Polyethylene glycol radical-initiated oxidation of benzylic alcohols in compressed carbon dioxide

Article abstract of DOI:10.1039/b900128j

The PEG radical from oxidative degradation of polyethylene glycol was first used to initiate the oxidation of benzylic alcohols to carbonyl compounds without the need of a catalyst and/or additive in a viable synthetic, cost-effective and environmentally benign way, in which PEG/O ₂/CO₂ acts as initiator, oxidant and solvent. Compressed CO₂ in this study not only provides a safe environment to conduct the oxidation employing molecular oxygen as an oxidant, but also could improve the reaction and adjust the selectivity of the target product by altering CO ₂ pressure. Moreover, this methodology could be used to oxidize a set of benzyl alcohols. The findings introduced here provide useful examples for developing free-radical chemistry from PEG thermal oxidative degradation.

Full text of DOI:10.1039/b900128j

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www.rsc.org/greenchem | Green Chemistry
Polyethylene glycol radical-initiated oxidation of benzylic alcohols in
compressed carbon dioxide†
Jin-Quan Wang, Liang-Nian He* and Cheng-Xia Miao
Received 6th January 2009, Accepted 31st March 2009
First published as an Advance Article on the web 16th April 2009
DOI: 10.1039/b900128j
The PEG radical from oxidative degradation of polyethylene glycol was first used to initiate the
oxidation of benzylic alcohols to carbonyl compounds without the need of a catalyst and/or
additive in a viable synthetic, cost-effective and environmentally benign way, in which
PEG/O
2
/CO
2
acts as initiator, oxidant and solvent. Compressed CO in this study not only
2
provides a safe environment to conduct the oxidation employing molecular oxygen as an oxidant,
but also could improve the reaction and adjust the selectivity of the target product by altering CO
pressure. Moreover, this methodology could be used to oxidize a set of benzyl alcohols. The
2
findings introduced here provide useful examples for developing free-radical chemistry from PEG
thermal oxidative degradation.
Introduction
benign media for chemical reactions and phase transfer catalysts
8
as well as having an almost negligible vapor pressure. On the
The oxidation of primary and secondary alcohols to aldehydes
or acids and ketones, respectively, is of significant importance
in organic chemistry, both for fundamental research and in-
dustrial manufacturing. Traditionally, such transformation has
been performed with stoichiometric inorganic oxidants, notably
9
other hand, the oxidative thermal degradation of PEG was also
encountered because PEG is susceptible to free radical oxidative
attack in the presence of oxygen at an elevated temperature over
1
◦
7
0 C. In this respect, PEG and oxygen can react to form PEG
peroxide in excess air through a so-called random chain scission
2
chromium reagents. However, with the increasing environmen-
9
process. We reasoned that the PEG radicals in situ generated
tal and economical concerns in recent years, much attention
has been directed towards the development of environmentally
benign methods for oxidation of alcohols, using molecular
oxygen as a primary oxidant, which is readily available and
produces water as the sole by-product. Recently, efficient systems
using molecular oxygen have been developed for the catalytic
from oxidative degradation could be used to induce organic
reactions, specifically oxygenation of alcohols. To the best of
our knowledge, this methodology has not yet been reported in
the literature so far. In this work, we found that the PEG radical
from oxidative degradation of polyethylene glycol could be used
to initiate the oxidation of benzylic alcohols to produce carbonyl
compounds without the need for a catalyst and/or additive, in
3–6
oxidation of alcohols.
Compressed CO
2
has been touted as a suitable solvent
which PEG/O
shown in Scheme 1. Compressed CO
could improve the oxidation but adjust the selectivity of the
target product by altering CO pressure.
2
/CO
2
acts as initiator, oxidant and solvent, as
for organic synthesis, offering economical and environmental
2
in this study not only
benefits due to its favorable properties and readily tunable
7
solvent parameters, particularly, compressed CO
2
appears to be
2
an ideal solvent for use in oxidation. Unlike almost any organic
solvent, CO will not be oxidized further, and hence the use
of CO as a reaction medium eliminates by-products originating
from solvents. At the same time, compressed CO provides a safe
2
2
2
reaction environment with excellent mass and heat transfer for
aerobic oxidations. As a consequence, novel chemistry relevant
to enhancing selectivity towards desired products, improving
reactivity and ease of product separation could be created when
Scheme 1 Oxidation of benzyl alcohol by the PEG radical in com-
pressed CO
2
.
utilizing compressed CO as a reaction medium.
2
Polyethylene glycol (PEG) and its derivatives are commonly
known to be inexpensive, thermally stable, environmentally
Results and discussion
Optimization
To validate our hypothesis, we initially examined oxygenation of
State Key Laboratory and Institute of Elemento-Organic Chemistry
Nankai University, Tianjin, 30071, P.R. China. E-mail: heln@
nankai.edu.cn; Fax: +86 22-2350 4216; Tel: +86 22-2350 4216
benzyl alcohol and the reaction was carried out in the presence
◦
of PEG and 2.5 MPa O
2
at 100 C for 12 h. Under these
conditions, the benzyl alcohol furnished the benzaldehyde (19%)
and benzoic acid (13%) concomitant with 3% benzyl formate
(Table 1, entry 2). The conversion was further improved by
†
Electronic supplementary information (ESI) available: General infor-
mation, EPR spectrum, GC chart, ESI-MS spectrum and NMR charts.
See DOI: 10.1039/b900128j
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Table 1 Oxidation of benzyl alcohol initiated by PEG radical arising
Table 2 The influence of the PEG molecular weight on the oxidation
a
a
from oxidative/thermal degradation of PEG. Screening
of benzyl alcohol
◦
b
b
b
Entry T/ C Conv.(%) Yield of aldehyde (%) Yield of acid (%)
Yield of
Conv. (%) aldehyde (%)
Yield of b
acid (%)
b
Entry PEG
1
2
3
4
5
6
7
8
9
100
100
100
100
80
120
100
100
100
77
36
0
0
8
97
98
88
0
22
19
0
0
7
1
6
8
0
50
13
0
0
0
89
84
61
0
c
d
e
1
2
3
4
5
6
300
1000
2000
6000
95
77
73
9
1
8
7
22
28
8
1
12
83
50
43
0
0
69
20 000
f
PEG-dimethyl
ether (MW = 1000)
g
h
a
All the reactions were conducted with 0.2 g (1.93 mmol) benzyl alcohol,
a
◦
b
All the experiments were carried out with 1.93 mmol of benzyl alcohol,
.7 mmol of PEG-1000, 2.5 MPa O
0.7 g PEG, 2.5 MPa O
by GC.
2 2
, 13.5 MPa CO at 100 C for 12 h. Determined
0
2
and 13.5 MPa CO
2
d
at 12 h, unless
In the absence
b
c
otherwise stated. Determined by GC. Without CO
2
.
e
f
g
of PEG. Without O
PEG-1000, 2 MPa O
2
. 24 h. 77.2 mmol of benzyl alcohol, 2.1 mmol of
◦
h
2
, 100 C, 18 h. TEMPO (0.193 mmol) was added.
Table 3 The influence of PEG-1000 amount on the oxidation of benzyl
alcohol
a
introducing 13.5 MPa CO
2
(entry 1), and reached up to 98% with
Substrate/PEG
(molar ratio)
Yield of
aldehyde (%)
Yield of b
acid (%)
b
Entry
Conv. (%)
benzoic acid as a main product when elongating reaction time
to 24 h (entry 7). Furthermore, the reaction temperature had
great influence on this transformation. The reaction gave only
1
2
3
4
11
1
3
77
82
1
0.2
22
21
0
0.4
50
55
5.5
2.8
1.4
◦
with 8% conversion at 80 C (entry 5), suggesting that an elevated
◦
9
temperature (over 80 C) is required to induce thermal oxidative
◦
degradation of PEG; whereas, higher temperatures (120 C) pre-
a
All the reactions were conducted with 1.93 mmol benzyl alcohol,
2.5 MPa O , 13.5 MPa CO at 100 C for 12 h. Determined by GC.
2 2
◦
b
ferentially afforded benzoic acid with 97% conversion (entry 6).
More importantly, the reaction could be scaled up (entry 8),
making this method more synthetically viable in organic syn-
thesis. Notably, the reaction could not take place without
ratio of the substrate and PEG-1000 was around 1.4 under the
otherwise identical reaction conditions.
PEG or O (entries 3 and 4). In other words, both PEG
2
and molecule oxygen are prerequisites to performing those
reactions smoothly. In addition, the oxygenation was completely
suppressed by the presence of TEMPO (2, 2, 6, 6-tetramethyl-
piperidine-1-oxyl) (entry 9). In short, those results imply that
Effect of CO
2
pressure
Fig. 1 shows the effect of CO
2
pressure on the reaction. In the
pressure could significantly
-expanded
present study, an appropriate CO
enhance the reaction rate probably due to the “CO
effect of PEG” in PEG/compressed CO systems, thus resulting
2
9
PEG oxidative/thermal degradation plays a critical role in this
2
reaction and the “PEG/O
2
/CO
2
” possibly serves as a radical
2
initiator, oxidant and solvent.
in changes in the physical properties of the PEG, including
lowered melting points, lowered viscosity, increased gas/liquid
diffusion rates. Indeed, as judgedbyvisualinspectionthrough a
window-equipped high-pressure reactor, we found that PEG and
11
Influence of PEG molecular weight
its derivatives are expandible with CO
the solubility of O and CO in PEG reported in the literature.
Interestingly, the selectivity noticeably depended on the pressure
of CO . Lower pressure favored the formation of benzoic acid,
and higher pressure preferentially afforded benzaldehyde.
2
, which possibly improves
We next investigated what influence PEGs of different molecular
weights had on the reaction. The conversion was gradually
decreasing with the PEG molecular weight increasing from
8,12
2
2
3
00 to 20 000 (Table 2), being presumably ascribed to the
increasing mass transport limitation of gaseous oxygen in highly
2
10
viscous PEG with a long chain. Interestingly, the product
distribution between aldehyde and acid was also correlative with
the PEG molecular weight (Table 2, entries 1–5). The reaction
preferentially furnished the corresponding acid with PEG-300
(
entry 1), and exclusively afforded the targeted aldehyde with
PEG-6000 although the lower conversion was achieved (entry 4).
In addition, the PEG with protected hydroxyl group also proved
effective (entry 6).
Influence of PEG amount
Furthermore, the effect of PEG amount on this oxygenation
were also studied as shown in Table 3. The conversion was
enhanced by increasing the quantity of PEG, the suitable molar
2
Fig. 1 CO pressure effect on oxidation of benzyl alcohol. Reaction
conditions: 1.93 mmol benzyl alcohol, 0.7 g PEG-1000, 2.5 MPa O at
2
◦
100 C for 12 h.
1
014 | Green Chem., 2009, 11, 1013–1017
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Table 4a The influence of O
2
pressure on the oxidation of benzyl
Yield of
Table 5 Oxidation of primary and secondary alcohols using PEG/O
2
/
a
alcohol
2
CO system
b
Entry Substrate
1
Product
Conv. (%) Yield (%)
b
b
Entry
O
2
/MPa
Conv. (%)
aldehyde (%)
Yield of acid (%)
100
100
97
93
90
89
1
2
3
4
5
1.5
2.5
2.5
3
9
77
36
97
0
8
22
19
4
0
50
13
88
0
c
c
2
3
0.1
0
a
All the reactions were conducted with 1.93 mmol benzyl alcohol,
◦
b
1
3.5 MPa CO
2
at 100 C for 12 h, unless otherwise stated. Determined
c
by GC. Without CO
2
.
Influence of O
2
pressure
c
The influence of the pressure of O
2
on oxidation of benzyl
4
60
50
53
30
alcohol was also examined. As shown in Table 4, the higher
pressure was beneficial for the formation of the acid and the
O
2
d
lower pressure preferentially afforded aldehyde although with
the low conversion (entry 1 vs. entry 2). This is understandable
because higher oxygen concentration favors further oxidation
of aldehyde to carboxylic acid. It is worth mentioning that
5
d
the oxidation reaction did not occur under 1 atm O
absence of CO under the otherwise identical reaction conditions
entry 5), whereas 2.5 MPa O gave 36% conversion (entry 3),
is required for this transfor-
2
in the
6
—
0
0
2
(
2
hinting that enough pressure of O
mation (entries 1–4).
2
c
7
83
73
Substrate scope
a
All the experiments were carried out with 1.93 mmol substrate,
◦
The generality of this methodology was also evaluated. As
shown in Table 5, primary and secondary alcohols can be
oxygenized into the corresponding carbonyl compounds in fair
to high yields (entries 1–4). In general, substrates with an
electron donating group generally showed higher activity than
those having an electron withdrawing group. The 4-methoxy-
phenyl methanol afforded the aldehyde in high selectivity at
low temperature (entry 4). The 4-nitrophenyl methanol showed
relatively poor activity (entry 5). In addition, the 2-nitro-
phenyl methanol only recovered starting material (entry 6),
possibly due to the steric hindrance effect besides the electron
effect. With regards to selectivity, secondary benzylic alcohols
showed higher selectivity towards ketone (entries 1 and 2),
while primary ones afforded carboxylic acid as a dominant
product. On the other hand, the substrate with an electron-
withdrawing group on the benzene ring predominantly gave a
carboxylic acid (entry 5). Moreover, b-phenylethanol (entry 7)
gave predominantly the formylated ester rather than the oxidized
product, further supporting the proposed radical mechanism for
0.7 mmol PEG, 2.5 MPa O and 13.5 MPa CO at 120 C for 12 h,
unless otherwise stated. Isolated yields. 100 C. 24 h.
2
2
d
b
c
◦
Scheme 2 The proposed mechanism of oxidation of benzyl alcohols by
the PEG radical from oxidative degradation of PEG.
of PEG with oxygen is prone to generating PEG radicals,
which induces the hydrogen at the benzylic position, then
goes through a free radical process. In our study, the free
radical signals from EPR (electron paramagnetic resonance)
experiments (Fig. S1, ESI†) and the formylated product from
PEG oxidative degradation using b-phenylethanol as a substrate
9
PEG degradation. This is reasonable because the main product
was produced via formylation of alcohol with formic acid in situ
generated during the degradation of PEG as previously
9
reported.
Possible mechanism
(
Table 5, entry 7) presumably supports the proposed radical
Based on aforementioned findings, a plausible mechanism of
mechanism.
this protocol may be considered to involve PEG radical in situ
formed during PEG thermal degradation. A mechanistic
scheme was proposed as delineated in Scheme 2. The reaction
The proposed mechanism was also supported as evidenced by
the following facts. Those include producing trace amounts of
formylated products from degradation of PEG (Fig. S2, ESI†),
13
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broadening PEG molecule weight distribution (Fig. S3, ESI†),
generating a peroxide intermediate as detected by a KI/starch
test, and the suppressing effect of TEMPO on this reaction
2014) equipped with a capillary column (HP-5 30 m ¥ 0.25 mm)
using a flame ionization detector. The structure and the purity
of the products were further identified using NMR (BRUKER-
300 MHz, VARIAN-400 MHz), GC-MS (HP G1800A), HPLC-
MS (LCQ Advantage) and GC, HPLC by comparing reten-
tion times and fragmentation patterns with those of authentic
samples. The EPR (Bruker EMX-6) experiments were carried
(
Table 1, entry 9).
Conclusions
We have herein disclosed that the PEG radical presumably
from oxidative degradation of PEG could be utilized to trigger
oxygenation of benzylic alcohols without the need of a catalyst
and additive under metal-free conditions in efficient, cost-
out with 5.7 mmol benzyl alcohol and 2.1 mmol PEG-300
◦
at 100 C in presence of 1 atm O
2
after 4 h. NMR spectral
characterizations of the products are as follows:
1
Benzaldehyde. H NMR (CDCl
3
, 400 MHz): d = 7.50 (t,
effective and environmentally benign fashion. Compressed CO
could improve the oxidation and also adjust the selectivity of
target product by altering CO pressure. Those findings intro-
2
3
3
J (H, H) = 7.7 Hz, 2H; PhH), 7.60 (t, J (H, H) = 7.4 Hz,
3
1
H; PhH), 7.84 (d, J (H, H) = 7.9 Hz, 2H; PhH), 9.98 (s, 1H,
2
1
3
1
CHO); C { H} NMR (CDCl , 100.6 MHz): d = 128.85, 129.58,
3
duced here provide a useful example for developing free-radical
chemistry from PEG thermal oxidative degradation. Further
broadening this protocol to a wide set of organic reactions with
enormous synthetic potentials requiring radical chemistry is in
progress. Given more time for in-depth investigation, we believe
that free-radical chemistry of PEG thermal degradation will
become an even more powerful tool in organic synthesis.
1
34.33, 136.19, 192.28.
1
Acetophenone. H NMR (400 MHz, CDCl ): d = 2.60 (s,
3
3
3
3H, CH ), 7.46 (t, J (H, H) = 7.7 Hz, 2H; PhH), 7.56 (t, J (H,
3
3
H) = 7.5 Hz, 1H; PhH), 7.96 (d, J (H, H) = 7.8 Hz, 2H; PhH);
1
3
1
C { H} NMR (CDCl , 100.6 MHz): d = 26.54, 128.25, 128.51,
3
133.04, 137.10, 198.06.
1
4
-Methoxy-benzaldehyde. H NMR (300 MHz, CDCl
3
): d =
3
Experimental
3.87(s, 3H, CH ), 7.00 (d, J(H, H) = 8.72 Hz, 2H; PhH), 7.82
3
3
13
1
(
d, J(H, H) = 8.78 Hz, 5H; PhH), 9.87 (s, 1H, CHO); C { H}
3
Safety warning
NMR (75 MHz, CDCl
190.7.
): d = 55.5, 114.3, 130.0, 131.9, 164.6,
Experiments using large amounts of compressed gases, espe-
cially molecular oxygen and supercritical fluids, are potentially
hazardous and must only be carried out by using the appropriate
equipment and under rigorous safety precautions. In particular,
1
Diphenyl-methanone. H NMR (300 MHz, CDCl
3
): d = 7.48
3
3
(
t, J(H, H) = 7.9 Hz, 4H; PhH), 7.58 (t, J(H, H) = 7.9 Hz, 2H;
3
1
3
1
PhH), 7.81 (t, J(H, H) = 5.7 Hz, 4H; PhH); C { H} NMR
oxygen is introduced into the substrate-loaded reactor after CO
To avoid an explosive regime the following order should be used:
substrate > CO > oxygen. Moreover, the oxygen content should
2
.
(
75 MHz, CDCl
3
): d = 128.3, 130.1, 132.4, 137.6, 196.7.
1
2
Formic acid phenethyl ester. H NMR (300 MHz, CDCl
3
):
), 4.39 (t, J(H, H) =
), 7.21–7.32 (m, 5H, PhH), 8.03 (s, 1H, OCH);
3
3
not exceed 14 vol% when CO
2
is used as a reaction medium.
d = 2.97(t, J(H, H) = 7.01 Hz, 2H; CH
2
7
.03 Hz, 2H; CH
2
1
3
1
Materials
C { H} NMR (75 MHz, CDCl ): d = 34.9, 64.3, 126.7, 128.5,
3
1
28.8, 137.4, 160.8.
The alcohols were purchased from J & K CHEMICAL. Carbon
dioxide with a purity of 99.99% was commercially available. The
other organic compounds from Tianjin Guangfu Fine Chemical
Research Institute were used without further purification except
for the solvents, which were distilled by the known method prior
to use.
Acknowledgements
Financial support from National Science Foundation (Grant
no. 20421202, 20672054 and 20872073), and the 111 project
(B06005) and Tianjin Natural Science Foundation is gratefully
acknowledged.
Oxidation reaction
A mixture of substrate (1.93 mmol) and PEG-1000 (0.7 g,
Notes and references
0
.7 mmol) was placed in a 25 mL autoclave equipped with
1
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2
2
sequentially introduced into the autoclave and heated to the
reaction temperature. Then the final pressure was adjusted to
the desired pressure at the reaction temperature by introducing
2
002, 35, 774; B. Z. Zhan and A. Thompson, Tetrahedron, 2004, 60,
the amount of CO
the designed reaction time, then the reactor was placed into
ice water and CO was released slowly passing through a cold
trap containing diethyl ether to absorb the trace amount of
reactant and product entrained by CO . After depressurization,
2
. The mixture was stirred continuously for
2917.
2
3
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