Chiral auxiliaries onto conducting polymers

Article abstract of DOI:10.1016/S0040-4039(00)02144-4

The synthesis of new chiral auxiliaries was performed onto conducting polythiophene. The electrochemical behavior of such a matrix was investigated and one of them present a noticeable stability when an adequate spacer is introduced between the redox centre and the chiral unit.

Full text of DOI:10.1016/S0040-4039(00)02144-4

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 867–869
Chiral auxiliaries onto conducting polymers
Pascal Pellon,a,* Elisabeth Deltela and Jean-Franc¸ois Pilardb
aLaboratoire de Physicochimie Structurale 2, UMR 6510, Beaulieu, Universite´ de Rennes 1, 35042 Rennes, France
bLaboratoire d’Electrochimie Mole´culaire et Macromole´culaire, UMR 6510, Beaulieu, Universite´ de Rennes 1,
35042 Rennes, France
Received 19 October 2000; accepted 21 November 2000
Abstract—The synthesis of new chiral auxiliaries was performed onto conducting polythiophene. The electrochemical behaviour
of such a matrix was investigated and one of them present a noticeable stability when an adequate spacer is introduced between
the redox centre and the chiral unit. © 2001 Elsevier Science Ltd. All rights reserved.
Apart from the large number of asymmetric reactions
involving soluble diphosphine–metal complexes,1 the
use of supported catalysts in order to get stereoselective
synthesis at a solid interface is a highly developed, yet
still evolving, topic.2 Despite the tremendous activity
devoted to this route and in connection with previous
studies based on optically-active acetals,3 we focused
our efforts on the development of chiral auxiliaries
onto conducting polymers. This would allow the design
of a large variety of modified electrodes as well as their
potential use in catalytic reactions involving electrogen-
erated species. To this purpose, different thiophene
monomers 1 and 2 linked to a chiral acetal ring were
synthesised (Fig. 1). Their further electropolymerisation
at a solid anode readily led to the desired polymer
without any expensive and time-consuming polystyrene
functionalisation.
Compounds 1 and 2 were obtained according to the
following sequence (Scheme 1). The first step consisted
of a standard acetal formation starting from aldehydes
3a,b and diol 4. The last step leading to the di-mesylates
2a,b was performed by an ethyl ester reduction fol-
lowed by mesylation of the subsequent diol.4
The electrochemical behaviour of monomers 1a,b and
2a,b was studied as well as their anodic polymerisation.
The cyclic voltammetry behaviour of both monomers
and polymers was investigated within the range 0 to +2
V in 0.1 M Bu4NBF4+CH2Cl2. All electrochemical data
are summarised in Table 1. At 100 mV s−1, 1a exhibited
a single irreversible oxidation peak at Epa=1.77 V,
while 1b presented a less anodic value (Table 1, entry
2). A more noticeable difference present between 2a
Table 1. Electrochemical data for compounds 1 and 2
Epa (V)
Ef (V)
1a
1b
2a
2b
1.77
1.70
1.78
1.57
1.475
1.40
1.60
1.50
Figure 1.
Scheme 1. (i) H+/C6H6/D; (ii) LiAlH4/Et2O; (iii) MsCl/Et3N/CH2Cl2.
* Corresponding author.
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)02144-4

868
P. Pellon et al. / Tetrahedron Letters 42 (2001) 867–869
Figure 2.
Scheme 2.
and 2b indicating the direct influence of the length of the
intermediate spacer, which links the acetal ring and the
thiophene unit. This observation can be explained by the
inductive effect of the two oxygen atoms that, when
directly attached to the electroactive thiophene, decrease
its electronic density and render the electronic transfer
more difficult.5
As measured by chronoamperometry, different experi-
ments conducted at higher potentials led to polymer
destruction by over-oxidation. The high electrochemical
stability of poly-(2b) as well as its easy access allowed us
to investigate its properties as a solid-phase asymmetric
reagent. For this purpose, the derivatisation of the
mesylate groups in phosphine boranes was performed
first on the monomer. As demonstrated using monomer
5 obtained from 2b according to known procedure,7 a
model hydrogenation8 of alkene 6 led to the desired
reduced S product in quantitative yield with an enan-
tiomeric excess of 68% (Scheme 2). The efficiency of such
a catalyst appeared identical to that of the DIOP
complex. First attempts at heterogeneous catalysis on
poly-(5) modified electrodes gave unsatisfactory results
(ee close to 0). On the contrary, the use of a co-polymer
between thiophene and 5 led to promising results (57%
ee and 20% hydrogenation).
The progressive electrochemical growth (cyclic voltam-
metry)6 of poly-(2) was performed in CH2Cl2+0.1 M
Bu4NBF4 as inert electrolyte since no polymerisation
was found to occur using CH3CN. Fig. 2 shows a typical
response of a conducting phase covering the working
electrode. As an example, a resin corresponding to
poly-(2b) was obtained after six recurrent potential
cycles (Fig. 2b). Contrary to poly-(2a) (Fig. 2a), it
exhibited a noticeable stability without any displacement
of potentials during the experiment toward more anodic
values.
In conclusion, we have developed a new class of con-
ducting polythiophenes bearing chiral auxiliaries. We
have found that poly-(2b) was stable toward oxidative
cycling, and we are currently investigating the improve-
ment of such a process in order to increase the stereose-
lectivity of the heterogeneous reaction.
The potentiostatic growth of the title poly-(2) matrix was
investigated. Thus, the obtaining of a growing redox
system at Ef=1.60 or 1.50 V for poly-(2a,b), respectively,
accounts for the deposition of the polymer layer. The
same observations occurred when starting from poly-(1).

P. Pellon et al. / Tetrahedron Letters 42 (2001) 867–869
869
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2×CH2OCO); 1.30 (6H, 2t, 2×CH3). 1b: yield: 70% 1H
NMR (CDCl3): lppm 7.15 (1H, m, Hthiophene); 6.80 (2H, m,
H
thiophene); 5.20 (1H, m, O-CH-O); 4.75–4.60 (2H, AB
A mixture of (RhCODCl)2 (0.0014 mmol), diphosphine 3
(5%) and a-acetamidoacrylic acid (1.120 mmol) in
methanol (2.6 ml) was stirred in the hydrogenation
apparatus under a hydrogen atmosphere for 6 h. After
concentration to dryness, the residue was dissolved in
water and the catalyst was eliminated by simple filtration.
The N-acetyl alanine was finally obtained by further evap-
oration of the title solution.
system, J 3.9, 2×O-CH); 4.15 (4H, 2q, 2×CH2OCO); 2.90
(2H, t, CH2-thiophene); 2.10 (2H, m, CH2); 1.20 (6H, 2q,
2×CH3).
2a: overall yield: 45% H NMR (CDCl3): lppm 7.45 (1H,
m, Hthiophene); 7.30 (1H, m, Hthiophene); 7.15 (1H, m,
1
H
thiophene); 6.10 (1H, s, O-CH-O); 4.35 (6H, m, 2×O-CH
and 2×CH2-O); 3.10 (3H, s, CH3); 2.99 (3H, s, CH3).
1
2b: overall yield: 30% H NMR (CDCl3): lppm 7.45 (1H,
.
.