2,3-Dichloro-5,6-dicyano-1,4-benzoquinone-catalyzed aerobic oxidation reactions via multistep electron transfers with iron(II) phthalocyanine as an electron-transfer mediator

Article abstract of DOI:10.1039/c6ra08921f

A new biomimetic catalytic oxidation system which employs 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) as the catalyst, molecular oxygen as the terminal oxidant and iron(ii) phthalocyanine (Fe^IIPc) as the electron-transfer mediator has been developed. This system can be applied for oxidative deprotection of PMB ethers, alcohol oxidation, aromatization and α,β-unsaturated aldehyde formation. After immobilizing Fe^IIPc on multi-walled carbon nanotubes, it can be reused without loss of activity.

Full text of DOI:10.1039/c6ra08921f

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COMMUNICATION
2,3-Dichloro-5,6-dicyano-1,4-benzoquinone-catalyzed aerobic
oxidation reactions via multistep electron transfers with iron(II)
Received
Accepted
phthalocyanine as an electron-transfer mediator
†
Yiming Hong,a Tiantian Fang,b Meichao Li,a Zhenlu Shen,*a Xinquan Hu,*b Weimin Mo,a Baoxiang
Hu,a Nan Sun,a and Liqun Jina
DOI:
www.rsc.org/
molecular oxygen to their active forms.2 However, this approach is
A new biomimetic catalytic oxidation system which employed 2,3-
dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) as the catalyst,
molecular oxygen as the terminal oxidant and iron(II)
phthalocyanine (FeIIPc) as the electron-transfer mediator has been
developed. This system can be applied for oxidative deprotection
of PMB ethers, alcohol oxidation, aromatization and α,β-
unsaturated aldehyde formation. After immobilized FeIIPc on
multi-walled carbon nanotube, it can be reused without loss of
activity.
not universal, which fails in some cases, due to the electron transfer
between catalyst and molecular oxygen is high in energy barrier.
Therefore, an alternative method using electron-transfer mediators
(ETMs) in the catalysts and molecular oxygen is developed. The
ETMs would transfer electrons from catalyst to molecular oxygen
along a low-energy pathway (Figure 1).3 This can be described as a
biomimetic approach due to the similarities with biological systems.
2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) is a powerful
oxidant, and it has been successfully applied in many reactions.4
However, the use of stoichiometric DDQ may result in purification
difficulties because of the concomitant by-product 2,3-dichloro-5,6-
dicyano-hydroquinone (DDHQ). To avoid this inherent disadvantage,
Introduction
5
Oxidation reactions play an important role in organic synthesis, and
numerous oxidants and oxidation methodologies have been
developed. Today, there is an increasing demand for the use of
molecular oxygen as the terminal oxidant in the oxidation reactions
due to its remarkable advantages, including great abundance, low
cost, and generating water as the only byproduct. This kind of
oxidant meet the requirement for sustainable and environmentally
benign processes. However, the direct oxidation of common organic
compounds by the molecular oxygen is difficult because of the high
energy barrier in the electron transfer from the organic substrates
to molecular oxygen. This is also the reason why the molecular
oxygen has a high oxidative potential but shows extraordinary
stability. Therefore, oxidation systems which employed substrate-
selective catalysts with molecular oxygen as the terminal oxidant
have been developed to circumvent the above problem.1 In these
systems, the organic substrates can be oxidized by the substrate-
selective catalysts, and the reduced catalysts can be reoxidized by
catalytic oxidation systems with DDQ as the catalyst and Mn(OAc)3
6
or MnO2 as the terminal oxidants were developed, while it should
be noted that using of a great amount of these inorganic oxidants
brought new environmental effluents. We are interested in using
molecular oxygen as the terminal oxidant, but it is impossible to
directly regenerate HDDQ to DDQ with molecular oxygen, and
therefore an additional ETM is needed. Previously, our group
developed a DDQ/tert-butyl nitrite (TBN)/O2 catalytic oxidation
system for the aerobic oxidation of alcohols,
a College of Chemical Engineering, Zhejiang University of Technology, Hangzhou
310014, China. e-mail: zhenlushen@zjut.edu.cn, xinquan@zjut.edu.cn
b College of Pharmaceutical Sciences, Zhejiang University of Technology, Hangzhou
310014, China
† Electronic Supplementary Informaꢀon (ESI) available: Experimental procedure,
characterization data and copies of 1H NMR spectra. See DOI: 10.1039/x0xx00000x
Figure 1 Comparison of a) direct reoxidation catalysts by O2 and b)
reoxidation catalysts via low-energy electron transfer using ETMs.
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ARTICLE
Journal Name
oxidative deprotection of ethers, and oxidation of diarylmethane
sp3 C–H bonds,7 which was also utilized by other groups.8 The key
point of this aerobic oxidation is that NO released from TBN can act
as the ETM between DDQ and molecular oxygen. In fact, this
concept was also applied in our other catalytic oxidation systems,
Scheme 1 Proposed DDQ/FeIIPc/O2 catalytic oxidation system.
9
10
such
as
TEMPO/Br2/NaNO2/O2
11
,
TEMPO/HBr/TBN/O2 ,
12
TEMPO/TBN/O2
TEMPO/KPF6/NaNO2/O2
,
and
Considering that DDQ is a quinone compound, we reasoned that
FePc could act as the ETM between DDQ and molecular oxygen.
Thus we wish to establish DDQ/FeIIPc/O2 system for the oxidation
reactions as described in Scheme 1. This new process involves two
redox systems: DDQ/DDHQ–FePcox/FeIIPc. Herein, we reported our
research results of this catalytic oxidation system for oxidative
deprotection of p-methoxybenzyl (PMB) ethers, alcohol oxidation,
aromatization and α,β-unsaturated aldehyde formation. Although
DDQ-catalyzed oxidative deprotection of PMB ethers, alcohol
oxidation and aromatization using molecular oxygen as the terminal
oxidant were reported recently, to the best of our knowledge, this
work reported the first example of DDQ-catalyzed conversion of
aldehyde to α,β-unsaturated aldehyde using molecular oxygen as
the terminal oxidant.
TEMPO/DDQ/TBN/O2 system.13 In addition, azobisisobutyronitrile
(AIBN) also can be used as the ETM in the DDQ-catalyzed aerobic
oxidations.14
Iron phthalocyanine (FePc) is a macrocyclic compound, whose
central iron ion is bonded so strongly that it only can be removed by
breaking the macrocycle.15 Having a similar structure with iron
porphyrins,16 the efficiencies of FePc as catalyst in many oxidation
reactions, such as catalytic oxidation of arenes,17 alkanes,18
phenols,19 olefins20 and other compounds,21 have been
demonstrated. Based on the FePc-catalyzed oxidation reactions of
hydroquinones to benzoquinones,18 several aerobic oxidation
reactions employing palladium as the catalyst, benzoquinone (BQ)
as the co-catalysts and FePc as the ETM have been developed.22
Table 1 Optimization of DDQ/FeIIPc/O2 for the oxidative deprotection of 1aa
DDQ
(mol %)
-
FeIIPc
(mol %)
20
Additive
(mol %)
T
t
(h)
24
24
24
24
24
5
P
(MPa)
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
Conv. b
(%)
1
Select. [b]
Entry
Solvent
(°C)
120
120
120
120
120
120
120
120
120
120
120
80
(%)
-
1
2
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
CH3CN
DMF
-
20
-
-
16
-
3
20
20
-
70
43
79
87
90
96
96
91
92
88
98
98
98
96
98
-
4
20
20
2,2′-bpy (20)
4,4′-bpy (20)
4,4′-bpy (20)
4,4′-bpy (10)
4,4′-bpy (8)
pyrazole (10)
imidazole (10)
pyridine (10)
4,4′-bpy (10)
4,4′-bpy (10)
4,4′-bpy (10)
4,4′-bpy (10)
4,4′-bpy (10)
4,4′-bpy (10)
4,4′-bpy (10)
4,4′-bpy (10)
4,4′-bpy (10)
70
5
20
20
100
100
100
86
6
20
20
7
10
10
5
8
8
8
5
9
10
10
5
52
10
11
12
13
14
15
16
17
18
19
20
10
10
5
98
10
10
5
54
10
10
5
90
10
10
80
5
78
10
10
80
6
82
10
10
80
8
100
9
10
10
80
8
10
10
80
8
10
GDMEc
10
10
80
8
25
-
PhCl
TeCAd
10
10
80
8
100
<1
96
-
10
10
80
8
a The reactions were performed by using 1a (1 mmol), and solvent (20 mL). b The conversion and selectivity were determined by GC with area normalization. c GDME:
ethylene glycol diethyl ether. d TeCA: 1,1,2,2ꢀtetrachloroethane.
2
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Table 2 Oxidative deprotection of PMB ethers with DDQ/FeIIPc/O2
systema
Results and discussion
By applying it to the oxidative deprotection of PMB ethers, we
study the feasibility of the DDQ/FeIIPc/O2 system. In the initial
screening experiments, the oxidative deprotection of PMB ether of
2-octanol (1a) was selected as the model reaction (Table 1). Firstly,
the oxidative deprotection of 1a was carried out in toluene at 120
oC under 0.5 MPa of oxygen for 24 h. Without DDQ or FeIIPc, the
conversion of 1a was poor (entries 1 and 2). In the presence of 20
mol% of FeIIPc and 20 mol% of DDQ, the conversion of 1a was
increased to 70%, but the selectivity to 2-octanol (2a) was only 43 %
(entry 3).
t
Conv.b
(h) (%)
Select.b
(%)
Entry Substrate
1
1a
1b 14 100
1c 10 100
8
100
96
96
97
2c
3
CH3(CH2)7OPMB
It was well known that nitrogenous ligands could affect the
catalytic performance of metalloporphyrin via changing the spin
state of the catalyst.23 It also has been reported that nitrogenous
compounds could modulate the physicochemical properties of FePc
via changing the spin property of the iron atom due to the strong
ligand field created by the nitrogenous ligand.15,24 Thus we
reasoned nitrogenous compounds could regulate the catalytic
performance of FePc, and 2,2'-dipyridyl (2,2′-bpy) was added in our
DDQ/FeIIPc/O2 system. To our delight, the selectivity to 2a was
dramatically increased to 79%, though the conversion of 1a
remained 70% (entry 4). When 2,2′-bpy was replaced by 4,4′-bpy,
100% conversion of 1a with 87% selectivity to 1b was obtained
(entry 5). Then the reaction time was shortened, it was found the
oxidative deprotection could be completed within 5 h (entry 6).
Later on, the loads of DDQ, FeIIPc and 4,4′-bpy were also attempted
to be reduced (entries 7 and 8). The experimental results showed
that 1a could be fully converted to 2a in 96% selectivity with 10
mol% of FeIIPc, 10 mol% of DDQ and 10 mol% of 4,4′-bpy (entry 7).
Other nitrogenous compounds were also screened, 4,4′-bpy gave
the best result (entries 7, 9–11).
4c
1d 20 100
98
98
5
1e
14 100
6d
7c
1f
24 100
18 100
93
96
Boc
N
1g
OPMB
8
1h 16 100
98
9
1i
1j
8
100
98
95
10c
10 100
Cutting down the reaction temperature and pressure of oxygen,
the conversion of 1a would be decreased, but not unacceptable
(entries 12 and 13). When the reaction time was prolonged to 8 h,
the complete conversion of 1a with 96% selectivity to 2a could be
achieved at 80 oC under 0.4 MPa of oxygen (entry 15). Among the
screened solvents, toluene and chlorobenzene gave the excellent
results (entries 15–20). Toluene was selected as the solvent for the
next studies because it was more environmentally benign than
chlorobenzene.
11
12
13
14
15
16
HO(CH2)6OPMB
1k
1k′
1l
8
8
8
100
100
100
92
95
96
95
95
96
PMBO(CH2)6OPMB
MeO(CH2)6OPMB
MOMO(CH2)6OPMB
AcO(CH2)6OPMB
BnO(CH2)6OPMB
1m 16 100
1n
1o
8
8
100
100
a Reaction conditions: PMB ether (1 mmol), toluene (20 mL), DDQ (10 mol %),
FeIIPc (10 mol %), 4,4′-bpy (10 mol%), O2 (0.4 MPa), 80 oC. b The conversion
and selectivity were determined by GC with area normalization. c DDQ (15
mol %), FeIIPc (15 mol %), 4,4′-bpy (15 mol%).d DDQ (20 mol %), FeIIPc (20
mol %), 4,4′-bpy (20 mol%).
With the optimized reaction conditions in hand, the results of
aerobic oxidative deprotection of a variety of PMB ethers in
presence of DDQ, FeIIPc and 4,4′-bpy are summarized in Table 2. All
of the substrates could be smoothly converted to their
corresponding alcohols. PMB ethers of 1-octanol and cyclohexanol
(1b and 1c) underwent a complete deprotection to produce the
corresponding alcohols in excellent selectivities (entries 2 and 3).
When PMB ethers with high steric hindrance (1d and 1e) were
subjected to deprotection, increased catalyst loads or prolonged
reaction time were needed (entry 4 and 5). PMB ethers containing a
heterocyclic moiety (1f–1i) could also be fully deprotected (entries
6–9). The successful deprotection of 1g showed that the oxidative
deprotection can endure the Boc group (entry 7). When mono- or
di-PMB protected 1,6-hexanediol (1k and 1k′) were used as the
substrates, hexanediol (2k) could be obtained in excellent yields
(entries 11 and 12). The substrates with two different hydroxyl
protecting groups (1l–1n) were also submitted to the oxidative
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Table 3 Oxidative deprotection of PMB ethers with DDQ/FeIIPc-
In order to demonstrate the stability and recyclability of FeIIPc-
suported, FeIIPc-MWCNT was examined by the oxidative
MWNT/O2 systema
Entry
1
2c
Substrate
1a
T (oC)
80
t (h)
8
Yieldb (%)
(93)
87
100
80
60
40
20
0
1b
80
14
10
20
14
7
3
4c
5c
6d,e
7c
1c
80
(96)
(98)
82
1d
80
1e
80
1f
120
80
65
1g
18
16
16
10
16
24
16
16
16
8
88
8
9d
1h
80
91
1i
80
78
10d
11d
12d
13d
14d
15d
16d
1j
100
120
120
100
100
100
100
84
1
2
3
run
4
5
1k
79
Figure 2 Stability and recyclability of FeIIPc-MWCNT in the
oxidative deprotection of 1a
1k′
1l
78
71
1m
1n
85
84
deprotection of 1a under optimized reaction conditions. After
completion of each reaction run, FeIIPc-MWCNT could be recovered
by simple filtered and washed with toluene. The recovered catalyst
was then successfully used in four runs at the expense of a slight
decline in catalytic performance (Figure 2).
1o
91
a Reaction conditions: PMB ether (1 mmol), toluene (20 mL), DDQ (10 mol %),
FeIIPc-MWNT (10 mol %), 4,4′-bpy (10 mol%). Isolated yields, values in
b
c
parentheses were determined by GC internal standard method. DDQ (15
mol %), FeIIPc (15 mol %), 4,4′-bpy (15 mol%).
Then DDQ/FeIIPc/O2 system was examined in other aerobic
oxidation reactions to explore the generality of the process. Alcohol
oxidation, aromatization and α,β-unsaturated aldehyde formation
were selected as examples (Scheme 2). Oxidation of alcohol is quite
effective, with cinnamyl alcohol (3) being converted to
cinnamaldehyde (4) in 90% isolated yield. This DDQ/FeIIPc/O2
system could also be applied to the aromatization, as seen in the
conversion of indoline (5) to indole (6). Hayashi et al reported a
practical conversion of aldehydes into α,β-unsaturated aldehydes
with a catalytic amount of DDQ and stoichiometric MnO2.27 Herein,
our DDQ/FeIIPc/O2 system successfully replaced DDQ/MnO2 system,
and 3-phenylpropanal (7) was converted to cinnamaldehyde (4) in
deprotection. The results showed that the PMB group could be
selectively removed to give the corresponding alcohols without
affecting the other protecting groups (entries 13–15). Compared
with Bn group, PMB group was easy to deprotect. Entry 16 showed
that the PMB group was selectively cleaved without affecting the
Bn group.
To increase the synthetic utility of the reaction, the recyclability
of FePc was considered, and the immobilization of FePc on
insoluble supports appears to be a good way to make it practicable.
Multi-walled carbon nanotubes (MWCNTs) exhibit extraordinary
physical and chemical characteristics, and they are good carriers for
catalytic applications because of specific metal support interactions
given by their graphitic structure as well as specific surface area.25
FePc and Fe-porphyrin could be easily adsorbed on MWCNTs via
non-covalent π–π interactions, and immobilization of FePc and Fe-
porphyrin on the MWCNTs also have been reported.26 Therefore,
the MWCNT-supported FeIIPc (FeIIPc-MWCNT) was prepared and
applied to the oxidative deprotection of PMB ethers. Under the
reaction conditions of 0.4 MPa of oxygen and toluene as the
solvent, the results of oxidative deprotection of PMB ethers are
summarized in Table 3. The data in Table 3 show the yields of
alcohols to be good to excellent (entries 1–16). Most of the PMB
ethers showed high reactivities affording the desired products.
Compared with the oxidative deprotection reactions performed
with non-supported FeIIPc, most of the reactions performed with
FeIIPc-MWNT needed longer reaction time, higher catalyst loads or
reaction temperature.
4
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Except that FePc was replaced by FeIIPc-MWCNT, the experimental
process was as same as the above description. After the reaction
was finished, the reaction mixture was filtered, and FeIIPc-MWCNT
was washed with toluene. Then the filtrate was concentrated by
rotary evaporator and the residue was purified by column
chromatography on silica gel to afford 2h in 91% yield as a white
solid. 1H NMR (500 MHz, CDCl3): δ 7.39 (d, J = 8.10 Hz, 1H), 7.15 (d, J
= 8.05 Hz, 1H), 4.68-4.70 (t, 1H), 3.61 (s, 1H), 2.83-2.72 (m, 2H),
2.24-2.23 (m, 1H), 2.02-2.00 (m, 1H), 1.85-1.80 (m, 2H).
Scheme 2 DDQ/FeIIPc-catalyzed aerobic oxidation reactions.
Acknowledgements
This work was financially supported by the National Natural Science
Foundation of China (21376224, 21206147) and Hangzhou
Qianjiang Distinguished Experts Project.
70% isolated yield. DDQ/TBN/O2 system was also applied to this α,β-
unsaturated aldehyde formation reaction, but no desired product Notes and References
could be obtained.
1
J. Piera and J.-E. Backvall, Angew. Chem. Int. Ed., 2008, 47,
3506–3523.
Conclusions
2
(a) L. L. Joyce, R. A. Batey; Org. Lett., 2009, 11, 2792–2795; (b) A.
Acharya, S. V. Kumar and H. Ila, Chem. Eur. J., 2015, 21, 17116–
17125; (c) S. H. Zhang, P. C. Qian, M. L. Zhang, M. L. Hu and J.
Cheng, J. Org. Chem., 2010, 75, 6732–6735.
In conclusion, a new catalytic oxidation system has been developed,
which employed DDQ as the catalyst, molecular oxygen as the
environmentally benign terminal oxidant, FeIIPc as the ETM and
4,4′-bpy as the additive. This system was successfully applied for
oxidative deprotection of PMB ether, alcohol oxidation,
aromatization and transformation of aldehydes into α,β-
unsaturated aldehydes. Under the optimal reaction conditions, a
variety of PMB ethers can be deprotected to their corresponding
alcohols in excellent conversions and selectivities. For the purpose
of reuse of FeIIPc, the MWCNT-supported FeIIPc (FeIIPc-MWCNT)
was prepared, which also exhibited good activity in the DDQ-
catalytic aerobic oxidative depreotection of PMB ethers. FeIIPc-
MWCNT can be easily recovered and reused for at least four times
without affecting its catalytic performance.
3
(a) B. W. Purse, L.-H. Tran, J. Piera, B. Akermark and J.-E.
Backvall, Chem. Eur. J., 2008, 14, 7500–7503; (b) S. Luo, F. X. Luo,
X. S. Zhang and Z. J. Shi, Angew. Chem. Int. Ed., 2013, 52,
10598–10601; (c) J.-E. Backvall, R.B. Hopkins, H. Grennberg, M.
M. Mader and A. K. Awasth, J. Am. Chem. Soc., 1990, 112,
5160–5166. (d) Z. K. Wickens, K. Skakuj, B. Morandi and R. H.
Grubbs, J. Am. Chem. Soc., 2014, 136, 890–893; (e) A. E.
Wendlandt and S. S. Stahl, Angew. Chem. Int. Ed., 2015, 54,
14638–14658.
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(a) B. A. McKittrick and B. Ganem, J. Org. Chem., 1985, 50,
5897–5898; (b) B. V. Rokade, S. K. Malekar and K. R. Prabhu,
Chem. Commun., 2012, 48, 5506–5508; (c) Y. H. Jang and S. W.
Youn, Org. Lett., 2014, 16, 3720–3723; (d) Z. M. Wang, H. J. Mo,
D. P. Cheng and W. L. Bao, Org. Biomol. Chem., 2012, 10, 4249–
4255.
Experimental Section
General Procedure for oxidative deprotection of PMB ethers with
DDQ/FeIIPc/O2 system (Table 2, Entry 1)
(a) G. V. M. Sharma, B. Lavanya, A. K. Mahalingam and P. R.
Krishna, Tetrahedron Lett., 2000, 41, 10323–10326; (b) C. C.
Cosner, P. J. Cabrera, K. M. Byrd, A. M. Thomas and P. Helquist,
Org. Lett., 2011, 13, 2071-2073.
To a teflon-lined 316L stainless steel autoclave (300 mL) was added
1 mmol PMB ether of 1a, 10 mol% of DDQ, 10 mol% of FePc, 10
mol% of 4,4ˊ-Bpy and 20 mL of toluene. Then closed the autoclave
and charged O2 to 0.4 MPa. Put the autoclave into an oil bath,
6
7
(a) H. Yi, Q. Liu, J. Liu, Z. Q. Zeng, Y. H. Yang, A and W. Lei,
ChemSusChem, 2012, 5, 2143–2146; (b) L. Liu and P. E.
Floreancig, Org. Lett., 2010, 12, 4686-4689.
o
which was preheated to 80 C. After the reaction was finished, the
autoclave was cooled to room temperature and carefully
depressurized. Diluted the sample with CH3CN and detected the
conversion and selectivity by GC without any purification.
(a) Z. L. Shen, J. L. Dai, J. X, X. J. He, W. M. Mo, B. X. Hu, N. Sun
and X. Q. Hu, Adv. Synth. Catal., 2011, 353, 3031–3038; (b) Z. L.
Shen, L. L. Sheng, X. C. Zhang, W. M. Mo, B. X. Hu, N. Sun and X.
Q. Hu, Tetrahedron Lett., 2013, 54, 1579–1583; (c) J. Q. Ma, Z.
General Procedure for oxidative deprotection of PMB ethers with
DDQ/FeIIPc-MWCNT/O2 system (Table 3, Entry 8)
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DOI: 10.1039/C6RA08921F
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RSC Advances
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DOI: 10.1039/C6RA08921F
2,3-Dichloro-5,6-dicyano-1,4-benzoquinone-catalyzed aerobic oxidation
reactions via multistep electron transfers with iron(II) phthalocyanine as an
electron-transfer mediator
Yiming Hong,a Tiantian Fang,b Meichao Li,a Zhenlu Shen,*a Xinquan Hu,*b Weimin Mo,a Baoxiang
Hu,a Nan Sun,a and Liqun Jina
A new biomimetic catalytic oxidation system was developed for oxidative deprotection of PMB ethers,
alcohol oxidation, aromatization and α,β-unsaturated aldehyde formation.