Aromatic Monochlorination Photosensitized by DDQ with Hydrogen Chloride under Visible-Light Irradiation

Article abstract of DOI:10.1002/asia.201600083

Photochlorination of aromatic substrates by hydrogen chloride with 2,3-dichloro-5,6-cyano-p-benzoquinone (DDQ) occurs efficiently to produce the corresponding monochlorinated products selectively under visible-light irradiation. The yields for the chlorination of phenol were 70 % and 18 % for p- and o-chlorophenol, respectively, without formation of further chlorinated products. The photoinduced chlorination is initiated by electron transfer from Cl^- to the triplet excited state of DDQ. The radical intermediates involved in the photochemical reaction have been detected by time-resolved transient absorption measurements.

Full text of DOI:10.1002/asia.201600083

DOI: 10.1002/asia.201600083
Communication
Chlorination
Aromatic Monochlorination Photosensitized by DDQ with
Hydrogen Chloride under Visible-Light Irradiation
Kei Ohkubo,*[a, b, c] Atsushi Fujimoto,[a] and Shunichi Fukuzumi*[b, c]
was reported as 3:4 with producing further chlorinated phe-
Abstract: Photochlorination of aromatic substrates by hy-
drogen chloride with 2,3-dichloro-5,6-cyano-p-benzoqui-
none (DDQ) occurs efficiently to produce the correspond-
ing monochlorinated products selectively under visible-
light irradiation. The yields for the chlorination of phenol
were 70% and 18% for p- and o-chlorophenol, respective-
ly, without formation of further chlorinated products. The
photoinduced chlorination is initiated by electron transfer
from ClÀ to the triplet excited state of DDQ. The radical in-
termediates involved in the photochemical reaction have
been detected by time-resolved transient absorption
measurements.
nols. Recently, a highly selective ortho-chlorination of phenol
has been reported by using of CuCl2, where the ratio of ortho/
para was 1:10.[3] However, this method can be employed only
with phenolic compounds because the copper-phenolate (or
phenoxyl radical) complex is an important intermediate in the
selective chlorination.
We report herein selective para-chlorination of phenol and
methoxybenzene by hydrogen chloride (HCl) as a chlorine
source with DDQ, which acts as an efficient photosensitizer
rather than a chlorinating reagent under visible-light irradia-
tion. The photochemical reaction mechanism is clarified based
on laser flash photolysis measurements and quantum yield de-
termination.
Photoirradiation of a deaerated acetonitrile (MeCN) solution
containing DDQ (2.0 mm), phenol (2.0 mm), and HCl (50 mm)
with a xenon lamp (500 W, l>390 nm) yielded monochloro-
substituted phenol [Eq. (1)].
Aromatic substitution reactions generally pose a challenge in
terms of separation, owing to selectivity considerations in the
product isomer distributions. Especially, chlorophenol is one of
the important chemicals and precursors in pharmaceutical, ag-
ricultural, and dye industries for the production of drugs, insec-
ticides, herbicides, and liquid crystals.[1–3] General methods for
the preparation of chlorophenol involve electrophilic aromatic
halogenation reactions using various chlorinating agents such
as chlorine,[4] sulfuryl chloride,[5–7] hypochlorites,[8] N-chlorosuc-
cinimide,[9–11]
2,3-dichloro-5,6-dicyano-p-benzoquinone
(DDQ),[12] and a guanidine-based chlorinating reagent.[13] For
The time course of the photochemical reaction is shown in
Figure 1. The yields after 2 h photoirradiation of p- and o-chlor-
ophenol were 70 and 18%, respectively, as determined by GC-
MS and 1H NMR analyses (Figure 1). No further chlorinated
product was observed under the present reaction conditions.
The pertinent data of the photoinduced mono-chlorination re-
actions of phenol, methoxybenzene, and naphthalene deriva-
tives are summarized in Table 1. For the naphthalene deriva-
tives, selective monochlorinated products were obtained as
only one isomer (see the Supporting Information, Figure S1).[15]
Time-resolved transient absorption spectroscopy was em-
ployed to clarify the photochlorination mechanism. Nanosec-
ond laser pulse excitation (lex =430 nm) of the absorption
band of DDQ in deaerated MeCN results in the appearance of
the triplet–triplet (T–T) absorption at 630 nm at 1.5 ms as
shown in Figure 2a.[15] The T–T absorption decayed obeying
second-order kinetics due to T–T annihilation (Figure 2b). The
lifetime of the triplet excited state of DDQ (3DDQ*; *denotes
the excited state) was determined from the slowest single-ex-
ponential component to be 2.4 ms. The T–T decay rate con-
stant of DDQ (kobs) increased with an increase in the concentra-
example, the ratio of ortho/para chlorination of phenol by Cl2
[a] Prof. Dr. K. Ohkubo, A. Fujimoto
Department of Material and Life Science
Graduate School of Engineering
Osaka University and ALCA and SENTAN, Japan Science and Technology
Agency (JST)
Suita, Osaka 565-0871 (Japan)
Fax: (+81)6-6879-4732
E-mail: ookubo@chem.eng.osaka-u.ac.jp
[b] Prof. Dr. K. Ohkubo, Prof. Dr. S. Fukuzumi
Department of Chemistry and Nano Science
Ewha Womans University
Seoul 120-750 (Korea)
E-mail: fukuzumi@chem.eng.osaka-u.ac.jp
[c] Prof. Dr. K. Ohkubo, Prof. Dr. S. Fukuzumi
Faculty of Science and Technology
Meijo University
ALCA and SENTAN, Japan
Science and Technology Agency (JST)
Nagoya, Aichi 468-8502 (Japan)
Supporting information and the ORCID identification number(s) for the au-
thor(s) of this article can be found under http://dx.doi.org/10.1002/
asia.201600083.
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Figure 2. (a) Transient absorption spectra of DDQ (1.2 mm) in the absence
(red) and presence of PhOH (0.10 mm, blue) in deaerated MeCN taken at
1.5 ms after nanosecond laser excitation (l=355 nm). (b) Decay time profiles
at 630 nm due to DDQ* with various concentrations of PhOH. Inset: Plot of
kobs vs. [C6H5OH].
Figure 1. (a) GC charts and (b) Irradiation time profiles of the photoinduced
chlorination of phenol (4.0”10À3 m) in deaerated MeCN (0.6 mL) containing
HCl (1.0”10À1 m) at 298 K; [DDQ]=5.0”10À3 m. Phenol: black, p-chlorophe-
nol: red, o-chlorophenol: blue.
3
tion of phenol (0–0.10 mm). The quenching rate constant
(kq) was determined from the slope of kobs versus phenol
concentration to be 9.5”109 mÀ1 sÀ1 (Inset in Figure 2b).
The free energy change of electron transfer from phenol
Table 1. Product yields for chlorination of phenol, methoxybenzene and naph-
thalene derivatives photosensitized by DDQ with HCl in deaerated MeCN at
298 K.[a]
3
to DDQ* is significantly negative (DGet =À1.09 eV), be-
cause the one-electron reduction potential of 3DDQ*
(2.7 V vs. SCE) is smaller than the oxidation potential of
phenol (1.6 V vs. SCE).[16] Thus, electron transfer is ener-
getically favorable to produce the radical ion pair. The
transient absorption spectra indeed showed formation of
Entry Substrate
Conversion Product
[%]
Yield [%]
t
[h]
1
2
88
98
18
16
70 2.0
82 2.5
the radical species composed of DDQ* (lmax =590 nm)
À
and phenol radical cation (lmax =440 nm).[17–19]
Electron transfer from ClÀ (Eox =1.15 V vs. SCE)[20] to
3DDQ* is also favorable. The T–T quenching experiments
were also carried out when phenol was replaced by HCl
as a quencher as shown in Figure 3. The rate constant of
T–T absorption decay increased with an increase in the
concentration of HCl. The transient absorption spectrum
3
4
75
71
4.7
2.0
trace
trace
5
6
7
50
50
78
98
4.7
2.5
2.0
of DDQ with HCl revealed formation of DDQ* at 590 nm
À
by electron transfer from ClÀ to DDQ. The quenching
rate constant was determined from the slope of the
linear plot in the inset of Figure 3b to be 4.1”108 mÀ1 sÀ1.
78
3
100
The quenching rate constant of DDQ* by phenol was 23
times larger than the quenching by ClÀ.
The quantum yield of photochlorination (F) was deter-
mined from the initial rate of the formation of products
by use of a standard actinometer, potassium ferrioxalate
(see the Experimental Section).[21] The plot of F value
versus concentration of HCl is shown in Figure 4a, where
8
99
98
2.0
[a] Conditions: [Sub.]=4 mm, [DDQ]=5.0 mm, [HCl]=0.1m, CD3CN=0.6 mL.
Chem. Asian J. 2016, 11, 996 – 999
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Communication
a maximum value (4.1%) at a high concentration (120 mm).
When the concentration of HCl was fixed at 20 mm, the F
value first increased linearly with increasing concentration of
phenol (up to 0.5 mm), but then gradually decreased with fur-
ther increase in the concentration of PhOH as shown in Fig-
ure 4b.
Based on above-mentioned results, the photochlorination
mechanism is summarized in Scheme 1. Electron transfer from
Figure 3. (a) Transient absorption spectra of DDQ (1.2 mm) in the absence
(red) and presence of HCl (2.0 mm, blue) in deaerated MeCN taken at 2.5 ms
after nanosecond laser excitation (l=355 nm). (b) Decay time profiles at
3
630 nm due to DDQ* with various concentrations of HCl. Inset: Plot of kobs
vs. [HCl].
Scheme 1. Plausible mechanism of photoinduced chlorination of phenol.
phenol to the triplet excited state of DDQ (3DDQ*) occurs to
give phenol radical cation and DDQ* in competition with
À
electron transfer from ClÀ to 3DDQ* to produce Cl* and
DDQ*À. Back electron transfer from DDQ to the phenol radi-
*
À
cal cation is known to occur rapidly,[14] whereas Cl* reacts with
phenol to produce the chlorine-adduct radical. The selectivity
of chlorinations of phenol and naphthalene derivatives is con-
trolled by the thermodynamic stability of the chlorinated radi-
cal, which is confirmed by DFT calculations (see Figure S2, Sup-
porting Information). Hydrogen atom transfer from the chlor-
ine-adduct radical to DDQ* yields the final products, chloro-
À
phenols and 2,3-dichloro-5,6-dicyanohydroquinone (DDQH2) by
protonation with HCl. When the reaction was carried out at
low concentrations of HCl or high concentrations of PhOH,
electron transfer from PhOH to 3DDQ* occurred dominantly
3
without quenching of DDQ* by ClÀ. In such a case, no chlori-
nated product was produced due to the fast back electron
transfer from DDQ* to PhOH*
.
This is the reason of the
À
+ [14]
Figure 4. (a) Plot of quantum yields (F) vs. [HCl] in the photochlorination of
phenol (0.50 mm) with HCl (0–120 mm) and DDQ (2.0 mm) in deaerated
MeCN at 298 K. (b) Plot of quantum yields vs. [phenol] for formation of p-
and o-chlorobenzenes in the photochlorination of phenol (0–1.5 mm) with
HCl (20 mm) and DDQ (2.0 mm) in deaerated MeCN at 298 K.
sigmoidal dependence of F on the concentration of HCl (Fig-
ure 4a) and decrease in the F value at high concentrations of
HCl (Figure 4b). Thus, the quantum yield of the product forma-
tion is determined by the competition of electron transfer
3
from ClÀ versus phenol to DDQ*.
the F value gradually increased with increasing concentration
of phenol to exhibit a sigmoidal curve under the conditions of
low HCl concentration (0–20 mm), and the F value reached
In conclusion, photochlorination of aromatic substrates by
hydrogen chloride with DDQ occurs efficiently via electron
transfer from ClÀ to 3DDQ* to produce the corresponding
Chem. Asian J. 2016, 11, 996 – 999
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Communication
corded with
a
digitizing oscilloscope (Tektronix, TDS3032,
monochlorinated products selectively under visible-light irradi-
ation. This study provides a unique chlorination pathway of ar-
omatic hydrocarbons with chloride.
300 MHz). The transient spectra were recorded for fresh solutions
in each laser excitation. All experiments were performed at 298 K.
Acknowledgements
Experimental Section
Materials
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); ALCA and SENTAN
projects from JST, Japan (to S.F.).
Chemicals were purchased from a commercial source and used
without purification, unless otherwise noted. 2,3-Dichloro-5,6-di-
cyano-p-benzoquinone (DDQ) was obtained commercially and pu-
rified by recrystallization from hot dichloromethane. Potassium fer-
rioxalate used as an actinometer was prepared according to the lit-
erature and purified by recrystallization from hot water.[21] Acetoni-
trile was of spectral grade, obtained commercially, and used with-
out further purification. Deuterated [2H3] acetonitrile (CD3CN) was
obtained from Euriso-TOP, CEA, France, and used as received.
Keywords: chlorination · laser flash photolysis · photoinduced
electron transfer · quinone · radical
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Laser flash photolysis
The measurements of nanosecond transient absorption spectra in
the photochemical reactions of DDQ with phenol and HCl in MeCN
were performed according to the following procedure. Typically,
a nitrogen-saturated MeCN solution containing benzene (1.0”
10À2–1.0m) and DDQ (1.2 mm) was excited by a Nd:YAG laser (Con-
tinuum, SLII-10, 4–6 ns fwhm) at l=430 nm with the power of
10 mJpulseÀ1. Photoinduced events were measured by use of
a continuous Xe lamp (150 W) and an InGaAs-PIN photodiode (Ha-
mamatsu 2949) as a probe light and a detector, respectively. The
output from the photodiodes and a photomultiplier tube was re-
Manuscript received: January 21, 2016
Accepted Article published: February 19, 2016
Final Article published: March 9, 2016
Chem. Asian J. 2016, 11, 996 – 999
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999
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim