Surface-modified carbon felts: Possible supports for combinatorial chemistry

Article abstract of DOI:10.1021/jo025880+

It is possible to prepare carbon-based analogues of the Merrifield resin by electrochemical reduction of diazonium salts or oxidation of aryl acetates on high specifc surface area carbon felts. These modified felts can undergo further reactions: nucleophilic substitution, Suzuki reaction, and finally reductive electrochemical cleavage, taking advantage of the conductivity of the carbon felt. This provides a simple example of the possible use of electrochemistry in combinatorial synthesis.

Full text of DOI:10.1021/jo025880+

Su r fa ce-Mod ified Ca r bon F elts: P ossible Su p p or ts for
Com bin a tor ia l Ch em istr y
Estelle Coulon and J ean Pinson*
Laboratoire d’Electrochimie Mole´culaire, Universite´ Paris 7-Denis Diderot, 75251 Paris Cedex 05, France
J ean-Dominique Bourzat, Alain Commerc¸on, and J ean-Pierre Pulicani*
Centre de Recherche de Vitry, Aventis Pharma, 13 Quai J ules Guesdes, BP14,
94403 Vitry sur Seine Cedex, France
pinson@paris7.jussieu.fr; jean-pierre.pulicani@aventis.com
Received April 30, 2002
It is possible to prepare carbon-based analogues of the Merrifield resin by electrochemical reduction
of diazonium salts or oxidation of aryl acetates on high specific surface area carbon felts. These
modified felts can undergo further reactions: nucleophilic substitution, Suzuki reaction, and finally
reductive electrochemical cleavage, taking advantage of the conductivity of the carbon felt. This
provides a simple example of the possible use of electrochemistry in combinatorial synthesis.
SCHEME 1
In tr od u ction
We have shown recently that it is possible to prepare
carbon materials with high specific surface area and well-
defined functionalities covalently attached to the surface.1
This was achieved by electrochemical reduction of dia-
zonium salts2 or oxidation of aryl acetates3 using a carbon
felt with a high specific surface area (Actitex 1500-1, 1500
m2 g-1) as the working electrode. Three different surface-
modified felts were prepared; they are shown in Scheme
1. The organic groups present on the surface were
characterized by elemental analysis, by X-ray photoelec-
tron spectroscopy (XPS), by scanning microscopy/energy
dispersive X-ray spectrometry (EDS), and by IR spec-
troscopy. It was also possible to measure the capacity of
these materials, 1.73, 0.88, and 1.83 mmol g-1, and the
surface concentrations (calculated on the basis of 1500
m2 g-1 for the specific surface of the carbon felt), 1.47,
0.70, and 1.71 × 10-10 mol cm-2, for C1, C2, and C3,
respectively. These capacities can be favorably compared
to those of commercial resins for ion exchange or for
combinatorial chemistry; this is due to the very large
specific surface area of the felt. The attachment of the
organic groups to the surface is strong as these groups
cannot be removed by prolonged rinsing in an ultrasonic
cleaner in various organic solvents.1-3
styrene-divinylbenzene resins. Besides, the conductivity
of the carbon makes possible the inclusion of electro-
chemical steps (including the cleavage step) in the
combinatorial synthesis. In this paper we report a simple
sequence to demonstrate the possibility of solid-phase
chemistry on this carbon material including an electro-
chemical cleavage step. Preceding investigations dealing
with combinatorial chemistry including electrochemical
steps have been published by J . Simonet and co-workers;4
they made use of a polythiophene matrix electropolymer-
ized on a platinum electrode. The polythiophene was
substituted in the 3-position by a linker and the reactive
SO2Cl function. Cleavage of the amine previously at-
tached to the SO2Cl group was achieved by electrochemi-
These materials can find a large variety of applications
including ion exchange materials, catalysts, or supports
for combinatorial chemistry. As concerns this last ap-
plication, the carbon support is robust and insensitive
to the solvent and there is no swelling problem as with
(1) Coulon, E.; Pinson, J .; Bourzat, J .-D.; Commerc¸on, A.; Pulicani,
J .-P. Langmuir 2001, 17, 7102.
(2) (a) Delamar, M.; Hitmi, R.; Pinson, J .; Save´ant, J . M. J . Am.
Chem. Soc. 1992, 114, 5883. (b) Allongue, P.; Delamar, M.; Desbat,
B.; Fagebaume, O.; Hitmi, R.; Pinson, J .; Save´ant, J . M. J . Am. Chem.
Soc. 1997, 119, 201.
(3) Andrieux, C. P.; Gonzalez, F.; Save´ant, J . M. J . Am. Chem. Soc.
1997, 119, 4292.
(4) (a) Marchand, G.; Pilard, J . F.; Simonet, J . Tetrahedron Lett.
2000, 41, 883. (b) Marchand, G.; Pilard, J . F.; Fabre, B.; Rault-
Berthelot, J .; Simonet, J . New J . Chem. 1999, 23, 869. (c) Dubey, S.;
Fabre, B.; Marchand, G.; Pilard, J . F.; Simonet, J . J . Electroanal. Chem.
1999, 477, 121. (d) Pilard, J . F.; Marchand, G.; Simonet, J . Tetrahedron
1998, 54, 9401.
10.1021/jo025880+ CCC: $22.00 © 2002 American Chemical Society
Published on Web 11/02/2002
J . Org. Chem. 2002, 67, 8513-8518
8513

Coulon et al.
SCHEME 2
cal cleavage of the S-N bond. An electrochemical setup
specially designed for parallel electrosynthesis (spatially
addressable electrolysis platform) has been devised by
Yudin and co-workers.5 It was used for the anodic
oxidation of carbamates, amides, and sulfonamides,
which led to libraries of R-alkoxycarbamates, R-alkoxy-
amides, and R-alkoxysulfonamides.5 Similarly, the in-
tramolecular cyclization of hydroxamides provided het-
erobicyclic compounds in high yields, and hydrocoupling
of aldimines yielded vicinal diamines. The instrumenta-
tion for parallel electrochemical synthesis has been
described.5a
Resu lts a n d Discu ssion
Among the three carbon materials of Scheme 1 we
choose C1 and C3 for further studies as these materials
are analogues of a Merrifield resin and substitution of
the halogen should be easy, much easier than the
substitution of the bromine of C2. The modified carbon
felts C1 and C3 were prepared1 in the form of disks (22
mm diameter, 2 mm thick) in the following way: the
4-chloro- or bromomethylphenyl group was attached to
the carbon felt by electrochemical reduction of the tet-
rafluoroborate of 4-chloromethylbenzenediazonium or by
electrochemical oxidation of 4-bromophenylacetate tet-
ramethylammonium. This was achieved in a circulating
cell6 where the solution (ACN + 0.1 M NBu4BF4
+
substrate) flows through carbon felt disks packed in the
working compartment. Two counter electrodes are sepa-
rated from the working compartment by ion exchange
membranes. As the solution flows through the carbon felt
disks, the disk close to the inlet will be termed upstream
and that close to the outlet downstream. The potential
of the carbon felt is maintained by a potentiostat (see
the Experimental Section). After completion of the elec-
trolysis, the carbon felt disks were thoroughly rinsed
ultrasonically in different solvents and then dried. Char-
acterization of the modified carbon felts1 was achieved
by elemental analysis, XPS, scanning electron microscopy/
EDS, and IR spectroscopy.
The reaction sequence of Scheme 2 was then at-
tempted. The benzylic chlorine or bromine was first
substituted by an aromatic thiolate to give a sulfide.
Using differently 4-X substituted thiolates, we could
obtain C4 (X ) F), C5 (X ) Cl), C6 (X ) Br), C7 (X )
CF3), and C8 (X ) NO2). Starting from C5 (n )0) and
C6 (n)0), which are substituted by a chlorine or bromine,
respectively, it was then possible to perform a Suzuki
reaction to obtain C9 (Y ) CF3) and C10 (Y ) H).
Electrochemical cleavage of the benzylic carbon-sulfide
bond is then possible due to the conductivity of the
carbon, and 4 was obtained by reduction of C10.
The nucleophilic substitution of C1 was achieved by
placing the disks at reflux either in a round-bottom flask
or in a Quest Argonaut reactor. The results of these
experiments are reported in Table 1.
TABLE 1. Yield s of Nu cleop h ilic Su bstitu tion
Rea ction R4a
starting and
obtained
material
X (amt amt of
of thiol NEt3
(equiv)) (equiv)
yieldc ratio
(%) X/Sa
conditionsb
C1 f C4 (n ) 0) F (15)
C1 f C5 (n ) 0) Cl (15)
C1 f C6 (n ) 0) Br (16)
C1 f C7 (n ) 0) CF3 (15)
C1 f C8 (n ) 0) NO2 (15)
C3 f C4 (n ) 1) F (41)
C3 f C5 (n ) 1) Cl (15)
C3 f C6 (n ) 1) Br (13)
C3 f C7 (n ) 1) CF3 (31)
C3 f C8 (n ) 1) NO2 (15)
20
20
20
20
20
41
36
43
41
20
A, 80 °C, 6 h
44d
84
1
e
1
3
f
1
e
1
3
f
A, 80 °C, 10 h
B, 80 °C, 88 h
A, 80 °C, 4 h
A, 80 °C, 10 h
B, 80 °C, 108 h
B, 80 °C, 96 h
B, 80 °C, 108 h
B, 80 °C, 108 h
A, 80 °C, 10 h
100
13d
84
24d
63
43
43d
30
a
b
Determined from the elemental analysis. A ) Quest Argo-
naut reactor. B ) round-bottom flask with magnetic stirring.
c Determined from the %S obtained by elemental analysis. Tak-
d
ing into account the 2% F observed in the untreated felt. e An
excess of chlorine was observed, probably coming from CHCl3 used
for rinsing and incompletely eliminated. f The felt already contains
about 2% N.
The occurrence of the nucleophilic substitution was
ascertained by elemental analysis through the determi-
nation of sulfur in all experiments and of F (C4 and C7),
Cl (C5), and Br (C6), (see the Experimental Section). The
yields are satisfactory to good, but the surface reactions
are much slower than in solution, as usually observed.
Provided the reaction times are long enough, the yields
do not depend very much on the way in which the
experiments are performed either simply in a flask or in
(5) (a)Yudin, A. K.; Siu, T. Curr. Opin. Chem. Biol. 2001, 5, 269. (b)
Siu, T.; Li W.; Yudin, A. K. J . Comb. Chem. 2000, 2, 545. (c) Siu, T.;
Yekta S.; Yudin, A. K. J . Am. Chem. Soc. 2000, 122, 11787.
(6) (a) Landaez-Machado, H. J .; Darchen, A.; Moinet, C. Electrochim.
Acta 1980, 25, 1321. (b) J acob, G.; Moinet, C. Bull. Soc. Chim. Fr. 1968,
291. (c) Peltier, D.; Moinet, C. Bull. Soc. Chim. Fr. 1968, 2657. (d) Ng,
P. K. J . Electrochem. Soc. 1981, 128, 792. (f) Pollard, R.; Trainham, J .
A. J . Electrochem. Soc. 1983, 130, 1531.
8514 J . Org. Chem., Vol. 67, No. 24, 2002

Surface-Modified Carbon Felts
TABLE 2. F TIR Sp ectr a of Mod ified Ca r bon F elt
SCHEME 3
wavelength
material
(cm-1
)
assignment
C4, n ) 0 1586
ν CdC of aromatic ring
1487, 1227
F-substituted aromatic ring
para-F-substituted aromatic ring
para-disubstituted aromatic ring
para-Cl-disubstituted aromatic ring
Cl-substituted aromatic ring
para-disubstituted aromatic ring
para-Cl-substituted aromatic ring
para-Br-substituted aromatic ring
δ CH of aromatic rings
1154-1012
827
C5, n ) 0 1473, 1009
1092
811
485
C6, n ) 0 1469, 1005
1090, 1066
807
a reactor specially designed for combinatorial chemistry.
The ratios X/S are in good agreement with the formula
of the material, provided that great care is exercised to
achieve complete rinsing of the felt to be certain that no
supporting electrolytes remain adsorbed. Chloroform is
a good rinsing solvent, but further rinsing with ether is
necessary. If further rinsing is not done, some chlorine
may remain attached to the surface as with C5, where
the measured chlorine was higher than expected. Start-
ing from C3 (n )1), the analysis shows that the amount
of bromine which has reacted is much higher than the
yield of substituted material; for example, in the reaction
C3 f C8, 95% of the initial bromine has been consumed
but the yield is only 30%. This is likely due to the
solvolysis of the bromine by triethylamine. Analysis of
the solution after derivatization of the felt by oxidation
of 4-bromomethylphenylacetate tetramethylammonium
(C3 obtained through reaction R3) showed the presence
of cyclic dimers and trimers stemming from the nucleo-
philic reaction of the carboxylate on the benzylic bromine.
As such reactions could also take place on the surface to
give chemically inhomogeneous surfaces as shown in
Scheme 3, we discontinued the use of these processes to
derivatize the felts.
para-disubstituted aromatic ring
para-Br-substituted aromatic ring
para-disusbstituted aromatic ring
with a CF3
477
C7, n ) 0 1395
1322, 1165, 1126 CF3
1061, 1011
823
493
para-CF3-substituted aromatic ring
para-disubstituted aromatic ring
para-CF3-substituted aromatic ring
para-NO2-substituted aromatic ring
NO2
C8, n ) 0 1574
1510, 1534
1182
δ CH of aromatic rings
851, 838
469
para-disubstituted aromatic ring
para-disusbstituted aromatic ring
with a NO2
The substituted felts prepared by reduction of the
diazonium salt and nucleophilic substitution by thiolates
were further characterized by FTIR spectroscopy and by
scanning microscopy/energy dispersive X-ray spectrom-
etry. The FTIR spectrum of the untreated Actitex 1500-1
carbon felt is shown in Figure S1 in the Supporting
Information; it shows three large IR bands around 3440,
1970, 1170 cm-1. Figure S2 in the Supporting Informa-
tion shows the spectrum of C2 after modification of the
same felt by 4-bromophenyl groups. The FTIR spectra
of the different materials are reported in Table 2, and
the spectrum of C6 (n ) 0) is shown in Figure 1. The
spectra of C4-C8 are shown, respectively, in Figures
S3-S6 in the Supporting Information. These spectra
permit the presence of both the substituents and the
aromatic rings to be observed.
The scanning microscopy images permit observation
that the fibers of the modified felts1 are similar to those
of untreated felts, indicating that they are not damaged
during the chemical reaction and that no thick polymeric
layer is formed on the surface. The EDS spectra permit
characterization of the substituents as shown in Table
3. In all these spectra one can observe the presence of
sulfur (albeit in variable amounts from material to
material) as well as that of chlorine as the yields are not
quantitative except in the case of C6; one also observes
the unreacted halogen substituents.
F IGURE 1. FTIR spectrum of C6.
TABLE 3. Sca n n in g Micr oscop y/EDS Sp ectr a a
material
analysis (mass %)
C4
C5
C6
C7
C8
C, 75.0; O, 12.1; Cl, 0.9; S, 0.7; F, 0.4
C, 72.1; O, 13.7; S, 4.1; Cl, 5
C, 75.8; O, 14.5; Cl, nd; S, 4.3; Br, 12.8
C, 76.8; O, 8.7; Cl, 2.5; S, 1.0; F, 0.8
C, 77.0; O, 8.6; Cl, 4.1; S, 8.3
a
Other minor elements are detected: P, Na, Ca, Mg, P.
along a Suzuki reaction (R6, Scheme 2) to give C9 and
C10. The results concerning C9 are reported in Table 4.
The occurrence of the Suzuki reaction was ascertained
in the case of C9 by (i) elemental analysis through the
increase of the amount of fluorine from 0.2% (as in
untreated felts) to 1.89% (starting from C5, n ) 0) and
1.68% (from C6, n ) 0) and (ii) FTIR as shown in Table
5 and Figure S6. In the case of C10 (n ) 0) as there is no
characteristic substituent we used it directly in the next
step.
The above results indicate that the Suzuki reaction is
indeed possible but that the yields are low.
The next step is the reaction of C5 and C6 (n ) 0) with
phenylboronic acid and its 4-fluoromethyl derivative
As the carbon felts are electronic conductors, it should
be possible to cleave electrochemically the molecules
J . Org. Chem, Vol. 67, No. 24, 2002 8515

Coulon et al.
TABLE 4. F or m a tion of C9 fr om C5 a n d C6 th r ou gh a Su zu k i Rea ction
starting and
substituted material
amt of catalyst
Pd(PPh3)4 (equiv)
amt of base
Na2CO3 (equiv)
boronic acid
(amt (equiv))
time
(h)
yield
(%)
C5 f C9 (n ) 0)
C6 f C9 (n ) 0)
1.9
1.1
95
55
trifluoromethyl (76)
trifluoromethyl (44)
96
72
32
23
TABLE 5. F TIR Sp ectr a of C9
wavelength
(cm-1
)
assignment
1475
1385
816
para-disubstituted aromatic ring
CF3
para-disubstituted aromatic ring
F IGURE 3. Cyclic voltamogram (first wave and reversible
thiolate/disulfide system) of 5 (c ) 2 mM) in ACN + 0.1 M
NBu4BF4 on a glassy carbon electrode. Reference SCE. Scan
rate v ) 0.2 V/s.
SCHEME 4
F IGURE 2. Cyclic voltamogram of 5 (c ) 2 mM) in ACN +
0.1 M NBu4BF4 on a glassy carbon electrode. Reference SCE.
Scan rate v ) 0.2 V/s.
attached to the surface. We attempted the cleavage of
C6 (X ) Br), C9 (Y ) CF3), and C10 (Y ) H). The
cleavage of carbon-sulfur bonds has been previously
investigated.7,8 Upon electrochemical reduction, C6H5SR
sulfides (R ) CH3, C2H5, i-Pr, t-Bu) are cleaved at about
-2.7 V/SCE to give C6H5S- and RH.9 To determine the
cleavage potential of C6, we synthesized 4-methylbenzyl
4′-bromophenyl sulfide (5). Its cyclic voltammetry (on
glassy carbon in ACN + 0. 1 M NBu4BF4) presents two
and of Br from 15.25% to 3.49%, from which a yield of
86% could be calculated.
Analysis of the electrolysis solution after extraction
with ethyl acetate indicated the presence of BrC6H4-
SSC6H4Br (6) obtained by oxidation of the thiolate during
the workup. Minor products were also detected, resulting
from the cleavage of the C-Br bond. If the extraction is
performed with CH2Cl2, a 17% yield of BrC6H4SCH2-
SC6H4Br (7) is obtained, resulting from the attack of the
thiolate on dichloromethane and giving additional evi-
dence of the formation of the thiolate and the occurrence
of the cleavage reaction.
irreversible cathodic waves at Ep ) -2.36 V/SCE (which
1
remains irreversible up to 200 V s-1) and Ep ) -2.58
2
V/SCE (Figure 2). The first one is assigned to the cleavage
of the carbon-sulfur bond and the second one to the
cleavage of the C-Br bond (Scheme 4). If the cathodic
potential scan is limited to -2.50 V/SCE, one can observe
a reversible system at E° ) -0.78 V/SCE which corre-
sponds to the couple thiolate/diphenyl disulfide (BrC6H4S-/
[BrC6H4S]2) (Figure 3). The cleavage of the benzylic
carbon-sulfur bond of C6 was then accomplished in a
circulation cell6 in ACN + 0.1 M NBu4BF4 at a potential
of -2.3 V/SCE for 1 h. Analysis of the felt after electroly-
sis indicated a decrease of the %S from 5.65% to 0.77%
To determine the reduction potential of C9 and C10,
we synthesized 4-methylbenzyl 4′-trifluoromethylbiphen-
yl sulfide (8) and 4-methylbenzyl biphenyl sulfide (9). The
cyclic voltammetry of 8 presents first a 2e- irreversible
reduction wave located at Ep ) 2.05 V/SCE correspond-
1
ing to the cleavage of the benzylic carbon-sulfur bond
(7) Lund, H. Cleavage and Deprotection. In Organic Electrochem-
istry; Lund, H., Hammerich, O., Eds.; M. Dekker: New York, 2001; p
969.
(8) (a) Simonet, J . The Chemistry of Sulphur Containing Functional
Groups. In The Chemistry of Functional Groups (supplement); Patai,
S. S., Rappoport, Z., Eds.; J . Wiley: New York, 1993; pp 448-449. (b)
Chambers, J . Q. Organic sulfur Compounds. In Encyclopedia of
Electrochemistry of the Elements; Bard, A. J ., Lund, H., Eds.; M.
Dekker, New York, 1978; Vol. XII, p 329.
(Figure 4). The second 6e- irreversible reduction wave
at Ep ) -2.42 V/SCE corresponds to the stepwise
2
cleavage of the CF bonds (Scheme 5).10 On the basis of
these results, we attempted the cleavage of the C-S bond
of C9 at a potential of -2.0 V/SCE for 6 h. Analysis of
the felt showed that the %S and %F decreased to
negligible values, but analysis of the resulting solution
(9) Gerdil, R. J . Chem. Soc. B 1966, 1071.
8516 J . Org. Chem., Vol. 67, No. 24, 2002

Surface-Modified Carbon Felts
resins. We have now shown that these modified felts
could be used for solid-phase synthesis. Many other
reactions besides those described in this paper could be
used, and they should permit inclusion of electrochemical
steps in the synthesis. For example, as suggested by a
reviewer, C2 could be substituted by suitably chosen
nucleophiles such as thiolates via an SRN1 reaction;
oxidation by H2O2 into sulfones and then electrochemical
cleavage into sulfinates (easily converted to sulfonyl
chlorides) would open the way to easy and very versatile
supported synthesis using a very convenient and cheap
substrate. Among the large number of organic electro-
chemical reactions which have been investigated up to
now, one can find many more examples of electrochemical
cleavage7 reactions which could be useful for combina-
torial synthesis: (i) Carbon-carbon bonds in pinacols,
diketones, and chalcones can be cleaved by reduction or
oxidation. (ii) Carbon-oxygen bonds can be cleaved by
reduction of esters of aromatic acids and alcohols. Acyl
derivatives of phenols can be cleaved to phenolate and
acyl radical. Carbonyls conjugated with aromatic systems
cleave rapidly such as in benzyl benzoate, which cleaves
rapidly to benzoate and benzyl radical, which leads to
toluene. (iii) Besides thioethers described in this paper,
sulfonium ions, sulfoxides, thioesters, and sulfones can
be cleaved upon electrochemical reduction.8 (iv) Carbon-
nitrogen bonds can be cleaved upon electrochemi-
cal reduction in quaternary ammoniums and in benz-
amides,12 which give good yields of benzylamines upon
electrochemical reduction. But the use of electrochemistry
in combinatorial synthesis could be expanded beyond
cleavage steps to modification of attached groups by
reduction or oxidation and to coupling reactions, for
example, the coupling of activated alkenes,13 one of the
alkenes already being attached to the support and the
other one being in solution. One can also think of
oxidative couplings14 such as with aryl ethers or amines
or to the Kolbe reaction. It should therefore be possible
to use already known reactions in a new context to
provide new and hopefully interesting compounds. How-
ever, the failure to prepare the trifluoromethylbiphen-
ylthiol by reduction of C9 indicates that some improve-
ments should be made in the design of the cell to control
as perfectly as possible the potential inside the whole
volume of the felt.
F IGURE 4. Cyclic voltamogram of 8 (c ) 1.27 mM) in ACN
+ 0.1 M NBu4BF4 on a glassy carbon electrode. Reference SCE.
Scan rate v ) 0.2 V/s.
SCHEME 5
showed that it consisted of a complex mixture likely
stemming from the cleavage of the C-F bonds. This is
likely due to an unproper control of the potential inside
the felt (see the Experimental Section, Circulation Cell).
We then investigated the cyclic voltammetry of 9 as a
model of C10. It presents a 2e- irreversible cathodic wave
at Ep ) -2.23 V/SCE, and on the anodic return scan the
oxidation wave of biphenyl thiolate is observed at Ep
)
an
-0.12 V/SCE. 9 was electrolyzed at -2.2 V/SCE, and
biphenylthiol (4, Y ) H) was identified by comparison
with an authentic sample.11 In the same way, C10 was
electrolyzed at -2.2 V/SCE in a circulating cell,6 and the
resulting solution was shown to contain biphenylthiol
identical with the reduction product of 9 and with an
authentic sample. Thiphenylphosphine which had been
retained in the felt despite vigorous rinsing and triethy-
lamine coming from the supporting electrolyte were
detected as impurities.
Exp er im en ta l Section
Ca r bon F elt. All the experiments were made with Actitex
1500-1 with a specific surface area of 1500 m2 g-1. Typical
analysis (undegassed): C, 79.53; H, 0.64; N, 0.98; O, 3.04; S,
ꢀ; I, ꢀ; Cl, ꢀ; Br, ꢀ; F, 0.20 (ꢀ ) not quantifiable). The FTIR
spectrum of the felt shows two large absorption bands at 1560
and 1140 cm-1 which correspond to the vibrations of the carbon
skeleton. The EDS spectrum shows the following: C, 87.0; O,
10.6; Cl, 0.2; P, 1.5; Na: 0.1. Disks with 22 mm diameter were
cut into 2 mm thick sheets of the felt.
Con clu sion
We have shown previously that analogues of Merrifield
resins could be prepared on carbon felts and that the
loading of these felts was equivalent to that of commercial
Cir cu la tion Cell.6 All the derivatizations of the felts as
well as the further electrochemical cleavage reaction were
(10) (a) Coleman, J . P.; Gilde, M. G.; Utley, J . H. P.; Weedon, B. L.
C.; Eberson, L. J . Chem. Soc., Perkin Trans 1973, 2, 1903. (b)
Saboureau, C.; Troupel, M.; Sibille, S.; Pe´richon, J . J . Chem. Soc.,
Chem. Commun. 1989, 1138. (c) Andrieux, C. P.; Combellas, C.;
Kanoufi, F.; Save´ant, J . M.; Thie´bault. A. J . Am. Chem. Soc. 1997,
119, 9527. (d) Clavel, P.; Le´ger-Lambert, M. P.; Biran, C.; Serein-Spiro,
F.; Bordeau, M.; Roques, N.; Marzouk, H. Synthesis 1999, 5, 829. (e)
Clavel, P.; Lessene, G.; Biran, C.; Bordeau, M.; Roques, N.; Trevin, S.;
de Montauzon, D. J . Fluorine Chem. 2001, 107, 301.
(12) Wagenknetch, J . H.; Eberson, L.; Utley, J . H. P. Carboxylic
Acids and Derivatives. In Organic Electrochemistry; Lund, H., Ham-
merich, O., Eds.; M. Dekker: New York, 2001; p 453.
(13) Nielsen, M. T.; Utley, J . P. Reductive Coupling. In Organic
Electrochemistry; Lund, H., Hammerich, O., Eds.; M. Dekker: New
York, 2001; p 795.
(14) Scha¨fer, H. J . Electrolytic Oxidative Coupling. In Organic
Electrochemistry; Lund, H., Hammerich, O., Eds.; M. Dekker: New
York, 2001; p 883.
(11) Testaferri, L.; Tiecco, M.; Tingoli, M.; Chianelli, D.; Montanucci,
M. Synthesis 1983, 751.
J . Org. Chem, Vol. 67, No. 24, 2002 8517

Coulon et al.
performed in a circulating cell analogous to that described by
Moinet.6 The cell is made of Teflon and glass assembled as in
filter press cells. The working electrode is made from eight
carbon felt disks pressed together inside a Teflon cylinder. The
contact to the carbon felt disks is made with graphite paper.
There are two carbon counter electrodes separated from the
working electrode by ion exchange membranes. The reference
electrode is a Ag/AgCl electrode inserted close to the working
electrode through a hole in the Teflon cylinder. The flow of
electrolyte and substrate is pumped by a peristaltic pump
through the felt in the Teflon cylinder, which constitutes the
compartment of the working electrode. A potentiostat main-
tains the potential of one of the counter electrodes (current I1
flows through this electrode) to obtain the desired potential
difference between the working and the reference electrodes;
the current of the second counter electrode (I2) is maintained
by a galvanostat under automatic control by I1 so that I1/I2 )
1.9, where I1 and I2 are the currents flowing to or from the
upstream or downstream counter electrode, respectively.6 At
the end of the electrolysis the disks were rinsed for 5 min in
each of the solvents acetonitrile, chloroform, acetone, and
diethyl ether and dried for 24 h in an oven at 40 °C.
During the electrolysis of C9 it has been shown above that
the C-S bond is cleaved but that C-F bonds are also cleaved
although the peak potential corresponding to this process is
about 0.4 V more negative. It therefore seems that at least
some parts of the felts must reach more negative potentials
than that imposed by the potentiostat. With the complex
structure of such felts including meso- and micropores, it is
likely that the felt is not entirely equipotential and that ohmic
drop should lower the potential, not increase it. Further
studies should be carried out to solve this problem.
4-Meth ylben zyl 4′-Br om op h en yl Su lfid e (5). A 1.04 g
sample of 4-methylbenzyl bromide (5.52 mM, 1 equiv) and 1.07
g of 4-bromobenzenethiol (5.66 mM, 1 equiv) were dissolved
in 60 mL of acetonitrile. A 1 mL sample of triethylamine was
then added (7.17 mM, 1.3 equiv), and the solution was refluxed
under argon. The reaction was followed by CCM (cyclohexane/
ethyl acetate, 6/4). After 1 h of reaction, the thiol had
completely reacted. The solvent was evaporated and the
resulting product extracted with CH2Cl2 in the presence of 1
M HCl. The residue was purified on a silica gel column eluted
with cyclohexane to give a white solid (1.37 g, 4.68 mM, yield
83%). Mp: 90 °C. Anal. Calcd for C14H13BrS: C, 57.33; H, 4.43;
S, 10.92; Br, 27.30. Found: C, 57.48; H, 4.51; 11.06; Br, 27.06.
1H NMR (DMSO-d6): δ (ppm) 2.33 (s, 3H, CH3), 4.07 (s, 2H,
CH2), 7.00-7.37 (m, 8H, aromatics). MS (EI, 70 eV): m/z )
294 (MH+), 187 (SC6H4Br+), 105 (CH3PhCH2+).
4-Meth ylben zyl 4′-Tr iflu or om eth ylbip h en yl Su lfid e
(8). 8 was prepared as 9 below. White solid (yield 62%). Anal.
Calcd for C21H17SF3: C, 70.39; H, 4.74; S, 9.93. Found: C,
70.68; H, 5.06; S, 10.02. 1H NMR (DMSO-d6): δ (ppm) 2.34 (s,
3H, CH3), 4.16 (s, 2H, CH2), 7.1-7.9 (m, 12H, aromatics). MS
(EI, 70 eV): m/z ) 358 (MH+), 253 (+SC6H4C6H4-CF3), 105
(CH3PhCH2+).
4-Meth ylben zyl 4′-Bip h en yl Su lfid e (9). A 0.40 g sample
of 4-methylbenzyl 4′-bromophenyl sulfide (1.37 mM, 1 equiv)
was stirred under argon in the presence of tetrakis(triphen-
ylphosphine)palladium (0.14 mM, 0.1 equiv) in 20 mL of DME
for 5 min at room temperature. Phenylboronic acid (0.34 g,
2.74 mM, 2.0 equiv) and 2.4 mL of a 2 M aqueous solution of
Na2CO3 (4.8 mM, 3.5 equiv) were then added to the orange
solution. The reaction medium was refluxed, and the solution
became brown and then black. The reaction was followed by
CCM (cyclohexane/dichloromethane, 8/2). After 6 h, heating
was discontinued and the solvent evaporated. The brown
residue was purified on a silica gel column (cyclohexane/
dichloromethane, 8/2) to give 0.28 g (0.96 mM, 71% yield) of a
white solid. Mp: 116 °C. Anal. Calcd for C20H18S: C, 82.75;
Cyclic volta m m etr y was performed in ACN + 0.1 M NBu4-
BF4 as supporting electrolyte on 3 mm diameter carbon disk
electrodes previously polished with 1 µm diamond paste and
ultrasonically rinsed in ACN.
Ch a r a cter iza tion of th e F elts.
1
H, 6.20; S, 11.03. Found: C, 82.31; H, 6.27; S, 10.92. H NMR
Elemental analyses where made at CRVA (Aventis, Vitry,
France) with undegassed felts, which explains that the sum
of the elements does not amount to 100%.
Sca n n in g Electr on ic Micr oscop y/EDS. The mass per-
cent of the elements is normalized to 100%.
Mod ifica tion of th e Ca r bon F elts. C1 was prepared by
electrolysis of a 5 × 10-3 M solution of -BF4+N2C6H4CH2Cl at
-0.8 V SCE for 2 h and then rinsed in an ultrasonic cleaner
for 5 min in each of the solvents acetonitrile, chloroform,
acetone, and ether and dried for 24 h in an oven at 40 °C.
Elemental analysis: C, 77.30; H, 2.07; N, 2.05; Cl, 6.13; F,
(DMSO-d6): δ (ppm) 2.26 (s, 3H, CH3), 4.15 (s, 2H, CH2), 7.0-
7.7 (m, 13H, aromatics). MS (EI, 70 eV): m/z ) 290 (MH+),
184 (+SC6H4C6H5 - H), 152 (+C6H4C6H4), 105 (CH3PhCH2+).
Bip h en ylth iol (4)11 was obtained as described. It was
identified with the products obtained by cleavage of either 9
or C10 by TLC, NMR, and MS (m/z ) 186, M+).
C9 and C10 were prepared in the same way as 8 and 9, but
after addition of the boronic acid, reflux was maintained for 4
days. The felts were then rinsed in an ultrasonic cleaner
successively in water, ethanol, acetone, chloroform, and diethyl
ether and dried for 24 h in an oven at 40 °C.
0.44. The XPS spectrum shows the doublet of Cl2p and
3/2
Cl2p1/2: 1.7%. The EDS spectrum shows the following: C, 87.0;
O, 11.3; Cl, 0.5; P,1.1; Ca, 0.1; S, 0.1. FTIR spectrum (cm-1):
2923 (νa CH2 alkyl), 2853 (νs CH2 alkyl), 1503 (ν CdC aromatic
rings), 1016-1084 (δ CH aromatic rings), 808 (para-disubsti-
tuted aromatic rings).
Ack n ow led gm en t. We are highly indebted to Marc
Vuilhorgne, Head of the Analytical Department of the
Centre de Recherche de Vitry-Alfortville (Aventis), for
the numerous analyses performed during this investiga-
tion. We are very grateful to Genevie`ve Lespinasse and
Michel Moreau (Aventis) for the careful interpretation
of the EDS and FTIR spectra, respectively. We thank
Ms. Bounine and Kochanek (Aventis) for performing
elemental analysis. We thank P.-E. Blanc, Sales and
Marketing Director, Actitex, Levallois, France, for the
gift of the carbon felts. We acknowledge the help of
Michel Druet (Laboratoire d’Electrochimie Mole´culaire,
Universite´ Paris 7-Denis Diderot) for designing and
building the electronic equipment. Professor Claude
Moinet is thanked for helpful discussion on circulation
cells.
C3 was prepared by electrolysis of a 1.4 × 10-2 M solution
of +Me4N-BrCH2C6H4 in ACN + 0.1 M NBu4BF4 at +1.2
V/SCE for 2 h and then rinsed and dried as above. Elemental
analysis: C, 76.49; H, 1.00; N, 1.46; O, 6.25; Br, 6.32. The XPS
spectrum shows the doublet of Br3d and Br3d at 71 and 72
5/2
3/2
eV: 0.45%. The EDS spectrum shows the following: C, 82.3;
O 14.1; Cl, 0.5; Br, 1.4; P, 1.0.
E lem en t a l An a lysis. C4: C, 74.26; H, 1.41; N, 2.04; F,
1.18; Cl, 9.06; S, 1.96. C5: C,78.39; H, 1.20; N,1.69; Cl, 9.14;
S, 3.78; O, 4.50. C6: C, 66.44; H, 1.50; N, 0.33; Br, 14.25; S,
5.65. C7: C, 78.81; H, 1.57; N, 1.93; F, 1.10; Cl, 3.36; S, 0.60.
C8: C, 74.57; H, 1.60; N, 3.07; O, 8.15; S, 3.76; Cl, 5.30. C9
(obtained from C5): C, 78.42; H, 1.90; N, 1.06; S, 1.85; F, 1.89;
Cl, 2.96. C9 (obtained from C6): C, 65.70; H, 1.80; N, 1.40; S,
0.18; F; 1.68; Cl, 2.26. C10: C, 78.66; H, 1.63; N, 1.84; S, 1.73.
Th e loa d in gs of th e felts were calculated as in commercial
resins: for example, with -C6H4CH2Cl, the %Cl being x, the
loading is 10x/35.5 mol of Cl/g of modified felt.
Su p p or tin g In for m a tion Ava ila ble: FTIR spectra of Ac-
titex 1501, C2, C4, C5, and C7-C9. This material is available
free of charge via the Internet at http://pubs.acs.org.
J O025880+
8518 J . Org. Chem., Vol. 67, No. 24, 2002