Photooxygenation of alkanes by dioxygen with: P -benzoquinone derivatives with high quantum yields

Article abstract of DOI:10.1039/c6pp00102e

Alkanes were oxygenated by dioxygen with p-benzoquinone derivatives such as p-xyloquinone in alkanes which are used as solvents to yield the corresponding alkyl hydroperoxides, alcohols and ketones under visible light irradiation with high quantum yields (Φ = 1000, 1600%). The photooxygenation is started by hydrogen atom abstraction from alkanes by the triplet excited states of p-benzoquinone derivatives as revealed by laser-induced transient absorption spectral measurements.

Full text of DOI:10.1039/c6pp00102e

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Photooxygenation of alkanes by dioxygen
with p-benzoquinone derivatives with
high quantum yields†
Kei Ohkubo,*a,b,c Kensaku Hiroseb and Shunichi Fukuzumi*c,d
Cite this: Photochem. Photobiol. Sci.,
2016, 15, 731
Received 11th April 2016,
Accepted 29th April 2016
DOI: 10.1039/c6pp00102e
www.rsc.org/pps
Alkanes were oxygenated by dioxygen with p-benzoquinone is initiated by electron transfer from Cl− to the mesitylene
derivatives such as p-xyloquinone in alkanes which are used as sol- radical cation moiety of the electron-transfer state of the
vents to yield the corresponding alkyl hydroperoxides, alcohols 9-mesityl-10-methylacridinium ion to produce Cl• that
and ketones under visible light irradiation with high quantum abstracts a hydrogen atom from cyclohexane to generate a
yields (Φ = 1000, 1600%). The photooxygenation is started by cyclohexyl radical, which is readily oxygenated by O2.13 There
hydrogen atom abstraction from alkanes by the triplet excited has been no report on the photochemical oxygenation of cyclo-
states of p-benzoquinone derivatives as revealed by laser-induced hexane with an organic photooxidant and without an in-
transient absorption spectral measurements.
organic compound such as HCl.
We report herein inorganic-free oxygenation of cyclohexane
by O2 using p-benzoquinone derivatives as the only organic
photocatalyst in O2-saturated cyclohexane under visible light
irradiation. Other alkanes were also oxygenated by O2 with
p-benzoquinone derivatives in O2-saturated alkanes under
visible light irradiation. The photooxygenation mechanism is
clarified by laser-induced transient absorption measurements.
Photoirradiation of an O2-saturated cyclohexane solution
(3.0 cm3) containing p-xyloquinone (PXQ, 2.0 mM) for 26 h
with a xenon lamp (λ > 390 nm) resulted in the production
of oxygenated products (Fig. 1), which were detected by
Oxidation of alkanes, which are saturated hydrocarbons, to
produce basic raw materials in chemical industry is quite
difficult and requires harsh conditions. For example, the con-
ventional industrial process for oxidation of cyclohexane to a
mixture of cyclohexanol and cyclohexanone known as KA-oil,
which are important intermediates for the manufacture of
nylon-6 and nylon-6,6, requires the use of soluble cobalt or
manganese salts as homogeneous catalysts at 140–180 °C and
0.8–2 MPa (7.9–19.8 atm) in air.1,2 So far extensive efforts have
been devoted to develop catalytic oxidation of cyclohexane
using hydrogen peroxide (H2O2),3–6 tert-butyl hydroperoxide
(TBHP),6,7 iodosylbenzene (PhIO),8 ozone (O3),9 and dioxygen
(O2)3,10–12 as oxidants. Among various oxidants, O2 is an ideal
oxidant because it is ubiquitous and the reduced product
(H2O or H2O2) is safe. Oxygenation of cyclohexane by O2 was
made possible by using the 9-mesityl-10-methylacridinium ion
as an organic photocatalyst combined with HCl in O2-saturated
acetonitrile (MeCN) at room temperature under visible light
irradiation.13 The photochemical oxygenation of cyclohexane
aDepartment of Applied Chemistry, Graduate School of Engineering, Osaka
University, Suita, Osaka 565-0871, Japan. E-mail: ookubo@chem.eng.osaka-u.ac.jp
bDepartment of Material and Life Science, Graduate School of Engineering, Osaka
University, ALCA and SENTAN, Japan Science and Technology Agency (JST), Suita,
Osaka 565-0871, Japan
cDepartment of Chemistry and Nano Science, Ewha Womans University,
Seoul 120-750, Korea. E-mail: fukuzumi@chem.eng.osaka-u.ac.jp
dFaculty of Science and Technology, Meijo University, ALCA and SENTAN,
Japan Science and Technology Agency (JST), Nagoya, Aichi 468-8502, Japan
†Electronic supplementary information (ESI) available: Experimental details and
GC data for the product analyses. See DOI: 10.1039/c6pp00102e
Fig. 1 Reaction time profiles for the formation of cyclohexyl hydroper-
oxide (blue), cyclohexanol (black) and cyclohexanone (red) in the photo-
chemical oxygenation of an oxygen-saturated cyclohexane solution
containing PXQ (2.0 mM) under photoirradiation by the use of a xenon
lamp (500 W; λ > 390 nm) at 298 K. The GC charts for the determination
of the yields are shown in Fig. S1 (ESI†).
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GC-MS (Fig. S1 in the ESI†).14 The yields based on PXQ were
Nanosecond laser flash photolysis was employed to observe
580% for cyclohexyl hydroperoxide, 180% for cyclohexanol the hydrogen-transfer dynamics of the triplet excited state of
and 130% for cyclohexanone (Scheme 1). Cyclohexyl hydro- PXQ (3PXQ*: * denotes the excited state) in cyclohexane. The
peroxide was also detected by the titrations of sodium transient absorption spectra were recorded at 0.1 μs after nano-
iodide15 and triphenylphosphine12 (see the Experimental second pulse irradiation at 355 nm of a deaerated cyclohexane
section in the ESI†). The conversion of neat cyclohexane was solution (spectrum in black) and acetonitrile solution (red)
22%. The quantum yield of the photooxygenation was deter- containing PXQ as shown in Fig. 2a. The transient absorption
mined to be as high as 100% from the initial rate using bands appearing at 440 nm and 405 nm are assigned to
monochromatised light (λ = 420 nm) (see the Experimental 3PXQ* 16 and PXQH•,17 respectively. The decay at 440 nm
section in the ESI†).
obeyed pseudo-fist-order kinetics as shown in Fig. 2b. The
3
When PXQ was replaced by p-benzoquinone (Q), the yields decay rate constants (kobs) of PXQ* increased with increasing
were decreased to 16% for cyclohexanol and 14% for cyclo- concentrations of cyclohexane (Fig. 2c). The rate constant of
hexanone (Fig. S2a in the ESI†). In the case of tetramethyl- hydrogen transfer from cyclohexane to 3PXQ* (kH) was deter-
p-benzoquinone, no oxygenated product was formed under the mined to be 1.3 × 107 M−1 s−1 from the slope of the plot of kobs
otherwise same experimental conditions. No photoreactivity of vs. [cyclohexane] (Fig. 2c). The value in the case of cyclo-
tetramethyl-p-benzoquinone may result from the steric effect hexane-d12 (kD) was also determined to be 4.3 × 106 M−1 s−1
of the methyl group.
(Fig. 2c; red plot). The deuterium kinetic isotope effect (KIE =
Photooxygenation of branched alkanes such as 3-methyl- kH/kD) was determined to be 3.0. Such a large KIE value indi-
pentane with PXQ occurred to produce the corresponding cates that the rate-determining step of the photochemical oxy-
alcohol and ketone products (Table 1, entry 2) with the total genation is hydrogen-atom transfer from cyclohexane to the
yield of 1000% based on the initial concentration of PXQ triplet excited state of PXQ (3PXQ*), which produces a cyclo-
(2.0 mM). In the case of normal-chain alkanes such as hexyl radical and PXQH•. The hydrogen-atom abstraction from
n-hexane and n-pentane, the efficient catalytic oxygenation also various substrates by the triplet excited states of ketones has
took place with the high quantum yields (510% for n-hexane been known for a long time.18–21 However, the hydrogen-atom
and 140% for n-pentane) as shown in Table 1 (GC data are abstraction from cyclohexane has been difficult, because cyclo-
shown in Fig. S3–S5 in the ESI†).
hexane has been used as an inert solvent for transient absorp-
tion measurements of the triplet excited states of ketones.22
The photooxygenation of cyclohexane (CyH) is initiated by
3
hydrogen abstraction from cyclohexane by PXQ* to form the
radical pair of a cyclohexyl radical (Cy•) and a PXQ semi-
quinone radical (PXQH•) (Scheme 2). The addition of O2 to Cy•
affords a cyclohexylperoxyl radical (CyOO•), which abstracts a
hydrogen atom from cyclohexane to produce the cyclohexyl
radical (Cy•) and cyclohexyl hydroperoxide (CyOOH). Cyclo-
hexanol (CyOH) and cyclohexanone (CyvO) as final oxygenated
products are formed via the disproportionation of CyOO•.13,23
Scheme 1
Table 1 Product yields and quantum yields (Φ) for the photooxygenation of alkanes with PXQ in O2-saturated alkanes under visible light irradiation
Entry
1
Substrate
Product (yields vs. [PXQ]0)a
Φb%
1000
2
1600
3
4
510
140
a [PXQ]0 = 2.0 mM, T = 298 K. b Total quantum yield for the formation of oxygenated products. Excitation wavelength: 420 nm.
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The extremely high quantum yields of the photooxygena-
tion such as 1600% for 3-methylpentane and 1000% for cyclo-
hexane result from these autoxidation radical chain processes.
In conclusion, efficient oxygenation reactions of alkanes are
initiated by hydrogen-atom transfer from alkanes to the triplet
excited state of p-xyloquinone acting as a photocatalyst under
the solvent-free and ambient conditions, where molecular
oxygen acts as an oxidant. Radical chain autoxidation pro-
cesses with high concentrations of substrates (solvent) have
enabled us to obtain the high quantum yields. Thus, the
present study provides an environmentally benign way for
photooxygenation of alkanes by O2 with unusually high
quantum yields.
This work was supported by Grants-in-Aid (no. 16K13964,
26620154 and 26288037 to K. O.) from the Ministry of Edu-
cation, Culture, Sports, Science and Technology (MEXT); ALCA
and SENTAN projects from JST, Japan (to S. F.).
Notes and references
Fig. 2 (a) Transient absorption spectra of a deaerated acetonitrile solu-
tion (red) or a deaerated cyclohexane solution (black) containing PXQ
(12 mM) recorded at 0.1 μs after nanosecond laser excitation at 355 nm.
(b) Decay time profiles at 440 nm due to 3PXQ* with cyclohexane [0 mM
(black), 150 mM (red), 300 mM (blue)]. (c) Plots of kobs for the decay at
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