Photocatalytic monofluorination of benzene by fluoride via photoinduced electron transfer with 3-cyano-1-methylquinolinium

Article abstract of DOI:10.1021/jp408315a

The photocatalytic fluorination of benzene occurs under photoirradiation of an oxygen-saturated acetonitrile (MeCN) of the 3-cyano-1-methylquinolinium ion (QuCN⁺) containing benzene and tetraethylammonium fluoride tetrahydrofluoride (TEAF·4HF) with a xenon lamp (500 W) attached to a colored-glass filter (λ < 290 nm) to yield fluorobenzene and hydrogen peroxide. The quantum yield of formation of fluorobenzene was 6%. Nanosecond laser flash photolysis measurements were performed to elucidate the mechanistic details for photocatalytic fluorination. Transient absorption spectra taken after the nanosecond laser excitation at 355 nm of a degassed MeCN solution of QuCN⁺ and benzene exhibited absorption bands due to QuCN ^? (λ_max = 500 nm) and the benzene dimer radical cation (λ_max = 900 nm), which were generated by photoinduced electron transfer from benzene to the singlet excited state of QuCN⁺. The decay rate of the transient absorption band due to the benzene dimer radical cation was accelerated by the addition of TEAF·4HF. The observed rate constant increased with increasing concentration of TEAF·4HF. The rate constant of the electrophilic addition of fluoride to the benzene radical cation was determined to be 9.4 × 10⁹ M^-1 s^-1. Thus, the photocatalytic reaction is initiated by intermolecular photoinduced electron transfer from benzene to the single excited state of QuCN⁺. The benzene radical cation formed by photoinduced electron transfer reacts with the fluoride anion to yield the F-adducted radical. However, QuCN^? can reduce O₂ to O₂^?-, and this is followed by the protonation of O₂^?- to afford HO₂^?. The hydrogen abstraction of HO₂^? from the F-adduct radical affords fluorobenzene and H₂O₂ as the final products.

Full text of DOI:10.1021/jp408315a

Article
pubs.acs.org/JPCA
Photocatalytic Monofluorination of Benzene by Fluoride via
Photoinduced Electron Transfer with 3‑Cyano-1-methylquinolinium
Kei Ohkubo,† Atsushi Fujimoto,† and Shunichi Fukuzumi*,†,‡
†Department of Material and Life Science, Graduate School of Engineering, Osaka University and ALCA, Japan Science and
Technology Agency (JST), 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan
‡Department of Bioinspired Science, Ewha Womans University, Seoul 120-750, Korea
S
* Supporting Information
ABSTRACT: The photocatalytic fluorination of benzene occurs under photoirradiation of
an oxygen-saturated acetonitrile (MeCN) of the 3-cyano-1-methylquinolinium ion
(QuCN+) containing benzene and tetraethylammonium fluoride tetrahydrofluoride
(TEAF·4HF) with a xenon lamp (500 W) attached to a colored-glass filter (λ < 290
nm) to yield fluorobenzene and hydrogen peroxide. The quantum yield of formation of
fluorobenzene was 6%. Nanosecond laser flash photolysis measurements were performed
to elucidate the mechanistic details for photocatalytic fluorination. Transient absorption
spectra taken after the nanosecond laser excitation at 355 nm of a degassed MeCN
solution of QuCN+ and benzene exhibited absorption bands due to QuCN• (λmax = 500
nm) and the benzene dimer radical cation (λmax = 900 nm), which were generated by
photoinduced electron transfer from benzene to the singlet excited state of QuCN+. The
decay rate of the transient absorption band due to the benzene dimer radical cation was
accelerated by the addition of TEAF·4HF. The observed rate constant increased with
increasing concentration of TEAF·4HF. The rate constant of the electrophilic addition of
fluoride to the benzene radical cation was determined to be 9.4 × 109 M−1 s−1. Thus, the photocatalytic reaction is initiated by
intermolecular photoinduced electron transfer from benzene to the single excited state of QuCN+. The benzene radical cation
formed by photoinduced electron transfer reacts with the fluoride anion to yield the F-adducted radical. However, QuCN• can
•
reduce O2 to O2•−, and this is followed by the protonation of O2•− to afford HO2 . The hydrogen abstraction of HO2• from the
F-adduct radical affords fluorobenzene and H2O2 as the final products.
(MeCN).37,47,48 We have recently found that the 3-cyano-1-
INTRODUCTION
■
methylquinolinium ion (QuCN+) has an extremely strong
Aromatic fluorination reactions have merited special attention
oxidizing ability to oxidize benzene easily by electron transfer
because of useful application in materials science, the chemical
upon photoexcitation in MeCN.37 The reduced radical of
industry, and medicine.1−10 Fluorinated substitution has been
carried out by the Balz-Schiemann reaction using HBF4 with
QuCN+ (QuCN•) as a product of photoinduced electron
transfer is readily oxidized by O2 to regenerate QuCN+. Thus,
HNO3, which requires an amino group.11,12 Direct substitution
QuCN+ can be an effective photocatalyst for the fluorination of
of the fluorine group on the benzene ring is also known by
benzene with O2 via electron-transfer oxidation of benzene with
electrochemical fluorination and the use of XeF2; however, their
synthetic utility has been limited because of low yield and poor
a weak nucleophile, the fluoride anion. The nucleophilic
selectivity.13−21 In particular, it has been difficult to perform
addition of the fluoride anion to radical cations would enable
selective fluorination via the electron-transfer oxidation of
selective monofluorination of aromatic compounds. The best
substrates. However, there has been no report on the selective
oxidant is obviously dioxygen from the standpoint of green
chemistry.22−24
Radical cations produced by photoinduced electron-transfer
reactions are known to react with nucleophiles to form various
monofluorination of aromatic compounds via electron-transfer
oxidation using oxygen as an oxidant.
We report herein that 3-cyano-1-methylquinolinium per-
chlorate (QuCN+ClO4 )37 acts as an efficient photocatalyst for
the selective monofluorination of benzene with tetraethylam-
monium fluoride tetrahydrogen fluoride salt (TEAF·4HF) as a
fluorine source and O2 as an oxidant under photoirradiation to
produce fluorobenzene without further fluorination. The
−
adducts.25−38 Among aromatic radical cations, the benzene
radical cation is a very strong electrophile. Thus, the electron-
transfer oxidation of benzene is expected to undergo an
efficient nucleophilic aromatic substitution. The nucleophilic
substitution of aromatic radical cations has been mainly
investigated in the gas phase.39−46 However, the electron-
transfer oxidation of benzene in solution is relatively difficult
because of the high one-electron oxidation potential of
benzene, Eox = 2.48 V versus SCE in acetonitrile
Received: August 20, 2013
Revised: September 17, 2013
Published: September 19, 2013
© 2013 American Chemical Society
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photocatalytic reaction is initiated by photoinduced electron
transfer from benzene to the singlet excited state of QuCN+.
The photocatalytic mechanism is clarified by detecting radical
intermediates involved in the photocatalytic reactions with the
use of laser flash photolysis. The present study provides a
unique fluorination pathway of aromatic hydrocarbons with
fluoride and oxygen as a green oxidant.
recorded in a FluoroMax-4 equipped with a 150 W xenon
lamp. Time-resolved florescence measurements were made with
the technique of time-correlated single-photon counting
(TCSPC) in a FluoroMax-4-TCSPC. Samples were excited
with a 350 nm NanoLED with a fwhm of 1.4 ns and a
repetition rate of 100 MHz.
Laser Flash Photolysis. Measurements of nanosecond
transient absorption spectra in the photochemical reactions of
QuCN+ with benzene, TEAF·4HF, and O2 in MeCN were
performed according to the following procedures. Typically, the
nitrogen-saturated MeCN solution containing benzene (10
mM−1.0 M) and QuCN+ (40 μM) was excited with a Nd:YAG
laser (Continuum, SLII-10, 4−6 ns fwhm) at λ = 355 nm with a
power of 10 mJ/pulse. Photoinduced events were monitored by
the use of a continuous Xe lamp (150 W) and an InGaAs-PIN
photodiode (Hamamatsu 2949) as a probe light and a detector,
respectively. The output from the photodiodes and a
photomultiplier tube was recorded with a digitizing oscilloscope
(Tektronix, TDS3032, 300 MHz). The transient spectra were
recorded for fresh solutions in each laser excitation. All
experiments were performed at 298 K.
EXPERIMENTAL SECTION
■
Materials. 1-Methyl-3-cyanoquinolinium iodide
(QuCN+I−) was prepared by the reaction of 3-cyanoquinoline
with methyl iodide in acetonitrile, converted to the perchlorate
salt (QuCN+ClO4 ) by the addition of magnesium perchlorate
−
to QuCN+I− in methanol and purified by recrystallization from
methanol.49,50 Potassium ferrioxalate used as an actinometer
was prepared according to the literature and purified by
recrystallization from hot water.51,52 Benzene, fluorobenzene,
chlorobenzene, bromobenzene, and iodobenzene used as
substrates were obtained commercially. Tetraethylammonium
fluoride tetrahydrofluoride (TEAF·4HF) was purchased from
Tokyo Chemical Industry Co., Ltd. Acetonitrile was spectral
grade, obtained commercially and used without further
purification. Deuterated [2H3] acetonitrile (CD3CN) was
obtained from Euri SO-TOP, CEA, France, and used as
received.
Theoretical Calculations. Density functional theory
(DFT) calculations were performed with Gaussian 09.55 The
calculations were performed on a 32-processor QuantumCube
at the B3LYP/6-311+G(d,p) level of theory by applying the
polarizable continuum model (PCM) using the integral
equation formalism variant (IEF-PCM). Electronic charges
were obtained by natural population analysis (NPA).
Reaction Procedure. A CD3CN solution (0.6 cm3)
−
containing QuCN+ClO4 (2.0 × 10−3 M) and TEAF·4HF
(5.0 × 10−2 M) in a sample tube sealed with a rubber septum
was saturated with oxygen by bubbling with oxygen through a
stainless steel needle for 5 min. Then, benzene (1.2 μmol, 2.0 ×
10−2 M) or halogenated benzene (1.2 μmol, 2.0 × 10−2 M) was
added to the solution. The solution was then irradiated with a
500 W xenon lamp (Ushio Optical model X SX-UID
500XAMQ) through a colored-glass filter transmitting λ >
290 nm at room temperature. After photoirradiation, the
corresponding oxygenated product was identified and quanti-
RESULTS AND DISCUSSION
■
The photoinduced fluorination of benzene occurs under the
photoirradiation of an oxygen-saturated acetonitrile (MeCN)
solution containing QuCN+ ClO4 (2.0 mM), benzene (20
−
mM), and TEAF·4HF (50 mM) with a xenon lamp (500 W)
through a colored-glass filter transmitting λ > 290 nm to yield
fluorobenzene and hydrogen peroxide (H2O2) (eq 1). The time
course of the photocatalytic reaction is shown in Figure 1.
1
fied by a comparison of the H NMR spectra with that of an
authentic sample. The1H NMR measurements were performed
on a Japan Electron Optics JMN-AL300 (300 MHz) NMR
spectrometer. The yield of H2O2 was determined by titration
with excess NaI (100 mm). The amount of I3− formed was then
determined from the UV−vis spectrum (λmax = 361 nm, ε361 nm
= 2.5 × 104 M−1 cm−1).53,54
Quantum Yield Determinations. A standard actinometer
(potassium ferrioxalate)51 was used for the quantum yield
determination of the QuCN+ photosensitized fluorination of a
benzene with oxygen and fluoride anions. A square quartz
cuvette (10 mm i.d.) that contained an MeCN solution (3.0
cm3) of QuCN+ClO4− (5.0 × 10−3 M), TEAF·4HF (5.0 × 10−2
M), and benzene (0−1.0 × 10−1 M) was irradiated with λ = 334
nm monochromatized light from a Shimadzu RF-5300PC
fluorescence spectrophotometer. Under the conditions of
actinometry experiments, the actinometer and QuCN+
absorbed essentially all of the incident light of λ = 334 nm.
The light intensity of monochromatized light of λ = 334 nm
was determined to be 1.2 × 10−8 einstein s−1. The
photochemical reaction was monitored using a Shimadzu
GC-17A gas chromatograph and a Shimadzu MS-QP5000 mass
spectrometer. The quantum yields were determined from the
increase in the quantity of fluorinated benzene derivatives.
Photophysical Measurements. UV−visible absorption
measurements were performed using a Hewlett-Packard 8453
spectrophotometer. Steady-state fluorescence spectra were
The yield of fluorobenzene after 50 min of photoirradiation was
20% with a 40% conversion of benzene, which was determined
1
by GC-MS and H NMR analyses. H2O2 was detected by an
iodometric titration. Phenol was also detected as a side product
because of the photocatalytic oxygenation of benzene with
QuCN+ in the presence of a small amount of H2O containing
MeCN.36,56−58 No further fluorination occurred under the
present reaction conditions. When benzene was replaced by
fluorobenzene as a substrate, no fluorination occurred to yield
difluorobenzene. The quantum yield of the formation of
fluorobenzene (Φ) was determined from the initial rate of
product formation by the use of potassium ferrioxalate as a
standard actinometer (Experimental Section).51,52 The Φ value
increased with increasing concentration of benzene to approach
a limiting value (Φ∞) (Figure 2). Such a saturated dependence
of Φ on the benzene concentration is expressed by
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kobs = Kobsτ−1
(3)
From the curve fitting using eq 2 in Figure 2, the values of Φ∞
and Kobs were determined to be 6.0% and 90 M−1, respectively.
The kobs value was determined to be 1.0 × 1010 s−1 as calculated
from eq 3 using τ = 9.0 ns for 1QuCN+*.59 The pertinent data
of the photoinduced fluorination of benzene and halogenated
benzene are summarized in Table 1.
Photoirradiation of the absorption band of QuCN+ (λex
=
350 nm) results in fluorescence in MeCN at 430 nm. The
fluorescence lifetimes were determined by time-resolved
fluorescence decay measurements as shown in Figure 3a. The
Figure 1. Irradiation time profiles of the photoinduced fluorination of
benzene (2.0 × 10−2 M) in O2-saturated MeCN (0.6 mL) containing
−
TEAF·4HF (5.0 × 10−2 M) at 298 K; [QuCN+ ClO4 ] = 2.0 × 10−3
M. Benzene, black; hydrogen peroxide, blue; fluorobenzene; red;
phenol, orange.
Figure 2. Plot of quantum yield vs [C6H6] for the formation of
fluorobenzene in the photofluorination of benzene (0−100 mM) with
TEAF·4HF (50 mM) and QuCN+ (2.0 mM) in O2-saturated MeCN
at 298 K.
Φ Kobs[C6H6]
1 + Kobs[C6H6]
∞
Φ =
(2)
Figure 3. (a) Fluorescence decays of QuCN+ at 430 nm with benzene
(0−10 mM) in deaerated MeCN containing H2O (100 mM). A pale
gray line denotes the instrument response. Excitation wavelength, 350
nm. (b) Plot of fluorescence decay rate constant (kobs) vs benzene
concentration.
where Kobs is the quenching constant. The Kobs value can be
converted to the corresponding rate constant (kobs) provided
that the lifetime of the excited state involved in the reaction (τ)
is known as eq 3.
Table 1. Product Yields for the Photocatalytic Fluorination of Benzene Derivatives and Rate Constants of Photoinduced
Electron Transfer from Substrates to 1QuCN+* (kq) and the Addition Reaction of F− to Substrate Radical Cations (kF) in MeCN
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1
fluorescence decay rate constant of QuCN+* (kobs) increased
with an increase in the concentration of benzene (0−10 mM).
The quenching rate constant (kq) was determined from the
slope of kobs versus [C6H6] to be 1.1 × 1010 M−1 s−1 (Figure
3b), which is close to the rate constant determined from the
quantum yield dependence on the benzene concentration
(Figure 2) given by eqs 2 and 3. Thus, the rate-determining
step is photoinduced electron transfer from benzene to
1QuCN+*.
The free-energy change of photoinduced electron transfer
1
from benzene to QuCN+* was determined from the one-
electron oxidation potential of benzene (Eox = 2.48 V vs
SCE),36,46,47 the one-electron reduction potential of QuCN+
(Ered = −0.60 V vs SCE),57 and the singlet excited energy of
QuCN+ (1E* = 3.32 eV)36 using eq 4 to be negative (ΔGet =
−0.24 eV).
ΔGet = e(Eox − Ered) − 1E*
(4)
Thus, the photocatalytic fluorination of benzene is made
possible by the electron-transfer oxidation of benzene by
1
1QuCN+*. The rate constants of quenching of QuCN+* by
halogenated benzenes were also determined, and the values are
summarized in Table 1. (Data are shown in Supporting
Figure 4. (a) Transient absorption spectrum observed upon 355 nm
nanosecond laser excitation of QuCN+ (40 μM) with benzene (1.0 M)
in deaerated MeCN at 298 K taken after 0.4 μs. (b) Decay time
profiles of absorbance at 900 nm in the presence of TEAF·4HF (0, 0.1,
0.2, and 0.5 mM). Kinetic traces were drawn by single-exponential
curve fitting. (Inset) kobs vs [TEAF·4HF].
1
Information Figures S1−S4.) QuCN+* was also efficiently
quenched by fluorobenzene, which is a reason that the
conversion of benzene was limited in the photocatalytic
fluorination of benzene (Figure 1).
The occurrence of photoinduced electron transfer from
benzene to 1QuCN+* was confirmed by nanosecond laser flash
photolysis measurements. Nanosecond laser excitation at 355
nm of QuCN+ in deaerated MeCN containing benzene (1.0 M)
afforded a transient absorption spectrum at 0.4 μs, in which
new absorption bands appeared at 520 and 900 nm as shown in
Figure 4a. The transient absorption band at 520 nm is assigned
to the one-electron reduced QuCN+ (QuCN•).36,60 The near-
IR absorption around 900 nm is assigned to the benzene dimer
radical cation.36,57,61 The absorption bands at 520 and 900 nm
decay, obeying second-order kinetics resulting from the
bimolecular back electron transfer from QuCN• to the benzene
dimer radical cation or the benzene radical cation. When
TEAF·4HF was introduced into the QuCN+/benzene system,
the broad absorption due to the benzene dimer radical cation
disappeared as shown in Figure 4b, whereas the absorption
band at 520 nm due to QuCN• remained. The decay time
profile in the presence of TEAF·4HF obeyed first-order
kinetics. The observed decay rate constant (kobs) increased
linearly with increasing concentration of TEAF·4HF as shown
in the inset of Figure 4b. The rate constant of the reaction of
the benzene dimer radical cation or the benzene radical cation
with fluoride (kF) was determined to be 9.4 × 109 M−1 s−1 from
the slope shown in the inset of Figure 4b. No quenching of the
benzene dimer radical cation was observed when TEAF·4HF
was replaced by tetra-n-butylammonium chloride or bromide
under otherwise the same experimental conditions. Thus, no
photocatalytic chlorination and bromination of benzene
occurred with QuCN+. The rate constants of the benzene
radical cation and halogenated benzene radical cations with
TEAF·4HF were also determined to be 1.0 × 1010−1.3 × 1010
M−1 s−1, respectively (Table 1) (data are shown in Figures S5−
S7 in Supporting Information). No dependence of kq on the
free-energy change of electron transfer was observed because
the kq values are close to the diffusion rate constant in MeCN
because of the highly exergonic photoinduced electron-transfer
reactions.
On the basis of the above-mentioned results, the photo-
catalytic mechanism is proposed as shown in Scheme 1, where
the photocatalytic reaction is initiated by photoinduced
1
electron transfer from benzene to QuCN+*. The benzene
radical cation, formed by photoinduced electron transfer, exists
in equilibrium with the benzene dimer radical cation because of
the large excess of benzene. The benzene radical cation reacts
with the fluoride of TEAF·4HF to yield the F-adduct radical.
However, radical QuCN•, formed by the photoinduced
1
electron transfer with QuCN+*, can reduce O2 to O2•−, and
62
•
•−
this is followed by the protonation of O2 to afford HO2 .
•
The hydrogen abstraction of HO2 from the F-adduct radical
yields fluorobenzene and H2O2 (Scheme 1).64 The formation
of phenol as a side product (Figure 1) is ascribed to the
addition of OH− to the benzene radical cation.
Figure 5 shows electronic charges of the radical cations
calculated by the DFT method to clarify the reason that no
photofluorination of fluorobenzene and iodobenzene occurred
as shown in Table 1. In the case of benzene radical cation, OH−
can attack toward the carbon nucleus with a positive charge
(+0.081 shown in Figure 5a). However, the electronic positive
charge of the fluorobenzene radical cation is mainly localized on
the ipso carbon (+0.609) rather than on the other carbons
(−0.196 for the ortho postion, −0.151 for the meta position,
and +0.056 for the para position) as a result of the electron-
withdrawing effect of the fluorine atom (Figure 5b). F− of
TEAF·4HF may attack toward the ipso carbon to form the 1,1-
difluorocyclohexadienyl radical. Then, C−F bond cleavage
occurs to yield a starting material, fluorobenzene, without
fluorination (Scheme 2). In contrast, the iodobenzene radical
cation has no positive charge on the carbon atoms as a result of
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Scheme 1. Photocatalytic Mechanism of the Fluorination of Benzene with QuCN+
ASSOCIATED CONTENT
■
S
* Supporting Information
Transient absorption spectral data to determine rate constants.
Complete author list for ref 55. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*Fax: +81-6-6879-7370. E-mail: fukuzumi@chem.eng.osaka-u.
ac.jp.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by grants-in-aid (nos.
20108010 and 23750014) from MEXT, Japan, and NRF/
MEST of Korea through the WCU (R31-2008-000-10010-0)
and GRL (2010-00353) programs.
Figure 5. Electronic charges on the carbon and halogen atoms of
radical cations of (a) benzene, (b) fluorobenzene, (c) chlorobenzene,
(d) bromobenzene, and (e) iodobenzene obtained by natural
population analyses (NPA). The geometry optimizations were carried
out with density functional theory at the B3LYP/6-311+G(d,p) level
by applying the polarizable continuum model (PCM) using the
integral equation formalism variant (IEF-PCM).
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