Selective photosensitized oxidation and its catalytic regulation of monoterpene with molecular oxygen in different reaction media

Article abstract of DOI:10.1016/j.jphotochem.2010.10.026

The photo-catalytic oxidation of α-pinene, β-pinene and limonene with molecular oxygen sensitized by tetrachlorotetraiodo-fluorescein sodium salt (RB) has been studied in different reaction media under green light irradiation. Simple and efficient photosensitized oxidation equipment was employed successfully to improve the efficiency of the photosensitized reaction. The results indicate that the photosensitized oxidation products can be directly obtained without after-treatment by reductive reagent. Furthermore, it was found that the product distributions are remarkably affected by reaction media, and that N,N-dimethylformamide (DMF) can regulate the selectivity to products. In particular, the good selectivity (85%) to the main product (myrtenal) and the excellent conversion (99%) were obtained in the absence of any other catalysts when DMF was used as reaction solvent in the photosensitized oxidation of β-pinene. Moreover, the possible photosensitized oxidation reaction mechanism in different media was suggested in the present work.

Full text of DOI:10.1016/j.jphotochem.2010.10.026

Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 321–325
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Selective photosensitized oxidation and its catalytic regulation of monoterpene
with molecular oxygen in different reaction media
Kuiyi Youa, Dulin Yinb,∗, Liqiu Maob, Pingle Liua, He’an Luoa,∗∗
a College of Chemical Engineering, Xiangtan University, Xiangtan 411105, PR China
b Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research, Ministry of Education, Institute of Fine Catalysis and Synthesis, Hunan Normal University,
Changsha 410081, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 April 2010
Received in revised form 24 July 2010
Accepted 28 October 2010
Available online 4 November 2010
The photo-catalytic oxidation of ␣-pinene, ␤-pinene and limonene with molecular oxygen sensitized
by tetrachlorotetraiodo-fluorescein sodium salt (RB) has been studied in different reaction media
under green light irradiation. Simple and efficient photosensitized oxidation equipment was employed
successfully to improve the efficiency of the photosensitized reaction. The results indicate that the pho-
tosensitized oxidation products can be directly obtained without after-treatment by reductive reagent.
Furthermore, it was found that the product distributions are remarkably affected by reaction media,
and that N,N-dimethylformamide (DMF) can regulate the selectivity to products. In particular, the good
selectivity (85%) to the main product (myrtenal) and the excellent conversion (99%) were obtained in the
absence of any other catalysts when DMF was used as reaction solvent in the photosensitized oxidation
of ␤-pinene. Moreover, the possible photosensitized oxidation reaction mechanism in different media
was suggested in the present work.
Keywords:
Photosensitized oxidation
Catalytic regulation
Monoterpene
Molecular oxygen
© 2010 Elsevier B.V. All rights reserved.
of 1,4-cycloaddition of 1O2, which would decompose to give ben-
zaldehyde at 100% yield as the final product. Li et al. [22] reported
1. Introduction
Selective oxidation of alkenes by molecular oxygen is one of the
current challenges in the manufacture of organic building blocks
and industrial intermediates [1–7], and photosensitized oxida-
tion holds special promise for yielding product specificity [8,9].
There are two well-established paths of dye-sensitized photo-
oxidation: the energy-transfer pathway and the electron-transfer
pathway [10,11]. Unfortunately, in many cases, the two paths of
photo-oxidation occur simultaneously. Although the photosensi-
tized oxidation has been found many applications in the field of
synthesis, control of regio- and stereo selectivity are still difficult
issues and the selectivity of oxidation reactions is not high [12]. To
gain excellent selectivity in these reactions, various efforts have
been made in the past decades, and remarkable control of the
reaction pathway by the use of organized and constrained media
[13–20]. In previous works, Tung et al. [21] used trans-stilbene as a
substrate and successfully demonstrated that a good control of the
selectivity in the photo-oxidation process might be achieved by
isolation of the substrate within Na-ZSM-5 zeolites from the sen-
sitizer 9,10-dicyanoanthracene (DCA) in the surrounding solution.
The photo-oxidation yields the endoperoxide as an intermediate
the photo oxidation of pinene sensitized by DCA under ultravio-
let light irradiation (with a 450-W medium pressure Hg lamp as
light source) in homogeneous solution, followed by reduction of
the reaction mixture with sodium sulfite solution, giving the ‘ene’
products and ‘non-ene’ products. Recently, Maldotti et al. [23] have
found that a composite palladium porphyrin photo-catalyst is an
excellent sensitizer in photo-oxidation of cyclohexene by molec-
ular oxygen. The reaction gives cyclohexene hydroperoxide as the
major product with a high selectivity up to 90%, whereas cyclo-
hexenol and cyclohexenone are only formed in a trace amount.
Sehlotho and Nyokong have employed zinc phthalocyanine as the
sensitizer [24]. This complex, as compared to palladium porphyrin,
can utilize a larger portion of solar light. The sensitized reaction,
however, results in several products with low selectivity. The cata-
lyst itself also suffers a serious bleaching during the reaction. In
addition, such undesirable degradation is also found with other
phthalocyanines of iron, cobalt and manganese, during the ther-
mally catalytic oxidation of alkenes [25–28].
In this work, a novel photosensitized catalytic oxidation for
monoterpene with molecular oxygen sensitized by tetrachlorote-
traiodo fluorescein sodium salt (RB) under high pressure sodium
lamp irradiation was studied. The catalyst was added into the
reaction system and the photosensitized oxidation products were
directly obtained without after-treatment by reductive reagent.
Furthermore, a special effect on the photosensitized oxidation reac-
∗
Corresponding author. Tel.: +86 731 88872576; fax: +86 731 88851226.
Corresponding author. Tel.: +86 731 58293545; fax: +86 731 58298267.
∗∗
E-mail addresses: dulinyin@126.com (D. Yin), heanluo@126.com (H. Luo).
1010-6030/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2010.10.026

322
K. You et al. / Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 321–325
3. Results and discussion
3.1. Photosensitized oxidation of ˛-pinene
The RB-sensitized photooxidation reaction of ␣-pinene with
molecular oxygen was examined in different reaction media.
After irradiation by the high-pressure sodium lamp, the ‘ene’
products verbenol (2), myrtenol (5), and the ‘non-ene’ products
myrental (4), verbenone (3), 4-Hydroxy-2,3-epioxypinane (6) and
1-Hydroxy-2,3-epioxypinane (7) have been obtained, as is shown
in Scheme 1(a). The product selectivity distributions in various
different reaction media are given in Table 1. The most proba-
ble photosensitized oxidation reaction path may be described as
through a hydroperoxide intermediate process.
Fig. 1. Experimental equipment for photosensitized oxidation.
In Table 1, it is clearly shown that the 2,6-lutidine and acetic
anhydride can efficiently catalyze the photosensitized oxidation
reaction of ␣-pinene in Table 1. It is worth mentioning that the
photosensitized oxidation products can be directly obtained in
the absence of any other catalysts and the residues (non-volatile
substances) are minimal when DMF is used as solvent. Further-
more, the selectivity to the main product verbenone is remarkably
improved also, and the selectivity to verbenone is up to 51% with
98% conversion in DMF solution. However, 97% conversion with
42% selectivity to verbenone is only afforded under the catalysis
of 2,6-lutidine combined with acetic anhydride in methyl alcohol
solution. The possible reason is that DMF itself possesses a zwitte-
rion resonance structure with characteristics of acidity and basicity,
which prompts decomposition or reduction of the hydroperoxide
intermediate to the corresponding aldehyde, ketone and alcohols.
In addition, the selectivity to the main product and conversion of
material are affected by the method of addition of the catalysts.
The selectivity and conversion are much higher when the compos-
ite catalysts are directly added before the reaction than after the
reaction. The results indicate that the selectivity and conversion
strongly depend on the reaction media.
tion was observed when N,N-dimethylformamide (DMF) was used
as solvent, and the selectivity to the main product can be efficiently
improved. This finding may be very significant to establish a new
synthesis process in organic photochemistry fields.
2. Experimental
2.1. Reagents and instrument
␣-Pinene, ␤-pinene and limonene were distilled under reduced
pressure from turpentine oil; 2,6-lutidine was purchased from
BDH Chemicals Ltd., Poole, England; acetic anhydride (AR),
tetrachlorotetraiodo-fluorescein sodium salt (RB), methyl alcohol
(AR), DMF (AR) and decalin (AR) were purchased from chemi-
cal reagent factory of Shanghai. A 110 W high-pressure sodium
lamp was used as the green irradiation source. Gas chromatog-
raphy (GC) analysis was performed on an Agilent 6890 equipped
with an FID detector and
a quartz capillary column (DB-5,
30 m × 0.25 mm × 0.25 ␮m). Mass spectra (MS) were run on an
America Varian Saturn 2100T GC–MS instrument.
3.2. Photosensitized oxidation of ˇ-pinene
2.2. Experiment process of photosensitized oxidation
As observed in the case of ␣-pinene, the RB-sensitized photooxi-
dation of ␤-pinene (8) was investigated by the same method. There
may be two probable reaction paths in photosensitized oxidation
of ␤-pinene. Firstly, the singlet oxygen reacts with the double bond
carbon atom to form a peroxide radical, and then the peroxide
radical abstracts the hydrogen to generate a hydroperoxide inter-
mediate. Finally, the intermediate is decomposed to myrtenal (4)
or reduced to myrtenol (5) under the action of catalysts. The second
probable path involves the allylic oxidation of ␤-pinene, and affords
pinocarveol (9) and pinocarvone (10), as is shown in Scheme 1(b).
The product distributions in methyl alcohol and DMF are given in
Table 2.
All the photosensitized oxidation reactions were carried out in
a water-bathed reactor with the inner-circulated molecular oxy-
gen as oxidant and the immersed high-pressure sodium lamp as
the green irradiation light source (see Fig. 1). Firstly, the methyl
alcohol or DMF solution of 0.5 mol/L substrate concentration was
prepared; then, the sensitizer (RB) and catalyst (5 wt.% of substrate)
were added into the reactor. After the photosensitized oxidation
was performed for 4 h at room temperature, the final products were
identified by GC–MS and the products were quantitatively ana-
lyzed by the internal standard method (decalin was used as internal
standard sample).
Table 1
Product distributions in the RB-sensitized photooxidation of ␣-pinene.a
Solvent
Catalyst
Conversion (%)
Selectivity (%)
2
3
4
5
6
7
Residuesb
CH3OH
CH3OH
CH3OH
CH3OH
CH3OH
DMF
None
86
94
94
89
97
98
13
17
13
28
36
33
32
35
36
36
42
51
7
5
5
8
13
8
7
10
2
4
5
5
3
1
6
2
2
1
5
35
25
43
16
1
2,6-lutidine
Acetic anhydride
Compositesc
Compositesd
None
tre
2
1
tr
6
0
a
Conversion and selectivity were determined by the internal standard method.
Residues refers to non-volatile substances in gas chromatography.
Composite catalyst stands for added 2,6-lutidine and acetic anhydride after the reaction.
Composite catalyst stands for added 2,6-lutidine and acetic anhydride before the reaction.
Trace amounts.
b
c
d
e

K. You et al. / Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 321–325
323
CH2OH
O
CH2OH
CHO
O
O2, hv, Sens.
+
+
+
+
+
(a)
Solv., Cat.
OH
OH
O
5
6
7
2
3
4
1
CHO
CH2OH
O2, hv, Sens.
OH
O
+
(b)
(c)
+
+
Solv., Cat.
9
10
4
5
8
OH
HO
OH
+
O
O2, hv, Sens.
+
+
solv., Cat.
13
11
12
14
15
Scheme 1. The photosensitized oxidation of three monoterpenes.
Table 2
Product distributions in the RB-sensitized photo oxidation of ␤-pinene.
Solvent
Catalyst
Conversion (%)
Selectivity (%)
4
5
9
10
Residues
CH3OH
CH3OH
CH3OH
CH3OH
DMF
None
63
78
75
92
99
33
49
41
74
85
26
11
4
9
6
2
2
2
2
2
tr
2
37
36
53
8
2,6-Lutidine
Acetic anhydride
Compositesa
None
trb
9
4
3
a
Composite catalyst stands for 2,6-lutidine and acetic anhydride.
Trace amounts.
b
Table 2 summarizes representative results for the photosensi-
Table 3, the strong effect of the monoterpene structure on the con-
version and selectivity has been observed in the photosensitized
oxidation of limonene. The conversion of limonene is also obviously
much lower than ␣-pinene and ␤-pinene under the same reaction
conditions. Generally, the photosensitized oxidation of limonene
takes place on its intra-cyclic double bond, while the external-cyclic
double bond is comparatively steady under the reaction conditions.
Maybe the reactivity is determined to itself molecular structure.
It is worth noting that the product distributions in DMF solu-
tion are also different from those in methyl alcohol solution in the
photosensitized oxidation reaction of limonene. In spite of the fact
that the conversion of limonene changes a little, the selectivity to
the main product 2,8-p-menthadiene-1-ol is remarkably improved
in DMF solution. The possible reason relates to the procedure of
decomposition or reduction of the hydroperoxide intermediate.
tized oxidation reaction of ␤-pinene, and clearly shows that the
2,6-lutidine, acetic anhydride and DMF can catalyze the photo-
sensitized oxidation reaction to some degree. In particular, the
photosensitized oxidation products can be directly obtained with-
out need for any other catalysts and the residues (non-volatile
substances) are minimal when DMF is used as solvent. Moreover,
the special effect is clearly observed that the selectivity to the main
product is remarkably improved in the DMF solution. The excel-
lent result of 99% conversion with 85% selectivity to myrtenal was
obtained in the DMF solution.
3.3. Photosensitized oxidation of limonene
According to the above-mentioned experimental process, the
photosensitized oxidation of limonene (11) was likewise examined
in the different reaction media. In this case, the oxidation products
carveol (14) and carveone (15), as well as 2,8-p-menthadiene-1-
ol (13) and 2-methylene-5-(1-methylethenyl) cyclohexanol (12),
have been obtained, as is shown in Scheme 1(c). The product dis-
tributions in methyl alcohol and DMF are listed in Table 3. From
3.4. The possible photosensitized oxidation reaction mechanism
of monoterpenes in different reaction media
On the basis of these results in this work, the most probable
reaction mechanism of photosensitized oxidation of monoterpenes
Table 3
Product distributions in the RB-sensitized photooxidation of limonene.
Entry
Solvent
Catalyst
Conversion
%
Selectivity (%)
12
13
14
15
Residues
1
2
3
4
CH3OH
CH3OH
CH3OH
DMF
2,6-Lutidine
Acetic anhydride
Compositea
None
25
21
33
34
14
16
13
14
32
26
39
49
24
22
28
24
9
6
9
7
21
30
11
6
a
Composite catalyst stands for 2,6-lutidine and acetic anhydride.

324
K. You et al. / Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 321–325
CH2OH
O
intramolecular epoxidation
CHO
decomposition
CH2OOH
path 1
AC2
,
CH2OH
reduction
O2, hv, Sens.
(a)
Solv., Cat.
decomposition
path 2
,
AC2
OOH
O
reduction
intramolecular epoxidation
O
OH
OH
CH3
CH3
H
C
H
C
H2C
O
OH
N
O
H
O
OH
CH2OH
-H2O
O
O
C
O
C H
DMF
H3C
H3C
H3C
N
N
H
H3C
CHO
CH2OOH
H
O
O
H2C
CH2
O
CH2OH
1
2
1O2
OH
-
O
H
sens* , O 2
H3C
N
C
H
(b)
H3C
C
N
H3C
DMF
H3C
OOH
H
O
H
O
O
1
2
1O2
-
H
O
OH
C
H3C
O
H
H3C
H3C
N
N
H
OH
DMF
H3C
OH
C
O
OH
CH3
H
C
N
O
O
H
O
CH3
H
OH
-H2O
H3C
H3C
N
C
H
DMF
O
Scheme 2. The probable reaction path of photosensitized catalytic oxidation of ␣-pinene in different media.

K. You et al. / Journal of Photochemistry and Photobiology A: Chemistry 217 (2011) 321–325
325
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monoterpenes. The possible reason is as follows. Acetic anhydride
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the hydroperoxide intermediate converting to corresponding oxi-
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In summary, photosensitized oxidation of three monoterpenes
in homogeneous solution was researched, and efficient photosen-
sitized oxidation equipment had been successfully employed in the
present work. The results indicate that the selectivity to products is
remarkably controlled by reaction media and that DMF can regulate
the selectivity to products. In addition, a probable photo-oxidation
reaction mechanism of monoterpene with molecule oxygen sensi-
tized by RB in different reaction media was proposed. This method
may also be suitable for photosensitized oxidation of other alkenes
with molecular oxygen. It is very significant to establish such a new
synthesis approach in the organic photochemistry field.
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The authors are grateful for the financial support for this
work by the National Natural Science Foundation of China (Nos.
21006081, 20976043 and 20976148), Scientific Research Fund of
Hunan Provincial Education Department (09C955) and Technolog-
ical Plan Key Project of Hunan Province (2009FJ2004). Moreover,
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