Microwave-assisted green synthesis of 1,1-diacetates (acylals) using selectfluor as an environmental-friendly catalyst under solvent-free conditions

Article abstract of DOI:10.1007/s11164-015-2168-1

An efficient and simple procedure has been developed for the acetylation of aldehyde by selectfluor [1-(chloromethyl)-4-fluoro-1,4-diazoniabicyclo[2,2,2]octane bis(tetrafluoroborate)] as a chemoselective and environmentally friendly catalyst under solvent-free conditions or microwave irradiation. The application of microwave irradiation improved the yields and reduced the reaction times. In this study, selective conversion of aldehydes was observed in the presence of ketones, and the deprotection of 1,1-diacetates has also been achieved using selectfluor in water as green solvent in reflux conditions. The methodology provides synergy of microwave irradiation which offers several advantages such the simple work-up procedure, short reaction time, excellent yields and environmentally benign procedure.

Full text of DOI:10.1007/s11164-015-2168-1

Res Chem Intermed
DOI 10.1007/s11164-015-2168-1
Microwave-assisted green synthesis of 1,1-diacetates
TM
(
friendly catalyst under solvent-free conditions
acylals) using selectfluor
as an environmental-
1
1
•
Sobhan Rezayati
2
Zahra Erfani
•
Rahimeh Hajinasiri
Received: 27 April 2015 / Accepted: 4 July 2015
Ó Springer Science+Business Media Dordrecht 2015
Abstract An efficient and simple procedure has been developed for the acetyla-
TM
tion of aldehyde by selectfluor
[1-(chloromethyl)-4-fluoro-1,4-diazoniabicy-
clo[2,2,2]octane bis(tetrafluoroborate)] as a chemoselective and environmentally
friendly catalyst under solvent-free conditions or microwave irradiation. The
application of microwave irradiation improved the yields and reduced the reaction
times. In this study, selective conversion of aldehydes was observed in the presence
of ketones, and the deprotection of 1,1-diacetates has also been achieved using
TM
selectfluor
in water as green solvent in reflux conditions. The methodology
provides synergy of microwave irradiation which offers several advantages such the
simple work-up procedure, short reaction time, excellent yields and environmentally
benign procedure.
Keywords Microwave assisted synthesis Á Protection Á Deprotection Á One-pot
TM
condensation Á Selectfluor
Introduction
The formation of 1,1-diacetates (acylals) through protection carbonyl groups of
aromatic and aliphatic aldehydes is the great importance in synthetic organic
chemistry and also it is extremely important steps in modern organic chemistry,
because of the stability of corresponding products and their easy conversion back to
the parent compounds [1]. Acylals are synthetically important precursors for the
preparation of a,b-unsaturated for Diels–Alder reactions, which are useful
&
Sobhan Rezayati
sobhan.rezayati@yahoo.com
1
2
Chemistry Department, Qaemshahr Branch, Islamic Azad University,
P. O. Box 163, Qaemshahr, Iran
Department of Chemistry, Faculty of Science, Ilam University, P. O. Box 69315516, Ilam, Iran
123

S. Rezayati et al.
intermediates in industries, as chiral allyclic esters and also as cross linking reagents
for the cellulose in cotton [2–5]. Therefore, the methods for their synthesis have
received considerable attention. Usually, the preparation of 1,1-diacetates has been
achieved by the reaction of aldehydes with acetic anhydride in the presence of
different catalysts and under microwave irradiation such as SBA-15-Ph-PrSO H [6],
3
Fe O @SiO [7], PEG-SO H [8], Poly(4-vinylpyridinium) perchlorate [9], TiCl
3
4
2
3
4
2
[
[
10], N-Sulfonic acid poly(4-vinylpyridinum) hydrogen sulfate [11], NaHSO -SiO
4
12], H SO -silica [13], SbCl [14], 1,3-Dibromo-5,5-dimethylhydantoin [15],
2
4
3
Silica-supported functional ionic liquid (Si-[SbSipim][PF6]) [16], ZSM-5-SO H
3
[
17], Amberlyst-15 [18], Zirconium hydrogen sulfate [19] Bismuth triflate [20],
Iron(III) fluoride [21] and etc. Although some of these methods 1,1-diacetates are
obtained in good to high product yields suffer from some disadvantages such as long
reaction time, require high temperature, tedious work-up procedure, the use of
homogenous catalysts or harmful solvents, expensive and highly toxic catalysts and
involve strongly acidic or oxidizing conditions.
TM
Selectfluor (Fig. 1) is important reagent in the reactions organic synthesis that
TM
is used as a fluorine donor. Selectfluor for the first time by Banks et al. [22–25]
described and it is used for in organofluorine chemistry in the field of electrophilic
fluorination.
Also one of the most valuable catalyst for various other functionalisations of
organic compounds. The salient features of these reagents are a commercially
available, nonvolatile, non hygroscopic, environmentally benign, non-toxicity,
storage and handling of this compound do not need special precautions.
TM
In recent years, the use of selectfluor
as catalysts has been studied in the
organic compounds widely. In this context, reported including Bromination [26],
Iodination [27], Oxidation of alcohols [28], Synthesis of 14-aryl-14H-
dibenzo[a,j]xanthenes [29], a-thiocyanation of ketones [30], Synthesis of 9-aryl-
1
,8-dioxooctahydro xanthenes [31], and synthesis of 2h-chromen-2-ones [32] be can
noted.
As a part of an ongoing research project to develop amicable organic reactions
environmentally, we report here a rapid and efficient procedure for the formation of
,1-diacetates (3a–u) using a reaction between aromatic and aliphatic aldehydes
1
TM
(
1a–u) and acetic anhydride (2) in the presence of the selectfluor
under solvent free conditions at room temperature or microwave irradiation
Scheme 1).
as catalyst
(
Fig. 1 Structure of
selectfluor
Cl
.
TM
N
BF4-
2
N
F
1
23

Microwave-assisted green synthesis of 1,1-diacetates…
SelectfluorTM (0.1 mmol)
m.w, solvent-free
AC O
2
OAc
H
AcO
O
R
R
1
H
3
a-u
a-u
SelectfluorTM (0.1 mmol)
solvent-free
R= alkyl, aryl
TM
Scheme 1 Schematic synthesis of gem-diacetate using catalyst selectfluor (0.1 mmol) under solvent-
free conditions
Experimental
General
All the solvents and reagents were purchased from Merck or Fluka with high-grade
TM
quality and used without any purification. The selectfluor
1
was a sample from
Aldrich. The nuclear magnetic resonance ( H-NMR) spectra were recorded on a
BRUKER DRX-400 AVANCE instrument at 400 MHz (in CDCl solution) using
3
TMS as internal standard. Melting points were taken on an Electrothermal capillary
melting point apparatus (UK) and are uncorrected. All the reactions were carried out
in scientific microwave system (Catalyst system ‘CATA-R’, 700 W). The reactions
were carried out in a round-bottom flask of 25 ml capacity. Monitoring of the
reactions and the purity check of the final products were carried out by thin layer
chromatography (TLC) using silica gel precoated aluminium sheets (60–120 mesh;
Merck) and visualization with ultraviolet light at 365 and 254 nm.
General procedure for the preparation of 1,1-diacetates
Method A A mixture of aldehyde (1 mmol), freshly distilled Ac O (2 mmol), and
2
TM
selectfluor (0.035 g, 0.1 mmol) was stirred at room temperature for appropriate
time as shown in Table 3. After the completion of the reaction was monitored by
TLC (hexane/ethyl acetate, 5:1), CH Cl (15 ml) was added to the mixture and
2
2
TM
filtered, then the solid material (Selectfluor ) was again washed with CH Cl
2
2
(
2 9 5 ml) and collected for reuse. Evaporation of the solvent followed by column
chromatography on silica gel afforded the pure compound corresponding to 1,1-
diacetates.
123

S. Rezayati et al.
Method B A mixture of aldehyde (1 mmol), freshly distilled Ac O (2 mmol), and
2
TM
selectfluor
(0.035 g, 0.1 mmol) was charged into a 25 ml flask. The reaction
mixture was then subjected to microwave irradiation at power level (280 W) for
appropriate time as shown in Table 1. After the completion of the reaction was
monitored by TLC (hexane/ethyl acetate, 5:1), CH Cl (15 ml) was added to the
2
2
TM
mixture and filtered, then the solid material (Selectfluor ) was again washed with
CH Cl (2 9 5 ml) and collected for reuse. Evaporation of the solvent followed by
column chromatography on silica gel afforded the pure compound corresponding to
2
2
1
,1-diacetates. All the products were fully characterized by IR and H NMR
1
spectroscopy and melting points, which were consistent with literature data.
Results and discussion
In continuation of our search for new catalysts [33–36] convenient Herein we wish
to report an extremely, facile, and highly chemoselective procedure for the
preparation 1,1-diacetates from both aromatic and aliphatic aldehydes and acetic
TM
anhydride in the presence of selectfluor
as efficient catalyst under solvent-free
conditions at room temperature and under microwave irradiation (Scheme 1).
To optimize of amount of catalyst-loading in model reaction between benzalde-
hyde (1 mmol) and acetic anhydride (2 mmol) in the presence of different amount
TM
selectfluor as catalyst under solvent-free conditions at room temperature. In the
absence of a catalyst the reaction was slow and without yield even after 120 min.
Finally, we observed that the best results were obtained using 0.035 g of catalyst
under solvent-free conditions at room temperature. The obtained results based on
time and yield are recorded (Table 2, Method A).
Table 1 Optimization of reaction condition for the synthesis of 1,1-diacetates under microwave
irradiation
a
Entry
Power levels in Watt
Time/min
Yield %
Purity of product
1
2
3
4
5
6
7
8
9
140
210
240
280
350
420
450
490
560
700
15
10
8
68
73
86
99
96
91
84
77
53
43
TM
5
Excellent yield
Purity was fine
5
5
5
Loss of yield
5
5
1
0
5
Low yield
Model reaction: benzaldehyde (1 mmol), AC
irradiation at different power level
2
O (2 mmol) and selectfluor (0.035 g) under microwave
a
Yield of isolated products
1
23

Microwave-assisted green synthesis of 1,1-diacetates…
TM
Table 2 Acetylation of aldehydes using Ac
conditions
2
O in the presence of selectfluor
under solvent-free
a
c
Entry
Ar/R-CHO
Product
Method A
Method B
m.p./°C
b
b
Time/min
d
Yield %
Time/min
d
Yield %
1
2
3
4
5
6
7
8
9
C
6
H
5
3a
3b
3c
3d
3e
3f
70
80
30
55
60
50
50
60
65
50
80
80
45
50
45
86
79
85
85
81
80
83
84
77
85
76
85
84
79
85
–
5
15
10
15
10
15
10
10
5
99
87
90
84
90
89
93
89
90
91
96
91
94
88
88
–
44–45
79–80
57–58
84–86
91–92
Oil
2-CNC
2-ClC
2-OEtC
3-NO
3-OMeC
3-ClC
4-CNC
4-PhC
4-MeOC
4-ClC
6 4
H
6 4
H
6 4
H
2 6 4
C H
6
4
H
4
6
H
3g
3h
3i
64-65
6
H
4
100–102
128–129
63–64
80–82
58–59
126–128
80–82
92–94
–
6 4
H
10
11
12
13
14
15
16
17
18
19
20
21
a
6
4
H
4
3j
12
7
6
H
3k
3l
4-Acetylbenzaldehyde
10
7
4-NO
4-MeC
4-BrC
4-NMe
2
C
6
H
4
3m
3n
3o
3p
3q
3r
3s
3t
6
H
4
5
6
H
4
5
e
24
e
24
2
C
6
H
4
1-Naphthaldehyde
Terephthalaldehyde
Furfural
60
75
55
90
80
80
82
80
75
73
15
10
7
85
91
98
90
89
101–102
174–176
50–52
Oil
Butyraldehyde
Isobutyraldehyde
15
15
3u
183–185
1
All the products were characterized by H NMR spectroscopy
Reactions were run till the completion as indicated by TLC
b
c
Melting points are uncorrected and by comparison with those reported in the literature
d
Yield of isolated products
In hour
e
To investigate the influence of microwave irradiation on a reaction mixture, the
effect of microwave irradiation with different power levels on this procedure was
investigated (Table 1).
The results are shown in Table 2, that all the titled compound 1,1-diacetates
derivatives (3a–u) can be synthesized with high purity, high yield and short reaction
time at power level 4 (280 W) and 5 (350 W) (Table 1, Entry 4 and 5). Finally, we
observed that the reaction was very efficient and clean in the presence of
TM
selectfluor as catalyst 0.035 g under microwave irradiation at power 280 W and
the corresponding 1,1-diacetates derivatives product was formed after 5 min in
excellent yield and short reaction time (Table 1, Entry 4).
123

S. Rezayati et al.
The all reactions worked well and the results are summarized in Table 2. The
results are shown in Table 2 that in all cases, the use of microwave irradiation
resulted in faster reaction (5–15 min) and in good to high excellent yields
(
84–99 %). Benzaldehyde (Table 2, Entry 1), 3-chloro benzaldehyde (Table 2,
Entry 7), 4-chloro benzaldehyde (Table 2, Entry 11), and 4-nitro benzaldehyde
Table 2, Entry 13) was converted to the corresponding acylals in excellent yields
(
and short reaction times. 2-cyano benzaldehyde (Table 2, Entry 2), 2-ethoxy
benzaldehyde (Table 2, Entry 4), 3-methoxy benzaldehyde (Table 2, Entry 6), and
4
-cyano benzaldehyde (Table 2, Entry 8) gave slightly decreased the yield and
increased the reaction time. When the reaction was carried out using aliphatic
aldehydes, we obtained good yields and enquired a longer reaction time (Table 2,
Entries 20–21). It must be mentioned here that compound 3p was not converted to
Table 3 Chemoslective acetylation of various aldehydes in the presence of ketones
H
Me
O
H
O
Me
Me
O
O
O
Me Me
O
O
O
Me
Me
SelectfluorTM
O
O
O
O
O
O
1
2
Ac O, solvent-free
2
(1 mmol) (1 mmol)
method A
method B
86.0%
99.0%
0.0%
0.0%
H
O
Me
O
O
H
O
Me
Me
O
Me Me
SelectfluorTM
O
Ac O, solvent-free
2
NO2
NO2
NO2
NO2
(1 mmol)
(1 mmol)
method A
method B
84.0%
0.0%
94.0%
0.0%
H
O
Me
O
O
H
O
Me
Me
O
O
Me Me
O
Me
SelectfluorTM
O
O
O
3
4
Ac O, solvent-free
2
Me
1 mmol)
Me
(1 mmol)
Me
79.0%
Me
0.0%
(
method A
method B
88.0%
0.0%
H
O
Me
O
O
H
O
Me
Me
O
O
Me Me
O
Me
SelectfluorTM
O
O
O
Ac O, solvent-free
2
OMe
OMe
(1 mmol)
OMe
OMe
(
1 mmol)
85.0%
91.0%
0.0%
0.0%
method A
method B
1
23

Microwave-assisted green synthesis of 1,1-diacetates…
the corresponding acylals, due to the strong electron donating nature of the amino
group. Moreover, sensitive rings furfural was successfully converted to corre-
sponding acylals in excellent yield and short reaction time (Table 2, Entry 19). In
this reaction sequence, the alcoholic group is also acylated apart from acylal
formation in more than 85 % yield and 15 min (Table 2, Entry 17). All the
products were characterized by comparison (TLC and physical constants) with
TM
authentic samples prepared by the conventional method, using selectfluor as the
catalyst.
Finally, In order to show the high chemoselectivity of the method, a
competition reaction between mixtures of some aldehydes and ketones such
benzaldehyde and acetophenone, 4-nitro benzaldehyde and 4-nitro acetophenone,
4
benzaldehyde and 4-methoxy acetophenone with acetic anhydride (2 mmol) in
-methyl benzaldehyde and 4-methyl acetophenone, and also 4-methoxy
TM
the presence of selectfluor
(0.035 g) under solvent-free condition at room
Table 4 Comparison of different catalysts
g
Catalyst
Solvent
T (°C)
Time/min Yield %
References
f
Si-[SbSipim][PF
6
]
Solvent-free 30 °C
Solvent-free r.t
Solvent-free r.t
3
90
97
95
86
99
99
90
98
92
90
76
94
90
85
86
99
[16]
[37]
[38]
[39]
[40]
[41]
[42]
[43]
[44]
[45]
[46]
[47]
[48]
[48]
–
NbCl
MCM-41-SO
/Al
Sulphated zirconia
5
10
3
3
H
P
2
O
5
2
O
3
Neat
r.t
45
f
5
Solvent-free 0 °C
Solvent-free r.t
2?a
2
c-Fe O
3
@HAp-Ni
10
SbCl
3
Solvent-free Microwave-assisted 60
Solvent-free r.t
Solvent-free Microwave-assisted 120
Phospho sulfonic acid
Silica gel
5
f
Saccharin sulfonic acid
Solvent-free 90 °C
0.8
10
Ruthenium(III) Chloride n-hexane
r.t
f
4
Cu(OTf)
2
CH
CH
CH
2
2
2
Cl
2
Cl
2
Cl
2
r.t
b
f
8
TBBDA
c
PBBS
Reflux
Reflux
f
5
TMd
TMe
Selectfluor
Selectfluor
Solvent-free r.t
70
5
Solvent-free Microwave-assisted
–
a
b
c
d
2?
2 3
Ni @hydroxyapatite-core@shell c-Fe O nanoparticles
0
0
N,N,N ,N -tetrabromobenzene-1,3-disulfonamide
0
Poly(N,N -dibromo-N-ethyl-benzene-1,3-disulfonamide)
TM
O (2 mmol) and selectfluor (0.035 g) was stirred at
Model reaction: benzaldehyde (1 mmol), AC
room temperature under solvent-free conditions
2
e
TM
Model reaction: benzaldehyde (1 mmol), AC
wave irradiation and solvent-free conditions
2
O (2 mmol) and selectfluor
(0.035 g) under micro-
f
In hour
g
Yield of isolated products
123

S. Rezayati et al.
temperature and microwave irradiation was studied. The results are shown in
Table 3 that aldehydes were converted to the corresponding acylals, but ketones
remained practically intact.
To illustrate the efficiency of the proposed method, the numbers of methods for
the preparation of acylals under different conditions are given in Table 4.
Ghorbani-Vaghei et al. reported for the synthesis of 1,1-diacetates using TBBDA
and PBBS as a catalyst. This method relies on the use of toxic solvent like CH Cl
2
2
and used in reflux conditions, that the reaction conditions is dangerous. On the
other hand, for completion of the reaction requires long reaction time and product
yields are low. Reported using Ruthenium(III) Chloride as a catalyst for the
synthesis of 1,1-diacetates, which used from solvent toxic like n-hexane. Also
gave product yields of 76 %, that product yields are low. Reported using saccharin
sulfonic acid as a catalyst for the synthesis of 1,1-diacetates, which for completion
of the reaction requires very long reaction time (8 h) and high temperature
(90 °C). And also gave product yields of 90 %. Recently, Kang et al. in [16]
reported the synthesis of 1,1-diacetates by addition silica-supported functional
ionic liquid Si-[SbSi-pim][PF ] as a catalyst. Which for completion of the reaction
6
requires high temperature, high catalyst loading, and excess amount of Ac O. Also
2
long reaction time and gave product yields of 90 %. As shown in Table 4, our
procedure in compared to some of the reported the methods has higher
regioselectivity, shorter reaction time, excellent yield and solvent-free conditions.
From this study it can be concluded that the novel catalyst has much higher than
others activity.
TM
In addition to these results, we investigated the application of selectfluor
as catalyst for deprotection of 1,1-diacetates to their corresponding aldehydes.
For this purpose, after the completion of the reaction for the formation of
phenyl ethylene diacetate from benzaldehyde and acetic anhydride, water as a
green solvent was added to mixture reaction and the reaction temperature
increased to 100 °C. By this procedure, related acylals has been completely
transformed into benzaldehyde in short reaction time and high yields
(
Scheme 2).
The suggest mechanism for the preparation of 1,1-diacetates from reaction
TM
between benzaldehyde and acetic anhydride promoted by selectfluor under mild
reaction conditions through an electrophilic reaction is shown in Scheme 3 can be
selected as the most probable one. The mechanism for the reaction is assumed to be
the same as that proposed by Poor Heravi in 2009 for the Synthesis of 9-Aryl-1,8-
dioxooctahydroxanthanes derivatives [31].
H
Me
O
O
Me
O
SelectfluorTM (0.1 mmol)
O
O
H
ο
Water, 100 C, 15 min, 98%
TM
Scheme 2 Deprotection of phenyl ethylene diacetate in the presence of selectfluor
1
23

Microwave-assisted green synthesis of 1,1-diacetates…
Cl
N
.
2BF4
N
F
O
O
O
O
Ar
H
Me
O
Me
O
Cl
Ar
H
N
.
2BF4
N
H C
O
3
O
H
CH3
Ar
O
AcO
Ar
OAc
F
N
H
.
N
2BF4
Cl
-
SelectfluorTM
Scheme 3 Proposed mechanism of reaction between aldehyde and acetic anhydride in the presence of
selectfluor
TM
Conclusions
TM
In summary, selectfluor is a highly efficient and chemoselective catalyst for gem-
diacetate synthesis from both aromatic and aliphatic aldehydes under solvent-free
conditions at ambienet temperature or microwave irradiation. The advantages of this
methodology over the reported methods is low cost of the catalyst used, excellent
chemoselectivity, exceptionally fast reactions, applicability to large-scale reactions,
short reaction time and operational simplicity without need of any solvent. On the
other hand, microwave irradiation increased the catalytic activity and higher product
yields and short reaction times were obtained.
Acknowledgments The authors wish to thank the Islamic Azad University Qaemshahr for financial
support to carry out this research. We also thank Dr. Rahimeh Hajinasiri for her assistance.
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