Synthesis of 1,4,5,8,9,12-hexabromododecahydrotriphenylene and its application in constructing polycyclic thioaromatics

Article abstract of DOI:10.1039/b904477a

A new route for constructing PAHs from cyclohexanone by trimerization, bromination, trisannulation and aromatization, and the synthesis of a novel sulfur-containing polycyclic heteroaromatic have been described. The Royal Society of Chemistry 2009.

Full text of DOI:10.1039/b904477a

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Synthesis of 1,4,5,8,9,12-hexabromododecahydrotriphenylene and its
application in constructing polycyclic thioaromaticsw
Junfa Wei,* Xiaowei Jia, Jun Yu, Xianying Shi, Congjie Zhang and Zhanguo Chen
Received (in Cambridge, UK) 5th March 2009, Accepted 4th June 2009
First published as an Advance Article on the web 24th June 2009
DOI: 10.1039/b904477a
A new route for constructing PAHs from cyclohexanone by
trimerization, bromination, trisannulation and aromatization,
and the synthesis of a novel sulfur-containing polycyclic
heteroaromatic have been described.
distinguishes itself from the usual methods by absence of high
temperature or FVP. Herein, we describe our route to syn-
thesize a polycyclic pentathioaromatic 8 from cyclohexanone
as well as the structures of 8 and the key intermediate,
all-trans-1,4,5,8,9,12-hexabromododecahydrotriphenylene 7.
The availability of 7 presented an opportunity to explore the
synthesis of various compounds by replacement of bromines
with uni- or divalent groups.
Polycyclic aromatic hydrocarbons (PAHs) have attracted
extensive attention due to their potential applications for
liquid crystalline materials, semiconductors, and organic
photovoltaic cells.1,2 Amongst the PAHs, coronene 1,
sumanene 2, and trithiosumanene 3 are of special interest.3
Triphenylene 4 has the fundamental skeleton of many PAHs,
polycyclic heteroaromatics, graphenes, buckminsterfullerenes,
as well as carbon nanotubes, and thus is expected as a
precursor to construct PAHs and heterocyclic heteroaromatics
via bridging each of the bay regions by a divalent group
between two aromatic rings.
Dodecahydrotriphenylene 6 was prepared by the ZrCl4-
catalyzed trimerization of cyclohexanone. In contrast to the
reported method,5 in which cyclohexanone trimerized in an
autoclave at 200 1C, we found that the trimerization proceeded
smoothly at normal pressure and solvent-free condition. The
best result was obtained when cyclohexanone was refluxed
with 4 mol% ZrC14 as catalyst for 10 h. The reaction can be
simply monitored whenever the reaction mixture is solidified.
After recrystallization in chloroform–ethanol, 6 was obtained
as a white powder in 71% yield.6
The hexabromo intermediate 7 was obtained via the bromi-
nation of 6 with bromine in a water–ice bath while illuminating
with a 300 W incandescent bulb. A glass vessel is preferred for
the ice–water bath due to its good light transmittance, and
liquid bromine was added dropwise over 30–40 min. Under
these conditions, the yield was 87% with good purity.
Elemental and spectral analysesz indicate that 7 is extremely
symmetrical. There are only three signals, appearing at d 6.13
(m, 6 H), 2.71–2.78 (d, 6 H), 2.44–2.47 (d, 6H) and at d 28.26,
46.30, 136.48 in the 1H and 13C NMR spectra, respectively. An
X-ray crystallographic studyy confirms that the molecule of 7
has a surprisingly regular structure with C3d symmetry
(Fig. 1). The six bromo atoms are alternately disposed
‘‘above’’ and ‘‘below’’ the aromatic ring plane, forming an
all-trans stereochemistry. Each of the fused cyclohexane rings
is in approximately half-chair conformation, in which bromine
and hydrogen atoms are separately in pseudo-axial
and -equatorial positions. This all-trans stereoisomer exists
as a pair of enantiomers with all R- or S-configuration
stereogenic centers.7
However, it is difficult to introduce three bridges into the
bay regions of triphenylene simultaneously.4 The Klemm
group reported the formation of 5 by means of hetero-
geneously catalyzed sulfur bridging by hydrogen sulfide at
500–550 1C.4a,b The Imamura group synthesized the triannulated
compound 3 in overall yield of 2% using a different route
based on tribenzannulation of benzotrithiophene by flash
vacuum pyrolysis (FVP) at 1000 1C and 0.005 Torr.3e Moreover,
sumanene 2 was obtained via a route involving tandem
ROM–RCM reaction3a while FVP of tribromomethyltri-
phenylene gives mono- and di-bridged products rather than 2.4c
Since previous attempts to synthesize PAHs through tri-
sannulation of triphenylene with three bridges had been
unsuccessful, we designed a new synthetic strategy for
constructing PAHs from cyclohexanone in a reaction sequence
of tricondensation, bromination, trisannulation and aromati-
zation, and successfully synthesized a novel sulfur-containing
polycyclic heteroaromatic
8 (Scheme 1). Our protocol
School of Chemistry and Materials Science, Shaanxi Normal
University, Xi’an, 710062, P. R. China. E-mail: weijf@snnu.edu.cn;
Fax: +86-029-85307774
w Electronic supplementary information (ESI) available: Experimental
details, spectral data, and X-ray crystallographic data. CCDC 723162
and 723163. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/b904477a
Scheme 1 Reagents and conditions: (i) ZrCl4 4 mol%, reflux, 10 h,
71%; (ii) Br2, CCl4, hn, 0 1C, 87%; (iii) (1) Na2Sꢀ9H2O 450 mol%,
DMSO, 0 1C to rt, 10 h; 98.7%; (2) DDQ 720 mol%, PhCl, reflux,
24 h, 20%.
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Fig. 2 Molecular structure of 8 showing top and side view (thermal
Fig. 1 Molecular structure of 7 showing top and side view (thermal
ellipsoids: 30% probability).
ellipsoids: 30% probability).
Bond alternation (Mills–Nixon effect8) in the central
benzene ring was observed, meaning that the benzene ring
has a cyclohexatriene-like geometry in which the bond lengths
are alternately shorter (endocyclic bonds, average 1.399 A)
and longer (exocyclic bonds, average 1.420 A) with a
DR (Rexo ꢂ Rendo) of 0.021 A.
The stereochemical outcome indicates that the bromination
of 6 was highly stereoselective and exclusively gave the
all-trans isomer 7. We have no definitive hypothesis for this
from the point of mechanistic view, however it may be
assumed that undesired isomers, i.e., a di-trans isomer, formed
by the random collision of bromine atoms, would transform
into the all-trans isomer via a mechanism of reversible free
radical substitution (see ESIw). The fact that 7 readily decomposes
at 105 1C to form triphenylene 4 provides support for this
assumption.9
Fig. 3 Packing diagram structure of 8: views along c- and b-axis
(thermal ellipsoids: 30% probability).
The views along the b- and c-axis show that the packing of 8
is dominated by alternating stacked arrays of molecules tilted
to the column axis. Overlap between intracolumn neighbors
involves approximately four fused rings. Significant face-to-face
interaction in the crystal structure was observed via p–p
stacking interactions between neighboring layers at the rings
1–50, 5–10; 4–60 and 6–40 with core–core separation distances
of 3.518, 3.752, 3.640, and 3.626 A, respectively.
With the intermediate 7 in hand, the trisannulation with
sulfur bridges connecting the adjacent benzylic positions was
explored to access tristhiasumanene 3, a bowl-shaped molecule
resembling sumanene 2 geometrically and coronene 1 iso-
electronically. The addition of hexabromide 7 into a stirred
mixture of Na2Sꢀ9H2O in DMSO–H2O at 0 1C with a gradual
temperature raise to room temperature within 10 h resulted in
a pale yellow solid crude product which was not purified owing
to its low solubility in common solvents. Aromatizing this
crude product with DDQ in chlorobenzene at reflux for 24 h
generated a yellow solid in 20% yield. Chromatography with
silica gel and recrystallization in hexane–ethyl acetate gave a
The UV-Vis spectrum of 8 in CH2Cl2 shows a weak
absorption band at lmax 431 nm (log e 3.16), which causes
the yellow coloration for 8. In addition, there are three strong
bands centered at 383 nm (log e 4.01), 348 nm (log e 4.44) and
289 nm (log e 4.56).
In summary, this work has shown that PAHs can be
synthesized via a trisannulation reaction wherein the bridging
agents are introduced concurrently into the three bay regions
of 1,4,5,8,9,12-hexabromododecahydrotriphenylene. To our
knowledge, this is the first report of synthesis of PAHs via
such a simultaneous triannulation reaction. In view of its
simplicity and the mild conditions under which the individual
steps can proceed, it would be found to be a versatile and
convenient route to PAHs. The investigations on syntheses of
the Se- and Te-containing polycyclic heteroaromatics and
other PAHs as well as the substitution of the bromo atoms
by other nucleophiles, i.e., N3ꢂ, SCNꢂ, CNꢂ, etc., are now on
progress.
1
yellow crystalline product 8.z The H NMR spectrum of the
crystalline product shows three peaks at d 7.26 (s, 2 H), 7.75
(d, 2 H, J = 9 Hz), 8.01 (d, 2 H, J = 9 Hz). Elemental and
EI-MS analyses indicate that the product has a molecular
formula of C18H6S5. All these results suggest that there should
be one S bridge and two S–S bridges inserted in the three bay
regions of triphenylene, respectively. The structure of 8 was
verified by X-ray crystallography (Fig. 2 and 3).10y
The authors are grateful to the National Foundation of
Natural Science in China (Grant No. 20572066).
Fig. 2 and 3 show that compound 8 has a flying squirrel-like
structure in which thiophene ring 1, central benzene ring 5,
and benzene rings 4 and 6 around the thiophene ring are
approximately coplanar, while benzene ring 7 is clearly not
coplanar with central ring 5 (see Fig. 2). All of the benzene
rings are not regular hexagons. The six carbon atoms of the
central ring are coplanar within 0.047 A, forming a somewhat
boat-like structure. The disulfur-containing six-membered
rings 2 and 3 are highly deviated from a planar structure with
C–S–S–C dihedral angles of 55.85 and 56.611.
Notes and references
z Selected data for 7: C18H18Br6: colorless plate crystals from
petroleum ether (bp 90–120 1C)–ethyl acetate; mp 4 300 1C; dH
(CDCl3, 300 MHz): 6.13 (m, 6H), 2.75 (d, J = 21 Hz, 6H), 2.46
(d, J = 9.0 Hz, 6H); dC (CDCl3, 75 MHz): 28.26, 46.30, 136.48.
Elemental analysis: calc. for C18H18Br6: C, 30.29; H, 2.54; found: C,
30.17; H, 2.26%.
Selected data for 8: Yellow crystals from ethyl acetate–hexane.
mp 4300 1C. dH (CDCl3, 300 MHz): 7.26 (s, 2H), 7.50 (d, J = 9.0 Hz,
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2H), 8.17 (d, J = 9.0 Hz, 2H); MS (m/z): 381.9 (M+); elemental
analysis: calc. for C18H6S5: C, 56.51; H, 1.58; S, 41.91; found: C, 56.17;
H, 1.62; S, 42.21%.
y Crystal data for 7: C18H18Br6, M = 713.78, tetragonal, space group
I41/a, a = b = 10.724(2), c = 36.099(14) A, V = 4151(2) A3,
T = 296(2) K, Z = 8, m(Mo-Ka) = 0.71073 A, colorless square-plate
crystal, crystal dimensions: 0.25 ꢃ 0.20 ꢃ 0.09 mm, crystal density:
2.284 Mg mꢂ3. Full-matrix least squares based on F2, gave R1 = 0.0413
and H. Sakurai, J. Am. Chem. Soc., 2008, 130, 8592–8593; (d) J. T.
M. van Dijk, A. Hartwijk, A. C. Bleeker, J. Lugtenburg and
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Heterocycl. Chem., 1987, 24, 1749–1755; (b) L. H. Klemm, E. Hall,
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Commun., 1993, 1006–1008.
and wR2 = 0.0926 for 1849 (I Z 2s(I)) observed reflections,
GOF = 1.012 for 110 parameters. CCDC 723162.
Crystal data for 8: C18H6S5: M = 382.53, orthorhombic, space
group: Pnma, a = 6.9134(5), b = 19.5698(14), c = 10.8775(8) A,
V = 1471.66(18) A3, T = 296(2) K, Z = 4, m(Mo-Ka) = 0.71073 A,
block yellow crystal, crystal dimensions: 0.37 ꢃ 0.28 ꢃ 0.15 mm,
crystal density: 1.726 Mg mꢂ3. Full-matrix least squares based on F2,
gave R1 = 0.0418 and wR2 = 0.1207 for 2624 (I Z 2s(I)) observed
reflections, GOF = 1.070 for 208 parameters. CCDC 723163.
5 H. Shirai, N. Amano, Y. Hashimoto, E. Fukui and Y. Ishii, J. Org.
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6 Cyclopentanone, acetophenone and their analogs can also trimerize
to the corresponding products in high yields with our protocol.
7 The B3LYP/3-31g* calculated structure of 7 is in agreement with
that determined by X-ray crystallography. The computational
result also reveals that the all-trans structure is the most stable
structure.
8 For the Mills–Nixon effect, see: (a) T. Rosenau, G. Ebner,
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that determined by X-ray crystallography (see ESIw).
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4716 | Chem. Commun., 2009, 4714–4716