Asymmetric dibenzophospholes: New phosphorus-based chiral liquid crystals

Article abstract of DOI:10.1016/S0040-4039(01)01661-6

(S_P,S_C)-5-Ethyl-3-(2-methylbutoxy)-7-(4-undec-10- enoxyphenoxycarbonyl)-5H-dibenzophosphole 5-oxide, the first example in the literature of a new asymmetrically substituted dibenzophosphole-based group of systems with liquid-crystal properties is described.

Full text of DOI:10.1016/S0040-4039(01)01661-6

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 7791–7793
Asymmetric dibenzophospholes: new phosphorus-based chiral
liquid crystals
Ernesto Dura´n,a Esther Gordo,a Jaume Granell,b Dolores Velascoa and Francisco Lo´pez-Calahorraa,*
aDepartament de Qu´ımica Orga`nica, Facultat de Qu´ımica, Universitat de Barcelona, Mart´ı i Franque`s, 1-11,
E-08028 Barcelona, Spain
bDepartament de Qu´ımica Inorga`nica, Facultat de Qu´ımica, Universitat de Barcelona, Mart´ı i Franque`s, 1-11,
E-08028 Barcelona, Spain
Received 31 August 2001; revised 3 September 2001; accepted 4 September 2001
Abstract—(SP,SC)-5-Ethyl-3-(2-methylbutoxy)-7-(4-undec-10-enoxyphenoxycarbonyl)-5H-dibenzophosphole 5-oxide, the first
example in the literature of a new asymmetrically substituted dibenzophosphole-based group of systems with liquid-crystal
properties is described. © 2001 Elsevier Science Ltd. All rights reserved.
in order to generate chiral liquid-crystal monomeric
and macromolecular systems. Here we report a chiral
phosphorus-containing structure with liquid-crystal
properties, specifically a molecule derived from the
dibenzophosphole system, (SP,SC)-5-ethyl-3-(2-methyl-
butoxy)-7-(4-undec-10-enoxy-phenoxycarbonyl)-5H-
dibenzophosphole 5-oxide (1).
1. Introduction
Liquid-crystal synthesis has been investigated in recent
years mainly because successful applications of liquid
crystals have been developed, particularly in the area of
electro-optical displays.1 At present our research group
is working on the synthesis of several chiral structures
ii, iii
i
Br
NH2
Br
P
P
P
O Et
O Et
O Et
vii, viii
iv, v, vi
Br
O
HOOC
O
P
P
O Et
O Et
ix, x
O
9
O
( )
O
4
6
P
O
O
Et
1
Scheme 1. Reagents and conditions: (i) Br2, AcOH, MeOH, 45°C, 53%; (ii) HNO3, H2SO4, 0°C, 100%; (iii) Fe, HCl/H2O, EtOH,
reflux, 82%; (iv) NaNO2, H2SO4/H2O, 0°C; (v) Cu(NO3)2·3H2O, Cu2O, rt, 87% (steps (iv) and (v)); (vi) S-2-methylbutyltosylate,
K2CO3, DMF, reflux, 95%; (vii) KCN, Pd(OAc)2, PPh3, Ca(OH)2, DMF, 100°C, 92%; (viii) HCl/H2O, reflux, 80%; (ix) DCC,
DMAP, CH2Cl2, rt; (x) 4-undec-10-enoxyphenol, rt, 80% (steps (ix) and (x)).
Keywords: liquid crystals; phosphorus heterocycles; resolution.
* Corresponding author. Tel.: 0034 93 4021252, fax: 0034 93 3397878; e-mail: flopez@qo.ub.es
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)01661-6

7792
E. Dura´n et al. / Tetrahedron Letters 42 (2001) 7791–7793
2. Synthesis
nation of the enantiomeric excess9,10 of Lewis bases
such as phosphines and amines. In addition, phosphine
oxides are easily reduced to phosphines by trichloro-
silane.11 Therefore, we used the following procedure to
resolve our new dibenzophosphole 5-oxide (Scheme 2):
the first step was to reduce the oxide 1 by use of
trichlorosilane; then the reaction between the phosphine
2 obtained and the optically active dinuclear cyclopalla-
dated compound 3 gave a mixture of diastereomers (4)
that were separated by column chromatography, using
a SiO2 column with CHCl3–acetone (100/3) as eluent.
Up to 85% of each diastereomer was isolated, with
d.e.>95%.
The synthesis is summarised in Scheme 1. It should be
emphasised that descriptions in the literature of me-
thods of synthesis of asymmetrically substituted diben-
zophospholes are very scarce and limited in scope.
3. Optical resolution and determination of the absolute
configuration of the phosphorus atom
The synthesis procedure yields a 1:1 mixture of two
diastereomers, with different configuration on the phos-
phorus atom. The diastereomers were too similar to be
separated by physical methods, so we had to find a
resolution method for this kind of compound. The
literature only gave a resolution method of dibenzo-
phospholes based on the recrystallization of chiral
amides prepared from dibenzophospholes with a car-
boxylic acid function and a pure enantiomeric amine.2
However, we needed a general method applicable not
only to dibenzophospholes with a carboxylic acid
function.
In the same way, we obtained (R)-[1-(1-aminoethyl)-
naphthyl - C2,N][3 - cyano - 5 - methyl - 5H - dibenzophos-
phole-P][chloro]palladium(II) 4a, a related molecule
simpler and easier to study than 4b. After the dia-
stereomers were separated, the crystal structure of (S)-
4a was determined by X-ray diffraction§ in order to
establish the absolute configuration of the phosphorus
atom, showing that the phosphorus atom in the diben-
zophosphole ring is S.
Dimeric cyclopalladated compounds react easily with a
wide range of Lewis bases to give monomeric com-
plexes of formulae [Pd(C-N)ClL] or [Pd(C-N)ClL2].
This simple reaction, using optically pure cyclopalla-
dated dimers, is used for the resolution3–8 and determi-
The absolute configuration of the phosphorus atom and
the NMR data were correlated, as has been done in
other coordinated chiral phosphines,12,13 and the data
obtained extrapolated to analogous compounds (4b).
The 1H NMR spectra of (S)-4a and (R)-4a showed
R1 = CN
R1 =
R2 = H
a)
R3 = Me
HSiCl3
NEt3
R2
R2
R1
R1
O
P
toluene
100%
P
R3
O
R3
1
R2 =
b)
2
COO
O
( )
9
P(R) / P(S) = 1:1
R3 = Et
23 24
Cl
R1
C
P(S)
Pd
NH2
Pd
Pd
*
*
2
(S)-4
(R)-4
3
N
Cl
separation
Me
P
R2
H
R3
2
Pd
C
N
P(R)
Cl
THF
100%
Me
NH2
4
Cl
PPh2
C
P(S)
Ph2P
H2O2
(R)-2
R2
R1
Pd
*
THF
93%
N
Cl
4
6
P
O
R3
(S)-4
(S)-1
C
P
P
Cl
Pd
*
N
Scheme 2. Optical resolution of 1.
§ Crystallographic data (excluding structure factors) for the structure reported in this paper have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publication no. CCDC-150264. Copies of the data can be obtained free of charge on application
to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK [fax: +44(0)-1223-336033 or e-mail: deposit@ccdc.cam.ac.uk].

E. Dura´n et al. / Tetrahedron Letters 42 (2001) 7791–7793
7793
clear differences in the aromatic protons H4 and H6, the
nearest to the phosphorus atom in the dibenzophos-
phole ring (see Scheme 2). In (S)-4a, the l values of these
protons are respectively 8.62 and 7.58 ppm, and in (R)-4a
are 7.89 and 8.22 ppm. That is, the proton H4 appears
in the diastereomer (S)-4a shifted to higher fields than
in diastereomer (R)-4a (Dl(H4)=0.73 ppm), and the
opposite occurs in the case of the proton H6 (Dl(H6)=
−0.64 ppm). This difference can only be explained by the
relative positions of the planes determined by the naph-
thyl group and by the phenyl groups of the dibenzophos-
phole ligand. Exactly the same phenomenon is observed
in the 1H NMR spectra of the diastereomers (S)-4b and
(R)-4b: the d values of H4 and H6 are 7.88 and 8.23 ppm
in (S)-4b and 7.07 and 9.02 ppm in (R)-4b (Dl(H4)=0.88
ppm and Dl(H6)=−0.79 ppm), which shows that such
criteria can be used for the determination of the absolute
configuration of the phosphorus atom in our complexes.
As such, (S)-4b is the SP diastereomer.
The differential scanning calorimetry (DSC) study of
(S)-1b showed two phase transitions, the first from the
isotropic liquid at 25°C with an associated enthalpy of
0.6 J/g and a second one at −4°C with an enthalpy of 0.8
J/g. Observation of the compound in the polarised light
optical microscope showed a clear Grandjean-type
cholesteric texture below 25°C, and also a blue phase on
the borders of the preparation after several days at 24°C.
This cholesteric structure of (S)-1b was confirmed by
X-ray diffraction; however, in both small-angle and
wide-angle zones, reflections corresponding to smectic
fluctuations or microstructures inside the cholecteric
phase were also observed. The transition at −4°C could
not be studied by X-ray diffraction for technical reasons.
Acknowledgements
This research was financially supported by the Spanish
Ministerio de Educacio´n y Cultura, grant PB96-1491,
and a postgraduate fellowship to Dr. E. Dura´n from the
Generalitat de Catalunya. We would also like to thank
Dr. C. Minguillo´n for her help in the HPLC study and
Professor H. Finkelmann for help in the study of
liquid-crystal behaviour.
The third step of the resolution process was to decoor-
dinate dibenzophosphole through the reaction of the
pure
diastereomer
with
1,2-bis(diphenylphos-
phino)ethane; and the last step was to oxidate the
optically pure ligand by hydrogen peroxide. The action
of 1,2-bis(diphenylphosphino)ethane (dppe) on the opti-
cally pure cyclopalladated diastereomers (S)-4 led to the
enantiopure free dibenzophospholes (R)-2. The displace-
ment proceeds by retention of the configuration at the
phosphorus atom, as verified by the quantitative re-
generation of the starting material (S)-4 from the free
ligand (R)-2 and the dinuclear cyclopalladated derivative
3. Change in nomenclature is due to priority changes of
the phosphorus-attached groups.
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of configuration.14 Optical purity of (S)-1b was deter-
mined by 1H NMR and by adding 0.6 equiv. of europium
(III) tris{3-(heptafluoropropylhydroxy-methylene)-(+)-
camphorate} as resolution agent. Signals of the aromatic
H23 and H24 (see Scheme 2) showed different l values for
each diastereomer and can be used to confirm that the
(R)-1b diastereomer was not detected after the oxidation
of (R)-2b with hydrogen peroxide. In addition, HPLC
with a CSP-1,15 a non-commercial chiral stationary phase
derived from N-[N-(3,5-dinitrobenzoyl)-(S)-phenyl-
alanyl]-(3-triethoxysilyl)propylamide, was used to deter-
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for (S)-1b.
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4. Liquid-crystal properties
Just as practically no synthetic nor optical general
resolution methods are described in the literature for the
kind of molecules reported here, so this is the first
example of liquid-crystal dibenzophospholes.