4-CH3CONH-TEMPO/Peracetic Acid System for a Shortened Electron-Transfer-Cycle-Controlled Oxidation of Secondary Alcohols

Article abstract of DOI:10.1002/cctc.201500214

We have developed a 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) derivative catalyzed oxidation of secondary alcohols with peracetic acid as the oxidant, which was generated from H₂O₂ and acetic acid catalyzed by strongly acidic resins. The oxidation of alcohols proceeded well through a shortened electron-transfer cycle under metal-free conditions, avoiding the use of any other electron-transfer mediators such as halides. In addition, we demonstrated that the present system exhibited excellent efficiency under mild conditions for the oxidation of aromatic, aliphatic, and allylic secondary alcohols. Shortcut to ketones: The 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-derivative-catalyzed oxidation of secondary alcohols employing peracetic acid generated from H₂O₂ and acetic acid with strongly acidic resins proceeds through a shortened electron-transfer cycle without halide additives. The system not only exhibits excellent efficiency at room temperature but also has a wide substrate scope.

Full text of DOI:10.1002/cctc.201500214

DOI: 10.1002/cctc.201500214
Full Papers
4
-CH CONH–TEMPO/Peracetic Acid System for a Shortened
3
Electron-Transfer-Cycle-Controlled Oxidation of Secondary
Alcohols**
[
a]
Shufang Zhang, Chengxia Miao,* Chungu Xia, and Wei Sun*
We have developed
a
2,2,6,6-tetramethylpiperidine-1-oxyl
electron-transfer cycle under metal-free conditions, avoiding
the use of any other electron-transfer mediators such as hal-
ides. In addition, we demonstrated that the present system ex-
hibited excellent efficiency under mild conditions for the oxi-
dation of aromatic, aliphatic, and allylic secondary alcohols.
(
TEMPO) derivative catalyzed oxidation of secondary alcohols
with peracetic acid as the oxidant, which was generated from
H O and acetic acid catalyzed by strongly acidic resins. The
2
2
oxidation of alcohols proceeded well through a shortened
Introduction
Oxidation reaction consisting of a series of complicated elec-
tron-transfer procedures is one of the most significant transfor-
mations in organic chemistry. Especially the chemoselective ox-
idation of alcohols into carbonyl compounds plays an impor-
tant role, because aldehydes and ketones are vital intermedi-
ates ranging from pharmaceuticals to plastic additives as well
as perfume, flavoring compounds and certain dyes in the tex-
oxygen as environmentally friendly oxidants, multicomponent
systems were developed to achieve the oxidation through sev-
eral cycles. Inspired by biological oxidations, the use of transi-
[7]
[8]
[9]
[10]
tion metals such as Cu, Fe, V, Ru in combination with
TEMPO as electron-transfer mediator has been widely reported
in the oxidation of alcohols. In addition, TEMPO/transition-
metal-free aerobic catalytic systems have gradually drawn
[
1,2]
[11–13]
tile industry.
Great efforts have been made to modify the
more and more attention.
In 2004, Hu, Liang, et al. ingeni-
catalytic system to facilitate electron-transfer procedures of the
alcohol oxidation. However, most oxidants such as peracid, hy-
drogen peroxide, or molecular oxygen could not directly
achieve the oxidation because of high activation energy for
the electron-transfer from the organic substrate to the oxi-
ously described the oxidation of a series of primary and secon-
dary alcohols into the corresponding aldehydes and ketones
by TEMPO/Br /NaNO in dichloromethane with the participa-
2
2
[14]
tion of air. Iwabuchi et al. also reported that oxoammonium
+
À
nitrate (5-F-AZADO NO ) worked as a bifunctional catalyst
3
[
3,4]
dants.
In addition, the unfavorable kinetics of oxidation of
comprising 5-fluoro-2-azaadamantane N-oxyl (5-FAZADO) and
[15]
alcohols with an oxidant directly demands electron-transfer
mediators (ETMs) to facilitate the procedure by transporting
the electrons from the catalyst to the oxidant along a low-
energy pathway, then increasing the efficiency of the oxidation
process and realizing the direct oxidation reactions.
NOx to realize direct oxidation of alcohols.
Major break-
through has been made in TEMPO-catalyzed oxidation of alco-
hols, and NO or other additives except TEMPO acting as elec-
x
tron-transfer mediator to activate the oxidant is necessary for
the reaction to achieve several catalytic cycles.
Recently, the utilization of the stable nitroxyl radical 2,2,6,6-
tetramethylpiperidine-1-oxyl (TEMPO) compound as electron-
transfer mediator for the oxidation of alcohols has appeared
Compared with hydrogen peroxide or molecular oxygen,
peracetic acid (PAA, CH CO H) as the environmentally friendly
3
3
oxidant that releases nontoxic acetic acid as co-product has
[
5]
[16]
very appealing. Extra additives such as halides were usually
added in the developed systems with NaClO, Ca(ClO)2,
been widely applied to oxidation of olefins. PAA in combina-
tion with TEMPO has been also used in the oxidation of alco-
[17]
m-chloroperbenzoic acid (m-CPBA), or NaIO , and so on as the
hols, giving the corresponding acids or ketones. However,
halides as additives for electron-transfer mediators and a large
amount of PAA were indispensable for the developed systems
4
[6]
oxidant.
In addition, employing hydrogen peroxide or
(
Scheme 1A). Moreover, the commercially available PAA was
[
a] S. Zhang, Dr. C. Miao, Prof. C. Xia, Prof. W. Sun
State Key Laboratory for Oxo Synthesis and Selective Oxidation
Lanzhou Institute of Chemical Physics
Chinese Academy of Sciences
Lanzhou, 730000 (China)
Fax: (+86)931-827-7088
usually obtained by a H SO -catalyzed method. Recently, Pratt,
2
4
Valgimigli, et al. discovered the possibility that protonation of
TEMPO under the acid conditions in organism may yield an ef-
fective H-atom donor, and TEMPOHC will be transformed into
+
[18]
TEMPOnium with the help of peroxyl radicals. TEMPOnium
E-mail: chxmiao@licp.cas.cn
was regarded as the active species in many reported systems
wsun@licp.cas.cn
[11–15]
for the oxidation of alcohols.
Therefore we supposed
[
**] TEMPO=2,2,6,6-Tetramethylpiperidine-1-oxyl.
whether TEMPO could be directly transformed into the possi-
ble active species through a shortened electron-transfer cycle
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201500214.
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1
mol%, the oxidation showed
weaker activity, only giving less
than 50% yield (entry 4). The use
of 2 equivalents CH CO H and
3
3
5
mol% of T6 could result in up
to 99% conversion and 95%
yield by prolonging the reaction
time to 10 h (entry 5). Besides,
the reduction of the amount of
CH CO H also led to lowering
3
3
the yield (entries 6 and 7). In ad-
dition, only adding 1 mol%
nBu NBr to the optimized reac-
4
tion conditions would lead to
nearly complete conversion
within 10 min (entry 8), which
was more efficient than reported
[17]
before.
However, using 3.8%
CH CO H generated from 30%
3
3
H O and acetic acid instead of
Scheme 1. Oxidation of alcohols catalyzed by TEMPO derivative with CH
3
CO
3
H (PAA) as the oxidant. A) Reaction
2
2
with commercially available PAA; B) reaction with PAA generated with the help of strongly acetic resins.
6% CH CO H prepared from
3 3
5
0% H O and acetic acid would
2 2
without any other electron-transfer mediators, with the aid of
[a]
CH CO H prepared from H O and acetic acid catalyzed by
Table 1. Screening of the activities of TEMPO derivatives.
3
3
2
2
[16a–c,19]
strongly acidic resins.
As we expect, 4-CH CONH–
3
TEMPO/peracetic acid system could realize the oxidation of
a variety of aromatic, aliphatic and allylic secondary alcohols at
room temperature, giving up to 95% yield through a shortened
electron-transfer cycle (Scheme 1B).
Entry
Catalyst
t
Conv.
[%]
Yield
[%]
[
b]
[b]
[
h]
[
[
[
[
c]
d]
e]
f]
1
2
3
4
5
6
7
8
9
1
T1
–
24
24
24
24
24
24
24
24
24
6
trace
26
trace
18
Results and Discussion
[g]
[g]
T1
T1
T1
T2
T3
T4
T5
T6
T7
NR
NR
NR
NR
The exploratory experiments were started by testing the proto-
col given in Scheme 1B and screening the reaction conditions
with 1-phenylethyl alcohol as the model substrate, and the re-
sults are summarized in Table 1. Clearly, TEMPO and CH CO H
[g]
[g]
98
77
71
81
82
94
71
66
78
78
98
97
3
3
were essential for the oxidation of 1-phenylethyl alcohol
Table 1, entries 1 and 2). Only 26% conversion and 18% yield
were obtained employing 2 equivalents CH CO H as the oxi-
(
0
11
>99
>99
3
3
24
dant (entry 2). And the reaction was inert with H O or H O /
2
2
2
2
[
3 3
a] Reaction conditions: 1-phenylethyl alcohol (1 mmol), 6% CH CO H
CH CO H mixture as the oxidant (entries 3 and 4). After that,
3
2
(2 equiv.), and catalyst (20 mol%) at room temperature for certain time.
various TEMPO derivatives were examined with a 20 mol% cat-
alysts loading and 2 equivalents CH CO H in CH CN at room
[b] Determined by GC using n-decane as an internal standard. [c] No
CH
3
CO
(2 equiv.) and 20 equiv. CH
g] NR=No reaction.
3
H. [d] No catalyst. [e] 50% H
2
O
2
(2 equiv.) as the oxidant. [f] 50%
CO H as the oxidant.
3
3
3
H
2
O
2
3
CO H instead of CH
2
3
3
temperature for a certain time. To our delight, 4-CH CONH–
3
[
TEMPO (T6) turned out to be the most effective one, affording
complete conversion and 98% yield within 6 h (entry 10). Be-
sides, the oxidation of alcohol catalyzed by TEMPO or 4-
CH COO–TEMPO needed 24 h to obtain considerable conver-
lead to poor reactivity, providing 28% yield under optimized
reaction conditions (entry 9).
3
sion and yield (entries 5 and 11). In addition, the other TEMPO
derivatives performed lower activities, giving less than 80%
yield after 24 h (entries 6–9).
To examine the practicability of the catalytic system for the
oxidation of alcohols, a variety of secondary alcohols including
aliphatic, aromatic, allyl, and heterocyclic alcohols were tested,
and the results are summarized in Table 3. As expected, most
of the aromatic secondary alcohols were smoothly converted
into the corresponding ketones, giving 73–95% isolated yield
(Table 3, entries 1–13). Notably, electronic properties and steric
Subsequently, the loading of TEMPO derivatives (T6) and
CH CO H was further investigated in detail. The yield reduced
3
3
sharply with the loading of T6 decreasing from 20 mol% to
5
6
mol% when the reaction proceeded at room temperature for
h (Table 2, entries 1–3). Further reducing the loading of T6 to
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[a]
Table 2. Screening of the loading of TEMPO derivatives and CH
oxidation of 1-phenethyl alcohol.
3
CO
3
H for
Yield
Table 3. Catalytic oxidation of various secondary alcohols.
[
a]
Entry
Cat.
%]
Amount of CH
[equiv.]
3
CO
3
H
t
[h]
Conv.
[%]
[
b]
[b]
[
[%]
[b]
Entry
Substrate
t [h]
Yield [%]
1
2
3
4
5
6
7
20
10
5
1
5
5
5
5
5
2
2
2
2
6
6
6
>99
84
66
50
99
83
67
99
30
98
83
62
45
95
80
65
96
28
6
2
10
10
10
0.17
10
1.6
1.2
2
[c]
1
2
3
4
R=H
R=p-F
R=p-Br
10
10
10
10
95
90
95
90
[
c]
8
9
[
d]
2
R=o-CF
3
[
a] Reaction conditions: 1-phenylethyl alcohol (1 mmol), certain amount
of 6% CH CO H and T6 at room temperature for certain time. [b] Deter-
mined by GC using n-decane as an internal standard. [c] 1 mol% nBu NBr
was added into the optimized reaction conditions. [d] 3.8% CH CO
generated from 30% H and acetic acid instead of 6% CH CO H pre-
pared from 50% H and acetic acid.
3
3
5
10
88
4
3
3
H
2
O
2
3
3
2
O
2
hindrance of substituents have certain influence on the reactiv-
ity in the cases of 1-phenylethyl alcohols with different sub-
stituents. 2-Trifluoromethyl-1-pheneylthyl alcohol also showed
good activity (entry 4). However, increasing the length of the
chain or the size of the ring on one side of the hydroxyl would
result in certain reduction of the yields (entries 5–8). Fortunate-
ly, diphenylethanols with either electron-deficient or electron-
rich groups such as chlorine or methyl performed excellent re-
activities, the desired ketones were isolated in more than 90%
yields (entries 9–12). In addition, methyl mandelate provided
up to 81% isolated yield (entry 13). In the case of 1-(pyridin-2-
yl)ethanol, it also could be oxidized to the desired ketone in
a 51% yield (entry 14). For the oxidation of unsaturated alco-
hols, such as 1,3-diphenylprop-2-yn-1-ol or 1,3-diphenylprop-2-
en-1-ol, only less than 25% yields were obtained (entries 15
and 16). However, 1,3-diphenylprop-2-en-1-ol and 1-cyclohexe-
nylpentan-1-ol worked well under the reaction conditions after
prolonging the reaction time to 24 h, affording 61% and 87%
yield respectively (entries 17 and 18). Several aliphatic alcohols
were well tolerated with 75–93% yields, if the reaction time
was prolonged to 24 h (entries 19 and 21–24). Only the oxida-
tion of 1-cyclopropylethanol resulted in a 30% yield for 24 h,
perhaps for the steric hindrance (entry 20). Delightedly, the
present alcohol oxidation protocol can be smoothly conducted
on a gram scale [Eq. (1)]. 82% yield was given in the oxidation
of 1-phenylethyl alcohol under optimized reaction conditions
for 10 h. In addition, nearly complete conversion and 94%
yield could be obtained after prolonging the reaction time to
6
7
8
n=1
n=2
n=3
10
10
10
83
86
73
9
R=H
R=p-Cl
R = o-Cl
10
9
9
89
94
93
92
10
1
1
12
R=p-CH
3
10
13
10
81
14
12
10
51
23
1
1
5
6
10
24
25
61
17
[c]
1
8
9
24
10
87
92
1
16 h.
[
[
c]
c]
2
0
1
24
24
30
93
2
[c]
22
24
75
Studies on acid-promoted disproportionation of TEMPO to
[
20]
TEMPOnium and TEMPOH have been reported.
In this
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PAA and m-CPBA as the oxidant in combination with TEMPO.
TEMPO and m-CPBA could give moderate yield in the previous
Table 3. (Continued)
Entry Substrate
[b]
[6d]
t [h]
Yield [%]
report and good yield could be provided with m-CPBA and
[
22]
T6 system.
Using m-CPBA as the oxidant would lead to
[c]
[23]
2
2
3
4
24
86
86
TEMPOnium according to the previous reported. Therefore,
TEMPOnium seems to be the active oxidant in the oxidation of
alcohol, perhaps, it was generated from the oxidation of PAA
[c]
10
(
Scheme 2C). Subsequently, intermediate b was formed after
[
(
a] Reaction conditions: alcohol (1 mmol), T6 (5 mol%), and 6% CH
2 equiv.) at room temperature for certain time. [b] Isolated yield. [c] De-
3
CO
3
H
nucleophilic attack of alcohol a to the oxoammonium species,
giving the product and TEMPOH by the hydrogen transfer.
[24]
termined by GC using nitrobenzene or n-decane as an internal standard.
As a whole, the catalytic system shortened the electron-trans-
[17]
fer cycle compared with the previous systems,
avoiding the cycle with halides as electron-transfer
mediators.
Conclusions
We have developed a mild and metal-free system in-
cluding 4-CH CONH–TEMPO and CH CO H, which
3
3
3
was generated from H O and acetic acid catalyzed
2
2
by strongly acidic resins for oxidation of secondary
alcohols. The system was suitable for a variety of sec-
ondary alcohols including aromatic, aliphatic and al-
lylic groups, affording up to 95% yield at room tem-
perature. In addition, a mechanism involving TEMPO-
nium was proposed. The present protocol represents
a simple pathway for oxidation of secondary alcohols
into ketones, proceeding through a shortened elec-
tron-transfer cycle without any other electron-transfer
mediators. Further investigations about the scopes
and detailed mechanism are in progress.
Experimental Section
General
GC–MS spectra were recorded on an Agilent Technolo-
1
13
gies 7890A/5975C. The H and C NMR spectra were re-
corded on a Bruker Avance III 400 MHz spectrometer.
The chemical shifts (d) are reported in ppm and coupling
constants (J) in Hz. GC analysis was performed on Agi-
lent Technologies 6820 with a flamed-ionization detec-
tor.
Scheme 2. TEMPO derivatives/peracetic acid system for the oxidation of secondary alco-
hols. A) Disproportionation of TEMPO to TEMPOnium under acid conditions; B) TEMPOni-
um generated under organic acid conditions; C) proposed reaction mechanism.
system, disproportionation of TEMPO to TEMPOnium is possi-
ble under acetic acid conditions (Scheme 2A). Another proto-
[
16c,g]
Preparation of peracetic acid
col for generating TEMPOnium may also exist because the
combination of commercial available PAA (generated from
H SO -catalyzed method) and TEMPO did not give satisfactory
Glacial acetic acid (50 g) was stirred in a bottle, and 50% H
5.7 g) was slowly added at room temperature. Once homogenous,
1.7 g of Amberlite IR-120 was added, and the mixture was stirred
at room temperature for 24 h. After that, the peracetic acid solu-
tion was filtered and stored in a polyethene bottle at À208C.
O
2
2
(
2
4
[
21]
results for the oxidation of alcohols. Pratt and Valgimigli re-
ported that TEMPO can react with oxygen-centered radicals
+
under acidic conditions and EPR spectra of TEMPOH C have
[
18]
been previously reported in concentrated acids. So path B
may also exist in our system and weakly acidic conditions may
be important (Scheme 2B). Only employing hydrogen peroxide
or hydrogen peroxide and glacial acetic acid instead of PAA as
the oxidant could not promote the oxidation reaction, which
further confirmed PAA was the real terminal oxidant. Then,
a control experiment was done to test the similarity between
Typical procedure for the oxidation of secondary alcohols
Substrate (0.5 mmol), TEMPO or its derivatives (0.025 mmol), and
6
% CH CO H (1 mmol) were successively added into CH CN (1 mL),
3 3 3
and the mixture was stirred at room temperature for certain time.
The progress of the reaction was checked by TLC. After the reac-
tion, conversions and yields were obtained by GC measurements
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employing nitrobenzene as an internal standard or by column
chromatography.
Keywords: acidity · alcohols · electron transfer · oxidation
1
1
7
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(
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3
1
29.4, 128.9, 31.8, 8.2 ppm.
[
1
2
7
7
,3-Dihydro-1H-inden-1-one: H NMR (400 MHz, CDCl ): d=7.80–
3
.73 (m, 1H), 7.59 (td, J=7.6, 1.2 Hz, 1H), 7.53–7.45 (m, 1H), 7.42–
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3
8
2
,4-Dihydronaphthalen-1(2H)-one: H NMR (400 MHz, CDCl ): d=
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.97 (t, J=6.1 Hz, 2H), 2.70–2.62 (m, 2H), 2.17–2.11 ppm (m, 2H).
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3
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3
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1
27.2, 126.6, 39.2, 29.7, 23.3 ppm.
1
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6
,7,8,9-Tetrahydro-5H-benzo[7]annulen-5-one: H NMR (400 MHz,
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1
2
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1
3
2
013, 17, 1247; d) L. Liu, J. J. Ma, Z. Sun, J. P. Zhang, J. J. Huang, S. Z. Li,
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5.2, 20.9 ppm.
1
Benzophenone: H NMR (400 MHz, CDCl ): d=7.88–7.77 (m, 4H),
3
[
8] a) N. W. Wang, R. H. Liu, X. M. Liang, Chem. Commun. 2005, 5322; b) L. Y.
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7
4
1
.59 (ddd, J=8.4, 2.3, 1.1 Hz, 2H), 7.48 ppm (dd, J=10.4, 4.6 Hz,
H). C NMR (100 MHz, CDCl3): d=196.8, 137.6, 132.4, 130.1,
28.3 ppm.
1
3
1
(
4-Chlorophenyl)(phenyl)methanone: H NMR (400 MHz, CDCl ): d=
3
[
9] a) Z. D. Du, J. P. Ma, H. Ma, J. Gao, J. Xu, Green Chem. 2010, 12, 590;
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7
.84–7.71 (m, 4H), 7.65–7.56 (m, 1H), 7.55–7.41 ppm (m, 4H).
1
3
C NMR (100 MHz, CDCl ): d=195.5, 138.9, 137.3, 135.9, 132.7,
3
[
10] a) A. Dijksman, A. Marino-Gonzalez, A. A. i Payeas, I. W. C. E. Arends, R. A.
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1
31.5, 129.9, 128.7, 128.4 ppm.
1
(
2-Chlorophenyl)(phenyl)methanone: H NMR (400 MHz, CDCl ): d=
3
1
26, 3242; c) V. N. Korotchenko, K. Severin, M. R. Gagne, Org. Biomol.
7
7
1
.82 (dt, J=8.5, 1.5 Hz, 2H), 7.65–7.55 (m, 1H), 7.53–7.41 (m, 4H),
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1585.
13
.41–7.32 ppm (m, 2H). C NMR (100 MHz, CDCl ): d=195.3, 138.6,
3
36.5, 133.7, 131.3, 131.1, 130.1, 129.1, 128.6, 126.7 ppm.
1
Phenyl(p-tolyl)methanone: H NMR (400 MHz, CDCl ): d=7.79 (dt,
3
[
[
11] a) Y. Xie, W. M. Mo, D. Xu, Z. L. Shen, N. Sun, B. X. Hu, X. Q. Hu, J. Org.
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13
(
m, 2H), 2.44 ppm (s, 3H). C NMR (101 MHz, CDCl ): d=196.5,
3
1
43.2, 138.0, 134.9, 132.2, 130.3, 129.9, 129.0, 128.2, 21.7 ppm.
1
1
-(Pyridin-2-yl)ethanone: H NMR (400 MHz, CDCl ): d=8.70 (ddd,
3
7210; Angew. Chem. 2007, 119, 7348; c) S. Wertz, A. Studer, Adv. Synth.
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351, 2209.
1
.7 Hz, 1H), 7.48 (ddd, J=7.6, 4.8, 1.2 Hz, 1H), 2.74 ppm (s, 3H).
[
1
3
C NMR (101 MHz, CDCl ): d=200.1, 153.5, 148.9, 136.9, 127.1,
3
1
21.7, 25.8 ppm.
[
[
14] R. H. Liu, X. M. Liang, C. Y. Dong, X. Q. Hu, J. Am. Chem. Soc. 2004, 126,
1
Methyl 2-oxo-2-phenylacetate: H NMR (400 MHz, CDCl ): d=8.06–
3
4112.
7
.98 (m, 2H), 7.71–7.63 (m, 1H), 7.57–7.48 (m, 2H), 4.02–3.94 ppm
m, 3H). C NMR (101 MHz, CDCl ): d=186.1, 164.0, 135.0, 132.4,
3
15] M. Shibuya, Y. Osada, Y. Sasano, M. Tomizawa, Y. Iwabuchi, J. Am. Chem.
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1
3
(
1
30.1, 128.9, 52.8 ppm.
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Acknowledgements
We are grateful to the National Natural Science Foundation of
China (NO. 21103207 and 21133011) and the western light pro-
gram of the Chinese Academy of Sciences (2012).
3
53, 3014; h) S. J. Yu, C. X. Miao, D. Q. Wang, S. F. Wang, C. G. Xia, W.
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