NaBrO3/guanidinium-based sulfonic acid: As a transition metal- and strong inorganic acid-free oxidation system for alcohols and alkyl arenes

Article abstract of DOI:10.1039/c5nj02215k

The bromate anion as a safe, cheap, stable and easily stored oxidizing system has been utilized for the oxidation of a wide range of alcohols and alkyl arenes to the corresponding aldehydes and ketones. The reaction was performed in the presence of guanid

Full text of DOI:10.1039/c5nj02215k

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Journal Name
ARTICLE
NaBrO3/guanidinium-based sulfonic acid: as a transition metal-
and strong inorganic acid-free oxidation system for alcohols and
alkyl arenes
Received 00th January 20xx,
Accepted 00th January 20xx
Ahmad Shaabani,* Mohammad Sadegh Laeini, Shabnam Shaabani, Mozhdeh Seyyedhamzeh
DOI: 10.1039/x0xx00000x
Bromate anion as a safe, cheap, stable and easily stored oxidizing system has been utilized for the oxidation of a wide range
of alcohols and alkyl arenes to the corresponding aldehydes and ketones. The reaction was performed in the presence of
guanidinium-based sulfonic acid as a Brønsted acid organocatalyst in aqueous acetonitrile at room temperature. Products
are isolated in high yields and catalyst could be easily recovered and recycled.
www.rsc.org/
Introduction
tungstophosphoric acid(H3PW12O40),25 ruthenium(II)complex26
In recent years, development of many chemical transformations and vanadyl alkyl phosphonate.27
such as oxidation processes as cheapest routes to synthesis of Guanidines and their corresponding guanidinium salts have
useful molecules from natural materials have increasingly been known to chemists because of widespread use as strong
gained attention. The selective oxidation plays a crucial role in and weak Brønsted base catalysts in useful chemical
organic syntheses and is among the most common classes of transformations, respectively.28 Recently, a number of excellent
reactions. Given that carbonyl compounds are versatile reviews on guanidine organocatalysis exist because of their high
intermediates of valuable products such as pharmaceuticals, charge dispersion, excellent thermal and chemical stabilities
agricultural, and fine chemicals,1-6 most of them are produced owing to their special structures.29-35 Despite this wealth of
from the oxidation of alcohols and alkyl arenes. Recently, a large functionality present in the guanidine, the synthesis of
number of organic and inorganic oxidants has been conducted guanidinium-based Brønsted acid is reported very recently.36 In
in this area.7-11
continuation of our efforts to introduce new and efficient
Sodium bromate is an important oxy-halo reagent which has strategy for various organic transformations,37-41 herein, we
employed as a useful oxidizing agent for this purpose because report the oxidation of a wide range of alcohols and alkyl arenes
of its capability in multi-electron transfer.12-15 In spite of the fact to the corresponding aldehydes and ketones using sodium
that sodium bromate is thermodynamically a strong oxidant, bromate in the presence of guanidinium-based sulfonic acid
according to the literature, bromate anion itself cannot oxidize (1,1,3,3-tetramethylguanidine sulfonic acid: TMG-SO3H) as a
organic compounds.16 The standard redox potential of the Brønsted acid organocatalyst in aqueous acetonitrile at room
ˉ
bromate ion, BrO3 , is 0.61 V in alkaline aqueous solution,17 1.10 temperature (Scheme 1).
V in pH 7,18 while in aqueous acidic media, it has a potential of
1.42 V.17 Because of the low standard redox potential in neutral
O
R1
R1
or weak acidic media and also very low solubility in most of
NaBrO3, TMG-SO3H
organic solvents, it is usually utilized in aqueous media in the
or
OH
CH3CN/H2O (2:1), r.t.
or
O
21
presence of inorganic acids such as HBr,19 H2SO4,20 NaHSO3
R2 R3
R2 R3
and HCl-TEMPO,22 in which final isolation of the product
demands quenching and neutralization steps to remove the
acid from the reaction media, or expensive and toxic metal
R1, R2 : Alkyl, Aryl
R3: H, Alkyl, Aryl
catalysts such as cerium(IV) ammonium nitrate (CAN),23,24 Scheme 1. Oxidation of alcohols and alkyl arenes using sodium
bromate in the presence of TMG-SO3H
*Faculty of Chemistry, Shahid Beheshti University, G. C., P. O. Box 19396-4716,
Tehran, Iran.
E-mail: a-shaabani@sbu.ac.ir,
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Table 2. Comparative study between various solveVnietws A(RrtieclaecOtniolinne
conditions: benzyl alcohol (1 mmol), NaBrO3 (1.50 mmol), TMG-
SO3H (10% mol), r.t.)
DOI: 10.1039/C5NJ02215K
Results and discussion
In our previous report, a simple procedure for the synthesis of
TMG-SO3H as an acidic organocatalyst for the first time was
disclosed.36 The catalyst is simply prepared by dropwise
addition of chlorosulfonic acid to n-hexane mixture of
tetramethyl guanidine (TMG) at 0 °C (Scheme 2).
Entry
Solvent
Time (h)
Yield (%)
1
2
3
4
5
6
H2O
EtOAc
CH3CN
8
8
8
8
8
3
5
68
75
72
80
99
CH2Cl2
H
SO3H
n-Hexane
CH3CN/H2O (2:1)
N
NH
ClSO3H
N
N
N
N
0 oC, n-hexane
Cl
Thus the reaction of benzyl alcohol with NaBrO3 in 1:1.50 molar
ratio in the presence of TMG-SO3H (10 mol%) in CH3CN/H2O
(2:1) as solvent gave the optimum yield of the product.
Subsequently reaction of different alcohols was attempted with
the above optimized reaction conditions. As indicated in Table
3, secondary alcohols are converted into the corresponding
ketones in good yields (Entries 7-11) and aldehydes are
achieved in comparable yields from the oxidation of primary
alcohols (Entries 1-6). In addition, it was observed that
aldehydes did not undergo subsequent oxidation to carboxylic
acids. After screening reactions of a series of alcohols and in
order to further assess the ability of this reagent, the oxidation
of alkyl arenes to related carbonyl compounds in the similar
optimizing reaction conditions was investigated. The results of
the mentioned reactions are provided in the Table 4.
One of advantages of TMG-SO3H is its ability to function as a
recyclable organocatalyst. Recyclability of the TMG-SO3H was
examined in the oxidation reaction of benzyl alcohol. After
extracting of the product with dichloromethane, solvents were
evaporated under vacuum. Then, TMG-SO3H was extracted
from inorganic compounds, sodium bromide and un-reacted
sodium bromate, with acetonitrile and reuses it for subsequent
reactions. This procedure was performed for four repetitive
cycles (Figure 1).
Scheme 2. Preparation of catalyst
In this work, we investigated the performance of TMG-SO3H as
an organocatalyst in the oxidation process using sodium
bromate as an oxidant. In a pilot experiment, benzyl alcohol was
performed as a model substrate and reactions were carried out
at room temperature under various conditions. As shown in
Table 1, firstly the reaction was attempted using 1:0.50 molar
ratio of benzyl alcohol: NaBrO3 and 10% mol of TMG-SO3H in
CH3CN/H2O (2:1) as solvent at room temperature. The reaction
was incomplete after 6 h, but showed the formation of
benzaldehyde by TLC and GC. After workup and separation, 54%
of benzaldehyde was obtained (Table 1, Entry 1). The reaction
was then attempted using 1:1 ratio of benzyl alcohol: NaBrO3 at
former conditions and it was incomplete after 6 h. This time,
80% of benzaldehyde was obtained (Table 1, Entry 2). Finally,
for improving the rate of the reaction and producing better
yield, we used 1:1.50 molar ratio of benzyl alcohol: NaBrO3. The
reaction completed after 3 h and yielded 99% was obtained
(Table 1, Entry 3). Also the reaction was carried out in various
amount of TMG-SO3H. As indicate in Table 1, when 10% mol of
TMG-SO3H was used, the reaction performed in good yield
(Table 1, Entries 3-5). In order to obtain the best solvent, we
have examined various solvents such as H2O, EtOAc, CH3CN,
CH2Cl2, n-hexane and CH3CN/H2O (2:1). As exposed in Table 2,
CH3CN/H2O (2:1) is the best solvent for this reaction in terms of
yield and time. In the cases of, EtOAc, CH3CN, CH2Cl2 and n-
hexane the reaction times were longer in order to produce
reasonable yields (68-80) (Table 1, Entries 2-5).
The results of our catalyst with respect to the yields, solvents,
temperature and reaction times have been compared with
previously reported catalysts in the oxidation of organic
compounds. Using eco-friendly co-catalyst with minimal
environmental impact in the absence of transition metals, also
reasonable reaction time at room temperature are notable
features of this procedure in comparison with the existing
methods (Table 5).
Table 1. Optimizing the amount of NaBrO3 and TMG-SO3H in the
oxidation reaction (Reaction conditions: benzyl alcohol (1
mmol), CH3CN/H2O (3 ml), r.t.)
100
80
60
40
20
0
TMG-SO3H
(mol%)
Yield
(%)
Entry
Substrate:NaBrO3
Time (h)
1
2
3
4
5
1:0.50
1:1
1:1.50
1:1.50
1:1.50
10
10
10
5
6
6
3
6
6
54
80
99
78
66
1
2
3
4
Run
3
Figure 1. Recycling of catalyst in the oxidation of benzyl
alcohol
Table 3. Oxidation of various alcohols with NaBrO3 in the presence of TMG-SO3H (Reaction conditions: organic compounds (1 mmol),
NaBrO3 (1.50 mmol), TMG-SO3H (10% mol), CH3CN/H2O (2:1), r.t.)
Entry
Alcohol
OH
Product
Time (h)
Yield (%)a
Conversation (%)a
O
1
3
99(98,95,92)b
99
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OH
OH
O
O
View Article Online
DOI: 10.1039/C5NJ02215K
2
3
3
4
95
96
99
98
Cl
Cl
OH
O
4
4
97
97
O2N
O2N
OH
O
CHO
5
6
3
95
90
99
95
MeO
MeO
7
6
OH
O
OH
OH
7
8
9
98
90
98
99
94
99
O
6
5
OH
O
O
OH
10
11
7
7
82
88
97
95
OH
O
aConversion and yield determined by GC analysis.
b Isolated yields after recycling of catalyst.
Table 4. Oxidation of various arenes using NaBrO3 in the presence of TMG-SO3H (Reaction conditions: organic compound (1 mmol), NaBrO3
(1.50 mmol), TMG-SO3H (10% mol), CH3CN/H2O (2:1), r.t.)
Entry
Arene
Product
Time (h)
Yield (%)a
Conversation (%)a
O
1
20
96
96
O
2
24
90
99
O
O
3
4
5
6
24
24
20
24
80
88
99
90
98
98
99
99
O
O
O
aConversion and yield determined by GC analysis.
Table 5. Comparison of various oxidation systems in the oxidation of benzyl alcohol with sodium bromate as an oxidant
Entry
Catalyst
4-Benzamido-TEMPO, HCl
CAN
Conditions
DCM/H2O, r.t.
CH3CN/80 ᵒC
CH3CN/H2O, r.t.
H2O, 23 ᵒC
Yield (%)
Time (h)
Ref.
22
1
2
3
4
5
99
81
90
72
99
1
1.5
3
23
H3PW12O40
cis-[Ru(2,9-Me2phen)2(OH2)2]2+
25
10
3
26
Guanidinium-based sulfonic acid (TMG-SO3H)
CH3CN/H2O, r.t.
This work
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9
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arenes to the corresponding carbonyl compounds using NaBrO3
in the presence of guanidinium-based sulfonic acid as a
Brønsted acid catalyst. The advantages of this bronsted acid
catalyst make it a promising candidate to be employed as a co-
catalyst for performing highly efficient oxidation reactions.
Further examination of the application of this co-catalyst for
other oxidation processes and also other organic
transformations are currently in progress.
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All chemicals were purchased from Aldrich or Merck companies and
used without further purification. Identification and quantification
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containing a magnetic stirring bar containing CH3CN/H2O (2:1) (3 mL)
at room temperature, while the progress of the reaction was
followed by TLC and GC. GC conversions and yields were obtained
using n-decane as an internal standard based on the amount of
organic compound employed relative to authentic standard product.
After completion of the reaction, the product was extracted from the
reaction mixture with dichloromethane (3×5 mL). Evaporation of the
solvent gave the product with sufficient purity for most purposes.
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Acknowledgements
This investigation was financially supported by the Iran National
Elites Foundation (INEF) and the Research Council of Shahid
Beheshti University.
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A new catalytic system for the oxidation of organic compounds in good to excellent yields at room temperature