D. Jiang, et al.
MolecularCatalysis494(2020)111127
spectra of the tungsten–bromine lamp is shown in Fig. S2) came with an
UV light filter (Osram brand) was immersed in the acetone solution
containing DHA (0.05 mmol) and EB (0.5 mmol). The reaction mixture
was stirred magnetically under pure oxygen (1 atm) or air atmosphere
and sustained visible light irradiation, and the temperature of reaction
solution increased to about 25 °C because of the heating effect of light
irradiation. After the desired irradiation time had elapsed, the reaction
was stopped and the oxidative products of EB were quantitatively
analyzed on an Agilent 6890 N gas chromatograph (GC) with a DB-17
polysiloxane capillary column (30 m × 0.32 mm × 0.50 μm) and flame
ionization detector (FID) using cyclohexanone as an internal standard,
the oxidative products of DHA were determined on an Agilent 1260
HPLC (Agilent, USA) with an external standard method and the specific
detection conditions were as follows: 4.6 × 250 mm of Agilent SB-C 18
chromatographic column, acetonitrile/ water 65:35 (v/v) of mobile
phase, 0.5 mL/min of flow rate, 260 nm UV detector, column tem-
perature 30 °C, injection volume 20 μL. The conversion of EB was cal-
culated by dividing the actual added molar mass of EB by the reacted
molar mass of EB obtained from the oxygenated products. The se-
lectivity of ACP was calculated by dividing the molar mass of oxyge-
nated products by the molar mass of ACP. The yield of AQ was calcu-
lated by dividing the actual produced molar mass of AQ by the
theoretical molar mass of AQ.
and 99.8 % ACP selectivity (Entry 3), which was almost equal to the
current standard synthesis protocol. The current photooxidation was
further studied with a monochromatic visible light as an external illu-
mination to make clear the specific work wavelength (the reaction
device sees Fig. S3). As shown in Entry 4, the auto-photooxidation of
DHA by O2 could be driven by a violet light of 400 nm and achieved
24.4 % AQ yield, along with the occurrence of a trace EB photooxida-
tion (1% conversion). And AQ more efficiently catalyzed the photo-
oxidation of EB with O2 under this violet light illumination, providing a
much higher 12.6 % EB conversion than the DHA-triggered EB photo-
xidation (Entry 5). But the photooxidation of EB by O2 could not occur
under the illumination of a blue light of 500 nm whether DHA or AQ
was used as a source of photo-catalyst or a photo-catalyst (Entries 6
and 7). These findings indicate that DHA-triggered EB photoxidation
can function only using a violet light part of tungsten-bromine lamp and
its goal product AQ can work well owing to its stronger absorption to
this violet light than DHA, as supported by UV–vis spectral character-
izations in Fig. S4. Notably, AQ can be accumulated by preferentially
catalyzing the photooxidation of DHA because the C–H bond dissocia-
tion energy (78.0 kcal⚫ mol−1) of DHA is smaller than that of EB (87
kcal⚫ mol−1). And the accumulated AQ, as an active photo-catalyst,
should be responsible for the photooxidation of EB to ACP. Three blank
experiments conducted in the absence of O2 or DHA or without light
irradiation confirmed that O2, DHA and light irradiation are necessary
for this photo-synthetic protocol (Entries 8 and 10).
Results and discussion
Photooxidation of various substrates
Photooxidation of EB under different conditions
In order to verify the adaptability of the above DHA-based photo-
synthetic protocol, the photooxidation of various hydrocarbons with
primary or/and secondary C–H bonds as well as four alcohols and a
thioether was examined using the above standard reaction conditions.
As shown in Table 3, in the examined hydrocarbons with secondary
C–H bonds, DHA could very efficiently trigger the photo-oxidations of
three aromatic EB, tetrahydronaphthalene and diphenylmethane, re-
spectively affording 87.7, 90.2 and 97.5 % conversions with an almost
100 % selectivity for the corresponding ketones (Entries 1-3). But its
synthetic efficiency was significantly reduced when p-bro-
moethylbenzene, especially cyclohexene and cyclohexane were used as
substrates (Entries 4-6). Entries 7–10 show that in the presence of
DHA, benzyl alcohol, n-hexanol, especially cyclohexanol and thioani-
sole were also efficiently oxidized by O2 to the corresponding oxyge-
nated products under illumination, giving 71.4, 79.9, 94.8 and 96.5 %
conversions, respectively. But DHA exhibited a relatively low efficiency
for the photooxidation of furfuryl alcohol (Entry 11, 48.0 % conver-
sion). Entry 12 shows that with DHA, the photooxidation of toluene to
benzaldehyde and benzoic acid could also proceed but afforded a low
conversion of 24 %. And its reactivity was influenced by the aromatic
ring substituent, which was significantly improved when introducing a
p-methyl (Entry 13, 66.1 % conversion) but significantly decreased
when introducing a meso-chlorine (Entry 14, 10 % conversion) and
almost lost when introducing a para-NO2 (Entry 15, 1% conversion). In
addition, DHA exhibited a poor synthetic efficiency for the photo-
oxidation of 2-methylfuran to furfural and furanoic acid (Entry 16,
10.3 % conversion) and was even completely ineffective for the pho-
tooxidation of n-pentane although this linear alkane contained sec-
ondary C–H bonds (Entry 17). The reactivity of some hydrocarbons
diphenylmethane, tetrahydronaphthalene, EB, toluene and cyclohexane
is obviously inversely proportional to their C–H bond dissociation en-
ergies (BDEs) at the reactive sites (see Table 3), supporting that their
photo-activation complies with a hydrogen atom transfer (HAT) me-
chanism [36,37]. But the photo-activation of other hydrocarbons does
not seem to comply with the HAT mechanism because the above-
mentioned inverse relationship is controversial on them. Four typical
examples are that p-xylene, toluene, p-nitrotoluene and n-pentane have
almost equivalent BDE values at the reactive sites to each other, but
Table 2 lists the data for the DHA-triggered EB photooxidation to
ACP under various reaction conditions. Under the standard conditions,
with acetone solvent, 1 atm pure O2 and 12 h of sustained visible light
irradiation under room temperature, the DHA-based photosynthetic
protocol could achieve 87.7 % EB conversion and 99.5 % selectivity
(Entry 1), representing a mild and efficient character. Even if using air
instead of pure O2, the protocol still exhibited a good efficiency, af-
fording 62.9 % EB conversion and 86.4 % ACP selectivity (Entry 2).
And the use of AQ instead of DHA might achieve 86.5 % EB conversion
Table 2
The photooxidation of EB by molecular oxygen under different conditions.
Entry Change from standard conditions
Conv. (%) Sel. (%)
ACP α-PEA
99.5 0.5
AQ yield
(%)
a
1
2
3
4
5
6
Standard
87.7
62.9
86.5
1.0
71.8
56.7
–
24.4
0
Air instead of O2
AQ instead of DHA
Violet light, 6 h
Blue light, 6 h
Violet light, 6 h, AQ instead of
DHA
96.3 3.7
99.8 0.2
60.3 29.7
b,d
b,c
b,d
0
0
0
12.6
76.6 23.4
–
b,c
e
7
8
9
Blue light, 6 h, AQ instead of DHA
Without light illumination
Without DHA
0
0
0
0
0
0
0
0
0
–
0
–
10
N2 instead of O2
0.1
36.1 63.9
0.8
aStandard reaction conditions: EB, 0.5 mmol; DHA, 0.05 mmol; Pure O2, 1 atm;
Acetone, 5 mL; 35 W tungsten-bromine lamp as a built-in light source, illumi-
b
c
nation time, 12 h; By use of an external radiation; Using a monochromatic
green light with a center wavelength of 500 nm obtained by a 300 W xenon
lamp equipped with a light filter of 500 nm; DHA, 0.4 mmol; Acetone, 20 mL; d
Using a monochromatic violet light with a center wavelength of 400 nm ob-
tained by a 300 W xenon lamp equipped with a light filter of 400 nm; DHA,
e
0.4 mmol; Acetone, 20 mL; 35 °C, in the dark;
3