CL-140182
Received: March 3, 2014 | Accepted: March 25, 2014 | Web Released: April 4, 2014
Efficient Synthetic Photocyclization for Phenacenes Using a Continuous Flow Reactor
Hideki Okamoto,*1 Takamitsu Takane,2 Shin Gohda,3 Yoshihiro Kubozono,1 Kaori Sato,3 Minoru Yamaji,4 and Kyosuke Satake1
1Division of Earth, Life, and Molecular Sciences, Graduate School of Natural Science and Technology,
Okayama University, 3-1-1 Tsushima-Naka, Okayama 700-8350
2Department of Chemistry, Faculty of Science, Okayama University, 3-1-1 Tsushima-Naka, Okayama 700-8350
3Material Science Research Group, NARD Institute, Ltd., 2-6-1 Nishinagasu-cho, Amagasaki, Hyogo 660-0805
4Division of Molecular Science, Faculty of Science and Technology, Gunma University,
1-5-1 Tenjin-cho, Kiryu, Gunma 376-8515
(E-mail: hokamoto@okayama-u.ac.jp)
The continuous flow reaction technique has been applied to
the photocyclization of 1,2-diarylethenes, the so-called Mallory
reaction, to afford phenacenes in high chemical yields and
efficiencies (114-288 mg h¹1). The present technique will allow
us to produce several grams of phenacenes at a time.
continuous flow reaction technique to the Mallory photoreaction.
Flow photoreaction technique has merits to overcome the
drawbacks due to conventional batch photolysis, i.e., shortening
irradiation time, suppressing the secondary photoprocesses, no
limitations in the reaction scale, etc.13 Since very few literature
reports are available for the Mallory photocyclization using a
flow technique,14 we have fabricated a flow photolysis system
according to the paper by Booker-Milburn.15 With the estab-
lished system, various phenacenes have been efficiently pre-
pared. Herein, we report the results of the flow photolysis.
The flow photoreactor was fabricated according to the
literature.15 It consists of a tetrafluoroethylene-hexafluoropro-
pylene copolymer (FEP) tube (2 mm © 10 m), a 450-W high-
pressure Hg arc lamp (Pyrex-filtered, - > 280 nm), and a
ceramic plunger pump (Figure S1, Supporting Information).16
The 1,2-diarylethene substrates used in this study were prepared
by the Wittig reaction between the corresponding arylaldehydes
and (arylmethyl)triphenylphosphonium salts. The E-Z mixtures
thus obtained were used for the photocyclization without
separation because the E-diarylethene, which was unfavorable
for the photocyclization, isomerized to the favorable Z-isomer
under the photolysis conditions.12
We investigated the photocyclization of 2-methylstilbene
(2MS) to 1-methylphenanthrene (1MP) as a pilot reaction to
examine the flow-photolysis conditions because the latter is an
important platform for the synthesis of higher phenacenes.17,18
Figure 1 compares the photolysis results obtained by conven-
tional batch irradiation with those by the flow reaction.
As seen in Figure 1a, upon the batch photolysis of 2MS
(5 mM in cyclohexane) in the presence of I2 (0.1 equiv), 2MS
was consumed almost completely after 8 h of irradiation and
1MP was formed in up to 80% yield.
Figure 1b summarizes the flow reaction results, showing the
2MS consumption and 1MP yields for different residence times.
The residence time implies the period during which the reactant
solution flows through the flow-reactor tube. Thus, it is
equivalent to the irradiation time. Six minutes of irradiation
resulted in complete 2MS consumption and quantitative 1MP
formation. By using the developed flow system, the irradiation
time was drastically shortened, and the 1MP yield was
significantly improved as compared to those in batch photolysis.
For a preparative scale flow reaction, 2.00 g of 2MS was
converted to 1MP (1.83 g, 92%) in 6.7 h. By contrast, for a
batch photolysis of 0.56 g of 2MS with 28 h irradiation, 0.44 g
(76%) of 1MP was obtained after chromatographic separation.
These results clearly show the advantages of the flow reaction
technique over the conventional batch photolysis.
Aromatic molecules with an extended π system have
attracted considerable interest because of their importance from
the aspects of organic electronic materials such as organic
semiconductors.1 In the past two decades, acenes, represented by
pentacene, have been extensively studied as a potential active
layer in organic field-effect transistors (OFETs).2 Acenes with
extended π conjugation are, however, generally unstable on
exposure to oxygen and light.3,4 Thus, it is necessary to develop
a post-pentacene organic material possessing high chemical
stability and excellent OFET performance.
We have demonstrated that picene ([5]phenacene) shows
very high p-channel OFET performance. The field-effect carrier
¹1 5
mobility ® of picene was ca. 1 cm2 V¹1 s
. Additionally,
picene displays superconductivity upon doping with alkali
metals such as potassium.6 Recently, we have found that
fulminene ([6]phenacene) is a suitable material for the active
layer of p-channel OFETs (® = 7.4 cm2 V¹1 s¹1).7 [7]Phenacene
also serves as the active layer of OFETs (® = 0.8 cm2 V¹1 s¹1).8
As phenacenes are quite stable on exposure to air and light, they
are promising organic electronic materials.9 Moreover, concern-
ing the fundamental photophysical behavior, picene displays
fluorescence emission from the second lowest excited singlet
(S2) state in the vapor phase.10 Thus, the photophysical features
of phenacenes are also of interest. In order to promote more
extensive and intensive investigations on the solid-state elec-
tronics and photophysical features of phenacenes, a simple and
efficient synthetic process for supplying them on a practical
scale is highly desired.11
Conventionally, phenacene skeletons have been constructed
via the photocyclization of 1,2-diarylethene (the Mallory photo-
cyclization, Scheme 1).12 Although the Mallory photoreaction is
an excellent procedure for phenacene synthesis, it is necessary to
increase the efficiency. In the present study, we have applied a
hν, I2, O2
Scheme 1. General reaction scheme of the Mallory photo-
cyclization.
© 2014 The Chemical Society of Japan