Synthesis and structures of multifunctionalized helicenes and dehydrohelicenes: An efficient route to construct cyan fluorescent molecules

Article abstract of DOI:10.1002/chem.201001655

Spiral up! A series of multifunctionalized benzo[5]helicene derivatives and a benzo[7]helicene were conveniently synthesized. Moreover, the oxidative cyclodehydrogenation of the benzo[5]helicenes by DDQ in trifluoroacetic acid or trifluoromethanesulfonic acid quantitatively provided the corresponding dehydro[5]helicene derivatives, which showed the cyan fluorescent properties (shown here).

Full text of DOI:10.1002/chem.201001655

COMMUNICATION
DOI: 10.1002/chem.201001655
Synthesis and Structures of Multifunctionalized Helicenes and
DehydroACHTUNGTRENNUNGhelicenes: An Efficient Route to Construct Cyan Fluorescent
Molecules
Jun-Dao Chen,[a, b] Hai-Yan Lu,[b] and Chuan-Feng Chen*[a]
Helicenes, a class of molecules that consist of ortho-fused
aromatic rings with helical chirality,[1] have attracted great
interest owing to their unique structural features and wide
potential applications in chiral materials,[2] biomolecular rec-
ognition,[3] and asymmetric synthesis.[4] Since Newman and
Lednicer reported the first helicene in 1956,[5] various meth-
ods for synthesis of helicenes including successive Diels–
Alder reactions,[6] cyclotrimerizations of acetylenes,[7] carbe-
noid couplings,[8] electrophilic aromatic cyclizations,[9] radical
cyclizations,[10] olefin metathesis,[11] cycloisomerization,[12]
and Friedel–Crafts-type cyclizations[13] have been described.
However, in most cases, complicated precursors, expensive
and sensitive metallic catalysts or harsh reaction conditions
were needed, and the products were usually prepared on a
small scale. In particular, functionalized helicenes were ob-
tained by multistep reactions, and the precursors were gen-
erally difficult to access.[7,11,14] The direct functionalization of
helicenes is undoubtedly the most effective way to prepare
the multifunctionalized helicenes, but few of them have
been reported to date, which to a certain extent restricts the
practical applications of the helicene derivatives.
a thermal cyclodehydogenation of [5]heli
vacuum pyrolysis at 10008C, but under the same conditions,
significant portion of benzo[5]helicene stayed un-
ACHTUNGTRENUNcNG ene by flash
a
changed.[16] Herein, we report 1) a convenient and efficient
synthesis of a dimethoxy-substituted benzo[5]helicene and a
benzo[7]helicene, 2) direct bromination of the benzo[5]heli-
cene and synthesis of a series of multifunctionalized [5]heli-
cenes by the Suzuki–Miyaura cross-coupling reactions, and
3) the further oxidative cyclodehydrogenation of the [5]hel-
icene derivatives to afford a series of multifunctionalized de-
hydro[5]helicene (or naphthoperylene) derivatives with the
cyan fluorescent properties and high quantum yields.
Synthesis of the helicenes is depicted in Scheme 1. Start-
ing from the commercially available 7-methoxy-3,4-dihydro-
naphthalen-1ACHTUNTRGNEUNG(2H)-one 1, the diene 2 was obtained in 60%
yield for two steps. The Diels–Alder reaction between diene
2 and the benzyne from 2-carboxybenzenediazonium chlo-
ride gave the compound 3, which was then dehydronated by
2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) to afford the
benzo[5]helicene 4 in 97% yield. According to a similar ap-
Dehydrohelicenes[15] are compounds in which the two
heliACHTUNGTRENNUNGcal termini of a helicene are connected by a s bond. As
a special class of polycyclic aromatic hydrocarbon (PAH),
they could have the wide potential applications in materials
science. However, few reports on the cyclodehydrogenation
of helicenes were known. Recently, Xue and Scott reported
[a] J.-D. Chen, Prof. C.-F. Chen
Beijing National Laboratory for Molecular Sciences
CAS Key Laboratory of Molecular Recognition and Function
Institute of Chemistry, Chinese Academy of Sciences
Beijing 100190 (P.R. China)
Fax : (+86)10-62554449
E-mail: cchen@iccas.ac.cn
[b] J.-D. Chen, Prof. H.-Y. Lu
Scheme 1. Preparation of benzo[5]helicene 4 and benzo[7]helicene 7:
a) Al (1 equiv), HgCl2, toluene, ethanol, reflux; b) CH3CO2H,
(CH3CO2)2O, reflux; c) 2-carboxybenzene-diazonium chloride (2 equiv),
CH2ClCH2Cl, 2-methyloxirane, reflux; d) DDQ (10 equiv), xylene,
reflux.
Graduate School, Chinese Academy of Sciences
Beijing 100049 (P.R. China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201001655.
Chem. Eur. J. 2010, 16, 11843 – 11846
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Table 1. Suzuki–Miyaura cross-coupling of 8 with various arylboronic
acids.
proach, the benzo[7]helicene 7 was conveniently synthesized
in four steps in 21% total yield starting from 2,3-dihydro-
Entry
Product
Ar
Yield [%]
phenanthren-4ACHTUNGTRENNUNG(1H)-one 5 (Scheme 1).
Method 1[a]
Method 2[b]
It was known that halide-substituted aromatic compounds
can be used as important precursors for the further function-
alization by, for example, Suzuki–Miyaura cross-coupling re-
actions.[17] Thus, we carried out the bromination of the ben-
zo[5]helicene 4 with N-bromosuccinimide (NBS) in di-
chloromethane, which provided the dibromo-substituted 8 in
quantitative yield (Scheme 2). The bromination selectively
1
2
3
4
5
6
7
8
9a
9b
9c
9d
9e
9 f
9g
9h
phenyl
72
46
62
75
51
66
53
49
83
79
70
78
76
73
75
56
4-methoxy-phenyl
4-chloro-phenyl
4-carboxyl-phenyl
4-amino-phenyl
4-formyl-phenyl
4-cyano-phenyl
3-thienyl
[a] Solvent: DME, H2O. [b] Solvent: toluene, ethanol, H2O.
Scholl reaction conditions.[20] As shown in Scheme 3, the
[5]helicene derivatives 9a–h could be selectively cyclodehy-
drogenated by DDQ to give the dehydro[5]helicenes 10a–h
in quantitative yields in both trifluoroacetic acid and tri-
Scheme 2. Bromination of benzo[5]helicene 4 and the Suzuki–Miyaura
cross-coupling reactions of 8 with various arylboronic acids: a) NBS
(2.2 equiv), CH2Cl2, RT; b) Pd
ACHTUNGTERN(NUNG PPh3)4, arylboronic acid (3 equiv), DME,
H2O, K2CO3, reflux; c) Pd(PPh3)4, arylboronic acid (3 equiv), toluene,
AHCTUNGTRENNUNG
ethanol, H2O, K2CO3, reflux.
took place at the 5- and 12-positions of the [5]helicene,
which was proved by its single-crystal structure (Figure 1).[18]
It was found that the dihedral angle of the two twisted
phenyl rings is 30.688, which is smaller than that of its ana-
Scheme 3. Synthesis of dehydro[5]helicene derivatives 10a–h.
fluoromethanesulfonic acid systems. Moreover, it was found
that the dehydrogenation only took 5–30 min; however,
longer reaction time and more oxidant (up to 1.5 equiv
DDQ) were needed for the [5]helicene derivatives with
electron-withdrawing groups in the substituted aromatic
subunits. Under the same conditions, we also found that the
oxidative cyclodehydrogenation of [7]helicene 7 did not
take place, mainly due to the overlapped steric situation of
the molecule. Furthermore, a single crystal of 10a suitable
for X-ray diffraction was obtained by slow evaporation of a
solution of 10a in chloroform and cyclohexane. The crystal
structure[18] (Figure 2a and b) showed that the molecule 10a
was nonplanar because of the two close methoxy groups,
and the dihedral angle of the two twisted phenyl rings is
31.768. It was also found that the two substituted phenyl
groups are noncoplanar, and each of them shows a 62.918 di-
hedral angle to its adjacent phenyl ring in the [5]helicene.
Moreover, the packing pattern of 10a includes both edge-to-
face and face-to-face arrangements (Figure 2c), in which
one molecule faces another while it forms a T-shape with
the edges of two other molecules. The face-to-face arrange-
ment consists of a pair of enantiomers; the interplanar dis-
Figure 1. Top view (left) and side view (right) of crystal structure of 8.
Hydrogen atoms are omitted for clarity.
logue [5]helicene (35.68) without the methoxy groups.[19]
Moreover, the distance between the two oxygen atoms is
4.80 ꢁ, and no intramolecular hydrogen-bonding interac-
tions were found in the molecule. With the [5]helicene de-
rivative 8 in hand, we then examined its Suzuki–Miyaura
cross-coupling reactions in two systems with [PdACHTNUTRGNE(UNG PPh3)4] as
catalyst and K2CO3 as base, which provided a series of 5,12-
disubstituted [5]helicene derivatives 9a–h in middle or good
yield (Table 1). Moreover, the results showed that the reac-
tions in toluene/water/ethanol gave higher yields than those
in DME/water solvent mixtures, which is probably caused
by the favorable solubility of the PAH substrates in toluene.
Furthermore, the oxidative cyclodehydrogenation reac-
tions of the helicenes were carried out under the established
À
tance is 3.37 ꢁ and p–p and C H···p interactions are pres-
ent. The face-to-edge arrangement shows a 70.128 dihedral
angle between the two adjacent molecules, which results in
a herringbone-like packing (Figure 2c).
All dehydro[5]helicene (or naphthoperylene) derivatives
showed good solubility in common organic solvents, such as
11844
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Helical Structures
COMMUNICATION
Table 2. The photophysical properties of dehydrohelicenes 10 in cyclo-
hexane.[a]
Abs [nm]
Em [nm]
F[b]
10a
10b
10c
10d
10e
10 f
10g
10h
436
436
435
432
436
433
433
438
478
479
480
480
478
479
480
481
0.51
0.78
0.45
0.33
0.58
0.40
0.47
0.75
[a] [10]=1ꢂ10À6 m. [b] Perylene as the standard (cyclohexane, 0.98).
cence in cyclohexane. Because of the nonplanarity of the
substituted aromatic rings with the dehydro[5]helicene core,
only small emission wavelength shifts in the spectra were
observed with the changes of the substitution groups. How-
ever, it was found that the quantum yields of the dehydrohe-
licenes differ significantly (Table 2), in which electron-with-
drawing groups generally gave low F values, while electron-
donating groups gave high F values.
In conclusion, we have shown a convenient and efficient
synthesis of a dimethoxy-substituted benzo[5]helicene and a
benzo[7]helicene. By direct bromination of the benzo[5]heli-
cene and then Suzuki–Miyaura cross-coupling reactions, we
have also synthesized a series of multifunctionalized ben-
zo[5]helicene derivatives. Moreover, we found that the con-
venient oxidative cyclodehydrogenation of the [5]helicenes
by DDQ in trifluoroacetic acid or trifluoromethanesulfonic
acid quantitatively provided the corresponding dehydro[5]-
helicene (or naphthoperylene) derivatives, which showed
the cyan fluorescent properties and high quantum yields. We
believe that these easily available, multifunctionalized heli-
cenes and dehydro[5]helicene derivatives will find wide ap-
plications in materials science and supramolecular chemis-
try; further investigations along these lines are in progress
in our laboratory.
Figure 2. a) Top view and b) side view of crystal structure of 10a. c) Her-
ringbone-like packing of 10a viewed along the a axis.
cyclohexane, CH2Cl2, CHCl3, toluene, and so forth. As
shown in Figure 3, the UV/Vis spectrum of dehydro[5]heli-
cene 10a in cyclohexane exhibited the maximum absorption
Experimental Section
Full experimental details and characterization data are given in the Sup-
porting Information.
Figure 3. UV/Vis (dashed line) and emission (solid line) spectra of 10a in
cyclohexane at room temperature. [10a]=1ꢂ10À6 m.
Synthesis of compound 3: A mixture of diene 2 (10 g, 31 mmol) and 2-
carboxybenzenediazonium chloride (11.6 g, 62 mmol) in 1,2-dichloro-
ethane (250 mL) and propylene oxide (20 mL) was refluxed for 6 h. The
reaction mixture was concentrated under reduced pressure, and the resi-
due was recrystallized from ethyl acetate to give pure product 3 (8.7 g,
71%) as white powder.
band at 436 nm, a bathochromic shift of 51 nm compared to
benzoACHTUNGTRENNUNG
[g,h,i]perylene,[21] which is probably due to the larger
Synthesis of the benzo[5]helicene 4: Compound 3 (1.10 g, 2.79 mmol)
and DDQ (6.3 g, 27.9 mmol) were added to dried xylene (50 mL). After
being refluxed for 16 h, the reaction mixture was cooled to room temper-
ature, filtered, and then washed with CH2Cl2 (10 mLꢂ3). The solvent
was removed from the dark red filtrate under reduced pressure. The resi-
due was subjected to flash column chromatography with
CH2Cl2:petroleum ether (3:1) as eluent to afford 4 (1.05 g, 97%) as pale
yellow solid.
conjugated p system and the electron-donating effect of me-
thoxy groups in 10a. In the fluorescent spectrum, the maxi-
mum emission wavelength of 10a was observed at 478 nm,
which was consistent with cyan fluorescence. Under the
same conditions, the photophysical properties of the other
dehydro[5]helicenes were also investigated, and the results
were summarized in Table 2. Similar to 10a, dehydroheli-
cenes 10b–h all showed the strong dazzling cyan fluores-
General procedure for oxidative dehydrogenation of the [5]helicene de-
rivatives 9: Trifluoroacetic acid or trifluoromethanesulfonic acid (1 mL)
Chem. Eur. J. 2010, 16, 11843 – 11846
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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C.-F. Chen et al.
was added by a syringe to a mixture of the [5]helicene 9 (0.096 mmol)
and DDQ (0.096 mmol for 9a, 9b, 9e and 9h; 0.144 mmol for 9c, 9d, 9 f
and 9g) in dry CH2Cl2 (10 mL) at 08C under argon atmosphere. The mix-
ture was then stirred and monitored by TLC. After the reaction was com-
pleted, the mixture was quenched by saturated NaHCO3 solution
(20 mL), and then extracted with CH2Cl2 (20 mLꢂ2). The combined or-
ganic layer was dried over anhydrous MgSO4, and concentrated under re-
duced pressure. The residue was subjected to column chromatography to
give the desired dehydro[5]helicene 10 as greenish yellow powder.
d) M. C. CarreÇo, ꢇ. Enrꢄquez, S. Garcꢄa-Cerrada, M. J. Sanz-
Cuesta, A. Urbano, F. Maseras, A. Nonell-Canals, Chem. Eur. J.
2008, 14, 603–620; e) A. Latorre, A. Urbano, M. C. CarreÇo, Chem.
Commun. 2009, 6652–6654.
ˇ
ˇ
[7] a) F. Teply, I. G. Starꢅ, I. Stary, A. Kollꢅroviꢈ, D. Saman, L. Rulisek,
P. Fiedler, J. Am. Chem. Soc. 2002, 124, 9175–9180; b) F. Teply, I. G.
ˇ
ˇ
Starꢅ, I. Stary, A. Kollꢅroviꢈ, D. Lustinec, Z. Krausovꢅ, D. Saman,
P. Fiedler, Eur. J. Org. Chem. 2007, 4244–4250; c) P. Sehnal, I. G.
ˇ
ˇ
ˇ
Starꢅ, D. Saman, M. Tichy, J. Mꢄsek, J. Cvaꢈka, L. Ruꢄsek, J. Cho-
ˇ
cholousovꢅ, J. Vacek, G. Goryl, M. Szymonski, I. Cꢄsaøovꢅ, I. Stary,
Proc. Natl. Acad. Sci. USA 2009, 106, 13169–13174; d) K. Tanaka,
N. Fukawa, T. Suda, K. Noguchi, Angew. Chem. 2009, 121, 5578–
5581; Angew. Chem. Int. Ed. 2009, 48, 5470–5473.
Acknowledgements
[8] a) F. Dubois, M. Gingras, Tetrahedron Lett. 1998, 39, 5039–5040;
b) M. Gingras, F. Dubois, Tetrahedron Lett. 1999, 40, 1309–1312.
[9] G. Pieters, A. Gaucher, D. Prim, J. Marrot, Chem. Commun. 2009,
4827–4828.
[10] a) D. C. Harrowven, M. I. T. Nunn, D. R. Fenwick, Tetrahedron Lett.
2002, 43, 3189–3191; b) D. C. Harrowven, M. I. T. Nunn, D. R. Fen-
wick, Tetrahedron Lett. 2002, 43, 7345–7347; c) D. C. Harrowven,
I. L. Nanson, Angew. Chem. 2006, 118, 2300–2303; Angew. Chem.
Int. Ed. 2006, 45, 2242–2245.
We are grateful to the National Natural Science Foundation of China
(20625206), and the National Basic Research Program of China
(2009ZX09501-018) for financial support.
Keywords: fluorescence · functionalization · helicenes ·
helical structures · synthetic methods
[11] a) S. K. Collins, A. Grandbois, M. P. Vachon, J. Cꢉntꢃ, Angew.
Chem. 2006, 118, 2989–2992; Angew. Chem. Int. Ed. 2006, 45, 2923–
2926; b) A. Grandbois, S. K. Collins, Chem. Eur. J. 2008, 14, 9323–
9329.
[1] For reviews, see: a) A. Urbano, Angew. Chem. 2003, 115, 4116–
4119; Angew. Chem. Int. Ed. 2003, 42, 3986–3989; b) R. H. Martin,
Angew. Chem. 1974, 86, 727–738; Angew. Chem. Int. Ed. Engl.
1974, 13, 649–660.
ˇ
[12] J. Storch, J. Sykora, J. ðermꢅk, J. Karban, I. Cꢄsaøovꢅ, A. Rꢊsiꢈka,
J. Org. Chem. 2009, 74, 3090–3093.
[13] J. Ichikawa, M. Yokota, T. Kudo, S. Umezaki, Angew. Chem. 2008,
120, 4948–4951; Angew. Chem. Int. Ed. 2008, 47, 4870–4873.
[14] a) T. J. Katz, L. Liu, N. D. Willmore, J. M. Fox, A. L. Rheingold, S.
Shi, C. Nuckolls, B. H. Rickman, J. Am. Chem. Soc. 1997, 119,
10054–10063; b) S. D. Dreher, D. J. Weix, T. J. Katz, J. Org. Chem.
1999, 64, 3671–3678; c) K. Paruch, L. Vyklicky, D. Z. Wang, T. J.
Katz, C. Incarvito, L. Zakharov, A. L. Rheingold, J. Org. Chem.
2003, 68, 8539–8544.
[15] a) J. H. Dopper, D. Oudman, H. Wynberg, J. Org. Chem. 1975, 40,
3398–3401; b) K. Yamamoto, T. Harada, M. Nakazaki, J. Am.
Chem. Soc. 1983, 105, 7171–7172; c) K. Yamamoto, Pure Appl.
Chem. 1993, 65, 157–163.
[16] X. Xue, L. T. Scott, Org. Lett. 2007, 9, 3937–3940.
[17] L. Kꢋrti, B. Czakꢆ in Strategic Applications of Named Reactions in
Organic Synthesis (Ed.: J. Hayhurst), Elsevier, Amsterdam, 2005,
pp. 448–449.
[18] CCDC-780282 (8) and 779937 (10a) contain the supplementary crys-
tallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
[2] a) K. E. S. Phillips, T. J. Katz, S. Jockusch, A. J. Lovinger, N. J. Turro,
J. Am. Chem. Soc. 2001, 123, 11899–11907; b) M. A. Shcherbina,
X. B. Zeng, T. Tadjiev, G. Ungar, S. H. Eichhorn, K. E. S. Phillips,
T. J. Katz, Angew. Chem. 2009, 121, 7977–7980; Angew. Chem. Int.
Ed. 2009, 48, 7837–7840; c) S. Graule, M. Rudolph, W. Shen,
J. A. G. Williams, C. Lescop, J. Autschbach, J. Crassous, R. Rꢃau,
Chem. Eur. J. 2010, 16, 5976–6005; d) L. Norel, M. Rudolph, N.
Vanthuyne, J. A. G. Williams, C. Lescop, C. Roussel, J. Autschbach,
J. Crassous, R. Rꢃau, Angew. Chem. 2010, 122, 103–106; Angew.
Chem. Int. Ed. 2010, 49, 99–102.
[3] a) Y. Xu, Y. X. Zhang, H. Sugiyama, T. Umano, H. Osuga, K.
Tanaka, J. Am. Chem. Soc. 2004, 126, 6566–6567; b) K. Shinohara,
Y. Sannohe, S. Kaieda, K. Tanaka, H. Osuga, H. Tahara, Y. Xu, T.
Kawase, T. Bando, H. Sugiyama, J. Am. Chem. Soc. 2010, 132,
3778–3782.
[4] a) I. Sato, R. Yamashima, K. Kadowaki, J. Yamamoto, T. Shibata, K.
Soai, Angew. Chem. 2001, 113, 1130–1132; Angew. Chem. Int. Ed.
2001, 40, 1096–1098; b) S. D. Dreher, T. J. Katz, K.-C. Lam, A. L.
Rheingold, J. Org. Chem. 2000, 65, 815–822; c) M. T. Reetz, S. Sost-
mann, J. Organomet. Chem. 2000, 603, 105–109; d) N. Takenaka,
R. S. Sarangthem, B. Captain, Angew. Chem. 2008, 120, 9854–9856;
Angew. Chem. Int. Ed. 2008, 47, 9708–9710.
[19] D. J. Morrison, T. K. Trefz, W. E. Piers, R. Mcdonald, M. Parvez, J.
Org. Chem. 2005, 70, 5309–5312.
[20] L. Zhai, R. Shukla, R. Rathore, Org. Lett. 2009, 11, 3474–3477.
[21] N. Nijegorodov, R. Mabbs, W. S. Downey, Spectrochim. Acta Part A
2004, 57, 2673–2685.
[5] M. S. Newman, D. Lednicer, J. Am. Chem. Soc. 1956, 78, 4765–
4770.
[6] a) M. C. CarreÇo, S. Garcꢄa-Cerrada, A. Urbano, J. Am. Chem. Soc.
2001, 123, 7929–7929; b) M. C. CarreÇo, S. Garcꢄa-Cerrada, A.
Urbano, Chem. Eur. J. 2003, 9, 4118–4131; c) M. C. CarreÇo, M.
Gonzꢅlez-Lꢆpez, A. Urbano, Chem. Commun. 2005, 611–613;
Received: June 11, 2010
Published online: September 6, 2010
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