Solvent-Free Photooxidation of Alkanes by Dioxygen with 2,3-Dichloro-5,6-dicyano-p-benzoquinone via Photoinduced Electron Transfer

Article abstract of DOI:10.1002/asia.201600828

Photooxidation of alkanes by dioxygen occurred under visible light irradiation of 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) which acts as a super photooxidant. Solvent-free hydroxylation of cyclohexane and alkanes is initiated by electron transfer from alkanes to the singlet and triplet excited states of DDQ to afford the corresponding radical cations and DDQ^??, as revealed by femtosecond laser-induced transient absorption measurements. Alkane radical cations readily deprotonate to produce alkyl radicals, which react with dioxygen to afford alkylperoxyl radicals. Alkylperoxyl radicals abstract hydrogen atoms from alkanes to yield alkyl hydroperoxides, accompanied by regeneration of alkyl radicals to constitute the radical chain reactions, so called autoxidation. The radical chain is terminated in the bimolecular reactions of alkylperoxyl radicals to yield the corresponding alcohols and ketones. DDQ^??, produced by the photoinduced electron transfer from alkanes to the excited state of DDQ, disproportionates with protons to yield DDQH₂.

Full text of DOI:10.1002/asia.201600828

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Title: Solvent-free Photooxidation of Alkanes by Dioxygen with 2,3-Dichloro-5,6-
dicyano-p-benzoquinone via Photoinduced Electron Transfer
Authors: Kei Ohkubo; Kensaku Hirose; Shunichi Fukuzumi
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Solvent-free Photooxidation of Alkanes by Dioxygen with 2,3-
Dichloro-5,6-dicyano-p-benzoquinone via Photoinduced Electron
Transfer
Kei Ohkubo,*[a,b,c] Kensaku Hirose,[b] and Shunichi Fukuzumi *[c,d]
Abstract: Photooxidation of alkanes by dioxygen occurred under
visible light irradiation of 2,3-dichloro-5,6-dicyano-p-benzoquinone
(3DDQ*: * denote the excited state) is 3.18 V vs. SCE, which is high
enough to oxidize benzene by electron transfer to produce benzene
radical cation as a key intermediate.[14,15] Formation of radical cations
(DDQ) which acts as
a
super photooxidant. Solvent-free
hydroxylation of cyclohexane and alkanes is initiated by electron
transfer from alkanes to the singlet and triplet excited states of DDQ
to afford the corresponding radical cations and DDQ•– as revealed by
femtosecond laser-induced transient absorption measurements,
Alkane radical cations readily deprotonate to produce alkyl radicals,
which react with dioxygen to afford alkylperoxyl radicals. Alkylperoxyl
radicals abstract hydrogen atoms from alkanes to yield alkyl
hydroperoxides, accompanied by regeneration of alkyl radicals to
constitute the radical chain reactions, so called autoxidation. The
termination of the radical chain is the bimolecular reactions of
alkylperoxyl radicals to yield the corresponding alcohols and ketones.
On the other hand, DDQ•– produced by the photoinduced electron
transfer from alkanes to the excited states of DDQ disproportionates
with protons to yield DDQH2.
of
benzonitrile
(PhCN),
nitrobenzene
(PhNO2),
and
trifluoromethylbenzene (PhCF3) for the hydroxylation also became
possible by the singlet excited state of DDQ (1DDQ*: * denote the
excited state), which is 3.8 V vs. SCE.[16] The singlet excited state
(1DDQ*), which is rapidly relaxed to DDQ* exhibited a short lifetime
3
of a few picoseconds.[16,17] The lifetime of singlet excited state was
too short to be able to oxidize PhCN at low concentrations.[16] Thus,
neat PhCN was used as a solvent, when electron transfer from
PhCN to 1DDQ* occurred efficiently.[17] The ionization potentials of
PhCN (9.72 eV) is somewhat lower than that of cyclohexane (9.88
eV).[18,19] Electron transfer from cyclohexane to 1DDQ* may be
thermodynamically feasible to activate the C-H bond.[20] However,
there has been no report on photoinduced electron-transfer oxidation
of cycloalkanes or normal alkanes under the homogeneous
conditions.[21]
We report herein solvent-free photooxidation of cyclohexane,
hexane, 3-methylpentane and pentane by O2 using the singlet
excited state of DDQ as an effective photooxidant under visible light
irradiation at ambient conditions. This is the first report for the direct
photochemical hydroxylation of alkanes by O2 via photoinduced
electron transfer. The mechanism is discussed based on
femtosecond and nanosecond laser-induced transient absorption
measurements to directly monitor the photoinduced electron-transfer
reactions.
Recently, photoredox catalysts have merited increasing attention,
because they are well applied in photoinduced organic syntheses
using inorganic[1-9] and organic photosensitizers.[10-13] The
photoredox catalyst is initiated by photoinduced electron-transfer
oxidation/reduction of substrates. Photoinduced electron-transfer
reactions produce highly reactive radical cations and anions, leading
to the finely tuned novel organic synthetic transformations.[1-13]
We have reported the hydroxylation of benzene to phenol under
ambient conditions via photoinduced electron transfer from benzene
to 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ).[14] The one-
electron reduction potential of the triplet excited state of DDQ
Photooxidation of cyclohexane by O2 occurred under visible light
irradiation of DDQ (3.0 mM) in O2-saturated cyclohexane with a
xenon lamp (500 W) attached with a color glass filter (> 390 nm) to
produce
cyclohexane
hydroperoxide,
cyclohexanol
and
cyclohexanone. The yields were determined by GC analysis for
cyclohexanol and cyclohexanone (see Figure S1 in the supporting
information (SI)) and by iodometric titration for cyclohexane
hydroperoxide. The reaction time profile is shown in Figure 1. It is
known for long time that photooxidation of cyclohexane by O2 occurs
under UV light irradiation of the charge-transfer complex formed
between cyclohexane and O2.[22] It was confirmed, however, no
photooxidation of cyclohexane by O2 occurred without DDQ under
visible light irradiation.
[a]
[b]
Prof. Dr. K. Ohkubo
Division of Applied Chemistry
Graduate School of Engineering, Osaka University
Suita, Osaka 565-0871 (Japan)
Tel: (+81) 6-6879-4131
E-mail: ookubo@chem.eng.osaka-u.ac.jp
Prof. Dr. K. Ohkubo, K. Hirose
Department of Material and Life Science
Graduate School of Engineering, Osaka University and
SENTAN, Japan Science and Technology Agency (JST)
Suita, Osaka 565-0871 (Japan)
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
Prof. S. Dr. Fukuzumi
Faculty of Science and Technology, Meijo University
SENTAN, Japan Science and Technology Agency (JST)
Nagoya, Aichi 468-8502 (Japan)
When cyclohexane was replaced by n-hexane, the
photooxidation of neat n-hexane by O2 occurred under visible light
irradiation of DDQ (3.0 mM) in O2-saturated n-hexane to produce the
corresponding hydroperoxides, alcohols and ketones (Figure 2).[23]
Similarly 3-methylpentane and pentane were oxidized under visible
light irradiation of O2-saturated neat solvents containing DDQ
(Figures S3 and S4 in SI). The product yields are summarized in
Table 1.
[c]
[d]
Supporting information for this article is given via a link at the end of
the document.
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Figure 1 Reaction time profiles for formation of cyclohexanol (black) and
cyclohexanone (gray) in the photooxidation of cyclohexane by O2 with DDQ
(3.0 mM) under visible light irradiation of an O2-saturated cyclohexane solution
by use of a xenon lamp (500 W; 
shown in Figure S1 (SI).
Figure 3 a) Transient absorption spectra of DDQ (3.0 mM) in deaerated
cyclohexane after femtosecond laser excitation at 420 nm; b) Time profile of
absorbance at 630 nm.
Table 2 Ionization potentials of alkanes and the rate constant of electron
transfer from alkanes to 1DDQ*
Figure 2 Reaction time profiles for formation of 2-hexanol (black) and 3-
hexanol (red), 2-hexanone (blue) and 3-hexanone (green) in the
photooxidation of n-hexane by O2 with DDQ (5.0 mM) under visible light
irradiation of an O2-saturated n-hexane by use of a xenon lamp (500 W; >
390 nm) at 298 K. The GC charts are shown in Figure S2 (SI).
Substrate
IP,[a] eV
9.79
ket, s–1
9.2 x 1011
(9.2 x 1011) [b]
9.0 x 1010
1.4 x 1011
3.8 x 1011
Table 1 Product yields and quantum yields () for the photooxidation of
10.18
10.08
10.34
alkanes by O2 with DDQ in O2-saturated alkanes under visible light irradiation[a]
[a] Taken from refs. 18, 19. [b] Value in parenthesis is determined in
cyclohexane-d12
.
the photooxidation of cyclohexane by O2 with DDQ increased with
increasing concentration of O2 to reach a constant value. The 
values of cyclohexane and 3-methylpentane are significantly larger
than 100 %, 280% for cyclohexane and 300% for 3-methylpentane
(Table 1). This indicates that the photochemical step induces radical
chain processes (vide infra).
[a] [DDQ]0 = 2.0 mM, T = 298 K. [b] Total quantum yield for formation of
oxygenated products.
The mechanism of photooxidation of alkanes was examined by
laser-induced time-resolved transient absorption experiments. A
singlet-singlet absorption spectrum of DDQ was observed at 470 nm
The quantum yields () of the formation of oxygenated products
were determined by use of a standard actinometer, potassium
ferrioxalate (see the experimental section in SI).[24] The  value of
in cyclohexane at 1.3 ps after femtosecond laser excitation of λex
=
1
420 nm (see Figure 3). The singlet-singlet absorption due to DDQ*
decayed, accompanied by an increase in absorption at 630 nm due
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to the triplet-triplet absorption band of DDQ. The absorption at λmax
=
The photooxidation of cyclohexane (CyH) by O2 with DDQ is
605 nm is assigned to the semiquinone radical anion of DDQ (DDQ•–
).[25] This absorption band agrees with that observed in the electron-
transfer reduction of DDQ by decamethylferrocene in 1,2-
dichloroethane (λmax = 605 nm) (Figure S5 in SI).[16,25] We also
confirmed the formation of DDQ•– by the ESR measurements (Figure
S6 in SI). Thus, efficient electron transfer from cyclohexane to 1DDQ*
occurs to produce the radical ion pair (CyH•+ and DDQ•–), in which
only DDQ•– has the absorption in the visible region (Figure 3a).
The rate constant of formation radical ion pair (ket) was
determined from the first-order rate constant of the rise in
absorbance at 605 nm (kobs = 1.1 × 1012 s–1 in Figure 3b) using
Equation (1), to be 9.2 × 1011 s–1. Similarly the ket values were
initiated by electron transfer from CyH to DDQ* form the radical ion
1
pair of cyclohexene radical cation (CyH•+) and DDQ•– (Scheme 1).
CyH•+ is deprotonated rapidly to produce cyclohexyl radical (Cy•) and
2,3-dichloro-5,6-dicyano-1,4-hydroquinone (DDQH2). The addition of
O2 to Cy• is known to occur rapidly to afford cyclohexylperoxyl radical
(CyOO•), which abstracts a hydrogen atom from cyclohexane to
produce cyclohexyl hydroperoxide (CyOOH), accompanied by
regeneration of cyclohexyl radical (Cy•) to constitute the radial chain
reactions. Because the quantum yield became constant with
increasing concentration of O2 (Figure 3) , the O2 addition process is
not the rate-determining process in O2-saturated cyclohexane. The
chain reactions are terminated by the disproportionation of CyOO• to
yield cyclohexanol (CyOH) and cyclohexanone (Cy=O).[35-38] The
reason why the yield of cyclohexanol is larger than that of
cyclohexanone may result from the radical chain decomposition of
CyOOH to CyOH via cyclohexanoxyl radical (CyO•) as shown in
Scheme 2.
ket = kobs – kISC
(1)
determined for electron transfer from other alkanes to 1DDQ* as
listed in Table 2 (data are shown in Figure S7-S9 in SI). The ket
value of n-hexane (9.0 × 1010 s–1) is one-order of magnitude smaller
than the value of cyclohexane in accordance with the higher
ionization potential of n-hexane (10.13 eV) as compared with the
value of cyclohexane (9.88 eV).[19] No kinetic isotope effect was
observed in the case of deuterated cyclohexane (C6D12) (see Table
1 and Figure S10 in SI), indicating that the observed behavior is an
electron-transfer process rather than a hydrogen abstraction from
the substrate. It has been well known that triplet excited ketones are
quenched by hydrogen abstraction from substrates.[26-32] In the case
In conclusion, efficient photooxidation of alkanes by O2 are
initiated by photoinduced electron transfer from alkanes used as
neat solvents to the singlet excited state of DDQ (1DDQ*) to produce
the radical ion pair (alkane radical cation and DDQ•–). Deprotonation
of alkane radical cation produces the alkyl radical to which O2 is
added rapidly to give the alkylperoxyl radical. Then, the well-known
chain propagation proceeds via the hydrogen abstraction from the
alkane by the alkylperoxyl radical to yield the alkyl hydroperoxide.
The corresponding alcohols and ketones are produced by the
termination step, that is the disproportionation of the alkylperoxyl
radicals. Such radical chain autoxidation processes with high
concentrations of substrates (neat solvent) have enabled to obtain
the high quantum yields. Thus, the present study provides an
environmentally benign way for photooxidation of alkanes by O2
initiated by photoinduced electron transfer.
1
of DDQ, DDQ* is quenched by electron transfer from alkanes prior
to the rapid intersystem crossing to the triplet excited state.[33,34]
Experimental Section
Materials. Chemicals were purchased from commercial source and used
without purification, unless otherwise noted. DDQ was recrystallized from
hot ethanol. Potassium ferrioxalate used as an actinometer to determine
the quantum yield was prepared according to the literature and purified
by recrystallization from hot water.[24]
Reaction procedures. The photooxygenation of saturated hydrocarbons
with DDQ was carried out by the following procedure. Typically, a
cyclohexane solution (3.0 cm3) containing DDQ (2.0 mM) with acetonitrile
as a co-solvent (2%) in a square quartz cuvette (10 mm x 10 mm) with a
silicon septum was saturated with oxygen by bubbling oxygen through a
stainless tube for 5 min. The solution was then irradiated with a 500 W
xenon lamp (Ushio Optical Module X SX-UID 500XAMQ) through a color
filter glass (Asahi Techno Glass Y43) transmitting  > 390 nm at room
temperature. After photoirradiation, the corresponding oxygenated
products were identified and quantified by comparison of the GC
retention time and the MS spectra (Shimadzu QP-5000) in comparison
with those of authentic samples.
The amounts of hydroperoxides were determined by titration by
iodide ion as follows. The photoirradiated solution was titrated with
–
Scheme
1
Proposed reaction mechanism for the photooxidation of
excess amount of NaI (0.1 M). The amount of I3 formed was then
determined from the UV-vis spectrum (max = 361 nm, 361 = 2.5 x 104 M–1
cyclohexane by O2 with DDQ via photoinduced electron transfer
cm–1).[39]
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Acknowledgements
This work was supported by JSPS KAKENHI (Nos.
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•
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Entry for the Table of Contents
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K. Ohkubo,* K. Hirose and S. Fukuzumi*
Page No. – Page No.
Solvent-free Photooxidation of
Alkanes by Dioxygen with 2,3-
Dichloro-5,6-dicyano-p-benzoquinone
via Photoinduced Electron Transfer
Aerobic photooxidation of alkanes occurred under visible light irradiation of 2,3-
dichloro-5,6-dicyano-p-benzoquinone via the photoinduced electron-transfer
process.
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