any electrophile other than the aryl or vinyl triflate itself,
homocoupling reactions were observed affording biaryls[7]
or conjugated dienes,[6] respectively. In the presence of car-
bon dioxide as the electrophile, aromatic carboxylic acids[4]
and α,β-unsaturated carboxylic acids[6] were synthesized in
good yields with PdCl2(PPh3)2 as the catalyst. We now wish
to report more details concerning the carboxylation reac-
tion.
Mechanism of the electrochemical reduction of aryl tri-
flates is depicted in Eqs. 1Ϫ7.[7]
Table 1. Reduction peak potentials of aryl triflates and arylpalladi-
um(II) chloride complexes (Scheme 1, eq. a)
ArϪ
ArϪOTf
Red
ArϪPdϪCl(PPh3)2
Red
[a]
[a]
Ep
Ep
p-CNϪC6H4Ϫ
p-TfOϪC6H4Ϫ
p-CF3ϪC6H4Ϫ
p-EtO2CϪC6H4Ϫ
p-ClϪC6H4Ϫ
p-BrϪC6H4Ϫ
p-FϪC6H4Ϫ
Ϫ1.80
Ϫ2.24
Ϫ2.21
Ϫ1.82
Ϫ2.50
Ϫ2.44
Ϫ2.54
Ϫ2.63
Ϫ2.71
Ϫ2.67
Ϫ2.74
Ϫ2.70
Ϫ1.95
Ϫ2.01
Ϫ1.75
Ϫ2.15
Ϫ2.00
Ϫ1.75
Ϫ2.41
Ϫ2.16
Ϫ2.54
Ϫ2.20
Ϫ2.53
Ϫ[b]
Ϫ2.68
Ϫ2.46
Ϫ1.89
Ϫ1.90
Results and Discussion
Palladium-Catalysed Electrosynthesis of Aromatic Carboxylic
Acids from Aryl Triflates and Carbon Dioxide
C6H5Ϫ
As we have previously reported,[4] aryl triflates, ArOTf,
are electroactive compounds (Table 1). Their electrochemi-
cal reduction leads, after hydrolysis, to a mixture of phenol
ArϪOH and arene ArϪH, as illustrated in Table 2 (entry
p-MeϪC6H4Ϫ
p-MeOϪC6H4Ϫ
p-tBuϪC6H4Ϫ
o-MeϪC6H4Ϫ
1-naphthylϪ
1) for 1-naphthyl triflate. When the electrolysis was per- 2-naphthylϪ
formed at the controlled potential of Ϫ2.0 V at room tem-
perature in DMF, 1-naphthol was formed as the major
compound (86% yield), showing that the cleavage of the
[a]
Volts vs. SCE; reduction peak potentials were determined at a
steady gold disc electrode (i.d. 0.5 mm) at a scan rate of 0.2 VsϪ1
in DMF containing nBu4NBF4 (0.3 ) at 20°C. Ϫ No reaction
[b]
OϪS bond (Eq. 2) in the intermediate radical anion ArϪO- at 20°C.
Eur. J. Org. Chem. 1998, 1811Ϫ1821
WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1998
1434Ϫ193X/98/0909Ϫ1811 $ 17.50ϩ.50/0
1811

´
A. Jutand, S. Negri
FULL PAPER
As a consequence, when the electrochemical reduction of Scheme 1. Mechanism of the palladium-catalyzed electrocarboxy-
lation of aryl triflates
1-naphthyl triflate was performed in the presence of carbon
dioxide at Ϫ2.0 V (Table 2, entry 2), only a small amount
of the desired 1-naphthoic acid was formed by car-
boxylation of the electrogenerated 1-naphthyl anion (Eq. 6);
1-naphthol was still the major product of the reaction (Eqs.
4, 5). A possible side reaction, namely the carboxylation
of the electrogenerated 1-naphthoxide anion to afford
1-hydroxynaphthalene-2-carboxylic acid as in a Kolbe-
Schmitt reaction did not take place.[8] The introduction of
a palladium catalyst such as PdCl2(PPh3)2 resulted in the
exclusive formation of the desired 1-naphthoic acid (Table
2, entry 3, Eq. 8) at a potential of Ϫ1.7 V, which is less
negative than the reduction potential of 1-naphthyl triflate
(Ϫ1.95 V).
dition with aryl triflates, ultimately leading to the corre-
sponding biaryl ArAr[7] (eqs. f and g in Scheme 1).[11]
For example, p-CF3ϪC6H4ϪOTf, 3 mmol dmϪ3, in
Moreover, no 1,1Ј-binaphthyl was detected, which was DMF (containing nBu4NBF4, 0.3 ) was reduced at EpR0 ϭ
the major compound formed when the electrolysis was car- Ϫ2.0 V (Figure 1a), whereas the reduction of CO2 took
ried out in the absence of CO2 (Table 2, entry 4, Eq. 9).[7]
This shows that the Pd-catalysed carboxylation (Eq. 8) is
place at Ϫ2.32 V. When the reduction was performed with
stoichiometric mixture of p-CF3ϪC6H4ϪOTf and
a
considerably more efficient than the Pd-catalysed homo- PdCl2(PPh3)2, we first observed the reduction peak of
coupling (Eq. 9).
PdCl2(PPh3)2 at EpR1 ϭ Ϫ0.74 V (Figure 1b). This complex
The mechanism of the Pd-catalysed carboxylation of aryl was then reduced to palladium(0) complex
a
triflates (Scheme 1) can be described with reference to the Pd0(PPh3)2ClϪ, which underwent an oxidative addition
mechanism of the Pd-catalysed homocoupling of aryl tri- with the aryl triflate (eq. a in Scheme 1). The oxidative ad-
flates that we have described previously.[7] The precursor dition was attested by the quasi-absence of the oxidation
PdCl2(PPh3)2 is reduced to a palladium(0) complex ligated peak of Pd0(PPh3)2ClϪ, which is usually observed on the
by one chloride ion: Pd0(PPh3)2ClϪ.[9] This complex then reverse scan at Ep ϭ ϩ0.08 V (Figure 1c). Since the elec-
O1
undergoes an oxidative addition with the aryl triflate (eq. a trogenerated palladium(0) species, Pd0(PPh3)2ClϪ, was
in Scheme 1), resulting in the formation of a neutral com- ligated by one chloride ion, the oxidative addition resulted
plex ArPdCl(PPh3)2,[2] which is more easily reduced than in the formation of
a
neutral complex, p-
the starting aryl triflate (see Table 1). The electrochemical CF3ϪC6H4ϪPdCl(PPh3)2.[2] This complex was reduced at
reduction of neutral ArPdX(PPh3)2 complexes consumes Ep ϭ Ϫ1.83 V, i.e. it was more easily reduced than the
R2
two electrons per mol (eq. b in Scheme 1) and affords an starting aryl triflate (Figure 1b). The cyclic voltammogram
anionic arylpalladium(0) complex ArPd0(PPh3)2Ϫ, which is exhibited a third reduction peak at EpR3 ϭ Ϫ2.12 V (Figure
in rapid equilibrium with the free anion ArϪ and 1b), which was assigned to the dimer p-4,4Ј-
Pd0(PPh3)2 (eq. c).[10] It has been established that the car- CF3ϪC6H4ϪC6H4ϪCF3 (Eq. 9). The magnitude of the re-
boxylation proceeds from the anion ArϪ (eq. d),[10] whereas duction peak R2 grew as the concentration of the aryl trifl-
ArPd0(PPh3)2Ϫ complexes are able to undergo oxidative ad- ate was increased (Figure 1d), indicating a catalytic reaction
Table 2. Uncatalysed and palladium-catalysed electrochemical reduction of 1-naphthyl triflate;[a] Ar ϭ 1-naphthyl
Entry
Catalyst (10%)
CO2 [atm]
E[b] [V]
T [°C]
F/mol[c]
ArϪOH
ArϪH
ArϪCO2H ArϪAr
(%)[d]
(%)[d]
(%)[d]
(%)[d]
1
2
3
4
no
no
1
Ϫ2.0
Ϫ2.0
Ϫ1.7
Ϫ1.7
20
20
90
90
2.0
2.2
2.2
1.5
86
80
0
13
0
Ϫ
0
0
no
13
83[e]
Ϫ
PdCl2(PPh3)2
PdCl2(PPh3)2
1
0
0
no
13
33
50
[a]
[b]
[c]
1 mmol of 1-naphthyl triflate in 50 ml of DMF containing nBu4NBF4 (0.3 ); divided cell. Ϫ Electrolysis potential vs. SCE. Ϫ
Faradays per mol ϭ number of electrons per mol involved in the electrolysis. Ϫ [d] Yields are relative to that obtained with 1-naphthyl
1
triflate, which was completely converted; values were determined from the crude mixture after work-up by H-NMR spectroscopy using
[e]
CHCl2CHCl2 as internal standard. Ϫ Isolated yield.
1812
Eur. J. Org. Chem. 1998, 1811Ϫ1821

Activation of Aryl and Vinyl Triflates by Palladium and Electron Transfer
FULL PAPER
leading to the biaryl (Eq. 9).[7] In the presence of CO2, the the carboxylation step (Scheme 1, eq. d) is not the rate-
reduction of p-CF3ϪC6H4ϪPdCl(PPh3)2 at R2 afforded the determining step. As illustrated in Table 3, a wide variety
carboxylic acid, p-CF3ϪC6H4ϪCO2H.
of aromatic carboxylic acids, substituted by electron donor
or acceptor groups, has been synthesized by electrocarbox-
ylation of aryl triflates with CO2 under atmospheric pres-
sure. The main by-products were phenols, which could eas-
Figure 1. Cyclic voltammetry performed in DMF (containing
nBu4NBF4, 0.3 ) at a stationary gold disc electrode (i.d. ϭ 0.5
mm) at a scan rate of 50 mVsϪ1 at 20°C; (a) p-CF3ϪC6H4ϪOTf,
3
mmol dmϪ3 mmol dmϪ3 and p- ily be recycled to the aryl triflates.
; (b) PdCl2(PPh3)2, 3
CF3ϪC6H4ϪOTf, 3 mmol dmϪ3; (c) same solution as in (b) but
the direction of the scan has been reversed just after the reduction
of PdCl2(PPh3)2 at Ϫ1.2 V. (d) PdCl2(PPh3)2, 3 mmol dmϪ3 and p-
CF3ϪC6H4ϪOTf, 6 mmol dmϪ3
The electrocarboxylation proved to be regiospecific and
compatible with most functional substituents on the aryl
ring (e.g. CN, CF3, CO2Et, Cl, F). However, the reaction
failed when starting from p-NO2ϪC6H4ϪOTf, due to pref-
erential reduction of the nitro group. Moreover, no car-
boxylation occurred when the substituent was too strongly
donating (entry 15). In such cases, direct reduction of the
aryl triflate to the corresponding phenol was observed, as
the oxidative addition was too slow. Indeed, we have re-
ported previously that the oxidative addition of aryl triflates
to palladium(0) complexes is favoured when the aryl tri-
flates are substituted by electron-withdrawing groups
(Hammett correlation with a positive value of ρ).[2] With p-
BrϪC6H4ϪOTf as the substrate, some para-terephthalic
acid was formed after consumption of 2e per mol (entry
7), showing that the palladium(0) underwent a competitive
oxidative addition into both the ArϪBr and ArϪOTf
bonds. We have previously established that palladium(0)
complexes ligated by PPh3 are only 1.7 times more reactive
towards ArOTf than towards ArBr.[2][13] With the bromide
substituent in the ortho position, double carboxylation was
not observed, but reduction of the CϪBr bond to CϪH
took place instead (entry 17). On the other hand, oxidative
addition of palladium(0) complexes with aryl chlorides is
known to be very slow, which is why a good compatibility
was observed in the case of p-ClϪC6H4ϪOTf (entry 9). At-
tempts to use NiCl2(dppe) as a catalyst resulted in lower
yields when the substituent was an acceptor such as an ester
group (entry 5), whereas this complex proved to be an ef-
ficient catalyst in cases with donor substituents (compare
The carboxylation reaction is in fact more efficient than entries 15 and 16). In an experiment aimed at elucidating
the dimerization, since the oxidative addition of the mechanism of the Pd-catalysed carboxylation, electroly-
Ϫ
ArPd0(PPh3)2 with ArOTf (Scheme 1, eq. f) is consider- sis was performed in the presence of a stoichiometric
ably slower than the nucleophilic attack of the anion ArϪ amount of Pd0(PPh3)4, the oxidative addition of which to
on CO2 (eq. d). Indeed, this anion is very basic as it is ArOTf, in the absence of any chloride ions, affords a cat-
generated in the absence of any stabilizing cation, the only ionic complex ArPdII(PPh3)2ϩ [2] The high yield of the reac-
.
cation present being the nBu4Nϩ of the supporting electro- tion (entry 12) demonstrates that the carboxylation can also
lyte. The rate-determining step of the Pd-catalysed car- take place by the reduction of cationic arylpalladium(II)
boxylation is the first chemical step (Scheme 1, eq. a), i.e. complexes.[14]
the oxidative addition of the aryl triflate to the palladium(0)
Thus, by supplying electrons, it is possible to invert the
complex. The rate constant of the oxidative addition has reactivity of aryl triflates, thereby causing them to react
been estimated by cyclic voltammetry as 5.5 Ϫ1 sϪ1 for 1- with an electrophile such as carbon dioxide.[15] To date, this
naphthyl triflate at 20°C.[7][12]
represents a unique route for the synthesis of aromatic car-
The electrolyses were performed in DMF in a divided boxylic acids from carbon dioxide and phenols via the trifl-
cell, at the reduction potentials of the arylpalladium(II) ate derivatives, by substitution at the ArϪO bond (Eq. 10).
complexes, which were found to be more easily reduced
Indeed, reaction of phenoxide with carbon dioxide as in
than the precursor aryl triflates (Table 1). The reactions the Kolbe-Schmitt reaction (Eq. 11) results in the formation
were performed at 90°C to accelerate the rate of the oxida- of the ortho-carboxylated phenols without, of course, any
tive addition. Even though the solubility of CO2 at 90°C is activation of the ArϪO bond.
most probably less than that at room temperature, the reac-
Our process is an alternative to the electrocarboxylation
tion works equally well at low CO2 concentration because of aryl halides, in the absence[16] or the presence of pal-
Eur. J. Org. Chem. 1998, 1811Ϫ1821
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´
A. Jutand, S. Negri
FULL PAPER
Table 3. PdCl2(PPh3)2-catalysed electrocarboxylation of aryl triflates in DMF at 90°C (Eq. 8)[a]
Entry
ArϪOTf
E[b] [V]
F/mol[c]
ArϪCO2H (%)[d]
ArϪOH
(%)[d]
1
p-CNϪC6H4ϪOTf
p-CF3ϪC6H4ϪOTf
p-CH3COϪC6H4ϪOTf
p-EtO2CϪC6H4ϪOTf
p-EtO2CϪC6H4ϪOTf[f]
p-TfOϪC6H4ϪOTf
p-BrϪC6H4ϪOTf
p-BrϪC6H4ϪOTf
p-ClϪC6H4ϪOTf
p-FϪC6H4ϪOTf
Ϫ1.6
Ϫ1.6
Ϫ1.4
Ϫ1.6
Ϫ1.6
Ϫ1.8
Ϫ1.8
Ϫ1.8
Ϫ2.0
Ϫ2.0
Ϫ2.0
Ϫ2.0
Ϫ2.0
Ϫ2.1
Ϫ2.4
Ϫ1.9
Ϫ1.6
Ϫ1.8
Ϫ1.8
1.9
2.1
2.2
2.1
1.3
3.7
2.0
4.0
3.0
2.3
2.3
2.3
2.2
2.2
3.0
2.1
2.8
2.6
2.2
p-CNϪC6H4ϪCO2H, 35[e]
p-CF3ϪC6H4ϪCO2H, 64 [50]
p-CH3COϪC6H4ϪCO2H, 22
p-EtO2CϪC6H4ϪCO2H, 52
p-EtO2CϪC6H4ϪCO2H, 10
p-CO2HϪC6H4ϪCO2H, [71]
22
4
2
3
48
28
47
0
4
5
6
7
p-BrϪC6H4ϪCO2H, 46 ϩ p-CO2HϪC6H4ϪCO2H, 31
p-CO2HϪC6H4ϪCO2H, 63 ϩ C6H5ϪCO2H, 26
p-ClϪC6H4ϪCO2H, [71]
0
0
8
9
0
10
11
12
13
14
15
16
17
18
19
p-FϪC6H4ϪCO2H, [92]
0
C6H5ϪOTf
C6H5ϪCO2H, [96]
0
C6H5ϪOTf[g]
C6H5ϪCO2H, [88]
0
p-CH3ϪC6H4ϪOTf
p-MeOϪC6H4ϪOTf
p-tBuϪC6H4ϪOTf
p-tBuϪC6H4ϪOTf[h]
o-BrϪC6H4ϪOTf
o-CH3ϪC6H4ϪOTf
2-naphthylϪOTf
p-CH3ϪC6H4ϪCO2H, [75]
p-MeOϪC6H4ϪCO2H, 13
p-tBuϪC6H4ϪCO2H, 0
0
49
100
17
0
p-tBuϪC6H4ϪCO2H, [45]
C6H5ϪCO2H, [41]
o-CH3ϪC6H4ϪCO2H, 44
2-naphthylϪCO2H, [95]
33
0
[a]
[b]
ArOTf/PdCl2(PPh3)2: 1 mmol/0.1 mmol in 50 ml of DMF containing nBu4NBF4 (0.3 ); divided cell. Ϫ
Electrolysis potential in
[c]
[d]
volts vs. SCE. Ϫ Faradays per mol ϭ number of electrons involved in the electrolysis. Ϫ Yields are relative to initial ArOTf; values
were determined from the crude mixture after work-up by 1H-NMR spectroscopy using CHCl2CHCl2 as internal standard; isolated yields
[g]
[e]
[f]
are given in brackets. Ϫ Formation of 47% of 4,4Ј-CNϪC6H4ϪC6H4ϪCN. Ϫ NiCl2(dppe) as catalyst; ArOTf recovered: 26%. Ϫ
[h]
Stoichiometric reaction ArOTf/Pd(PPh3)4: 1 mmol/1 mmol. Ϫ NiCl2(dppe) as catalyst; ArOTf recovered: 22%.
initial product immediately after extraction and prior to
heating in vacuo exhibited, besides the signals characteristic
of compound 2,[24] additional signals attributable to the
enol 3.[25] The tautomeric equilibrium between the ketone
and the enol was probably due to some traces of acid[26]
originating from the acidic work-up. Under these con-
ditions, the formation of compound 4 is a condensation of
three molecules of the ketone 2 via the enol, with the con-
current elimination of three molecules of water in vacuo.[27]
Since we were developing new reactions of aryl triflates
through activation of the ArϪO bond by palladium com-
plexes and electron transfer, we took the opportunity to
transform compound 4 into functional terphenylene deriva-
tives that might represent new materials or be precursors of
new materials. Compound 4 was found to be reducible in
ladium[10][17] or nickel complexes[18][19] as catalysts. How-
ever, the challenge is higher for aryl triflates than for aryl
halides since the latter can be reduced, after cleavage of the
ArϪBr bond, to the anion ArϪ, which then reacts with
CO2. As shown above, the reduction of aryl triflates is a
more complex process since a carbanion ArϪ cannot be
quantitatively generated by cleavage of the ArϪO bond,
and thus a catalyst is required. It is only in the presence of
palladium that activation of the ArϪO bond takes place,
leading to an ArϪPdII complex, which is the source of the
anion ArϪ after activation by electron transfer.
DMF at Ep ϭ Ϫ1.67, Ϫ1.85, and Ϫ1.97 V vs. SCE (at a
red
gold disc electrode with a scan rate of 0.2 VsϪ1). An elec-
trolysis performed at Ϫ1.9 V afforded, after consumption
of 6 electrons per mol, a white solid that was isolated in
67% yield and characterized as 1,3,5-tris(4-hydroxyphen-
yl)benzene (5) (Scheme 2). All attempts to synthesize com-
pound 5 by the direct condensation of compound 1 in
acidic media were unsuccessful.[28]
Application to the Synthesis of Substituted 1,3,5-Triarylbenzene
Derivatives from 4-Hydroxyacetophenone
In an attempt to synthesize 4-(trifluoromethylsulfonyl-
An electrolysis of compound 4 performed at Ϫ1.6 V, in
oxo)acetophenone (2) by treating triflic anhydride with 4- the presence of gaseous carbon dioxide and 30% of
hydroxyacetophenone (1), we found that a modification of PdCl2(PPh3)2 according to Eq. 8, afforded, after consump-
the reported procedure[20] and of the usual work-up[23] (i.e. tion of 6.6 electrons per mol, a white solid that was isolated
heating the crude product at 140Ϫ150°C in vacuo) did not in 75% yield and characterized as 1,3,5-tris(4-carboxyphen-
result in the formation of the desired aryl triflate 2. Instead, yl)benzene (6) (Scheme 2). When the electrosynthesis was
a new compound was isolated as white crystals in 97% yield performed with only 10% of the catalyst, compound 6 was
and was characterized as 1,3,5-tris[4-(trifluoromethylsulfon- obtained in 43% yield along with a by-product which was
yloxo)phenyl]benzene (4). The 1H-NMR spectrum of the isolated and characterized as compound 7 (34% yield).
1814
Eur. J. Org. Chem. 1998, 1811Ϫ1821

Activation of Aryl and Vinyl Triflates by Palladium and Electron Transfer
Scheme 2
FULL PAPER
Figure 2. Cyclic voltammetry of vinyl triflate 8b, 3 mmol dmϪ3 in
DMF (containing nBu4NBF4, 0.3 ) at a stationary gold disc elec-
trode (i.d. ϭ 0.5 mm) at a scan rate of 50 mVsϪ1 at 20°C
over, they may have interesting properties with regard to
nonlinear optics.[32]
Palladium-Catalysed Electrosynthesis of α,β-Unsaturated
Carboxylic Acids from Vinyl Triflates and Carbon Dioxide
Vinyl triflates 8 are electroactive compounds (Table 4).[6]
Their electrochemical reduction affords, after hydrolysis, the
ketone 9 in almost quantitative yield (Table 5, entries
1Ϫ2).[33] The cyclic voltammogram of compound 8b (Fig-
ure 2), obtained at a scan rate of 50 mVsϪ1, exhibits two
Table 4. Reduction peak potentials of vinyl triflates and vinylpalla-
dium(II) chloride complexes (Scheme 4, eq. a)
In conclusion, it is possible to synthesize new 1,3,5-triar-
ylbenzene derivatives possessing triflate, carboxyl, and/or
hydroxy groups as substituents. Some functionalized deriva-
tives of 1,3,5-triarylbenzene (terphenylene) have been
shown to be precursors of discotic liquid crystals and of
materials possessing self-assembling properties.[29] Com-
pounds 5-7 can be easily transformed into esters or ethers
and are thus potential precursors of liquid crystals.[30] They
[a]
Volts vs. SCE; reduction peak potentials were determined at a
steady gold disc electrode (i.d. 5 mm) at a scan rate of 0.2 V sϪ1 in
can be used as reticulating agents for polymers.[31] More- DMF containing nBu4NBF4 (0.3 ) at 20°C.
Eur. J. Org. Chem. 1998, 1811Ϫ1821
1815

´
A. Jutand, S. Negri
FULL PAPER
Table 5. Uncatalysed and PdCl2(PPh3)2-catalysed electrochemical reduction of vinyl triflates in DMF at 20°C[a]
[a]
[b]
1 mmol of vinyl triflate 8 in 50 ml of DMF containing nBu4NBF4 (0.3 ); divided cell. Ϫ Electrolysis potentials in volts vs. SCE.
Ϫ
Ϫ
[c] Faradays per mol ϭ number of electrons involved in the electrolysis. Ϫ [d] Yields are related to the vinyl triflate completely converted.
[e]
[f]
49% of recovered 8f. Ϫ Formation of 12b (51%), 13b (25%).
successive reduction peaks. The first one, at Ϫ2.02 V, Scheme 3. Mechanism of the electrochemical reduction of vinyl tri-
flates
characterizes the two-electron reduction of the vinyl triflate
8b, while the second one, at Ϫ2.62 V, characterizes the re-
duction of the α-tetralone 9b, identified by comparison with
an authentic sample. Similar behaviour was observed in the
case of 8a, where a second reduction peak was detected at
Ϫ2.61 V and assigned to acetophenone.
The first electron transfer affords a radical anion 8•Ϫ
(Scheme 3). After cleavage of the OϪS bond, a vinyloxy
radical 10 is formed, which is more easily reduced than the
vinyl triflate. Its reduction furnishes the enolate 11 and,
after protonation, the ketone 9 (route A), with the overall
process consuming 2 electrons per mol (Eq. 12). Thus, as in
the case of aryl triflates, electrochemical reduction of vinyl
triflates results in an activation and cleavage of the OϪS
bond of the triflate group.
The electrochemical reduction of vinyl triflates 8 in the
presence of carbon dioxide is a complex process. As illus-
trated in Table 4, the vinyl triflates 8 considered in this work
can be divided in two groups, depending on the value of
their reduction potential relative to that of CO2 (Ϫ2.32 V,
determined at a gold disc electrode at a scan rate of 0.2
VsϪ1). Vinyl triflates 8a, b possessing a double bond conju-
gated to a phenyl group are more easily reduced than CO2.
In such cases, it is possible to selectively reduce the vinyl
triflates 8a, b without reducing CO2. For example, the elec- was formed by carboxylation of the electrogenerated enol-
trolysis of 8b, performed in the presence of CO2 in a divided ate 11, as illustrated in Scheme 3 (route B, Eq. 13), while
cell, afforded the ketone 9b, the β-oxocarboxylic acid 12b, compound 13b probably resulted from the carboxylation of
and the carbonate 13b (Table 5, entry 6). Compound 12b the enolate at the oxygen atom.
1816
Eur. J. Org. Chem. 1998, 1811Ϫ1821

Activation of Aryl and Vinyl Triflates by Palladium and Electron Transfer
FULL PAPER
The non-conjugated vinyl triflates 8c؊f are reduced at try, which has provided a mechanistic insight into the Pd-
very negative potentials (Table 4), at around Ϫ3 V vs. SCE, catalysed carboxylation (Scheme 4).
and are thus less easily reduced than CO2 (Ϫ2.32 V).
Scheme 4
Consequently, in these cases, electrolyses performed at the
reduction potentials of 8c؊f also involved the reduction of
CO2. An electrolysis of 8f, performed in a divided cell at a
controlled potential of Ϫ2.9 V, afforded, after consumption
of 4 electrons per mol, the ketone 9f in 46% yield. Some
49% of the vinyl triflate 8f was recovered (Table 5, entry 3)
and no reaction of the electrogenerated enolate with CO2
was observed.[6] The consumption of more than 2 electrons
per mol of 8f converted indicates that reduction of CO2
occurred concurrently with that of 8f, and that the direct
reduction of 8f to ketone 9f was not affected by the re-
duction of CO2. Tokuda has recently reported that β-ox-
ocarboxylic acids 12 are formed exclusively in good yields,
even from vinyl triflates that are less easily reduced than
CO2.[34] The success of this reaction is due to the presence
of magnesium salts released by oxidation of the magnesium
anode during the electrolyses, which were carried out in un-
divided cells.[34]
Figure 3a shows the cyclic voltammogram obtained for
the reduction of compound 8d (3 mmol dmϪ3 in DMF con-
taining nBu4NBF4, 0.3 ) at a scan rate of 50 mV sϪ1. Un-
der these conditions, the reduction peak of 8d was not de-
tected before the reduction of the solvent.[35] In the pres-
ence of PdCl2(PPh3)2, 3 mmol dmϪ3, the reduction peak R1
of PdCl2(PPh3)2 was first observed at EpR1 ϭ Ϫ0.74 V (Fig-
However, in none of these cases were α,β-unsaturated
carboxylic acids 16 produced, since activation of the CϪO
bond of the vinyl triflates could not be achieved by simple
electron transfer. This reaction requires activation of the
CϪO bond by a transition metal. In the presence of a cata-
lytic amount of PdCl2(PPh3)2, the formation of ketone 9
was completely inhibited and a conjugated diene 14 was
obtained in quantitative yield (Table 5, entry 4, Eq. 14).
This shows that activation of the CϪO bond of the vinyl
triflate via a palladium complex occurred, thereby pre-
venting the direct reduction of the vinyl triflate to the corre-
sponding ketone.
ure 3b), followed by a second reduction peak at Ep
ϭ
R2
Ϫ1.90 V (Figure 3b). PdCl2(PPh3)2 was reduced to a pal-
ladium(0) complex Pd0(PPh3)2ClϪ, which underwent an
oxidative addition with compound 8d (eq. a in Scheme 4).
Indeed, the oxidation peak O1, characteristic of the electro-
generated complex Pd0(PPh3)2ClϪ, was not observed on the
reverse scan of Figure 3c. Stille has established that the oxi-
dative addition of vinyl triflates to palladium(0) complexes
yields cationic σ-vinylpalladium(II) complexes, whereas
neutral σ-vinylpalladium(II) chloride complexes are formed
when the oxidative addition is performed in the presence of
chloride anions.[3] Thus, in the present case, the oxidative
addition
to the
electrogenerated
chloride-ligated
Pd0(PPh3)2ClϪ led to a neutral σ-vinylpalladium(II) chlo-
ride complex (Scheme 4, eq. a). This complex was reduced
at R2, i.e. at a less negative potential than the reduction
potential of the vinyl triflate 8d. The magnitude of the re-
duction peak R2 grew as the concentration of the vinyl trifl-
ate was increased (Figure 3d), indicating a catalytic reaction
leading to the conjugated diene (Eq. 14). The α,β-unsatu-
rated carboxylic acid was produced when the reduction was
performed at R2 in the presence of CO2 (Eq. 15). The
mechanism of the carboxylation step can reasonably be de-
scribed as depicted in Scheme 4, by analogy with the
mechanism outlined above for the aryl triflates (Scheme 1).
The electrochemical reduction performed in the presence
of PdCl2(PPh3)2 and carbon dioxide resulted in the forma-
tion of α,β-unsaturated carboxylic acids 15 (Eq. 15, Table
5, entries 5, 7).[6]
We note from Table 5 (entries 4, 5, 7) that the Pd-cata- Thus, initial activation of the vinylpalladium(II) chloride
lysed homocoupling (Eq. 14) and carboxylation (Eq. 15) complex by electron transfer affords a vinyl anion, which is
were performed at a less negative potential than the re- then able to react with CO2 (eqs. bϪd). In all cases investi-
duction potential of the vinyl triflates, and that this poten- gated here, the σ-vinylpalladium(II) complexes were found
tial is also less negative than that of CO2. The values of the to be more easily reduced than the precursor vinyl triflate
electrolysis potentials were determined by cyclic voltamme- (Table 4). Thus, the electrosyntheses of the α,β-unsaturated
Eur. J. Org. Chem. 1998, 1811Ϫ1821
1817

´
A. Jutand, S. Negri
FULL PAPER
Figure 3. Cyclic voltammetry performed in DMF (containing
nBu4NBF4, 0.3 ) at a stationary gold disc electrode (i.d. ϭ 0.5
mm) at a scan rate of 50 mVsϪ1 at 20°C; (a) vinyl triflate 8d, 3
mmol dmϪ3; (b) PdCl2(PPh3)2, 3 mmol dmϪ3 and 8d, 3 mmol
dmϪ3; (c) same solution as in (b) but the direction of the scan has
been reversed just after the reduction of PdCl2(PPh3)2 at Ϫ1.2 V.
(d) PdCl2(PPh3)2, 3 mmol dmϪ3 and 8d, 6 mmol dmϪ3
Table 6. PdCl2(PPh3)2-catalysed electrocarboxylation of vinyl trifla-
tes in DMF at 20°C (Eq. 15)[a]
carboxylic acids 15 were performed at the reduction poten-
tial of the vinylpalladium(II) complexes, i.e. at a potential
less negative than that of the vinyl triflates, thereby avoiding
their direct reduction to enolates, which are source of ke-
tones or α-oxocarboxylic acids. Moreover, the σ-vinylpal-
ladium(II) complexes were more easily reduced than CO2.
[a] VinylϪOTf 8/PdCl2(PPh3)2: 1 mmol/0.1 mmol in 50 ml of DMF
[b]
containing nBu4NBF4 (0.3 ). Ϫ
Electrolysis potential in volts
vs. SCE. Ϫ [c] Faradays per mol ϭ number of electrons involved in
the electrolysis. Ϫ [d] Yields are relative to the vinyl triflate that was
[e]
[f]
totally converted. Ϫ
Divided cell. Ϫ
Undivided cell with a
[g]
The reduction of CO2 did not occur during the electrolyses magnesium sacrificial anode. Ϫ Undivided cell with Li2C2O4 as
and the carboxylation could proceed normally. Car-
compound oxidized at a platinum anode. Ϫ [h] 71% of 8b recovered.
[i]
Ϫ
Formation of 8% of 1-hexene.
boxylations were performed with cyclic and linear vinyl tri-
flates (Table 6) and were regiospecific. Most electrolyses
were performed in divided cells, although they could be per-
formed in an undivided cell, provided that a sacrificial
anode was used (compare entries 2Ϫ4 of Table 6). Vinyl
triflates are more reactive than aryl triflates since their elec-
trocarboxylation could be performed by bubbling CO2 at
room temperature, rather than at 90°C as in the case of the
aryl triflates. This enhanced reactivity is probably due to a
faster oxidative addition of the palladium(0) complex with
vinyl triflates than with aryl triflates. Attempts to car-
boxylate a vinyl trifluoroacetate derivative such as com-
whereas linear vinyl triflates (compounds 8a, d) are synthe-
sized by addition of HOTf to alkynes.[37]
This work shows that cyclic ketones and alkynes are
pound 16c, which would be expected to be less reactive in transformed into α,β-unsaturated carboxylic acids in the
the oxidative addition than the triflate derivative, were in- presence of CO2, a palladium catalyst, and an electron
deed unsuccessful (Table 6, entry 9).
source, via the vinyl triflates (Eqs. 16 and 17). These new
Vinyl triflates are synthesized by two different routes. reactions offer an alternative route to the palladium-cata-
Cyclic vinyl triflates (compounds 8b, c, f, g) are synthesized lysed synthesis of α,β-unsaturated carboxylic acids from vi-
by reaction of ketone enolates with a triflating reagent,[36] nyl triflates and carbon monoxide under basic con-
1818
Eur. J. Org. Chem. 1998, 1811Ϫ1821

Activation of Aryl and Vinyl Triflates by Palladium and Electron Transfer
ditions.[38] Uncatalysed electrocarboxylation of vinyl hal-
FULL PAPER
1,3,5-Tris[4-(trifluoromethylsulfonyloxo)phenyl]benzene (4): To
ides has recently been reported by Tokuda.[39] However, this
reaction is restricted to phenyl-substituted vinyl bromides,
i.e. to vinyl halides that are easily reduced to the vinylic
anion, which then reacts with CO2. For alkyl-substituted
vinyl bromides, i.e. those which are not electroreducible, a
nickel catalyst is required.[40] Our reaction is more general
in the sense that it is not restricted to phenyl-substituted
vinyl triflates. Moreover, as in the case of aryl triflates, the
a solution of 2.0 g (14.7 mmol) of 4-hydroxyacetophenone (1) in
150 ml of CH2Cl2 at Ϫ50°C, was added 2.16 ml (31.3 mmol) of
triethylamine and 4.37 g (15.5 mmol) of trifluoromethanesulfonic
anhydride. The solution was stirred for 1 h at Ϫ50°C, then for 8 h
at room temperature, and then added to aqueous HCl (1 ). The
organic phase was dried and the dichloromethane was evaporated.
1
The H-NMR spectrum of the crude product revealed, besides the
signals of compound 2,[24] the presence of 7.5% of enol 3.[25] The
crude product was then heated at 140Ϫ150°C in vacuo for 1.5 h to
challenge is higher since we react vinyl triflates the direct afford brown crystals, which were recrystallised from ethanol. 3.56
g (97%) of white crystals were collected, m.p. 153°C. When the
enol was not present in the 1H-NMR spectrum, 0.045 g (0.3 mmol)
triflic acid was added to the crude product and the resulting mix-
ture was treated as above. Ϫ 1H NMR (250 MHz, CDCl3): δ ϭ
7.42 (d, J ϭ 8.5 Hz, 6 H, m-H), 7.75 (s, 3 H, central H), 7.76 (d,
J ϭ 8.5 Hz, 6 H, o-H). Ϫ 13C NMR (62 MHz, CDCl3): δ ϭ 118.74
(q, JCF ϭ 315 Hz), 121.90 (d), 125.82 (d), 129.09 (d), 140.84 (s),
141.02 (s), 149.29 (s). Ϫ IR (KBr): ν˜ ϭ 1611, 1510 cmϪ1 (CϭC),
1420, 1214, 614 (SO3CF3), 848, 751, 721 (CH). Ϫ MS (70 eV); m/z
(%): 750 [Mϩ], 617 (100) [M Ϫ SO2CF3], 484 [M Ϫ 2 SO2CF3],
351 [M Ϫ 3 SO2CF3]. Ϫ C27H15F9O9S3 (749.97): calcd. C 43.20, H
2.02; found C 43.38, H 2.08.
electrochemical reduction of which does not lead to vinyl
anions by cleavage of the CϪO bond, but rather to enolates
by cleavage of the OϪS bond of the triflate group. When
vinyl triflates are generated from terminal alkynes, our pro-
cess has the advantage of selectively affording the α,β-un-
saturated carboxylic acids in which the carboxyl group re-
sides at the more substituted position (Eq. 17). Nickel-cata-
lysed electrocarboxylation of terminal alkynes has been re-
ported.[41] However, this reaction is not selective and
affords a mixture of two α,β-unsaturated acids.
Conclusion
Cyclic vinyl triflates 8b, c, f, g were synthesized from commercial
cyclic ketones;[36] acyclic vinyl triflates 8a, d from commercial
alkynes.[37]
Aryl and vinyl triflates usually react with nucleophiles in
the presence of a palladium catalyst. By supplying electrons,
their reactivity has been reversed so that they react with
electrophiles such as carbon dioxide, affording aromatic
and α,β-unsaturated carboxylic acids, respectively.
Electrochemical Set-up and Procedure for Cyclic Voltammetry and
Electrolyses: Cyclic voltammetry was performed with a home-made
potentiostat and a PAR Model 175 wave-form generator. The cyclic
voltammograms were recorded with a Nicolet 3091 digital oscillo-
scope. Experiments were carried out in a three-electrode cell con-
nected to a Schlenk line. The counterelectrode consisted of a plati-
num wire of ca. 1 cm2 macroscopic surface area. The reference was
a saturated calomel electrode (Tacussel) separated from the solu-
tion by a bridge filled with 3 ml of a solution of nBu4NBF4 (0.3
) in DMF. 20 ml of DMF containing nBu4NBF4 (0.3 ) was
poured into the cell, 0.06 mmol of the appropriate aryl of vinyl
triflate was added, and cyclic voltammetry was performed with a
stationary disc electrode [a gold disc made from a cross section of
wire (i.d. 0.5 mm) sealed into glass] at a scan rate of 0.05 VsϪ1 or
0.2 VsϪ1. 42 mg (0.06 mmol) of PdCl2(PPh3)2 was then added and
cyclic voltammetry performed once more.
Carboxylations generally proceed in one of two ways:
Uncatalysed carboxylations proceed by nucleophilic attack
of CO2 by a carbanion (e.g. a Grignard reagent), whereas
catalysed carboxylations usually proceed through activation
of CO2 by a transition metal complex.[42] Our work is rep-
resentative of a third route, which proceeds by a double
activation. Indeed, the carboxylation reaction requires a
transition metal, but this is responsible for the activation of
the CϪO bond of the aryl or vinyl triflate by an oxidative
addition. This first activation affords an aryl- or vinylpal-
ladium(II) complex, activation of which by electron transfer
is required so as to generate an aryl or vinyl carbanion able
to react with CO2. The process is of particular interest since
aryl or vinyl carbanions cannot be directly generated by
reduction of aryl of vinyl triflates in the absence of a cata-
lyst.
The electrolyses were performed at a controlled potential using
a Tacussel PJT 35-2 potentiostat in a divided three-electrode cell
with the two compartments separated by a sintered-glass frit. The
cathodic compartment contained 50 ml of anhydrous DMF with
nBu4NBF4 (0.3 ) and was equipped with a carbon-cloth cathode
(10 cm2, Carbone Lorraine). The anodic compartment contained 5
ml of DMF with nBu4NBF4 (0.3 ) and was equipped with a mag-
nesium rod. The reference was a saturated calomel electrode sepa-
rated from the solution by a bridge filled with 3 ml of a solution
of nBu4NBF4 (0.3 ) in DMF.
This work has been supported by the Centre National de la Re-
cherche Scientifique (CNRS, URA 1679, “Processus dЈActivation
`
´
´
Moleculaire”) and the Ministere de lЈEnseignement Superieur et de
´
la Recherche (Ecole Normale Superieure).
Typical Procedure for the Electroreduction of Aryl Triflates in the
Absence of CO2 and Catalyst Ϫ Preparation of 1,3,5-Tris(4-
hydroxyphenyl)benzene (5): 0.375 g (0.5 mmol) of compound 4 was
placed in the cathodic compartment. The electrolysis was per-
formed at room temperature at a controlled potential of Ϫ1.9 V
vs. SCE and was terminated when the current dropped to about
5% of its initial value. 6 F/mol were passed through the cell. The
contents of the cathodic compartment were then added to aqueous
HCl (1 ) and after leaving the mixture overnight, it was extracted
with diethyl ether. After evaporation of the solvent from the com-
Experimental Section
IR (KBr pellets): Nicolet Impact 400D. Ϫ NMR: Bruker AM
1
13 C,
400 (400 MHz and 100.6 MHz for H and
respectively, with
TMS as internal standard), Bruker AC 250 (250 MHz and 62.9
MHz for 1H and 13 C, respectively, with TMS as internal standard).
Ϫ MS: Nermag R 10-10C.
Materials: The aryl triflates listed in Tables 2 and 3 were synthe-
sized from commercial phenols according to published pro-
cedures.[21][36a]
Eur. J. Org. Chem. 1998, 1811Ϫ1821
1819

´
A. Jutand, S. Negri
FULL PAPER
bined ether extracts, 0.119 g (67%) of white, greasy crystals was
isolated. Ϫ H NMR (250 MHz, [D4]methanol): δ ϭ 7.03 (d, J ϭ treating the mixture with dichloromethane. Compound 6 is insol-
presence of the two compounds 6 and 7. They were separated by
1
8 Hz, 6 H, o-H), 7.69 (d, J ϭ 8 Hz, 6 H, m-H), 7.73 (s, 3 H, central
uble in dichloromethane, whereas compound 7 is freely soluble.
H), 9.47 (s, 3 H, OH) Ϫ 13C NMR (62.9 MHz, [D4]methanol): δ ϭ Thus, filtration afforded 0.188 g (43%) of pure compound 6. Evap-
115.11 (d), 122.45 (d), 127.74 (d), 132.56 (s), 141.80 (s), 156.73 (s). oration of dichloromethane from the filtrate afforded 0.139 g (34%)
Ϫ MS (70 eV); m/z (%): 354 (100) [Mϩ], 338.[27b]
of pure compound 7 as white crystals. Ϫ 1H NMR (250 MHz,
[D6]acetone): δ ϭ 6.93 (dd, J ϭ 7 Hz, J ϭ 1.5 Hz, 2 H, o-H vs.
OH), 7.65 (m, 2 H), 7.80 (m, 2 H, m-H vs. OH), 7.89 (m, 5 H),
8.03 (d, J ϭ 7 Hz, 4 H, o-H vs. CO2H). Ϫ IR (KBr): ν˜ ϭ
3600Ϫ3000 cmϪ1 (OH), 1620 (CϭO), 1475, 1430 (CϭC), 1090
(CϪO), 750, 740, 705, 690 (CH). Ϫ MS (70 eV); m/z (%): 410 (100)
[Mϩ], 393 [M Ϫ OH], 384.
Typical Procedure for the Palladium-Catalysed Electrocarboxyl-
ation of Aryl Triflates Ϫ Preparation of 4-Fluorobenzoic Acid: The
cathodic compartment was charged with 0.244 g (1 mmol) of 4-
fluoro(trifluoromethylsulfonyloxo)benzene followed by 0.07 g (0.1
mmol) of PdCl2(PPh3)2. Carbon dioxide was then bubbled into the
cathodic compartment and the electrolysis was conducted at 90°C
at a controlled potential of Ϫ2 V, until the current dropped to
5% of its initial value. The electrolysis consumed 2.3 F/mol of 4-
fluoro(trifluoromethylsulfonyloxo)benzene. The mixture was then
hydrolysed overnight with aqueous HCl (1 ). After extraction with
diethyl ether and evaporation of the solvent, the crude product was
analysed by 1H-NMR spectroscopy, which revealed the presence of
some phosphane oxide. The crude product was then treated with
an aqueous solution of NaOH (1 ) so as to solubilize the corre-
sponding carboxylate salt. After washing with diethyl ether, the
basic aqueous solution was treated with aqueous HCl (1 ) and
extracted with diethyl ether, to afford 0.129 mg of pure 4-fluoroben-
zoic acid. Ϫ 1H NMR (250 MHz, CDCl3): δ ϭ 7.17 (t, JHH ϭ 9
Typical Procedure for the Palladium-Catalysed Electrocarboxyl-
ation of Vinyl Triflates Ϫ Preparation of 6-Methylcyclohex-1-ene-1-
carboxylic Acid (15g): The cathodic compartment was charged with
0.244 g (1 mmol) of 8g, followed by 70 mg (0.1 mmol) of
PdCl2(PPh3)2, and carbon dioxide was bubbled through the catho-
lyte. The electrolysis was conducted at room temperature at a con-
trolled potential of Ϫ2 V, until the current dropped to 5% of its
initial value. The mixture was then hydrolysed overnight with aque-
ous HCl (1 ). After extraction with diethyl ether and evaporation
1
of the solvent, the crude product was analysed by H-NMR spec-
troscopy. The signals of the α,β-unsaturated carboxylic acid were
detected, but the compound was shown to be contaminated with
some triphenylphosphane oxide. The carboxylic acid was purified
by solubilization in aqueous NaOH (1 ). After washing with di-
ethyl ether, the basic aqueous phase was treated with aqueous HCl
(1 ). The acidic aqueous phase was then extracted with diethyl
ether. Evaporation of the solvent from the combined extracts af-
forded 120 mg (86%) of pure 15g as white crystals, m.p. 102°C. Ϫ
1H NMR (250 MHz, CDCl3): δ ϭ 1.04 (d, J ϭ 7 Hz, 3 H, CH3),
1.45Ϫ1.60 (m, 4 H, CH2CH2), 2.06Ϫ2.17 (m, 2 H, CH2CHϭ),
2.54Ϫ2.67 (m, 1 H, CHCH3), 7.03 (t, 1 H, J ϭ 4 Hz, ϭCH), 11.60
(br. s, 1 H, OH). Ϫ 13C NMR (62.9 MHz, CDCl3): δ ϭ 16.99,
20.10, 26.12, 27.40, 29.43, 134.69, 142.17, 173.14. Ϫ IR (KBr pel-
let): ν˜ ϭ 3200Ϫ2700 cmϪ1 (OϪH), 2965 (ϭCH), 2945, 2918, 2870
(CH), 1677 (CϭO), 1635 (CϭC), 1422, 1274, 1251 (CO), 1068, 939.
Ϫ MS (70 eV); m/z (%): 140 [Mϩ], 125 [M Ϫ CH3], 95 (100) [M
Ϫ CO2H].[44]
Hz, JHF ϭ 9 Hz, 2 H, o-H vs. F), 8.15 (dd, JHH ϭ 9 Hz, JHF
ϭ
5.5 Hz, 2 H, m-H vs. F). Ϫ MS (70 eV); m/z (%): 140 [Mϩ], 123
(100) [M Ϫ 17], 95 [M Ϫ 45].
1-Naphthoic acid, 4-cyanobenzoic acid, 4-trifluoromethylbenzoic
acid, 4-acetylbenzoic acid, para-terephthalic acid, 4-bromobenzoic
acid, 4-chlorobenzoic acid, 4-fluorobenzoic acid, benzoic acid, 4-
methylbenzoic acid, 4-methoxybenzoic acid, 4-tert-butylbenzoic acid,
2-bromobenzoic acid, 2-methylbenzoic acid, and 2-naphthoic acid
were characterized by 1H-NMR (250 MHz) spectroscopy, mass
spectrometry, and by comparison of their analytical data with those
of commercial samples. para-Terephthalic acid monoethyl ester was
characterized by 1H-NMR (250 MHz) spectroscopy and mass spec-
trometry, and by comparison of its analytical data with those re-
ported in the literature.[43]
1,3,5-Tris(4-carboxyphenyl)benzene (6): 0.75 g (1 mmol) of com-
pound 4 was placed in the cathodic compartment, followed by 0.21
g (0.3 mmol) of PdCl2(PPh3)2, and carbon dioxide was bubbled
through the catholyte. The electrolysis was performed at 90°C at a
controlled potential of Ϫ1.6 V vs. SCE and was terminated when
the current dropped to 5% of its initial value. 6.6 F/mol were passed
through the cell. The catholyte was then added to aqueous HCl (1
) and the mixture was left overnight. A white precipitate was
formed, which was filtered off and washed with dichloromethane,
to give 0.328 g (75%) of 6, Ϫ m.p. 235°C. Ϫ 1H NMR (400 MHz,
[D6]DMSO): δ ϭ 8.262 (d, J ϭ 7.5 Hz, 6 H, m-H), 8.291 (d, J ϭ
7.5 Hz, 6 H, o-H), 8.305 (s, 3 H, central H). Ϫ 13C NMR (100.6
MHz, [D6]DMSO): δ ϭ 125.61 (d), 127.45 (d), 129.94 (d), 130.04
(s), 140.76 (s), 143.87 (s), 167.19 (s). Ϫ IR (KBr): ν˜ ϭ 3600Ϫ2400
cmϪ1 (OH), 1680 (CϭO), 1600, 1560, 1420 (CϭC), 1290 (CϪO),
850, 765, 700 (CH). Ϫ MS (70 eV); m/z (%): 439 (100) [M ϩ 1]. Ϫ
C27H18O6 (438.11): calcd. C 73.97, H 4.14; found C 73.75, H 4.38.
The isolated α,β-unsaturated carboxylic acids 2-phenylacrylic
acid (15a),[45] 3,4-dihydronaphthalene-1-carboxylic acid (15b),[46] 4-
tert-butyl-cyclohex-1-enecarboxylic acid (15c),[47] 2-butylacrylic acid
(15d),[48] and 4-phenylcyclohexyl-1-enecarboxylic acid (15f)[38] were
characterized by 1H-NMR (250 MHz) spectroscopy, mass spec-
trometry, and by comparison of their analytical data with those
reported in the literature.
The α-oxocarboxylic acid, 1-oxo-1,2,3,4-tetrahydronaphthalene-2-
carboxylic acid (12b) was characterized by 1H-NMR (250 MHz)
spectroscopy.[49]
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1820
Eur. J. Org. Chem. 1998, 1811Ϫ1821

Activation of Aryl and Vinyl Triflates by Palladium and Electron Transfer
FULL PAPER
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C. Courtot, V. Ouperoff, C. R. Acad. Sci. Fr. 1930,
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NMR spectrum, and heating this pure compound in vacuo did
not lead to compound 4. However, the addition of a catalytic
amount of triflic acid (2%) to pure compound 2 resulted in the
formation of some enol 3, which was detectable in the 1H-NMR
spectrum. Under these conditions, heating the mixture of ke-
tone and enol afforded compound 4 in a quantitative yield
(97%).
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C. Amatore, A. Jutand, F. Khalil, M. F. Nielsen, J. Am. Chem.
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M. Blanchard-Desce, A. Jutand, M. Mladenova, S. Negri, J. B.
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1821