Sulfonic acid anchored on silica, SiO2@SO3H: A superior solid acid catalyst for quick and solvent-free reductive-deoxygenation of ketones with NaBH3CN

Article abstract of DOI:10.1002/jccs.201800250

NaBH₃CN as a modified hydroborate agent and due to a strong withdrawing CN group does not show any reducing ability to reduce functional groups in the absence of acidic media (pH ~ 3–4). In this study, the immobilized sulfonic acid on silica, SiO₂@SO₃H, was prepared and applied as a new solid acid catalyst for extremely enhancing the reducing ability of NaBH₃CN. The influence of SiO₂@SO₃H was highlighted by performing the quick and green reduction of structurally diverse carbonyl compounds involving aldehydes, ketones, α,β-unsaturated enals and enones, α-diketones, and acyloins to the corresponding alcohols or alkanes with NaBH₃CN. By the NaBH₃CN/SiO₂@SO₃H system, aldehydes were reduced to the corresponding alcohols and ketonic compounds to alkanes as reductive-deoxygenation products. All reduction reactions were carried out within 3 min at room temperature and under solvent-free conditions to afford the products in high to excellent yields (90–98%).

Full text of DOI:10.1002/jccs.201800250

Received: 29 June 2018
Accepted: 15 October 2018
DOI: 10.1002/jccs.201800250
A R T I C L E
Sulfonic acid anchored on silica, SiO2@SO3H: A superior solid
acid catalyst for quick and solvent-free reductive-deoxygenation
of ketones with NaBH3CN
Behzad Zeynizadeh1 | Mehri Kouhkan2
1Faculty of Chemistry, Urmia University,
NaBH3CN as a modified hydroborate agent and due to a strong withdrawing CN
Urmia, Iran
2Department of Pharmacy, Urmia University of
Medical Sciences, Urmia, Iran
group does not show any reducing ability to reduce functional groups in the
absence of acidic media (pH ~ 3–4). In this study, the immobilized sulfonic acid
on silica, SiO2@SO3H, was prepared and applied as a new solid acid catalyst for
extremely enhancing the reducing ability of NaBH3CN. The influence of SiO2@-
SO3H was highlighted by performing the quick and green reduction of structurally
diverse carbonyl compounds involving aldehydes, ketones, α,β-unsaturated enals
and enones, α-diketones, and acyloins to the corresponding alcohols or alkanes
with NaBH3CN. By the NaBH3CN/SiO2@SO3H system, aldehydes were reduced
to the corresponding alcohols and ketonic compounds to alkanes as reductive-
deoxygenation products. All reduction reactions were carried out within 3 min at
room temperature and under solvent-free conditions to afford the products in high
to excellent yields (90–98%).
Correspondence
Behzad Zeynizadeh, Faculty of Chemistry, Urmia
University, Urmia 5756151818, Iran.
Email: bzeynizadeh@gmail.com
Funding information
research council of Urmia University
KEYWORDS
alcohols, alkanes, NaBH3CN, SiO2@SO3H
reagent suffers from strong acidic conditions, limitation to
use acid-sensitive functional groups, and the formation of
1
| INTRODUCTION
Reduction is one of the most important reactions in organic
synthesis and a vast variety of reducing agents have been
introduced for this achievement. In this context, considerable
attention has been paid to modified hydroborate agents for
reduction of organic compounds.[1–4] NaBH3CN with an
electron withdrawing cyano group is a remarkable stable and
selective reducing agent and has been found many applica-
tions in organic synthesis.[5–16] In addition, it is well known
that the reducing capability of NaBH3CN for reduction of
functional groups is greatly modified with pH variations.
Thus in basic or neutral media, functional groups such as the
carbonyl moiety are practically inert toward NaBH3CN, and
the adequate reduction rate is obtained when the reaction
medium gets acidic conditions (pH ~ 3–4).[5,17] Although
NaBH3CN has been used in many organic reactions, this
some side products. Therefore, the development and intro-
duction of new protocols that use green and mild reaction
criteria as well easily separable heterogeneous catalysts is a
subject of more interest.
In line with the outlined strategies and continuation of
our research program directed to modified hydroborate
agents in organic synthesis,[18–22] herein, we wish to intro-
duce sulfonic acid anchored on silica, SiO2@SO3H, as a
new and superior solid acid catalyst for dramatically raising
the reducing ability of NaBH3CN toward green and quick
reduction of various carbonyl compounds such as aldehydes,
ketones, α,β-unsaturated enals and enones, and α-diketones
and acyloins to the corresponding alcohols or alkanes at
room temperature and under solvent-free conditions
(Scheme 1).
© 2018 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
J Chin Chem Soc. 2018;1–8.
http://www.jccs.wiley-vch.de
1

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ZEYNIZADEH AND KOUHKAN
that the preparation of the catalyst is carried out with the
reaction of cheaply available counterparts. After synthesis of
SiO2@SO3H, its promoter activity toward reducing potential
of NaBH3CN was studied by reduction of carbonyl com-
pounds in the absence of any acidic medium.
SCHEME 1 Reduction of carbonyl compounds with the NaBH3CN/
SiO2@SO3H system
2.1 | Catalytic reduction of aldehydes to alcohols
To optimize the reaction conditions, we performed reduction
of benzaldehyde as a model compound with NaBH3CN in
the presence of SiO2@SO3H under solvent-free conditions.
The experiments proved that using 3 mmol NaBH3CN and
0.1 g SiO2@SO3H per 1 mmol of PhCHO at room tempera-
ture is the requirement for complete reduction of benzalde-
hyde to benzyl alcohol (Table 1, entry 2) (Scheme 3).
SCHEME 2 Preparation of the immobilized sulfonic acid on SiO2
2
| RESULTS AND DISCUSSION
In recent years, heterogeneous catalysts due to economic
concerns have attracted a great deal of attention. They are
easily handled, as well as low cost, more reactive and more
environmentally benign alternatives than their homogeneous
counterparts. As mentioned here, exploring the reducing
capability of NaBH3CN due to a strong withdrawing cyano
group completely depends on applying vigorous acidic con-
ditions. This situation puts some restrictions to use
NaBH3CN for bench-top and practical purposes. To over-
come the mentioned drawbacks, we recently introduced wet
SiO2 as a solid acid promoter for reduction of carbonyl com-
pounds with NaBH3CN under acid-free and solvent-free
conditions.[23] Although the reported method has own
merits; however, the requirement for oil bath (70–80ꢀC) con-
ditions (accompanied with the prolonged reaction times) or
microwave irradiation as an unconventional energy source
are its disadvantages. Therefore, keeping a new strategy in
mind encouraged us to investigate the promoter activity of
sulfonic acid anchored on silica as a new solid acid catalyst
for enhancing the reducing capability of NaBH3CN. Accord-
ing to the reported procedure,[24] the catalyst was prepared
by simply mixing and stirring of SiO2 and ClSO3H under
solvent-free conditions for 2.5 hr (Scheme 2). It is notable
For highlighting the promoter activity of SiO2@SO3H,
we also examined the influence of other solid acids, which
were prepared by simple mixing of Amberlyst A-15 and
Al2O3 with H2SO4/H3PO4 as well molecular sieve 4 Å with
H3PO4, on the reducing potential of NaBH3CN under sol-
vent-free conditions. The summarized results in Table 1
show that in comparison to SiO2@SO3H, the influence of
examined solid acids was not satisfactory. In addition, a
comparison of the obtained results with the current protocol
and reported wet SiO2 shows that the present system exhibits
a superior promoter activity toward NaBH3CN (Scheme 4).
The usefulness and generality of the NaBH3CN/SiO2@
SO3H system were further studied by performing reduction
of structurally diverse aliphatic and aromatic aldehydes
under the optimized reaction conditions. The summarized
results in Table 2 show that all reduction reactions were car-
ried out at room temperature and under solvent-free condi-
tions to afford the corresponding primary alcohols in high to
excellent yields. As seen in Table 2, all reactions were car-
ried out dramatically within 3 min in the absence of any
aqueous acidic medium. In addition, because of a dramatic
TABLE 1 Optimization experiments for reduction of benzaldehyde and acetophenone with NaBH3CN in the presence of solid acids
Entry
Reaction components (molar ratio)
PhCHO/NaBH3CN (1:2)
PhCHO/NaBH3CN (1:3)
PhCHO/NaBH3CN (1:4)
PhCHO/NaBH3CN (1:4)
PhCHO/NaBH3CN (1:4)
PhCHO/NaBH3CN (1:4)
PhCHO/NaBH3CN (1:4)
PhCOCH3/NaBH3CN (1:2)
PhCOCH3/NaBH3CN (1:3)
Support (g)
Condition
Time (min)
Conversion (%)
1
2
3
4
5
6
7
8
9
SiO2@SO3H (0.1)
Solvent-free/r.t.
Solvent-free/r.t.
Solvent-free/80ꢀC
Solvent-free/80ꢀC
Solvent-free/80ꢀC
Solvent-free/80ꢀC
Solvent-free/80ꢀC
Solvent-free/r.t.
Solvent-free/r.t.
120
3
80
100
0
SiO2@SO3H (0.1)
Amberlyst A-15@H3PO4 (0.1)
Amberlyst A-15@H2SO4 (0.1)
Molecular sieve 4 Å@H3PO4 (0.1)
Al2O3@H3PO4 (0.1)
Al2O3@H2SO4 (0.1)
SiO2@SO3H (0.1)
180
180
180
180
180
180
3
0
0
0
30
60
100
SiO2@SO3H (0.1)
SCHEME 3 Reduction of benzaldehyde with the NaBH3CN/SiO2@SO3H system

ZEYNIZADEH AND KOUHKAN
3
2.2 | Catalytic reduction of ketones to alkanes
Reactivity of ketones toward the NaBH3CN/SiO2@SO3H
system was also examined by the reduction of acetophe-
none under solvent-free conditions. The examinations
showed that the optimized reaction conditions for reduction
of benzaldehyde by the NaBH3CN/SiO2@SO3H system
(Table 1, entry 9) were also suitable for reduction of aceto-
phenone. Thus, using 3 mmol NaBH3CN and 0.1 g SiO2@-
SO3H per 1 mmol of acetophenone was sufficient to afford
ethylbenzene as a reductive-deoxygenation product at room
SCHEME 4 Comparing the promoter activity of SiO2@SO3H and wet
SiO2 in reduction of benzaldehyde
influence of SiO2@SO3H, the presence of electron releasing
or withdrawing substituents on aromatic rings did not affect
the rate of reductions.
TABLE 2 Solvent-free reduction of aldehydes with the NaBH3CN/SiO2@SO3H systema
Entry
Substrate
Product
Molar ratio (subs./NaBH3CN)
SiO2@SO3H (g)
Time (min)
Yield (%)b
1
1:3
0.1
3
92
2
3
1:3
1:3
0.1
0.1
3
3
95
91
4
1:3
0.1
3
91
5
6
1:3
1:3
0.1
0.1
3
3
95
94
7
8
1:3
1:3
0.1
0.1
3
3
95
95
9
1:3
1:3
0.1
0.1
3
3
94
92
10

4
ZEYNIZADEH AND KOUHKAN
TABLE 2 (Continued)
Entry
Substrate
Product
Molar ratio (subs./NaBH3CN)
SiO2@SO3H (g)
Time (min)
Yield (%)b
11
1:3
0.1
3
95
12
13
1:3
1:3
0.1
0.1
3
3
95
93
a
All reactions were carried out at room temperature.
Isolated yields.
b
by SiO2@SO3H produces alkanes through the reductive-
deoxygenation pathway (Scheme 6).
2.3 | Catalytic reduction of α,β-unsaturated aldehydes
and ketones
SCHEME 5 Reduction of acetophenone with the NaBH3CN/SiO2@SO3H
system
Next, the reducing ability of the NaBH3CN/SiO2@SO3H
system toward regioselective reduction of α,β-unsaturated
aldehydes and ketones was studied. The results of solvent-
free reduction of cinnamaldehyde with 3 mmol NaBH3CN
and 0.1 g SiO2@SO3H showed that the reaction was car-
ried out within 3 min to afford a mixture of cinnamyl alco-
hol (62%) and 3-phenyl-1-propanol (38%). The
examinations for reduction of benzalacetone, benzalaceto-
phenone, and citral with NaBH3CN/SiO2@SO3H and pro-
ducing the mixture of products showed that the current
protocol did not have any regioselectivity toward reduction
of conjugated enals and enones. In addition, conjugated
aldehydes as the simple aldehydes were reduced to the cor-
responding alcohols (Table 4, entries 1 and 4), and conju-
gated ketones as the simple ketones were converted to the
conjugated alkanes with this reducing system (Table 4,
entries 2 and 4).
temperature (Scheme 5). This result exhibited that the
reducing pathway of the formyl moiety (–CHO) and ben-
zoyl function (ArCO–) with the NaBH3CN/SiO2@SO3H
system is entirely different. It means that aldehydes pro-
duce primary alcohols; however, the benzoyl function
affords a reductive-deoxygenation product without forma-
tion of secondary alcohol.
To explore the synthetic utility of the NaBH3CN/
SiO2@SO3H system toward reductive-deoxygenation of
ketones, structurally diverse aromatic ketones were sub-
jected to 3 mmol NaBH3CN in the presence of 0.1 g
SiO2@SO3H under solvent-free conditions. The results of
this investigation are summarized in Table 3. The
Table exhibited that all reduction reactions without influ-
ence of electron releasing or withdrawing substituents on
aromatic rings were carried out successfully within 3 min
to afford the corresponding alkanes in excellent yields. In
addition, an examination for 4-phenylcyclohexanone with
the NaBH3CN/SiO2@SO3H system shows that in the case
of aliphatic ketones, the current protocol obeys from the
same mechanistic pathway of aromatic ketones to afford
4-phenylcyclohexane as a reductive-deoxygenation prod-
uct (Table 3, entry 7). Comparison of the results for wet
SiO2 (previously reported) and SiO2@SO3H as solid acid
catalysts for NaBH3CN reduction of ketones shows that
both of solid acids promote the reduction reaction through
different mechanistic pathways. It means that by the pro-
motion with wet SiO2, reduction of ketones leads to the
formation of secondary alcohols; whereas, the promotion
2.4 | Catalytic reduction of acyloins and α-diketones
Reactivity of α-diketones and acyloins toward the
NaBH3CN/SiO2@SO3H system was further studied
through the reduction of benzil (1 mmol) with NaBH3CN
(3 mmol) and SiO2@SO3H (0.1 g) under solvent-free con-
ditions. The examination showed that the reaction was
completed within 3 min to afford 1,2-diphenylethane as a
reductive-deoxygenation product in 92% yield (Table 5,
entry 1). Reducing capability of the NaBH3CN (3 mmol)/
SiO2@SO3H (0.1 g) system was further examined by the
reduction of other α-diketones and acyloins at room tem-
perature and under solvent-free conditions. The results of

ZEYNIZADEH AND KOUHKAN
5
TABLE 3 Solvent-free reduction of ketones with the NaBH3CN/SiO2@SO3H systema
Entry
Substrate
Product
Molar ratiob
SiO2@SO3H (g)
Time (min)
Yield (%)c
1
1:3
0.1
3
94
2
1:3
0.1
3
95
3
4
1:3
1:3
0.1
0.1
3
3
95
92
5
6
1:3
1:3
0.1
0.1
3
3
96
94
7
8
1:3
1:3
0.1
0.1
3
3
98
90
9
1:3
1:3
0.1
0.1
3
3
94
98
10
11
12
1:3
1:3
0.1
0.1
3
3
96
96
a
b
c
All reactions were carried out at room temperature.
Molar ratio as: Subs./NaBH3CN.
Isolated yields.

6
ZEYNIZADEH AND KOUHKAN
3.2 | Preparation of sulfonic acid anchored on silica,
SiO2@SO3H
In a round-bottom flask containing silica gel (1 mmol, 60 g,
70–230 mesh) and a magnetic stirrer, ClSO3H (0.2 mol,
23.3 g) was added within 30 min at room temperature.
During the addition of ClSO3H, the mixture was stirred mag-
netically and the evolution of HCl gas was taken place. Stir-
ring of the reaction mixture was continued for 2 hr to
completely release HCl gas. The immobilized silica
(SiO2@SO3H) was obtained as a white and fine granular
powder in 76 g and 39% w/w.[24]
SCHEME 6 Comparing the promoter activity of SiO2@SO3H and wet
SiO2 toward the reduction of acetophenone
this investigation are illustrated in Table 5. The obtained
results represent that the current reducing system affords
reductive-deoxygenation products instead of secondary
alcoholic products (Table 5).
3.3 | A typical procedure for solvent-free reduction of
carbonyl compounds with the NaBH3CN/SiO2@SO3H
system
3
| EXPERIMENTAL
In an agate mortar, a mixture of SiO2@SO3H (0.1 g), aceto-
phenone (0.12 g, 1 mmol), and NaBH3CN (0.187 g, 3 mmol)
was ground at room temperature for 3 min. Progress of the
reaction was monitored by TLC (eluent; n-hexane/EtOAc:
5/2). After completion of the reaction, the mixture was
washed with ethyl acetate. Evaporation of the solvent under
reduced pressure affords the pure liquid ethylbenzene
(0.99 g, 94% yield) (Table 3, entry 1).
3.1 | General
All reagents and substrates were purchased from commer-
cial sources with the best quality and they were used with-
out further purification. SiO2 was used in the form of
silica gel 60 (70–230 mesh) and was purchased from
Merck company. FT-IR and 1H/13C NMR spectra were
recorded on Thermo Nicolet Nexus 670 FT-IR and Bruker
Avance 300 MHz spectrometers, respectively. The prod-
ucts were characterized by a comparison with authentic
samples (melting or boiling points) and their spectral data.
All yields refer to isolated pure products. TLC was
applied for the purity determination of substrates, prod-
ucts, and reaction monitoring over the silica gel 60 F254
aluminum sheet.
4
| CONCLUSIONS
In this study, the immobilized sulfonic acid on silica, SiO2@-
SO3H, was used as a superior solid acid catalyst for extremely
raising the reducing ability of NaBH3CN. Structurally diverse
TABLE 4 Solvent-free reduction of conjugated carbonyl compounds with the NaBH3CN/SiO2@SO3H systema
Yield of products (%)b,c
Entry
Substrate
Time (min)
–CH=CH-CHOH–
–CH=CH-CH2–
–CH2CH2CH2–
–CH2CH2CHOH–
1
3
62
—
—
38
2
3
—
64
36
—
3
4
3
3
—
60
40
—
54
—
—
46
a
b
c
All reactions were carried out in a molar ratio of Subs./NaBH3CN (1:3) in the presence of 0.1 g SiO2@SO3H at room temperature.
Isolated yields.
The percentage of products was determined by 1H NMR.

ZEYNIZADEH AND KOUHKAN
7
TABLE 5 Solvent-free reductive-deoxygenation of acyloins and α-diketones with the NaBH3CN/SiO2@SO3H systema
Molar ratio (subs./
NaBH3CN)
SiO2@SO3H
(g)
Time
(min)
Yield
(%)b
Entry
Substrate
Product
1
1:4
0.1
3
92
2
1:4
0.1
3
95
3
3
1:4
1:4
0.1
0.1
3
3
95
93
5
1:4
0.1
3
94
a
All reactions were carried out at room temperature and under solvent-free conditions.
Isolated yields.
b
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carbonyl compounds were reduced by the NaBH3CN
(3 mmol)/SiO2@SO3H (0.1 g) system at room temperature
and under solvent-free conditions. All reduction reactions
were carried out within 3 min to afford the corresponding
alcohols or alkanes in high to excellent yields. The obtained
results showed that via this reducing system, reduction of
aldehydes led to the formation of primary alcohols, and
ketones to the corresponding alkanes. Easy preparation of the
catalyst, its dramatic influence on reducing potential of
NaBH3CN, quick and mild reaction conditions, high to excel-
lent yield of the products, and easy workup procedure as well
as the benefits of solvent-free conditions are the advantages
that make this protocol a prominent choice for the reduction
of carbonyl compounds by NaBH3CN.
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How to cite this article: Zeynizadeh B, Kouhkan M.
Sulfonic acid anchored on silica, SiO2@SO3H: A
superior solid acid catalyst for quick and solvent-free
reductive-deoxygenation of ketones with NaBH3CN.
J Chin Chem Soc. 2018;1–8. https://doi.org/10.1002/
jccs.201800250
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