Thermic conversion of benzene into 6-phenylfulvene with high yield mediated by GaP nanocrystals

Article abstract of DOI:10.1039/b208076a

With GaP nanocrystals being used in a close reaction system, 6-phenylfulvene is successfully obtained via a high yield thermic conversion from benzene, which provides the possibility of applying nanocrystals to mediate organic reactions.

Full text of DOI:10.1039/b208076a

Thermic conversion of benzene into 6-phenylfulvene with high yield
mediated by GaP nanocrystals†
Shanmin Gao, Jun Lu, Yan Zhao, Nan Chen and Yi Xie*
Structure Research Laboratory and Department of Chemistry, University of Science and Technology of
China, Hefei, Anhui 230026, P.R.China. E-mail: yxielab@ustc.edu.cn
Received (in Cambridge, UK) 19th August 2002, Accepted 10th October 2002
First published as an Advance Article on the web 31st October 2002
With GaP nanocrystals being used in a close reaction system,
6-phenylfulvene is successfully obtained via a high yield
thermic conversion from benzene, which provides the
possibility of applying nanocrystals to mediate organic
reactions.
In recent years, research in academia and industry has
increasingly emphasized the search for efficient transforma-
tions of easily available starting materials into complex organic
molecules.1,2 From the green chemistry point of view, it is very
important and even desirable to develop an atom-economy
Scheme 1 Schematic illustration of the atom economic protocol for the
thermic rearrangement and dimerization of benzene induced by GaP
nanocrystals at 450–500 °C.
strategy3–6 to synthesise series of materials in order to reduce
consumption and pollution. Currently, another intensive re-
search topic focuses on nanomaterials, which exhibit many
intriguing and even unexpected properties derived from the
dramatic reduction of the particle size, such as pronounced
quantum size effects,7 large surface and high catalytic activity.8
This gives us an inspiration that nanoparticles may provide a
potential opportunity for the syntheses of organic compounds.
Based on this idea, we firstly explore here the possibility of
nanocrystalline GaP in an organic reaction to induce thermic
rearrangement of benzene, and successfully realize a high yield
conversion of benzene to 6-phenlyfulvene under supercritical
conditions. It is well known that benzene can undertake
photochemical rearrangement easily,9 but its thermic rearrange-
ment has proved very difficult to achieve. To our best
knowledge, this is the first and yet an indirect observation of the
thermic rearrangement of benzene. Nanocrystalline GaP plays
an important role in the thermic conversion of benzene,
supporting our strategy that the use of inorganic nanoparticles
would favor some organic reactions and thus open a new
application field of inorganic nanoparticles.
Fulvenes are non-aromatic cyclic polyolefins, which undergo
the gamut of radical, electrophilic, nucleophilic and concerted
reactions.10 and they also serve as excellent models in judging
electronic theories of organic reactivity. Over the past several
decades, there were many reports on the syntheses of fulvenes
and their corresponding electronic theories of organic re-
activity.11–14 However, little research touched upon the lumi-
nescence properties of these compounds mainly because of the
low yield and the difficulties in isolation from the complicated
reaction systems in previously reported methods. In this
communication, a novel atom-economic reaction is established
to prepare 6-phenylfulvene from benzene in the presence of
GaP nanocrystals, and the luminescence properties of this
compound are investigated. The diagram of the preparation
process is illustrated in Scheme 1.
resulting in an ashen precipitate with mass of 70.35 g. Products
were dissolved in chloroform and the solution was filtered to
remove the GaP powder. The final products with a mass of
69.70 g were obtained after vaporizing the chloroform in the
solution. According to the mass of original reagent, benzene, the
yield of the final product was 99%.
The structure of the product was analyzed by mass spectros-
1
copy, H and 13C NMR, IR and UV-vis absorption spectros-
copy, the analytical data are listed in Table 1. The mass spectra
were performed with a GC-TOF Mass Spectrometer. The NMR
studies were carried out using a General Electric QE 300 (7.05
T). The IR spectra were recorded in the wavenumber range of
4000–350 cm21 with a Nicolet Model 759 Fourier transform
infrared (FTIR) spectrometer, using a KBr wafer, and the UV–
vis absorption spectra were performed on a UV–vis spec-
trophotometer (Specord 200) in the wavelength range 190–1100
nm (Analytic Jena GmbH, Germany). The morphology of the
product and GaP nanocrystals, were observed using scanning
electronic microscopy (SEM) which was performed on an X-
650 scanning electron micro-analyzer, and TEM measurements
which were made on a Hitachi H-800 transmission electron
microscope with accelerating voltage of 200 kV, respectively. A
Hitachi 850 fluorescence spectrometer with a Xe lamp at 25 °C
(lex = 320 nm) was used to measure the fluorescence properties
of 6-phenylfulvene. The MS16 spectrum showed that the
molecular formula of the product was C12H10. The other data
Table 1 Characterization of the product
UV
lmax/nm
IR (cm21) (CCl4/CS2) (C6H12
1H NMR
d (ppm)
13C NMR
d (ppm)
)
Details of a typical experiment are as follows: 0.5 g newly-
prepared GaP nanocrystals which were synthesized in the
procedure reported in our previous work15 were added into a
stainless steel autoclave with 100 ml capacity, then 80 ml (70.32
g) analytical grade benzene was added into the autoclave to 75%
of the total volume. The autoclave was maintained at 450–500
°C for 8–10 h and then naturally cooled to room temperature,
H-C(1) 6.70
H-C(2) 6.64
H-C(3) 6.48
H-C(4) 6.32
H-C(6) 7.20
C(1) 120.3
C(2) 136.0
C(3) 130.9
C(4) 128.2
C(5) 145.0
1625s
923m
900s
882w–m
833w–m
761s
230
237
318
374
1492m-s
1472m–s
1445m
1397m
1338m
H-C(8- 7.0–7.7 12) C(6) 138.4
710m–s
1086w–m 690s
1080m 620s
1028w-m 601m
988m 482m
† Electronic supplementary information (ESI) available: MS and 13C NMR
spectra of the as-prepared products. See http://www.rsc.org/suppdata/cc/b2/
b208076a/
(CDCl3)
(CDCl3)
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This journal is © The Royal Society of Chemistry 2002

Fig. 1 The mechanism for thermic rearrangement and dimerization of
benzene catalyzed by GaP nanocrystals at 450–500 °C.
were all in accord with the literature for the molecular structure
of this compound.17–19
The yield of this approach for 6-phenylfulvene was assessed
to be roughly 100%, according to the following facts: (1) From
the experimental data of the mass of reagents and products, the
transformation rate of benzene to 6-phenylfulvene is near to
100%. (2) Spectral analysis (Table 1) showed that the ashen
precipitate left was qualitatively pure 6-phenylfulvene. This
high yield makes it possible to investigate the luminescence
properties of this organic compound without a complicated
isolation procedure.
From the point of view of the reactant and the end product,
the reaction should undergo a process of dimerization of
benzene and thermic rearrangement of intermediates. It is well
known, however, that benzene was hardly subjected to thermic-
rearrangement, because it is a ground-state rearrangement.
Therefore, why did benzene achieve thermic rearrangement
under the current conditions? The primary reason must be that
nanocrystalline GaP plays a crucial role in the procedure. The
mechanism is not clear yet, although a relatively reasonable
mechanism can be proposed for the thermic rearrangement of
benzene, as shown in Fig. 1. The whole reaction may experience
three steps. Step (I): benzene molecules are chemically
adsorbed onto the surface of GaP nanocrystals and undergo
dimerization successively at the temperature of 450–500 °C to
form compound 1. Step (II): compound 1 transforms into
compound 2 since it is unstable compared with compound 2;
which has been proved by their free enthalpies through
theoretical calculation using Gaussian 98. Step (III): based on a
similar theoretical calculation, compound 2 would further
transform into the end product, 6-phenylfulvene, via elimina-
tion of two hydrogen atoms. Due to the larger surface and higher
catalytic activity of the newly-prepared GaP nanocrystals, the
whole reaction would occur in rapid reaction steps. Thus, this
explains why the yield was high enough to form an atom-
economy approach for 6-phenylfulvene. Control experiments
were carried out to probe the role of GaP nanocrystals. Keeping
other conditions unchanged, nanocrystals of GaAs, InP, CdS,
were used respectively instead of GaP nanocrystals in the
system. No reaction was observed, which suggested the novel
effect of GaP nanocrystals. The TEM images of the catalysis
nanocrystals before and after the reaction are shown in Fig. 2A
and B, respectively, from which one can clearly see that the
surface of GaP was capped with something other than benzene
molecules and their sizes became obviously larger after
reaction.
Fig. 2 (A) and (B), TEM images of GaP nanocrystals before and after
reaction, respectively. (C) SEM image of the as-prepared 6-phenylfulvene.
(D) Photoluminescence (PL) spectrum (with the excitation wavelength 450
nm) of 6-phenylfulvene prepared at 450–500 °C. (E) The photograph of
luminescent phenomenon observed under the irradiation of a UV-lamp.
fulvene, this idea has achieved great success with high yield.
Benzene, a remarkably inexpensive and fertile source, was used
as the raw material, which can greatly reduce the cost, and can
also offer an opportunity for the industrial application of this
important organic luminophor of 6-phenylfulvene.
Notes and references
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The SEM image of the obtained 6-phenylfulvene, as shown in
Fig. 2C, reveals an interesting net-cross morphology. Fig. 2D
shows the photoluminescence (PL) spectra for 6-phenylfulvene
in 2 ppm chloroform solution, which consists of a broad
emission with a maximum at about 565 nm. When fabricated
into thin film, 6-phenylfulvene emits strongly in the yellow–
green region of the spectrum (550–570 nm). This bright yellow–
green luminescence can easily be observed with the unaided eye
under UV excitation (Fig. 2E).
12 H. Kurata, T. Ekinaka, T. Kawase and M. Oda, Tetrahedron Lett., 1993,
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16 For more detailed MS and 13C NMR data, please see ESI†.
17 P. Bonzli, A. Otter and M. Neuenschwander, Helv. Chim. Acta, 1986,
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In conclusion, the application of inorganic nanocrystals in
mediating organic rearrangement reactions is first explored. In
the case of the thermic conversion of benzene into 6-phenyl-
18 H. Bircher and M. Neuenschwander, Helv. Chim. Acta, 1989, 72,
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19 M. Neuenschwander and R. Iseli, Helv. Chim. Acta, 1977, 60, 1061.
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