Synthesis and characterization of new hexahelicene derivatives

Article abstract of DOI:10.1016/j.tetlet.2007.01.058

The synthesis and structural characterization of a new hexahelicene derivative 2 is reported. This compound features a new type of structure bearing a bromine atom and a methoxy group. Suitable crystals of the latter indicate that its conformation closely

Full text of DOI:10.1016/j.tetlet.2007.01.058

Tetrahedron Letters 48 (2007) 2017–2020
Synthesis and characterization of new hexahelicene derivatives
a
a
b
a,
*
´
´
Faouzi Aloui, Riadh El Abed, Angela Marinetti and Bechir Ben Hassine
a
`
´
Laboratoire de Synthese Organique Asymetrique et Catalyse Homogene (O1UR1201), Faculte des Sciences,
´
Avenue de l’environnement, 5019 Monastir, Tunisia
`
bInstitut de Chimie des Substances Naturelles, CNRS UPR 2301, Avenue de la Terrasse, 91198 Gif-sur-Yvette Cedex, France
Received 23 November 2006; revised 4 January 2007; accepted 10 January 2007
Available online 14 January 2007
Abstract—The synthesis and structural characterization of a new hexahelicene derivative 2 is reported. This compound features a
new type of structure bearing a bromine atom and a methoxy group. Suitable crystals of the latter indicate that its conformation
closely resembles that of the unsubstituted [6]helicene, whose idealized symmetry is C2. This 3-bromo-14-methoxyhexahelicene was
subjected to lithiation/phosphinilation and yielded the new 3-methoxy-14-(diphenylphosphino)hexahelicene 3.
ꢀ 2007 Elsevier Ltd. All rights reserved.
Helicenes constitute a class of polycyclic aromatic mole-
cules derived from phenanthrene, in which the extra
ortho-condensed rings give rise to a regular cylindrical
helix due to the repulsive steric overlap of the terminal
aromatic nuclei. These organic molecules present left-
OMe
PPh2
PPh2
R
and right-handed chiral helical structures of M and P
configuration, respectively.1 They have been intensively
studied owing to their chiroptical and nonlinear optical
properties.2 Furthermore, enantiomerically enriched
helicenes have been used as chiral catalysts3 and ligands4
in asymmetric syntheses.
1
2: R= Br
3: R= PPh2
Figure 1.
In the hexahelicene series, derivatives usually have
substituents in positions 1, 2 and 15, 16 of the helicene
skeleton.5 To our knowledge, only one monodentate
phosphine and one bidentate phosphine have been
previously described in these series.
We have already described the syntheses of 3,14-di-
methoxy[6]helicene and 3,14-dihydroxy[6]helicene and
we have demonstrated that chelation of transition
metals may be possible by these two bidentate deriva-
tives.8 We report here the syntheses and the crystal
structure analysis of helicene 2 prepared via a Heck
reaction followed by photocyclodehydrogenation.
In an independent study, Reetz et al. described the
synthesis of 2,15-bis(diphenylphosphino)hexahelicene 1
in an enantiomerically pure form (Fig. 1).4 Its use as a
helical ligand in Pd-allylic substitution confirms that it
behaves as a monodentate ligand.6 Teply´ et al. prepared
3-(diphenylphosphino)hexahelicene, but they did not
report a resolution or its use in catalysis.7
The Heck reaction of 2-bromo-6-methoxynaphthalene 6
(2.0 g, 8.4 mmol) with 4-bromostyrene 7 (1.65 mL,
12.6 mmol) in the presence of sodium acetate and
Hermann’s catalyst in N,N-dimethylacetamide (20 mL)
afforded diarylethene 8 in 42% yield.9a,b The latter was
irradiated with a 150 W high-pressure mercury lamp,
on a 500 mg scale, to give 2-bromo-10-methoxy-
benzo[c]phenanthrene 9 in 70% yield (Scheme 1).10
Keywords: Hexahelicene; Heck reaction; Photodehydrocyclization;
Helical phosphine; Helical alcohol.
*
The Heck coupling9a,b of benzo[c]phenanthrene deriva-
tive 9 (1.0 g, 2.9 mmol) with 3-bromostyrene (1.5 equiv),
Corresponding author. Tel.: +216 73500279; fax: +216 73500278;
e-mail: bechirbenhassine@yahoo.fr
0040-4039/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.01.058

2018
F. Aloui et al. / Tetrahedron Letters 48 (2007) 2017–2020
OCH3
OCH3
Br
(b)
(a)
+
Br
1.5 h
CH3O
Br
Br
6
7
8
9
OCH3 Br
OCH3
OCH3 PPh2
Br
Br
(c)
(b)
2 h
(a)
10
2
3
(e)
(d)
OH
Br
OCH3 P(O)Ph2
5
4
Scheme 1. Reagents and conditions: (a) Hermann’s catalyst (1%), NaOAc (1.1 equiv), N,N-DMA, 140 ꢁC, 48 h, 42% for 8, 65% for 10; (b) hm, I2
(1.1 equiv), propylene oxide (excess), cyclohexane, 70% for 9, 60% for 2; (c) n-BuLi (1.1 equiv), THF, À78 ꢁC, 1.5 h, then ClPPh2 (1.2 equiv), À78 ꢁC
to rt, 60%; (d) BBr3 (1.5 equiv), CH2Cl2, 0 ꢁC, 3 h, 94%; (e) H2O2, CH2Cl2, rt, 30 min, 98%.
using 1% of Hermann’s catalyst, produced the coupled
for about 120 min, on a 200 mg scale in the presence
of a stoichiometric amount of iodine and an excess of
propylene oxide10 and afforded 3-bromo-14-methoxy-
[6]helicene 214 in 60% yield (Scheme 1).
product 10 (65% yield) possessing the E-stereochemistry
1
at the double bond, based on the H NMR spectrum.
The photolysis of 10 was performed in cyclohexane,
˚
Figure 2. Crystal structure of ( )-2: ORTEP drawing, selected dihedral angles (ꢁ) and nonbonding distances (A).

F. Aloui et al. / Tetrahedron Letters 48 (2007) 2017–2020
2019
Chem. 2003, 68, 8539–8544; (b) Wachman, C.; Weber, E.;
Czugler, M.; Seichler, W. Eur. J. Org. Chem. 2003, 2863–
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Lovinger, A. J.; Turro, N. J. J. Am. Chem. Soc. 2001, 123,
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66, 200–205; (e) Dreher, S. D.; Paruch, K.; Katz, T. J.
J. Org. Chem. 2000, 65, 806–814; (f) Thongpanchang, T.;
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Brown, J. M.; Field, I. P.; Sidebottom, P. J. Tetrahedron
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Krieger, C. Tetrahedron Lett. 1994, 45, 8357–8360.
6. Reetz, M. T.; Sostmann, S. J. Organomet. Chem. 2000,
603, 105–109.
Suitable crystals of compound 2 were obtained as pale-
yellow plates by the slow evaporation of a methylene
chloride solution. The X-ray analysis of [6]helicene 2
was carried out on a single crystal obtained from race-
mic 2 as shown in Figure 2. The torsion angles along
the inner helical rim (C16c–C16b–C16a–C16, C16c–
C16d–C16e–C1, C16e–C16d–C16c–C16b, C16a–C16b–
C16c–C16d), which vary from À11.91ꢁ to À29.02ꢁ, are
also a convenient measure of the helicity and are in
accordance with those observed in other hexahelicenes.
The terminal inner helical torsion angles 1 and 2
[C16c–C16d–C16e–C1 and C16c–C16b–C16a–C16]
show somewhat unequal but relatively small opening
at À15.55ꢁ and À11.91ꢁ.15
The demethylation of 2 using boron tribromide led to 3-
bromo-14-hydroxy[6]helicene 514 in 94% yield (Scheme
1).11
ˇ
´
´
´
ˇ
7. Teply´, F.; Stara, I. G.; Stary, I.; Kollarovic, A.; Saman,
ˇ
D.; Vyskocˇil, S.; Fiedler, P. J. Org. Chem. 2003, 68, 5193–
5197.
The metallation of 3-bromo-14-methoxy[6]helicene 2
was achieved by metal-halogen exchange using n-butyl-
lithium at À78 ꢁC.12 Reaction of the lithiated species
with excess chlorodiphenylphosphine yielded the desired
3-methoxy-14-(diphenylphosphino)[6]helicene 314 in
60% yield, after chromatography under argon. The solid
was stable to brief exposure to air and light.
8. Aloui, F.; El Abed, R.; Guerfel, T.; Ben Hassine, B. Synth.
Commun. 2006, 11, 1557–1567.
9. (a) El Abed, R.; Ben Hassine, B.; Geneˆt, G.-P.; Gorsane,
M.; Marinetti, A. Eur. J. Org. Chem. 2004, 1517, 1522;
(b) Harrmann, W. A.; Brossmer, C.; O}fele, K.; Reisinger,
C.-P.; Priermeir, T.; Beller, M.; Fisher, H. Angew. Chem.,
Int. Ed. Engl. 1995, 34, 1844–1848.
10. Liu, L.; Yang, B.; Katz, T. J.; Poindexter, M. K. J. Org.
Chem. 1991, 56, 3769–3775.
Helical phosphine 3 was converted to its corresponding
phosphine oxide 4, in an excellent yield (98%), using a
35% hydrogen peroxide solution.13 Compound 4 is more
stable than phosphine 3 and could be resolved using
chiral HPLC.
11. Reetz, M. T.; Sostmann, S. Tetrahedron 2001, 57, 2515–
2520.
12. Terfort, A.; Go¨rls, H.; Brunner, H. Synthesis 1997, 79–
86.
13. Fuk Yee, K.; Qingchuan, Y.; Thomas, C. W. M.; Albert,
S. C. C.; Kin Shing, C. J. Org. Chem. 2002, 67, 2769–2777.
14. Selected spectroscopic data: 3-Bromo-14-methoxyhexa-
helicene 2: pale yellow solid; mp = 268–270 ꢁC; Rf 0.29
(cyclohexane/ethyl acetate 98:02); 1H NMR (300 MHz,
CDCl3): d (ppm) 3.88 (s, 3H, OCH3), 6.42 (dd, J = 2.7,
J = 9.3 Hz, 1H, H-15), 6.81 (dd, J = 2.4, J = 9.3 Hz, 1H,
H-2), 6.21 (d, J = 2.7 Hz, 1H, H-13), 7.50 (d, J = 8.7 Hz,
1H, H-16), 7.52 (d, J = 7.8 Hz, 1H, H-1), 7.81 (d,
J = 8.4 Hz, 1-H), 7.87 (d, J = 8.7 Hz, 1-H), 7.92- 7.98
(m, 6H), 8.01 (d, J = 8.1 Hz, 1H); 13C NMR (75 MHz,
CDCl3): d (ppm) 55.6 (OCH3), 107.5 (C-13), 116.2 (C-15),
119.8 (C-3), 123.9 (C), 124.8 (C), 126.3 (CH), 127.0 (CH),
127.1 (CH), 127.5 (CH), 127.6 (CH), 127.8 (CH), 127.9
(CH), 128.0 (CH), 128.1 (C-2), 128.2 (C), 128.4 (C), 128.8
(C), 129.5 (C-16), 129.7 (C-1), 130.0 (CH), 130.6 (C), 131.4
(C), 133.4 (C), 133.7 (2C), 157.6 (C-14); MS (EI):
m/z = 437 [M+]. Anal. Calcd for C27H17BrO: C, 74.15;
H, 3.92. Found: C, 74.10; H, 3.83.
In summary, we have prepared and characterized, in
racemic form, a new class of hexahelicene derivatives
unsymmetrically disubstituted at positions 3 and 14.
3-Methoxy-14-(diphenylphosphino)hexahelicene 3 could
serve as a P–O bidentate ligand in asymmetric synthesis.
Its resolution is under investigation in our laboratory.
References and notes
1. (a) Urbano, A. Angew. Chem. 2003, 115, 4116–4119,
Angew. Chem., Int. Ed. 2003, 42, 3986–3989; (b) Diedrich,
C.; Grimme, S. J. Phys. Chem. A 2003, 107, 2524–2539; (c)
Newmann, M. S.; Lednicer, D. J. Am. Chem. Soc. 1956,
78, 4765.
2. (a) Van Elshocht, S.; Verbiest, T.; Busson, B.; Kauranen,
M.; Snauwaert, J.; Hellemans, L.; Persoons, A.; Nuckolls,
C.; Phillips, K. E.; Kats, T. J. Synth. Met. 2000, 115, 201–
205; (b) Fox, J. M.; Katz, T. J.; Van Elshocht, S.; Verbiest,
T.; Kauranen, M.; Persoons, A.; Thongpanchang, T.;
Krauss, T.; Brus, L. J. Am. Chem. Soc. 1999, 121, 3453–
3459.
3-Methoxy-14-(diphenylphosphino)hexahelicene 3: pale
yellow solid, showing a violet fluorescence when dissolved;
Rf 0.34 (cyclohexane/ethyl acetate 60:40); 1H NMR
(500 MHz, CDCl3): d (ppm) 3.95 (s, 3H, OCH3), 6.53
(dd, J = 3, J = 9 Hz, 1H, H-2), 6.70 (ddd, J = 1.5,
J = 6.5 Hz, J = 9 Hz, 1H, H-15), 7.22 (d, J = 3 Hz, 1H,
H-4), 7.23–7.45 (m, 10H, 2Ph), 7.53 (d, J = 9 Hz, 1H),
7.62 (d, J = 8.5 Hz, 1H), 7.82–7.86 (m, 3H), 7.91
(d, J = 8.5 Hz, 1H), 7.93 (d, J = 8.5 Hz, 1H), 7.94–7.98
(m, 3H), 8.01 (d, J = 8.5 Hz, 1H); 13C NMR (75 MHz,
CDCl3): d (ppm) 55.1 (OCH3), 106.9 (C-4), 115.8 (C-2),
123.8 (C), 124.7 (C), 125.9 (CH), 126.6 (CH), 126.8 (CH),
127.1 (CH), 127.2 (CH), 127.4 (2CH), 127.6 (C), 127.7
(2CH), 127.8 (C), 128.1 (C), 128.3 (C-H), 128.5 (3CH),
128.6 (CH), 128.7 (CH), 129.1 (d, Jcp = 15.4 Hz, CH),
129.4 (CH), 130.1 (d, Jcp = 7.4 Hz, C), 131.5 (C), 131.6
3. Dreher, S. D.; Katz, T. J.; Lam, K. C.; Rheingold, A. L.
J. Org. Chem. 2000, 65, 815–822.
4. (a) Reetz, M. T.; Beuttenmuller, E. W.; Goddart, R.
¨
Tetrahedron Lett. 1997, 38, 3211–3214; (b) Tsuneomi, S.;
Kenta, S.; Emanuela, L.; Alberto, B.; Stefano, M.; Kenso,
S. Tetrahedron: Asymmetry 2006, 17, 2050–2053.
5. (a) Paruch, K.; Vyklicky´, L.; Wang, D. Z.; Katz, T. J.;
Incarvito, C.; Zakharov, L.; Rheingold, A. L. J. Org.

2020
F. Aloui et al. / Tetrahedron Letters 48 (2007) 2017–2020
(C), 131.7 (C), 133.2 (C), 133.3 (CH), 133.6 (CH), 133.7
129.6 (CH), 130.1 (C), 131.2 (C), 133.4 (C), 133.5 (C),
134.0 (C), 155.2 (C-14); MS (EI): m/z = 423 [M+].
(CH), 133.6 (d, Jcp = 22.0 Hz, CH), 134.0 (CH), 134.1 (d,
Jcp = 10.0 Hz, C), 137.0 (d, Jcp = 10.20 Hz, C), 137.5 (d,
Jcp = 10.65 Hz, C), 157.2 (C-14); 31P NMR (202 MHz,
CDCl3): d (ppm)—5.62 (s); MS (ES+): m/z = 543.2
[M+H]+.
15. Crystal data for C27H17BrO were recorded on a Nonis
MACH3/CAD4 diffractometer, M = 437.32, orthorhombic,
˚
˚
˚
Pbca, a = 10.54(7) A, b = 16.563(6) A, c = 22.525(7) A,
3
3
˚
V = 3937.5(3) A , Z = 8, Dc = 1.475 g/cm , X-ray source
Mo Ka (radiation), k = 0.71073 A, F(000) = 1776, T =
˚
3-Bromo-14-hydroxyhexahelicene 5: light-yellow solid;
mp = 230–232 ꢁC; Rf 0.21 (cyclohexane/ethyl acetate
90:10); 1H NMR (300 MHz, acetone-d6): d (ppm) 6.39
(dd, J = 3, J = 9.5 Hz, 1H, H-15), 6.88 (dd, J = 2,
J = 9 Hz, 1H, H-2), 7.29 (d, J = 3 Hz, 1H, H-13), 7.42
(d, J = 9.5 Hz, 1H, H-16), 7.55 (d, J = 9 Hz, 1H, H-1),
7.89 (d, J = 8.5 Hz, 1H), 7.99 (d, J = 8.5 Hz, 1H), 8.00 (d,
J = 8.5 Hz, 1H), 8.05 (d, J = 8.5 Hz, 1H), 8.07–8.15 (m,
5H), 8.59 (s, 1H, OH); 13C NMR (75 MHz, acetone-d6): d
(ppm) 110.3 (C-13), 115.9 (C-15), 118.8 (C-3), 123.1 (C),
123.6 (C), 125.6 (CH), 126.5 (CH), 126.6 (CH), 127.1
(CH), 127.3 (CH), 127.4 (CH), 127.5 (CH), 127.6 (C),
127.7 (2CH), 127.9 (C), 128.5 (C), 128.9 (CH), 129.2 (CH),
293(2) K, yellow plates 0.42 · 0.25 · 0.18 mm. The struc-
ture was solved by direct methods and refined anisotrop-
ically using full-matrix least squares based on F2 to give
R1 = 0.0532, wR2 = 0.1392 for 4285 independent observed
reflexions and 318 parameters. Crystallographic data
(excluding structure factors) for the structure in this paper
have been deposited with the Cambridge Crystallographic
Data Centre as supplementary publication number CCDC
292056. 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).