New boronic-acid- and boronate-substitued aromatic compounds as precursors of fluoride-responsive conjugated polymer films

Article abstract of DOI:10.1002/(SICI)1099-0690(200005)2000:9<1703::AID-EJOC1703>3.0.CO;2-S

New electropolymerizable aromatic compounds (i.e. pyrrole, thiophene, aniline) bearing boronic acid and ester substituents have been synthesized and their electrochemical behavior has been investigated. Functionalized polythiophene and polypyrrole films could be anodically generated in acetonitrile, whereas the polyaniline derivative was electroformed in an acidic aqueous solution. The electrochemical responses of some of these materials were changed when fluoride ions were added to the electrolytic solutions. The strongest modifications, caused by binding of fluoride by the immobilized boron, were observed for the polypyrrole derivative in hydroorganic media.

Full text of DOI:10.1002/(SICI)1099-0690(200005)2000:9<1703::AID-EJOC1703>3.0.CO;2-S

FULL PAPER
New Boronic-Acid- and Boronate-Substituted Aromatic Compounds as
Precursors of Fluoride-Responsive Conjugated Polymer Films
Mael Nicolas,[a] Bruno Fabre,*[a] Gilles Marchand,[a] and Jacques Simonet[a]
Keywords: Conjugation / Boron / Sensors / Cyclic voltammetry / Polymers
New electropolymerizable aromatic compounds (i.e. pyrrole,
thiophene, aniline) bearing boronic acid and ester substitu-
troformed in an acidic aqueous solution. The electrochemical
responses of some of these materials were changed when flu-
ents have been synthesized and their electrochemical be- oride ions were added to the electrolytic solutions. The
havior has been investigated. Functionalized polythiophene
strongest modifications, caused by binding of fluoride by the
and polypyrrole films could be anodically generated in ace- immobilized boron, were observed for the polypyrrole deriv-
tonitrile, whereas the polyaniline derivative was elec- ative in hydroorganic media.
Introduction
vided that the binding site is in close proximity to the poly-
mer.
In recent years, cation recognition by functionalized
redox-active polymer-film-coated electrodes has been a sub-
ject of considerable interest. For example, much work has
been devoted to the sensing properties of certain linear pol-
Results and Discussion
yether-
or
crown-ether-functionalized
conjugated
Synthesis of Monomers
polymers.[1Ϫ8] In contrast, redox-active polymer-film-
based selective anion detection has surprisingly been largely
With a view to electrochemically generating versatile con-
ignored, even though an extensive range of receptors have jugated polymer films showing electroactivity in organic
been used in anion transport and sensing.[9,10] Among such and aqueous media, the functionalization of various elec-
receptors, Shinkai and co-workers have demonstrated that tropolymerizable aromatic units, specifically aniline, pyr-
boronic acid and boronate groups are able to strongly bind role, and thiophene, by a boronic acid group or its protected
hard-base anions such as FϪ, resulting in specific orbital form, ‘‘pinacol borane’’ (4,4,5,5-tetramethyl-1,3,2-diox-
changes from sp2- to the more stable sp3-hybridized aborolane), was envisaged. Direct attachment of the func-
boron.[11Ϫ14] In the case of a boron site being covalently tional group to the aromatic ring was considered in the
bound to ferrocene, the boronϪFϪ interaction was found cases of 3-substituted aniline and thiophene. 3-Aminophen-
to perturb the electrochemical signal of the electroactive ylboronic acid (1) and thiophene-3-boronic acid (2) are
moiety in solution. This encouraging result was clearly in- commercially available, while the boronate derivative 3 of
dicative of an anion-recognition phenomenon. However, thiophene could easily be obtained in 90% yield by reaction
the immobilization of these functional groups on electrode of 2 with pinacol in anhydrous diethyl ether (Scheme 1).
surfaces has not hitherto been achieved. In continuation of
The incorporation of an alkyl spacer arm between the
our investigations concerning the development of new sens- aromatic ring and the recognition site was achieved by hy-
ory materials, we describe here the synthesis of various droboration of the appropriate alkenes.[15] Thus, 3-vinyl-[16]
boronic-acid- and boronate-substituted electropolymeriz- (4a) and 3-allylthiophene[17] (4b) were treated with diisopin-
able aromatic compounds with the aim of electrochemically ocampheylborane[18] (Ipc2BH), previously prepared from
generating new anion-sensitive layers. Precursors differing (ϩ)-α-pinene and boraneϪdimethyl sulfide, to give the di-
in the nature and length of the spacer arm between the ethyl boronate derivatives (Scheme 1). Treatment with aque-
monomeric unit and the boron site were chosen, as such ous acid or pinacol yielded 5a and 6a؊b, respectively, in
parameters have strong effects on the electropolymerization 22Ϫ26% overall yield. The same protocol was used for the
process and the electroactivity of the resulting conjugated vinyl derivative of the protected pyrrole 7, which was syn-
polymers, and are also central to fast and sensitive recogni- thesized in four steps from pyrrole.[19,20] Deprotection of
tion. Thus, the electronic and/or conformational changes the functionalized pyrrole was carried out electrochemically
induced by boronϪfluoride binding can be expected to be at Ϫ2.45 V (vs. 10Ϫ1  Agϩ/Ag) using pyrene as a redox
detectable through the modification of a characteristic (e.g. mediator. Compared to the classical chemical deprotection
electrochemical) response of the conjugated polymer, pro- method (i.e. using concentrated aqueous NaOH solution),
this procedure was found to be more efficient.
[a]
`
`
´
Laboratoire de Synthese et Electrosynthese Organiques (Unite
The steric and electronic effects of the substituents on the
electropolymerization efficiency, as well as on the elec-
troactive and recognition properties of the corresponding
´
´
Mixte de Recherche no. 6510 associee au CNRS), Universite de
Rennes 1,
Campus de Beaulieu, 35042 Rennes Cedex, France
Eur. J. Org. Chem. 2000, 1703Ϫ1710
WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2000
1434Ϫ193X/00/0509Ϫ1703 $ 17.50ϩ.50/0
1703

M. Nicolas, B. Fabre, G. Marchand, J. Simonet
FULL PAPER
Scheme 1. Reagents and conditions: (a) Ipc2BH (1.2 equiv.), THF, Ϫ40 °C, then addition of the vinylated compound, room temp., 24 h;
(b) acetaldehyde (10.0 equiv.), 0 °C, 24 h; (c) pinacol (1.0 equiv.), THF, room temp., 24 h; (d) 1  HCl; (e) pyrene (20%), reduction at
Ϫ2.45 V (vs. 10Ϫ1  Agϩ/Ag) in CH3CN ϩ 10Ϫ1  Bu4NClO4
polymers, were also examined by introducing a phenyl ring Anodic Behaviour of Boronic-Acid- and Boronate-
in the spacer arm, in some cases in conjunction with an Substituted Aromatic Compounds ؊ Effect of F؊ on the
ether group. The isomerically pure trans compound 11 was Oxidation of the Aniline Derivative
prepared in 42% yield by means of a Wittig reaction be-
tween p-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benz-
aldehyde and
Cyclic voltammetric characterization of the five-mem-
triphenyl(3-thienylmethyl)phosphonium bered heterocycles in CH3CN ϩ 10Ϫ1  Bu4NClO4 revealed
bromide (10) using lithium diisopropylamide as the base an irreversible oxidation peak Epa in the range 1.40Ϫ1.60
(Scheme 2).[21] As observed previously for thiophenes bear- V (at 100 mV sϪ1) for the thiophene derivatives, whereas 9
ing arylsulfonamide substituents in their 3-positions,[22] was oxidized at a lower potential of 0.82 V (Table 1). Com-
only the trans isomer was detected when a short spacer was pared with its analogues, the easier oxidation of 11 may
used. Catalytic hydrogenation of 11 yielded 12, in which the be rationalized in terms of charge delocalization along the
‘‘pinacol borane’’ moiety is electronically insulated from the alkene. From the data gathered in Table 1, it is apparent
thiophene ring.
that the nature and length of the substituent grafted to the
Compounds 13a؊b, incorporating an ether function, thiophene ring has a significant effect on Epa. Firstly, the
were readily obtained (80Ϫ92% yield) by condensation of slightly less positive oxidation potential exhibited by the
3-thienylalkoxides with p-bromobenzyl bromide (Scheme 3) boronic esters can be ascribed to a decrease in the electron-
according to a similar procedure developed by Garnier and withdrawing character of the boron atom. Moreover, the
co-workers.[23] The Grignard reagents prepared from 13a؊b introduction of an ethyl spacer between the thiophene and
were treated with trimethyl borate to give the dimethyl the boronic unit can be seen to contribute to the decrease
boronate derivatives, which were then converted to the re- in Epa, whereas no such clear changes are observed for a
quired monomers in about 25% yield by treatment with propyl spacer (3 compared to 6b). In fact, the electronic
aqueous acid or pinacol.
effect of the boronic group on the oxidation of the thio-
1704
Eur. J. Org. Chem. 2000, 1703Ϫ1710

Precursors of Fluoride-Responsive Conjugated Polymer Films
FULL PAPER
Scheme 2. Reagents and conditions: (a) N-bromosuccinimide, CCl4, hν; (b) PPh3, DMF; (c) pinacol (1.1 equiv.), Et2O, room temp.,
overnight; (d) lithium diisopropylamide (1.3 equiv.), EtOH, reflux, 2Ϫ3 h; (e) 10% Pd/C, MeOH, H2 (50 atm), overnight
Scheme 3. Reagents and conditions: (a) tBuOK (1.2 equiv.), THF, room temp., 15 min; (b) p-bromobenzyl bromide (1.3 equiv.), THF,
reflux, 4 h; (c) Mg, THF, reflux, 1Ϫ2 h; (d) B(OMe)3, THF, Ϫ78 °C, 1Ϫ2 h, then room temp. overnight; (e) 1  HCl; (f) pinacol (1.1
equiv.), THF, room temp., 12 h
phene ring is seen to be strongly attenuated with a short fact, their oxidation rendered the electrode passive after a
linear chain, whereas the more flexible propyl spacer facilit- few potential scans without the apparent formation of a
ates direct interaction between the boronic group and the deposit. The ClO4Ϫ- or PF6Ϫ-doped films were potentiodyn-
oxidizable ring. The Epa values exhibited by 6a and 12 may amically electroformed with an anodic limit close to the ir-
be explained in terms of the steric hindrance and electron- reversible oxidation peak potential of the monomer, e.g.
withdrawing character of the phenyl group. As has already 0.75 V vs. 10Ϫ1  Agϩ/Ag for 9. For example, the successive
been demonstrated for oxyalkyl-substituted polythioph- cyclic voltammograms recorded for 9 showed the emergence
enes,[24] the steric effect of the phenyl group may be negated of new redox systems attributable to the doping-undoping
by incorporating an ether function in the spacer arm.
It was found that the thiophene and pyrrole derivatives
process of the electroformed polymer (Figure 1).
In contrast to the facile electropolymerization of the five-
could be anodically electropolymerized in thoroughly dried membered heterocycles, 1 was found to be inefficiently elec-
acetonitrile containing Bu4NClO4 or Bu4NPF6 as the sup- tropolymerized in aqueous solution containing 5 ϫ 10Ϫ1

porting electrolyte. The boronate-substituted compounds H2SO4. Nevertheless, the process was strongly promoted
proved to be the most efficiently electropolymerized, prob- when FϪ was added to the electrolyte (Figure 2). No such
ably as a result of the strong adsorption of the boronic acid effect was observed with ClϪ or with unfunctionalized poly-
containing derivatives on the drying agent used in situ (i.e. aniline in the presence of FϪ. This catalytic phenomenon
alumina). Compounds 5a, 11, and 14b could not be electro- can only be explained in terms of an enhanced reactivity of
polymerized, irrespective of the experimental conditions. In 1 stemming from a boronϪFϪ interaction. Thus, the elec-
Eur. J. Org. Chem. 2000, 1703Ϫ1710
1705

M. Nicolas, B. Fabre, G. Marchand, J. Simonet
FULL PAPER
Table 1. Voltammetric data of boronic-acid- and boronate-substi-
tuted aromatic compounds and their corresponding polymers in
CH3CN ϩ 10Ϫ1  Bu4NClO4 (electroformation charge: 65 mC
cmϪ2); potential scan rate: 100 mV sϪ1; reference electrode: 10Ϫ1
 Agϩ/Ag
Compound
Epa [V][a]
E°Ј polymer [V][c]
1
0.44[b]
1.60
1.61
1.47
1.43
1.62
0.82
0.25 and 0.44[b]
0.69
2
3
0.83
5a
6a
6b
9
Ϫ[d]
0.52
0.86
Ϫ0.35
Ϫ0.08[e]
Ϫ[d]
11
12
14b
15a
15b
1.04 and 1.14
1.58
0.77
1.56
Ϫ[d]
1.53
0.65
0.56
1.50
[a]
Anodic peak potential relative to the compound at 2 ϫ 10Ϫ2 M.
[b] vs. SCE in H2O ϩ 5 ϫ 10Ϫ1  H2SO4. Ϫ [c] Average of anodic
Ϫ
and cathodic peak potentials corresponding to the p-doping/un-
doping process. Ϫ [d] Passivation of the electrode occurred. Ϫ [e] vs.
SCE in H2O/CH3CN (1/1 v/v) ϩ 5ϫ10Ϫ1  LiClO4.
Figure 2. Successive cyclic voltammograms of 1 (10Ϫ1 ) (a) in the
absence and (b) in the presence of 10Ϫ1  KF; electrolyte: H2O ϩ
5 ϫ 10Ϫ1  H2SO4; potential scan rate: 100 mV sϪ1
Electrochemical Characteristics of the Electroformed
Polymers: Effect of F؊
Following their electrosynthesis, the substituted polythio-
phene and polypyrrole films were examined in monomer-
free acetonitrile solutions. They exhibited stable reversible
redox systems corresponding to the p-doping/undoping
process (Table 1) and the doping level was deduced from
the cyclic voltammograms to be in the range 0.15Ϫ0.30.
Additionally, below Ϫ2.2 V, a moderately stable n-doped
state was observable for some polythiophenes, such as po-
ly(6a) (Figure 3, a). The electrochemical behaviour of the
polythiophene films proved to be very sensitive to substitu-
ent effects. As shown in Figure 4, the electronic and steric
properties of the substituents exerted similar effects as in
the monomers.
Figure 1. Successive cyclic voltammograms of 9 (10Ϫ2 ) in
CH3CN ϩ 10Ϫ1  Bu4NPF6 (final electropolymerization charge:
70 mC cmϪ2); potential scan rate: 100 mV sϪ1
In contrast to the polythiophene films, the 3-substituted
polypyrrole electroformed from 9 remained electroactive in
trogenerated polymer may be considered as being self- aqueous media. However, its electroactivity was weaker and
doped. The binding of FϪ was confirmed by recording 11B- less reversible than in organic electrolytes, probably owing
NMR spectra (96 MHz) of 1 in solution before and after to the presence of the hydrophobic pinacol moiety. A better
the addition of KF. In H2O, containing 5 ϫ 10Ϫ1  H2SO4, electrochemical response was observed in hydroorganic me-
the 11B signal of 1 was observed at δ ϭ 27.7 vs. BF3 Et2O. dia, such as H2O/CH3CN (Figure 3, b). In this medium, the
Upon addition of KF (from 0 to 3 equiv.), another signal addition of KF led to a decrease of the p-doping/undoping
emerged at δ ϭ 3.4, the magnitude of which increased with system at Ϫ0.11 V vs. SCE, while a new redox peak
the amount of FϪ added. These results are thus perfectly emerged at Ϫ0.37 V, the magnitude of which increased with
consistent with a transformation from sp2- to sp3-hybrid- the FϪ concentration. For a film electropolymerized using
ized boron upon binding of FϪ.
a charge of 70 mC cmϪ2, only the second system was ob-
1706
Eur. J. Org. Chem. 2000, 1703Ϫ1710

Precursors of Fluoride-Responsive Conjugated Polymer Films
FULL PAPER
Figure 4. Epa of the thiophene derivatives as a function of E°Ј cor-
responding to the p-doping/undoping process of their respective
polymers; data were extracted from Table 1
ditions. For the polyaniline derivative, it was found that
films electrosynthesized in the presence of various amounts
of KF showed different electrochemical responses when
cycled in a weakly acidic buffer at pH ϭ 4.0 (Figure 5, a).
These observations suggest that the interaction in solution
between the boron atom and fluoride ion has a modifying
Figure 3. Cyclic voltammograms of (a) poly(6a) in CH3CN ϩ 10Ϫ1 effect on the structural arrangement of the resulting poly-
 Bu4NClO4 and (b) poly(9) in H2O/CH3CN (1:1, v/v) ϩ 5 ϫ 10Ϫ1
mer. Moreover, the response of a poly(1) film electrosyn-
 LiClO4 in the absence and presence of 2 m KF; electroforma-
tion charge of the polymers: 65 mC cmϪ2; potential scan rate: 100
mV sϪ1
thesized in the absence of KF was also modified, albeit to
a lesser extent, upon addition of this salt to a buffered solu-
tion (Figure 5, b). In this case, the binding event apparently
induces a post-polymerization modification of the polymer
structure. However, the electrochemical perturbations
caused by the boronϪfluoride interaction were lower in
magnitude than those observed for the polypyrrole system.
Indeed, the polypyrrole response was strongly shifted when
only 2 m KF was added to the electrolytic solution (Fig-
ure 3, b), whereas with poly(1) the effect of FϪ only became
electrochemically detectable above 5 m. Further work is
necessary to assess the effect of the film thickness on the
sensitivity of these two potential sensor materials.
served at a high FϪ concentration, e.g. 2 m (Figure 3, b).
It must be pointed out that the electrochemical response of
poly(9) was not modified when FϪ was replaced by ClϪ or
BrϪ up to tested concentrations of 20 m. Moreover, the
interaction with FϪ was specific to the boronate-containing
polymers; the voltammetric responses of the unfunc-
tionalized polymer films were found to be unaffected by the
presence of this anion. Thus, all these results are consistent
with a selective electrochemical recognition of FϪ by
poly(9). The negative shift of the redox potential indicates
that the FϪ-bound polymer is more easily oxidizable. This
may be rationalized in terms of stabilization of the FϪ
bound to the boron atom by the positive charges along the
backbone of the oxidized polymer.
Conclusions
Of the various boronic-acid- and boronate-functionalized
The effect of FϪ on the other functionalized polymers conjugated polymer films described here, the polypyrrole
was also investigated. The results obtained for the polythi- derivative poly(9) has been shown to exhibit the most signi-
ophene derivatives in acetonitrile using Bu4NF were not ficant and interesting recognition properties towards FϪ.
wholly satisfactory owing to the well-known instability and Thus, the electrochemical response of this polymer was
hygroscopic nature of the fluoride salt.[25] Generally, reac- strongly modified in the presence of this anion, whereas ClϪ
tions that have been reported to proceed in the presence of or BrϪ had no effect. Such specific changes may be ascribed
Bu4NF have involved hydrated FϪ or bifluoride ions. In to electronic and/or conformational perturbations caused
view of the lack of electroactivity of these materials in aque- by the interaction between the immobilized boron atoms
ous media, we did not test the effect of FϪ under such con- and fluoride ions. UV/Vis spectroelectrochemical experi-
Eur. J. Org. Chem. 2000, 1703Ϫ1710
1707

M. Nicolas, B. Fabre, G. Marchand, J. Simonet
FULL PAPER
possible ways of improving the hydrophilicity of these mat-
erials.
Experimental Section
Synthesis of Monomers
General: All organic reactions were carried out under nitrogen.
Solvents were freshly distilled from Na or P2O5 prior to use. Ϫ 1H-
and 13C-NMR spectra were recorded in CDCl3 solution with a
Bruker AC 300 spectrometer using TMS as an internal reference.
Ϫ Melting points were determined with an Electrothermal melt-
ing point apparatus. Ϫ Silica gel (70Ϫ230 mesh) for column chro-
matography was purchased from Merck. Ϫ Mass spectra were
measured with a Varian MAT 311 spectrometer. Ϫ The boronic-
acid-substituted compounds 1 and 2 are commercially available
(Lancaster; 98%) and were used without further purification.
3: At room temperature, 2 (1.28 g, 10.0 mmol) and anhydrous pina-
col (1.30 g, 11.0 mmol) were mixed in anhydrous diethyl ether
(10 mL) and the reaction mixture was stirred for 24 h. After wash-
ing with water, the ether phase was dried with MgSO4 and concen-
trated to dryness in vacuo to give 3 in 90% yield. White solid; m.p.
1
82 °C. Ϫ H NMR (300 MHz): δ ϭ 1.24 (s, 12 H, CH3), 7.24 (dd,
4
3
4
3J ϭ 4.9 Hz, J ϭ 2.7 Hz, 1 H, CH), 7.32 (dd, J ϭ 4.9 Hz, J ϭ
4
4
1.0 Hz, 1 H, CH), 7.83 (dd, J ϭ 2.7 Hz, J ϭ 1.0 Hz, 1 H, CH).
Ϫ MS: m/z ϭ 210.0864 [Mϩ•]; calcd. 210.0886.
Hydroboration of Alkenes: The functionalized monomers con-
taining an alkyl chain were prepared by hydroboration[15] of 3-vi-
nylthiophene, 3-allylthiophene, and 1-(phenylsulfonyl)-3-vinylpyr-
role. Ipc2BH was used as a regio- and stereoselective hydroboration
agent. It was prepared from (ϩ)-α-pinene and boraneϪdimethyl
sulfide according to a known procedure.[18] 4a and 4b were synthe-
sized in two steps in yields of 70% and 40% from 3-(2-hydroxyethyl)-
thiophene and 3-bromothiophene, respectively, according to previ-
ously described procedures.[16,17] 7 was prepared in 55% overall
yield from pyrrole.[19,20] All these reagents exhibited physical char-
acteristics similar to those reported in the literature. Ϫ For the
synthesis of ‘‘pinacol borane’’-substituted compounds, the typical
procedure was as follows: 5 mmol of the alkene was added dropwise
to a well-stirred white suspension of Ipc2BH (1.70 g, 6.0 mmol) in
anhydrous THF (8 mL) maintained at Ϫ40 °C. Following the addi-
tion, the reaction mixture was allowed to very slowly warm to room
temperature and stirring was continued for 24 h. The mixture was
then cooled to 0 °C, whereupon freshly distilled acetaldehyde
(50.0 mmol) was added. After 24 h, the excess acetaldehyde was
Figure 5. (a) Cyclic voltammograms in an acetate buffer at pH ϭ
4.0 of poly(1) films electrosynthesized in 5 ϫ 10Ϫ1  H2SO4 con-
taining (i) 1 at 10Ϫ1 , (ii) KF at 2 ϫ 10Ϫ1 , and (iii) KF at
4 ϫ 10Ϫ1 ; (b) effect of the FϪ concentration in an acetate buffer
at pH ϭ 4.0 on the electrochemical response of a poly(1) film elec-
trosynthesized in the absence of this anion; KF at (i) 0 , (ii)
5 ϫ 10Ϫ3 , (iii) 10Ϫ2 , and (iv) 2 ϫ 10Ϫ2 ; potential scan rate:
20 mV sϪ1; electroformation charge of the polymers: 125 mC cmϪ2
removed in vacuo and
a solution of recrystallized pinacol
(5.0 mmol) in anhydrous THF (5 mL) was added. The resulting
mixture was stirred for 24 h and then the THF was evaporated in
vacuo. The residue was transferred to a Büchi kugelrohr apparatus
in order to distil off (ϩ)-α-pinene and pinacol, and then the ‘‘pina-
col borane’’-substituted crude product was purified by column
chromatography (silica gel).
ments are currently in progress aimed at providing further
information on the effect of FϪ. Several parameters, such
as the nature and length of the spacer arm between the
boron atom and the monomer, need to be optimized in or-
der to obtain a sensory material capable of operating over a
wide FϪ concentration range. Moreover, considering some
potential technological applications, an enhancement of the
electroactive properties of poly(9), as well as those of the
polythiophene derivatives, in aqueous media would be re-
quired. The introduction of oligooxyethylene segments in
the spacer or the electrochemical copolymerization of the
6a: Eluent: CH2Cl2/petroleum ether, 1:1. Colorless oil. Yield: 26%.
Ϫ
1H NMR (300 MHz): δ ϭ 1.06 (t, 3J ϭ 7.9 Hz, 2 H, CH2B),
1.11 (s, 12 H, CH3), 2.67 (t, 3J ϭ 7.9 Hz, 2 H, Th-CH2), 6.83Ϫ6.86
3
4
(m, 2 H, Th-H), 7.10 (dd, J ϭ 4.9 Hz, J ϭ 3.0 Hz, 1 H, Th-H).
Ϫ
144.8. Ϫ 11B NMR (96 MHz): δ (vs. external BF3 Et2O) ϭ 33.8. Ϫ
MS: m/z ϭ 223.0969 [M Ϫ CH3]ϩ; calcd. 223.0964.
13C NMR (75 MHz): δ ϭ 24.6, 24.8, 83.1, 119.4, 125.0, 128.2,
6b: Eluent: Diethyl ether/petroleum ether, 1:1. Colorless oil. Yield:
functionalized pyrrole with unsubstituted pyrrole might be 22%. Ϫ 1H NMR (300 MHz): δ ϭ 1.17 (t, 3J ϭ 7.6 Hz, 2 H,
1708
Eur. J. Org. Chem. 2000, 1703Ϫ1710

Precursors of Fluoride-Responsive Conjugated Polymer Films
FULL PAPER
CH2B), 1.25 (s, 12 H, CH3), 1.57 (qt, 3J ϭ 7.6 Hz, 2 H, CH2), 2.62 latter was used directly for the next step. Ϫ In a 25-mL three-
(t, 3J ϭ 7.6 Hz, 2 H, Th-CH2), 6.84Ϫ6.92 (m, 2 H, Th-H),
necked flask, 10 (0.49 g, 1.1 mmol) and p-(4,4,5,5-tetramethyl-
7.18Ϫ7.22 (m, 1 H, Th-H). Ϫ 13C NMR (75 MHz): δ ϭ 24.8, 25.4, 1,3,2-dioxaborolan-2-yl)benzaldehyde (0.26 g, 1.1 mmol) were dis-
32.8, 82.9, 120.0, 124.9, 128.4, 143.0.
solved in 10 mL of freshly distilled absolute EtOH. Lithium diisop-
ropylamide (0.16 g, 1.5 mmol) was added dropwise at room temper-
ature and the mixture was then refluxed for 2Ϫ3 h. After cooling
to room temperature, the solution was poured into water (10 mL)
and extracted with diethyl ether. The organic phase was separated,
dried with MgSO4, and concentrated to dryness. The crude product
was chromatographed on silica gel (eluent: diethyl ether/petroleum
ether, 1:10) to give 0.15 g (0.5 mmol) of 11 as a yellow powder.
Yield: 42%. Ϫ 1H NMR (300 MHz): δ ϭ 1.27 (s, 12 H, CH3), 6.88
8: Eluent: CH2Cl2. Colorless oil. Yield: 23%. Ϫ 1H NMR (300
3
MHz): δ ϭ 1.01 (t, J ϭ 7.8 Hz, 2 H, CH2B), 1.15 (s, 12 H, CH3),
2.50 (t, 3J ϭ 7.8 Hz, 2 H, Py-CH2), 6.16 (dd, 3J ϭ 3.3 Hz, 4J ϭ
4
4
1.5 Hz, 1 H, Py-H), 6.89 (dd, J ϭ 2.2 Hz, J ϭ 1.5 Hz, 1 H, Py-
3
H), 7.05 (dd, J ϭ 3.3 Hz, 4J ϭ 2.2 Hz, 1 H, Py-H), 7.43Ϫ7.55 (m,
3 H, Ph-H), 7.79Ϫ7.82 (m, 2 H, Ph-H). Ϫ 13C NMR (75 MHz):
δ ϭ 21.1, 24.8, 83.1, 114.8, 116.9, 120.8, 126.7, 129.3, 132.0, 133.6,
139.3. Ϫ MS: m/z ϭ 361.1534 [Mϩ•]; calcd. 361.1519.
3
and 7.12 (AB system, J ϭ 16.3 Hz, 2 H, vinyl-H), 7.17Ϫ7.28 (m,
3 H, Th-H), 7.40 and 7.71 (AAЈBBЈ system, 3J ϭ 8.1 Hz, 4 H, Ph-
H). Ϫ Reduction of the alkene was performed as described below.
A well-stirred and deaerated solution of 11 (0.2 g, 0.6 mmol) in
anhydrous MeOH (20 mL) was hydrogenated overnight under a hy-
drogen pressure of 50 atm in the presence of 10% Pd/C (0.1 mmol).
The resulting mixture was then filtered through Celite, which was
carefully washed with MeOH. The combined filtrate and washings
were concentrated to dryness and the residue was purified by col-
umn chromatography (eluent: diethyl ether/petroleum ether, 1:1) to
give 0.17 g (0.5 mmol) of 12 as a yellowish solid. Yield: 83%; m.p.
Compound 8 was electrochemically deprotected using pyrene as a
redox mediator. At 2 ϫ 10Ϫ2  in 10Ϫ1  Bu4NClO4/CH3CN, 8
showed an irreversible cathodic peak at Ϫ2.87 V vs. 10Ϫ1  Agϩ/
Ag (100 mV sϪ1), which was assigned to cleavage of the SϪN bond.
In order to avoid the formation of by-products at this highly nega-
tive potential, pyrene at 4 ϫ 10Ϫ3  was added to the electrolytic
medium and the cleavage peak potential was lowered to Ϫ2.50 V.
Ϫ The preparative-scale cathodic reduction of 8 was performed
at Ϫ2.45 V in 10Ϫ1  Bu4NClO4/CH3CN, thoroughly dried with
alumina. The progress of the reaction was followed by thin-layer
chromatography (eluent: CH2Cl2) and the quantity of electricity
passed. After the passage of 2 F per mol, the starting product had
been entirely consumed and the electrolytic solution was then fil-
tered in order to remove alumina. The solvent was evaporated and
the residue was taken up in CH2Cl2 and chromatographed on silica
gel (eluent: CH2Cl2/diethyl ether, 1:1) to remove the electrolyte. A
second column chromatography (eluent: CH2Cl2) was required to
separate 9 from pyrene.
1
77Ϫ78 °C. Ϫ H NMR (300 MHz): δ ϭ 1.32 (s, 12 H, CH3), 2.92
4
(m, 4 H, CH2), 6.87 (dd, J ϭ 3.0 Hz, 4J ϭ 1.2 Hz, 1 H, CH), 6.90
3
(dd, J ϭ 5.0 Hz, 4J ϭ 1.2 Hz, 1 H, CH), 7.20 (dd, 3J ϭ 5.0 Hz,
3J ϭ 3.0 Hz, 1 H, CH), 7.18 and 7.73 (AAЈBBЈ system, 3J ϭ 8.0 Hz,
4 H, Ph-H). Ϫ 13C NMR (75 MHz): δ ϭ 24.9, 32.0, 37.2, 83.7,
120.5, 125.3, 128.0, 128.3, 135.0, 142.0, 145.1. Ϫ 11B NMR
(96 MHz): δ (vs. external BF3 Et2O) ϭ 30.9.
Thiophene Derivatives Containing Both a Phenyl and an Ether
Group: In a first step, 25.0 mmol of 3-thienylmethanol (Lancaster;
97%) or 3-thienylethanol (Fluka; 98%) was added to a solution of
tBuOK (3.37 g, 30.0 mmol) in THF (50 mL). After stirring for ca.
1
9: Brownish oil. Yield: 58%. Ϫ H NMR (300 MHz): δ ϭ 1.04 (t,
3J ϭ 8.1 Hz, 2 H, CH2B), 1.15 (s, 12 H, CH3), 2.55 (t, 3J ϭ 8.1 Hz,
2 H, Py-CH2), 6.00Ϫ6.04 (m, 1 H, Py-H), 6.47Ϫ6.50 (m, 1 H, Py-
H), 6.57Ϫ6.61 (m, 1 H, Py-H), 8.02 (s, 1 H, NH). Ϫ 13C NMR (75
MHz): δ ϭ 21.6, 25.2, 83.4, 108.7, 114.9, 117.9, 126.7. Ϫ MS:
m/z ϭ 221.1584 [Mϩ•]; calcd. 221.1587.
15 min,
a solution of p-bromobenzyl bromide (Acros; 98%)
(32.5 mmol, 8.13 g) in THF (40 mL) was added and the reaction
mixture was refluxed for 4 h. It was then cooled to room temper-
ature and treated with diethyl ether/1  aq. HCl. The organic phase
was separated, washed with water, dried with MgSO4, and concen-
trated to dryness under reduced pressure. The crude product was
purified by column chromatography (eluent: diethyl ether/petro-
leum ether, 1:1) to afford ca. 6.0 g (20 mmol) of 13 in good yield.
The boronic-acid-substituted thiophene 5a was prepared in a sim-
ilar manner as 6 and 8, but the mixture was treated with aqueous
acid instead of pinacol, as described below. After removal of the
excess acetaldehyde, the mixture was diluted with pentane (5 mL)
and then treated with 1  aqueous HCl. The organic layer was
separated and the aqueous phase was extracted with diethyl ether.
The combined organic phases were then extracted with 3  aqueous
NaOH solution. The alkaline solution was acidified to pH ϭ 1
with 1  HCl, which resulted in precipitation of the boronic acid
derivative. The collected yellowish solid could be recrystallized
from water but its purification proved rather intricate. Yield: 25%.
Ϫ 1H NMR (300 MHz): δ ϭ 1.06 (t, 3J ϭ 7.9 Hz, 2 H, CH2B), 2.67
(t, 3J ϭ 7.9 Hz, 2 H, Th-CH2), 5.15 [s, 2 H, B(OH)2], 6.83Ϫ6.86 (m,
2 H, Th-H), 7.10 (dd, 3J ϭ 4.9 Hz, 4J ϭ 3.0 Hz, 1 H, Th-H). Ϫ
13a: Yellowish liquid. Yield: 92%. Ϫ 1H NMR (300 MHz): δ ϭ
4.40 (s, 2 H, CH2O), 4.48 (s, 2 H, OCH2Ph), 6.92Ϫ7.25 (m, 3 H,
3
Th-H), 7.13 and 7.40 (AAЈBBЈ system, J ϭ 8.4 Hz, 4 H, Ph-H).
13b: Colorless liquid. Yield: 78%. Ϫ 1H NMR (300 MHz): δ ϭ
3
2.95 (t, 3J ϭ 6.8 Hz, 2 H, CH2), 3.68 (t, J ϭ 6.8 Hz, 2 H, CH2O),
4.47 (s, 2 H, OCH2Ph), 6.95Ϫ7.02 (m, 2 H, Th-H), 7.17 and 7.45
(AAЈBBЈ system, 3J ϭ 8.4 Hz, 4 H, Ph-H), 7.23Ϫ7.27 (m, 1 H, Th-
H). Ϫ 13C NMR (75 MHz): δ ϭ 30.6, 70.5, 72.0, 121.1, 125.2,
128.4, 129.1, 131.3, 137.3, 139.0.
FT-IR (KBr): νOH ϭ 3250 cmϪ1
.
‘‘Pinacol Borane’’-Substituted Thiophenes Containing One Phenyl
The boronic acid and ester derivatives were synthesized using the
Group: The synthesis of the phosphonium salt 10 from 3-methylthi-
Grignard reagent of 13 and trimethyl borate, followed by treatment
ophene has been described previously by Greenwald et al.[21] p- with aqueous acid or pinacol, respectively. Thus, a solution of 13
(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)benzaldehyde could
easily be prepared from a stirred solution of p-formylbenzene-
boronic acid (Lancaster; 97%) and pinacol (1.1 equiv.) in anhydrous
diethyl ether. The mixture was allowed to react at room temper-
ature overnight. After washing with water, the organic phase was
separated, dried with MgSO4, and concentrated under reduced
pressure to give the expected product in quantitative yield. The
(10.0 mmol) in anhydrous THF (10 mL) was slowly added to a
well-stirred suspension of magnesium (11.0 mmol) in THF
(1Ϫ2 mL). The mixture was heated under reflux for 1Ϫ2 h with
activation by a few drops of dibromoethane until the Grignard re-
agent had been formed. The Grignard reagent was then carefully
and slowly poured into a solution of trimethyl borate (2.0 equiv.,
Fluka; 99%) in THF (20 mL) maintained at Ϫ78 °C by means of
Eur. J. Org. Chem. 2000, 1703Ϫ1710
1709

M. Nicolas, B. Fabre, G. Marchand, J. Simonet
an acetone/liquid air bath. After 1Ϫ2 h at this temperature, the model 173 potentiostat and monitored with an EGG PAR model
FULL PAPER
reaction mixture was allowed to slowly warm to room temperature
and stirring was continued overnight. Then, a solution of pinacol
(1.1 equiv., 1.30 g, 11.0 mmol) in anhydrous THF (11 mL) was ad-
ded and the reaction was allowed to proceed for 12 h. Thereafter,
diethyl ether (50 mL) was added and the resulting mixture was
washed with water. The organic phase was dried with MgSO4 and
concentrated to dryness under reduced pressure. The crude product
was purified by column chromatography (eluent: diethyl ether/pet-
roleum ether, 1:1) to give 15 in 25% yield.
175 signal generator and an EGG PAR model 179 digital coulo-
meter. Cyclic voltammograms were plotted with an X-Y Sefram
type TGM 164 recorder. Ϫ The preparative-scale electrochemical
reduction of 8 was performed using a Tacussel potentiostat/gal-
vanostat type PJT 120Ϫ1 connected to a U-shaped two-compart-
ment cell (total volume 100 mL). The working and counter elec-
trodes were made of platinum and glassy carbon plates (area
10 cm2), respectively.
1
Acknowledgments
Drs. M. Vaultier and D. Carrie are gratefully acknowledged for
their help in synthesizing the monomers. The Region de Bretagne
is also acknowledged for a research grant to one of the authors
(M. N.).
15a: Yellow oil. Ϫ H NMR (300 MHz): δ ϭ 1.25 (s, 12 H, CH3),
4.44 (s, 2 H, CH2O), 4.47 (s, 2 H, OCH2Ph), 6.95 (dd, 3J ϭ 5.0 Hz,
´
4J ϭ 1.3 Hz, 1 H, CH), 7.07 (dd, J ϭ 3.0 Hz, J ϭ 1.3 Hz, 1 H,
4
4
´
3
3
CH), 7.15 (dd, J ϭ 5.0 Hz, J ϭ 3.0 Hz, 1 H, CH), 7.27 and 7.72
(AAЈBBЈ system, 3J ϭ 8.1 Hz, 4 H, Ph-H). Ϫ 13C NMR (75 MHz):
δ ϭ 24.9, 67.3, 71.9, 83.8, 122.9, 126.0, 127.0, 127.4, 135.0, 139.3,
141.4. Ϫ 11B NMR (96 MHz): δ (vs. external BF3 Et2O) ϭ 30.8. Ϫ
MS: m/z ϭ 315.1223 [M Ϫ CH3]ϩ; calcd. 315.1226.
[1]
B. Fabre, J. Simonet, Coord. Chem. Rev. 1998, 178Ϫ180,
1211Ϫ1250.
[2]
B. Fabre, J. Simonet, Curr. Top. Electrochem. 1998, 6, 175Ϫ187.
15b: Colorless oil. Ϫ 1H NMR (300 MHz): δ ϭ 1.30 (s, 12 H, CH3),
2.89 (t, J ϭ 6.8 Hz, 2 H, CH2), 3.62 (t, J ϭ 6.8 Hz, 2 H, CH2O),
[3]
T. M. Swager, Acc. Chem. Res. 1998, 31, 201Ϫ207.
3
3
[4]
P. Bäuerle, S. Scheib, Adv. Mater. 1993, 5, 848Ϫ853.
[5]
3
4
H. Korri Youssoufi, M. Hmyene, A. Yassar, F. Garnier, J. Elec-
4.49 (s, 2 H, OCH2Ph), 6.98 (dd, J ϭ 5.0 Hz, J ϭ 1.3 Hz, 1 H,
CH), 7.02 (dd, 4J ϭ 2.9 Hz, 4J ϭ 1.3 Hz, 1 H, CH), 7.25 (dd, J ϭ
3
troanal. Chem. 1996, 406, 187Ϫ194.
[6]
A. Ion, I. Ion, A. Popescu, M. Ungureanu, J. C. Moutet, E.
5.0 Hz, 3J ϭ 2.9 Hz, 1 H, CH), 7.28 and 7.80 (AAЈBBЈ system,
3J ϭ 7.6 Hz, 4 H, Ph-H). Ϫ 13C NMR (75 MHz): δ ϭ 25.4, 31.3,
71.0, 73.3, 84.2, 121.7, 125.7, 127.0, 129.0, 135.5, 139.7, 142.2. Ϫ
11B NMR (96 MHz): δ (vs. external BF3 Et2O) ϭ 30.6. Ϫ MS:
m/z ϭ 344.1622 [Mϩ•]; calcd. 344.1617.
Saint-Aman, Adv. Mater. 1997, 9, 711Ϫ713.
[7]
´
L. M. Goldenberg, I. Levesque, M. Leclerc, M. C. Petty, J.
Electroanal. Chem. 1998, 447, 1Ϫ3.
[8]
F. Sannicolo, E. Brenna, T. Benincori, G. Zotti, S. Zecchin, G.
Schiavon, T. Pilati, Chem. Mater. 1998, 10, 2167Ϫ2176.
P. D. Beer, Acc. Chem. Res. 1998, 31, 71Ϫ80.
M. M. G. Antonisse, D. N. Reinhoudt, Chem. Commun.
1998, 443Ϫ448.
[9]
[10]
The boronic acid 14b was prepared in a similar manner as 5a, but
using 1  HCl solution in place of pinacol. As for 5a, the purifica-
tion of this product was rather intricate. It was obtained as a yel-
lowish solid. Yield: 25%. Ϫ 1H NMR (300 MHz): δ ϭ 2.89 (t, 3J ϭ
[11]
[12]
[13]
[14]
T. D. James, K. R. A. Samankumara Sandanayake, S. Shinkai,
Angew. Chem. Int. Ed. Engl. 1996, 35, 1910Ϫ1922.
C. R. Cooper, N. Spencer, T. D. James, Chem. Commun.
1998, 1365Ϫ1366.
H. Yamamoto, A. Ori, K. Ueda, C. Dusemund, S. Shinkai,
Chem. Commun. 1996, 407Ϫ408.
C. Dusemund, K. R. A. Samankumara Sandanayake, S. Shin-
kai, J. Chem. Soc., Chem. Commun. 1995, 333Ϫ334.
N. Miyaura, A. Suzuki, Chem. Rev. 1995, 95, 2457Ϫ2483.
M. Nicolas, B. Fabre, J.-F. Pilard, J. Simonet, J. Heterocycl.
Chem. 1999, 36, 1105Ϫ1106.
X. Wu, T. A. Chen, L. Zu, R. D. Rieke, Tetrahedron Lett. 1994,
35, 3673Ϫ3674.
I. Paterson, J. M. Goodman, M. A. Lister, R. C. Schumann,
C. K. McClure, R. D. Norcross, Tetrahedron 1990, 46,
4663Ϫ4684.
3
6.8 Hz, 2 H, CH2), 3.62 (t, J ϭ 6.8 Hz, 2 H, CH2O), 4.49 (s, 2 H,
OCH2Ph), 4.51 [s, 2 H, B(OH)2], 6.98 (dd, 3J ϭ 5.0 Hz, 4J ϭ
4
4
1.3 Hz, 1 H, CH), 7.02 (dd, J ϭ 2.9 Hz, J ϭ 1.3 Hz, 1 H, CH),
7.25 (dd, 3J ϭ 5.0 Hz, 3J ϭ 2.9 Hz, 1 H, CH), 7.28 and 7.80
(AAЈBBЈ system, 3J ϭ 7.6 Hz, 4 H, Ph-H). Ϫ FT-IR (KBr):
[15]
[16]
νOH ϭ 3200 cmϪ1
.
[17]
[18]
Electrochemical Experiments and Instrumentation: Tetra-n-butylam-
monium tetrafluoroborate (Bu4NBF4) (Fluka) was recrystallized
three times from methanol/water (1:1, v/v) and then dried for 48 h
in vacuo. Tetra-n-butylammonium perchlorate (Bu4NClO4) and
tetra-n-butylammonium hexafluorophosphate (Bu4NPF6) (Fluka)
were used as received (puriss, electrochemical grade). All salts were
stored in a vacuum desiccator over silica gel. Acetonitrile (Merck;
max. 50 ppm water) was used without further purification and was
stored under dry argon. All electrolytic solutions were dried in situ
with neutral alumina (Merck), previously activated at 450 °C in
vacuo for several hours. The solutions were thoroughly deaerated
and kept under a positive pressure of dry argon throughout each
run. Ϫ Cyclic voltammetry measurements were performed in a clas-
sical three-electrode cell. The working electrode was a platinum disc
(area: 8 ϫ 10Ϫ1 mm2) and the counter electrode was a glassy car-
bon rod. All potentials were referenced to the system 10Ϫ1  Ag-
NO3|Ag in acetonitrile. The cell was connected to an EGG PAR
[19]
[20]
[21]
[22]
[23]
[24]
[25]
M. Kakushima, P. Hamel, R. Frenette, J. Rokach, J. Org.
Chem. 1983, 48, 3214Ϫ3219.
R. Settambolo, R. Lazzaroni, T. Messeri, M. Mazzetti, P. Sal-
vadori, J. Org. Chem. 1993, 58, 7899Ϫ7902.
Y. Greenwald, G. Cohen, J. Poplawski, E. Ehrenfreund, S.
Speiser, D. Davidov, J. Am. Chem. Soc. 1996, 118, 2980Ϫ2984.
G. Marchand, J.-F. Pilard, B. Fabre, J. Rault-Berthelot, J. Si-
monet, New J. Chem. 1999, 23, 869Ϫ875.
M. Lemaire, R. Garreau, D. Delabouglise, J. Roncali, H. K.
Youssoufi, F. Garnier, New J. Chem. 1990, 14, 359Ϫ364.
M. Lemaire, R. Garreau, J. Roncali, D. Delabouglise, H. K.
Youssoufi, F. Garnier, New J. Chem. 1989, 13, 863Ϫ871.
R. K. Sharma, J. L. Fry, J. Org. Chem. 1983, 48, 2112Ϫ2114.
Received October 22, 1999
[O99601]
1710
Eur. J. Org. Chem. 2000, 1703Ϫ1710