Efficient oxidation of secondary alcohols to ketones by NaOCl catalyzed by salen-Mn(III)/NBS

Article abstract of DOI:10.1134/S1070363214100272

An efficient catalytic system salen-Mn(III)/NBS for oxidation of secondary alcohols to ketones by inexpensive and readily available oxidizing agent NaOCl has been developed. The process resulted in good to excellent yields under the action of 2 mol % of salen-Mn(III) and 13 mol % of NBS at room temperature. However, such system was not efficient in oxidation of secondary benzyl alcohols with a strong electronicdonating substituent attached to the benzene ring due to bromination of the alcohols.

Full text of DOI:10.1134/S1070363214100272

ISSN 1070-3632, Russian Journal of General Chemistry, 2014, Vol. 84, No. 10, pp. 2021–2026. © Pleiades Publishing, Ltd., 2014.
Efficient Oxidation of Secondary Alcohols to Ketones
1
by NaOCl Catalyzed by Salen-Mn(III)/NBS
Y. C. Zhang, F. L. Lü, M. J. Cui, and J. Q. Zhao
School of Chemical Engineering and Technology, Hebei University of Technology, Tianjin 300130, PRChina
e-mail: zhaojq@hebut.edu.cn
Received August 1, 2014
Abstract—An efficient catalytic system salen–Mn(III)/NBS for oxidation of secondary alcohols to ketones by
inexpensive and readily available oxidizing agent NaOCl has been developed. The process resulted in good to
excellent yields under the action of 2 mol % of salen–Mn(III) and 13 mol % of NBS at room temperature.
However, such system was not efficient in oxidation of secondary benzyl alcohols with a strong electronic-
donating substituent attached to the benzene ring due to bromination of the alcohols.
Keywords: salen–Mn(III), NaOCl, NBS, oxidation, secondary alcohols
DOI: 10.1134/S1070363214100272
Selective oxidation of alcohols into the corres-
ponding carbonyl compounds is an important transfor-
mation in organic synthesis. Traditionally such pro-
cesses had been performed with stoichiometric amounts
of organic or inorganic oxidizing agents including di-
methyl sulfoxide, lead acetate, chromates and manga-
nese oxides [1]. The reaction had been accompanied
by formation of significant quantities of toxic side
products. Therefore, efficient catalysis of the process
became an objective of intensive research. A number
of TEMPO-based (2,2,6,6-tetramethylpiperidyl-1-oxy)
catalytic systems were efficient in oxidation of
alcohols to aldehydes and ketones by oxygen [2].
organic reactions. Xia et al. [8] utilized chiral salen–
Mn(III) complexes as catalysts in combination with
(diacetoxyiodo)benzene [PhI(OAc) ] as terminal
2
oxidant in the oxidative kinetic resolution (OKR) of
secondary alcohols. Key points of the mechanism of
the process proposed by Corey and co-workers [9]
included formation of Mn(V)–salen dibromide, its
subsequent reaction with an alcohol to give alkoxy
Mn(V) species and carbonyl-forming elimination to
give the corresponding ketone via a highly organized
transition state with intramolecular transfer of
hydrogen from carbon to oxygen of the salen ligand.
Based on this mechanism there was developed a new
protocol with N-bromosuccinimide (NBS) oxidant
Anelli and coworkers [3] had developed another
TEMPO-based protocol for oxidation of alcohols to
the carbonyl compounds that involved NaOCl as the
stoichiometric oxidant. The reaction had been carried
out at 0°C in a biphasic CH Cl –water mixture with an
instead of PhI(OAc) which proved to be highly
2
efficient in oxidative kinetic resolution of a variety of
secondary alcohols catalyzed by chiral salen–Mn(III)
complexes [10]. Recently we determined that the OKR
of secondary alcohols catalyzed by chiral salen–Mn(III)
complexes could be achieved with a catalytic amount
2
2
excess of NaOCl, NaHCO , KBr, and catalytic amount
3
of 4-MeO-TEMPO giving aldehydes in high yields in
the course of minutes. Oxidation of secondary alcohols
required for clear reasons longer reaction time. Sodium
hypochlorite acted as an oxidizing agents of alcohols
with high efficiency in accordance with other protocols
of Br as a cycling agent and stoichiometric oxidant
2
NaOCl [11]. This method was inexpensive and simple
at the step of products purification due to formation of
NaCl from NaOCl.
[
4, 5] that involved also somewhat problematic
In the course of the method development we
replaced chiral salen–Mn(III) complex with its achiral
counterpart as a catalyst of oxidation of secondary
alcohols by stoichiometric amount of NaOCl and
catalytic amount of NBS. Herein, we report the ef-
ficient oxidation of secondary alcohols of such system.
TEMPO-based systems [2, 6, 7]. In this respect salen–
Mn(III) complexes had been versatile catalysts in
1
The text was submitted by the authors in English.
2
021

2
022
ZHANG et al.
without Salen Mn(III) and NBS (Table 2, entry 1)
OH
O
salen−Mn(III)/bromine source
almost no reaction took place. Upon slow addition of
NaOCl (2.3 mmol), salen–Mn(III) (2 mol %), and NBS
(13 mol %) oxidation of 1-phenylethanol to aceto-
phenone completed smoothly in 10 min (Table 2, entry 5).
Without salen–Mn(III) 1-phenylethanol was converted
to acetophenone in 1 h (Table 2, entry 2). Probably it
proceeded via the initial NBS reaction with water
leading to HOBr species with positively charged
bromine that acted as an oxidant of 1-phenylethanol
giving acetophenone along with HBr as by-product
o
NaOCl, CH Cl , 25 C
2
2
R1
R2
R1
R2
N
O
N
O
Mn
salen−Mn(III) =
[
14]. HBr was oxidized by NaOCl to Br , which
2
Oxidation of secondary alcohols to ketones by NaOCl
catalized by salen–Mn(III).
formed HOBr in presence of water. Replacement of
salen–Mn(III) by manganese acetate resulted in very
low rate of oxidation indicating that salen–Mn(III)
itself could catalyze oxidation of 1-phenylethanol by
NaOCl to acetophenone without bromine but rate of
the reaction was lower than in the process supported
Corey’s mechanism led to a conclusion that
substances that can be oxidized to Br in situ act as the
2
sources of bromine in salen–Mn(III) complex
catalyzed oxidation of alcohols. Therefore, several
bromides and bromines were tested in oxidation of 1-
phenylethanol (see figure) as a reference reaction.
by NBS or Br . Such effect was consistent with the
2
chiral salen–Mn(III) catalyzed oxidative kinetic resolu-
tion of secondary alcohols [9]. Without NaOCl or with
molecular oxygen the process resulted in low conver-
sion of 1-phenylethanol (Table 2, entries 3, 6). Overall,
the data presented in Table 2 indicated that each com-
ponent was essential for the efficient oxidation reaction.
Results of the reaction carried out under various
conditions (Table 1) can be associated with formation
of HOBr and its direct oxidizing of alcohols to
aldehydes or ketones [13]. Therefore, NBS was chosen
as bromine cycling agent in the novel catalytic system.
The effect of salen–Mn(III) amount on the reaction
was studied at room temperature with NBS (13 mol %)
and NaOCl (2.3 mmol). Generally under same
conditions conversion rate of 1-phenylethanol became
Activity of each component of the catalytic system
was studied (Table 2). According to the presented data
Table 1. Effect of various bromine sources on oxidation
Table 2. Oxidation of 1-phenylethanol to acetophenone
a
a
reaction of 1-phenylethanol. Reation condition: 1-phenyl-
catalyzed by salen–Mn(III). Reation condition: 1-phenyl-
ethanol (1 mmol), salen–Mn(III) (2 mol %), bromine source
ethanol (1 mmol), dichloromethane (2 mL), room tempera-
ture, NaOCl addition time 10 min
(
13 mol %), dichloromethane (2 mL), room temperature,
NaOCl (2.3 mmol)
Mn(III), NBS, NaOCl, Reaction Conversion,
Entry
mol % mol % mmol
time, h
1.0
%
–
Bromine Reaction Conversion, Selectivity,
Entry
1
2
3
4
5
0
0
2
2
2
0
2.3
2.3
2.3
0
source
time, h
%
%
13
0
1.0
52
53
3.5
1
2
NBS
0.17
>
99
>99
6.0
Br
Br
2
2
0.67
13
13
1.0
>99
>99
2.3
0.17
>
99
22
3.0
3b
4
3.0
2.0
5.0
68
>99
>99
>99
6b
7c
2
2
13
13
2.3
0
5.0
2.0
KBr
53
60
a
5
NaBr
mol % vs. the substrate as indicated by GC; selectivity > 99%.
The reaction was carried out with 2 mol % manganese acetate
without Salen Mn(III).
b
a
NaOCl addition time 10 min. The products were tested by GC.
b
c
Br
2
(6 mol %).
2
The reaction was carried out under O without NaOCl.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 84 No. 10 2014

EFFICIENT OXIDATION OF SECONDARY ALCOHOLS TO KETONES
2023
a
Table 3. Effect of each component rate on oxidation of 1-phenylethanol . Reation condition: 1-phenylethanol (1 mmol),
dichloromethane (2 mL), room temperature
Entry Salen Mn(III), mol %
NBS, mol %
NaOCl, mmol
Reaction time, h
Conversion, %
1
2
3
0
1.5
2
13
13
13
2.3
2.3
2.3
1.0
52
95
0.17
0.17
>99
4
5
2
2
13
13
2.0
2.3
0.83
0.17
85
>
99
99
6
2
13
2.5
0.17
>
7
8
9
2
2
2
2
5
8
2.3
2.3
2.3
2.3
8.0
4.5
70
88
94
10
16
2.0
1
0
0.17
>99
a
mol % vs. the substrate; tested by GC; selectivity > 99%.
higher with the increase of salen–Mn(III) content
Table 3, entries 1–3). Similar relationship was
observed for NaOCl with the optimum rate of it being
the electron-donating nature of the substituent which
promoted the electrophilic substitution in the benzene
ring. Bromination reaction consumed the source of
bromine which retarded the process of oxidation.
(
2
.3 mmol. The data accumulated were higher than that
in Anelli’s protocol [3] and comparable with the
process carried out in acetic acid [5].
Due to steric effects ortho-substituted secondary
alcohols had lower activity than their para-substituted
analogues (Table 4). The low reaction rate of ortho-
substituted secondary alcohols can be ascribed to steric
hindrance between a substituent and Salen ligand in
Influence of NBS amount on the reaction was
studied within the margins 5 to 16 mol % with
constant amounts of salen–Mn(III) (2 mol %) and
NaOCl (2.3 mmol) at room temperature. Conversion of
+
the alkoxy complex [salen–Mn(V)–Br–OR] which
1
-phenylethanol increased from 70% in 7 h to higher
was the key reaction intermediate and precursor of the
ketones formed [9]. The size of aliphatic chain of 1-phe-
nyl alcohols also had some influence upon the process
than 99% in 10 min upon increased NBS rate. Higher
content of NBS did not influence upon the reaction
rate. We applied 13 mol % as the most efficient
amount of NBS in the subsequent experiments.
(
Table 4).
This system was efficient in oxidation of secondary
Based on the above data we correlated influence of
the novel catalytic system with the structure of
secondary alcohols (Table 4). 1-Phenylethanol and its
para-substituted derivatives including those bearing
electron-withdrawing or weak electron-donating
alcohols in fused rings and secondary aliphatic
alcohols. 1,2,3,4-Tetrahydro-1-naphthol was converted
into 1,2,3,4-tetrahydro-1-naphthalenone in 1.5 h.
Secondary aliphatic alcohols were converted to the
corresponding ketones with high yield. Higher content
of NaOCl and longer reaction time were required
groups (F, Cl, Br, and CH ) were selectively converted
3
to corresponding ketones under the optimized condi-
tions (Table 4, entries 1–5). However, in the presence
of the OMe substituent this catalytic system had low
activity in oxidation of 1-(4-methyloxyphenyl)ethanol.
Higher content of NaOCl did not improve the yield
(
Table 4, entries 14–7). Even cyclohexanol and its
derivative menthol were oxidized to ketones with
yields higher than 90% (Table 4).
In conclusion, an active catalytic system salen–
Mn(III)/NBS for oxidation of secondary alcohols by
inexpensive and readily available NaOCl has been
(
Table 4, entry 6). The reaction was accompanied by
formation of bromine-substituted by-products due to
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 84 No. 10 2014

2
024
ZHANG et al.
Table 4. Selective oxidation of secondary alcohols by salen–Mn(III)/NBS/NaOCl. Reation conditions: substrate (1 mmol),
salen–Mn(III) (2 mol %), NBS (13 mol %), CH
Cl
2 2
(2 mL), room temp., NaOCl addition time 10 min
Reacti
on
NaOCl, Reaction
NaOCl,
Entry
1
Substrate
OH
Entry
10
Substrate
OH
mmol time, h
mmol
2.5
time, h
2.3
0.17
>99
>99
0.17
93.0
84.0
98
F
2
3
4
2.7
2.3
2.3
0.17
0.17
0.17
0.17
98.4
>99
>99
>99
>99
>99
11
12
13
2.3
0.17
0.17
>99
>99
OH
OH
3
.0
9
8.4
F
Cl
Br
3.0
.2
0.17
0.17
80.0
84.0
>99
OH
OH
OH
>
99
3
>99
>99
2.3
1.5
>99
>99
OH
OH
5
6
2.3
2.5
14
15
2.7
3.0
0.17
1.0
>99
>99
>99
OH
5.0
5.0
65
65
85
86
95.0
OH
OH
OH
3
.0
O
7
2.5
.0
4.5
4.5
28
30
96
97
16
3.0
0.17
>99
>99
OH
3
O
8
9
2.5
2.5
0.17
0.17
86
34
>99
79
17
18
3.0
2.5
0.17
0.17
90.0
55
>99
>99
OH
OH
OH
OH
Cl
a
1
The products were tested by GC and confirmed by H NMR.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 84 No. 10 2014

EFFICIENT OXIDATION OF SECONDARY ALCOHOLS TO KETONES
2025
developed. This system had high efficiency in
oxidation of various secondary alcohols to the
corresponding ketones in good to excellent yields with
salen–Mn(III) (2 mol %) and NBS (13 mol %) in
CH Cl at room temperature. However, it is not
efficient in oxidation of secondary benzyl alcohols
containing strong electronic-donating substituents in
the benzene ring.
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ACKNOWLEDGMENTS
The authors are grateful for financial support from
the National Natural Science Foundation of China
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