Solvent-free one-step photochemical hydroxylation of benzene derivatives by the singlet excited state of 2,3-dichloro-5,6-dicyano-p-benzoquinone acting as a super oxidant

Article abstract of DOI:10.1002/chem.201404810

Photoinduced hydroxylation of neat deaerated benzene to phenol occurred under visible-light irradiation of 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ), which acts as a super photooxidant in the presence of water. Photocatalytic solvent-free hydroxylation of benzene derivatives with electron-withdrawing substituents such as benzonitrile, nitrobenzene, and trifluoromethylbenzene used as neat solvents has been achieved for the first time by using DDQ as a super photooxidant to yield the corresponding phenol derivatives and 2,3-dichloro-5,6-dicyanohydroquinone (DDQH₂) in the presence of water under deaerated conditions. In the presence of dioxygen and tert-butyl nitrite, the photocatalytic hydroxylation of neat benzene occurred with DDQ as a photocatalyst to produce phenol. The photocatalytic reactions are initiated by oxidation of benzene derivatives with the singlet and triplet excited states of DDQ to form the corresponding radical cations, which associate with benzene derivatives to produce the dimer radical cations, which were detected by the femto- and nanosecond laser flash photolysis measurements to clarify the photocatalytic reaction mechanisms. Radical cations of benzene derivatives react with water to yield the OH-adduct radicals. On the other hand, DDQC^?- produced by the photoinduced electron transfer from benzene derivatives reacts with the OH-adduct radicals to yield the corresponding phenol derivatives and DDQH₂. DDQ is recovered by the reaction of DDQH₂ with tert-butyl nitrite when DDQ acts as a photocatalyst for the hydroxylation of benzene derivatives by dioxygen.

Full text of DOI:10.1002/chem.201404810

DOI: 10.1002/chem.201404810
Full Paper
&
Organocatalysis
Solvent-Free One-Step Photochemical Hydroxylation of Benzene
Derivatives by the Singlet Excited State of 2,3-Dichloro-5,6-
dicyano-p-benzoquinone Acting as a Super Oxidant
Kei Ohkubo,* Kensaku Hirose, and Shunichi Fukuzumi*[a]
Abstract: Photoinduced hydroxylation of neat deaerated
benzene to phenol occurred under visible-light irradiation of
2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ), which acts
as a super photooxidant in the presence of water. Photoca-
talytic solvent-free hydroxylation of benzene derivatives with
electron-withdrawing substituents such as benzonitrile, ni-
trobenzene, and trifluoromethylbenzene used as neat sol-
vents has been achieved for the first time by using DDQ as
a super photooxidant to yield the corresponding phenol de-
rivatives and 2,3-dichloro-5,6-dicyanohydroquinone (DDQH2)
in the presence of water under deaerated conditions. In the
presence of dioxygen and tert-butyl nitrite, the photocatalyt-
ic hydroxylation of neat benzene occurred with DDQ as
a photocatalyst to produce phenol. The photocatalytic reac-
tions are initiated by oxidation of benzene derivatives with
the singlet and triplet excited states of DDQ to form the cor-
responding radical cations, which associate with benzene
derivatives to produce the dimer radical cations, which were
detected by the femto- and nanosecond laser flash photoly-
sis measurements to clarify the photocatalytic reaction
mechanisms. Radical cations of benzene derivatives react
with water to yield the OH-adduct radicals. On the other
Cꢀ
hand, DDQ produced by the photoinduced electron trans-
fer from benzene derivatives reacts with the OH-adduct radi-
cals to yield the corresponding phenol derivatives and
DDQH2. DDQ is recovered by the reaction of DDQH2 with
tert-butyl nitrite when DDQ acts as a photocatalyst for the
hydroxylation of benzene derivatives by dioxygen.
a key intermediate.[14] Formations of radical cations of benzoni-
trile (PhCN), nitrobenzene (PhNO2), and trifluoromethylbenzene
(PhCF3) for the hydroxylation are much more difficult than that
of benzene radical cation, because the one-electron oxidation
potentials of PhCN, PhNO2, and PhCF3 are much higher than
that of benzene (2.48 V vs. SCE)[15] due to electron-withdrawing
cyano, nitro, and trifluoromethyl groups. The one-electron oxi-
dation potentials of PhCN, PhNO2, and PhCF3 were not report-
ed. However, the ionization potentials of PhCN (9.72 eV),
PhNO2 (9.93 eV), and PhCF3 (9.70 eV) are significantly higher
than the value of benzene (9.23 eV).[16] The singlet excited
state of DDQ (1DDQ*) is expected to have a much stronger oxi-
dizing ability than that of 3DDQ because the singlet–triplet
energy gap in the excited state of a p-benzoquinone derivative
(p-chloranil) is known to be 0.64 eV.[17] However, the singlet excit-
Introduction
Photoredox catalysts have been widely investigated and uti-
lized to photoinduced organic syntheses by using inorgan-
ic[1–12] and organic photosensitizers.[13] The photocatalytic reac-
tion photosensitized by photoredox catalyst is initiated by
photoinduced electron-transfer oxidation/reduction of the sub-
strate. The photoinduced electron-transfer reaction has ena-
bled highly reactive radical cations and anions to be produced,
leading to the finely tuned new organic synthetic transforma-
tions.[1–13] We have reported the catalytic hydroxylation of ben-
zene to phenol under ambient conditions through photoin-
duced one-electron oxidation of benzene with 2,3-dichloro-5,6-
dicyano-p-benzoquinone (DDQ) as a triplet photooxidant.[14]
The one-electron reduction potential of the triplet excited
state of DDQ (3DDQ*: in which “*” denotes the excited state) is
3.18 V versus SCE, which is high enough to oxidize benzene by
electron transfer to produce the benzene radical cation as
3
ed state (1DDQ*) is spontaneously relaxed to DDQ* with a life-
time of a few picoseconds,[17] which is too short to be able to ox-
idize PhCN at low concentrations. If a neat aromatic substrate is
1
used as a solvent, DDQ* would be able to oxidize the substrate.
However, photoinduced oxidation of aromatic compound with
electron-withdrawing substituent by 1DDQ* is yet to be reported.
[a] Prof. Dr. K. Ohkubo, K. Hirose, Prof. Dr. S. Fukuzumi
Department of Material and Life Science
Graduate School of Engineering
Osaka University, ALCA, JST Suita
Osaka 565-0871 (Japan)
Fax: (+81)6-6879-7370
Results and Discussion
E-mail: ookubo@chem.eng.osaka-u.ac.uk
fukuzumi@chem.eng.osaka-u.ac.jp
Photochemical hydroxylation of benzene derivatives
Photohydroxylation of benzene occurred under visible-light ir-
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201404810.
radiation of DDQ (3.0 mm) in deaerated benzene and H2O
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(50 mm) with a xenon lamp (500 W) attached with a color
glass filter (l>390 nm) to produce phenol and DDQH2
[Eq. (1)].
The reaction time profile is shown in Figure 1. The yield of
phenol was 99% with 99% conversion (>99% selectivity)
based on the initial concentration of DDQ. No further oxygen-
ated product was observed under the present experimental
conditions as confirmed by the GC analysis (Figure 1a).
Figure 2. Reaction time profiles in the photohydroxylation of deaerated
(a) PhCN and (b) PhNO2 as solvents with DDQ (5.0 mm) and H2O (100 mm)
under photoirradiation by using a xenon lamp (500 W) at 298 K. GC charts
are shown in Figure S1 and S2 (the Supporting Information).
omethylbenzene with DDQ also occurred and the product
yields and selectivities are summarized in Table 1. When anisole
was used as a solvent, however, no photohydroxylation with
DDQ occurred. The reason will be discussed later. The quan-
tum yields (F) of the formation of phenol were determined by
use of a standard actinometer, potassium ferrioxalate (see the
Experimental Section). The F values were determined to be
3.0, 0.6, and 0.1% for the initial formation rates of phenol(s)
from benzene, PhCN and PhNO2, respectively.
Figure 1. (a) GC charts in photohydroxylation of benzene with DDQ (3.0 mm)
and H2O (50 mm) in deaerated benzene at 298 K under photoirradiation by
use of a xenon lamp (500 W). (b) Reaction time profile of formation of
phenol.
Table 1. Yields, selectivities, and turnover numbers (TON) of phenols for
the photochemical hydroxylation.
Substrate
Yield
[%][a]
Selectivity [%][b]
(o-, m-, p-)
TON[c]
0
When benzene was replaced by electron-withdrawing group
substituted benzene such as PhCN and PhNO2, the selective
photohydroxylation with DDQ also occurred under otherwise
the same reaction conditions (see [Eq. (1)] and Figure 2). The
product ratio of the oxidized PhCN was 44:18:38 for o-, m-
and p-cyanophenols, respectively with 87% total yield after
photoirradiation for 30 h, which was determined by the GC-MS
analyses (Figure S1 in the Supporting Information). The prod-
uct ratio of the photohydroxylation of PhNO2 was 45:40:15 for
o-, m-, and p-nitrophenols after photoirradiation for 48 h (GC
data is shown in Figure S2, the Supporting Information). Photo-
hydroxylation of tert-butylbenzene, chlorobenzene and trifluor-
anisole[d]
0
25
92
>99
8
87
–
tert-butylbenzene[d]
chlorobenzene[d]
benzene[d]
>99 (4:0:96)
>99 (17:0:83)
>99
>99 (37:59:4)
>99 (44:18:38)[f]
>99 (45:40:10)[f]
–
[f]
1.4
4.8
trifluoromethylbenzene[d]
benzonitrile[e]
nitrobenzene[e]
–
[f]
1.2
[f]
10
–
[a] Total yield of phenols based on the initial concentration of DDQ.
[b] Based on the consumption of DDQ. [c] In the presence of TBN
(3.0 mm). [d] [DDQ]=3.0 mm. [e] [DDQ]=5.0 mm. [f] Not determined due
to the low catalytic reactivity. GC data are shown in Figures S1–S5 in the
Supporting Information.
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In the presence of tert-butyl nitrite (TBN), the photocatalytic
hydroxylation of benzene occurred, in which DDQ was regen-
erated from DDQH2 by TBN with O2.[18] The catalytic turnover
number was determined to be 4.8 based on the initial concen-
tration of DDQ in oxygen-saturated benzene containing DDQ
(0.30 mm), H2O (50 mm) and TBN (3.0 mm) under visible-light
irradiation by a xenon lamp for 48 h. The overall stoichiometry
is shown in Equation (2).[19] Photocatalytic hydroxylation of
PhCN also occurred under the similar conditions to yield cya-
nophenols with turnover number (TON) of 1.2. Although the
TON is small, no further oxygenated products such as quinone,
diphenol, or CO2 were observed under the present photocata-
lytic conditions.
in benzene at 1.0 ps as shown in Figure 3b. The absorption at
lmax =605 nm is assigned to the semiquinone radical anion of
[20]
Cꢀ
DDQ (DDQ ).
This absorption band agrees with that ob-
served in the electron-transfer reduction of DDQ by ferrocene
in benzene (lmax =605 nm; see Figure S7 in the Supporting In-
formation). The broad absorption at near-IR region is assigned
to the p-dimer radical cation of benzene (lmax =900 nm),[21,22]
which is formed by the strong association of the benzene radi-
cal cation and the large excess of neutral benzene used as
a solvent. The rate constant of formation radical ion pair [ket(S)]
was determined from the rate constant of the rise in absorb-
ance at 605 nm (kobs =1.5ꢁ1012 sꢀ1 in Figure S8 in the Support-
ing Information) using Equation (3) to be 1.3ꢁ1012 sꢀ1
,
ketðSÞ ¼ kobsꢀkISC
ð3Þ
which is larger than the rate constant of intersystem crossing
in deaerated MeCN. This suggests that ultrafast electron trans-
1
fer occurred from the solvent to DDQ* to produce the corre-
Cꢀ
Photodynamics in photochemical hydroxylation
sponding p-dimer radical cation and DDQ .
The near-IR absorption bands due to p-dimer radical cations
of solvents were also observed when benzene was replaced by
PhCN and PhNO2 as shown in Figure 3c and 3d, respectively.
The rate constants of electron transfer from PhCN and PhNO2
to1DDQ* were determined to be kobs =3.5ꢁ1011 sꢀ1 and 2.5ꢁ
1011 sꢀ1, respectively (Figure 4). The ket(S) values were deter-
The mechanism of hydroxylation of benzene and its derivatives
was examined by time-resolved transient absorption experi-
ments. A singlet–singlet absorption spectrum of DDQ was ob-
served at 470 nm in deaerated MeCN at 1.0 ps after femtosec-
ond laser excitation of lex =420 nm (see Figure 3a). The sin-
1
glet–singlet absorption due to DDQ* decayed, accompanied
by an increase in absorption at 630 nm due to the triplet-trip-
let absorption band of DDQ. The rate constant of intersystem
crossing from 1DDQ* to 3DDQ* (kISC) was determined to be
1.8ꢁ1011 sꢀ1 in MeCN (Figure S6 in the Supporting Informa-
tion). In contrast, no singlet–singlet absorption was observed
Figure 4. Time profiles at 625 nm in transient absorption spectral changes
observed in deaerated (a) PhCN and (b) PhNO2 containing DDQ (10 mm)
after femtosecond laser excitation at 420 nm. Solid lines are drawn by the
single exponential curve fitting.
mined to be 1.7ꢁ1011 sꢀ1 for PhCN and 7.0ꢁ1010 sꢀ1 for
s
PhNO2, which are smaller than that of benzene (1.3ꢁ1012 ꢀ1).
In the cases of anisole and tert-butylbenzene possessing elec-
tron-donating substituents, the electron-transfer oxidation oc-
curred much more efficiently than that of benzene (Figures S10
and S12 in the Supporting Information). The rate constants of
photoinduced electron transfer are summarized in Table 2. The
one-electron oxidation potential (Eox) of PhNO2 has not been
reported previously because of the solvent oxidation prior to
the oxidation of PhNO2. The Eox value of PhNO2 is estimated
from the Eox value of benzene (2.48 V vs. SCE) and ionization
potentials (IPs) of benzene (9.25 eV)[16] and PhNO2 (9.93 eV)[16]
to be 3.16 V versus SCE. On the other hand, the one-electron
Figure 3. Transient absorption spectra of DDQ (10 mm) in deaerated
(a) MeCN, (b) C6H6, (c) PhCN, and (d) PhNO2 after femtosecond laser excita-
tion at 420 nm.
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Table 2. Ionization potentials (IPs), rate constants of the electron transfer
from substrates to the excited states of DDQ (ket(S) and ket(T)), and the
rate constant of substrate radical cation with H2O (kH2O).
Substrate
IP
[eV][a]
ket(S)
[sꢀ1
ket(T)
[mꢀ1 sꢀ1
KH O
2
[b]
[b]
]
]
[mꢀ1 sꢀ1
]
[d]
PhOMe
PhtBu
PhCl
8.39
8.68
9.08
9.25
9.70
9.72
9.93
3.1ꢁ1012
3.4ꢁ1012
1.6ꢁ1012
1.3ꢁ1012
1.4ꢁ1010
1.7ꢁ1011
7.0ꢁ1010
1.2ꢁ1010
6.4ꢁ109
6.0ꢁ109
5.3ꢁ109[c]
1.2ꢁ106
2.0ꢁ106
4.2ꢁ105
–
1.5ꢁ107
8.7ꢁ107
5.5ꢁ107[c]
4.9ꢁ107
1.5ꢁ107
6.2ꢁ106
C6H6
PhCF3
PhCN
PhNO2
[a] Ref. [16]. [b] In MeCN. [c] Ref. [14]. [d] Not determined due to no ob-
servation of radical cation by transient absorption measurements. The
rise and decay data to determine the rate constants are shown in Fig-
ure S8–S21 in the Supporting Information.
reduction potential of 1DDQ* is also estimated to be 3.8 V
3
versus SCE from the Ered value of DDQ* (3.18 V vs. SCE)[14] and
the singlet–triplet energy gap of p-chloranil (0.64 eV).[17] The Eox
1
value of PhNO2 is significant smaller than that of Ered of DDQ*.
1
Thus, electron transfer from PhNO2 to DDQ* is energetically
Figure 5. (a) Transient absorption spectra of deaerated MeCN (black) and
PhCN (gray) solutions containing DDQ (1.0 mm) taken at 100 ns after nano-
second laser excitation at 430 nm. Decay time profiles at 620 nm due to
3DDQ* with (b) PhCN (20–475 mm) and (c) PhNO2 (0.050–1.3m); (d) Plots of
kobs for the decay at 620 nm versus [substrate] in MeCN.
feasible to generate the radical ion pair. The ket(S) values of the
other substrates were also determined (Figures S9–S12 in the
Supporting Information) and listed in Table 2.
1
Quenching ratios of DDQ* by solvents (benzene, PhCN, and
PhNO2) through electron transfer were determined from the
above mentioned rate constants of kISC and ket(S) to be 88, 49,
3
and 28%, respectively. To evaluate the contribution of DDQ*
in electron-transfer oxidation of substrates, nanosecond laser
flash photolysis was also performed to observe the electron-
3
transfer processes of DDQ* with substrates. The time-resolved
transient absorption spectral measurements of a deaerated
MeCN solution containing DDQ exhibited the absorption
bands at 620 nm due to 3DDQ* at 100 ns after nanosecond
laser excitation (Figure 5a). The decay rate constants (kobs) of
3DDQ* increased with increasing concentrations of PhCN and
PhNO2 as shown in Figure 5b and 5c, respectively. The elec-
3
tron-transfer rate constant of DDQ* with PhCN [ket(T)] was de-
termined from the slope of the plot of kobs versus [PhCN] to be
2.0ꢁ106 mꢀ1 sꢀ1 (Figure 5d). This value is much smaller than
that of benzene (5.3ꢁ109 mꢀ1 sꢀ1), but larger than that of
Figure 6. Plots of kobs for the decay of radical cation versus [H2O] in benzene
~
&
*
( ), PhCN ( ), and PhNO2 ( ).
PhNO2 (4.2ꢁ105 mꢀ1
s
ꢀ1; see Table 2). Thus, efficient quenching
3
of DDQ* by photoinduced electron transfer occurs because of
the high concentrations of substrate: benzene (11m), PhCN
(9.7m) and PhNO2 (9.7m) under the solvent-free conditions.
The radical ion pair of the p-dimer PhCN radical cation and
H2O was determined from the slope of kobs versus [H2O] to be
k
H O =5.5ꢁ107 mꢀ1 sꢀ1 for benzene radical cation,[14] 1.5ꢁ
2
107 mꢀ1 sꢀ1 for PhCN and 6.2ꢁ106 mꢀ1 sꢀ1 for PhNO2
+
+
C
C
.
Cꢀ
DDQ was observed in neat PhCN as shown in Figure 3a (gray
Reaction mechanism for photohydroxylation
spectrum), disappearing through bimolecular back electron
transfer to the ground state in dry PhCN. The decay of absorb-
ance at 950 nm due to the p-dimer PhCN radical cation was
accelerated by addition of H2O. The decay time profile obeyed
first-order kinetics in the presence of large excess H2O as
shown in the Supporting Information, Figure S17. The decay
rate constant (kobs) increased linearly with increasing concen-
tration of H2O (Figure 6). The rate constant for the reaction of
substrate radical cation and/or the p-dimer radical cation with
On the basis of abovementioned results, the mechanism of
photohydroxylation of PhCN is summarized in Scheme 1.
+
C
PhCN formed by photoinduced electron transfer from PhCN
1
3
to DDQ* and DDQ* reacts with water to yield the OH-adduct
radical.[14,23]
Cꢀ
On the other hand, DDQ can react with the OH-adduct
radical to yield cyanophenols and DDQH2. When phenol was
used as a substrate in MeCN with H2O, no further hydroxyl-
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Figure 7. Electronic charges on carbon, nitrogen, and the oxygen nucleus of
(a) PhCN radical cation and (b) PhNO2 radical cation calculated by the DFT
B3LYP/6-31+G(d,p) basis set with natural population analyses (NPA). Hydro-
gens are omitted.
Scheme 1. Proposed reaction mechanism for the photohydroxylation of
PhCN.
of theory as shown in Figure 7. In the case of PhCN radical
cation, the minus charge at the meta position is larger than
that that at the ortho position, whereas the plus charge is lo-
cated at the para position. Thus, addition of OHꢀ to the radical
cation occurred mainly to the two ortho positions and the one
para position to yield o- and p-cyanophenols, respectively
(Table 1). In contrast to the case of PhCN radical cation, the
largely negative charge is located at the para position, result-
ing in the photohydroxylation to yield mainly o- and m-nitro-
phenols.
ation occurred under the otherwise the same experimental
conditions.[14]
Additionally, no 18O incorporation of phenol was observed in
photoirradiation of MeCN solution containing phenol and
H218O (see Figure S22 in the Supporting Information). These re-
Cꢀ
sults suggest that back electron transfer from DDQ to the
phenol radical cation is much faster than the reaction of
phenol radical cation with H2O, resulting in no net reaction
(Scheme 2). In contrast, back electron transfer from radical cat-
Conclusion
Solvent-free selective hydroxylation reactions of PhCN, ben-
zene, PhNO2, and PhCF3 are initiated by photoinduced electron
transfer from the substrates to the excited states of DDQ,
1
yielding the corresponding phenols, in which DDQ* is acting
Cꢀ
as a super oxidant. Back electron transfer from DDQ to radical
Scheme 2. No photochemical hydroxylation through fast back electron
cations of phenols is slow enough to complete the reactions
of the radical cations with H2O because of the large driving
force of the back electron transfer in the Marcus inverted
region. By the same reason, no further oxidation of phenols
occurred, enabling selective hydroxylation of benzene and
benzene derivatives with electron-withdrawing groups, be-
transfer.
ions of benzene and benzene derivatives with electron-with-
drawing substituents (PhCN, PhNO2, and PhCF3) is slow
enough to complete with reactions of the radical cations with
H2O (see the long-lived transient species (t>3 ns) remaining in
Figure 3 and the Supporting Information, Figure S9). The slow
back electron transfer results from the large driving force of
the back electron transfer, which may be deeply in the Marcus
inverted region, in which the rate of electron transfer become
slower with increasing the driving force.[24,25] By the same
reason, no photochemical reaction of anisole with DDQ oc-
curred (see above) because of the fast back electron transfer
Cꢀ
cause of fast back electron transfer from DDQ to radical cat-
ions of phenol derivatives, which is not in the Marcus inverted
region. The present study paves a new way to utilize the
1DDQ* as a super oxidant for solvent-free photooxidation
reactions.
Cꢀ
from DDQ to the anisole radical cation (kBET =4.9ꢁ1011 sꢀ1
,
Experimental Section
Figure S10 in the Supporting Information) due to the smaller
driving force of back electron transfer (Scheme 2). In the pres-
ence of tert-butyl nitrite, DDQH2 can be recycled by the reac-
tion with O2 through NO2 to regenerate DDQ.[14,18]
Materials
Chemicals were purchased from commercial source and used with-
out purification, unless otherwise noted. 2,3-Dichloro-5,6-dicyano-
p-benzoquinone (DDQ) was obtained commercially and purified by
recrystallization from hot dichloromethane.[26] Potassium ferrioxa-
late used as an actinometer was prepared according to the litera-
ture and purified by recrystallization from hot water.[27] Acetonitrile
(MeCN) was of spectral grade, obtained commercially and used
without further purification. Benzene, benzonitrile, nitrobenzene,
chlorobenzene, tert-butylbenzene, trifluoromethylbenzene, and ani-
sole used as solvents were purified by the standard method.[26]
DFT calculations
The regioselectivities of isomers in photohydroxylation of
PhCN and PhNO2 are shown in Table 1. The ratios of regioisom-
ers depend on the electronic charge of radical cation species,
calculated by the DFT method at the B3LYP/6-31+G(d,p) level
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phase modulation. A variable neutral density filter, an optical aper-
ture, and a pair of polarizer were inserted in the path to generate
stable white-light continuum. Prior to generating the probe contin-
uum, the laser pulse was fed to a delay line that provides an exper-
imental time window of 3.2 ns with a maximum step resolution of
7 fs. In our experiments, a wavelength at l=420 nm was irradiated
at the sample cell with a spot size of 1 mm diameter where it was
merged with the white probe pulse in a close angle (<108). The
probe beam after passing through the 2 mm sample cell was fo-
cused on a fiber optic cable that was connected to a CMOS spec-
trograph for recording the time-resolved spectra (l=410–800 nm).
Typically, 3000 excitation pulses were averaged for 3 seconds to
obtain the transient spectrum at a set delay time. Kinetic traces at
appropriate wavelengths were assembled from the time-resolved
spectral data. All measurements were conducted at room tempera-
ture, 295 K.
Reaction procedure
Typically, the stoichiometric photoreaction was performed by the
following procedure. A benzene solution (3.0 cm3) containing DDQ
(3.0 mm) and H2O (50 mm) in a quartz cuvette (1.0 cmꢁ1.0 cm)
with a rubber septum was deaerated by bubbling with nitrogen
through a stainless steel needle for 5 min. In the case of photoca-
talytic reaction, a benzene solution containing DDQ (0.30 mm) with
tert-butyl nitrite (TBN, 3.0 mm) and H2O (50 mm) in a square quartz
cuvette with rubber septum was saturated with dioxygen by bub-
bling dioxygen through a stainless steel needle for 5 min. The solu-
tion was then irradiated with a 500 W xenon lamp (Ushio Optical
Module X SX-UID 500XAMQ) through a color glass filter transmit-
ting l>390 nm at room temperature. After photoirradiation, the
corresponding oxygenated products were identified and quantified
by comparison of the GC-MS analyses with those of the authentic
samples. The photochemical reactions were monitored using a Shi-
madzu GC-17A gas chromatograph and Shimadzu MS-QP5000 GC-
MS spectrometer.
Theoretical calculations
Density functional theory (DFT) calculations were carried out with
Gaussian 09 (Revision D.01, Gaussian, Inc.).[28] The calculations were
performed on a 32-processor QuantumCube at the B3LYP/6–31+
G(d,p) level of theory. Graphical outputs of the computational re-
sults were generated with the GaussView software program (ver.
3.09) developed by Semichem, Inc.[29]
Quantum yield determinations
A standard actinometer (potassium ferrioxalate) was used for the
quantum yield determination of the DDQ photosensitized hydrox-
ylation of aromatic compound with water.[27] Typically, a square
quartz cuvette (10 mm i.d.) that contained a substrate solution
(3.0 cm3) of DDQ (1.0 mm) and H2O (50 mm) was irradiated with
monochromatized light of l=420 nm from a Shimadzu RF-5300PC
fluorescence spectrophotometer. Under the conditions of actino-
metry experiments, DDQ absorbed essentially 99% incident light
of l=420 nm (Abs420 ꢁ3). The light intensity of monochromatized
light of l=420 nm was determined as 7.2ꢁ10ꢀ9 einsteinsꢀ1. The
photochemical reaction was monitored using a Shimadzu GC-17 A
gas chromatograph and Shimadzu MS-QP5000 mass spectrometer.
The quantum yields were determined from concentrations of
phenol derivatives.
Acknowledgements
This work was supported by Grants-in-Aid (Nos. 26620154 and
26288037 to K.O.) from the Ministry of Education, Culture,
Sports, Science and Technology (MEXT) and an ALCA project
from JST, Japan (to S.F.).
Keywords: electron transfer · organocatalysis · oxidation ·
photochemistry · redox chemistry
Laser flash spectroscopy
Measurements of nanosecond laser flash photolysis for photoexci-
tation of DDQ in PhCN were performed according to the following
procedures. Typically, a nitrogen-saturated PhCN solution contain-
ing DDQ (1.2 mm) in the presence of H2O was excited by a Nd:YAG
laser (Continuum, SLII-10, 4–6 ns fwhm) at l=430 nm with the
power of 3.0 mJ/pulse. Photoinduced events were monitored by
use of a continuous Xe lamp (150 W) and an InGaAs-PIN photo-
diode (Hamamatsu 2949) as a probe light and a detector, respec-
tively. The output from the photodiodes and a photomultiplier
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Published online on && &&, 2014
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FULL PAPER
Super photooxidant: Photoinduced hy-
droxylation of neat deaerated benzene
derivatives to the corresponding phe-
nols occurred under visible-light irradia-
tion of 2,3-dichloro-5,6-dicyano-p-ben-
zoquinone (DDQ; see figure), which acts
as a super photooxidant in the presence
of water. The reactions are initiated by
electron-transfer oxidation of benzene
derivatives with the singlet and triplet
excited states of DDQ to form the corre-
&
Organocatalysis
K. Ohkubo,* K. Hirose, S. Fukuzumi*
&& – &&
Solvent-Free One-Step Photochemical
Hydroxylation of Benzene Derivatives
by the Singlet Excited State of 2,3-
Dichloro-5,6-dicyano-p-benzoquinone
Acting as a Super Oxidant
Cꢀ
sponding radical cations and DDQ .
&
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Chem. Eur. J. 2014, 20, 1 – 8
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8
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!