Solvent and additive-free selective aerobic allylic hydroxylation of β-pinene catalyzed by metalloporphyrins

Article abstract of DOI:10.1016/j.cclet.2016.11.020

Metallodeuteroporphyrins (MDPs) were employed as the catalysts for aerobic oxidation of β-pinene in absence of solvents and additives. Allylic hydroxylation products were found to be the main products from this protocol. The catalytic activity of MDPs with different metal nuclei and the influences of technological conditions on this reaction were investigated. This catalytic system has bright application prospect since only eco-friendly and readily available dioxygen were needed.

Full text of DOI:10.1016/j.cclet.2016.11.020

G Model
CCLET 3894 1–5
Chinese Chemical Letters xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
1
Original article
2
3
Solvent and additive-free selective aerobic allylic hydroxylation
of -pinene catalyzed by metalloporphyrins
b
Shi-Chao Xua,b,c,d,e, Shou-Ji Zhua,b,c, Liang-Wu Bia,b,c,d,e, Yu-Xiang Chena,b,c,d
,
4
5
Q1
Q2
Jing Wanga,b,c,d,e, Yan-Ju Lua,b,c,d,e, Yan Gua,b,c,e, Zhen-Dong Zhaoa,b,c,d,e
,
*
6
7
8
9
a
Institute of Chemical Industry of Forest Products, Chinese Academy of Forestry, Jiangsu Province, Nanjing, 210042, China
National Engineering Lab. for Biomass Chemical Utilization, Key and Open Lab. on Forest Chemical Engineering, State Forestry Administration, Jiangsu
Province, Nanjing, 210042, China
Key Lab. of Biomass Energy and Material, Jiangsu Province, Nanjing, 210042, China
Institute of Forestry New Technology, Chinese Academy of Forestry, Beijing, 100091, China
b
c
d
e
2011 Collaborative Innovation Center of Jiangxi Typical Trees Cultivation and Utilization in Jiangxi Agricultural University, Nanchang, 330045, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Metallodeuteroporphyrins (MDPs) were employed as the catalysts for aerobic oxidation of
b-pinene in
Received 10 October 2016
Received in revised form 7 November 2016
Accepted 8 November 2016
Available online xxx
absence of solvents and additives. Allylic hydroxylation products were found to be the main products
from this protocol. The catalytic activity of MDPs with different metal nuclei and the influences of
technological conditions on this reaction were investigated. This catalytic system has bright application
prospect since only eco-friendly and readily available dioxygen were needed.
ã 2016 Zhen-Dong Zhao. Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of
Medical Sciences. Published by Elsevier B.V. All rights reserved.
Keywords:
Turpentine transformation
b-Pinene
Metalloporphyrin
Biomimetic catalysis
Aerobic allylic hydroxylation
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1. Introduction
of 2 and 3 [10–12]. These procedures have been now rejected
because a large amount of toxic waste could be produced in these
systems and caused great environmental impacts. In contrast to
stoichiometric oxidants, molecular oxygen is an excellent oxidant
due to its inexpensive, eco-friendly and easily available character-
istics [13]. However, because of the inertness of molecular oxygen
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19
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25
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With the rapid decrease of the fossil resources, there is an ever-
increasing interest for the utilization of renewable biomass
resources in making more valuable products [1,2]. Turpentine,
obtained by collecting and isolating the oleosus exudates of living
pine trees, is the most important and cheapest monoterpene
resources all over the world [3]. As the major component of
and the complex molecular structure of
b-pinene, selective
aerobic oxidation of -pinene by molecular oxygen is still
b
turpentine,
turpentine materials [4]. Due to the presence of a double bond
and a strained four-membered ring, -pinene can be transformed
b-pinene (1, Fig. 1) contributes to 30% in raw
amongst the major challenges in academic and industrial research
[14].
b
Metalloporphyrins (MPs) are efficient selective catalysts
applied widely for direct aerobic allylic oxidation of hydrocarbons
[15–17]. But most are used under the assistance of solvents,
reductants and cocatalysts. MPs catalyzed aerobic oxidation of
hydrocarbons in absence of solvents and additives has bright
industry application prospect since only eco-friendly and readily
available molecular oxygen were needed. Aerobic oxidation of
simple hydrocarbons catalyzed by MPs in solvent and additive free
system has been studied [18,19]. However, to the best of our
knowledge, selective allylic oxidation of complex alkenes in MPs
catalyzed aerobic oxidation systems in absence of solvents and
additives has not been reported before.
readily to produce a number of useful products for pharmaceutical
and other industrial applications [5–7].
b
-Pinene oxidation products such as myrtenol (2) and
pinocarveol (3) are valuable pharmaceuticals, fragrance ingre-
dients or chemical intermediates [8,9]. Generally, direct allylic
oxidation of b-pinene by stoichiometric oxidants such as peracids,
SeO2 or H2O2 is the most efficient processes for the production
* Corresponding author.
E-mail address: zdzhao@189.cn (Z.-D. Zhao).
http://dx.doi.org/10.1016/j.cclet.2016.11.020
1001-8417/ã 2016 Zhen-Dong Zhao. Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights
reserved.
Please cite this article in press as: S.-C. Xu, et al., Solvent and additive-free selective aerobic allylic hydroxylation of
metalloporphyrins, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.11.020
b-pinene catalyzed by

G Model
CCLET 3894 1–5
2
S.-C. Xu et al. / Chinese Chemical Letters xxx (2016) xxx–xxx
Fig. 1. Structure of
b-pinene and its aerobic oxidation products over the catalysis of metallodeuteroporphyrins (MDPs).
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59
60
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73
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79
80
In this work, an efficient metallodeuteroporphyrins (MDPs)
catalyzed selective aerobic allylic oxidation method of -pinene
(Fig. 1, containing one C
C bond and six different CꢀꢀH bonds in a
selectivity (S) of allylic hydroxylation products is much lower than
the values under the catalysis of MDPs, indicating that MDPs acted
as critical regioselective catalysts in this reaction. The accumula-
tion of HPs was detected at the initial of this reaction. But HPs could
be decomposed under the catalysis of MPs [19]. When this reaction
proceeded for 5 h, the yield of HPs decreased to a stable value of
b
¼
molecule) was established. Allylic hydroxylation products could be
obtained with high selectivity under the catalyzing of metal-
lodeuteroporphyrin dimethyl esters (Fig. 2, MDPDMEs, 4a–4d) in
absence of solvents and additives because of the high reactivity of
about 0.5% while the conversion (C) value of
b-pinene reached
allylic CꢀꢀH bonds of
b
-pinene in this aerobic oxidation system.
18.6%, indicating a completely decomposition of HPs in this
protocol.
The effects of reaction parameters and metal nuclei of MDPDMEs
on this reaction were investigated. The possible reaction mecha-
nism and the role of MDPDMEs in this procedure were initially
discussed.
81
2.2. Optimization of the working conditions
82
MPs are important biomimetic catalysts with bright industry
application prospect. In order to obtain the optimal reaction
conditions, FeClDPDME was selected as a model to investigate the
influence of various working conditions on this reaction. Table 1
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62
83
2. Results and discussion
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85
2.1. Aerobic oxidation of
b-pinene under the catalysis of MDPs
86
listed the C values of b-pinene and the S values of various oxidation
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64
65
66
67
68
69
70
71
87
According to GC, GC–MS and chemical analysis data, the
oxidation products consisted of 2, 3, pinocarvone (5) and trace
amount of 2,10-epoxypinane (6) and hydroperoxides (HPs). HPs are
proved to be myrtenyl-hydroperoxide (7) and pinocarvyl-hydro-
peroxide (8) [4]. Products 2, 3 and 5 belonged to the oxidation of
allylic CꢀꢀH bonds. Product 6 was attributed to the epoxidation of
products of this reaction at different temperatures. The results
showed that the C values increased with reaction temperature at
the temperature range from 70 ꢁC to 110 ꢁC. The total selectivity of
allylic hydroxylation products 2 and 3 reached a maximal value of
85.8% at 90 ꢁC. It has been reported that only high-spin unsteady
state perfluorinated or poly-halogenated MPs can activate
molecular oxygen under ambient pressure in this solvent and
additive free system [18]. The reductive potentials of simple MPs
such as metallotetraphenylporphyrines (MTPPs) and MDPs are so
low that they can be easily reduced to their high-spin unsteady
state by thermal decomposition reduction [18,19]. Thus, the
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90
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93
p
-bond. Products 7 and 8 belonged to the hydroperoxidation of
-Pinene could be oxidized by dioxygen in the
94
allylic CꢀꢀH bonds.
b
95
absence of any catalysts under similar parameters (Table 1). But the
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catalytic of MDPs on aerobic oxidation of b-pinene at low reaction
99
temperatures plays a trivial role, which leads to the decrease of S
values of hydroxylation products 2 and 3. When the reaction
temperature was higher than 100 ꢁC, the selectivity of 2 and 3
decreased dramatically due to the over-oxidation of initial
oxidation products.
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101
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103
104
105
106
107
108
In our previous research, catalyst concentration was found to be
a critical factor in MPs biomimetic catalyzed reactions. Fig. 3
showed the conversion of
b-pinene and the selectivity of various
oxidation products over the catalysis of different concentration
MDPs from 1 ppm to 9 ppm. The results indicated that the
20
16
12
8
100
90
80
70
60
50
C (%)
S (%)
Fig. 2. Formula of MDPDMEs mentioned in the text. M = FeCl (4a), Co (4b), MnCl
(4c), Cu (4d).
Table 1
Comparison for aerobic hydroxylation oxidation of
temperatures.
b-pinene at different reaction
Entry Cat. T (ꢁC) C (%) Product selectivity (%)
Shydroxyl (%) TON
2
3
5
4
1
2
3
4
5
6
4a
4a
4a
Non 90
4a
4a
70
80
90
5.1
30.5
59.9
60.9
34.6
63.3
59.8
20.0
25.1
24.9
11.3
16.4
14.6
11.8
6.9
4.9
2.6
4.7
1.6
50.5
85.0
85.8
45.9
79.7
74.4
4700
16.9
18.6
18.5
20.8
21.8
15,400
17,000
–
0
0
30
60
90
120
150
100
110
19,000
19,900
Concentration (mol/L)
Reaction conditions:
b
-pinene 0.73 (m), FeClDPDME 8 (
m
m), temperature 90 (ꢁ C),
Fig. 3. Effects of catalyst concentration C and S values of this reaction. Reaction
ambient pressure, oxygen flow rate 60 (mL/min), time 5 (h).
conditions: temperature 90 (ꢁC), ambient pressure and time 5 (h).
Please cite this article in press as: S.-C. Xu, et al., Solvent and additive-free selective aerobic allylic hydroxylation of
metalloporphyrins, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.11.020
b-pinene catalyzed by

G Model
CCLET 3894 1–5
S.-C. Xu et al. / Chinese Chemical Letters xxx (2016) xxx–xxx
3
Table 2
35
30
25
20
15
10
5
100
Comparison for aerobic oxidation of
b
-pinene under the catalysis of different
MDPs.a
C (%)
S (%)
90
80
70
60
50
Entry Catalyst
C (%) Product selectivity (%)
Shydroxyl (%) TON
2
3
5
1
2
3
4
5
6
7
8
4a
FeCl3
4b
CoCl2
4c
MnCl2
18.6
17.8
17.1
18.4
17.2
60.9
43.2
54.7
42.0
48.1
38.0
49.1
39.8
24.9
14.4
21.3
14.6
22.1
14.5
14.9
18.1
4.9
2.4
2.5
1.6
2.8
1.3
3.5
1.4
85.8
57.6
76.0
56.6
70.2
52.5
64.0
57.9
17,000
ꢂ
6H2O
6H2O
16,200
15,600
16,800
15,700
16,700
16,000
16,000
ꢂ
ꢂ
4H2O 18.3
4d
CuCl2
17.5
17.5
0
ꢂ
2H2O
20
40
60
80
100
a
Reaction conditions:
b
-pinene 0.73 mol, MDPs and metal salts 8(mmol,
Flow rate (mL/min)
temperature 90 ꢁC, ambient pressure, oxygen flow rate 60 mL/min, time 5 h.
Fig. 4. Effects of oxygen flow rate on C and S values of this reaction. Reaction
conditions: -pinene 0.73 mol, FeClDPDME 8
mol, temperature 90 ꢁC, ambient
pressure and time 5 h.
b
m
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142
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for allylic hydroxylation products in our procedure [19,20]. This
phenomenon might be attributed to the different redox potential
of various MDPs.
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110
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conversion of b-pinene and the selectivity of allylic C–H oxidation
144
2.4. Probable reaction mechanism
products increased with the increasing of FeClDPDME concentra-
tion when the catalyst concentration was lower than 5 ppm. Higher
concentration of FeClDPDME would lead to the formation of
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Aerobic oxidation of hydrocarbons in presence and in absence
of catalysts has been widely investigated in previous researches. It
is widely accepted that these processes are initiated by different
active radical intermediates, which generated from the decompo-
sition of HPs [4,22]. These radicals remove hydrogen atoms in
substrate molecules, yielding corresponding resonance-stabilised
inactive
m-oxo metalloporphyrin dimers [20,21]. Therefore,
although more catalyst was added, the efficient concentration of
active catalyst reduced and caused the decrease of C and S values in
this reaction.
The effect of oxygen flow rate on conversion and product
selectivity were also investigated. As illustrated in Fig. 4, the
alkyl radicals. As it is mentioned above,
by oxygen in or in absence of MPs. Aerobic oxidation of
b
-pinene could be oxidized
-pinene in
b
conversion of b-pinene increased with the increase of the flow rate
absence of catalysts has been explicitly investigated by Hermans
and coworkers and found to be propagated by different peroxyl
radicals [4]. These radicals abstract weakly bonded a-hydrogen
atoms in the substrate, yielding the corresponding hydroperoxides
and resonance-stabilised alkyl radicals (Scheme 1). O2 adds to
these allyl radicals and generates allylic hydroperoxidation,
allylic hydroxylation and epoxidation porducts via complex
approaches.
of oxygen at flow rate less than 60 mL/min. If the oxygen flow rate
was higher than 80 mL/min, the selectivity of product 2 and 3
decreased evidently with the increase of oxygen flow rate. This
phenomenon might be explained as follows: The dissolved oxygen
in liquid phase increased with the increase of the oxygen flow rate.
Thus, the production of 2 and 3 increased with the increase of
oxygen flow rate. However, the higher flow rate of oxygen would
accelerate the over-oxidation of 2 and 3 to other by-products.
In MPs biomimetic catalyzed system,
b-pinene might be
128
oxidized in a different procedure. Under the catalyst of MPs, HPs
could decomposed rapidly by MPs to corresponding hydroxylation
products (Scheme 2a). Meanwhile, MPs transformed to high-
valence MP radicals (e.g. ferric(IV) porphyrin radicals, [FePIV = O+],
Scheme 2b) [19]. These radicals are regarded as the active species
to initiate this reaction, which removes hydrogen atoms in alkyls
and yields caged pair of alkyl radicals and high-valence MPs. The
caged pair collapses to alcohol or over-oxidized ketone products
via a complex oxygen transfer process [19,23]. Perhaps, this is the
main reason for the high selectivity of allylic hydroxylation
products in this catalytic oxidation system. Aerobic oxidation of
2.3. Effect of central metal of MDPs
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133
134
135
136
137
138
139
140
As reported in our previous work, MPs with different central
metal possess different catalytic activities. Table 2 summarized the
data obtained from the aerobic oxidation of
b-pinene under the
catalysis of various MDPs. The results showed that the S values of
allylic hydroxylation products varied dramatically with the change
of central metal nuclei. Under the catalysis of various correspond-
ing metal salts, the S values were much lower, indicating this
variation is mainly caused by the different catalytic abilities of
MDPs, which follows a sequence of FeClDPDME > CoDPDME >
MnClDPDME > CuDPDME. It has been reported that the catalytic
activity of MPs can be influenced by the stability and redox
potential of central metal nuclei, resulting to the higher selectivity
b-pinene in absence of catalysts might occur as a competing
reaction in this system. Therefore, the increasement of the flow
rate increased the selectivity of HPs and other over-oxidation
Scheme 1. Aerobic oxidation of
b
-pinene in absence of catalysts.
Please cite this article in press as: S.-C. Xu, et al., Solvent and additive-free selective aerobic allylic hydroxylation of
metalloporphyrins, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.11.020
b-pinene catalyzed by

G Model
CCLET 3894 1–5
4
S.-C. Xu et al. / Chinese Chemical Letters xxx (2016) xxx–xxx
Scheme 2. Aerobic oxidation of
b-pinene under the catalysis of FeP.
176
177
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180
222
223
224
225
products. MPs with different central metal nuclei possess different
activities in this biomimetic catalytic process. Higher reactive
catalysts could promote the decomposing of HPs and the
propagating of MPs catalyzed process, resulting to the different
selectivity for allylic hydroxylation products.
JSBEM-S-201605), the National Natural Science Foundation of
China (No. 31600466) and the Fundamental Research Funds
for the Central Non-profit Research Institution of CAF (No.
CAFYBB2014QA022).
226
Appendix A. Supplementary data
181
3. Conclusion
227
228
Supplementary data associated with this article can be found, in
the online version, at 10.1016/j.cclet.2016.11.020.
182
183
184
185
186
187
188
189
Aerobic oxidation of b-pinene catalyzed by MDPs in absence of
solvents and additives at ambient pressure was studied. The
optimal reaction conditions of this protocol were evaluated to be
90 ꢁC, 5 ppm and 60 mL/min. The catalyst active of MDPs with
different central metal nuclei followed a sequence FeClDPDME >
CoDPDME > MnClDPDME > CuDPDME. This catalytic system has
bright application prospect since only eco-friendly and readily
available dioxygen were needed.
229
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208
4.1. Aerobic oxidation procedure
250
251
209 Q3
210
Aerobic oxidation of
necked glass flask containing a reflux condenser, a thermometer
and a breather pipe. -Pinene (100 g, 0.73 mol) and a certain
amount of MDPs were added. When the flask was heated to a
certain temperature (60–110 ꢁC), oxygen was fed into the mixture
with a flow rate of 20–100 mL/min. The reaction was sampled
every one-hour and analyzed by GC using n-nonane as an inert
internal standard. Hydroperoxides were determined according to
the method reported in our previous work [19]. Other components
were identified by GC–MS.
b-pinene was carried out in a 250 mL four-
211
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b
212
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¼
C and allylic
220 Q4
This work was funded by the Fundamental Research Funds
for Jiangsu Key Lab of Biomass Energy and Material (No.
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263
a
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Please cite this article in press as: S.-C. Xu, et al., Solvent and additive-free selective aerobic allylic hydroxylation of
metalloporphyrins, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.11.020
b-pinene catalyzed by

G Model
CCLET 3894 1–5
S.-C. Xu et al. / Chinese Chemical Letters xxx (2016) xxx–xxx
5
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Please cite this article in press as: S.-C. Xu, et al., Solvent and additive-free selective aerobic allylic hydroxylation of
metalloporphyrins, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.11.020
b-pinene catalyzed by