3,3′,5,5′-Tetra- tert -butyl-4,4′-diphenoquinone (DPQ)-Air as a New Organic Photocatalytic System: Use in the Oxidative Photocyclization of Stilbenes to Phenacenes

Article abstract of DOI:10.1055/s-0036-1588598

We report an organic photocatalytic system, namely 3,3′,5,5′-tetra-tert-butyl-4,4′-diphenoquinone (DPQ) and air, capable of coupling efficiently with the photocyclization of stilbenes to afford phenacenes. The potential of this new and mild process is shown with the synthesis of [5]- and [7]phenacene, two semiconductors recently implemented into organic electronic devices, with high yields and remarkable purity.

Full text of DOI:10.1055/s-0036-1588598

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© Georg Thieme Verlag Stuttgart · New York
2016, 27, A–E
letter
A
M. Carrera et al.
Letter
Synlett
3,3′,5,5′-Tetra-tert-butyl-4,4′-diphenoquinone (DPQ)-Air as a
New Organic Photocatalytic System: Use in the Oxidative Photo-
cyclization of Stilbenes to Phenacenes
Manuel Carrera
Mónica de la Viuda
Albert Guijarro*
Departamento de Química Orgánica and Instituto Universitario
de Síntesis Orgánica, Unidad Asociada del CSIC, Universidad de
Alicante, Campus de Sant Vicent del Raspeig, Apdo. 99, 03080,
Alicante, Spain
aguijarro@ua.es
Received: 26.03.2016
Accepted after revision: 29.08.2016
Published online: 19.09.2016
The synthesis of phenacenes has often taken advantage
of the oxidative photocyclization of stilbenes as the key
strategy of the process.7 Under UV irradiation, stilbene and
stilbene-like compounds undergo a fast E–Z isomeriza-
tion,8a followed by a singlet state intramolecular photocy-
clization of the Z isomer to give 4a,4b-dihydrophenan-
threnes as transient intermediates (Scheme 1). These inter-
mediates are energetically destabilized respect to the
stilbene precursors, and reversion to reagents, that is, (Z)-
stilbenes, is the main reaction channel at this point.8b But in
the presence of an efficient oxidant [Ox], evolution by dehy-
drogenation towards an even more stable compound, that
is, the aromatic hydrocarbon, results (Scheme 1). This is
one of the most powerful and direct methods to the synthe-
sis of PAH that have at least an armchair junction in the
scaffold, and it has proved to be of great utility in numerous
synthesis of PAH.
DOI: 10.1055/s-0036-1588598; Art ID: st-2016-b0215-l
Abstract We report an organic photocatalytic system, namely
3,3′,5,5′-tetra-tert-butyl-4,4′-diphenoquinone (DPQ) and air, capable
of coupling efficiently with the photocyclization of stilbenes to afford
phenacenes. The potential of this new and mild process is shown with
the synthesis of [5]- and [7]phenacene, two semiconductors recently
implemented into organic electronic devices, with high yields and re-
markable purity.
Key words photocatalysis, solar light, fused-ring aromatic system,
armchair junctions, diphenoquinone, phenacene
The use of organic molecules for electronic devices, or
organic electronics, has raised considerably the attention in
the development of new or improved synthetic methods for
many types of polycyclic aromatic hydrocarbons (PAH).1
Among them, phenacenes have recently entered the arena
of organic semiconductors.2 Phenacenes are armchair-
edged polycyclic aromatic hydrocarbons, more stable than
their zigzag counterparts acenes, with a comparatively larg-
er band gap. Picene, also called [5]phenacene, for instance,
can be used as an active layer of a p-channel organic field-
effect transistor (OFET),3 and so can its larger homologues.4
A different field of application is the superconductivity. Af-
ter intercalation with potassium, picene gave rise to a su-
perconductor material still to be appropriately character-
ized in what might be the first of a new family of PAH-
based superconductors.5 However, recent criticism on these
findings indicates that more efforts should be done in the
research of this topic.6 In order to facilitate the study of all
these materials, simple and easily accessible synthetic
methods should be made available to the researcher.
hν
hν
(E)-stilbenes
(Z)-stilbenes
hν
hν or Δ
[Ox]
H
H
phenanthrenes
(armchair junction)
4a,4b-dihydro-
phenanthrenes
Scheme 1 Typical strategy for the construction of armchair junctions
in PAH. [Ox] represents an oxidant that shifts the overall reaction to-
wards phenanthrene-type final products. The mixture I2 (cat.)/air or
I2/propylene oxide is customarily used, while the system DPQ (cat.)/air
is proposed in this work.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E

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M. Carrera et al.
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There have been two main landmarks in the develop-
ment of synthetically efficient versions of this reaction.
First, it was the introduction of iodine as a suitable oxidant
by Mallory.7 In many cases it could be used catalytically if
regeneration of I2 by dissolved oxygen was efficient enough.
However, for many stilbenes, photooxidative cleavage with
O2 and catalytic I2 is a main reaction outcome, which has
even been exploited as a general synthesis of aromatic alde-
hydes.9 Second, it was the introduction of propylene oxide
(C3H6O) as a HI scavenger by Katz.10 In the oxidation step,
iodine generates HI in the reaction media. HI is a very
strong Brønsted acid, and particularly so in hydrocarbon
solvents, so side reactions often occur and yields of the de-
sired photocyclization may drop. Under Katz’s conditions,
the reactions require an equivalent of iodine that generates
two equivalents of HI, which is removed from the reaction
media with propylene oxide in excess. Many photocycliza-
tions using stoichiometric iodine and propylene oxide af-
ford better yields, and the products were cleaner.11 Still,
propylene oxide is a highly volatile (bp 34 °C), extremely in-
flammable compound with a potential explosion risk that
presents also some health concerns.12 We propose in here
the use of 3,3′,5,5′-tetra-tert-butyl-4,4′-diphenoquinone-
air (DPQ-air) as a new photocatalytic process that can be
coupled to the photocyclization of stilbenes. It represents a
complementary synthetic protocol to the aforementioned
ones that avoids some of their inconveniences.
We considered first the synthesis of picene as a model
reaction for optimizing our photochemical conditions, giv-
en its potential applications in organic electronics and ma-
terials. As noticed by Malavasi,13 picene has usually not
been a readily available chemical. Shortage in commercial
supply and high prices became a problem ever since im-
portant applications such as superconductivity of K3picene
were first reported.5 Among the syntheses that have re-
ceived more attention, the recent photochemical prepara-
tion of picene from 1,2-di-(1-naphthyl)ethane using 9-fluo-
renone as a photosensitizer reported yields of 15–20%.14 In
general, nonphotochemical synthesis of picene rely on rath-
er sophisticated, heavily functionalized starting building
blocks. A Suzuki–Miyaura coupling followed by a double
cyclization involving a palladium-catalyzed C–H function-
alization allowed the synthesis of picene with an overall
yield of 13% for these two final steps.15 Back to the photo-
chemistry, excellent results were obtained by means of the
Mallory reaction,7 which was successfully adapted to flow-
reactor conditions with excellent yields (92%) starting from
1,2-di-(1-naphthyl)ethene (1) and I2.16 The photochemical
precursor of picene in this synthesis, 1, is a yellow com-
pound that has a broad absorption band at λmax = 341.5 nm
tailing down into the visible region (see Supporting Infor-
mation). This fact was elegantly exploited recently in a pi-
cene synthesis using sunlight with 69–72% yields, both
with I2 as catalyst.17 We proposed (E)-1 as a precursor for
the picene synthesis, which is easily obtained using a Horn-
er–Wadsworth–Emmons reaction, and differently from the
Z/E mixtures (obtained by the Wittig reaction),16,17 (E)-1 is
easily purified by recrystallization (see Supporting Infor-
mation). UV irradiation of (E)-1 using a high-pressure Hg
lamp in the presence of air and a suitable catalyst yields pi-
cene. The details of this study are collected in Table 1.
Table 1 Study of the Synthesis of Picene (2) by Oxidative Photocycliza-
tion of (E)-1,2-Di-(1-naphthyl)ethene [(E)-1]a
hν
air,
catalyst
(E)-1
2 (picene)
Entry Catalyst (mol%) Conditions
Time (h) Yield (%)b
1
2
–
–
5
8
38
83
62
93
81
79
81
58
28
11
24
27
69
55
48
54
83
68
53
83
I2 (5)
–
3
DPQ (1)
DPQ (5)
DPQ (5)
DPQ (10)
DPQ (100)
DPQ (5)
DPQ (5)
DPQ (5)
DPQ (5)
DPQ (5)
DPQ (10)
Cu(acac)2 (10)
DQ (5)e
TCQ (5)f
TCQ (100)
DPQ (5)
DPQ (5)
DPQ (5)
–
6
4
–
8
5
multigram scalec
–
17
8
6
7
degassed, under argon
Et3N (10 mol%)
K2CO3 (20 mol%)
in isooctane
in cyclohexane
in toluene
8.5
8
9
9
8.5
10
11
12
13
14
15
16
17
18
19
20
33
37.5
33
sunlightd
7 days
–
7
7
8
9
8
9
8
–
–
degassed, under argon
Ph2CO (5 mol%)
Ph2CO, Et3N (5 mol% each)
Rose bengal (5 mol%)
a Reaction carried out by external irradiation using a 400 W high-pressure Hg
lamp in CHCl3 at 20 °C.
b Isolated yields of a 2 mmol reaction scale after intensive washing of the pi-
cene with acetone to remove catalyst and byproducts. The resulting powder
was essentially pure picene by 300 MHz 1H NMR analysis.
c A 10 mmol scale (2.80 g of 1) using the same experimental setup.
d No lamp, only standing on the shelf.
e DQ = 2,3,5,6-tetramethyl-p-benzoquinone (duroquinone).
f TCQ = 2,3,5,6-tetrachloro-p-benzoquinone (chloranyl).
Direct photocyclization of (E)-1 open to the air is the
simplest experiment tested. As shown in Table 1, entry 1, it
is a viable although also low-yielding synthesis of picene
(38%). The main problem we found with this synthesis is
the photooxidative cleavage of the reagent with oxygen, af-
fording 1-naphthalaldehyde as well as 1-naphthoic acid as
the reaction times become longer.9 As expected, the role of
a 5 mol% of I2 is patent in Table 1, entry 2. Yields are clearly
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E

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M. Carrera et al.
Letter
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improved and an 83% of picene was isolated. We moved
now to DPQ as catalyst (Table 1, entries 3–7). The amount of
catalyst was adjusted to 5 mol% as the optimal with a 93%
isolated yield (Table 1, entry 4),18 seeing no improvement
when larger amounts were used in air (Table 1, entry 6).
A multigram-scale synthesis is also feasible, as shown in
Table 1, entry 5 (81%). Interestingly, stoichiometric DPQ
proved to be a suitable oxidant under anaerobic conditions
(81% yield, Table 1, entry 7), pertinent if a complete absence
of O2 were required. Regeneration of DPQ back from its re-
duced form (DPQH2) was initially one of our main concerns.
Deprotonation of DPQH2 should facilitate the process, but,
in practice, the use of basic additives such as triethylamine
or potassium carbonate had a deleterious effect on the
yields (Table 1, entries 8 and 9). Isooctane (2,2,4-trimethyl-
pentane) has an increased oxygen solubility compared to
chloroform,19 but the reaction turned out to be very slow
and the results very poor (Table 1, entry 10). Hydrocarbons
as cyclohexane and toluene also underperformed when
compared with chloroform (Table 1, entries 11 and 12). The
reaction also works with sunlight,17 although longer reac-
tion time is needed (Table 1, entry 13). Finally, some alter-
native oxidants with potential catalytic activity in the pres-
ence of O2 were assayed. Cu(acac)2, a relatively soluble form
of copper(II), gave only moderate results (Table 1, entry 14).
We also explored the performance of other quinones (Table
1, entries 15 and 16), above and below in the electrochemi-
cal series relative to DPQ. Duroquinone (DQ, Ered = –0.81 V)
and chloranyl (TCQ, Ered = +0.22 V) span their first reduction
potentials over a range larger than 1 V (both vs. NHE in ace-
tonitrile),20 but they both turned out to be worse catalysts
than DPQ (Ered = –0.28 V vs. NHE, converted from Ered = –0.52
V vs. SCE in acetonitrile).21 Still, stoichiometric TCQ in a de-
gassed reaction medium under argon afforded similar re-
sults to DPQ (Table 1, entry 17), indicating that, although it
is a good oxidant for the synthesis, does not perform well in
a catalytic manner.
Understanding better the role of DPQ under the photo-
chemical reaction conditions was also one of our objectives.
Most quinones, including 1,4-benzoquinone and many of
its derivatives, involve a reactive triplet state in their photo-
processes, so a rich photochemistry was initially expected
for this reagent.22 However, DPQ as well as other sterically
hindered quinones behave in a very different way. After ex-
citation, the quantum yield of intersystem crossing (ISC) to
the triplet state is vanishingly small, resulting in virtually
no photochemical activity in solution at room tempera-
ture.23,24 In our case, DPQ could also be recovered un-
changed (75% recovered after recrystallization from a crude
containing a 77% DPQ + 22% DPQH2 by 1H NMR analysis, Ta-
ble 1, entry 7). In an attempt to facilitate an ISC, a triplet
photosensitizer additive was tested with little success. Ben-
zophenone was chosen since its triplet energy (ET = 288 kJ
mol–1) was close to the absorption band energy of DPQ
(λmax = 427.00 nm in CHCl3) and therefore above the triplet
energy of DPQ. Yields were poorer than with DPQ alone (Ta-
ble 1, entry 18). The same outcome was observed for this
type of experiment in the presence of a base (Table 1, entry
19). The potential participation of a charge-transfer com-
plex between our substrate DNE (a π donor), and DPQ (a
potential π acceptor) was also considered, and later discard-
ed after UV-vis analysis (see Supporting Information). All
the above suggests that the DPQ acts as an oxidant in the
ground state, that is, it works under normal thermal activa-
tion. We named this first part of the mechanism the ther-
mal DPQ stage, or dark phase, in which a standard rearoma-
tization of the 4a,4b-dihydrophenanthre moiety takes place
affording the highly stable armchair (phenanthrene-like)
junction along with DPQH2 (Figure 1, top).
Figure 1 Simplified mechanistic view of the photocatalytic oxidative
step in the synthesis of phenacenes. It is divided in two phases: a) the
thermal DPQ stage (or dark phase), in which the dihydrocyclized prod-
uct is oxidized with DPQ under thermal activation, and b) the photo-
chemical DPQH2 stage (or light phase) in which the reduced DPQH2 is
photochemically reoxidized back to DPQ in the presence of air.
The next stage of the mechanism, that is, recycling of
DPQH2 back to DPQ, is not a straightforward process. Con-
trol experiments showed that, in the dark, DPQH2 was sta-
ble to air under our reaction conditions. We were very glad
to discover that this oxidation process was actually being
photochemically activated, and this occurred in the same
reaction vessel in parallel to the main photocyclization re-
action of Scheme 1.25 This is an unprecedent result that al-
lows us to introduce the DPQ-air system as a new photocat-
alytic method of general use, and that fits particularly well
for our specific goals concerning phenacene syntheses. We
call the second part of the mechanism the photochemical
DPQH2 stage or light phase, in which the main oxidant of
the reaction (DPQ) is regenerated back by means of molec-
ular oxygen (Figure 1, bottom). Since generating singlet ox-
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E

D
M. Carrera et al.
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Br
Cl
1. P(OEt)3
2. NaH
hν
3.
NBS
CHO
DPQ (5 mol%)
air, CHCl3
AIBN
CHCl3
4 (49%)
hν
5 (79%)
3 (87%)
1. P(OEt)3
2. NaH
3.
CHO
DPQ (5 mol%)
air, CHCl3
[7]phenacene (7, 92%)
6 (82%)
Scheme 2 Synthesis of [7]phenacene
ygen (1O2) by means of a specific photosensitizer (Rose ben-
gal) does not bring about an improvement in yield or reac-
Acknowledgment
This work was generously supported by the Spanish Ministerio de
Ciencia e innovación (CTQ2011-24165) and the Universidad de Ali-
cante. MC thanks the VIDI of the UA for a predoctoral grant.
tion times (Table 1, entry 20), reoxidation of DPQH2 (λmax
=
266.00 nm in CHCl3) must likely rely on the reactivity of its
own excited estate DPQH2*, which is thought to be the key
element of the light phase.
Supporting Information
Finally, we applied then this synthetic protocol to the
synthesis of [7]phenacene through a new disconnection ap-
proach for its armchair junctions, and using the same syn-
thetic strategies developed in here for picene (Scheme 2).
We started from commercial 1-(chloromethyl)naphthalene,
triethyl phosphite, and o-tolualdehyde in a Michaelis–
Arbuzov followed by a Horner–Wadsworth–Emmons reac-
tions to obtain 3 in good yield. Oxidative photocyclization
using DPQ-air afforded 1-methylchrysene (4),26 which was
brominated in the methyl group using N-bromosuccinim-
ide (NBS) and a catalytic amount of azobisisobutyronytrile
(AIBN) to 5. Repeating the former processes now with 1-
naphthaldehyde gave 6, which was submitted to another
round of oxidative photocyclization with DPQ-air to yield
[7]phenacene (7) in excellent yield.27 Again, 7 could also be
obtained using sunlight in a 74% yield after 7 days of expo-
sure (see Supporting Information).
In conclusion, we have discovered a new, entirely organ-
ic photocatalytic system based on DPQ-air that can be used
for general oxidative purposes using either natural (sun-
light) or lamp irradiation. DPQ works both alone (anaerobi-
cally) and also catalytically with air as co-oxidant and does
not produce strongly acidic reduction byproducts. It can be
coupled in situ to the oxidative photocyclization of stilben-
es, allowing the preparation phenacenes in high yields and
high standards of purity. Two important compounds in or-
ganic electronics, picene and [7]phenacene have been syn-
thetized in 93% and 92% yields, respectively, from stilbene-
type precursors. We are currently exploring further appli-
cations and establishing the precise mechanisms involved
in both phases of this new synthetic tool.
Supporting information for this article is available online at
http://dx.doi.org/10.1055/s-0036-1588598.
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References and Notes
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(18) Synthesis of [5]Phenacene (2)
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(23) Görner, H. Photochem. Photobiol. 2010, 86, 1202.
(24) This is backed up by the fact that DPQ was recovered unchanged
(84% after recrystallization) with no signs of any reaction after
50 d of irradiation with four 300 W tungsten filament lamps,
being described to be entirely photostable: Bruce, J. M.;
Chaudhry, A.-U.-H. J. Chem. Soc., Perkin Trans. 1 1974, 295.
(25) In control experiments, a sample of DPQH2 in CHCl3 (4·10–3 M,
standard reaction conditions) open to the air was stirred (160
rpm) in the dark for 7 d. Subsequent GLC analysis showed no
detectable amounts of DPQ. However, the same sample under
UV irradiation turned quickly into reoxidized DPQ, the conver-
sion being >90% complete in less than 1 h.
A solution of 1,2-dinaphthylethene (1, 0.5614 g, 2.0 mmol) and
DPQ (0.0409 g, 0.1 mmol) in CHCl3 (600 mL) was prepared in a 1
L jacketed Pyrex reactor and was mechanically stirred at ca. 160
rpm, open to the air. The recirculation chiller was turned on,
and the solution was irradiated with a 400 W high-pressure Hg
lamp for 8 h (Figure S1). The advance of the reaction was fol-
lowed by TLC (1 has a strong blue fluorescence). After the reac-
tion was completed, the solution was concentrated to dryness
at a reduced pressure (15 Torr). The CHCl3 was recycled, and
acetone was added to the residue (50 mL). After stirring, this
suspension was filtered off, washed with fresh acetone, and
dried at 110 °C to afford 0.5193 g (93%) of an off-white powder.
A sample of picene was recrystallized for analytical purposes
from toluene to give white crystals; mp 364.9 °C (Tonset by DSC).
1H NMR (300 MHz, CDCl3): δ = 7.63–7.70 (m, 1 H), 7.71–7.79 (m,
1 H), 7.99–8.07 (m, 2 H), 8.80 (d, J = 9.2 Hz, 1 H), 8.87 (d, J = 8.6
Hz, 1 H), 8.97 (s, 1 H). MS (EI): m/z = 280 (2.7) [M+ + 2], 279 (23)
[M+ + 1], 278 (100) [M+], 277 (11), 276 (25), 139 (13), 138 (13).
IR (neat): νmax = 3050, 1597, 1526, 1474, 1446, 1432, 1275,
1264, 1134, 1024, 945, 869, 852, 807, 754, 739, 671 cm–1. Anal.
Calcd for C22H14: C, 94.93; H, 5.07. Found: C, 94.73; H, 5.09. For
the reaction using sunlight (Figure S2), a solution of 1,2-dinaph-
thylethene (1, 0.0569 g, 0.2 mmol) and DPQ (0.0148 g, 0.02
mmol) in CHCl3 (10 mL) was exposed to sunlight and allowed to
stand for 7 d (this includes days and nights). The flask was
opened regularly to follow the advance of the reaction by TLC.
Working up the reaction as described above produced 0.0391 g
(69%) of picene as an off-white crystalline powder.
(26) Concerning the yield of 4, we noticed that our experimental set-
up works better for larger π-conjugated systems, so we suspect
that our current source of UV light is the limiting parameter. A
careful analysis of the reaction crude reveals small amounts of
chrysene (C18H12), indicating that an oxidative pathway affect-
ing the methyl substituent probably followed by decarboxyl-
ation might be taking place. As this byproduct only accounts for
a small amount of the missing yield (<3%), and we were not able
to identify the bulk of it, it is likely that radical polymerization
occurs accounting for the unexpected medium yield reported.
(27) Synthesis of [7]Phenacene (7)
A suspension of 1-[(E)-2-(1-naphtyl)vinyl]chrysene (6, 0.3841
g, 1 mmol) and DPQ (0.1223 g, 0.1 mmol) in CHCl3 (600 mL) was
irradiated following the same procedure explained for picene
(2). The advance of the reaction could be followed by TLC (6 has
a white-blue fluorescence). After the reaction was completed (8
h) the mixture was concentrated under reduced pressure (15
Torr), the solid was filtered off, washed with acetone, and dried
to give 0.3501 g (92%) of 7 as off-white plates. The product was
recrystallized for analytic purposes with 1,2-dichlorobenzene;
mp (decomp.) = 569.9 °C (Tpeak by DSC). 1H NMR (400.13 MHz,
(CDCl2)2 at 358 K): δ = 7.71–7.77 (m, 2 H), 7.79–7.85 (m, 2 H),
8.07–8.11 (m, 2 H), 8.14 (d, J = 9.2 Hz, 2 H), 8.90–8.95 (m, 4 H),
9.06 (d, J = 9.3 Hz, 2 H), 9.09 (s, 2 H), 9.15 (d, J = 9.2 Hz, 2 H). MS
(EI, DIP): m/z = 380 (5) [M+ + 2], 379 (32) [M+ + 1], 378 (100)
[M+], 376 (18), 189 (27), 188 (19), 187 (13). IR (neat): νmax
=
3056, 1469, 1442, 1432, 1282, 1234, 1145, 1128, 1028, 944,
866, 842, 805, 768, 740, 711 cm–1. Anal. Calcd for C30H18: C,
95.21; H, 4.79. Found: C, 95.21; H, 4.50. For the reaction using
sunlight (Figure S2), a solution of 6 (0.0076 g, 0.02 mmol) and
DPQ (0.0015 g, 0.002 mmol) in CHCl3 (30 mL) was exposed to
sunlight in a 50 mL flask and was allowed to stand for 7 d (this
includes days and nights). Working up the reaction as described
above gave 0.0056 g (74%) of [7]phenacene as off-white crystal-
line plates.
(19) The molar solubility of O2 in isooctane (1.6·10−2 mol O2/l at 20
°C, 1 atm) is a 76% higher than in CHCl3, a 41% higher than in
cyclohexane, and a 77% higher than in benzene, all of them
typical solvents for this reaction: (a) Battino, R. Rettich T. R.
1983, 12, 163. (b) Shirono, K.; Morimatsu, T.; Takemura, F.
J. Chem. Eng. Data 2008, 53, 1867.
(20) Wardman, P. J. Phys. Chem. Ref. Data 1989, 18, 1637.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E