Tetrathiafulvalene based phosphino-oxazolines: A new family of redox active chiral ligands

Article abstract of DOI:10.1039/b402877e

Reaction of the lithium salt of EDT-TTF-2-(4-methyl)oxazoline with chloro-diphenylphosphine afforded the novel redox active chiral chelating ligands, EDT-TTF-phosphino-oxazolines, for which a palladium (II) dichloride complex was synthesized and structura

Full text of DOI:10.1039/b402877e

Tetrathiafulvalene based phosphino-oxazolines: a new family of redox
active chiral ligands
Céline Réthoré, Marc Fourmigué* and Narcis Avarvari*
Laboratoire de Chimie, Ingénierie Moléculaire et Matériaux d’Angers, UMR 6200, Université d’Angers,
UFR Sciences, Bât. K, 2 Bd. Lavoisier, 49045 Angers Cedex, France.
E-mail: marc.fourmigue@univ-angers.fr; narcis.avarvari@univ-angers.fr; Fax: (33) 24173 5405; Tel: (33)
24173 5084
Received (in Cambridge, UK) 20th February 2004, Accepted 14th April 2004
First published as an Advance Article on the web 12th May 2004
Reaction of the lithium salt of EDT-TTF-2-(4-methyl)oxazoline
with chloro-diphenylphosphine afforded the novel redox active
chiral chelating ligands, EDT-TTF-phosphino-oxazolines, for
which a palladium (II) dichloride complex was synthesized and
structurally characterized.
An ultimate proof for their formation was provided by a single
crystal X-ray study of (R)-EDT-TTF-OX 2b, for which thin single
crystalline platelets could be grown by slow evaporation of an
AcOEt solution of donor. The compound crystallizes in the chiral
monoclinic group space P21.‡ The overall geometry of the donor is
nearly planar, only the methyl group C(12) at the asymmetric
carbon C(11), going out of the mean plane of the molecule (Fig.
1).
Furthermore, lithiation of the EDT-TTF-OX has been performed
with LDA; the reaction of the corresponding lithium salts with
Ph2PCl yielded the targeted phosphinooxazolines 3a–c (Scheme
2).
Note that the hydrogen abstraction and generation of the
intermediary carbanion is very likely facilitated by the presence of
the oxazoline ring.5 The 31P NMR spectroscopy demonstrates
unambiguously the formation of the PHOX ligands (d = 215.6
ppm), by comparison with chemical shifts of other TTF-phos-
phines.14 All the other analytical data and elemental analysis
confirm the successful preparation of the new redox-active
chelating ligands.† In order to test the coordination properties of the
EDT-TTF-PHOX thus obtained, and also to check the influence on
their electrochemical behaviour, we reacted the racemic donor 3a
with PdCl2, keeping in mind that many of the asymmetric catalytic
Since the original reports by the groups of A. Pfaltz,1 G. Helmchen2
and J. M. J. Williams,3 the phosphinooxazolines (PHOX) have been
widely used in a variety of asymmetric catalytic reactions such as
allylic substitution, Heck type reactions, enantioselective hydro-
genation, Diels–Alder reactions.4 Within this type of ligand, the
connectivity between the oxazoline ring and the phosphine is
ensured by a backbone which is very often an aryl group. Of special
interest is the case when the backbone is a ferrocenyl group,5
because of its planar chirality on the one hand, and its redox-active
character on the other hand. This last feature is very interesting
from a material point of view, since ferrocene derivatives can be
generally reversibly oxidized to the corresponding ferricinium
radical cations. Nevertheless, ferrocene based PHOX have not been
exploited so far in the synthesis of magnetic molecular materials.6
Another prominent redox-active class of compounds is represented
by the tetrathiafulvalene (TTF) derivatives which have been
extensively studied in the search for molecular conductors and
superconductors.7 In this context, the incorporation of TTF as
backbone for the phosphinooxazoline ligands opens the way to a
new class of redox-active chiral ligands which would eventually
allow an electrochemical modulation of the catalytic processes
thanks to the electroactive TTF core.8 In addition, these functional
chiral redox ligands or their transition metal complexes may be
activated by electrocrystallization into stable radical cation salts,
thus having a powerful access to chiral molecular materials with
conducting and/or magnetic properties. The coexistence and, more
interesting, but also more challenging, the interplay of conducting,
magnetic and optical properties hold much promise for the
development of multifunctional molecular materials, a field of
much current activity.9 Indeed, cooperative properties may lead to
interesting phenomena, such as the magnetoresistance.9b
Scheme 1 Reagents and conditions: i) 2-amino-1-propanol (± or R or S),
NEt3, THF, 12 h, RT; ii) NEt3, THF, MsCl at 0 °C, then 20 h at 50 °C.
There are several reports in the literature of enantiopure TTF
derivatives, the chirality being provided either by the appropriate
functionalization of the aliphatic backbone of BEDT-TTF and its
related compounds,10 or by a chiral binaphthyl block.11 A most
notable example consists in a series of TTF-oxazolines,8 the
catalytic activity of which proved to be rather modest. No example
of transition metal complexes with chiral TTFs has been described
so far. The PHOX ligands which we targeted possess a EDT-TTF
backbone since functional derivatives of the latter proved to be
appropriate precursors for interesting molecular materials.12 In-
deed, it is worthwhile to preserve an ethylenedithio bridge on one
side of the TTF core, because of its ability to establish more lateral
S–S intermolecular interactions, with the aim of increasing the
dimensionality of the material. The synthesis of the TTF-PHOX
ligands required three steps, the first two being achieved by the
reaction of EDT-TTF-COCl12a with racemic or enantiopure
alaninol, isolation and purification of the intermediary hydroxy-
amides 1a–c, followed by cyclization in the presence of methane-
sulfonyl chloride (MsCl)13 to give the chiral EDT-TTF-oxazolines
2a–c (Scheme 1).†
Fig. 1 ORTEP view of the enantiopure EDT-TTF-OX 2b (thermal ellipsoids
set at 50% probability, H atoms omitted). Selected bond lengths (Å): C(1)–
C(2) 1.34(3), C(1)–S(1) 1.75(2), C(2)–S(2) 1.66(3), C(9)–N(1) 1.20(3),
C(3)–C(4) 1.28(2).
Scheme 2 Reagents and conditions: i) LDA, THF, 278 °C, Ph2PCl; ii)
PdCl2, reflux in CHCl3 overnight.
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C h e m . C o m m u n . , 2 0 0 4 , 1 3 8 4 – 1 3 8 5
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

(60% yield). Reaction of 2a–c (1.32 mmol) with LDA (1.45 mmol) in THF
at 278 °C, 4 h, then addition of Ph2PCl (1.45 mmol), followed by stirring
the solution overnight at RT and a rapid column chromatography (CH2Cl2/
cyclohexane 4 : 1) provided the compounds 3a–c as red solids after
evaporation of solvent (27% yield).
1
Selected spectroscopic and analytical data: for 2a–c: H NMR (500.04
MHz, CDCl3) d 1.31 (d, 3J = 6.6 Hz, 3H, Me), 3.29 (s, 4H, S(CH2)2S), 3.88
(t, 3J = 2J = 7.9 Hz, 1H, OCH2), 4.31 (m, 1H, NCH), 4.45 (dd, 3J = 6.6
Hz, 2J
= 7.9 Hz, 1H, OCH2), 6.97 (s, 1H, = CH); Anal. Calc. for
Fig. 2 ORTEP view of the (EDT-TTF-PHOX)PdCl2 complex 4 (thermal
ellipsoids set at 50% probability, H atoms omitted). Selected bond lengths
(Å) and angles (°): Pd–P 2.2194(8), Pd–N 2.054(3), Pd–Cl(1) 2.3742(8),
Pd–Cl(2) 2.2927(9), C(1)–C(2) 1.349(5); Cl(1)–Pd–Cl(2) 90.43(3), Cl(1)–
Pd–N 92.30(8), Cl(2)–Pd–P 85.08(3), P–Pd–N 92.15(8).
C12H11NOS6: C 38.17, H 2.94, N 3.71; found: C 38.05, H 2.97, N
3.52%.
1
3
For 3a–c: H NMR (500.04 MHz, CDCl3) d 1.14 (d, J = 6.4 Hz, 3H,
Me), 3.25 (s, 4H, S(CH2)2S), 3.67 (t, 3J = 2J = 7.6 Hz, 1H, OCH2), 4.18
(m, 1H, NCH), 4.25 (dd, 3J = 6.4 Hz, 2J = 7.6 Hz, 1H, OCH2), 7.36–7.45
(m, 10H, Ph); 31P NMR (202.39 MHz, CDCl3) d 215.6 (s); Anal. Calc. for
C24H20NOPS6: C 51.31, H 3.59, N 2.49; found: C 51.66, H 3.73, N
reactions involve PHOX-palladium complexes (Scheme 2). The
complexation was monitored by 31P NMR spectroscopy, the
chemical shift of the complex (d = 14.8 ppm) being in the usual
range. The structure of the square planar complex 4 has been
determined by X-ray diffraction analysis. Suitable single crystals
have been grown by slow diffusion of pentane into a CH2Cl2
solution of 4. The complex crystallized in the triclinic centrosym-
2.55%.
For 4: 31P NMR (202.39 MHz, CDCl3) d 14.9 (s); MALDI-MS m/z: 666.9
[M]+; Anal. Calc. for C24H20NOPPdS6·CH2Cl2: C 36.44, H 2.69, N 1.70;
found: C 36.28, H 2.59, N 1.74%.
‡
Crystal data for 2b: C12H11NOS6, M = 377.58, monoclinic, space
group P21, a = 6.3686(10), b = 7.667(2), c = 16.419(3) Å, b = 99.60(2),
U = 790.4(3) Å3, Z = 2, T = 293(2) K, m = 0.858 mm21, Dc = 1.586 g
cm23, 6434 refl. measured, 682 refl. with I > 2s(I), R = 0.078, RW
=
¯
metric group space P1, with one enantiomer molecule in general
0.250.
position,‡ the other enantiomer being generated through the
inversion center. As expected, the palladium atom lies in a square
planar environment formed by two chlorine atoms, the N atom of
the oxazoline and the P atom of the phosphine (Fig. 2). Bond
lengths and angles are in the usual range.
For 4: C24H20Cl2NOPPdS6·CH2Cl2·H2O, M = 839.97, triclinic, space
group P1, a = 11.3583(3), b = 12.6818(4), c = 12.9393(2) Å, a =
¯
69.912(2), b = 85.895(2), g = 70.706(2), U = 1650.28(7) Å3, Z = 2, T =
293(2) K, m = 1.340 mm21, Dc = 1.690 g cm23, 31866 refl. measured,
5499 refl. with I > 2s(I), R = 0.041, RW = 0.098. CCDC 232992 (2b) and
CCDC 232993 (4). See http://www.rsc.org/suppdata/cc/b4/b402877e/ for
crystallographic data in .cif or other electronic format.
The TTF framework is twisted along the three S–S hinges, which
is not surprising for a neutral TTF derivative. Cyclic voltammetry
measurements on the series of EDT-TTF-OX 2a–c, EDT-TTF-
PHOX 3a–c and on the palladium complex 4 show for all the
compounds the two reversible single electron oxidation waves,
corresponding to the formation of the TTF+· radical cations, then
TTF2+, with values for E1/2 (V) as follows (ref. Ag/AgCl, 0.1 M
TBAPF6 in CH2Cl2, 0.1 V s21): 2 0.65 and 1.13, 3 0.63 and 1.09,
4 0.87 and 1.25. The anodic shift of about 0.24 V when comparing
3 and 4 is in the same range as that observed for a chelating TTF-
diphosphine coordinated to various transition metal fragments.15
This difference shows a significant electronic communication
between the TTF core and the coordinated metallic centre, an
important feature in the view of electrochemical controlled
catalytic processes based on TTF-PHOX ligands.
The synthesis of the new electroactive ligands EDT-TTF-OX
and EDT-TTF-PHOX offers further interesting developments such
as: (i) preparation of TTF based chiral molecular materials, with the
eventuality of chiral resolution of racemic anions by electro-
crystallization; (ii) utilization in asymmetric catalytic reactions.
Also, the synthetic path to access the EDT-TTF-PHOX opens up
the possibility to prepare bis(oxazoline) ligands in a modular
way.16
1 A. Pfaltz, Acta Chem. Scand. B, 1996, 50, 189.
2 G. Helmchen, S. Kudis, P. Sennhenn and H. Steinhagen, Pure Appl.
Chem., 1997, 69, 513.
3 J. M. J. Williams, Synlett, 1996, 705.
4 Extensive references can be found in: G. Helmchen and A. Pfaltz, Acc.
Chem. Res., 2000, 33, 336.
5 (a) C. Richards, T. Damaldis, D. E. Hibbs and M. B. Hursthouse,
Synlett, 1995, 74; (b) Y. Nishibayashi and S. Uemura, Synlett, 1995,
79.
6 J. S. Miller, Inorg. Chem., 2000, 39, 4392.
7 J. L. Segura and N. Martin, Angew. Chem. Int. Ed., 2001, 40, 1372.
8 A. Chesney and M. R. Bryce, Tetrahedron: Asymmetry, 1996, 7,
3247.
9 (a) E. Coronado, M. Clemente-Leon, J. R. Galan-Mascaros, C.
Gimenez-Saiz, C. J. Gomez-Garcia and E. Martinez-Ferrero, J. Chem.
Soc., Dalton Trans., 2000, 3955; (b) E. Coronado, J. R. Galan-
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(c) M. Minguet, D. Luneau, E. Lhotel, V. Villar, C. Paulsen, D. B.
Amabilino and J. Veciana, Angew. Chem. Int. Ed., 2002, 586.
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Financial support from the CNRS, the Ministère de l’Education
et de la Recherche (grant for C.R.) and University of Angers.
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R. Gomez, J. L. Segura and N. Martin, J. Org. Chem., 2000, 65,
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P. Batail, C. Coulon, R. Clérac, E. Canadell and P. Auban-Senzier, J.
Am. Chem. Soc., 2003, 125, 11583.
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Hayashi, Tetrahedron: Asymmetry, 1998, 9, 1779.
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643–644, 292.
16 S. Bellemin-Laponnaz and L. H. Gade, Angew. Chem. Int. Ed., 2002, 41,
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Notes and references
†
Compounds 1a–c were prepared by stirring 2-amino-1-propanol (9.84
mmol) with dry NEt3 (16 mmol) and EDT-TTF-COCl (8 mmol) in THF,
then purified by column chromatography on silica gel eluted with THF.
Then, to solutions of 1a–c (5.06 mmol) and NEt3 (10.8 mmol) in THF,
methanesulfonyl chloride (MsCl) (10.34 mmol) was added and, after 0.5 h
of stirring at 0 °C, NEt3 (45.2 mmol) was further added and the reaction
mixture thus obtained was heated at 50 °C for 20 h. After purification by
column chromatography on silica gel (AcOEt/cyclohexane 2 : 1) and
recrystallization in MeCN, 2a–c were recovered as orange crystalline solids
C h e m . C o m m u n . , 2 0 0 4 , 1 3 8 4 – 1 3 8 5
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