Solvent-free three component Strecker reaction of ketones using highly recyclable and hydrophobic sulfonic acid based nanoreactors

Article abstract of DOI:10.1039/b911388f

An efficient and environmentally benign system was developed for the one-pot three-component Strecker reaction of ketones under solvent-free conditions, with the use of a highly recoverable SBA-15 supported sulfonic acid. Also findings concerning the effects of functionalized inert groups and silica backbone pore size on substrate scope, catalytic activity and recycling behavior of the catalysts were briefly discussed. The simple experimental and product isolation procedure accompanied by easy recovery and reusability of the catalyst could be considered as attractive features of this new protocol which will hopefully develop a clean strategy for the synthesis of α-amino nitriles. The Royal Society of Chemistry 2009.

Full text of DOI:10.1039/b911388f

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Solvent-free three component Strecker reaction of ketones using highly
recyclable and hydrophobic sulfonic acid based nanoreactors†‡
*
Babak Karimi and Daryoush Zareyee
Received 10th June 2009, Accepted 7th September 2009
First published as an Advance Article on the web 2nd October 2009
DOI: 10.1039/b911388f
An efficient and environmentally benign system was developed for the one-pot three-component
Strecker reaction of ketones under solvent-free conditions, with the use of a highly recoverable SBA-15
supported sulfonic acid. Also findings concerning the effects of functionalized inert groups and silica
backbone pore size on substrate scope, catalytic activity and recycling behavior of the catalysts were
briefly discussed. The simple experimental and product isolation procedure accompanied by easy
recovery and reusability of the catalyst could be considered as attractive features of this new protocol
which will hopefully develop a clean strategy for the synthesis of a-amino nitriles.
have been several attempts in the literature to find alternative and
Introduction
environmentally benign synthetic routes for this transformation.
In this regard, water19,20 and ionic liquids21 have received
considerable attention and proved to be promising solvents in
the Strecker reaction due to their environmentally friendly and
polar nature. However, due to the reversible nature of the
Strecker reaction, most modified aqueous conditions still suffer
from certain drawbacks, including the tedious isolation of
pure a-amino nitriles from the reaction mixtures, which often
contain unreacted starting materials and by-products such as
cyanohydrins. Additionally, TMSCN is easily hydrolyzed in
the presence of water and it is also necessary to use excess
amounts of it.20
Compounds having versatile functional groups in close prox-
imity, such as a-amino nitriles, are extremely useful synthetic
intermediates. a-Amino nitriles are significantly valuable syn-
thons for the synthesis of a-amino acids,1 a variety of nitrogen
containing heterocycles2 such as imidazoles and thiodiazoles and
other pharmacologically useful molecules such as saframycin
A.1b They are generally prepared by the nucleophilic addition of
cyanide anion to imines, the so-called Strecker reaction.3
Although various cyanide ion sources such as HCN,1d KCN,4
(EtO)2P(O)CN,5 Et2AlCN,6 and Bu3SnCN7 have been utilized in
the Strecker reaction, TMSCN8 has been proved to be a safer,
more soluble, and more easily handled reagent for the Strecker
reaction. Along this line, many Lewis acids such as Cu(OTf)2,9
Sc(OTf)3,10 Yb(OTf)3,11 I2,12 InCl3,13 NiCl2,14 BiCl3,15 and
RuCl316 have been used to promote the Strecker reaction. Most
of these protocols require a high loading of expensive and non-
recoverable catalyst and prolonged reaction time. Furthermore
these methods, which are mainly limited to aldehydes, are not
satisfactorily applicable to ketones in most instances. To address
this issue, Olah and co-workers have recently demonstrated the
On the other hand, ionic liquids are often very expensive and
their use has always been accompanied by liberation of
hazardous BF3 and HF, in addition to issues such as dispos-
ability of the solvents and their biodegradability.22 Clearly green
chemistry not only requires the use of environmentally benign
reagents and solvents, but also it is very crucial to recover and
reuse the catalyst. To address the recyclability issue a number of
heterogeneous alternatives such as heteropoly acids,24 silica
supported heteropoly acid,25 montmorillonite KSF clay,26
guanidine hydrochloride,27 silica based Sc(OTf)3,28 Nafion-H
and Nafion-silica29 have also been proposed to be used in the
Strecker reaction. Although, these methods have been suitable
for certain synthetic conditions, the yields have not been always
satisfactory especially in the case of ketones, and the catalytic
activities have been lower than homogeneous catalysts in most
cases.
application
of
trimethylsilyl
trifluoromethanesulfonate
(TMSOTf)17 and Ga(OTf)318 as effective homogeneous catalysts
for Strecker synthesis of a-amino nitriles from aldehydes,
ketones and fluorinated ketones, at room temperature, and in
chlorinated solvents as the best solvent medium. However,
avoiding the use of harmful organic solvents is one of the most
fundamental strategies in green chemistry. For this reason, there
While acknowledging the pioneering advances in this area, we
believe that the development of novel and highly recoverable
systems, exhibiting more significant reactivity toward less reac-
tive hindered ketones under green conditions, is still a major
challenge.
Department of Chemistry, Institute for Advanced Studies in Basic Sciences
(IASBS), P.O.Box, 45195-1159 Gava Zang, Zanjan, Iran. E-mail:
karimi@iasbs.ac.ir; Fax: + 98 241 4249023; Tel: + 98 241 4153225
† This paper is part of a Journal of Materials Chemistry theme issue on
Green Materials. Guest editors: James Clark and Duncan Macquarrie.
‡ Electronic supplementary information (ESI) available: Experimental
procedure and characterization data (TEM, TGA, BET, BJH and N2
adsorption-desorption isotherm) for catalyst 5, general procedure for
the Strecker reaction, and copies of mass, 1H and 13C spectra for all
compounds. See DOI: 10.1039/b911388f
Herein, we describe the use of a variety of ordered nanoporous
sulfonic acid catalysts based on both functionalized MCM-41
and SBA-15 silica, and briefly discuss our findings concerning
the effects of functionalized inert groups and the pore size of
the silica backbone on substrate scope, catalytic activity, and
recycling behavior of catalysts.
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(100 mg) was added and the resulting mixture was stirred for
6 h after which the pH of the solution decreased to 2.19. The
acid amount of SBA-15-Ph-Pr-SO3H was determined to be
Experimental
Preparation of SBA-15-Pr-SH
1.61 mmolgꢁ1
.
The synthesis of SBA-15-PrSH was achieved using the procedure
described by Stucky and co-workers.23a This procedure involved
a synthetic strategy based on the co-condensation of tetrae-
thoxysilane (TEOS) and 3-mercaptopropyltrimethoxysilane
(MPTMS) in the presence of Pluronic P123 as the structure
directing agent. In a typical preparation procedure, 4.0 g of
Pluronic P123 (Aldrich, average Mw ¼ 5800) was dissolved in
125 g of 1.9 M HCl solution with stirring at room temperature.
The solution was heated to 40 ꢀC before adding 6.83g TEOS.
After 3 h pre-hydrolysis of TEOS, 1.6g thiol precursor MPTMS
General procedure for the Strecker reaction of aldehydes and
ketones
To the mixture of aldehyde/ketone (3 mmol), amine (3 mmol),
and catalyst 1 (93 mg; ꢂ5mol% –SO3H groups) in a sealed tube,
trimethylsilyl cyanide (98%, Merck; 3.6 mmol) was added at
ꢀ
50 C with constant stirring for the desired time as indicated in
Table 2. The reaction was monitored by TLC. After the
completion of the reaction, the reaction mixture was diluted with
dichloromethane and filtered to obtain the crude product. If it
was necessary, the crude products were further purified by
column chromatography (Table 2).
ꢀ
was added. The resultant solution was stirred for 20 h at 40 C,
after which the mixture was aged at 100 ꢀC for 24 h under static
conditions. The solid was recovered by filtration and air dried at
room temperature overnight. The template was removed from
the as-synthesized material by washing with ethanol using
a Soxhlet apparatus for 24 h.
Results and discussion
A method to overcome the problem of recyclability of the
traditional acid catalysts is to chemically anchor their reactive
center onto an inorganic solid support with large surface area, to
create a new organic-inorganic hybrid (interphase) catalyst.30 In
this type of solid, the reactive centers would are highly mobile,
similar to homogeneous catalysts, and as recyclable as hetero-
geneous catalysts. It has been shown previously that ordered
mesoporous materials, bearing sulfonic acid groups, combining
high acid strength with large surface area, were promising
alternatives to traditional homogeneous acids and sulfonic acid
resins in catalyzing chemical transformations.29 Moreover,
organic tailoring of the internal surface and consequently the
chemical behaviour of these mesoporous materials can easily be
achieved by either grafting of different functional groups or
via direct synthesis through co-condensation of siloxane and
organosiloxane species in the presence of various templating
surfactants.30 In other words, these solid catalysts combine, in
a single solid, both special chemical reactivity of organo-
functional groups and properties such as mechanically stable
structure, high surface area and large ordered pores with narrow
size distribution of the inorganic backbone.
Preparation of SBA-15-Ph-Pr-SH
To a suspension of SBA-15-Pr-SH (3 g) in dry toluene,
PhSi(OEt)3 (PTES, 4 mmol) was added. The resulting mixture
was first stirred at room temperature for 1 h and then refluxed for
a further 24 h. The solid materials were filtered off and succes-
sively washed with toluene, EtOH, and Et2O, and dried over-
night at 120 ꢀC to afford the corresponding SBA-15-Ph-Pr-SH.
Preparation of SBA-15-Ph-Pr-SO3H
Conversion of thiol groups of the catalyst to sulfonic acid
moieties was accomplished using hydrogen peroxide. Typically,
0.3 g of solid hydrophobic material was suspended in 10 g of
aqueous 30 wt% H2O2. This suspension was stirred at room
temperature in an Ar atmosphere for 24 h. After the oxidation
treatment, the resulting solution was filtered and washed sepa-
rately with water and ethanol. Finally the wet material was
suspended in 1M H2SO4 solution for 2 h, washed several times
with water and ethanol, and dried at 60 ꢀC under vacuum
overnight to give the corresponding nanoreactor 1.
We have shown recently among the different kinds of sulfonic
acid based nanoreactors which had different pore sizes and
organic groups, the catalyst in which both acidic sites and phenyl
groups were located inside the mesochannels of SBA-15 was
mostly active in the Pechmann reaction (Table 1, Scheme 1).31
pH Analysis of the nanocatalyst 1
The concentration of sulfonic acid groups was quantitatively
estimated by ion-exchange pH analysis. To an aqueous solution
of NaCl (1 M, 25 mL) with a primary pH 5.1, the catalyst 1
Table 1 Physicochemical and textural properties of sulfonic acid based nanoreactors
Catalysta
SBET
R
Pore sizec
Vp
Proton capacitye
b
d
1
2
3
4
5
453
458
586
430
356
—
13.0
12.9
12.9
31.5
27.8
0.24
0.11
0.07
0.67
0.58
1.46
1.17
1.20
1.77
1.61
Mef
Phf
—
Phg
a
1 ¼ MCM-Pr-SO3H,23b R ¼ none; 2 ¼ MCM-Me-Pr-SO3H,31 R ¼ Me; 3 ¼ MCM-Ph-Pr-SO3H,31 R ¼ Ph; 4 ¼ SBA-15-Pr-SO3H,31 R ¼ none;
b
c
d
e
5 ¼ SBA-15-Ph-Pr-SO3H,31 R ¼ Ph. BET surface area (m2 gꢁ1). BJH pore size rp (A). Total pore volume (cm3 gꢁ1). mmol (–SO3H) gꢁ1
;
˚
f
determined by pH analysis after ion-exchange. The loadings of methyl and phenyl groups in 2 and 3 were 0.70 and 0.33 mmol gꢁ1, respectively,
based on TGA and elemental analysis. g The loading of phenyl group in 5 was 0.37 mmol gꢁ1 based on TGA and elemental analysis.
8666 | J. Mater. Chem., 2009, 19, 8665–8670
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120 and 55 minutes, respectively, and the corresponding a-amino
nitriles were obtained in excellent yields (Table 2, entry 3).
To clarify the above observations, the substrates to acid sites
molar ratio was kept constant, and it was found that the higher
conversion of starting materials in the case of catalysts 4 and 5, in
comparison to nanoreactors 1–3, might likely be ascribed to the
larger pore size of the acid sites in these catalysts. Furthermore,
the beneficial influence of phenyl groups in close proximity to
sulfonic acid groups was manifested by significant enhancement
of the catalytic activity for nanoreactor 5 in comparison with
those observed for nanoreactor 4 (Table 2, entry 3).
Based on these observations, it was clear that the pore size and
hydrophobic character of catalysts 4 and 5 were crucial param-
eters in the effectiveness of sulfonic acid based catalysts in
catalyzing the Strecker reaction. To explain the higher activity of
nanoreactor 5 compared to nanoreactor 4, it could be said that
the higher affinity of the silica framework of SBA-15 in 4 to water
byproduct probably allowed a gradual desorption of the starting
materials from the catalyst surface, thus retarding the progress of
reaction. In nanoreactor 5, in which both sulfonic acid and
phenyl groups were covalently assembled inside the meso-
channels of SBA-15, this problem had been obviated (Scheme 2).
After establishing the optimal reaction conditions, we pro-
ceeded to investigate further the scope of this new catalytic
reaction with nanoreactor 5 under solvent-free conditions. The
catalytic activity of 5 is clearly recognized by the fact that no
reaction was observed in the absence of this catalyst. The results
showed that this solvent-free protocol could be employed in
highly efficient Strecker reaction of various types of ketones,
affording the corresponding a-amino nitriles in high yields and
with >99% purity (Table 2). Various types of substituted aceto-
phenones, bearing both electron donating and withdrawing
groups on their aromatic rings, also furnished the related
a-amino nitriles in good to excellent yields (Table 2, entries 4–8).
Even in the cases of the most challenging highly hindered and
enolizable ketones, the corresponding products were obtained in
good to excellent yields through a similar three-component
procedure (Table 2, entries 9–15).
Scheme 1 A typical schematic pathway for the preparation of sulfonic
acid based nanoreactor 5 ¼ SBA-15-Ph-Pr-SO3H.
We also demonstrated that both the functionalized ligand and
the pore size of the silica backbone had a crucial effect on the
catalytic activity of the supported sulfonic acids 1–5 for the
Pechmann reaction.
In this study we show that catalyst 5 (1–5 mol% equivalent
to –SO3H group) is a highly powerful and recyclable catalyst for
the three-component Strecker reaction of ketones, in good to
excellent yields under solvent-free conditions.
First, we focused our attention on the Strecker reaction of
benzaldehyde (3 mmol) with aniline (3 mmol) and TMSCN
(3.6 mmol) in the presence of 1 mol% of nanoreactors 1–5, under
solvent-free conditions. The corresponding a-amino nitriles were
obtained rapidly in quantitative yields, within 15 minutes at room
temperature. Nanoreactors 1–5 were equally effective in one-pot
reaction of benzaldehyde, TMSCN and p-anisidine, as amine
source, under the same reaction conditions (Table 2, entry 2).
Encouraged by these results of the Strecker reaction with
aldehydes, we then directed our attention toward more chal-
lenging unreactive ketones under similar conditions. Interest-
ingly, it was found that the reactions of acetophenone with
aniline and TMSCN, using 1 mol% nanoreactor 1–5, were very
sluggish at room temperature and afforded the corresponding
a-amino nitrile in poor yields. To further optimize the reaction,
we then turned our attention to reaction temperature and also
to reactants/catalyst ratios and we found that the best results
were obtained in a ratio of ketone:amine:TMSCN:catalyst ¼
1:1:1.2:0.05 at 50 ꢀC. However. it is very important to note that
while increasing the reaction temperature to 50 ꢀC and catalysts
loading to 5 mol% improved the rate and yield of reaction, a low
isolated yield of corresponding a-amino nitrile derivative was
obtained even after prolonged reaction time (24 h) when nano-
Products are separated by simple purification in pure form in
most cases where quantitative conversion are obtained and it is
not normally necessary to further purify the products by column
chromatography which requires large amounts of solvents.
The stability of the sulfonic acid groups was also studied by
repeating the Strecker reaction of acetophenone with aniline and
TMSCN over 5 mol% of catalyst 5.
It is worth noting that nanoreactor 5 could be used in more
than fifteen successive runs and exhibited consistent activity with
an average yield of >98%. It was concluded that the activity of
the catalyst remained unchanged throughout these fifteen runs
(Fig. 1).
Although there was no considerable change in the activity of the
catalyst during recovery experiments, we performed transmission
electron microscopy (TEM) analysis of catalyst 5 after the 15th
reaction cycle to determine if any structural change had occurred
in the catalyst or not. The TEM images of catalyst 5 after 15 times
recovery showed that the morphology remained unchanged after
recovery (Fig. 2). The left image in Fig. 2 clearly shows the
hexagonal array of uniform channels with the typical honeycomb
appearance of hydrophobic SBA-15 solid sulfonic acid.
˚
reactors 1–3 (average pore size 12.9–13.0 A), were used (Table 2,
entry 3). On the other hand, when nanoreactors 4 and 5 were
employed instead, the reaction time dramatically decreased to
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Table 2 Solvent-free Strecker reaction of aldehydes/ketones using 5 mol% of catalyst 5
Yielda b (%)
,
Entry
1c
Aldehyde/Ketone
Amine
Product
Nanoreactor
Time min/[h]
1
2
3
4
5
15d
15d
15d
15d
5d
100
100
100
100
100
2c
1
2
3
4
5
45d
45d
45d
45d
25d
100
98
99
100
100
3
1
2
3
4
5
[24]
[24]
[24]
120
55c
45
40
40
96
98 (98)
4
5
5
30
97
91
5
[3]
6
5
[3]
92
7
8
5
5
[4]
86
87
[3.5]
9
5
5
[8]
[8]
95
93
10
11
5
[7]
75
8668 | J. Mater. Chem., 2009, 19, 8665–8670
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Table 2 (Contd.)
Yielda b (%)
,
Entry
12
Aldehyde/Ketone
Amine
Product
Nanoreactor
5
Time min/[h]
[7]
78
13
5
[18]
75
14
15
a
5
5
[19]
[19]
65
72
Isolated yields of pure products. b Conditions were as follows: aldehydes/ketones (3 mmol), amine (3 mmol), TMSCN (3.6 mmol), catalyst (5 mol%), at
50 ꢀC. c Reactions were performed at room temperature. d 1 mol% of catalyst was used.
Fig. 1 Recyclability of catalyst 1 for the Strecker reaction of benzalde-
hyde, aniline and TMSCN.
Scheme 2 Plausible Strecker reaction model using nanoreactor 5 under
solvent- free condition
Conclusions
In summary, we showed that catalyst 5, having both acidic sites
and hydrophobic groups located inside the mesochannels of
SBA-15, was an effective catalyst for the one-pot synthesis of
a-amino nitriles from a variety of hindered ketones. These
materials could easily be prepared from commercially available
and relatively inexpensive starting materials. Catalyst 5 was in
particular highly stable and could be reused in fifteen successive
runs with no considerable loss of activity and with no significant
structural change. Based on these observations, it could be
Fig. 2 Transmission electron micrographs of nanoreactor 5 after the
15th reaction cycle: (left) perpendicular and (right) in the direction of the
pore axis.
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