Synthesis and characterisation of a new helically chiral ruthenium complex

Article abstract of DOI:10.1016/j.tetlet.2010.11.104

A new helically chiral hexacyclic phosphine, containing one thiophene ring was prepared in good yield via a five-step sequence involving a palladium-catalysed Mizoroki-Heck coupling reaction and classical oxidative photocyclisation. A mononuclear air stab

Full text of DOI:10.1016/j.tetlet.2010.11.104

Tetrahedron Letters 52 (2011) 572–575
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Tetrahedron Letters
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Synthesis and characterisation of a new helically chiral ruthenium complex
⇑
Faouzi Aloui, Souad Moussa, Béchir Ben Hassine
Laboratoire de Synthèse Organique Asymétrique et Catalyse Homogène (O1UR1201), Faculté des Sciences, Avenue de l’environnement, 5019 Monastir, Tunisia
a r t i c l e i n f o
a b s t r a c t
Article history:
A new helically chiral hexacyclic phosphine, containing one thiophene ring was prepared in good yield
via a five-step sequence involving a palladium-catalysed Mizoroki–Heck coupling reaction and classical
oxidative photocyclisation. A mononuclear air stable ruthenium complex of this phosphine was also pre-
pared and characterised.
Received 28 September 2010
Revised 5 November 2010
Accepted 19 November 2010
Available online 1 December 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Helicenes
Phosphines
Heck reaction
Photocyclisation
Helical complexes
Helicenes are molecules comprised of ortho-fused aromatic
rings that adopt a helical conformation to avoid overlapping of
the terminal rings. These helices can wind in either clockwise or
counter-clockwise direction. The two structures are mirror images
of one another, are non-superimposable, and consequently are
considered chiral. Their specific chiral structure, which combines
recently, Katz prepared optically active bidentate phosphines con-
taining hexa- and heptahelicene skeletons.10 Finally, Teply et al.
´
prepared 3-(diphenylphosphino)hexahelicene using a multistep
procedure, but they did not report its resolution.11 Despite these
developments, it is still desirable to have easy access to a large
family of modular helically chiral phosphines for examination as
ligands.
electron delocalisation and non-planarity of the
p-electron net-
work, makes them very stable towards acids and bases and rela-
We recently developed methods for the synthesis and resolu-
tion of various interesting helically chiral aromatic structures
via palladium-catalysed Mizoroki–Heck coupling reactions and
classical oxidative photocyclisation.12 Thus, in view of the impor-
tance of helicenes in asymmetric synthesis and due to the small
number of known [6]helicene-based phosphines, we wanted to
extend our approach to the design and synthesis of new types
of phosphorus-containing species capable of forming metallated
complexes. Our synthetic approach is based on utilisation of the
four-membered ring system 1 as a suitable starting material to
provide the helicene-precursor 2 which is then converted into
the helically chiral hexacyclic system 3 by photolysis. The bro-
mine atom in this chiral helicene is an appropriate functional
group for introduction of a phosphorus function which could
serve as a metal chelating agent.
tively high temperature.1
Helicene backbones have attracted significant interest owing to
their properties and practical applications in photo- and optoelec-
tronics, as well as their ability to act as potentially useful compo-
nents in chiral discotic liquid crystalline materials,2 as building
blocks for helical conjugated polymers3 and as rotors.4 Further-
more, enantiomerically enriched helicenes have proved successful
as chiral catalysts5 and ligands6 in asymmetric synthesis because
of their rigid framework, high optical stability and their resistance
to isomerisation.
Several synthetic strategies for helicenes have been developed
in order to exploit the particular properties of these molecules. A
few reports have described the synthesis of phosphorus-containing
helicenes with applications in various branches of chemistry. The
first report on racemic helical-chiral phosphane ligands appeared
in 1997, when Terfort et al. prepared bis(diphenylphosphino)[5]-
and [6]-helicenes.7 In an independent study, Reetz et al. described
the synthesis of optically active 2,15-bis(diphenylphosphino)hexa-
helicene and reported its use in both enantioselective hydrogena-
tions8 and palladium-promoted allylic substitutions.9 More
Palladium-promoted Mizoroki–Heck reaction13 of 1,4-dibro-
mobenzene with vinyl derivative 4 (1.5 equiv) in the presence of
sodium acetate and Hermann’s palladacycle {trans-di(l-acetato)-
bis[o-(di-o-tolylphosphino)benzyl]dipalladium} as the catalyst, in
N,N-dimethylacetamide (DMA), gave the corresponding diaryleth-
ene 5 in 83% yield after heating for 48 h at 140 °C (Scheme 1). The
latter was then irradiated using a 500 W high pressure mercury
immersion lamp on 500 mg scale, for about two hours, to afford
the benzo[c]phenanthrene-like system 1 in 76% yield.
⇑
Corresponding author. Tel.: +216 73500279; fax: +216 73500278.
E-mail address: bechirbenhassine@yahoo.fr (B.B. Hassine).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.11.104

F. Aloui et al. / Tetrahedron Letters 52 (2011) 572–575
573
Br
Br
S
S
1%[Pd], NaOAc
+
DMA, 140 °C, 48 h
83%
Br
4
5
S
hν, toluene
propylene oxide, I2
76%
Br
1
Scheme 1. Synthesis of the benzo[c]phenanthrene-like system 1.
A second Mizoroki–Heck type reaction takes place between 1
and 4-bromostyrene (1.5 equiv) using 1% of Hermann’s palladacy-
cle. The desired helicene precursor 2, possessing (E)-stereochemis-
try at the double bond,14 was obtained in 79% yield after heating
for 48 h at 140 °C, as shown in Scheme 2. To complete the synthesis
of the helicene skeleton 3, olefin 2 underwent photocyclisation in
the presence of a stoichiometric amount of iodine as the oxidising
agent and an excess of propylene oxide as hydrogen iodide scaven-
ger.15 Photolysis of 2 was performed on 200 mg scale per run in a
one litre reactor for about two hours to afford the expected hexa-
helicene 3 in 65% yield, after purification by column chromatogra-
phy.16 No other regioisomer was isolated from the reaction
mixture, indicating that ring closure of the diarylethene 2 had oc-
curred to the peri position of the tetracyclic moiety. In addition, no
product of homo-coupling of 4-bromostyrene was evident.
Suitable crystals of compound 3 were obtained as pale yellow
plates by slow evaporation of a dichloromethane/hexane solution.
The product was highly stable in air and to light. The X-ray anal-
ysis was carried out on a single crystal obtained from racemic 3,
Figure 1.17
The final step of the synthetic sequence is formation of the tar-
get phosphine which can be achieved through lithiation/phosph-
inylation of 3. Thus, metallation of the bromohelicene
3
proceeded well via metal–halogen exchange using n-butyllithium
at À78 °C. Subsequent treatment of the lithiated species with chlo-
Figure 1. X-ray crystal structure of the helically chiral hexacyclic system 3: ORTEP
drawing.
S
S
Br
1%[Pd], NaOAc
DMA, 140 °C, 48 h
79%
hν, toluene, I2
+
propylene oxide
65%
Br
1
2
Br
S
S
S
Br
PPh2
P(O)Ph2
H2O2, CH2Cl2
a) n-BuLi, THF
rt, 30 min
95%
b) ClPPh2, THF
-78 °C to rt
70%
3
6
7
Scheme 2. Synthetic pathway for the synthesis of the helical phosphine 6.

574
F. Aloui et al. / Tetrahedron Letters 52 (2011) 572–575
S
S
Cl
Cl
Cl
Ru
Ph
Ph
Ru
CH2Cl2, rt
75%
Ru
1/2
+
PPh2
P
Cl
Cl
Cl
8
6
9
Scheme 3. Synthetic route towards the helically chiral ruthenium(II) complex 9.
9. Reetz, M. T.; Sostmann, S. J. Organomet. Chem. 2000, 603, 105–109.
rodiphenylphosphine yielded the desired 3-(diphenylphos-
phino)thiahexahelicene 6 in 70% yield and in 23% overall yield over
five steps, starting from the commercially available 1,4-dibromo-
benzene (Scheme 2).18 This helically chiral trivalent phosphine
was a slightly air-sensitive solid. It is better handled and stored un-
der an inert atmosphere or treated with a 35% hydrogen peroxide
solution to give the corresponding phosphine oxide 7 in excellent
yield.
´
10. Paruch, K.; Vyklicky, L.; Wang, D. Z.; Katz, T. J.; Incarvito, C.; Zakharov, L.;
Rheingold, A. L. J. Org. Chem. 2003, 68, 8539–8544.
11. Teply´, F.; Stará, I. G.; Stary´, I.; Kollárovic, A.; Šaman, D.; Vyskocil, Š.; Fiedler, P. J.
Org. Chem. 2003, 68, 5193–5197.
12. (a) Aloui, F.; El Abed, R.; Marinetti, A.; Ben Hassine, B. Tetrahedron Lett. 2008,
49, 4092–4095; (b) Aloui, F.; El Abed, R.; Ben Hassine, B. Tetrahedron Lett. 2008,
49, 1455–1457; (c) Aloui, F.; El Abed, R.; Marinetti, A.; Ben Hassine, B. C. R.
Chim. 2009, 12, 284–290; (d) Aloui, F.; El Abed, R.; Marinetti, A.; Ben Hassine, B.
Tetrahedron Lett. 2007, 48, 2017–2020; (e) Aloui, F.; El Abed, R.; Guerfel, T.; Ben
Hassine, B. Synth. Commun 2006, 11, 1557–1567.
Finally, with the new helical phosphine 6 in hand, we next
investigated its coordinating ability towards transition metals. This
was performed by reacting phosphine 6 with [(p-cymene)RuCl2]2
dimeric complex 8 in dichloromethane at room temperature for
30 min (Scheme 3). Flash chromatography of the resulting reaction
mixture gave the mononuclear ruthenium complex 9 in 75% yield.
This synthetic procedure is very simple and can be performed on
multigram scale. Good analytical data were obtained for the heli-
cally chiral ruthenium complex 9, which is an orange-red solid sta-
ble in air. Characteristic spectroscopic data of this compound
consisted of a single 31P NMR resonance at 27.06 ppm in CDCl3 at
room temperature.19
In summary, we have developed a short procedure for the prep-
aration of a new helically chiral hexacyclic phosphine from readily
available and inexpensive materials. We accomplished the synthe-
sis of phosphine 6 in five steps, with an overall yield of 23%. This
trivalent phosphine was found to be a useful ligand for the prepa-
ration of a highly stable helical ruthenium complex which may
possess interesting catalytic activity. Further work in this field is
in progress.
13. (a) El Abed, R.; Ben Hassine, B.; Genêt, G.-P.; Gorsane, M.; Marinetti, A. Eur. J.
}
Org. Chem. 2004, 1517–1522; (b) Herrmann, W. A.; Brossmer, C.; Ofele, K.;
Reisinger, C. P.; Priermeir, T.; Beller, M.; Fisher, H. Angew. Chem., Int. Ed. Engl.
1995, 34, 1844–1848.
14. Selected data for the diarylethene 2: colourless solid, showing
a violet
fluorescence when dissolved; Rf = 0.52 (cyclohexane/EtOAc, 98:2); mp = 158–
160 °C; 1H NMR (500 MHz, CDCl3): d (ppm): 7.27 (d, J = 16.5 Hz, 1H, Hvinyl),
7.44 (d, J = 16.5 Hz, 1H, Hvinyl), 7.50–7.56 (m, 5H), 7.66 (t, J = 7.5 Hz, 1H), 7.83
(d, J = 8.5 Hz, 1H), 7.86 (d, J = 9 Hz, 1H), 7.90 (d, J = 8.5 Hz, 1H), 8.00 (d, J = 8 Hz,
1H), 8.03 (d, J = 8 Hz, 1H), 8.90 (d, J = 8.5 Hz, 1H), 8.98 (s, 1H); 13C NMR
(75 MHz, CDCl3): d (ppm): 121.18 (CH), 121.57 (C),122.09 (CH), 122.71 (CH),
123.29 (CH), 124.68 (CH), 124.88 (CH), 125.35 (CH), 127.52(CH), 128.12 (2CH),
128.36 (CH), 129.01 (C), 129.86 (CH), 129.92 (CH), 130.95 (C),131.59 (C),
131.88 (2CH), 135.69 (C), 136.25 (C), 136.63 (C), 139.18 (C), 139.85 (C).
15. Liu, L.; Yang, B.; Katz, T. J.; Poindexter, M. K. J. Org. Chem. 1991, 56, 3769–3775.
16. Selected data for 2-bromothiahexahelicene 3: pale yellow solid, showing a violet
fluorescence when dissolved; Rf = 0.54 (cyclohexane/EtOAc, 98:2); mp = 196–
198 °C; 1H NMR (500 MHz, CDCl3): d (ppm): 6.84–7.12 (m, 2H), 7.35–7.39 (m,
1H), 7.54 (dd, J1 = 8.5 Hz, J2 = 1.5 Hz, 1H, H-2), 7.86 (d, J = 8.5 Hz, 1H), 7.89 (d,
J = 8.5 Hz, 1H), 7.94–7.99 (m, 3H), 8.01 (d, J = 9 Hz, 1H), 8.03 (d, J = 9 Hz, 1H),
8.09 (d, J = 1.5 Hz, 1H, H-1), 8.12 (d, J = 8.5 Hz, 1H); 13C NMR (75 MHz, CDCl3): d
(ppm): 120.12 (C), 121.78 (CH), 122.36 (CH), 122.58 (CH), 124.45 (C), 125.64
(CH), 125.78 (CH), 126.16 (CH), 126.67 (CH), 126.98 (CH), 127.56 (CH), 128.22
(CH), 128.79 (CH), 129.18 (CH), 129.87 (C), 131.03 (C), 131.23 (C), 131.33 (CH),
131.60 (C), 131.63 (C), 132.59 (C), 135.68 (C), 138.51 (C), 139.99 (C); ESI-MS:
m/z = 411.99 [M+]; Anal. Calcd for C24H13BrS: C, 69.74; H, 3.17. Found: C, 69.67;
H, 3.15.
17. Crystal data for compound 3 (C24H13BrS) were recorded on a Bruker SMART
CCD diffractometer, M = 413.31, monoclinic, space group P21/n.
a = 13.3335(4) Å, b = 7.2326(2) Å, c = 18.2038(13) Å, V = 1750.90(14) Å3, Z = 4,
Acknowledgement
q
calcd = 1.568 g/cm3, X-ray source Cu K
a, k = 1.54180 Å, T = 293(2) K, measured
The authors are grateful to the DGRS (Direction Générale de la
Recherche Scientifique) of the Tunisian Ministry of Higher Educa-
tion and Scientific Research.
reflections = 6591, independent reflections = 3100, reflections used = 3100,
refinement type = Fmls, parameters refined = 236, R1 = 0.088, wR2 = 0.306.
Crystallographic data for the structure in this paper have been deposited
with the Cambridge Crystallographic Data Centre as supplementary
publication number CCDC 781749. These data can be obtained free of charge
from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK; fax: +44(0) 1223 336033 or e-mail: deposit@ccdc.cam.ac.uk.
References and notes
1. (a) Martin, R. H.; Marchant, M. J. Tetrahedron 1974, 30, 347–349; (b) Yamada,
K.; Nakagawa, H.; Kawazura, H. Bull. Chem. Soc. Jpn 1986, 59, 2429–2432; (c)
Kim, C.; Marks, T. J.; Facchetti, A.; Schiavo, M.; Bossi, A.; Maiorana, S.; Licandro,
E.; Todescato, F.; Toffanin, S.; Muccini, M.; Graiff, C.; Tiripicchio, A. Org. Electron.
2009, 10, 1511–1520; (d) Sahasithiwat, S.; Mophuang, T.; Menbangpung, L.;
Kamtonwong, S.; Sooksimuang, T. Synth. Met. 2010, 160, 1148–1152; (e) Severa,
L.; Adriaenssens, L.; Vávra, J.; Šaman, D.; Císarová, I.; Fiedler, P.; Teply, F.
Tetrahedron 2010, 66, 3537–3552.
2. (a) Verbist, T.; Sioncke, S.; Persoons, A.; Vyklicky, L.; Katz, T. J. Angew. Chem., Int.
Ed. 2002, 41, 3882–3884; (b) Nuckolls, C.; Katz, T. J.; Katz, G.; Collings, P. J.;
Castellanos, L. J. Am. Chem. Soc. 1999, 121, 79–88.
3. (a) Fox, J. M.; Lin, D. J. Org. Chem. 1998, 63, 2031–2038; (b) Dai, Y.; Katz, T. J. J.
Org. Chem. 1997, 62, 1274–1285.
4. (a) Kelly, T. R.; Tellitu, I.; Pérez Sestelo, J. Angew. Chem., Int. Ed. 1997, 36, 1866–
1868; (b) Kelly, T. R.; Silva, R. A.; De Silva, H.; Jasmin, S.; Zhao, Y. J. Am. Chem.
Soc. 2000, 122, 6935–6949.
18. Selected data for the phosphine 6: pale yellow solid, showing
a violet
fluorescence when dissolved; Rf = 0.65 (cyclohexane/EtOAc, 98:2); 1H NMR
(300 MHz, CDCl3): d (ppm): 6.86–6.96 (m, 6H), 7.17–7.29 (m, 7H), 7.45 (td,
J1 = 8 Hz, J2 = 1.5 Hz, 1H), 7.86 (d, J = 8 Hz, 1H), 7.88 (d, J = 8 Hz, 1H), 7.95 (d,
J = 8 Hz, 1H), 7.97–8.01 (m, 5H), 8.04 (d, J = 8 Hz, 1H); 13C NMR (75 MHz,
CDCl3): d (ppm): 121.67 (CH), 122.27 (CH), 122.63 (CH), 124.63 (C), 125.32
(CH), 125.52 (C), 125.72 (CH), 126.01 (CH), 126.66 (CH), 126.87 (CH), 127.57
(CH), 127.68 (CH), 127.75 (d, JC–P = 10.87 Hz, CH), 128.19 (CH), 128.27 (CH),
128.35 (CH), 128.44 (CH), 128.57 (CH), 130.39 (C), 130.43 (d, JC–P = 7.05 Hz, C),
130.29 (d, JC–P = 20.47 Hz, CH), 131.27 (C), 131.61 (C), 132.38 (C), 133.43 (CH),
ˇ
´
133.47 (CH), 133.64 (CH), 133.68 (CH), 133.73 (CH), 133.90 (CH), 134.77 (d, JC–P
=
10.35 Hz, C), 135.77 (C), 136.37 (d, JC–P = 10.35 Hz, C), 136.48 (C), 138.86 (C),
139.48 (C); 31P NMR (121.5 MHz, CDCl3): d (ppm): À3.52 (s); ESI-MS: m/z =
519.1 [M+H]+; HRMS (MALDI-TOF) calcd for C36H23PS [M+H]+.: 519.12581.
Found: 519.12495.
19. Typical experimental procedure for the Ru-complex 9: A solution of phosphine 6
(100 mg, 0.193 mmol) and [(p-cymene)RuCl2]2 (59 mg, 0.096 mmol) in CH2Cl2
(5 mL) was stirred at room temperature under argon. The reaction was
complete in 30 min (TLC) and the solvent was removed under reduced
pressure. The crude product was purified by column chromatography with
CH2Cl2/EtOAc (95:5) as the eluent to give the mononuclear ruthenium complex
9 as an orange-red solid (119 mg, 75%); mp = 197–199 °C; 1H NMR (300 MHz,
5. Dreher, S. D.; Katz, T. J.; Lam, K. C.; Rheingold, A. L. J. Org. Chem. 2000, 65, 815–
822.
6. (a) Kawasaki, T.; Suzuki, K.; Licandro, E.; Bossi, A.; Maiorana, S.; Soai, K.
Tetrahedron: Asymmetry 2006, 17, 2050–2053.
7. Terfort, A.; Görls, H.; Brunner, H. Synthesis 1997, 79–86.
8. Reetz, M. T.; Beuttenmüller, E. W.; Goddart, R. Tetrahedron Lett. 1997, 38, 3211–
3214.

F. Aloui et al. / Tetrahedron Letters 52 (2011) 572–575
575
CDCl3): d (ppm): 0.85 (d, J = 6.9 Hz, 3H, CHMe), 1.01 (d, J = 6.9 Hz, 3H, CHMe),
1.96 (s, 3H, Me), 2.60 (m, 1H, CHMe), 4.46 (d, J = 6 Hz, 1H, CHp-cym), 4.61 (d,
J = 6 Hz, 1H, CHp-cym), 4.87 (d, J = 6.3 Hz, 1H, CHp-cym), 5.18 (d, J = 6 Hz, 1H,
CHp-cym), 6.82 (td, J1 = 8.4 Hz, J2 = 1.2 Hz, 1H), 6.97–7.27 (m, 13H), 7.72 (d,
J = 8.1 Hz, 1H), 7.77 (d, J = 8.4 Hz, 1H), 7.87–7.91 (m, 4H), 7.97 (d, J = 8.4 Hz,
1H), 8.01 (m, 1H), 8.19 (dd, JH–P = 10.8 Hz, J = 1.2 Hz, 1H, H-1); 13C NMR
(75 MHz, CDCl3): d (ppm): 17.99 (Me), 22.27 (Me), 22.37 (Me), 30.61 (CHp-
cym), 88.16 (CHp-cym), 88.26 (CHp-cym), 89.09 (CHp-cym), 89.12 (CHp-cym),
95.48 (Cp-cym), 110.90 (Cp-cym), 122.29 (CH), 122.44 (CH), 122.93 (CH),
124.94 (C), 125.74 (CH), 125.97 (CH), 126.15 (C), 126.30 (C), 126.34 (CH),
126.69 (C), 127.18 (C), 127.33 (CH), 127.81 (CH), 127.94 (CH), 128.03 (CH),
128.16 (2CH), 128.51 (CH), 128.59 (d, JC–P = 12.22 Hz, CH), 129.39 (C), 130.07
(C), 130.20 (C), 130.41 (CH), 130.48 (CH), 130.51 (CH), 130.55 (CH), 131.19 (d,
JC–P = 3.83 Hz, CH), 131.54 (C), 132.23 (CH), 132.42 (CH), 132.94 (C), 134.62 (d,
JC–P = 9.15 Hz, CH), 135.06 (d, JC–P = 9.52 Hz, CH), 136.28 (C), 139.56 (C), 139.89
(C); 31P NMR (121.5 MHz, CDCl3): d (ppm): 27.06 (s); ESI-MS: m/z = 824.1 [M+];
HRMS (MALDI-TOF) calcd for
824.07665.
C
46H37Cl2PRuS [M+]: 824.07741. Found: