Synthesis and resolution of 2-(diphenylphosphino)heptahelicene

Article abstract of DOI:10.1016/j.jorganchem.2006.11.022

Palladium catalysed Heck couplings have been applied to the two-step synthesis of a stilbene derivative bearing a diphenylphosphine oxide function which represents a suitable precursor for the photochemical generation of the corresponding [7]-helicene. After reduction of the phosphine oxide, resolution of the monodentate helical phosphine has been performed by means of the ortho-metallated (R)-1-(naphthyl)ethylamine-palladium complex. A ruthenium complex of (heptahelicen-2-yl)diphenylphosphine has also been prepared and fully characterized.

Full text of DOI:10.1016/j.jorganchem.2006.11.022

Journal of Organometallic Chemistry 692 (2007) 1156–1160
www.elsevier.com/locate/jorganchem
Synthesis and resolution of 2-(diphenylphosphino)heptahelicene
a
a
b
Riadh El Abed , Faouzi Aloui , Jean-Pierre Geneˆt ,
a,
c,
*
Bechir Ben Hassine *, Angela Marinetti
´
a
Laboratoire de Synthese Organique Asymetrique et Catalyse Homogene, Faculte des Sciences de Monastir,
´ ´
` `
Avenue de l’Environnement, 5019 Monastir, Tunisia
Laboratoire de Synthese Selective Organique et Produits Naturels, CNRS UMR 7573, E.N.S.C.P. 11, rue Pierre et Marie Curie,
b
`
´
75231 Paris Cedex 06, France
Institut de Chimie des Substances Naturelles, CNRS UPR 2301, 1, av. de la Terrasse, 91198 Gif-sur-Yvette Cedex, France
c
Received 26 September 2006; received in revised form 14 November 2006; accepted 14 November 2006
Available online 19 November 2006
Abstract
Palladium catalysed Heck couplings have been applied to the two-step synthesis of a stilbene derivative bearing a diphenylphosphine
oxide function which represents a suitable precursor for the photochemical generation of the corresponding [7]-helicene. After reduction
of the phosphine oxide, resolution of the monodentate helical phosphine has been performed by means of the ortho-metallated (R)-1-
(naphthyl)ethylamine–palladium complex. A ruthenium complex of (heptahelicen-2-yl)diphenylphosphine has also been prepared and
fully characterized.
Ó 2006 Elsevier B.V. All rights reserved.
Keywords: Helicenes; Phosphines; Enantiomeric resolution; Heck reaction; Photocyclization
1. Introduction
tutions [3]. More recently, enantiomerically enriched
diphosphines with [6]-helicene and [7]-helicene structures
were obtained by Katz by introduction of diphenylphosph-
ino groups on suitably functionalized, enantiomerically
enriched helicenes [4]. Finally, Stara and Stary reported
the synthesis of a monodentate phosphine, 3-diph-
enylphosphino-[6]-helicene, in racemic form [5].
Thus, due to the very small number of known helicene
based phosphines [6] and having in hand an alternative
synthetic approach to helical compounds [7], we envisioned
to apply this method to the preparation of phosphorus
containing species.
In recent years, helicene based phosphines have been
envisaged to serve as chiral ligands in enantioselective
catalysis. Literature reports however on only a few pre-
parative approaches and even fewer catalytic applications
for compounds of this family. The first synthesis of helical
phosphanes was reported by Brunner in 1997: bis(diph-
enylphosphino) substituted [5]- and [6]-helicenes were
obtained in albeit racemic form. Resolution was achieved
only on an analytical scale by chiral HPLC [1]. Shortly
after, Reetz succeeded in the antipode separation of the
2,15-bis(diphenylphosphino)-[6]-helicene (called PHelix)
by preparative HPLC and performed the first experiments
directed toward the use of PHelix in both enantioselective
hydrogenations [2] and palladium promoted allylic substi-
2. Results and discussion
Our synthetic approach to helicenes relies on the easy
preparation of stilbene derivatives via palladium promoted
Mizoroki–Heck reactions. The stilbenes are then converted
into helical compounds through the classical photocyclode-
hydrogenation reaction [8]. Variously functionalized penta,
hexa [9] and heptahelicenes [7] have been obtained by this
*
Corresponding authors. Tel.: +33 01 69 82 30 36 (A. Marinetti).
E-mail addresses: bechir.benhassine@fsm.rnu.tn (B. Ben Hassine),
angela.marinetti@icsn.cnrs-gif.fr (A. Marinetti).
0022-328X/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2006.11.022

R. El Abed et al. / Journal of Organometallic Chemistry 692 (2007) 1156–1160
1157
procedure. The same strategy could be suitably applied to
the synthesis of helical phosphines in one of two ways,
either by taking advantage of an appropriate functional
group to introduce a phosphorus function on a preformed
helical derivative or by using phosphorus derivatives as
starting materials. The second approach has been selected
in this work for the synthesis of 2-diphenylphosphinyl-
[7]-helicene 6, given that the highly versatile Heck coupling
should provide an easy access to the required phosphorus
functionalized stilbenes.
Starting materials are 3,6-dibromophenanthrene (1) and
(p-styryl)diphenylphosphine oxide (2). 3,6-Dibromophe-
nanthrene [10] is available in two steps by photocyclization
of 4,40-dibromostyrene which is conveniently prepared by a
Wittig reaction, according to the published procedure
[10b].
ryl)phosphine oxide afforded only a very small amount of
the expected product.
Starting from the bis-stilbene 4, the helical frame can be
formed by photocyclization in the presence of iodine. Thus,
photolysis of 4 in cyclohexane with a high-pressure mer-
cury immersion lamp (150 W) in standard conditions [13],
affords the expected helicene 5 in 57% yield. High dilution
conditions are required for this photolysis step, which has
been performed then on a rather small scale, with about
150 mg of starting material, 4, per run in a one litre reactor.
Photolysis must be run up to total conversion of 4
because the final helicene derivative 5 cannot be separated
from the starting material by chromatography in the condi-
tions given in the experimental section (see Scheme 2).
The phosphine oxide 5 has been fully characterized. In
the 1H NMR spectrum, it displays the typical patterns
for the H-15/H-17 protons of [7]-helicene moieties: high
field signals at d = 6.26 and 6.75 ppm for H-17 and H-16,
respectively (Fig. 1).
The (p-styryl)diphenylphosphine oxide 2 [11] is obtained
by H2O2 oxidation of the corresponding styrylphosphine
[12].
Compounds 1 and 2 undergo a Mizoroki–Heck coupling
in the conditions above: 1% of Herrman’s palladacycle
[trans-di(l-acetato)-bis[o-(di-o-tolylphosphino)benzyl]dipal-
ladium] as the catalyst, sodium acetate as the base and N,N-
dimethylacetamide as the solvent. The desired phosphine
oxide 3 is obtained in 54% yield after two days heating at
140 °C, according to Scheme 1. The use of the phosphine
oxide as starting material in this reaction is imperative as
both (p-styryl)diphenylphosphine and its borane complex
do not afford the expected stilbene derivative in analogous
conditions, due to catalyst deactivation.
A second Heck-type reaction between 3 and styrene
affords the phosphine oxide 4 in 90% yield after purifica-
tion by column chromatography, with a total 49% yield
over two steps (Scheme 2). The alternative synthetic
approach to 4 via Heck reactions, i.e. coupling of 2-
bromo-6-styrylphenanthrene [7] with diphenyl(4-sty-
The final step of the envisioned synthetic procedure is
the reduction of phosphine oxide 5 into 2-diphenylphosph-
ino-[7]-helicene (6). This has been performed in high yields
by using the HSiCl3/Et3N mixture as the reducing agent.
The trivalent phosphine 6 is a slightly air sensitive solid.
It is better handled and stored under inert atmosphere.
As far as we know, no examples of isolated transition
metal–helical phosphine complex have been reported to
date. It was then very attractive to check the coordinating
ability of 6 toward transition metals by synthesizing any
metal complex. This has been done by reacting 6 with the
[(p-cymene)RuCl2]2 complex, which quantitatively affords
complex 7 as an air stable, orange-red compound.
The reaction takes places easily at room temperature,
showing that the helical structure of phosphine 6 does
not affect the coordinating properties of phosphorus (see
Fig. 2).
ClMg
Ph2PCl
Br
Br
H2O2
(ii)
(
i
)
+
54%
P(O)Ph2
Ph2(O)P
Br
Br
Br
1
2
3
(i) 1% Herrmann’s catalyst, DMA, AcONa, 140˚C, 48h. (ii) I2, propylene oxide, h , 2h,
cyclohexane.
Scheme 1. Synthesis of the styryl derivative 3.

1158
R. El Abed et al. / Journal of Organometallic Chemistry 692 (2007) 1156–1160
P(O)Ph2
P(O)Ph2
4
16
15
1
(i)
(ii)
90%
P(X)Ph2
Br
5 X = O, 57%
(iii)
3
4
6 X = lone pair, 92%
(i) 1% Herrmann’s catalyst, DMA, AcONa, 140˚C, 48h. (ii) h , I2, propylene oxide,
cyclohexane, 1.5h. (iii) HSiCl3, Et3N, 100˚C, 16h.
Scheme 2. Synthesis of the helical phosphine oxide 5.
the resolution of monodentate phosphines is based on the
use of chiral cyclopalladated amine complexes [14]. Their
reactions with racemic phosphines afford diastereoisomers
which can be separated either by crystallization or by chro-
matography. Following this approach, enantiomeric reso-
lution of 6 has been conveniently performed by means of
the ortho-palladated (R)-1-(naphthyl)ethylamine complex
8 [15] which reacts with the helical phosphine 6 to afford
the diastereomeric adducts 9a,b. The diastereomers could
be satisfactorily separated by column chromatography on
silica gel: the palladium complex of the M-configurated
helical phosphine eluted first (Rf = 0.3) when an ether–ace-
tone 92:8 mixture was used as the eluent. The second dia-
stereomer was obtained then, with an Rf value of about 0.2.
Fig. 1. 1H NMR spectrum of the phosphine oxide 5 (CDCl3, 300 MHz)
1
In the H NMR spectra, the H-30 protons give charac-
teristic signals at 5.92 (dd, J = 8.5 Hz, JH–P = 5.5 Hz) and
5.59 (dd, J = 8.5 Hz, JH–P = 5.5 Hz), for 9a and 9b, respec-
tively (see Scheme 3).
Ph2P
Ru
Removal of the enantiomerically enriched phosphines
from their palladium complexes was carried out by reac-
tion with bis(diphenylphosphino)ethane (dppe) at room
temperature. The specific rotation value obtained for the
M-configurated phosphine is ꢀ2980 (c = 0.1, CHCl3).
The very high specific rotation value is a known feature
of helical structures, while the sign of optical rotation
allows reliable assignment of the helical configuration
[8a,16].
Cl
Cl
7
Fig. 2. (2-Helicenyl)diphenylphosphine ruthenium complex (7).
Finally, having in hand the new helical phosphine 6, we
have considered procedures for its possible enantiomeric
resolution. A well established and attractive method for
4
16
H2N
Pd
Cl
Cl
acetone
r.t.
1/2
1
+
Pd
NH2
2
PPh2
Ph2P
3'
4'
(R
)-8
rac-6
Scheme 3. Enantiomeric resolution of the helical phosphine 6.
9a+ 9b

R. El Abed et al. / Journal of Organometallic Chemistry 692 (2007) 1156–1160
1159
The synthetic approach to phosphine 6 described here,
combined with the simple and efficient resolution proce-
dure, should allow studying new applications of helical
phosphines in organometallic chemistry and enantioselec-
tive catalysis. Works in this field are in progress.
J = 15 Hz, 1H, CHvinyl), 7.4–7.8 (21H), 8.55 (s, 1H, H-4),
8.78 (d, J = 1.5 Hz, 1H, H-5). 31P{1H} NMR (CDCl3) d
30.2 ppm. Mass spectrum (EI) m/z 558 (M, 20%), 479
(MꢀBr, 100%).
4.3. Diphenyl{4-[2-(6-[2-phenylvinyl]-3-
phenanthryl)vinyl]phenyl}phosphine oxide (4)
3. Conclusion
We have developed a synthetic approach and enantio-
meric resolution of the new 2-(diphenylphosphino)-[7]-heli-
cene (6). Unlike all previous synthetic approaches to helical
phosphines, where the phosphino group is introduced, in a
late step, on a preformed functional helicene, the helical
framework of 6 has been built here from phosphorus-con-
taining starting materials. We have also demonstrated that
the cyclopalladated a-naphthylethylamine complex 8 is a
suitable resolving agent for the helical phosphine. This
method represents a convenient alternative to the chiral
HPLC separation techniques applied so far to the resolu-
tion of helical phosphines.
Phosphine oxide 4 has been prepared according to the
procedure described above for the Heck reaction (see Sec-
tion 4.2) starting from 2.1 g of 3 (4.5 mmol) and 0.75 mL
styrene (5 mmol). The final product was purified by column
chromatography on silica gel with an ether–MeOH 98:2
mixture as the eluent and subsequent crystallization from
AcOEt/hexane. The phosphine oxide 4 was obtained in
90% yield (2.3 g). 1H NMR (CDCl3): d 7.2–7.8 (29H),
8.68 (s, 1H, H-4 or 5), 8.70 (s, 1H, H-4 or 5). 31P{1H}
NMR d 31 ppm. Mass spectrum (CI, NH3.) m/z (%) 583
(M+1, 100%).
4.4. Heptahelicen-2-yl(diphenyl)phosphine oxide (5)
4. Experimental
A solution containing phosphine oxide 4 (150 mg,
0.26 mmol), I2 (145 mg, 0.57 mmol) and propylene oxide
(1.8 mL, 26 mmol) in cyclohexane (1 L) was placed in the
photoreactor equipped with a 150 W immersion lamp.
The mixture was irradiated for about 90 min. After evapo-
ration of the solvent, the final product was purified by chro-
matography with ether–MeOH 97:3 as the eluent. Yield:
4.1. General
All reactions were performed under argon. Photochem-
ical reactions were carried out in a 1-L photoreactor
equipped with a high pressure mercury immersion lamp
[Heraeus TQ 150]. Column chromatography was per-
formed on silica gel columns. Bis[(R)-1-(1-aminoethyl)-
naphthyl-C2,N]di-l-chlorodipalladium has been prepared
according to Ref. [15b]. NMR spectra have been recorded
either on a Bruker AM 300 or on a Bruker AVANCE 500
instrument. Optical rotations have been measured on a
Jasco P-1010 polarimeter. Mass spectra have been recorded
on a Hewlett–Packard HP 5989. High resolution mass
spectra have been recorded on a MALDI-TOF Perspective
Biosystems Voyager DE-STR.
1
90 mg (57%) of a pale yellow solid. H NMR (CDCl3): d
6.31 (ddd, J = 8.4 Hz, J = 6.9 Hz, J = 1.2 Hz, 1H, H-17),
6.82 (ddd, J = 7.8 Hz, J = 6.9 Hz, J = 0.9 Hz, 1H, H-16),
7.00 (2H, H-15 and H-18), 7.1–7.5 (15H), 7.7–8.0 (8H).
31P{1H} NMR (CDCl3) d 27 ppm. Mass spectrum (CI,
NH3.) m/z (%) 579 (M+1, 100%). HRMS (MALDI) calcd.
for C42H28OP: 579.1878. Found: 579.1886.
4.5. Heptahelicen-2-yl(diphenyl)phosphine (6)
3,6-Dibromophenanthrene was obtained by photolysis
of 4,40-dibromostilbene, according to Ref. [10].
Phosphine oxide 5 (0.2 g, 0.34 mmol) was dissolved in
anhydrous toluene (5 mL) under argon. Triethylamine
(0.95 mL) and trichlorosilane (0.17 mL, 1.7 mmol) were
added successively at room temperature. The mixture was
heated overnight at 100 °C. The solution was then cooled
to 0 °C and a 30% aqueous NaOH solution was added
dropwise. The biphasic mixture was extracted with ether
under argon. The organic layer was dried over MgSO4
and filtered through a short silica gel column with cyclo-
hexane–ethyl acetate 9:1 as the eluent to afford 6 in almost
quantitative yield. 1H NMR (CDCl3): d 6.43 (ddd,
J = 8.0 Hz, J = 7.0 Hz, J = 1.2 Hz, 1H, H-17), 6.8–6.9
(3H), 6.93 (t, J = 7.5 Hz, 1H, H-16), 7.1 (3H), 7.2–7.4
(8H), 7.46 (d, J = 8.5 Hz, 1 H), 7.5–7.6 (2H), 7.76 (d,
J = 8.5 Hz, 1H), 7.85 (d, J = 8.5 Hz, 1H), 7.9–8.1 (6H).
31P{1H} NMR (CDCl3) d ꢀ5 ppm. Mass spectrum (EI)
m/z (%) 562 (M, 30%), 377 (MꢀPPh2, 10%). HRMS
(MALDI) calcd. for C42H28P: 563.1926. Found: 563.1923.
4.2. Synthesis of {4-[(E)-2-(6-bromo-3-
phenanthryl)vinyl]phenyl(diphenyl)phosphine oxide (3)
A solution of 3,6-dibromophenanthrene (3.0 g, 9 mmol),
diphenyl(p-styryl)phosphine oxide (3.8 g, 12.5 mmol) and
dry sodium acetate (0.8 g, 10 mmol) in N,N-dimethylaceta-
mide (15 mL) was placed in a Schlenk tube, under an argon
atmosphere. The mixture was heated to 100 °C before addi-
tion of a solution of the Herrmann catalyst (85 mg, 1%) in
N,N-dimethylacetamide (5 mL). The mixture was heated
then to 140 °C and heating was maintained for about
48 h. The workup procedure involves hydrolysis, extraction
with CH2Cl2 and drying over MgSO4. After column chro-
matography with ether–methanol 98:2 as the eluent, the
final product 3 was obtained in 54% yield (2.3 g) as a
pale-yellow solid. 1H NMR (CDCl3):
d 7.27 (AB,

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R. El Abed et al. / Journal of Organometallic Chemistry 692 (2007) 1156–1160
Enantiomeric resolution was performed as follows.
J = 6.9 Hz, J = 1.0 Hz, H-16), 7.05 (d, J = 8.3 Hz, 1H,
H-18). 13C NMR (selected data, CDCl3) d 17.2 (Me),
21.6 (Me), 22.0 (Me), 30.14 (CHMe2), 85.6, 86.3, 90.0,
94.2 (CHp-cym). Anal. Calc. for C53H44Cl2PRu.CH2Cl2:
C, 66.95; H, 4.79. Found: C, 66.09; H, 4.77%.
Bis[(R)-1-(1-aminoethyl)-naphthyl-C2,N]di-l-chlorodipal-
ladium (60 mg, 0.09 mmol) was reacted with 6 (100 mg,
0.18 mmol) in acetone (12 mL) at room temperature for
about 30 min. The solvent was removed and the yellow
solid was purified by column chromatography on silica
gel with an ether–acetone mixture 92:8 as the eluent. These
conditions also allowed separation of the two diastereo-
meric complexes. Firstly (R,M)-9a was isolated (Rf = 0.3)
as a pure diastereomer in about 75% yield (60 mg), fol-
lowed by the second diastereomer (R,P)-9b (Rf = 0.2)
which was obtained in 60% yield (45 mg). 31P{1H} NMR
(CDCl3) 40 ppm. (R,M)-9a: 1H NMR (selected data,
500 MHz, CDCl3) d 2.06 (d, J = 6.5 Hz, 3H, Me), 3.66
(1H, NH), 4.14 (1H, NH), 5.26 (q, J = 6.5 Hz, CHMe),
5.92 (dd, J = 8.5 Hz, JH–P = 5.5 Hz, 1H, H-30), 6.42 (ddd,
J = 8.0 Hz, J = 6.5 Hz, J = 1.0 Hz, 1H, H-17), 6.66 (d,
J = 8.6 Hz, 1H, CH-40), 6.99 (td, J = 7.5 Hz, J = 1.0 Hz,
Acknowledgements
´
The authors are grateful to CMCU (Comite Mixte
´
Franco-Tunisien pour la Cooperation Universitaire), the
Ministry of Higher Education, Scientific Research and
Technology in Tunisia and EGIDE for financial support.
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(18 mg) in dichloromethane at room temperature for 3 h.
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ˇ
´
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and [(p-cymene)RuCl2]2 (52 mg, 0.08 mmol) in dichloro-
methane (5 mL) was stirred under argon for 30 min. After
evaporation of the solvent, the crude product was crystal-
lized from a dichloromethane–ether mixture to afford 7
as an orange-red solid (93 mg, 60% yield). 31P{1H} NMR
(CDCl3) d 25 ppm. 1H NMR (selected data, CDCl3) d
0.89 (d, J = 6.8 Hz, 3H, Me), 0.91 (d, J = 6.7 Hz, 3H,
Me), 1.54 (d, J = 7.6 Hz, 3H, Me), 2.57 (m, 1H, CHMe2),
4.55 (d, J = 5.7 Hz, 1H, CHp-cym), 4.74 (d, J = 5.7 Hz,
1H, CHp-cym), 4.80 (d, J = 6.1 Hz, 1H, CHp-cym), 5.09 (d,
J = 6.1 Hz, 1H, CHp-cym), 6.42 (ddd, J = 8.4 Hz, J = 6.9
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