A simple synthesis of truxene, a building block for optoelectronics and fullerene fragments

Article abstract of DOI:10.1016/j.tetlet.2013.11.080

Truxene was efficiently synthesized by reduction of truxenone with excess hydrazine hydrate in diethylene glycol at 180 C without added base, a variation of the Huang-Minlon Wolff-Kishner reduction. The proposed mechanism highlights hydrazine as a nucleophile and a base, extracting protons from the hydrazone and diazene intermediates.

Full text of DOI:10.1016/j.tetlet.2013.11.080

Tetrahedron Letters 55 (2014) 636–638
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Tetrahedron Letters
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A simple synthesis of truxene, a building block for optoelectronics
and fullerene fragments
⇑
Yaacov Netanel Oded, Israel Agranat
Organic Chemistry, Institute of Chemistry, The Hebrew University of Jerusalem, Philadelphia Building #201/205, Edmond Safra Campus, Jerusalem 91904, Israel
a r t i c l e i n f o
a b s t r a c t
Article history:
Truxene was efficiently synthesized by reduction of truxenone with excess hydrazine hydrate in diethyl-
ene glycol at 180 °C without added base, a variation of the Huang-Minlon Wolff–Kishner reduction. The
proposed mechanism highlights hydrazine as a nucleophile and a base, extracting protons from the
hydrazone and diazene intermediates.
Received 1 October 2013
Revised 7 November 2013
Accepted 20 November 2013
Available online 28 November 2013
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Star-shaped PAHs
Fullerene fragments
Optoelectronics
Wolff–Kishner reduction
Huang-Minlon modification
Hydrazine
Truxene (also known as
a-truxene, 10,15-dihydro-5H-tribe-
nzo[a,f,k]trindene,tribenzylenebenzene),
a
C27H18 star-shaped
polycyclic aromatic hydrocarbon (PAH) with C3-symmetry, first
appeared in the chemical literature in 18891,2 (Fig. 1). It was synthe-
sized by the reaction of 4-phenylpropionic acid and phosphorus
pentoxide, and directly from 1-indanone and sulfuric acid–water
(1:1).1,2 These reactions of 1-indanone were in essence aldol-con-
densation cyclotrimerizations. Truxene was later synthesized by
cyclotrimerization of 1H-indene under harsh conditions.3 However,
the reaction had a major deficiency in that it was not regioselective;
truxene was accompanied by its constitutional isomer isotruxene
(also known as b-truxene, 14,15-dihydro-9H-tribenzo[a,f,l]trind-
ene)3,4 (Fig. 1). For example, heating 1H-indene at 300 °C gave trux-
ene and isotruxene in 9% and 5% yields, respectively.3 The
conversion of 3-phenylpropionic acid into truxene was improved
by using polyphosphoric acid. Truxene was obtained in 54% yield.
However, it was accompanied (38% yield) by the intermediate 2-
(indan-1-yliden)-indan-1-one, the aldol-condensation dimeriza-
tion product of 1-indanone.5 Oxidation of truxene2 gave truxenone
(also known as 5H-tribenzo[a,f,k]trindene-5,10,15-trione,tribenzo-
ylenebenzene), a C27H12O3 polycyclic aromatic ketone (PAK) with
C
3h symmetry6,7 (Fig. 1). The synthesis of truxenone by the acid-cat-
alyzed cyclotrimerization of 1,3-indandione6,7 afforded truxenone
in 90% yield,7 without any contamination by its constitutional
⇑
Corresponding author. Tel.: +972 2 6585221; fax: +972 2 6511907.
E-mail address: isri.agranat@mail.huji.ac.il (I. Agranat).
Figure 1. Structures of truxene, isotruxene, and truxenone.
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.11.080

Y. N. Oded, I. Agranat / Tetrahedron Letters 55 (2014) 636–638
637
isomer, isotruxenone (also known as 9H-tribenzo[a,f,l]trindene-
9,14,15-trione).8
allowed the determination of the truxene:truxenone ratio, based
on the 4.29 ppm singlet (truxene) and the 9.31 ppm doublet (tru-
xenone). There were no indications for the formation of truxenone
hydrazone, truxenone trihydrazone, truxenone ketazine, truxe-
none azine, or a truxene dimer (e.g., bitruxenylidene). At 140 °C
(40-fold excess of hydrazine hydrate, 65 h), the yield of truxene
was only 17%. At 160 °C (24 h), 190 °C (24 h), and 220 °C (30 h)
(40-fold excess of hydrazine hydrate), the yields of truxene were
52%, 80%, and 83%, respectively. The attempted reactions of truxe-
none with hydrazine hydrate in boiling ethanol, 1-butanol, or ace-
tic acid afforded only starting truxenone.
The successful carbonyl to methylene transformation, using
hydrazine hydrate in diethylene glycol solution at 180 °C without
an added base is presumably due to the ability of hydrazine to
act as a base (pKa = 8.1)21 toward the acidic hydrogen atoms of
the hydrazone and diazene intermediates at the high temperature
of the reaction. A proposed mechanism for the variation of the
Huang-Minlon modification is presented in Scheme 1. The first
equivalent of hydrazine acts as a nucleophile to give the hydra-
zone. The additional equivalent of hydrazine acts as a base, first
Truxene has been recognized as a building block and central
core for the construction of materials with promising and diverse
applications: larger polyarenes and bowl-shaped fragments of
the fullerenes, liquid crystals, C3 tripod materials in asymmetric
catalysis and chiral recognition, candidates for blue light-emitting
materials, optoelectronics as active materials in organic field-effect
transistors, acceptors in thin-film organic solar cells, liquid crystals
and photoresistors, thermally stable cathode buffer materials for
organic thin-film solar cells, and water-soluble anionic fluoro-
phores.9–15
We report here a straightforward, simple, and efficient synthe-
sis of truxene, uncontaminated with isotruxene, by a variation of
the Huang-Minlon modification (also known as Huang Minlon
reduction) of the Wolff–Kishner reduction.16,17 We take advantage
of the facile and efficient synthesis of pure truxenone.6,7 Reduction
of truxenone to truxene was affected by hydrazine hydrate in
diethylene glycol at 180 °C, without any added base. In addition,
we offer
a plausible mechanism for a hydrazine-promoted
R1R2C@O ? R1R2CH2 transformation, without promotion by an
added base.
extracting a proton from the NH2 group of the hydrazone
R1R2C@NANH2 to give R1R2C@NANHꢀ, and later extracts a proton
from the intermediate diazene, R1R2CHAN@NH. The resulting
diazene anion R1R2CH-N@Nꢀ is now susceptible to the departure
of the electrofugal NBN leaving group with the concomitant for-
mation of the carbanion R1R2CHꢀ, which is then converted into
the methylene product R1R2CH2. It is interesting to note that
hydrazine does not act as a double-hydride donor in the reduction
process, but by formal donation of its nitrogen electron lone pairs.
The two hydrogen atoms in the resulting methylene product are
donated as protons, not as ‘‘hydrides’’. Only one equivalent plus a
catalytic amount of hydrazine are required. According to the pro-
posed mechanism, three equivalents of hydrazine hydrate are
needed, whereas 40 equivalents were utilized to obtain the
maximum yield. The excess of hydrazine hydrate, bp 114 °C com-
pensates for its loss at the reaction temperature of 180 °C.
It is generally agreed that the Wolff-Kishner reduction involves
the formation of a hydrazone R1R2C@NANH2 (and H2O) in a
manner analogous to the formation of an imine in the reaction be-
tween a carbonyl compound (aldehyde or ketone) and a primary
amine.16 The role of the added base (e.g., OHꢀ) is to extract two
protons, upon heating, first from the NH2 group of the hydrazone
R1R2C@NANH2 and later from the NH group of the diazene inter-
mediate R1R2CHAN@NH.16,18,19 For the Huang-Minlon modifica-
tion, the carbonyl compound and hydrazine hydrate, as well as
the added base are heated in diethylene glycol, and upon comple-
tion of the hydrazone formation, the temperature is raised to
190–200 °C to drive off water and excess hydrazine hydrate.16 It
has been suggested that in the case of fluorenone, a six-fold excess
of hydrazine hydrate is required for its conversion into fluorene,
and that fluorenone ketazine is presumably formed as an interme-
diate, which is then reduced to fluorene by hydrazine.20 A neces-
sary step in these pathways is the departure of the electrofugal
leaving group NBN.
Although it has been taken for granted that an alkaline catalyst
is necessary to promote the Wolff-Kishner reduction,17 cases
where the reductions have been affected at high temperatures in
the absence of a base have been reported, in particular the
In our hands, truxenone6,7 reacted with 40 equivalents of
hydrazine hydrate (13.3 equiv per carbonyl group) in diethylene
glycol at 180 °C for 24 h to give, after a straightforward work-up,
truxene in 85% yield. The structure of truxene was verified by its
mp, and by 1H NMR and 13C NMR spectroscopy. Complete assign-
ments were made through 2-dimensional correlation spectroscopy
(DQF-COSY, HSQC, HMBC, and NOESY). 1H NMR (CDCl3, 500 MHz):
d (ppm) = 4.29 (10H,5H,15H, s, 6H), 7.40 (12H,7H,2H, dt, J1 = 7.4 Hz,
J2 = 1.1 Hz, 3H), 7.50
11H,6H,1H, br d, J = 7.4 Hz, 3H), 7.97 (14H,9H,4H, br d, J = 7.6 Hz,
3H). 13C NMR (CDCl3, 125 MHz): d = 36.57 (10C,5C,15C, 3C), 121.87
(
13H,8H,3H, br t, J = 7.5 Hz, 3H), 7.70
(
(
14C,9C,4C, 3C), 125.10 11C,6C,1C, 3C), 126.28 12C,7C,2C, 3C),
( (
126.92 (13C,8C,3C, 3C), 135.30 (9cC,4cC,14cC, 3C), 137.13 (14bC,9bC,4bC,
3C), 141.71 (14aC,9aC,4aC, 3C), 143.62 (10aC,5aC,15aC, 3C).
When the reaction was carried out with 3.3 and 10 equiv of
hydrazine hydrate (1.1 and 3.3 equiv per carbonyl group) at
180 °C for 24 h, the yields of truxene were 17% and 73%, respec-
tively. The unreacted truxenone was identified from its NMR spec-
tra: 1H NMR (CDCl3, 500.2 MHz): d (ppm) = 7.58 (13H,8H,3H, td,
1J = 7.4 Hz, 2J = 0.5 Hz, 3H), 7.74
(
12H,7H,2H, td, 1J = 7.7 Hz,
2J = 0.7 Hz, 3H), 7.87 (14H,9H,4H, dq, 1J = 7.4 Hz, 2J = 1.0, 3H), 9.31
(
11H,6H,1H, td, 1J = 7.7 Hz, 2J = 0.9 Hz, 3H). 13C NMR (CDCl3,
125.78 MHz): d = 123.98 (14C,9C,4C, 3C), 128.70 (11C,6C,1C, 3C),
129.64 (14bC,9bC,4bC, 3C), 131.77 (13C,8C,3C, 3C), 135.50 (12C,7C,2C,
3C), 136.00 (14aC,9aC,4aC, 3C), 141.59 (10aC,5aC,15aC, 3C), 147.72
Scheme 1. Proposed mechanism of the Huang-Minlon reduction without added
base.
(
9cC,4cC,14cC, 3C), 191.98 (10C,5C,15C, 3C). The 1H NMR spectra also

638
Y. N. Oded, I. Agranat / Tetrahedron Letters 55 (2014) 636–638
reduction of 9H-fluoren-9-one to 9H-fluorene.17,20,22,23 An interest-
ing variation of this motif, including the departure of the NBN
leaving group, is the intramolecular reaction of 1,10-biphenyl-
2,20-dicarboxaldehyde with hydrazine in boiling acetic acid which
gave phenanthrene.24 Likewise, 1,4,5,8-tetrabenzoylnaphthalene
underwent a double intramolecular cyclization under the Huang-
Minlon modification procedure to give 1,4,7,8-tetraphenylace-
The CH2Cl2 was removed by evaporation under reduced pressure.
The resulting crude product (0.821 g) consisted mostly of truxene.
The solid contained only truxene (0.020 g). The crude product was
triturated with hot acetone to give pure truxene in 85% yield; mp
26
368–369 °C (dec) [lit., mp 369–370 °C (dec)].
References and notes
25
naphtho[5,6-de][1,2]diazepine.
1. Hauausmann, J. Ber. Dtsch. Chem. Ges. 1889, 22, 2019.
2. Kipping, F. S. J. Chem. Soc. 1894, 269.
3. Dietzel, E. U.S. Patent 2,216,001, 1940; Chem. Abstr. 1941, 35, 4584.
4. Lang, K. F.; Zander, M.; Teiling, E. A. Chem. Ber. 1960, 93, 321.
5. Merz, G. Synthesis 1972, 614.
6. Loguercio, D. Jr. ‘‘Studies toward a convergent synthesis of C60’’, Ph.D. Thesis,
UCLA, Los Angeles, CA, 1988 Diss. Abstr. Int. B. 1988, 49.1796.
7. Dehmlow, E. V.; Kelle, T. Synth. Commun. 1997, 27, 2021.
8. Yang, J. S.; Huangm, H.-H.; Lin, S.-H. J. Org. Chem. 2009, 74, 3974.
9. Zhang, H.; Wu, D.; Liu, S. H.; Yin, J. Curr. Org. Chem. 2012, 16, 2124.
10. Gomez-Lor, B.; de Frutos, O.; Ceballos, P. A.; Granier, T.; Echavarren, A. M. Eur. J.
Org. Chem. 2001, 2107.
In conclusion, application of the Huang-Minlon reduction pro-
cedure to the reduction of truxenone with excess hydrazine, but
without added base, provides a simple and efficient synthesis of
pure truxene in excellent yields. A mechanism for the Huang-
Minlon reduction is proposed in which hydrazine acts not only
as a nucleophile, but also as a base. The application of the
Huang-Minlon reduction without added base is expected to be
useful for the reduction of aldehydes and ketones, which are
sensitive to the added base.
11. Cao, X.-Y.; Zhang, W.; Zi, H.; Pei, J. Org. Lett. 2004, 6, 4845.
12. Amsharov, K.; Jansen, M. Chem. Commun. 2009, 2691.
13. Tsuji, H.; Ota, Y.; Furukawa, S.; Mitsui, C.; Sato, Y.; Nakamura, E. Asian J. Org.
Chem. 2012, 1, 34.
14. Earmrattana, N.; Sukwattanasinitt, M.; Rashatasakhon, P. Dyes Pigm. 2012, 93,
1428.
15. Bisoyi, H. K.; Kumar, S. Chem. Soc. Rev. 2010, 39, 264.
16. Zang, W. In Comprehensive Organic Name Reactions and Reagents; Li, J., Ed.;
Wiley: New York, 2009; pp 3084–3089. Vol. 3.
Experimental
Truxenone (1.10 g, 2.86 mmol)6 in diethylene glycol (100 ml)
was added to a round-bottom flask, containing a magnetic stir
bar, a dropping funnel and a Y adaptor. The mixture was heated
to 180 °C. When all the truxenone has dissolved, a solution of
hydrazine hydrate (3.6 ml, 114 mmol) in diethylene glycol
(23 ml) was added dropwise with stirring. The original yellow–
greenish solution turned gradually brown–yellow. The mixture
was stirred at 180 °C for 24 h and then poured into H2O (500 ml)
and stirred overnight. CH2Cl2 (100 ml) was then added with stir-
ring. After 6 h, the resulting two liquid phases and solid were sep-
arated, first by filtration to give the solid, followed by separation of
the organic and aqueous layers. The aqueous fraction was ex-
tracted with CH2Cl2 (1 ꢁ 100 ml) and the combined organic frac-
tions were washed with H2O (2 ꢁ 100 ml) and dried (MgSO4).
17. Todd, D. Org. React. 1948, 4, 378.
18. Wolff-Kishner reduction, Name reactions, organic chemistry portal, http://
www.organic-chemistry.org/namedreactions/wolff-kishner-reduction.shtm
[last accessed 28/08/2013].
19. Szmant, H. H Angew. Chem., Int. Ed. 1968, 7, 120.
20. Weisburger, J. H.; Grantham, P. H. J. Org. Chem. 1956, 21, 1160.
21. Hall, H. K., Jr. J. Am. Chem. Soc. 1957, 79, 5441.
22. Staudinger, H.; Kupfer, O. Ber. Dtsch. Chem. Ges. 1911, 44, 2197.
23. Borsche, W.; Sinn, F. Justus Liebigs Ann. Chem. 1937, 532, 146.
24. Bacon, R. G. R.; Lindsay, W. S. J. Chem. Soc. 1958, 1375.
25. Bergmann, E. D.; Agranat, I. Tetrahedron 1966, 135. Supplement 8P1.
26. Stobbe, H.; Zschoch, F. Ber. Dtsch. Chem. Ges. 1927, 60, 457.